Rup nanoparticles anchored on n-doped graphene aerogels for hydrazine oxidation-boosted hydrogen production

Abstract

‘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. RuIII-polyethyleneimine (RuIII-PEI) complexes were adsorbed on the GA surface. Phytic acid (PA) was further adsorbed to form RuIII-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 N- doped 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.