Latest advances in solar fuels

Resume : Due to the intermittent nature of most renewable energy sources (such as solar and wind), practical large scale renewable energy utilization demands both efficient energy conversion and large scale energy storage or alternative usage. Earth-abundant but highly active electrocatalysts need to be developed to enable efficient and sustainable production of energy using electrocatalytic and photoelectrochemical (PEC) water splitting. We developed earth-abundant nanoscale electrocatalysts, such as exfoliated nanosheets of MoS2, for enhancing hydrogen evolution reaction (HER). We established nanotructures of ternary pyrite-type cobalt phosphosulfide (CoPS) as the best earth-abundant HER catalyst in acidic conditions to date. Our recent efforts on the conversion of biomass derived molecules utilizing the increasingly affordable renewable electricity to value-added chemicals will be discussed. Efficient photoelectrochemical hydrogen generation systems that integrate these earth-abundant electrocatalysts with efficient semiconductor materials have been rationally designed and demonstrated. I will also discuss novel hybrid solar-charged storage devices that integrate photoelectrochemical solar cells and redox flow batteries (RFBs). In these integrated solar flow batteries (SFBs), converted solar energy is used to directly charge up the redox couples without external electric bias; which can be discharged to generate the electricity when electricity is needed.

Resume : Direct photoelectrochemical (PEC) splitting of water into hydrogen and oxygen remains a promising route for producing sustainable and renewable hydrogen fuel. From chemical stability concerns, metal oxides are the desirable photoelectrodes. Unfortunately, most metal oxides have too large bandgaps to effectively absorb the main portion of the sunlight spectrum. Through density-functional theory (DFT) and experimental synthesis, we demonstrate a new approach to effectively engineer the bandgap of barium bismuth niobate double perovskites (Ba2BiNbO6) (BBNO). We find that the film with the composition of Ba2Bi1.4Nb0.6O6 forms single pure phase and exhibits a nearly direct bandgap of about 1.64 eV, which is small enough to absorb visible light at wavelengths below 756 nm. We further find that the PEC activity of BBNO-based electrodes can be dramatically enhanced by coating thin BBNO layers on tungsten oxide (WO3) nanosheets to solve the poor conductivity issue while maintaining strong light absorption. The PEC activity of BBNO/WO3 nanosheet photoanodes can be further enhanced by applying Co0.8Mn0.2Ox nanoparticles (CoMnO NPs) as a co-catalyst. A photocurrent density of 6.02 mA cm-2 at 1.23 V (vs. RHE) is obtained using three optically stacked, but electrically parallel BBNO/WO3 nanosheet photoanodes. The BBNO/WO3 nanosheet photoanodes also exhibit excellent stability in a high-pH alkaline solution; the photoanodes are stable with negligible photocurrent density decay for more than 7 h. Our work suggests a viable approach to improve the PEC performance of BBNO absorber-based devices.

Resume : The seek for the most promising renewable energy source, brings hematite (α-Fe2O3) photoelectrode a promising candidate for hydrogen production via water splitting induced by sunlight irradiation. However, its poor electronic characteristics together with its complex scale-up engineering production have prevented its commercial applications. This work describes different approach for improving hematite nanocrystalline structure / transparent conductor oxide substrate (TCO) interface and also, hematite nanocrystalline surface for chemical reactions with controlled addition of Tin (Sn). Herein, α-Fe2O3 electrodes were obtained by coating deposition of an alcoholic solution composed of Fe3+ complexed polymeric solution synthesized via Pechini method and annealed at 750 oC for 30 min under N2 atmosphere. Modified electrodes were obtained by 2 different routes: i) In the route here named (Sn-R1), 20 µL of a 5 µmol SnCl4 alcoholic solution was added on top of the unmodified α-Fe2O3 calcined films followed by re-calcination at 750 ⁰c for 30 min under N2 atmosphere. ii) In the second route named (Sn-R2), 1% excess of SnCl4 were added into the synthesized α-Fe2O3 Pechini solution, followed by spin-coating deposition and the same annealing protocol. Unmodified hematite films showed considerable enhancement on photocurrent density J, increasing from a range of ~ 30 µA.cm-2 [1, 2] up to 0.127 mA.cm-2 at 1.23 VRHE, with the onset of the photocurrent curve starting at 0.8 VRHE. Such improvement has been mainly attributed to a better uniformity of the films and an improved interfacial adherence of the hematite thin film to the TCO provoked primarily by the substitution of water to ethanol as a solvent into the polymeric deposition solution. When modified with Sn, hematite films showed an increasing in photocurrent density up to ~ 40% at 1.23 VRHE in both Sn-R1 and Sn-R2 routes. Specifically, in Sn-R1 route, (SnCl4 added dropwise on top of the electrode), a slight alteration in the onset of the curve were observed followed by a significant change upright on the slope. Likewise, in Sn-R2 route (when Sn are added into the Pechini solution), a pronounced alteration on the onset of the photocurrent curve, moving towards anodic direction from ~0.8 VRHE for unmodified electrode to ~0.95 VRHE has been observed, together with a change on the slope to upright direction, where surfaces effects are predominant [3] . Although increasing in photocurrent are shown in both routes, Sn-R2 provides more homogenous Sn addition through the hematite film nanostructure, enabling single-step preparation process in addition of better reproducibility. Acknowledgments The authors acknowledge the financial support from the Brazilian agency FAPESP (Grants 2015/23775-3), Capes and CNPq. D. N. F. M., F. L. S., M. A. M., R. V. G and A. F. N gratefully acknowledge support from FAPESP (Grant 2017/11986-5) and Shell and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. [1]. F. L. Souza, K. P. Lopes, P. A. Nascente and E. R. Leite, Solar energy materials and solar cells, 2009, 93, 362-368. [2]. D. A. Bellido-Aguilar, A. Tofanello, F. L. Souza, L. N. Furini and C. J. L. Constantino, Thin Solid Films, 2016, 604, 28-39. [3]. B. Yao, J. Zhang, X. Fan, J. He and Y. Li, Small, 2018, 1803746.

Resume : Natural photosynthesis has served as inspiration for solar driven fuels synthesis (e.g., hydrogen) as a potentially inexpensive technology to store solar energy into chemical bonds via photoelectrochemical water splitting. Water oxidation is still considered the bottleneck of the overall water splitting process due to its slow kinetics, however the mechanism of reaction and its kinetic consequences remains not yet well understood. Despite the 4 photogenerated holes this reaction requires to complete, recently, we have found that 3 of such species are required to diffuse together at a hematite surface to overcome the rate determining step of the water oxidation reaction. [1,2] In this talk, recent advances on the kinetics of water oxidation taking place on different metal-oxide photoanodes will be presented, focusing particularly on hematite photoanodes. A detailed mechanistic analysis will be presented by the study of the water oxidation rate law under different physicochemical conditions. Here we employed light-induced spectroelectrochemical techniques to investigate the photogeneration, recombination and reaction of the photogenerated holes and the correlation between surface density and the current densities produced. The experimental results combined with DFT calculations enabled us to propose a detailed molecular water oxidation mechanism on hematite, which is found to have parallels with proposed mechanisms for Photosystem II (PSII) in the natural photosynthesis. [3] These findings suggest that understanding the natural photosynthesis may lead to further developments in the artificial photosynthesis field. References [1] Le Formal, F. et al. Rate Law Analysis of Water Oxidation on a Hematite Surface. J. Am. Chem. Soc. 2015, 137, 6629–6637. [2] Francàs, L. et al. Rate law analysis of water splitting photoelectrodes. in: Advances in photoelectrochemical water splitting: Theory, experiment and systems analysis. Ed: Royal Society of Chemistry 2018. [3] Mesa, C. A. et al., Multihole catalysis of water oxidation on metal oxide photoanodes: Insights from in operando light-induced spectroelectrochemistry and density functional theory, Nature Chem., Under Revision.

Resume : Photocatalytic water splitting is a promising route to store solar energy in the form of hydrogen. The exact mechanism of photocatalytic water splitting remains a largely unexplored area. However, knowledge on surface intermediates and catalytic sites will allow fabrication of photoelectrodes with a maximum number of catalytic sites and higher water splitting efficiencies. Here, we report on a novel approach to study the surface intermediates using operando IR spectroscopy. An ZnSe internal reflection element (IRE) serves as substrate for the photoelectrode. This approach has several advantages over geometries in which a thin electrolyte layer separates the photoelectrode from the IRE [1,2]. Firstly, the surface groups are probed from the “back” which avoids IR absorption by the electrolyte. Secondly, since the IRE is not in contact with the electrolyte, the pH of the electrolyte is not limiting regarding the chemical stability of the IRE. Thirdly, the IR operando cell geometry is similar to conventional PEC cells. We will demonstrate the working principle of this approach using Fe2O3 as case study material. We will confirm the intermediate species predicted by simulations [3,4]. We will discuss different OER mechanisms and address implications for material design. [1] Zandi et al. Nat. Chem. (2016) [2] Zhang et al. J. Am. Chem. Soc. (2018) [3] Zhang, Bieberle-Hütter et al. J. Phys. Chem. C (2016) [4] George, Bieberle-Hütter et al. (2018) submitted

Resume : Energy issues have recently become globally important. Low-carbon green energy will have a key role in the Third Industrial Revolution. To reduce global greenhouse gas emissions in response to globalization and increasingly strict carbon emission policies, green energy technologies must be developed. A new age of human demand for green energy is coming and material scientists are devoted to finding new functional materials to produce clean and sustainable energy. Improving the energy conversion/generation/storage efficiency of energy materials has always been a great challenge. Monitoring the atomic/electronic structures close the interface in many important energy materials, such as nanostructured catalysts, artificially photosynthesizing materials, smart materials, and energy storage devices, is of great importance. Designing such a material with improved performance without understanding its fundamental properties, such as chemical states, atomic and electronic structures, and their changes under operating conditions, is difficult. Understanding and controlling the interfacial electronic structures of energy conversion/storage materials require in-situ/operando characterization tools, of which synchrotron x-ray spectroscopy is one with many unique features. The last decade has witnessed a golden age of operando and in situ synchrotron x-ray spectroscopy for energy materials. X-ray absorption spectroscopy can be used to determine local unoccupied electronic structures (conduction band) while X-ray emission spectroscopy can be utilized to examine the occupied electronic structure (valence band). The additional use of resonant inelastic X-ray scattering reveals inter- or intra-electric transitions (d-d or f-f excitation or charge transfer excitation) that reflect the chemical and physical properties of the material. The in situ/operando approach enables modifications of atomic and electronic structures of an energy material in an operational environment to be tracked. This presentation will report recent studies and perspectives of the application of in situ/operando synchrotron x-ray spectroscopy to energy materials. It will also introduce the Tamkang University (TKU) end-station, which was constructed at the Taiwan Photon Source (TPS) 45A beamline for the in situ/operando x-ray spectroscopic investigation of energy materials.

Resume : Photoelectrochemical (PEC) water splitting promises to deliver a carbon-free, high-energy density fuel but has not yet been commercialized. Partially, because we do not fully understand fundamental electronic processes occurring during PEC device operation. The operando methodology enables addressing these questions by studying devices in their actual working environment. Especially suited are hard X-ray absorption spectroscopies (XAS) offering high penetration depth and element selectivity. However, currently their broader application is limited due to lack of standard PEC cells compatible with X-ray methods. Here, we present an operando cell that simultaneously offers high X-ray detection efficiency and desired photoelectrochemical measures with low electrical noise. Most importantly, switching thin film materials for sequential operando measurements is realized in a standardized manner. We demonstrate the successful operation by collecting Fe K-edge spectra of hematite and ferrite oxide thin film photoelectrodes during water oxidation using high energy resolution fluorescence-detected (HERFD)-XAS. The cell enables to routinely conduct operando XAS experiments on photoelectrochemical thin films.

Resume : Electrochemical catalytic materials are a key component of various electrochemical devices for the storage of renewable electricity and low-carbon-footprint chemical production. For the oxygen evolution reaction (OER), it is applied to a variety of electrolysis systems. Their successful development and optimization require insight into the relations between the atomic-scale structure of the catalytic interface and the electrolyte to improve the catalytic activity and stability. In this presentation, we will focus on recent research on the design and understanding of Ir-oxide based materials and electronic structure by using in-situ/operando X-ray absorption spectroscopy technique. Based on these results, we will outline the preparation, characterization, and catalytic activity of Ir-oxide model catalysts for water splitting and discuss fundamental aspects of their structure-activity and -stability relationships.

Resume : The purpose of this talk is to demonstrate how the combination of soft x-ray and electron spectroscopies (in particular using high-brilliance synchrotron radiation) can be used to gain insights into the electronic and chemical surface properties of candidate materials for solar water splitting. Focus will be placed on compound semiconductors, with particular emphasis of chalcopyrite-based material systems, and how ambient conditions and deliberate tailoring can modify and impact the surface of such materials. It will be shown that photoelectron spectroscopy (PES), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band edge positions, band gaps, electronic level alignment, and insights into chemical stability, both with experiments in ultra-high vacuum, as well as under in situ and operando conditions.

Resume : Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage, and/or c) recombination. In this regard, we have been researching on III-V alloys involving GaN and GaP alloyed with antimony. Our recent results suggest that Sb alloying into GaN resulted in GaSbxN1-x alloys with band gaps ranging from 3.4 to 1.5 eV with appropriate band edge locations. Similarly, Sb alloying into GaP resulted in GaSbxP1-x alloys with band gaps ranging from 2.26 to 1.7 ev. In the case of GaSb0.03P0.97, a photovoltage in excess of 750 mV an onset potential of -0.5V vs RHE and a current density around 7 mAcm-2 under zero applied bias and 10 sun illumination was obtained. These promising results can be explained by the favorable direct (2.68eV) and indirect (2.26eV) energetic transitions and a sufficient negative conduction band edge at -0.7V vs RHE. This presentation will highlight recent advances with progress on both GaSbxN1-x and GaSbxP1-x materials and their growth using halide vapor phase epitaxy technique. The growth rates are modeled using a combination of thermodynamic and kinetic models and compared with experimental results.

Resume : Photoelectrochemical (PEC) water splitting is an attractive method for efficient renewable hydrogen production. However, the cost, efficiency and durability of lab-scale systems are not yet at the level required to make this technology economically attractive. Amongst all materials studied to date, the chalcopyrite class is arguably one of the most promissing classes for PEC water splitting, as it has already demonstrated low-cost and high photoconversion capabilities as a photovoltaic material. In the context of PEC, our group has shown that co-evaporated 1.65 eV bandgap (EG) CuGaSe2 is capable of evolving hydrogen with Faradaic efficiency greater than 85% and generate photocurrent densities over 15 mA.cm-2, as measured in a 3-electrode configuration in 0.5M H2SO4 under simulated AM1.5G illumination. Unfortunately, CuGaSe2’s narrow EG limits its integration as top absorber into a dual junction stacked PEC device (also known as hybrid photoelectrode, HPE). In the present communication, we report on our latest efforts to synthesize wide-EG (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating saturated photocurrent densities greater than 10 mA/cm2. We present specifically results on EG tunable Cu(In,Ga)(S,Se)2, Cu(In,Ga)S2, CuGa(S,Se)2 and CuGa3Se5. We also discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of In2S3 n-type buffer layers. We also include recent durability testing of chalcopyrite photocathodes coated with metal oxide protection layers. Finally, we present solid-state techniques and theoretical modeling used by our team to identify possible pitfalls in these material systems and discuss potential paths for improvement.

Resume : The development of robust, inexpensive and high performance semiconducting materials are needed to make the direct solar-to-fuel energy conversion by photoelectrochemical cells economically viable. In this presentation our laboratory’s progress in the development new light absorbing materials and co-catalysts will be discussed along with the application toward overall solar water splitting tandem cells for H2 production. Specifically, this talk will highlight recent results with the ternary oxides (CuFeO2 and ZnFe2O4) 2D transition metal dichalcogenides, and organic (π-conjugated) semiconductors as solution-processed photoelectrodes. With respect to ternary oxides, in our recent work [1,2] we demonstrate state-of-the-art photocurrent with optimized nanostructuring and address interfacial recombination by the electrochemical characterization of the surface states and attached co-catalysts. In addition, we report an advance in the performance of solution processed two-dimensional (2-D) WSe2 for high-efficiency solar water reduction by gaining insight into charge transport and recombination by varying the 2D flake size[3]and passivating defect sites[4]. Finally, with respect to π-conjugated organic semiconductors, the main approaches for employing organic semiconductor-based devices towards solar H2 generation via water splitting are reviewed[1] and new benchmark results are described. [1] X. Zhu, N. Guijarro, Y. Liu, P. Schouwink, R. A. Wells, F. L. Formal, S. Sun, C. Gao, K. Sivula, Adv. Mater. 2018, 30, 1801612. [2] Guijarro, N.; Bornoz, P.; Prevot, M.; Yu, X.; Zhu, X.; Johnson, M.; Jeanbourquin, X.; Le Formal, F.; Sivula, K., Sustainable Energy Fuels 2018, 2, 103-117. [3] Yu, X.; Sivula, K., Chem. Mater. 2017, 29, 6863-6875. [4] Yu, X.; Guijarro, N.; Johnson, M.; Sivula, K. Nano Lett. 2018, 18, 215-222. [5] L. Yao, A. Rahmanudin, N. Guijarro, K. Sivula, Adv. Energy Mater. 2018, 1802585.

Resume : Water oxidation (WO) is the ideal reaction to provide electrons and protons for the photo- and electro-synthesis of renewable fuels, generating exclusively oxygen as a benign by-product. Besides being demanding from the thermodynamic point of view, WO is also extremely difficult from the kinetic one. This has caused an explosion of interest for developing efficient water oxidation catalysts (WOCs) [1]. Parallel to the search for efficient non-noble metal WOCs [2], many efforts are directed forward noble-metal atom economy in WOCs [3]. Three strategies have been proposed to minimizing the atomic contents of noble metal: 1) exploiting molecular WOCs, under the assumption that all metal centres are catalytically active, differently from what happens in heterogeneous WOCs; 2) anchoring a well-defined molecular WOC onto a suitable support, thus obtaining a heterogenized WOC, combining the positive aspects of homogeneous and heterogeneous catalysis; 3) diluting active metal centres in a suitable material with features that maximize the metal-accessibility and performances. Herein, recent results of our efforts aimed at pursuing all three strategies for developing efficient WOCs based on iridium are reported [3]. [1] Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Chem. Rev. 2014, 114, 11863. Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W.; Chem. Rev. 2015, 115, 12974. [2] Kärkäs, M. D.; Åkermark, B. Dalton Trans. 2016, 45, 14421. [3] Macchioni, A. Eur. J. Inorg. Chem. 2019, 7.

Resume : The replacement of fossil fuels by a clean and renewable energy source is one of the mostn urgent and challenging issues our society is facing today, which is why intense research is devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidize water and generate carbohydrates (a solar fuel) for over a billion years. Inspired but not constrained by nature, artificial systems [1] can be designed to capture light and oxidize water and reduce protons or other organic compounds to generate useful chemical fuels. In this context this contribution will present how molecular water oxidation catalysts can be anchored on solid supports to generate powerful hybrid electro- and photo-anodes for water splitting. [2] References: 1. S. Berardi, S. Drouet, L. Francàs, L.; C. Gimbert-Suriñach, M. Guttentag, C. Richmond, T. Stoll, A. Llobet, Chem. Soc. Rev., 2014, 43, 7501-7519. 2. (a) R. Matheu, M. Z. Ertem, J. Benet-Buchholz, E. Coronado, V. S. Batista, X. Sala, A. Llobet, J. Am. Chem. Soc. 2015, 137, 10786-10795. (b) J. Creus, R. Matheu, I. Peñafiel, D. Moonshiram, P. Blondeau, J. Benet-Buchholz, J. García-Antón, X. Sala, C. Godard, A. Llobet, A. Angew. Chem. Int. Ed. 2016, 55, 15382-15386. (c) R. Matheu, I. A. Moreno-Hernández, X. Sala, H. B. Gray, B. S. Brunschwig, A. Llobet, A; N. S. Lewis, J. Am. Chem. Soc. 2017, 139, 11345-11348.

Resume : Photocatalytic overall water splitting without using sacrificial agents is very appealing for converting solar energy into chemical fuels. Recently, polymer photocatalysts with tunable molecular structures have emerged as promising alternatives to inorganic photocatalysts for overall water splitting. Unfortunately, developing single-component photocatalysts for overall water splitting has been proven rather difficult, due to the fact that very few polymer photocatalysts possess suitable band structures with sufficient redox potentials to simultaneously drive water oxidation and proton reduction reactions. With inspiration from the natural photosynthesis, coupling two different polymer photocatalysts to form a Z-scheme structure could potentially lead to overall water splitting. The key for developing polymer-based Z-scheme systems lies mainly in identifying the H2-evolving and O2-evolving polymer photocatalysts with suitable band structures. Moreover, efficient charge transfer between the interfaces of heterostructures is crucial to achieve high photocatalytic activity. Herein, 1 nm-thick ultrathin graphitic carbon nitride nanosheets (CNNS) were firstly synthesized by a two-step ultrasonication-calcination method. With the unique structural features for efficient electron excitation and charge transport, the resultant ultrathin CNNS exhibited excellent activity for photocatalytic hydrogen evolution. Subsequently, by introducing homogeneous defect and dopant to CNNS via NaBH4 thermal treatment, we modulated the band structures of this ultrathin CNNS to achieve higher over potential for water oxidation. Finally, taking use of electrostatic self-assembly strategy, the photogenerated holes and electrons produced by CNNS and NaBH4 treated CNNS, respectively, could be promptly recombined at the interfaces of heterostructures whereas other excited electrons and holes were retained on different counterparts to drive redox reactions, leading to an efficient two-dimensional ultrathin Z-scheme photocatalytic system.

Resume : The true electrocatalyst of water oxidation in a number of cases proved to be an in situ developed CuO/OH film instead of (or beside) the original Cu complex itself, since the breaking of metal-ligand interactions and release of Cu ions under the applied conditions often successfully compete with the homogeneous process. The in situ decomposition of catalyst candidates is obviously unwanted; on the other hand, inexpensive and controllable precursor complexes represent an exciting platform to fabricate nanostructured CuO/OH coatings utilized in artificial photosynthesis. We investigated a family of low-cost and environmentally benign Cu-tripeptide complexes with tuned stability, but uniform {N,N,N,O}eq peptide binding mode throughout the surveyed pH range of 7 to 11. Boundary conditions were obtained for their alternating functioning between molecular catalyst and precursor to CuO, in borate buffers. At pH <8.5 and pH >10 homogeneous catalysis via an emblematic Cu(III)-peptide intermediate was confirmed, assisted by borate. In contrast, at pH 8.6-10 the development of robust CuO coatings consisting of nanoparticles occurs upon water oxidation on indium tin oxide. Importantly, a fine CuO coating could be also fabricated on a α-Fe2O3 nano-array without affecting its anisotropic morphology. We identified the interplay between the co-existent borate equilibrium species and the Cu complexes as the key factor to determine the dominant process. Financial support of the NKFI-128841 grant, the VEKOP-2.3.2-16-2016-00011 grant supported by the European Structural and Investment Funds, and the János Bolyai Research Scholarship from the Hungarian Academy of Sciences (J. S. Pap) are gratefully acknowledged.

Resume : The topic of generating fuels in a sustainable manner has never been more relevant than today. The pollution originating from hydrocarbon fuels has started causing obvious changes to the environment. Solar photoelectrochemical (PEC) fuel generation is a potential future green technology that utilizes the photocatalytic properties of semiconducting materials for converting energy in the ultraviolet to the near infrared region of the solar spectrum to fuels through processes such as water splitting. Although various materials and strategies have been developed for making the technology commercially feasible, low process efficiency and stability of the electrodes are still outstanding issues. Several of the stable photocatalysts do not have the ability for broad spectrum light absorption and hence, do not offer high solar-to-fuel conversion efficiency. We have recently demonstrated that the heterostructures formed by narrow and wide band gap low dimensional (0, 1 or 2D) materials could utilize solar photons in a broad energy range. When such heterostructured materials were used as electrodes in the PEC cell for water splitting, a significant enhancement in the performance was observed. The low dimensional materials facilitated even an uphill transfer of electrons at the hetero-interface. The advances in this work will be discussed in this presentation.

Resume : Lack of a highly active, stable, earth-abundant electrocatalyst for carrying out hydrogen evolution and oxidation reactions (HER and HOR) in strongly acidic conditions is a major technical challenge for developing economical polymer electrolyte membrane (PEM)-based electrolyzers and fuel cells. We have found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material. We propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt. This material demonstrates excellent HER and HOR activities in acids using non-platinum group metal (PGM) catalysts, opening new opportunities to develop technologically and economically viable unitized regenerative fuel cell (URFC) systems and cost-effective PEM electrolyzers. Furthermore, these results have broad implications for using nitrogen incorporation (e.g. by N2 plasma treatment) to activate other non-conductive compounds and films to form new active electrocatalysts.

Resume : This talk describes emerging electrocatalyst and photoelectrode architectures for which the active sites for electrochemical reactions are encapsulated by nanomembrane overlayers that are permeable to the electroactive species of interest. These overlayers can be thought of as multifunctional exoskeletons that can be engineered to enhance the stability, charge transfer rates, and/or reaction selectivity of electrodes used in solar fuels generators. Here, I will present several case studies based on work from our lab that demonstrate each of these benefits using silicon oxide (SiOx) nanomembranes coated onto Pt-modified Si photocathodes and Pt thin films. We show that well-defined thin film electrodes are especially useful for quantifying transport properties of these nanomembranes and developing a deeper mechanistic understanding about (photo)electrocatalysis that occurs at buried interfaces. Importantly, these studies also show that properties of oxide nanomembranes can be highly tunable, providing new control knobs that be used to optimize the stability and efficiency of solar fuels generators for a variety of electrochemical reactions.

Resume : Solar hydrogen is a promising sustainable energy vector for mobility and power generation, and steady progress has been made towards the development of efficient water splitting materials. While most demonstrated solar water splitting systems run on liquid water, the idea of using water contained in ambient air is very appealing. Capturing water from the air implies that no liquid water is needed for operation. This potentially enables the construction of free-standing devices, in areas which don’t usually have a nearby water supply i.e. near to roads or remote areas. Since no ambient air testing protocols exist, we have developed a solid-state PEM-PEC cell inspired by the polymeric electrolyte membrane (PEM) electrolysers. In the PEM-PEC cell, the two electrochemical half-reactions are separated by a solid electrolyte. In this design the hydrogen generation and water dissociation are taking place in different compartments, allowing the exposure of the photoanode to ambient air. We show here how the functionalisation of porous TiO2 or WO3 photoanodes with ionomers enables the capture of water from ambient air. Functionalised photoanodes operating at 60-80% RH were able to recover 60-70% of the performance obtained when water is introduced in liquid phase. In addition long term experiments have shown remarkable stability at 60% RH for 80h of cycling (8h continuous illumination - 8h dark) demonstrating that the concept can be applicable outdoors.

Resume : While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are have sparked intense research interest - particularly due to their much higher synthetic flexibility. While some parallels can be drawn to organic photovoltaics, the fundamental understanding of photocatalytic processes has stayed behind the rapid development of more efficient materials. In this study,[1] we investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies with implications for the development of new and more efficient organic photocatalysts. References [1] Sachs, M.; Sprick, R. S.; et al. Nat. Commun. 2018, 9 (1), 4968.

Resume : In an effort to probe into the fundamentals of charge carriers dynamics relevant to photo-catalytic materials, we have performed femtosecond time resolved spectroscopy (TRS) on anatase and rutile single crystals in the gas phase as well as in aqueous environment. By studying the effect of pump fluence, differences in charge carrier recombination rates and nature between both crystals were observed. The time constants for carrier recombination are two orders of magnitude slower for anatase (101) when compared to those of rutile (110) single crystal. Bulk defects introduced by sample reduction via annealing in ultra-high vacuum resulted in faster recombination rates for both polymorphs. Both surfaces (fresh and reduced) probed by pump fluence dependence measurements revealed that the major recombination channel in fresh and reduced anatase and reduced rutile is the first-order Shockley–Reed mediated. The effect of a hole trapping agent (ethanol) and of electron trapping agents (Ag+ and Ce4+ cations) is investigated over TiO2(110) rutile single crystal. Ethanol adsorption resulted in increasing the transient absorption signal in the 700-950nm with a slight decrease of their lifetime. No change was seen in the 500nm region commonly attributed to the presence of holes. The decay of the 700-950 nm signal mainly composed of two components is considerably perturbed by the presence of Ag+ or Ce4+ cations, which may indicate its surface origin. Work in progress to extract kinetic information from the study of the concentration effect of these metal cations in both the “nominally” hole and electron transient absorption regions

Resume : Nowadays, the circular economy of carbon dioxide constitutes one of the major world challenges. The conversion of CO2 into value-added chemicals and/or fuels, using renewable energy and earth-abundant elements as well as environmental friendly materials is a key priority. Under this scenario, carbon dioxide can give rise to different reduction reactions pathways with several sub products. Consequently, catalyst materials are essentials for increasing selectivity and productivity towards determined sub product such as CO, syngas, formic gas, methanol or even methane or double bonded carbon molecules. So, aside of the system design constraints, parameters as the over-potential values or charge transfer resistances are determining the final energy balance as well as the overall efficiency and productivity. In this presentation, the influence of the catalyst characteristics on the final performances, energy balances and final productivity will be presented and discussed considering over-potentials, Tafel plots and the overall characteristics of the selected catalyst according to the target CO2R products and used electrolytes. Catalyst mechanisms considering bimetallic, metal combination with P, S or Se or the role of individual catalyst atoms using metal organic frameworks, MOF, will be discussed in order to design the more adequate catalyst for enhancing the electrode performances. Special attention will also be paid on the conditions for diminishing as much as possible the overall cell voltage. For it, aside the development of highly efficient cathode catalysts, the development of high-performance catalysts for the oxygen-evolution reaction (OER) is also of paramount importance for achieving a cost-effective conversion of renewable electricity to fuels and chemicals. Considering these boundaries, we will demonstrate industrial feasibility and competitiveness of the electrochemical conversion of CO2 as alternative for the future energy models considering solar and dark refineries as energy paradigms delivering high current densities at low voltage cells or under bias free conditions, improving energy balance with high stability.

Resume : Iron titanate has recently surfaced as a promising photocatalyst for the O2-generating water splitting half reaction. Compared to other photocatalysts, it shows improved structural and photochemical features, which result in better conductivity, smaller band gap, longer charge carriers diffusion and adequate valence band energy. It is also cheap and non-toxic. However, Fe2TiO5 has been scarcely explored for the solar energy conversion into fuels, especially as the main absorber. In this investigation, orange phase-pure nanosized Fe2TiO5 (30 to 100 nm), with band gap of 2.0 eV, was obtained through a hydrothermal method. The nanoparticles were suspended in water and irradiated by a Xe lamp (λ>400 nm), producing 59.2 μmol h-1 g-1 of O2, using NaIO4 as electron scavenger. This is the first time O2 production using suspended Fe2TiO5 is reported. The evolution was boosted to 92.4 μmol h-1 g-1 after the loading of 1 wt% NiO nanoparticles as cocatalysts. Further loading impairs the reaction yield. Photoelectrochemical measurements in phosphate buffer (pH 7) corroborate these observations. For example, overpotentials of 0.34 and 0.18 on the water oxidation potential ( 0.82 V vs NHE) were observed for pristine and Fe2TiO5 loaded with 1wt% NiO. Thus, NiO leads to lower energetic barrier for the minority charge extraction, also favoring the charge carriers separation. Overall, this work illustrates how structural alterations of newly discovered photocatalysts can improve photochemical energy conversion. The authors acknowledge support from FAPESP (Grant 2017/11986-5) and Shell and the strategic importance of the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation.

Resume : One of the biggest challenges of sustainable development is efficient solar energy harvesting and storage. Huge progress in photovoltaics enabled economically feasible solar-driven electricity generation, however an issue of the energy storage remains unresolved. One emerging possibility is photocatalytic oxygen reduction to the high-energy product hydrogen peroxide. Hydrogen peroxide is easier to store than hydrogen. Herein we present our recent work on photocatalysis and photoelectrocatalysis with organic semiconductors which efficiently produce hydrogen peroxide. When the organic semiconductor is in contact with oxygenated aqueous solution, the light driven 2-electron/2-proton redox cycle leads to photoreduction of oxygen to hydrogen peroxide. We show examples of n type, p type, and ambipolar organic semiconductors performing photochemical oxygen reduction, both as stand-alone photocatalysts and as components in photoelectrochemical cells. Many of these catalytic systems are stable and effective in a wide pH range from 2-12. This demonstration of efficient organic semiconductors as aqueous photocatalysts possibly opens new avenues for their catalytic applications.

Resume : Conjugated polymer (CP) semiconductors can be used to photocatalytically produce hydrogen from water using solar radiation. Most organic semiconductors have required a metal co-catalyst, usually photodeposited Pt, to produce quantifiable amounts of H2.(1) It is accepted that Pt accelerates the hydrogen evolution reaction (HER) by facilitating electron extraction from the semiconductor and subsequent electron transfer to protons in the aqueous phase.(2) However, many CPs contain significant levels of residual Pd, originating from their synthesis via Pd catalyzed polymerization.(3) There are conflicting views in the literature about the role of residual Pd in CP photocatalytic systems, which impedes the development of guiding structure–property relationships for efficient polymer design. In this presentation the effect of residual Pd on the hydrogen evolution activity in CP photocatalytic systems is discussed using CP nanoparticles as a model system. Residual Pd is observed in the form of homogeneously distributed nanoparticles within the polymers, and in the absence of additional co-catalysts is essential for any hydrogen evolution to be observed from the polymers. Low Pd concentrations (<40 ppm) are sufficient to have a significant effect on the HER rate, which increases linearly with increasing Pd concentration from <1 ppm to approximately 100 ppm, at which point the HER rate begins to saturate. Transient absorption spectroscopy experiments suggest that residual Pd mediates electron transfer from the F8BT nanoparticles to protons in the aqueous medium, thereby acting as a proton reduction co-catalyst. Because the morphology and distribution of residual Pd within CPs cannot effectively be controlled, its presence can make it impossible to make rational comparisons between the photocatalytic performance of different CPs. Furthermore, the presence of Pd could hinder a range of reduction chemistries, including CO2 reduction, as it will act as an electron trap, promoting proton reduction at the expense of the species of interest. Purification methods are presented that remove residual Pd from CPs, enabling rational comparisons between different organic semiconductor photocatalysts to be made. References: (1) Banerjee, T.; Gottschling, K.; Savasci, G.; Ochsenfeld, C.; Lotsch, B. V. H 2 Evolution with Covalent Organic Framework Photocatalysts. ACS Energy Lett. 2018, 3 (2), 400–409. (2) Yang, J.; Wang, D.; Han, H.; Li, C. Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis. Acc. Chem. Res. 2013, 46 (8), 1900–1909. (3) Krebs, F. C.; Nyberg, R. B.; Jørgensen, M. Influence of Residual Catalyst on the Properties of Conjugated Polyphenylenevinylene Materials: Palladium Nanoparticles and Poor Electrical Performance. Chem. Mater. 2004, 16 (7), 1313–1318.

Resume : Solar energy conversion to different types of energy which might be subsequently released in diverse ways remains an unsaturated need, since the global demand for energy is continuously growing and will continue to rise. However, currently the field of renewable energy suffers from lack of an ideal material able to drive conversion of the solar energy with high efficiency and being stable enough to provide a long-term productivity. Therefore, multi-directional efforts including development of new materials and catalysts, incorporation of plasmonic nanostructures or creation of low-level sub- stoichiometry through different approaches are continuously undertaken and devoted to minimisation of the required bias voltage, improvement of light capture or charge transport properties. Our recent achievements regrading employment of different in structure polyoxometalates as catalysts, sub-stochiometric semiconducting oxides, newly designed and engineered crystal structures in solar driven arrangements as well as their performance for photo- induced processes will be presented and discussed in detail. Besides the extensive characterization of the above-mentioned systems, the kinetic assessment of the transient absorption phenomena will be approached. Use of transient absorption spectroscopy to study charge carrier behaviour in semiconductors-based systems allows to relate a photoelectrode architecture to the charge carrier origin, separation, collection, trapping, lifetime and therefore final efficiency. Therfore, comparison of the charge carrier dynamics occurring in semiconducting systems: bare, modified as well as mixed, are particularly useful in clarifying the observed differences in their photoelectrochemical performance.

Resume : Abstract: Photoelectrochemical (PEC) water splitting offers a promising way to convert the intermittent solar energy into the renewable and storable hydrogen energy. However, the most studied semiconductor photoanodes generally exhibit a poor PEC performance due to sluggish water oxidation. In this work, we demonstrate that the bottleneck of PEC water oxidation can be overcome via atomic manipulating proton-transfer on polar surfaces of silicon carbide (SiC) photoanodes [1]. On the typical carbon-face SiC, where proton-coupled electron transfer governed the interfacial hole transfer for water oxidation, substantial energy loss was inevitable due to the highly-activated proton-transfer steps. Via preferentially exposing the silicon-face, we enabled the surface-catalyzed barrierless O-H breaking with a facile proton exchange and migration character. In particular, we show that among SiC polytypes, cubic SiC (3C-SiC) is a very promising semiconductor for PEC water splitting due to its relatively small bandgap of 2.36 eV, which is close to the hypothetical ideal bandgap of 2.03 eV for the theoretical maximum of the solar water splitting efficiency. [1] Hao Li, Huan Shang, Yuchen Shi, Rositsa Yakimova, Mikael Syvajarvi, Lizhi Zhang and Jianwu Sun*, “Atomically Manipulated Proton-Transfer Energizes Water Oxidation on Silicon Carbide Photoanodes”, Journal of Materials Chemistry A, 6, 24358, (2018).

Resume : Metal oxides are prone to oxygen deficiencies, which can have significant impact on photoanode performance. Bismuth vanadate has demonstrated the highest performing photoanodes to date, yet, the role these deficiencies play on charge carrier dynamics and thus the overall photocatalytic process remains controversial. In this talk, we will discuss the in situ modulation of the redox state associated with these defect induced sub-bandgap states through electrochemical, optical and thermal approaches, and explore the resulting impact on charge carrier dynamics and photocatalysis. The redox states of these defects are not only found to be essential for driving holes to the surface for water oxidation, but they are also associated with advantageous electron trapping. These trapped electrons are subsequently transported through the material facilitated by redox-mediated electron transport channels, giving rise to photocurrent generation. On the other hand, holes trapped into these states are rendered redundant to surface oxidation processes. As such, we show how incorporating the control of defect redox state in the photoelectrode fabrication and design process can be instrumental to optimising the overall performance.

Resume : BiVO4 is one of the most promising materials used as a photoanode for photoelectrochemical water splitting. The carrier recombination dynamics of BiVO4 is one of the main obstacles for the water oxidation reaction and it is commonly addressed employing the Ultrafast Transient Absorption Spectroscopy (UTAS). The aim of this study is two-folded: to understand the details of carrier recombination dynamics of BiVO4 and to reveal the impacts of surface engineering that leads to enhanced water oxidation activity. The proposed models for the recombination dynamics of BiVO4 are usually referred to UTAS investigations on TiO2 and Fe2O3, and the absorption spectra are interpreted in terms of excited state absorption, hole absorption, and ground state bleach. The electronic structure of BiVO4, therefore the possible optical pump induced transitions in the conduction and valence bands, are different from the TiO2 and Fe2O3, and more comprehensive analysis is required. In this study, we investigate the charge carrier dynamics of bare and modified BiVO4 (e.g. CoOx/BiVO4 and N-doped BiVO4) through UTAS and transient photocurrent spectroscopy (TPS). UTAS investigations in air, in different media (i.e. hole scavenger), and the numerical modelling help us to offer two promising dynamical models for BiVO4. The operando UTAS is utilized to identify the role of the interface on the catalytic activity and on the suppression of the carrier recombination. The co-catalyzed BiVO4 shows better activity which is associated to longer carrier life-time. TPS becomes handy to determine the change of depletion width and hole-diffusion length between engineered and bare BiVO4.

Resume : The demand for utilization of clean and renewable energy has motivated many researchers to study on various possible strategies for harvesting solar energy. Among them, solar water splitting, which generate usable hydrogen from water and sunlight using photoelectrochemical (PEC) system is a promising approach towards generation of clean fuel. The PEC water splitting system based on photoelectrodes of photoanode and photocathode and electrolyte with the main component is the semiconductor photolectrodes. In general, metal oxides with high stability for photoelectrocatalytic water oxidation are often used as photoanodes [1]. Recently, by using high throughput calculations and experiments, a list of promising VO4-based ternary vanadate photoanodes was reported. Among them, FeVO4 which contains earth abundant elements of Fe and V, with favourable bandgap of 2.06 eV is a potential candidate for photoanode [2]. However, the PEC performance reported is still low due to its low carrier mobility [3]. This motivated us to explore alteration of multiple properties of the FeVO4 by metal alloying or doping to improve its photoelectrochemical performance. In order to find effective metal doping for FeVO4 photoanode, we use combinatorial synthesis and high-throughput characterizations of libraries of materials. FeVO4 thin films was fabricated using inkjet printing machine; with its drop-on-demand feature, this is a versatile method which enables printing a material with certain composition at a desired area while allowing deposition of multiple compositions on a single substrate. In our study, we explore doping of Fe-V oxides by adding Mo, Ni, Bi, Cr, Zn or W. After that, we perform scanning droplet cell to our photoanode materials as a quick assessment method of their photoelectrochemical behaviour. The best photoresponse to date was obtained from Fe-V-Cr oxide with 7% doping ratio of Cr/(Fe +V). Other samples with Mo, Bi, or W doping also showed higher current density compared with undoped FeVO4. This result opens a direction for further improvement of photoanode efficiency of PEC system for accelerating toward practical application. [1] B. D. Alexander, P. J. Kulesza, I. Rutkowska, R. Solarska and J. Augustynski, J. Mater. Chem., 2008, 18, 2298-2303. [2] Q. Yan, J. Yu, S. K. Suram, L. Zhou, A. Shinde, P. F. Newhouse, W. Chen, G. Li, K. A. Persson, J. M. Gregoire and J. B. Neaton, PNAS, 2017, 114, 3040-3043. [3] M. Zhang, Y. Ma, D. Friedrich, R. V. D. Krol, L. H. Wong, F. F. Abdi, J. Mater. Chem. A, 2018, 6, 548–555

Resume : Investigating the influence of iron on nickel oxide nanosheets for enhanced overall water splitting through in-situ Raman and impedance spectroscopy Zhen Qiu, Gunnar A Niklasson, Tomas Edvinsson Department of Engineering Sciences, Solid State Physics, Uppsala University, Box 534, 75121 Uppsala, Sweden Contact email: zhen.qiu@angstrom.uu.se, tomas.edvinsson@angstrom.uu.se Mixed iron-nickel-based systems with tuned microstructure have recently emerged as promising non-noble electrocatalysts for alkaline water splitting. The understanding of interfacial reaction induced charge-transfer mechanisms and active phases, however, is still limited in overall water splitting. Herein, we report a detailed investigation of active surface phases and mechanisms during both the oxygen evolution (OER) and hydrogen evolution (HER) reactions in an alkaline electrolyte through in-situ Raman and impedance spectroscopy. The frequency response of electrical behaviour is interpreted by a full theoretical equivalent circuit model and is related to the Raman spectra. The results show that the reaction resistance exhibits a strong dependence on applied bias and electrode materials in natural correlation with the reaction rate under both OER and HER process. The presence of iron (Fe) results in a less inductive feature observed in HER impedance spectroscopy, which is associated with the coverage relaxation of involved adsorbed intermediates. By in-situ Raman spectroscopy, it is clear to see that the main function of nickel (Ni) and Fe sites are dependent on the applied energy. When the Femi level shifts to more negative potentials, the hydroxyl groups are prone to adsorb on Fe3 sites to form Fe oxyhydroxides, whereas the hydrogen groups show the tendency to adsorb (or migrate) to Ni sites, which accelerates water reduction and thus enhances HER activity. Moreover, the presence of Fe promotes the formation of high Ni valency (γ-NiOOH), leading to an improved OER catalytic performance. Our findings provide insights into the active phases formed in-situ under both the HER and OER reactions and are expected to be valuable for design strategies for efficient and earth-abundant Ni-Fe based catalytic systems.

Resume : Photoelectrochemical water oxidation still represents a major obstacle toward efficient solar fuel generation. In this regard, dye sensitized photoanodes embedding molecular water oxidation catalysts (WOC) are promising candidates to match solar emission spectrum at low energy radiation while combining fast catalysis. In the present work, poly-cationic perylene bisimides (PBIs) dyes have been used in combination with the deca-anionic tetra-ruthenium polyoxometalate (Ru4POM) WOC: the former feature a strong and broad absorption in the Vis range, great oxidizing power and stability toward oxidation; the latter consists of a tetra-ruthenium(IV) core, favouring sequential holes accumulation, stabilized by oxidation resistant POM ligands. The combination of electrostatic interactions with the pi-stacking ability of the dye offers a simple non-covalent approach for the assembly of these two components onto semiconducting metal oxides. When nanostructured WO3 is used, the resulting photoelectrodes show extended absorption up to 600 nm and light harvesting efficiency up to 40%, and show promising results in photoelectrochemical water oxidation, with quantitative faradaic yield towards O2 and absorbed photon to current efficiency (APCE) up to 1.4%. Further studies aim at improving the stability of the molecular assembly onto the semiconducting substrate. Bonchio et al., Nat. Chem. 2019. DOI: 10.1038/s41557-018-0172-y

Resume : Photocatalytic Z-Scheme water splitting uses a low-cost dispersion of two types of semiconducting particles to split water. Although simple and cost effective, this design strategy typically sufferers from low efficiencies. An improved design, known as the photocatalyst sheet, embeds particles in a conducting medium to facilitate inter-particle electron transfer. With appropriate co-catalysts, this highly scalable device architecture can split water unassisted with an quantum efficiency of 26 %. However, the key factors behind this remarkable efficiency are not yet fully understood. Herein, we focus on the proton reducing semiconductor in the Z-Scheme, La,Rh:SrTiO3 and present the first in-operando study of charge carrier dynamics of this material. Using photocatalyst sheet half-electrodes, we find that that in the absence of La, a re-organisation of the electronic structure of Rh:SrTiO3, via the reduction of Rh(IV) is a necessary prerequisite to observe the accumulation of persistent electrons. These can then be utilised by the co-catalyst to drive proton reduction. One way to achieve such Rh(IV) reduction is by applying a strong negative bias. This induces in-situ the reorganisation of electronic structure necessary to accumulate charge, however, such potentials are inaccessible in the complete photocatalyst sheet device. A different strategy must therefore be deployed in order to accumulate persistent electrons in Rh:SrTiO3.

Resume : Driving chemical reactions with renewable energy is a pathway to sustainable and carbon-neutral society. Combining photoabsorber and electrocatalytic components into integrated devices, we developed copper oxide-based photocathodes for water reduction to hydrogen [1,2], as well as carbon dioxide reduction to carbon monoxide [3], in demonstrations of efficient solar-to-fuels conversion. Such devices exhibit benchmark efficiencies among metal oxide based photoelectrodes.   Recently, oxide-derived copper emerged as an intriguing electrocatalyst for the conversion of carbon dioxide (CO2) into hydrocarbon products. In an effort to modify the product selectivity of copper oxide nanowire arrays, we used atomic layer deposition to apply conformal monolayers of tin oxide to the electrode surface. This copper-tin-oxide hybrid exhibited nearly unity selectivity for carbon monoxide (CO) as CO2 reduction product, deviating significantly from the behavior expected for copper or tin alone, neither of which typically produce CO at high selectivity. This result suggests atomic layer composition modification of electrode surfaces can be a powerful strategy for tuning electrocatalyst selectivity. (Work performed at previous affiliation: Ecole polytechnique fédérale de Lausanne EPFL, Switzerland)   1. L. Pan, J. H. Kim, M. T. Mayer, M-K Son, A. Ummadisingu, J. S. Lee, A. Hagfeldt, J. Luo, M. Grätzel, Nature Catalysis, 2018, 1, 412. 2. P. Dias, M. Schreier, S. D. Tilley, J. Luo, J. Azevedo, L. Andrade, D. Bi, A. Hagfeldt, A. Mendes, M. Grätzel and M. T. Mayer, Adv. Energy Mater., 2015, 5, 1501537. 3. M. Schreier, J. Luo, P. Gao, T. Moehl, M. T. Mayer and M. Grätzel, J. Am. Chem. Soc., 2016, 138, 1938. 4. M. Schreier, F. Héroguel, L. Steier, S. Ahmad, J. S. Luterbacher, M. T. Mayer, J. Luo and M. Grätzel, Nature Energy, 2017, 2, 17087.

Resume : The conversion of solar energy into fuels such as hydrogen through photoelectrochemical (PEC) cells is an attractive way to solve the storage problems present in renewable energies and also in transport applications.(1) Many advances have been made until today, but it is still necessary to develop new materials and cell configurations in order to take this technology to a commercial level. At the present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in PEC cells. However, achieving high STH (Solar To Hydrogen) by using a single material as photoelectrode is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2) In this work, we present a tandem PEC cell composed by two hybrid photoelectrodes formed by the heterojunction of a novel synthesized organic conductive polymer (Nano-CMPDTT) and inorganic nanocrystals (TiO2 and CuI). Nano-CMPDTT is based on dithiothiophene moiety and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure of this material, HOMO and LUMO orbital positions were determined by cyclic voltammetry and fermi level location through electrochemical impedance spectroscopy and XPS. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer both as photocathode and as an electron injector to TiO2 in photocatalytic reactions (to former the photoanode). In this way, TiO2 NCs suspensions and Nano-CMPDTT has been deposited by spin coating in ITO glasses to build the photoanode and CuI (hole collector) and Nano-CMPDTT for the photocathode. The formed hybrid photoelectrodes have been characterized by X-ray diffraction, SEM, EDX and AFM. A series of photoelectrochemical measurements have been performed in a three electrode cell configuration, using the hybrid materials as working electrodes. The photoelectrodes presents high values of photovoltages and photocurrents. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid photoelectrodes. Finally, we build a tandem PEC cell in a monolithic configuration and perform a series of photoelectrochemical measures in two electrode configuration and we test and quantify the hydrogen evolution reaction. References 1. Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting (Springer New York, New York, NY, 2013; http://link.springer.com/10.1007/978-1-4614-8298-7), SpringerBriefs in Energy. 2. T. Bourgeteau et al., Enhancing the Performances of P3HT : PCBM − MoS3 ‑ Based H2 ‑ Evolving Photocathodes with Interfacial Layers. ACS Appl. Mater. Interfaces. 7, 16395 (2015).

Resume : Artificial photosynthesis is a very promising method to turn solar energy into fuels.(1) Photoelectrochemical (PEC) cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.(2) The setup of a successful PEC cell requires the preparation of stable photoelectrodes with high light absorption and efficient charge separation. Substoichiometric copper telluride, Cu2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been employed in photovoltaic cells due to its small band gap (around 1.5 eV) and its high conductivity.(3) In this work, we prepared thin films of Cu2-xTe nanocrystals synthesized by a colloidal method(4) on ITO coated glass and characterized them as photoelectrodes. In order to prepare these photoelectrodes and keep the original optical and electrical properties of Cu2-xTe, the organic capping was successfully removed by a ligand exchange method. XPS (Fermi level-to-valence band distance), linear sweep voltammetry (photocurrents, flat band potential) and UV-Vis-nIR spectroscopy (band gap) allowed us to build the bands energy diagram, which confirms the thermodynamic ability of this material to reduce water to hydrogen. This means that Cu2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of H2 generation by water splitting in a two-electrode PEC cell connected to gas chromatography. A tandem PEC cell built with TiO2 and Cu2-xTe showed higher H2 generation than TiO2 or Cu2-xTe alone. Also, using Cu2-xTe as a photocathode and TiO2 as a photoanode enhances the photocurrent. The mechanisms and efficiency of these and other possible configurations will be discussed in the presentation. References (1) Kevin Sivula & Roel van de Krol, Semiconducting materials for photoelectrochemical energy conversion, Nature Reviews Materials 1, 1–16 (2016) (2) James L. Young, Myles A. Steiner, Henning Döscher, Ryan M. France, John A. Turner & Todd G. Deutsch, Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures, Nature Energy 2017, 17028, 1–8 (3) Nitiporn Rungtaweechai & Auttasit Tubtimtae, Cu2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells, Materials Letters 2015, 158, 70–74 (4) Enrico Mugnaioli, Mauro Gemmi, Renyong Tu, Jeremy David, Giovanni Bertoni, Roberto Gaspari, Luca De Trizio & Liberato Manna, Ab Initio Structure Determination of Cu2−xTe Plasmonic Nanocrystals by Precession-Assisted Electron Diffraction Tomography and HAADF-STEM Imaging, Inorganic Chemistry 2018, 57, 16, 10241-10248

Resume : Hybrid organic photoelectrochemical water splitting is gaining momentum in the field of solar conversion technologies. By taking advantage of organic semiconductors properties such as low cost, stability, tuneable electronic properties and ease of large area production, these materials can help to go beyond the limitations of standard inorganic photoelectrochemical water splitting. An efficient tandem cell must provide enough voltage to bias the overall water splitting reaction, overcoming the losses occurring in organic semiconductors-based systems. Therefore, we investigated the various donor and acceptor materials known from the field of OPV starting from electrochemical stability test. Complementary UV-Vis-NIR Absorption data are also collected for each D/A blend to assess the future integration in tandem systems. Hybrid photocathodes are then realized using different hole selective contacts and evaluating their effect on the performances. Materials belonging to the class of metal oxides, small molecules, and transition metal dichalcogenides are thereof studied and optimized for this peculiar application. Amongst the more promising architectures, PCDTBT:PC70BM based photocathodes delivered an open circuit potential of 0.85 V vs RHE, achieving a remarkable increase with respect to the benchmark P3HT:PCBM. The optimized architectures are then tested in organic-based tandem set-up with a perovskite solar cell to perform unbiased water splitting with STH efficiency above 2%.

Resume : We report on a flow-cell type device for bias-free solar conversion of CO2 to syngas (H2 + CO), which by design, is integrated, scalable to large areas, and compatible with earth-abundant and cheap photovoltaics and electrocatalysts. The presented device was composed of a photocathode performing the CO2-to-CO conversion and a dark anode for the oxygen evolution reaction (OER). The photocathode consisted of a triple junction thin-film silicon solar cell, which was chemically protected and electrically connected to three-dimensional (3D) Cu foam, coated with bimetallic Ag-Zn catalyst nanostructures (<50 nm). For the coating process, co-electrodeposition of Ag and Zn on the Cu foam was applied, which has been optimized regarding catalyst size/distribution and complete coverage of the 3D foam. In the continuous flow-cell setup (dark conditions), stable and tunable H2:CO ratios between 4 and 0.1, reaching ±100% Faradaic efficiency for a wide range of low cathode potentials (< 1.0 V) have been demonstrated for the Ag/Zn-on-Cu foam electrode. Furthermore, high CO Faradaic efficiencies of up to 91 % and partial current densities of 47.5 mA cm2 were achieved. For the OER anode, we employed colloidal Co3O4 nanoparticles (<10 nm) loaded on Ni foam. This 3D electrode configuration resulted in impressively low overpotentials for the electro-oxidation process of 330 mV for current densities of 50 mA/cm2. Both optimized electrodes were finally integrated in a complete flow-cell for solar-driven CO2 conversion that applied a bipolar membrane to separate catholyte and anolyte compartments. In this configuration, we demonstrated a stable and bias-free operating current density of 6.1 mA/cm2, reaching 98 % Faradaic efficiency for syngas production.

Resume : To mitigate the increasing atmospheric CO2 concentration, CO2 utilization techniques have been recently highlighted. Among many of the CO2 utilization techniques, electrochemical CO2 conversion is promising because it can combined with renewable energy sources such as a solar cell, which is fossil-fuel free process. In nature, photosynthesis system absorbs sunlight and convert CO2 and H2O to produce valuable bio-chemicals with the assistance of the solar energy, from which it provides sustainable carbon recycle. At KIST research team have been developed a monolithic and stand-alone photoelectrochemical device to convert CO2 to valuable chemicals. The device is composed of a photovoltaic cell module which supplies the required energy for electrochemical CO2 reduction and water oxidation reactions. We demonstrated that cost-effective Si module can be applied for the photovoltaic cell to achieve ~ 8% of solar to CO conversion efficiency. In order to increase the conversion efficiency, the device design is especially important to decrease the series resistance as the current density increases. In addition, in order to increase the economic feasibility, the conversion efficiency as well as the CO2 conversion rate have to be considered. In this talk, several electrocatalysts would be also introduced for selective CO2 reduction to value-added chemicals. For CO production, Ag or Zn metal based nanostructured catalysts have been demonstrated the surface modification is important to achieve over 90% of Faradaic efficiency for CO production. We found that one of the critical challenges is how to suppress the hydrogen evolution reaction from the aqueous electrolyte. For selective C2+ chemical production, Cu-based mono or hybrid catalysts have been applied in order to increase C-C coupling reaction efficiency (up to ~ 70% of Faradaic efficiency). We found that introducing heterogeneity on the Cu surface can contribute to CO dimerization. In-situ spectroscopic studies were performed to understand the active species of the catalysts. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules/materials are effective strategies to develop efficient and durable electrocatalysts for CO2 reduction reaction.

Resume : Titanium dioxide (TiO2) is the one of widely studied photocatalysts due to its non-toxicity and high chemical/physical stability. Nevertheless, it suffers through poor absorption in visible irradiation due to its large band gap energy of ~ 3.2 eV and high rate of recombination. Thus, bandgap engineering using doping or synthesizing with low band gap semiconductor materials is crucial to improve its photocatalytic properties. Recently, reduced TiO2 (TiO2-x) has received remote attention because of its wide light absorption ability as well as disordered structure, leading to better photocatalytic performances. Moreover, graphitic carbon nitride (g-C3N4), a non-metallic semiconductor having a bandgap of 2.6 eV, can be used to reduced its band gap energy as well as reducing recombination rates. This material has attracted much attention due to its easiness to manufacture as well as its environmentally benign and high thermal stability. Therefore, engineering heterostructure of reduced TiO2 and g-C3N4 would help enhancing photocatalytic evaluations. In the present work, we have tried to explore fabricating polymeric graphitic carbon nitride (g-C3N4) coating onto reduced TiO2 nanofibers via electrospinning followed by sintering method. Initially, TiO2 nanofibers were electrospinned and sintered by reducing nanofibers in H2/Ar atmosphere at 600 °C/4h. After that, as-fabricated reduced TiO2 nanofibers were sintered with thiourea solution at 520 °C/2h in air atmosphere to coat g-C3N4 structures onto reduced TiO2 nanofibers. This reduced TiO2 with g-C3N4 coating was analyzed to observe morphologies, its improvement in absorbance and reduction in recombination losses via different analytical techniques. Based on characterizations, CO2 photoreduction and H2 evolution photocatalytic measurements were performed to show better performance compared to that of the reduced TiO2 nanofibers only. Therefore, the optimized heterostructures were proven to be efficient photocatalytic materials which can be applied to artificial photosynthesis, solar cell, bio-applications.

Resume : Understanding the degradation mechanism and improving the stability of photoelectrodes are essential to realize solar to hydrogen conversion via photoelectrochemical (PEC) devices. Although amorphous TiO2 layer has been widely employed as a protection layer on top of p-type semiconductors to implement durable photocathodes, a gradual photocurrent degradation was still unavoidable. Herein, we elucidate the photocurrent degradation mechanism of TiO2-protected Sb2Se3 photocathode and propose a novel interface modification methodology in which fullerene (C60) is introduced as a photo-electron transfer promoter for significantly enhancing the long-term stability. It is demonstrated that the accumulation of photo-electrons at the surface of TiO2 layer induces its reductive dissolution, accompanying the photocurrent degradation. In addition, the insertion of C60 photo-electron transfer promoter at Pt/TiO2 interface facilitates the rapid transfer of photo-electrons out of TiO2 layer, thereby enabling the enhanced stability. Our Pt/C60/TiO2/Sb2Se3 device revealed the high photocurrent density of 17 mA cm−2 at 0 VRHE and outstanding stability over 10 h-operation, representing the best PEC performance and stability as compared to previously reported Sb2Se3-based photocathodes. Our research not only provides in-depth understanding of the degradation mechanism for TiO2-protected photocathodes, but also suggests a new direction to achieve durable photocathodes for PEC water splitting.

Resume : Chemical storage of solar energy by H2 production is an attractive approach for stabilization of the electrical grid and on-demand power generation. Currently the poor transparency of the thick low-cost catalyst layers required for water oxidation hinders the large-scale fabrication of efficient photo-electrochemical cells for artificial photosynthesis and hydrogen production. Flame synthesis is a scalable technology for the production of ultra-porous layers of photo-catalysts and optoelectronic devices [1]. Nevertheless, the fragility of this gas-phase self-assembly has limited their application in liquid environments with most studies requiring very high temperature sintering to obtain sufficient mechanical stability. Here, we report the multi-scale engineering of robust high performance photo- and electrocatalyst layers with tunable porosity and composition [2]. We discuss the critical parameters controlling the performance and present a flexible approach for their rapid in-situ mechanical and chemical stabilization. We apply this concept to the fabrication of photo-electrochemical cells for water splitting demonstrating high turn-over frequencies, controllable light absorption and efficient electron collection. We envision that this scalable synthesis approach can be readily implemented for the commercial production of low-cost devices for chemical energy storage and renewable fuel production. 1. N. Nasiri, A. Tricoli, et al., Adv. Mater. 2015, 27, (29), 4336 2. G. Liu, A. Tricoli, et al., Adv Energy Mater 2016, 6, (15).

Resume : For many catalytic processes, solar and chemical energy conversion, thin-film molecular and functional nanoscale materials and their surface assemblies are becoming increasingly significant. In this regard, developing robust and high activity electrocatalytic materials that are obtained from thin-film molecular and functional nanoscale materials and their synergistic interfacing with efficient light-harvesting modules is imperative for electro- and photo-electrocatalysis catalysis application in water oxidation and CO2 conversion, chemicals synthesis and solar fuels production. We have developed methods and exploited various functional thin film and nanoscale materials for catalytic water splitting, CO2 reduction, and recently for biomass catalysis, solar energy conversion and electrochemical sensing schemes. Now we implement several molecular nanoclusters, thin film, inorganic nanomaterials and metal-oxides displaying great potential to be used in surface chemistry and electrocatalysis. Their effective interfacing with semiconductor photo-responsive materials and/or CO2 reduction systems can provide a potential platform to make renewable energy supplies for a renewable and sustainable future. References: [1] K. S. Joya*, L. Sinatra, L. G. AbdulHalim, C. P. Joshi, M. N. Hedhili, O. M. Bakr and I. Hussain, Nanoscale, 8 (2016) 9695–9703. [2] K. S. Joya*, Y. F. Joya, H. J. M. de Groot, Adv. Energy Mater., 4 (2014) 1301929. [3] M. de Respinis, K. S. Joya, H. J. M. De Groot, F. D’Souza, W. A. Smith, R. van de Krol, B. Dam, J. Phys. Chem. C, 119 (2015) 7275–7281. [4] K. S. Joya*, H. J. M. de Groot, ACS Catal. 6 (2016) 1768–1771. [5] K. S. Joya*, N.K. Subbaiyan, F. D'Souza, H. J. M. de Groot, Angew. Chem. Int. Ed., 51 (2012) 9601–9605. [6] K. S. Joya*, N. Morlanés, E. Maloney, V. Rodionov, K. Takanabe, Chem. Commun., 51 (2015) 13481–13484. [7] K. S. Joya*, Y. F. Joya, K. Ocakoglu, R. van de Krol, Angew. Chem., 125 (2013) 10618–10630; Angew. Chem. Int. Ed., 52 (2013) 10426–10437.

Resume : The energy materials and devices have been largely limited in a framework of thermodynamic and kinetic concepts or atomic and nanoscale. Soft x-ray spectroscopy characterization offers unique characterization in many important energy materials of energy conversion, energy storage and catalysis in regards to the functionality, complexity of material architecture, chemistry and interactions among constituents within. The presentation firstly gives some details on in situ/operando soft x-ray spectroscopy characterization of interfacial phenomena in energy materials and devices. It has been found that the microstructure and composition of materials as well as the microstructure evolution process have a great influence on performances in a variety of fields, e.g., energy conversion and energy storage materials, chemical and catalytic processes. However, it is challenging to reveal the real mechanism of the chemical processes. In-situ/operando x-ray spectra characterization technique offers an opportunity to uncover the phase conversion, chemical environment change of elements and other very important information of solid/gas and solid/liquid interfaces in real time. It will be shown how to use the powerful in-situ/operando soft x-ray spectroscopy characterization techniques, e.g. soft x-ray absorption spectroscopy (XAS) and resonant inelastic soft x-ray scattering (RIXS) to investigate the real electrochemical mechanism during the operation. A number of electrochemical liquid cells will be presented with success in revealing the catalytic and electrochemical reaction process at real time. The experimental results demonstrate that in-situ/operando soft x-ray characterization techniques can help researchers well understand the real reaction mechanisms.

Resume : Herein, we report on the role of electronically coupled phase boundaries on the water oxidation performance of hematite (-Fe2O3) photoanodes. Different metal oxides were tested as underlayer materials with regard to their absorption and charge-transfer properties. We demonstrate that uranium oxide (U3O8) underlayers grown by chemical vapor deposition or magnetite (Fe3O4) underlayers prepared via directed post-deposition plasma-chemical reduction of hematite effectively accelerate the OER in conjunction with -Fe2O3 overlayers through a built-in potential at the interface. We found that the differential chemical constitution and valence states of iron (Fe2+/Fe3+) and uranium (U5+/U6+) ions can enhance the charge separation and thus the overall electrical conductivity of the layer. Moreover, DFT simulations showed that the multivalence of U and Fe ions can induce the adjustment of the band alignment subject to the concentration of interfacial Fe ions. For both underlayer materials, a favored type II band edge alignment was calculated, which improves charge-transfer processes as supported by transient and X-ray absorption (TAS and XAS) spectroscopy.

Resume : The role of the single layer graphene (SLG) as overlayer on hematite (α-Fe2O3) photoanodes surface was investigated for water oxidation reaction via photoelectrochemical studies. SLG was synthesized by chemical vapour deposition (CVD) and transferred from the copper foil to the α-Fe2O3 photoanode surface. To ensure the purity and quality of SLG, elemental composition of the samples was determined in all steps and no metal (Cu) trace was found after the transference onto the photoanode surface. The photocurrent density of α-Fe2O3/graphene photoanode was increased from 1.25 to 1.64 mA cm-2 at 1.23 VRHE in comparison to bare α-Fe2O3 photoanode. The role of SLG added on α-Fe2O3 and the charge carrier dynamics were investigated combining transient absorption spectroscopy (TAS)[1] and surface photovoltage spectroscopy (SPS)[2]. These combined techniques revealed that the photogenerated holes had their lifetime increased as a function of applied bias and after the SLG deposition onto the α-Fe2O3 photoanode surface. As consequence, the photochemical separation and transfer of the photogenerated charge carriers became more efficient in the presence of SLG. The incorporation of SLG on α-Fe2O3 photoanode is believed to suppress the surface traps, enabling holes to diffuse into the solid-liquid interface and promoting water oxidation reaction driven by sunlight irradiation. [1] S. R. Pendlebury, M. Barroso, A. J. Cowan, K. Sivula, J. Tang, M. Grätzel, D. Klug, J. R. Durrant, Chem. Commun. 47 (2011) 716–718 [2] H. L. Tan, A. Suyanto, A. T. Denko, W, H. Saputera, R. Amal, F. E. Osterloh, Y. H. Ng, Part. Part. Syst. Charact. 34 (2017) 1600290

Resume : Sb2Se3 has emerged as a promising photocathode for water splitting owing to its high photoelectrochemical performance (~ 15 mAcm-2) and resistance to photocorrosion in the absence of any protection layers. It has been demonstrated that sulfurization treatment of Sb2Se3 improves the onset potential and hence the photovoltage. However, the origin of such an improvement in the photovoltage is not fully understood in the literature. In this work, we employ 2 techniques to understand this phenomenon. Firstly, by in-situ surface potential sensing, the photovoltage was found to increase from 220 mV to 340 mV upon sulfurization treatment. Next, from time-resolved microwave conductivity (TRMC), although the carrier mobilities remained the same, there was an increase in the diffusion lengths upon sulfurization treatment. Interestingly, this increase was more pronounced in shorter wavelengths (400-700 nm) than longer wavelength (700-1100nm) suggesting that the sulfurization treatment primarily reduces the surface recombination rather than bulk recombination.

Resume : SnS is a p-type semiconductor with a bandgap of 1.1-1.3 eV and large optical absorption coefficient in the visible. Its conduction band alignment is favorable for water reduction. Compare to CIGS (CuInGa(S,Se))or CdTe, tin and sulfur are both earth abundant and non-toxic elements. Thus, SnS is a promising photocathode material for low-cost water splitting. Solution-phase deposition of inorganic semiconductor thin films is facile and scalable method for manufacturing thin film photoelectrochemical (PEC) cells. In a binary solvent mixture of ethylenediamine (en) and 1,2-ethanedithiol(edt) in 10:1 vol/vol ratio, bulk SnS powder dissolves and form molecular Sn-S ink. Phase pure SnS thin film is then prepared by spin-coating the 0.5M Sn-S ink on Au (100nm) coated FTO (Fluorine doped Tin Oxide) and annealing in inert atmosphere at 350°C. Bare SnS shows very late onset potential for hydrogen reduction and is unstable in acidic conditions of PEC cell. To shift the onset voltage positively, n-type Ga2O3 (20nm) buffer layer was grown by Atomic Layer Deposition (ALD) to form a heterojunction. To improve the stability of the electrode, n-type TiO2 (50nm) was grown on Ga2O3 layer by ALD. As a hydrogen evolution catalyst, Pt nanoparticles were deposited on the surface of TiO2 by photoelectrodeposition. These photocathodes generates an onset voltage shift of 400mV and a photocurrent density of -0.8mA cm-2 at 0V versus reversible hydrogen electrode (RHE) for water splitting under 1 sun illumination. The photocurrent density does not decrease over time (30 min at 0V vs RHE) showing that the SnS/Ga2O3/TiO2/Pt structure device has improved stability for water splitting.

Resume : Cu-based materials (metal and related oxides) are among the most successful materials for CO2 photoreduction process, with some selectivity for CH4 production. However, the mechanism by that reaction takes place is still poorly understood, specially when using Cu oxides. Therefore we systematically investigated CO2 photoreduction catalyzed by CuO from different synthetic methods (solvothermal, coprecipitation and solid-state reaction) to understand which mechanism is prevailing for this reaction. The results showed that CuO acts as a reactant during CO2 reduction through copper carbonate formation rather than a catalyst, but the as-formed CuCO3.Cu(OH)2 (malachite) is active as catalyst for the same reaction. Significant CO2 conversion was observed during the CuO carbonation process, reducing to lower conversion levels after complete conversion to copper carbonate. To increase the stability of CuO, an heterojunction was proposed with Nb2O5, which was a candidate material with good performance in previous CO2 reduction studies. The heterojunction formation was also investigated for other reductive reactions, using electron and hole scavengers (sacrificial reactants) to understand the mechanisms. These results indicate that these catalysts are potential materials for solar fuel production trough CO2 reduction, as well as highlight the importance of adequate synthetic methods to produce them.

Resume : Heterojunction configurations are particularly attractive for photoelectrochemical applications such as solar water splitting, based on the general idea that recombination can be partially prevented by increasing the charge separation efficiency; as a consequence, the current may increase and current onset may shift to more favorable potentials. The WO3/BiVO4 heterojunction system has previously been shown to generate a much larger photocurrent density than observed for the respective individual materials, by combining the better light harvesting ability of BiVO4 with the better electron conductivity of WO3, however, the exact nature of the charge carrier processes in the sandwich structure is still unclear. In this work, WO3/BiVO4 heterojunction thin films have been fabricated by spin coating in a variety of multilayer sandwich configurations, and the structure, morphology and photoelectrochemical properties have been characterized. Intensity-modulated photocurrent spectroscopy (IMPS) measurements illustrate that the balance between charge separation, surface recombination and hole transfer to the electrolyte can be optimized. The results are interpreted based on a simple model, and are compared to the results from electrochemical impedance spectroscopy (EIS). In addition, the individual materials are characterized in detail in order to explain the significant increase in efficiency for the heterojunction, and suggestions for further improvements are presented and discussed.

Resume : Semiconducting materials hold the key for efficient photocatalytic and photoelectrochemical water splitting. In this talk, we will give a brief overview of our recent progresses in designing semiconductor metal oxides materials for photoelectrochemical energy conversion including photocatalytic solar fuel generation. In more details, we have been focusing the following a few aspects; 1) band-gap engineering of layered semiconductor compounds including layered titanate, tantalate and niobate-based metal oxide compounds for visible light phtocatalysis, and 2) facet-controlled TiO2, Fe2O3, WO3, BiVO4 as building blocks for new photoelectrode design，and 3) the combination of a high performance photoelectrode BiVO4 with perovskite solar cells can lead to unassisted solar driven water splitting process with unassisted solar-to-hydrogen conversion efficiency of >6.5%; The resultant material systems exhibited efficient visible light photocatalytic performance and improved power conversion efficiency in solar energy, which underpin important solar-energy conversion applications including solar fuel generation and simultaneous environmental application.

Resume : Proton coupled electron transfer (PCET) plays a fundamental role in the mechanism of water-splitting at the oxygen-evolving complex (OEC) of photosystem II (PSII). We address the underlying reaction mechanism by structural studies of catalytic intermediates. Many physical techniques have provided important insights into the OEC structure and function, including X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy as well as mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopy, and Fourier transform infrared spectroscopy applied in conjunction with mutagenesis studies. However, experimental studies have yet to yield consensus as to the exact configuration of the catalytic metal cluster and its ligation scheme. Computational modeling studies, including density functional (DFT) theory combined with quantum mechanics/molecular mechanics (QM/MM) hybrid methods for explicitly including the influence of the surrounding protein, have proposed chemically satisfactory models of the fully ligated OEC within PSII that are maximally consistent with experimental results. The inorganic core of these models is similar to the crystallographic model upon which they were based, but comprises important modifications due to structural refinement, hydration, and proteinaceous ligation which improve agreement with a wide range of experimental data. The computational models are useful for rationalizing spectroscopic and crystallographic results and for building a complete structure-based mechanism of water-splitting assisted by PCET as described by the intermediate oxidation states of oxomanganese complexes. This talk summarizes recent advances on studies of the OEC of PSII and biomimetic oxomanganese complexes for artificial photosynthesis.