Influence of Stoichiometric Variation of 3d-4d Transition Metal Ions in Disordered Double Perovskite for Bifunctional Electrocatalysis

In pursuit of an efficient and sustainable energy conversion system, the design and development of an advanced bifunctional electrocatalyst for overall water splitting offers a promising approach for producing renewable hydrogen fuel. However, integrating bifunctionality into a single catalyst remains a considerable challenge, especially for materials derived from double-perovskite oxides. In this regard, three phase-pure polycrystalline disordered double-perovskite oxides, i.e., Ca 2 MnRuO 6 ( CMR ), Ca 2 Mn 1.25 Ru 0.75 O 6 ( CMR-1 ), and Ca 2 Mn 0.75 Ru 1.25 O 6 ( CMR-2 ), are synthesized to assess their potential as bifunctional electrocatalysts and evaluate the impact of mixed-valence 3d-4d transition metal ions (Mn/Ru) on the electrocatalytic performance of water splitting. Among the title compounds, the as-synthesized CMR-2 catalyst demonstrates remarkable hydrogen evolution reaction (HER) activity in an alkaline medium (1 M KOH) and shows a minimal overpotential of 270 mV at 10 mA/cm 2, outperforming both CMR (η 10 = 328 mV) and CMR-1 (η 10 = 280 mV) catalysts. Furthermore, when assessed for oxygen evolution reaction (OER) in the same electrolyte medium, the CMR-2 catalyst requires a very small overpotential of 300 mV @10 mA/cm 2 compared to the benchmark RuO 2 and various well-engineered oxide-based electrocatalysts with an F.E. of 77% at 1.62 V vs RHE. Coupled with its efficient and robust bifunctional catalytic performances, a prototype two-electrode-based water electrolyzer (H cell) was assembled with the CMR-2 catalyst, used as both the anode and cathode to perform overall water splitting studies. The fabricated water electrolyzer exhibits a comparable cell voltage and long-term stability, with negligible degradation over 24 h of continuous operation in a 1 M KOH electrolyte, highlighting its promising potential for future practical application. Moreover, the compelling composition-activity behaviors and detailed mechanistic investigations of the present disordered double perovskites are also corroborated by density functional theory (DFT) calculation studies.