Synthesis of Bis(diimino)palladium Nanosheets as Highly Active Electrocatalysts for Hydrogen Evolution

Development of efficient electrocatalysts for hydrogen evolution reactions (HERs) is necessary to achieve environmentally friendly and sustainable hydrogen production. To reduce cost and to circumvent the scarcity of platinum, the most efficient catalyst for HER, it is essential to develop catalysts using ubiquitous base metals or minimal amounts of precious metals. Bis(diimino)metal (MDI) coordination nanosheets are potential HER catalysts because their electric conductivities, two-dimensionality, and porous structures provide large surface areas and efficient mass and electron transfer. In addition, with sparse metal arrangements in their chemical structures, nanosheets can reduce the amount of metal needed. We synthesized bis(diimino)palladium coordination nanosheets (PdDI) as a coordination polymer composed of bis(diimino)palladium, with semiconducting characteristics, using gas-liquid interfacial synthesis and electrochemical oxidation. These electrochemically synthesized PdDIs exhibit remarkable catalytic performance with overpotential reaching 10 mA cm^-2 of 34 mV, a Tafel slope of 47 mV dec^-1, and an exchange current density of 2.1 mA cm^-2 after appropriate activation. This performance is closely comparable to that of metallic platinum. An ex-situ investigation of the activation process revealed that reduction of the divalent Pd center in bis(diimino)palladium produced a composite of Pd(0) species and PdDI, combining high catalytic activity with smooth electron transfer.