Pdδ+-Mediated Surface Engineering of AgMnO4 Nanorods as Advanced Bifunctional Electrocatalysts for Highly Efficient Water Electrolysis

As an extremely attractive technology for the efficient generation of O2 and H2, water electrolysis involving oxygen and hydrogen evolution reactions (OER, HER) mainly depends on efficient and affordable electrocatalysts. In this work, we initially synthesize silver permanganate, AgMnO4 (AMO), nanoparticles (NPs) with Pd0 through NaBH4 reduction. Subsequently, their surface is further modified using PdOx (x = 1, 1.5, 2) via annealing the AMO/Pd nanocomposite (NComp-1). For the optimization of catalytic properties, the chemical state of oxidic Pdδ+ is modulated by changing the annealing temperature from 160 to 360 °C. The electrocatalytic activity of NComp-1 is observed to improve gradually on increasing the temperature, and it reaches a maximum at 260 °C. This increase in temperature leads to an increase in the chemical state of Pdδ+ species produced at the AMO–Pd interface. Moreover, a temperature of 260 °C provides mixed-valence Pd (0, 2+, 3+), which strongly contributes to excellent OER/HER activities of AMO/PdOx/Pd-260 NComp (NComp-3). However, a temperature of 360 °C converts all Pd to Pd4+, which in turn decreases its activity, implying the intrinsic benefit of mixed-valence Pdδ+ toward OER/HER. The optimized NComp-3 features enhanced bifunctional properties, exhibiting extremely low overpotentials (η10) (160 mV for OER, 58 mV for HER at 10 mA cm–2) with small Tafel slopes (64.9 mV dec–1 for OER, 37.8 mV dec–1 for HER). Inspired by the superior bifunctionality, a symmetric alkaline electrolyzer is assembled with NComp-3, which needs only 1.50 V to reach a water-splitting current of 10 mA cm–2 and exhibits remarkable long-term stability. The enhanced electrocatalytic performance may be due to the synergetic effect among AMO, PdOx, and Pd, which distinctively improves the adsorption of reaction intermediates, surface area, electrical conductivity, charge-mass transport, and also stability. Therefore, our work highlights the importance of surface engineering through regulating the surface electronic status and also offers a feasible strategy for synthesizing efficient bifunctional electrocatalysts for renewable energy applications.