Materials for chemical and electrochemical energy storage

Resume : Among various materials for solid-state hydrogen storage, alloys and intermetallics forming hydrides are one of the most important classes due to their high-volume density, reversibility, and safety. Recently, a new paradigm of alloying in metallurgy has emerged based on the concept of high entropy alloys (HEAs), initially intended to enhance the mechanical properties. The principle is laid on the mixing of elements close to the equimolar proportion for systems up to five and more elements. This may lead to the formation of simple single-phased solid solutions (bcc, fcc and hcp) with large lattice distortion that can be favorable for hydrogen uptake. We report here the synthesis, the physicochemical and the hydrogen absorption/desorption properties of a series of HEAs (Ti0.30V0.25Zr0.10Nb0.25)X0.10 with X = Mg, Al, Cr, Mn, Fe, Co, Ni, Zn, Mo and Ta. A large pallet of experimental methods has been employed such as, laboratory/synchrotron X-ray diffraction, electron microscopy, in situ neutron diffraction, pressure-composition-isotherm, thermal desorption spectroscopy and differential scanning calorimetry. All the Ti-V-Zr-Nb-X alloys are single-phase bcc and undergo either a one-step (initial - dihydride) or a two-stage hydrogen sorption (initial - monohydride - dihydride). The role of additional element will be highlighted and compared to the quaternary Ti-V-Zr-Nb alloy as function of local lattice distortion and electronic properties. For example, adding light elements such as, Mg and Al, slightly decreases the capacity however, it improves the stability upon cycling as compared to the quaternary Ti-V-Zr-Nb alloy. The addition of Fe, Co, Ni or Zn strongly reduces the hydrogen uptake whereas, Cr, Mn, Mo and Ta improves the overall capacity. The most promising alloys have been the object of in situ neutron diffraction study to address the phase transition during desorption by heating under dynamic vacuum and the deuterium occupancy in the hydride phase. This approach is envisioned to clarify the role of additional elements on the hydrogen storage performances of refractory HEAs.

Resume : Hydrogen as a clean energy carrier is a viable future energy alternative. The lightweight and safe hydrogen storage is the key for the development of hydrogen-based clean energy carrier. There are several technologies for hydrogen storage. The solid-state storage through metal hydrides is a safe and compact option that has the potential to be used in mobile and stationary applications. Among the numerous studied metals, alloys and intermetallics, the high entropy alloys recently showed promising capacity of absorption. For example, the TiVZrNbHf alloy has a superior capacity of absorption than conventional alloys or individual components up to 2,5 H/M (hydrogen per metal atom) [1]. This work aims to study the effect of the addition of 10% of Mo into Ti0.325V0.275Zr0.125Nb0.275 on the hydrogen sorption properties. This composition was optimized to obtain single-phase BCC alloy based on our previous works [2]. The Ti0.30V0.25Zr0.10Nb0.25Mo0.10 alloy was prepared by high temperature arc melting and crystallizes into a single-phase BCC lattice. The alloy absorbs large hydrogen amount of 2.0 H/M (3 wt%) at ambient temperature forming a single-phase FCC dihydride. This value is higher as compared to 1.7 H/M (2.5 wt%) of the initial Ti0.325V0.275Zr0.125Nb0.275. Additionally, the study of the structural properties and phase transformation during Deuterium desorption were studied in situ by neutron diffraction and shows a sharp single-phase transition at 515 K from FCC dihydride phase to the pristine single-phase BCC. After 20 cycles of Abs/Desorption, the alloy experiences a small fading in the capacity by 13% after the first 6 cycles and stabilizes to around 1.73 H/M (2.5 wt%) for further cycling. [1] Sahlberg M., et al. Sci. Rep;6:36770.(2016). [2] Montero, J., et al. Molecules, 24, 2799.( 2019)

Resume : Hydrogen has a strong potential to be a replacement for fossil fuels but an important research effort is needed for finding efficient storage materials to be used in practical devices. H2 can be stored in solid-state as metal hydrides which possess the advantages of high safety as compared to gas storage, and high volumetric capacity exceeding those of liquid H2. Metal hydrides are well studied for solid-state H2 storage, but most alloys have not yet fulfilled all the needs for profitable and efficient storage. On the other hand, a new class of materials, known as multi-principal element alloys (MPEA) or high-entropy alloys (HEA), have captured the attention because of their simple crystalline structures (BCC, FCC, HCP) and large lattice distortion that could accommodate important hydrogen uptake, as proven for the TiVZrNbHf in past studies. In this report, we study the hydrogen storage properties of novel HEA’s containing refractory elements (Ti, V, Nb) and Al. The Alx(TiVNb)100-x (x = 0, 5, 10, 17.5 and 25 at.%) compositions were synthesized by arc melting method. We establish the influence of Al content on the formation of the BCC phase and on the hydrogen storage properties such as the storage capacity, desorption profiles, absorption kinetics, and cycling stability. As the first results, the increase of Al content maintains a single-phase BCC lattice and decreases the unit parameter as well as the lattice distortion. Moreover, XRD after Thermo-Desorption-Spectroscopy (TDS) proven that hydrogen absorption and desorption are fully reversible. In situ neutron diffraction on the composition Al10(TiVNb)90 demonstrated that the dihydride phase is FCC and the full desorption of deuterium induces a phase transition from FCC to a BCC lattice at around 530 K. Pressure-Composition-Isotherm measurements were carried out for this composition and show a two-plateaus behavior. The as-cast alloy absorbs hydrogen at very low pressure to form a BCC monohydride (0.8 H/M) followed by the formation of a dihydride phase at higher pressures (1.6 H/M). Further analysis is in progress in order to better assess the effect of Al addition on the other storage properties such as absorption/desorption cycling.

Resume : By deploying the gas pressure dependent thermal conductivity of porous structures vacuum insulation panels can be transformed into thermal barriers with controllable heat flux. The controllable vacuum insulation layer requires the concise adjustment of the gas pressure in the insulation panel. Reversible gas-solid reactions are a promising approach to set the gas pressure in a coupled porous insulation panel by tempering the solid reaction part along the equilibrium line. A suitable material combination is the metal hydride reaction system. Compatible metal hydrides for the application in variable conductance vacuum insulation layers are required to work reliably in vacuum regimes, show fast kinetics, and reach the vacuum pressure in technically interesting temperatures ranges between 0 °C and 200 °C. Metal hydrides that fulfil these requirements are lanthanum nickel or zirconium nickel alloys, for instance. A combination of these alloys in a cascading operation mode is considered to provide the appropriate gas pressure adjustment for the thermally controllable barrier. Thus, a dedicated understanding of the metal hydride materials ? individually and cascadingly interconnected ? in vacuum regime is prerequisite. The cascade connection of several metal hydride materials in the same reactor enables the possibility to provide a wide gas pressure range whilst applying a reduced required temperature range for the endo-/exothermal reaction. Our current work addresses characterization investigations on the manner of reaction of the alloys below atmospheric pressure. The main challenging question is the measurement of the pressure-concentration-isotherms of the metal hydrides in high vacuum. Usual experimental setups to conduct PCI-measurements dynamically are not able to cover high vacuum conditions, thus we built up a test bench ? including an appropriate reactor design - to carry out static PCI measurements. The driving parameter at this is the temperature of the metal hydride bulk while the according gas pressure is the dependent value. The hydrogen concentration is fixed and is stepwisely varied in different measurement series. The contribution will focus on the experimental investigations in terms of the characterization of zirconium nickel as well as lanthanum nickel alloys for high vacuum applications and will identify the potential of the cascade connection of different metal hydrides to adjust the gas pressure in the vacuum panel effectively. The specifically for static PCI measurements constructed test bench will be introduced as well as the results of PCI correlations will be outlined and discussed.

Resume : Hydrogen is a suitable carrier for renewable energy sources, enabling the transition towards CO2-free energy, and storage for long periods in solid-state materials. TiFe can store up to 1.86 wt.% hydrogen in mild conditions of temperature and pressure. Elemental substitution in TiFe enables easy activation and fine tuning of thermodynamics for hydrogen sorption, as a function of the envisaged application.[1] Ti, Mn, and Cu substitutions for Fe in TiFe have been explored, to select an optimised alloy composition for large-scale H2 storage at low pressure. The alloys were synthetized by induction melting and annealed at 1000 °C. After activation, fundamental hydrogenation properties have been determined (thermodynamics, kinetics and cycling). For more comprehensive structural studies, in-situ neutron diffraction experiments during deuterium loading have been performed in a custom-made stainless-steel sorption cell. First studies with Mn and Cu substitution for Fe demonstrate that the first hydrogenation (activation) can be achieved in mild conditions, enabling a broad use of TiFe-alloy.[2] The compositional exploration of the Ti-Fe-Mn system led to the determination of hydrogenation properties dependency over an extended Ti and Mn compositional range.[3] Ti and Mn substitutions at Fe site enlarge the TiFe cell volume, while decreasing the plateau pressures and hysteresis between absorption and desorption. Samples at the Ti-rich side have enhanced reversible storage capacities. The formation of secondary phases improves also the activation process. The crystal structures of TiFe(0.90-x)Mnx alloys (x = 0, 0.05, and 0.10) and their deuterides have been determined by in-situ neutron diffraction, while recording Pressure-Composition Isotherms at room temperature.[4,5] The investigations aimed at analysing the influence of stoichiometry and Mn for Fe substitution in TiFe-type alloys on structural properties during reversible deuterium loading. In conclusion, this study brings remarkable understanding on the hydrogen storage properties, basic structural knowledge, and support to the industrial usage of TiFe-type alloys for integrated hydrogen tank in energy storage systems. For large-scale hydrogen storage, TiFe-type alloys form room temperature hydrides under low gas pressure (below 5 MPa at RT) and are suitable for this purpose. The HyCARE project (www.hycare-project.eu) aims at developing an efficient integrated system to store approx. 50 kg of hydrogen in solid-state using such TiFe-based material. The system will be installed at the Engie Criogen Lab. in Paris in mid-2022. References: [1] E.M. Dematteis, N. Berti, F. Cuevas, M. Latroche, M. Baricco, Substitutional effects in TiFe for hydrogen storage: a comprehensive review, Mater. Adv. (2021). https://doi.org/10.1039/D1MA00101A. [2] E.M. Dematteis, F. Cuevas, M. Latroche, Hydrogen storage properties of Mn and Cu for Fe substitution in TiFe0.9 intermetallic compound, J. Alloys Compd. 851 (2021) 156075. https://doi.org/10.1016/j.jallcom.2020.156075. [3] E.M. Dematteis, D.M. Dreistadt, G. Capurso, J. Jepsen, F. Cuevas, M. Latroche, Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn, J. Alloys Compd. (2021) 159925. https://doi.org/10.1016/j.jallcom.2021.159925. [4] F. Cuevas, S. Deledda, E.M. Dematteis, B.C. Hauback, M. Latroche, L. Laversenne, J. Zhang, In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn-substituted Ti(Fe,Mn)0.9 alloys, Inst. Laue-Langevin. (2020). https://doi.org/10.5291/ILL-DATA.5-22-771. [5] E.M. Dematteis, B.C. Hauback, I. da Silva Gonzalez, S. Deledda, F. Cuevas, M. Latroche, G. Capurso, J. Barale, In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn,Cu-substituted Ti(Fe,Mn,Cu)0.9 alloys, STFC ISIS Neutron Muon Source. (2019). https://doi.org/10.5286/ISIS.E.RB1920559-1.

Resume : Mg is one of the most attractive hydrogen storage materials due to its high volumetric and gravimetric storage capacity and low material costs. However, the hydride phase MgH2 is too stable to desorb hydrogen at ambient pressure and temperature, and thus unsuited for practical stationary and onboard applications. One of the possible ways to destabilize MgH2 is the downsizing on a nanometer scale. We have first proposed to synthesize nanometer-sized MgH2 by combining immiscible metals with Mg. For Ti-rich Mg-Ti nonequilibrium alloy, nanometer-sized MgH2 clusters coherently embedded in a TiH2 matrix were formed upon hydrogenation1). Indeed, MgH2 is destabilized by the interface energy between the two metal hydrides2,3). The chemical segregation on a nanometer scale accompanies stress-induced strain and structural change of MgH24,5). However, stabilization of a part of MgH2 was also observed due to local dissolution of Ti into the Mg domains: since TiH2 is more stable than MgH2, hydrogen atoms occupying the interstitial sites are stabilized by the Ti coordination. Then we have focused on a nonhydride forming matrix which is again immiscible with Mg. We synthesized nonequilibrium Mg-Mn6) and Mg-Cr7) alloys, leading only to the destabilization effect of MgH2 without any interfering stabilization effect. Our results suggest that we can tune the thermodynamics of hydrogen absorption and desorption in Mg-H using the nanometer-sized effect, which contributes to the substantial development of lightweight and inexpensive hydrogen storage materials. Recently, we are expanding the target to nanometer-sized Mg2FeH6 embedded in Mg2Si8) and allotropic MgH2 formed by hydrogenation of Mg-Cu-Y alloy with a Long Period Stacking Ordered (LPSO) structure9). In this talk, those new systems developed by our group will be also introduced. This work was supported by JSPS Kakenhi Grants 17K06853 and 21H01744, the Photon and Quantum Basic Research Coordinated Development Program by MEXT, and the International Joint Research Program for Innovative Energy Technology by METI. References: 1) K. Asano, H. Kim, K. Sakaki, K. Page, S. Hayashi, Y. Nakamura, E. Akiba, J. Alloys Compd. 593, 132-136 (2014). 2) K. Asano, R. J. Westerwaal, A. Anastasopol, L. P. A. Mooij, C. Boelsma, P. Ngene, H. Schreuders, S. W. H. Eijt, B. Dam, J. Phys. Chem. C 119, 12157-12164 (2015). 3) K. Asano, R. J. Westerwaal, H. Schreuders, B. Dam, J. Phys. Chem. C 121, 12631-12635 (2017). 4) K. Asano, H. Kim, K. Sakaki, K. Jimura, S. Hayashi, Y. Nakamura, K. Ikeda, T. Otomo, A. Machida, T. Watanuki, Inorg. Chem. 57, 11831-11838 (2018). 5) H. Kim, H. Schreuders, K. Sakaki, K. Asano, Y. Nakamura, N. Maejima, A. Machida, T. Watanuki, B. Dam, Inorg. Chem. 59, 6800-6807 (2020). 6) Y. Lu, H. Kim, K. Sakaki, K. Jimura, S. Hayashi, K. Asano, Inorg. Chem. 58, 14600-14607 (2019). 7) Y. Lu, H. Kim, K. Jimura, S. Hayashi, K. Sakaki, K. Asano, J. Power Sources 494, 229742 (2021). 8) K. Asano, H. Kim, K. Sakaki, Y. Nakamura, Y. Wang, S. Isobe, M. Doi, A. Fujita, N. Maejima, A. Machida, T. Watanuki, R. J. Westerwaal, H. Schreuders, B. Dam, Inorg. Chem. 59. 2758-2764 (2020). 9) V. Charbonnier, K. Asano, H. Kim, K. Sakaki, Inorg. Chem. 59, 14263-14274 (2020).

Resume : Among compounds presenting reversible hydrogen storage properties at room temperature, FeTi-hydrides have recently re-attracted interest. These hydrides show good gravimetric and volumetric capacity and present relatively low-cost alternatives to other intermetallics. Due to its fast sorption kinetics and reversibility in mild conditions, this alloy is suitable for applications in hydrogen storage tanks. Moreover, multicomponent FeTi-based systems may have their thermodynamic and kinetic properties tuned by partially substituting Fe and/or Ti by other metallic elements, i.e., adjusting their composition to improve the performance needed by different technological applications. For this reason, developing multicomponent thermodynamic models for the description of stability of metal-hydrides may support the development of novel alloys to satisfy these requirements. In this regard, the CALPHAD method has been recognized as a powerful and consistent tool to accurately calculate the thermodynamic properties of multicomponent systems based merely on assessments of its unary, binary, and ternary subsystems. Therefore, metal-hydrides seem to be good candidates to benefit from computational thermodynamics in the design and the simulation stages. In this work, as an initial step in the modeling of the more complex FeTi-based hydride family, the pseudo-binary FeTi(1-x)H(x) system was chosen and assessed employing the CALPHAD method. The thermodynamic models of the phases were selected based on a critical analysis of literature information. New thermodynamic data was acquired by measuring Pressure-Composition-Isotherm (PCI) curves and by calculating crystallographic and thermochemical properties through Density Functional Theory (DFT). Reliable descriptions of key thermodynamic properties have been demonstrated using the proposed model. The dissociation pressure, PCI curves, formation enthalpies of hydrides, and phase diagrams were calculated and are in good agreement with our newly acquired data and with those reported in the literature. The present work may serve as a ground for higher-order thermodynamic assessments as well as consistent input data for kinetic models, helping the design of novel FeTi-based hydrides and their hydrogenation process simulation.

Resume : The hydrogen storage performance of the Reactive Hydride Composite Mg(NH2)2 + 2 LiH can be significantly improved by the addition of LiBH4 and the subsequent formation of the mixed amide-borohydride compound Li4(BH4)(NH2)3 during hydrogen release. We have investigated the structure and molecular motions of Li4(BH4)(NH2)3 by in situ synchrotron radiation powder X-ray diffraction (SR-PXD) (295-573 K) and quasielastic neutron scattering (QENS) (297-514 K). The highest temperature probed with QENS (514 K) is above the melting point of Li4(BH4)(NH2)3 and the neutron measurements confirm a long-range diffusive motion of hydrogen containing species with the diffusion coeffcient D ~ 10-6 cm2/s, which is comparable to the value of Li+ diffusion inferred from conductivity measurements. SR-PXD confirms the recrystallization of Li4(BH4)(NH2)3 into the α-phase during cooling from the melt. At temperatures below 514 K, localized rotational motions were observed which have been attributed to (BH4)- tetraheder units undergoing rotations mainly around C3 axes. The activation energy for this thermally activated process is found to be Ea = 15.5±0.9 and 17.4±0.9 kJ/mol for the two instrumental resolutions utilized in the QENS measurements, respectively, corresponding to observation times of 55 and 14 ps.

Resume : In this work, a systematic study of the CsNH2-CsH composite in the entire compositional range is investigated. In this context, the formation of the Cs(NH2)xH1-x solid solution underlying the CsNH2-CsH interaction is thoroughly investigated by means of in-situ synchrotron experiments. The Cs(NH2)xH1-x solid-solution in the CsNH2-CsH system is formed by the reaction mechanism of amide-hydride anion exchange, similar to the KNH2-KH and RbNH2-RbH composite systems. The interaction between the H+ ion in the amide and H- ion in the hydride helps to promote the formation of intermediate phases at the amide-hydride interface and in turn enhances the mobility of the ionic species at the interfaces, which is extremely important to achieve fast reaction kinetics of hydrogen storage systems, especially at low temperatures. Through the results of this study, a comprehensive insight into the structural properties of alkali metal amide/hydride systems and their reaction mechanism has been obtained. Therefore, this work could be the basis for further studies aimed at extending this line of research and finding similarities with other composite systems.

Resume : Mechanochemical approach was employed to synthesize K-based amides such as K2[Zn(NH2)4], K2[Mn(NH2)4], and KNH2 at room temperature and at low gas pressure. Potassium-based amide-hydride storage materials were developed by mixing these amides with hydrides. K2[Zn(NH2)4]-8LiH and K2[Mn(NH2)4]-8LiH showed extremely fast hydrogen absorption kinetics. For example, the dehydrogenated K2[Zn(NH2)4]-8LiH can be fully re-hydrogenated (ca. 3.0 wt% of H2) at 230 °C within 20 s, which is one of the fastest re-hydrogenation kinetics reported in literatures for amide-hydride systems. And these excellent hydrogenation properties can be maintained upon cycling. The ultra-fast hydrogenation reactions of these K-based amide-hydride composites may indicate a fast ionic conductor. Stimulated by this phenomenon, a new type of potassium ion conductor material KNH2 was discovered. Ionic conductivity of the as synthesized KNH2 is 9.84×10-9 and 4.84×10-5 S cm-1 at 40 and 150 ?. KNH2 after mechanochemical treatment at 150 rpm for 10 hours, the ionic conductivity increases to 1.39×10-6 and 3.56×10-4 S cm-1 at 40 and 150 ?, respectively. Experimental results showed that ball milling process improves the ionic conductivity of KNH2 by creating defects in its structure. To the best of our knowledge, this is the first reported case using amide as a new type of potassium fast ion conductor and has great potentials as potassium ion solid electrolyte.

Resume : The fossil fuels consumption is not only the main cause of air polluting but also the origin of global warming with more and more alarming consequences. These leads to major consideration for alternatives: solar, wind and sea waves, which requires energy storage for final usage. Hydrogen is one candidate that can be either used as fuel or charge carrier in Ni-MH batteries. Binary compounds Gd2Ni7, Y2Ni7, Sm2Ni7 and La2Ni7 absorb large amounts of hydrogen but show several plateaus and limited reversibility, which is not suitable for applications [1-3]. This behavior is mainly due to the stacked structure and the mismatch between subunits [A2B4] and [AB5]. This mismatch is also responsible for the hydrogen induced amorphization (HIA) of these compounds [4]. Since the atomic size of rare earths plays an important role on this geometrical parameter, it is worth to study the effect of different rare earths on the hydrogenation properties. Indeed, A can be almost all light rare earths (La to Gd), yttrium and alkaline earth metals (Mg or Ca), whereas B can contain various late transition metals (Mn to Ni). To understand the effects of composition on the physicochemical properties of La2Ni7-based compound, various pseudo-binary systems have been investigated: Gd2 xLaxNi7 (x = 0, 0.6, 1, 1.5), Sm2-xLaxNi7 (x = 0, 0.5, 1, 1.5), Y2-xLaxNi7 (x = 0, 0.4, 0.5, 0.6, 0.8, 1, 1.5, 1.75) and R0.5La1.1Mg0.4Ni7 (A = Sm, Gd and Y) [5]. To determine their crystallographic properties, X-ray diffraction analysis was performed, followed by Rietveld refinement. Thermodynamic properties regarding reversible hydrogen sorption were investigated at room temperature. Quaternary compounds present drastically improved sorption properties for practical energy storage applications with reversible capacity equivalent to 400 mAh/g for La1.1Sm0.5Mg0.4Ni7. 1. V. Charbonnier, J. Monnier, J. Zhang, V. Paul-Boncour, S. Joiret, B. Puga, L. Goubault, P. Bernard, M. Latroche, J. Power Sources. 326 (2016) 146. 2. J.-C. Crivello, N. Madern, J. Zhang, J. Monnier, M. Latroche, J. Phys. Chem. C. 123 (2019) 38. 3. K. Iwase, K. Sakaki, Y. Nakamura, E. Akiba, Inorg. Chem. 52 (2013) 10105. 4. F. Fang, Z. Chen, D. Wu, H. Liu, C. Dong, Y. Song, D. Sun, J. Power Sources. 427 (2019) 145. 5. J. Zhang, V. Charbonnier, N. Madern, J. Monnier, M. Latroche, J. Alloys Compounds 852 (2021) 157008.

Resume : Recently biochar, the carbon side-product in the pyrolysis/gasification of residual waste biomasses, started to receive a widespread attention in the field of the electrical energy-storage, thanks to its hierarchical porous structure inherited from biomass precursors, its excellent chemical and electrochemical stability, high conductivity, high surface area and inexpensiveness. In particular, biochar converted to activated carbon (SSA > 1000m2/g) through a chemical activation with KOH appears now to be a new cost-effective and environmentally-friendly carbon material with great application prospect in the field of energy-storage applications. We report here on the preparation of novel hierarchically-porous super-activated carbon materials originating from biochar derived by the pyrolysis of poultry litter (PL) and of rice bran. The chemical activation process proved to be efficient to remove the majority of impurities other than carbon, stabilizing highly porous hierarchical structures with local graphene-like morphology. The porous compound obtained by PL demonstrated to behave as an excellent electrode material for high-performance symmetric supercapacitors (SCs), reaching high specific capacitance up to 230 F/g. On the contrary, the compound obtained by rice bran shows a very good hydrogen storage ability, adsorbing up to 3.5 wt % of hydrogen in around 20 seconds at -196°C and 6 bar and around 1.5 wt% at room temperature. Work is in progress to optimize the pyrolysis and activation conditions and to improve the performance of the materials by decoration with transition metals. The availability, the biocompatibility and the inexpensiveness of the starting materials suggest possible large-scale applications for such devices, for example in the field of transportation or in renewable energy-grids, but also in the field of bio-medicine.

Resume : Motivated by a funding initiative by the Federal Ministry of Education and Research (BMBF) to develop and strengthen scientific collaborations between German institutions with partners within the Asian-Pacific Research Area (APRA) on the topic of green hydrogen, a large research consortium consisting of academic and industrial partners from New Zealand (NZ) and Germany was successfully brought together to establish and participate in the first jointly operated German-New Zealand Green Hydrogen Research Center. 80% of NZ’s electric energy already comes from renewables (i.e. hydro, wind, solar), which from a German perspective makes it a prime partner country to develop, field-test and upscale green hydrogen technologies. The center will be set up at the University of Otago in Dunedin, located in the southern part of NZ, and consists of laboratories, offices and seminar venues and will host partners from both countries to enable the study of generation, storage and utilization of green hydrogen from as many angles as possible. This presentation will introduce you to NZ’s massive potential for green hydrogen technologies, the members and the structure of the joint research center, the research activities and the future plans extending the scope of the collaborative project.

Resume : Phase change materials (PCMs) have attracted great interest due to their capability for the storage of thermal energy, which substantially contribute to the efficient use and conservation of residual heat and solar energy. Taking into account different thermal parameters (such as phase transition temperature, latent heat, specific heat, among others) PCMs can be used to store heat and/or “cold”. Accordingly, and depending on these parameters, PCMs can be used in a wide variety of applications: from heating buildings to cooling medical containers for transportation of blood or organs.[1] Nowadays, the most commonly used PCMs (such as eutectic water-salt solutions, hydrated salts, paraffins, fatty acids and clathrates) are these with a solid-liquid phase transition (melting) with a large latent heat. However, solid-liquid PCMs have some important drawbacks, such as volume shrinkage and leakage, which pose significant technological challenges.[2] Therefore, solid-solid PCMs are emerging as a very attractive alternative to solid-liquid PCMs because solids they significantly reduce the expansion and leakage problems. In this context, plastic crystals (PC), which are crystalline compound composed of weakly interacting molecules that possess some orientational or conformational degree of freedom, are emerging as attractive materials due to their potential applications in different fields, such as electric, caloric and optic.[3][4][5] Along the last years, PC have been intensively studied for thermal energy storage applications due to their typical first-order transition with large latent heat, which is associated with order-disorder transitions in their ions or molecules. However, most of these compounds operate near room temperature, and they are not used for “cold storage” applications, which traditionally requires operating temperatures well-below 293 K. In this work, we focus on the n-dibutylammonium tetrafluoroborate plastic crystal, which displays several solid-solid phase transitions in the range of 269 - 282 K, with the thermodynamic properties suitable for “cold storage”. From the structural point of view, this compound is very complex with several polymorphs as a function of temperature. These crystal structures were determined by using variable-temperature single crystal and powder X-ray diffraction, as well as NMR spectroscopy. The thermal analysis was carried out differential scanning calorimetry (DSC) and thermal gravimetric analyzer (TGA). Finally, the obtained results are compared with those of the principal PCMs used for “cold storage”, which up-to-date are only solid-liquid PCMs. References: [1] E. Oró et al, Appl. Energy, 2012, 99, 513. [2] H. Mehling el al, Heat and cold storage with PCM, 2008. [3] S. Santos-Moreno et al, Materials, 2020, 13, 1162. [4] B. Liu et al, Nat. Commun., 2014, 5, 3092. [5] J. Harada et al, J. Am. Chem. Soc., 2019, 141(23), 9349.

Resume : Energy storage in solids in the form of hydrogen or electrochemical reactions is still considered as safe method. The ultimate materials for this purpose remain unknown. Metal hydrides and complex metal hydrides may serve both purposes and these with favorable thermodynamic and kinetic properties are researched. That are metal hydrides resistant to hydrogen contamination not constituting any environmental hazards or solid state electrolytes with electrochemical stability window appropriate for Li or Na secondary batteries. The theoretical investigations are contributing to the research over the last decades. This is especially vital for development of new materials and manipulations on the atomic level, where methods based on quantum mechanics approach have established position. We summarize theoretical developments for the equilibrium properties of hydride crystalline materials, their equilibrium surface structures with an example of LiNi5, ZrV2 and TiFe alloys. The electrochemical stability of closo and nido borates of Li and Na is confronted with experimental evidence.

Resume : The so-called Lithium Reactive Hydride Composite (Li RHC) (2 LiH + MgB2 / 2 LiBH4 + MgH2) is a high-temperature material that can be considered a suitable candidate for hydrogen storage in solids. This work aims to develop a kinetic model of the Li RHC absorption reaction. For such a purpose, thermodynamic and kinetic investigations on the system were performed. Based on the thermodynamic assessment for the absorption reaction, the system’s enthalpy ∆H and entropy ∆S were determined to be 34 ± 2 kJ∙mol H2 1 and 70 ± 3 J∙K 1∙mol H2 1, respectively. The kinetic investigations performed between 325 °C and 412 °C and between 15 and 50 bar reveal that the system’s hydrogen absorption can be modeled as having a one-dimensional interface-controlled reaction with an apparent activation energy of 146 ± 3 kJ∙mol H2 1. Applying the model, the reaction rate can be calculated as a function of the reacted fraction as well as the pressure and temperature conditions. The development of such a model allows its use in the finite element method (FEM) simulations for an optimized design of hydrogen storage systems.

Resume : The necessity to store energy in large quantities will increase significantly in the future, due to the larger fraction that is going to be generated from renewable sources, which are only intermittently available. To store heat at high temperature levels, which is needed for concentrated solar power plants, molten salts are commonly used, owing to their low material costs. However, they are required in large quantities and this still impact the overall cost of the plant. Using thermo-chemical energy storage systems instead, can overcome this disadvantage, by having very high energy densities that makes much less material necessary. Metal hydride systems are very promising, due to the stable capability, the high reaction enthalpies and the good controllability, by simply regulating the hydrogen pressure. [1] It has already been demonstrated by Paskevicius et al. [2], that the MgH2 system can reversibly store heat at high temperatures for different operation scenarios. A more cost effective possibility is to use the hydrogenation of recycled Mg waste as thermochemical energy storage. Due to its high reaction enthalpy, the moderate pressure and appropriate temperature conditions, the broad abundance and the recyclability, the Mg/Al alloy is perfectly suitable for this purpose. Following a previous publication of Hardian et al. [3], in which the performance of recycled Mg/Al waste is presented, an extensive kinetic reaction model for hydro- and dehydrogenation is derived in the presented work. Temperature and pressure dependencies are determined, as well as the rate limiting step of the reaction using the reduced time method. First, experiments are carried out in an autoclave with a medium scale powder mass; by varying the temperatures conditions for absorption and desorption, the thermal behavior of the larger powder batch could be investigated systematically. This specific system is simulated by the finite element method, which proves the resilience of the derived reaction model. Based on this model, the behavior of large powder masses can be thoroughly predicted so that new systems can be designed for future thermochemical energy storage applications. [1] K. Manickam, P. Mistry, G. Walker, D. Grant, C. E. Buckley, T. D. Humphries, M. Paskevicius, T. Jensen, R. Albert, K. Peinecke, M. Felderhoff, International Journal of Hydrogen Energy 2019, 44 (15), 7738 – 7745. DOI: https://doi.org/10.1016/j.ijhydene.2018.12.011 [2] M. Paskevicius, D. A. Sheppard, K. Williamson, C. E. Buckley, Energy 2015, 88, 469 – 477. DOI: https://doi.org/10.1016/j.energy.2015.05.068 [3] R. Hardian, C. Pistidda, A.-L. Chaudhary, G. Capurso, G. Gizer, H. Cao, C. Milanese, A. Girella, A. Santoru, D. Yigit, H. Dieringa, K. U. Kainer, T. Klassen, M. Dornheim, International Journal of Hydrogen Energy 2018, 43 (34), 16738 – 16748. DOI: https://doi.org/10.1016/j.ijhydene.2017.12.014

Resume : TiFe intermetallic has been widely known for its preferable hydrogen storage properties near ambient conditions. As a stationary hydrogen storage material, it can store upto 1.9 wt% of hydrogen and offer a stable and safe medium for practical usage. The foremost difficulty in achieving its full potential is the surface oxidation that hinders hydrogen absorption and reversibility. Plenty of research have been carried out focusing on the activation of the material towards hydrogen storage. Even so, the mechanism of oxidation remains unclear. In this work, we attempt to elucidate the nature of the metal surface upon oxidation and how it affects the hydrogenation with a multifaceted modelling approach. The equilibrium crystal shape of TiFe is predicted and the dissociation and diffusion of hydrogen through a few layers of oxide on the lowest energy surface is discussed in detail. We were able to gain atomistic level insights into the oxidized surface and interfaces from Density Functional Theory (DFT) whilst machine learning based methods aided to further expand the study. The work also demonstrates the significance of integrating multiple modelling methods in material research.

Resume : All car manufacturers for electrical vehicles consider Ni-rich based positive electrode materials. They exhibit very good electrochemical performances. Nevertheless, if the cells are cycled at high voltage, to increase their capacity, a significant decrease of the performance is observed upon cycling. Therefore, the goal of the research consists in improving the lifetime of the cells. In the literature, this behavior is attributed to: (i) the instability of Ni4+ ions that are formed at the end of charge, (ii) the formation of cracks due to the large change in the unit cell volume when a too large amount of lithium is deintercalated. Most of the researches consist in: (i) optimizing the material electrode morphology to prevent the internal constraints, (ii) improving the material stability at high voltage by cationic substitution and/or foreign cation doping. In order to try to have a better understanding of the real electrochemical mechanism occurring at high voltage a very detailed study of the behavior of the cycling of the LiNiO2 parent material was done. In fact, the real formula of this layered material is Li1-zNi1+zO2 (z > 0.01). This deviation from their ideal stoichiometry result from the instability of Ni3+ ions at the synthesis temperature, which leads to the presence of the extra-nickel ions in the Li layer. This departure from the ideal stoichiometry has a dramatic effect on the cell capacity if z > 0.02. The synthesis of the pristine phase was done in optimized conditions (oxygen flow, 700°C). The Rietveld refinement of the XRD pattern and a magnetic study confirmed that the Li0.99Ni1.01O2 composition was obtained. This material was used in Li coin cells which were cycled in various conditions. After a first charge to 4.0V (4.2V, 4.3V respectively) several cycles were done in the 4.0V-3.8V range (4.2V-3.8V, 4.3V-3.8V respectively). Then for all experiments, a final discharge was done to 3.0 V in GITT conditions to intercalate as much lithium as possible. When the cell is cycled up to 4.0V the shape of the cycling curve is normal. On the contrary, for the higher voltage values, at the very beginning of the discharge there is an unexpected lower shoot, which increases with the number of cycles and the charge voltage cutoff. This is an evidence of an increase of the cell impedance due to the formation of an insulating layer, which starts to be formed at 4.2V. This leads to lower charge capacity on the high voltage plateau. Our experiments show that this behavior disappears if the charge is limited to 4.15V. At low voltage, around 3. 6V, for all Ni-rich materials, there is also a capacity loss, even at the first cycle that corresponds to 20% of the expected capacity. GITT experiments have shown that this limitation is due a kinetic problem. During the full discharge, our experiments show that the capacity loss at low voltage increases when the cycling is performed at high voltage. This indicates that both phenomena are linked.

Resume : Solid-state electrolytes offer advantages over conventional liquid electrolytes, like the possibility of constructing batteries in bipolar stacks and greater safety, e.g. because they cannot leak. One promising material group are sulfides, among which beta-Li3PS4 gathered much attention in the last years due to its good ionic conductivity and scalable synthesis route. Several studies determined the ionic conductivity, but the reported conductivity values scatter. Possible origins might be varying synthesis and measurement conditions. Therefore, a standardised measurement protocol is needed to gain comparable data. First attempts to identify influences of the measurement conditions on the obtained ionic conductivity of thiophosphate-based sulfides were made by Ohno et al. 2020 [1], in which an interlaboratory study was performed, and Doux et al. 2020 [2], who examined pressure effects. This work investigates effects of different electrode materials on the obtained data from electrochemical impedance spectroscopy (EIS) of beta-Li3PS4 under controlled measurement conditions. Beta-Li3PS4 was synthesised via wet-chemical synthesis in THF. The as-synthesized material was used to produce pellets in a standardised way in order to rule out preparation effects. A variety of common electrode materials was applied to perform impedance measurements. The choice of electrode material showed effects on the variance of the obtained data. [1] S. Ohno et al., ACS Energy Letters 2020 5 (3), 910-915; DOI: 10.1021/acsenergylett.9b02764 [2] J.-M. Doux et al., J. Mater. Chem. A, 2020,8, 5049-5055; DOI: 10.1039/C9TA12889A

Resume : In the last decades, organic plastic crystals (mainly polyalcohols) have been widely studied for thermal energy storage applications due to their extremely large thermal changes induced by modifications in the ambient temperature.[1] However, these materials exhibit some limitations and drawbacks, such as flammability, relatively low thermal stability and low density (which also reduces the density of stored energy). In the search for new plastic crystals with enhanced thermal properties, we focus on hybrid plastic crystals with general formula [A][FeCl4] (where A = organic cations). These materials have recently exhibited functional properties, of interest for different technological applications, such as magnetic or dielectric order and even thermal energy storage.[2] In the present work, we found that this emerging family can exhibit very large thermal changes that can be induced by thermal energy from sunlight, with charging and discharging temperatures suitable for domestic applications.[3] Moreover, these compounds are not flammable and they exhibit larger thermal stability than the traditional polyalcoholic plastic crystals. Interestingly, this specific combination of organic cations and inorganic anions leads towards antiferroelectric structures that can largely change their dielectric capacity when increasing the temperature. As a matter of fact, these materials can increase their electric energy capacity up to five times when the charging temperature is surpassed, and later release this energy when the discharging temperature is reached. In summary, the [A][FeCl4] plastic crystals materials here reported arise as a promising family of compounds that can simultaneously store multiple thermal and electric energy just by using the solar thermal energy, and use the store energy for domestic applications. [1] (a) J. Font et al., Sol. Energy Mater., 1987, 15, 299-310; (b) M. Barrio et al., Sol. Energy Mater., 1988, 18, 109-115. [2] (a) P. González-Izquierdo et al., J. Mater. Chem. C, 2021, 9, 4453-4465; (b) D. Li et al., Inorg. Chem., 2019, 58, 655-662. [3] J. Salgado-Beceiro et al., J. Mater. Chem. C, 2020, 8, 13686–13694.

Resume : Nitrogen-rich carbon materials have advantageous properties to be applied to energy research compared to general carbon materials. Nitrogen-rich carbon materials are more suitable for hybridization with transition metals than pristine carbon, and help transition metal active materials to be tuned to nano size. In addition, it can show better compatibility with various substances such as electrolytes or working solutions. Therefore, many existing studies have attempted to impart nitrogen to carbon materials. In general, carbon materials used in research are made through carbonization, and nitrogen-rich carbon materials can be made by using precursors that contain both carbon and nitrogen (melamine, urea, etc.). Also, physical doping using plasma or gas doping using ammonia is also used. We have discovered, through an entirely new method, a process for imparting nitrogen to a carbon material through a polymer that is completely removed during the experiment. In addition, we have developed a method for attaching various and complex nitrogen functional groups as well as nitrogen molecules depending on the type of polymer used. This method is composed of mixing a polymer having a specific functional group with the original carbon material and then completely removing the polymer through heat treatment under a mild vacuum. At this time, the degree of polymerization of the polymer has an effect on the attachment of nitrogen and functional groups. If the degree of polymerization is too high, additional carbonization of the polymer proceeds, and an undesired amorphous carbon structure is added to the original structure. In addition, if the degree of polymerization is too low, the polymer evaporates too quickly, so that nitrogen and functional groups can't be effectively imparted to the original structure. We found through experiments and analysis the appropriate degree of polymerization that can give nitrogen and functional groups to carbon. Additionally, we used a carbon structure including nitrogen and functional groups as an electrode of an organic electrolyte-based capacitor. The carbon structure containing nitrogen showed superior capacity and lifetime performance compared to the general carbon structure, and these results were attributed to the improvement of solvent affinity and conductivity by nitrogen addition. In addition, using a carbon electrode with various nitrogen functional groups to the capacitor, unique experimental results were obtained according to the change in surface energy and physical properties. In this study, we introduce a method to easily attach nitrogen and nitrogen functional groups to carbon materials, and include excellent electrochemical test results used the materials of the developed method. The result is mainly explained by the change in surface energy due to the participation of nitrogen in the carbon structure. This new method is expected to be applied in various fields of energy research, including energy storage.

Resume : As the price of renewable energy, e.g. sun and wind power, increasingly becomes competitive with fossil fuels, the main hurdle to overcome in the green transition is the storage of renewable energy to even out the intermittent energy production from these sources [1]. Metal carbonates have great potential as thermochemical energy storage materials through the endo- and exothermic release and uptake of CO2 with low cost and high energy density [2]. However, the major challenge is the loss of CO2 storage capacity, i.e. energy storage capacity, which drastically decreases over multiple cycles [3, 4]. Our recent research has highlighted multiple ways to significantly improve the cyclic stability and reaction kinetics of metal carbonates, e.g. by the addition of a molten salt catalyst [2], changing reaction pathway/thermodynamics by adding a metal orthosilicate [5], or by the formation of ternary metal oxide compounds [6]. A range of compounds operating in different temperature intervals have been investigated, i.e. dolomite, CaMg(CO3)2 at 550 °C, BaCO3 at 850 °C, and CaCO3 at 900 °C, and have been operated in energy storage and release cycles. The most promising system so far is the CaCO3-Al2O3 (20 wt%), which was cycled 500 times with a 90% energy capacity retention and has a cost of ~ 0.7 USD/kWh (materials-basis only) [6]. This presentation will give an overview of current research and an outline of future perspectives on thermochemical energy storage. References [1] International Renewable Energy Agency (IRENA), “Renewable Power Generation Costs in 2017: Key Findings and Executive Summary.” [2] T. Humphries, K. T. Møller, et al., J. Mater. Chem. A, 2019, 7, 1206. [3] G. S. Grasa, J. C. Abanades, Ind. Eng. Chem. Res. 2006, 45, 26, 8846. [4] J. Abanades, D. Alvarez, Energy Fuels 2003, 17, 2, 308. [5] K. T. Møller, et al., J. Mater. Chem. A, 2019, 8, 10935. [6] K. T. Møller, et al., J. Mater. Chem. A, 2019, 8, 9646.

Resume : Stable power generation from renewable energy requires the development of new materials that can be used for energy storage. A new reactive carbonate composite (RCC) based on SrCO3 is proposed as a material with high energy density for thermochemical energy storage. SrCO3-SrSiO3 can promote the thermodynamic destabilisation of SrCO3, making its operating temperature (700 °C) more suitable for concentrated solar thermal power applications. Utilising a eutectic mixture of salts as a catalyst, the reversible carbonation reaction achieves cycle stability of approximately 80% of efficiency over multiple cycles. The high volumetric (1878 MJ m-3) and gravimetric energy density (500 kJ kg-1) of the RCC, allows a compact thermal battery to be developed. Additionally, the low cost of SrCO3 makes the RCC a highly competitive alternative energy storage material in term of cost and efficiency when compared to existing molten salt based energy storage technology. Its cost-benefit is 60% cheaper than the molten salts currently used in concentrated solar thermal power application on a materials basis. A thermochemical energy storage system based on SrCO3-SrSiO3 requires a small footprint, which is essential for remote areas. It is also compatible with the Rankine-Brayton combined cycle and with a Stirling engine for thermal to electrical energy conversion and can be adapted for another renewable energy form such as photovoltaics and wind farms.

Resume : From hundreds of salts that have been considered for thermochemical heat storage, potassium carbonate (PC) has been selected as a promising candidate [1]. The main disadvantage of this compound, as well as of several other salts, is the large hysteresis between hydration and dehydration. This undesirable behaviour lowers the power output and limits the operating window of the material. Recent work showed that this hysteresis can be caused by multistep reaction processes mediated by a wetting layer on the surface of the salt [2]. Ionic mobility within that wetting layer is limited in a metastable zone around equilibrium conditions. Considering this, we have developed an approach that enhances the power output of PC by mixing it with another salt hydrate. This method makes use of deliquescence, which promotes mobility in the wetting layer. Our work shows that the power output can be tripled by mixing PC with a salt hydrate with a low deliquescence point depending on the reaction conditions. By testing a multitude of salt mixtures, we have developed an approach that helps in selecting a suitable additive for other salt hydrates that suffer from similar metastable behaviour. [1] Donkers, P.A.J., et al. "A review of salt hydrates for seasonal heat storage in domestic applications." Applied energy 199 (2017): 45-68. [2] Sögütoglu, L.C., et al. "Understanding the hydration process of salts: the impact of a nucleation barrier." Crystal Growth & Design 19.4 (2019): 2279-2288.

Resume : Potassium carbonate is a promising salt for thermochemical heat storage. For an application mm-sized salt hydrate particles are manufactured to be loaded inside a reactor. The step towards larger particles is necessary to prevent a large pressure drop over the reactor bed during hydration/ dehydration in a given air flow. Even though the performance of a single salt hydrate mm-sized particle is one of the key aspects in the application, its kinetics has not yet been extensively investigated. Therefore, in this work a systematic study on the hydration kinetics of mm-sized disc shaped salt hydrate (K2CO3) particles is presented for the first time. The effect of density, primary particle size and driving force on the hydration kinetics could be evaluated using a 1D diffusion model based on a shrinking core assumption. In this approach the overall reaction is limited by the diffusion of reactant towards the reaction front. The particle internal structure and powder kinetics are investigated to estimate the reaction front width. It is found that the particle density (porosity) greatly affects the hydration rate. The kinetic behavior is found to be independent of the primary powder size. Additionally, the effect of driving force is studied. The calculated transport mechanism is unaffected by changes in driving force whereas the power output is. The main conclusions are that the hydration kinetics of mm-sized salt hydrate particles is diffusion limited and that the particle density (porosity) is the main parameter controlling its performance. The results from this work can be used to predict kinetic behavior of particles with other generic shapes (2D and 3D). Since the particle hydration is expected to be similar for different salts, given the powder reaction rate constant is high enough, the model from this work can possibly be used to predict and optimize particles made from different salts as well. Therefore, this work provides input for designing future salt hydrate particles for thermochemical heat storage using various combinations of salts and particle shapes.

Resume : The unique advantage of concentrated solar power (CSP) plants implemented with thermal energy storage (TES) is their ability to store the solar heat generated during the daytime to produce electricity at night. Therefore, the future of this technology relies on the identification and development of cost-effective TES materials. The aim for next-generation CSP plants is to develop a TES material that can operate above 600 °C, increasing the overall energy efficiency of the plant. Metal hydrides, which can reversibly store heat via chemical reactions, have been identified as a potential next-generation TES material by the US Department of Energy as part of the SunShot research programme [1]. Calcium hydride is a promising high-temperature metal hydride due to its low operating hydrogen pressure at high temperature (1 − 5 bar at 1100 – 1400 °C) and high gravimetric (4939 kJ/kg) and volumetric energy densities (8396 MJ/m3) [2]. The melting point of both calcium hydride and calcium metal are 816 °C and 842 °C, respectively. The corrosive nature of molten calcium hydride at high temperature requires expensive containment tank materials [3]. The best solution to avoid these difficulties is to reduce the decomposition temperature of pure calcium hydride. Thermodynamic destabilization of calcium hydride, using suitable additives, is one approach for reducing the decomposition temperature of calcium hydride, transforming it into a suitable TES material for next generation CSP plants. The novelty of this study lies in the selection of relatively inexpensive and abundant materials such as aluminium oxide and zinc to be used as additives to reduce the decomposition temperature of calcium hydride. It has been demonstrated that adding aluminium oxide or zinc to calcium hydride, reduces the hydrogen equilibrium pressure (1 bar) from 1100 °C to 636 °C and 597 °C, respectively [4, 5]. This study provides an overview of the thermodynamic destabilisation of calcium hydride including thermodynamic and kinetic measurements, synthesis, characterisation and thermal analysis. The cost calculation and cycling stability of each system has been studied to check the feasibility of using the materials as a thermal battery for CSP applications. References 1. US Department of Energy, D.O.E. Sunshot Vision Study. https://www.energy.gov/eere/solar/sunshot-initiative. access date-09/06/2020. 2. Manickam, K., et al., Future perspectives of thermal energy storage with metal hydrides. International Journal of Hydrogen Energy, 2019. 44(15): p. 7738-7745. 3. Peterson, D. and V. Fattore, calcium-calcium hydride phase system1. The Journal of Physical Chemistry, 1961. 65(11): p. 2062-2064. 4. Balakrishnan, s., et al., Destabilised Calcium Hydride as a Promising High-Temperature Thermal Battery. The Journal of Physical Chemistry C, 2020, 124, 32, 17512- 17519. 5. Balakrishnan, S., et al., Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures. Physical Chemistry Chemical Physics, 2020. 22(44): p. 25780-25788.

Resume : The intermittent nature of renewable energy is a major challenge that can be overcome via cheap, effective energy storage[1]. Thermochemical energy storage is an upcoming technology that can improve efficiency and lower cost in applications such as concentrated solar power[1]. Metal carbonates have great potential as thermochemical energy storage materials through the reversible endo/exothermic desorb/absorption of carbon dioxide (CO₂)[2]. However, the major challenges include the loss of cyclic capacity and slow kinetics[3]. Previously, it has been established that barium carbonate (BaCO₃) can be thermodynamically destabilised by the addition of barium silicate (BaSiO₃)[4]. This lowers the operating temperature from ~1400 °C to 850 °C to allow operation with second-generation concentrated solar power plants. Moreover, the addition of a calcium carbonate (CaCO₃) catalyst improves kinetics by an order of magnitude [4]. This research explores the reactions of barium carbonate combined with various metal oxides, for example, iron (III) oxide (Fe₂O₃) or titanium (IV) oxide (TiO₂). This reduces the operating temperature from 1400 °C to 875 °C and 1050 °C respectively and improves reaction kinetics of carbon dioxide release and uptake. The performance of barium carbonate with titanium oxide is especially promising, as it shows minimal (~5 %) capacity loss over 75 cycles. This presentation will summarise current research and explore future directions. [1] Paskevicius, M.; Sheppard, D. A.; Williamson, K.; Buckley, C. E. Metal Hydride Thermal Heat Storage Prototype for Concentrating Solar Thermal Power. Energy 2015, 88, 469–477. https://doi.org/10.1016/j.energy.2015.05.068. [2] André, L.; Abanades, S. Evaluation and Performances Comparison of Calcium, Strontium and Barium Carbonates during Calcination/Carbonation Reactions for Solar Thermochemical Energy Storage. J. Energy Storage 2017, 13, 193–205. https://doi.org/10.1016/j.est.2017.07.014. [3] Humphries, T. D.; Møller, K. T.; Rickard, W. D. A.; Sofianos, M. V.; Liu, S.; Buckley, C. E.; Paskevicius, M. Dolomite: A Low Cost Thermochemical Energy Storage Material. J. Mater. Chem. A 2019, 7 (3), 1206–1215. https://doi.org/10.1039/C8TA07254J. [4] Møller, K. T.; Williamson, K.; Buckley, C. E.; Paskevicius, M. Thermochemical Energy Storage Properties of a Barium Based Reactive Carbonate Composite. J. Mater. Chem. A 2020, 8 (21), 10935–10942. https://doi.org/10.1039/D0TA03671D.

Resume : Producing green fuels from renewable electricity sources while capturing CO2 represents one of the main and fascinating challenges proposed to the scientific community for drastically reducing the even increasing level of the global warming gas CO2 and convert it into efficient and environmental friendly energy vectors. However, the direct transformation of CO2 into chemical green fuels is quite challenging due to the high stability of CO2 molecule and the relative expensive catalysts commonly used to activate most of the many processes reported in the current literature. For this reason, fervent research is still addressed to explore new and feasible ways to convert efficiently carbon dioxide by using abundant and cheap raw materials. Recently, we demonstrated how carbon dioxide can be efficiently converted to light hydrocarbons and carbonates, by mechanochemical activations of olivine, a natural and abundant mineral consisting of a solid solution of fayalite and forsterite, in presence of water. The olivine powders, once subjected to mechanical input through ball milling, react faster with water and carbon dioxide, producing hydrogen and light hydrocarbons with high degree of conversion: three order of magnitude superior than the hydrothermal approach reported in the literature. Within this context, this presentation aims to provide new insights and perspective into the study of the mechanically activated reaction between weathering olivine and carbon dioxide to form hydrogen, light hydrocarbons, and carbonates. The kinetics of gaseous products related to the in situ ball milling of olivine under carbon dioxide atmosphere will be reported, and the process efficiency compared with that of thermally activated. A detailed structural and microstructural characterization of the pre- and post-activated powders will be also presented, as well as the as-defined mechanism behind the mechanochemical process. The inferences of the here presented investigation are multiple: on the one hand, the production of light hydrocarbons (mainly methane) is significantly improved through ball milling with respect to thermal activation. On the other hand, the competitive carbonation process can represent an alternative way to fix carbon dioxide, of particular interest because able to simultaneously generate added values products for building materials.

Resume : In order to establish a sustainable society, hydrogen is seen as a promising energy carrier. Indeed, it is extremely light, highly energetic and does not emit greenhouse effect gases during use. But, because it is a gas, it suffers from poor volumetric energy density at ambient conditions of pressure and temperature. To overcome this problem, hydrogen can be stored in different ways: as a liquid at -253 °C, as a hydride or as a compressed gas. Among them, compressed gas is, today, the most mature technology and was chosen to store hydrogen in most Fuel Cell Vehicles (FCV). For cars, which are relatively small when compared to other FCV (such as trucks, buses or trains), get a high volumetric energy density is compulsory. Thus, hydrogen is often stored at the ultra-high pressure of 70 MPa. FCV can be filled up with 70 MPa-hydrogen in Hydrogen Refueling Stations (HRS). In these places, hydrogen is pressurized by mechanical compressors. The main drawbacks of these compressors are their cost (installation and maintenance), as well as their energy consumption. Here, we are looking for a solution that could advantageously replace mechanical compressors. Metal hydride (MH) compressors are silent, deliver high purity hydrogen and do not require frequent maintenance. In addition, they simply need heat energy to work. Thus, if this heat is provided by the waste heat of a nearby device, it might lead to a decrease in the pressurization energy consumption. A MH compressor is basically a tank containing a hydrogen storage alloy. This compound is able to absorb hydrogen at low pressure PL and low temperature TL, and, when heated at TH, it desorbs hydrogen and delivers it at higher pressure PH. In our concept, the MH compressor will be heated to TH thanks to the waste heat of nearby devices providing hydrogen (e.g. high pressure electrolyzer or electrochemical hydrogen compressor). This latter compressor delivers H2 at about 35 MPa, then the MH compressor takes over, compressing the hydrogen to 80 MPa. We are targeting a MH compressor with inlet hydrogen pressure PL = 20-30 MPa at TL = 30 °C, that would deliver hydrogen at outlet pressure PH = 80 MPa when heated at TH = 80 °C. Not only these operation pressure-temperature conditions, but also the slope of the plateau and hysteresis in the isotherms as well as the compression ratio, are important properties to consider when realizing a MH compressor. For that purpose, we are investigating various types of Ti-based hydrogen storage compounds with C14 structure. In this presentation, we would like to explain the effect of substitution on their hydrogenation properties. This work was supported by the Development of high efficiency hydrogen storage-compression-supply system by NEDO.

Resume : Magnesium (Mg) alloy are considered one of the candidates for engineering applications in the automobile and aeronautical industry, owing to their good castability and high strength-to-weight ratio (specific strength). However, the poor corrosion resistance of these alloys has significantly restricted their application in aerospace, rail transit, electronic communications and other fields. Mg is quite susceptible to corrosion due mainly to its negative potential (~2.37 VSHE) and low Pilling–Bedworth (P–B) ratio (<1). These factors are conducive for the occurrence of galvanic corrosion and hinder the formation of a protective passive film on the alloys surface. Therefore, Mg alloys are readily to corrode in humid environments. In this work, we designed a high corrosion resistant Mg-Sc-Y ternary peritectic alloys based on the first-principles calculations of density functional theory. The Mg-Sc-Y alloy is composed of the typical peritectic microstructure with cellular grains. Elemental Sc segregates (in general) into the intergranular cells and pushes Y to the surrounding areas. With the addition of Sc and Y, the difference of Volta potential and work function between intergranular cells and boundaries reduced, indicating a decrease in the driving force for corrosion. In addition, a more protective film (MgO/Mg(OH)2+Sc2O3+Y2O3) would form on the surface, which further improves the corrosion resistance of the alloy. Nevertheless, with the further increase of Y content, more micro-galvanic couples emerge, enhancing the cathodic hydrogen evolution. Therefore, the Mg-4Sc-1Y (wt.%) alloy exhibits the best corrosion resistance. The hydrogen evolution rate in 3.5wt.% NaCl solution is only 0.13ml/cm2/day, performing the same level as high-purity Mg.

Resume : In order to mitigate the ever-increasing CO2 concentration in the atmosphere, new materials and sustainable approaches, part of the carbon capture utilization strategies (CCU), are required. In this context, the present work focuses on the conversion of CO2 into light hydrocarbons through photochemical activated processes on Olivine-doped system. Olivine mineral group has been selected for the high content of magnesium, high abundance in nature and low suppling cost that makes it an ideal feedstock candidate for the strategies of CO2 storage and conversion. Different materials have been investigated as dopants to obtain the optimal energy band gap for the photochemical induced conversion process. The doped materials (Olivine with TiO2, NiO, Fe3O4, Fe2O3 with a weight fraction of the dopant in the range from 1 wt % to 15 wt %) have been prepared through mechanical ball milling and characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM). The energy band gaps of the prepared materials have been calculated by UV-vis absorbance measurements with the Tauc Plot method. The photocatalytic tests have been performed in a laboratory stainless steel gas reactor with quartz windows and a known amount of distilled water, under 1 bar of CO2 (99%) at room temperature. Pellet of the prepared materials have been tested for 12 h under solar light irradiation using a solar simulator with a Xenon lamp emission regulated with Air Mass filter 1.5 (A.M. 1.5). Gas chromatographic analysis performed on gasses upon solar irradiation tests, reveal methane, ethane and ethene evolution, especially on Olivine systems doped with 1 wt % of TiO2 anatase (methane evolution of 2.7(2) ∙ 104 μmol ∙ L-1). The XRD, SEM, EDX and FTIR characterization of pellet prepared through mechanical ball milling, post irradiation tests, show the absence of iron carbonates and traces of magnesium carbonates. This investigation show, for the first time by our knowledge, the effectiveness of CO2 conversion processes triggered by visible irradiation on Olivine-doped systems.

Resume : A very versatile class of catalyst materials that can be used for applications in (electro)chemical energy conversion are perovskite oxides. They have the general formula ABO3 and the high flexibility is due to the fact that the properties can be tuned by the choice of the cations A and B. These represent different sites in the crystal structure and either site can be occupied even by a combination of elements. This allows adapting a material to the specific catalytic problem, e.g. by doping with a catalytically highly active element. Furthermore, the stability can be tuned and highly thermally stable materials are achievable, suitable for high temperature applications. One of the reasons why perovskites excel in catalytic applications is their ability to exsolve metal nanoparticles consisting of the catalytically active doping elements, which consequently decorate the material surface. These nanoparticles are very stable towards sintering and coking due to being well socketed within the perovskite support. The catalytic ability of perovskite oxides has been demonstrated for the reverse water-gas shift reaction (rWGS), transforming CO2 and H2 to CO and H2O. This is an important reaction for energy conversion related processes. For example, CO could subsequently be used to produce fuels, effectively storing chemical energy. A very promising perovskite catalyst for this reaction was Nd0.6Ca0.4Fe0.9Co0.1O3-δ. During reaction, the Co-doped material showed exsolution of Co-nanoparticles, greatly enhancing catalytic activity. The material has been characterized with several methods, including in situ x-ray diffraction (XRD), in situ x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The composition of the material can be further tuned to the desired application, e.g. for optimized performance or lower costs (using less expensive elements). Also, the concept can be easily extended to other reaction systems such as methanol synthesis, thus directly producing a fuel. Acknowledgement: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 755744 / ERC - Starting Grant TUCAS).

Resume : Because of its relevance in catalysis, the interaction of hydrogen with metallic surfaces is generally well studied. However, knowledge on surface properties is limited when it comes to the class of metals, which form bulk hydrides, such as titanium. Hydride surfaces have peculiar properties for their application as heterogeneous hydrogenation catalysts, e.g., they may have even less hydro-gen on the surface than their metallic counterparts resulting in lower catalytic yields. Titanium dihydride TiH2 has been recently shown to catalyze the ammonia synthesis under Haber-Bosch conditions while titanium metal shows no activity. The key to understanding the catalytic properties of hydrides is their electronic structure. The standard electron spectroscopy methods are incompatible with hydrogen pressures needed to form hydrides, therefore these kind of experiments are usually restricted to post-mortem analysis. We have developed a method to hydrogenate thin films in-situ under UHV conditions compatible with electron spectroscopy measurements. We can measure pressure-composition isotherms (pcT) of the Ti-H system by electron energy loss spectroscopy (EELS) and investigate different titanium hydrides by electron spectroscopy methods. Furthermore the UHV system allows for introduction of nitrogen gas and the study of the resulting surfaces. The findings are supported by DFT calculations of the titanium hydrogen interactions.