A unique bifunctional oxide electrocatalyst capable of catalyzing both half-reactions of water splitting, namely, oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER), with the added capability of catalyzing HER in both acidic and basic media, a rare property in water-splitting catalysts, is reported. In addition to the bifunctional electrocatalytic properties, CaSrFeMnO 6−δ shows OER activity, which is even superior to the reported activities of precious-metal electrocatalysts, IrO 2 and RuO 2 . The correlation of electrocatalysis with the chargetransport properties in the series Ca 2 FeMnO 6−δ , CaSrFeMnO 6−δ , and Sr 2 FeMnO 6−δ is demonstrated. Given the dependence of charge transport on structure and defects, this work unravels the interesting correlations between the triangle of structure, charge transport, and electrocatalytic activity for water splitting.

An electrocatalyst for water splitting based on earth-abundant metals is reported. This perovskite–oxide catalyst, CaSrFe0.75Co0.75Mn0.5O6−δ (CSFCM), is examined using both experimental and computational methods. It demonstrates a combination of properties, which include (a) very high activity for the oxygen-evolution reaction with an overpotential of η = 0.19 V at 10 mA/cm2, (b) high stability over 1000 cycles of catalysis, (c) the ability to catalyze the hydrogen-evolution reaction effectively in both acidic and basic conditions, and (d) catalytic activity as a single-phase bulk material without the need for any additional processing, multicomponent composite preparation, or nanofabrication. Therefore, the catalytic activity of CSFCM is intrinsic, making it a good benchmark compound for future studies of electrocatalytic parameters. This work also highlights the impact of systematic structural design on electrocatalytic activity. Results from density functional theory calculations indicate that in addition to an optimal e g occupancy of ∼1, an additional descriptor, i.e., maximizing the number of free e g carriers, correlates with the electrocatalytic activity.

The diverse range of possible arrangements of oxygen vacancies in oxygen-deficient perovskites results in a variety of structure types and fascinating electrochemical properties. Here, we report Ca 3 GaMn 2 O 8 and SrCa 2 GaMn 2 O 8 , where the ordering of oxygen vacancies leads to cation and polyhedral order, resulting in a remarkable array of electrochemical properties coexisting in the same compound. Neutron and X-ray diffraction have been utilized to study the structure of these materials that feature simultaneous defect order and cation order. Remarkably, both materials show very high electrocatalytic activity for hydrogen-evolution reaction (HER) of water splitting in bulk form, without the need for composite formation or nanofabrication. The HER overpotential required to achieve a current density of 10 mA/cm 2 is as low as η 10 ≈ −315 mV. In addition, detailed pseudocapacitive studies show that both compounds are capable of energy storage as anion-based pseudocapacitors, arising from oxygen ion intercalation. The symmetric pseudocapacitor cells fabricated based on these materials show a combination of high energy density and power density. These pseudocapacitor cells are also extremely stable, maintaining their high activity over 1000 cycles of charge−discharge. Electrical charge-transport studies indicate that these compounds have semiconducting properties in a wide temperature range, 25− 800 °C. Magnetic studies using both magnetometry and neutron scattering indicate a transition to an antiferromagnetic state, with a G-type arrangement of spins, where the moment on each Mn is aligned antiparallel to all nearest neighbors. This combination of properties indicates the great potential of this class of defect-ordered systems and their importance to energy research.

We report the utilization of defect order for development of a bifunctional oxide electrocatalyst. This structure−activity relationship is demonstrated through a disorder−order transformation in LaA 2 Fe 3 O 8 (A = Ca, Sr). The defect-ordered oxide, LaCa 2 Fe 3 O 8 , shows electrocatalytic activity for oxygen evolution reaction, on par with that of the state-of-the-art precious metal catalyst RuO 2 . In addition, it is capable of catalyzing the other halfreaction of water-splitting, namely the hydrogen evolution reaction. Importantly, it is an iron-based catalyst and can be used as a single phase bulk material. The most important aspect of this work is the pronounced impact of defect order on the electrocatalytic properties of oxides.

Electrocatalytic splitting of water is a promising method of hydrogen generation. Here, we report an enhanced electrocatalytic performance for water electrolysis, achieved through a progressive increase in the ordering of oxygen vacancies in the structural network of oxides. In transition from Sr2FeCoO6−δ (disordered) to CaSrFeCoO6−δ (ordered) and Ca2FeCoO6−δ (highly ordered), the change in the average ionic radius of the A-site metals leads to an increase in the concentration and ordering of oxygen vacancies, resulting in a progressive enhancement of the electrocatalytic activity for both cathodic and anodic half-reactions of water splitting, i.e., hydrogen-evolution (HER) and oxygen-evolution (OER) reactions. The OER electrocatalysis is particularly important, as it is considered the bottleneck for water electrolysis. These electrocatalysts show better activity than the precious metal catalyst RuO2. In contrast to most bifunctional catalysts reported to date, these catalysts can be used in bulk form, without the need for nanofabrication, composite formation, or any additional processing. Density functional theory calculations indicate that the vacancy order leads to a shift of the electronic bands toward the Fermi level. We propose that the ordering of oxygen vacancies can be used as a handle for the design of highly active electrocatalysts.