Pseudocapacitive Energy Storage and Electrocatalytic Hydrogen-Evolution Activity of Defect-Ordered Perovskites Sr_xCa_3–xGaMn₂O₈ (x = 0 and 1)

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 Ca3GaMn2O8 and SrCa2GaMn2O8, 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/cm2 is as low as η10 ≈ −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.