Palladium Nanoparticle-Graphitic Carbon Nitride Porous Synergistic Catalyst for Hydrogen Evolution/Oxidation Reactions over a Broad Range of pH and Correlation of Its Catalytic Activity with Measured Hydrogen Binding Energy

The hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in aqueous medium are two fundamental reactions for the development of non-fossil energy storage and conversion devices. In the polymer electrolyte membrane fuel cell (PEMFC) carbon supported platinum (Pt/C) based catalysts are universally used in cathodes and anodes; however, the poor durability of Pt/C due to degradation of the catalyst in the strongly oxidizing environment prevents its widespread applications. It remains a great challenge to develop new electrocatalysts with superior activity and very high durability for the HER/HOR. Here, we report the synthesis of a porous palladium nanoparticle- carbon nitride composite (Pd-CNx) for its superior activity and high durability toward the HER/HOR in acidic and alkaline media. The Pd-CNx composites exhibited high catalytic activity for hydrogen evolution in acidic media with a small onset potential of -12 mV and a Tafel slope of 35 mV dec(-1). At a small Pd loading of 0.043 mg cm(-2), this catalyst also exhibits a current density of 10 mA cm(-2) at a low overpotential of -55 mV with an excellent stability. The HER activity on Pd-CNx composite is comparable to that of commercial Pt/C in acid media. The stability tests of this catalyst were done through a large number of repeated potential cycles and long-term electrolysis. These confirm the exceptional durability of this catalyst, which is much better than that of Pt/C catalysts. Furthermore, this catalyst has also displayed superior HOR activity, measured by a rotating-disk experiment with a broad range of pH (0-14) in different buffer solutions. The HER/HOR activities of porous Pd-CNx composite in different buffer solutions were correlated with the hydrogen binding energy (HBE) of the catalyst surface. The HER/HOR activity gradually decreases with an increase in the HBE as the solution pH increases. The superior HER/HOR activities and very high durability at porous Pd-CNx composite are due to strong bonding between Pd and carbon (Pd-C bond), the porous morphology, and synergistic interactions between Pd-NPs and the carbon nitride (CNx) support.