RuP Nanoparticles Anchored on N-doped Graphene Aerogels for Hydrazine Oxidation-Boosted Hydrogen Production

Green hydrogen' is a promising clean energy carrier for use instead of traditional fuels.For obtaining 'green hydrogen', electrochemical water splitting has been receiving considerable attention due to its ecofriendly and low-cost properties.However, the sluggish kinetics of the anodic oxygen evolution reaction (OER) reduces the efficiency of hydrogen production.Accordingly, the hydrazine oxidation reaction (HzOR) with low theoretical potential (-0.33 V vs. RHE) has been proposed as a reasonable alternative for the OER.In this study, graphene aerogel (GA) was utilized as a conductive substrate with a 3D porous framework.Ru III -polyethyleneimine (Ru III -PEI) complexes were adsorbed on the GA surface.Phytic acid (PA) was further adsorbed to form Ru III -PEI-GA-PA hybrids through the hydrogen bond interaction between PA and PEI, which can serve as a precursor to synthesize RuP nanoparticles anchored on Ndoped GA (RuP/N-GA) through the phosphorization reaction.In the pyrolysis process, the ultra-small RuP was formed at the GA surface.Additionally, the decomposition of PEI and PA can introduce abundant N and P heteroatoms into the structure of GA.As a result, RuP/N-GA hybrids achieve efficient HzOR with a low working potential of -54 mV at 10 mA•cm -2 .Moreover, the novel RuP/N-GA hybrids with low Ru loading also exhibit a promising hydrogen evolution reaction (HER) activity with an overpotential of -19.6 mV at 10 mA•cm -2 .Among various RuP/N-GA hybrids, the Tafel plot of HER at RuP/N-GA-900 reveals the smallest value to be 37.03 mV•dec -1 , which affords the fastest HER kinetics.Meanwhile, the result suggests that the HER at RuP/N-GA-900 undergoes a Heyrovsky mechanism similar to that of Pt.The theoretical results revealed that the anchored structure and the presence of N heteroatoms can promote the HzOR on RuP nanoparticles.The free energy of hydrazine molecular adsorption on RuP/N-GA was -0.68 eV, indicating that N-doping in the RuP/N-GA structure can adjust the electronic structure of the Ru active site, which also contributes to the enhanced HzOR activity of the Ru site.Additionally, RuP/N-GA hybrids exhibited excellent cycling and long-term stability for both HER and HzOR, superior to those of commercial Pt/C.Based on the bifunctional activity of RuP/N-GA hybrids, the constructed two-electrode hydrazine split system exhibits an extremely low cell voltage of 41 mV at 10 mA•cm -2 for the hydrogen production, which achieves the goal of energy-saved hydrogen production at low voltage.The excellent electrocatalytic activity of RuP/N-GA hybrids is attributed to the ultrasmall RuP nanoparticles for abundant Ru active sites.Meanwhile, the synergistic effect between N-doping in GA frameworks with RuP nanoparticles contributes to the activity enhancement of RuP/N-GA hybrids, in which the 3D porous N-GA with few-layer morphology accelerates the electron and mass transfer and the electron interaction between N-GA and RuP nanoparticles promotes the electrocatalytic activity of RuP nanoparticles for both HER and HzOR.This study extends the bifunctional electrocatalyst for the HER and HzOR to achieve energy-saved hydrogen production and sheds new light on the design and synthesis of advanced electrocatalysts via the adsorption-phosphatization method.