• Energy Research

Two-step, solar thermochemical splitting of water using nonstoichiometric redox-active metal oxides has emerged as an intriguing approach for large-scale hydrogen production. Perovskites have been ...

Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2020.11.016. [EN] Variable valence oxides of the perovskite crystal structure haveemerged as promising candidates for solar hydrogen productionvia two-step thermochemical cycling. Here, we report the excep-tional efficacy of the perovskite CaTi0.5Mn0.5O3–d(CTM55) for thisprocess. The combination of intermediate enthalpy, rangingbetween 200 and 280 kJ (mol-O) 1, and large entropy, ranging between 120 and 180 J (mol-O) 1K 1, of CTM55 create favorableconditions for water splitting. The oxidation state changes are domi-nated by Mn, with Ti stabilizing the cubic phase and increasing itsreduction enthalpy. A hydrogen yield of 10.0G0.2 mL g 1isachieved in a cycle between 1,350 C (reduction) and 1,150 C(watersplitting) and a total cycle time of 1.5 h, exceeding all previous fuelproduction reports. The gas evolution rate suggests rapid materialkinetics, and, at 1,150 C and higher, a process primarily limited bythe magnitude of the thermodynamic driving force. This research is funded by the U.S. Department of Energy, through the office of Energy Efficiency and Renewable Energy (EERE) contract DE-EE0008089. The supportfrom the European Union’s Horizon 2020 Research And Innovation Programme un-der the Marie Sklodowska-Curie grant agreement no. 746167 is also acknowledged.This work made use of the Jerome B. Cohen X-Ray Diffraction Facility and the Pulsed Laser Deposition Shared Facility at the Materials Research Center at Northwestern University supported by the National Science Foundation MRSEC program (DMR-1720139) and the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205). This work additionally made use of the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) supported by Northwestern University, The Dow Chemical Company, and DuPont de Nemours, Inc. The APS is a U.S. Department of Energy(DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. The authors acknowledge Dr. Timothy Davenport and Dr. Stephen Wilke for help in thermo-chemical hydrogen production measurements, and graduate student Louis Wangfor assistance with Rietveld refinements and for consultation on thermogravimetric measurements. Peer reviewed

Abstract Recently, CaMnO3 has been proposed as a promising candidate for high temperature thermochemical heat storage. The material reversibly releases oxygen in response to changes in oxygen partial pressure (pO2) in the temperature range (800-1000°C) suitable for Concentrated Solar Power (CSP) plants. However, it undergoes decomposition at pO2

[EN] CaMnO3 oxide can be considered a promising candidate for high temperature thermochemical heat storage, since it is able to release oxygen in a wide temperature range (800-1000 °C) at different oxygen partial pressures (pO2) suitable for Concentrated Solar Power (CSP) plants. Moreover, it is composed of earth abundant, inexpensive, non-toxic elements. However, it undergoes decomposition at pO2<0.01 atm and at temperature above 1100 °C. In order to overcome this limitation and to extent the operating temperature range, in this study B-site doping with Fe was used as approach for preventing decomposition. The reaction enthalpy was measured through equilibrium non-stoichiometry curves so that the heat storage capacity could be evaluated. It was demonstrated that Fe-doping prevented CaMnO3 decomposition up to 1200 °C at pO2=0.008 thus widening the operating temperature range and the oxygen reduction extent. In addition, the heat storage capacity (ΔH (kJ/molABO3)) of Fe-CaMnO3 (∼324 kJ/kgABO3) is remarkably higher than that of the un-doped CaMnO3 (∼250 kJ/kgABO3 This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement N° 74616. Support of the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Award DE-EE0008089.0000, is also acknowledged. Peer reviewed

Summary Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 m o l H 2 / g c a t h .

By co-doping CaMnO3 with La and Fe (LaxCa1−xMn1−yFeyO3), it is possible to tailor the material thermodynamics, reduction extent, together with operating temperature range for thermochemical heat storage application.

The CaFe0.1Mn0.9O3−δ0 oxide can reversibly release oxygen over a relatively wide range of temperatures and oxygen partial pressures and its favourable thermodynamic properties make it a promising candidate for thermochemical heat storage.