Manifolding surface sites of compositional CoPd alloys via pulsed laser for hydrazine oxidation-assisted energy-saving electrolyzer: Activity origin and mechanism discovery

Abstract

Energy-driven overall water splitting (OWS) for renewable hydrogen generation is critical for carbon neutrality. However, the OWS electrolyzer efficiency is significantly affected by the kinetically sluggish anodic oxygen evolution reaction (OER). An efficient approach is proposed to promote energy-saving hydrogen production by using a composition-dependent multifunctional CoPd alloy via pulsed laser ablation in liquids, where the OER is replaced with the hydrazine oxidation reaction (HzOR). The CoPd alloy improves N2H4 chemisorption onto the surface via dative bonding between the lone pair electrons of N and the empty 5 s orbital of Pd and 3d of low spin Co. The optimal Co1Pd9 alloy sample demonstrates excellent hydrogen evolution reaction and HzOR with the lowest overpotentials of 0.224 and 0.329 V at 10 mA/cm2 in 1.0-M KOH and 1.0-M KOH/0.5-M N2H4, correspondingly. An in situ/operando Raman study reveals that metal active site and structural transformation occurred during the oxidation (M*-O) and reduction (M*-H) reactions under an alkaline medium. Besides, the overall hydrazine splitting performed using the Co1Pd9 ‖ Co1Pd9 electrolyzer requires a cell voltage of ∼ 0.228 V only at 10 mA/cm2 with remarkable electrochemical and structural stability over 10 h, which is significantly lower than that of the traditional OWS electrolyzer (∼1.837 V). This study demonstrates the practical use of CoPd alloys in direct N2H4 fuel cells to produce both H2 fuel and electrical energy.