In a unified regenerative fuel cell(URFC)or reversible fuel cell,the oxygen bifunctional catalyst must switch reversibly between the oxygen reduction reaction(ORR),fuel cell mode,and the oxygen evolution reaction(OER),electrolyzer mode.However,it is often unclear what effect alternating between ORR and OER has on the electrochemical behavior and physiochemical properties of the catalyst.Herein,operando X-ray absorption spectroscopy(XAS)is utilized to monitor the continuous and dynamic evolution of the Co,Mn,and Fe oxidation states of perovskite catalysts Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF)and La0.4Sr0.6MnO3-δ(LSM),while the potential is oscillated between reducing and oxidizing potentials with cyclic voltammetry.The results reveal the importance of investigating bifunctional catalysts by alternating between fuel cell and electrolyzer operation and highlight the limitations and challenges of bifunctional catalysts.It is shown that the requirements for ORR and OER performance are divergent and that the oxidative potentials of OER are detrimental to ORR activity.These findings are used to give guidelines for future bifunctional catalyst design.Additionally,it is demonstrated how sunlight can be used to reactivate the ORR activity of LSM after rigorous cycling.

The development of unified regenerative fuel cells (URFCs) necessitates an active and stable bifunctional oxygen electrocatalyst. The unique challenge of possessing high activity for both the oxygen reduction (ORR) and oxygen evolution (OER) reactions, while maintaining stability over a wide potential window impedes the design of bifunctional oxygen electrocatalysts. Herein, two design strategies are explored to optimize their performance. The first incorporates active sites for the ORR and OER, Mn and Co, into a single perovskite structure, which is achieved with the perovskites Ba0.5Sr0.5Co0.8Mn0.2O3-delta (BSCM) and La0.5Ba0.25Sr0.25Co0.5Mn0.5O3-delta (LBSCM). The second combines an active ORR perovskite catalyst (La0.4Sr0.6MnO3-delta (LSM)) with an OER active perovskite catalyst (Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF)) in a physical mixed composite (BSCF/LSM). The success of the two strategies is investigated by measuring the catalysts' catalytic performance and response to alternating reducing and oxidizing potentials to mimic the dynamic conditions experienced during the operation of URFCs. Additionally, the continuous, potentiodynamic change in Mn, Co, and Fe oxidation states during the ORR and OER is elucidated with operando X-ray absorption spectroscopy (XAS) measurements, revealing key insights into the nature of the active sites. The results reveal important catalyst physiochemical properties and provide a guide for future research and design principles for bifunctional oxygen electrocatalysts.

Carbon is often used as a conductive additive in catalyst layers to increase conductivity and catalytic activity. However, the effect of carbon addition to perovskites on the oxygen reduction (ORR) and oxygen evolution (OER) reactions is convoluted. In this work, composites of perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) and conductive additives, carbon and indium doped tin oxide are compared. It is found that the conductive additives have differing effects on the ORR and OER activities and cobalt redox behavior, with carbon having a much more significant effect. In order to elucidate further these differences between BSCF and BSCF/carbon, operando X-ray absorption spectroscopy (XAS) is measured simultaneously with cyclic voltammetry into the ORR and OER regions and the continuous changes in the Co oxidation state are observed with high time resolution. We theorize that carbon is enhancing the Co redox activity and as a result, the ORR and OER activities are likewise improved.

With the increase in renewable energy usage comes the need for energy storage systems due to intermittency issues. Hydrogen storage systems have been identified as one solution. Unitized regenerative fuel cells (URFC) combine electrolyzers and fuel cells in one device, allowing electricity to be stored and used easily. However, the oxygen electrodes are still affected by high overpotentials and slow kinetics. Perovskite oxides have been identified as a class of materials, which are low-cost, tunable, and active for the oxygen reduction (ORR) and evolution (OER) reactions. Here, we investigate perovskites as bifunctional catalysts for ORR and OER in alkaline solution. We examine and compare two strategies for bifunctional catalysts: using one catalyst, which is able to perform OER and ORR vs. a combination of two catalysts, one active for ORR and one active for OER. Frequently, the catalysts’ performances for these two reactions are measured separately.1,2,3 Here, we investigate how these bifunctional catalysts respond to cycling between the OER and ORR regions. Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) is known to be a promising OER catalyst.4,5,6 However, without carbon, it lacks ORR activity.4 La(1-x)SrxMnO3 (LSM) is a promising ORR catalyst.3,7 However, without modification, it has been shown to have limited OER activity.3 Separately, these catalysts lack high performance for both reactions. Here, we combine the two catalysts into a BSCF/LSM/Carbon composite electrode and compare to electrodes prepared from the constituent single material components. In addition, we have synthesized single material perovskites containing both Co and Mn that to the best of our knowledge have never been tested as electrodes for ORR/OER. In order to understand the catalysts’ behaviors under OER and ORR conditions, X-ray adsorption spectroscopy (XAS) was measured continuously while performing cyclic voltammetry. We were able to monitor the continuous changes of the Co, Mn, and Fe oxidation states and local environment during OER and ORR with remarkably high time/applied potential resolution. Our findings illustrate the reversible and irreversible changes that can occur during OER and ORR and provide strategies for future bifunctional catalyst design. References Kirsanova, M. A.; Okatenko, V. D.; Aksyonov, D. A.; Forslund, R. P.; Mefford, J. T.; Stevenson, K. J.; Abakumov, A. M. Bifunctional OER/ORR Catalytic Activity in the Tetrahedral YBaCo 4 O 7.3 Oxide. Mater. Chem. A 2019, 7 (1), 330–341. Elumeeva, K.; Masa, J.; Sierau, J.; Tietz, F.; Muhler, M.; Schuhmann, W. Perovskite-Based Bifunctional Electrocatalysts for Oxygen Evolution and Oxygen Reduction in Alkaline Electrolytes. Acta 2016, 208, 25–32. Xu, W.; Apodaca, N.; Wang, H.; Yan, L.; Chen, G.; Zhou, M.; Ding, D.; Choudhury, P.; Luo, H. A-Site Excessive (La0.8Sr0.2)1+ XMnO3 Perovskite Oxides for Bifunctional Oxygen Catalyst in Alkaline Media. ACS Catal. 2019, 9 (6), 5074–5083. Fabbri, E.; Nachtegaal, M.; Cheng, X.; Schmidt, T. J. Superior Bifunctional Electrocatalytic Activity of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ /Carbon Composite Electrodes: Insight into the Local Electronic Structure. Energy Mater. 2015, 5 (17), 1402033. Fabbri, E.; Nachtegaal, M.; Binninger, T.; Cheng, X.; Kim, B.-J.; Durst, J.; Bozza, F.; Graule, T.; Schäublin, R.; Wiles, L.; Pertoso, M.; Danilovic, N.; Ayers, K. E.; Schmidt, T. J. Dynamic Surface Self-Reconstruction Is the Key of Highly Active Perovskite Nano-Electrocatalysts for Water Splitting. Mater. 2017, 16 (9), 925–931. Kim, B. J.; Fabbri, E.; Abbott, D. F.; Cheng, X.; Clark, A. H.; Nachtegaal, M.; Borlaf, M.; Castelli, I. E.; Graule, T.; Schmidt, T. J. Functional Role of Fe-Doping in Co-Based Perovskite Oxide Catalysts for Oxygen Evolution Reaction. Am. Chem. Soc. 2019, 141 (13), 5231–5240. Tulloch, J.; Donne, S. W. Activity of Perovskite La1−xSrxMnO3 Catalysts towards Oxygen Reduction in Alkaline Electrolytes. Power Sources 2009, 188 (2), 359–366.

One hindrance to the development of fuel cells and electrolyzers are the oxygen electrodes, which suffer from high overpotentials and slow kinetics. Perovskite oxides have been shown to be promising oxygen electrode catalysts because of their low cost, flexibility, and tailorable properties. In order to improve perovskite catalysts for the oxygen reduction (ORR) and oxygen evolution (OER) reactions, a better understanding of their reaction mechanisms is needed. This Perspective aims to inform researchers of the current proposed reaction mechanisms for ORR and OER on perovskites and perovskite/carbon composites in order to guide future catalyst development. Additionally, important experimental practices will be recommended. A recent development for OER is the lattice oxygen evolution reaction, which is a possible addition to the conventional four consecutive proton-coupled electron transfer mechanism. Carbon additives are consistently added to perovskites to enhance conductivity and ORR/OER activity. However, carbon plays an active role in ORR, and there is evidence of a synergistic relationship between perovskite and carbon in perovskite/carbon composites.

TiO$_2$ and WO$_3$ are two of the most important earth-abundant electronic materials with applications in countless industries. Recently alloys of WO$_3$ and TiO$_2$ have been investigated leading to improvements of key performance indicators for a variety of applications ranging from photo-electrochemical water splitting to electrochromic smart windows. These positive reports and the complexity of the ternary W-Ti-O phase diagram motivate a comprehensive experimental screening of this phase space. Using combinatorial thermal oxidation of solid solution W$_{1-x}$Ti$_{x}$ precursors combined with bulk and surface analysis mapping we investigate the oxide phase formation and surface passivation of tungsten titanium oxide in the entire compositional range from pure WO$_3$ to TiO$_2$. The system shows a remarkable structural transition from monoclinic over cubic to tetragonal symmetry with increasing Ti concentration. In addition, a strong Ti surface enrichment is observed for precursor Ti-concentrations in excess of 55 at.%, resulting in the formation of a protective rutile-structured TiO$_2$ surface layer. Despite the structural transitions, the optical properties of the oxide alloys remain largely unaltered demonstrating an independent control of multiple functional properties in W$_{1-x}$Ti$_{x}$O$_{n}$. The results from this study provide valuable guidelines for future development of W$_{1-x}$Ti$_{x}$O$_{n}$ for electronic and energy applications, but also novel engineering approaches for surface functionalization and additive manufacturing of Ti-based alloys.

TiO2 and WO3 are two of the most important, industrially relevant earth-abundant oxides. Although both materials show complementary functionality and are promising candidates for similar types of applications such as catalysis, sensor technology, and energy conversion, their chemical stability in reactive environments differs remarkably. In this study, anodic barrier oxides are grown on solid-solution W xTi1- x alloy precursors covering a wide compositional range (0 ≤ x ≤ 1) with the goal of creating functional oxides with tailored stability. A strong Ti-cation enrichment in the surface region of the grown W xTi1- xO n layer is observed, which can be controlled by both the anodizing conditions and precursor composition. For Ti concentrations above 50 at. %, a continuous nanometer-thick TiO2 protective coating is achieved on top of a homogeneous W xTi1- xO n film as evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy analyses. A comprehensive electrochemical assessment demonstrates a very stable passivation of the surface in both acidic and alkaline environments. This increase in chemical stability correlates directly with the presence of this protective TiO2 film. The results of this work provide insights into the oxidation behavior of W1- xTi x alloys, but more importantly demonstrate how controlled oxidation of self-passivating alloys can lead to oxide alloys with thin, protective surface layers that otherwise would require more sophisticated deposition methods.