Synthesis Of Self-Supported Nickel-Foam-Based Catalysts For Energy-Saving Hydrogen Production

As a green chemical fuel with high energy density and zero carbon emission,hydrogen is beneficial for reducing the excessive dependence on traditional fossil fuels and promoting the ultimate goal of"carbon peak and carbon neutrality".Water splitting powered by renewable energy is generally regarded as an environmentally friendly and sustainable technology.However,the application of water splitting technology is severely limited by expensive noble-metal-based catalysts for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）involving complex four-electron transfer,which requires high thermodynamic energy barrier and leads to huge power consumption.Therefore,developments of low-cost non-noble-metal catalysts with Pt-like activity and anodic small molecule oxidation reactions with lower onset potential are keys to design energy-saving hydrogen production system.Based on the advantages of self-supported structure to promote mass transfer and conductivity,and nickel foam（NF）substrate to facilitate bubble desorption,a series of highly efficient and stable NF based self-supported Fe/Ni/Cocatalysts are designed and applied to catalyze HER,OER and easily-oxidized hydrazine,urea and sulfion oxidation reactions.Above results promote the development of energy-saving hydrogen production and own theoretical significance with application values.Main research contents are as follows:（1）In order to solve the challenge that phosphides own weak activation ability of water,the self-supported FeNiP/MoO_x HER catalyst with multi-interface is successfully synthesized on NF through interface engineering strategy combining hydrothermal,Fe（NO₃）₃ immersing and phosphorization.Such catalyst exhibits higher HER activity than Pt/C in1.0 M KOH,and only needs overpotential of 97 m V to drive 100 m A·cm^-2with over 20 h stability.DFT calculations reveal that excellent activity is due to the multi-interface constructed between Fe₂P,Ni₅P₄ and MoO_x that efficiently activates water and promotes H*desorption.The overall water splitting system assembled by such catalyst only needs voltage of 1.62 V to afford 100 m A·cm^-2.（2）In view of the research status of insufficient understanding of reconstruction during alkaline HER,self-supported broccoli-like Ni₂P-Ni（PO₃）₂/MoO_x precursor catalyst（Ni-P-O/MoO_x）is successfully synthesized on NF through interface engineering strategy combining hydrothermal and phosphorization.The increased intrinsic activity during alkaline HER is attributed to the dissolution of MoO_x resulting in more exposed nickel sites as Lewis bases and surface amorphous hydroxyl-ligands serve as Lewis acids.DFT calculations reveal that the construction of Lewis pairs effectively promotes the adsorption of H*and activation of water,and the negative shift of Nid-band center caused by efficient electron transfer is conducive to H*desorption.Therefore,the restructured Ni-P-O catalyst shows higher HER activity than Pt/C,and only needs overpotential of 79 m V to drive 100 m A·cm^-2 in 1.0 M KOH with long-term stability.The overall water splitting system assembled by such catalyst only requires voltage of 1.57 V to afford 100 m A·cm^-2.（3）In view of the lack of research on the reconstruction of NiMoO₄-based catalysts under alkaline conditions,NiMoO₄ and monocrystalline P-NiMoO_x nanorods modified by CoP-Co₂P nanoparticles（CoP_x/P-NiMoO_x）are respectively synthesized on NF by hydrothermal reaction with subsequent phosphorization.Surface of NiMoO₄ is converted to highly active NiOOH during OER,and reconstructed NiOOH/NiMoO₄ catalyst only requires overpotential of 266 m V to drive 100 m A·cm^-2.Electrochemical reduction promotes the dissolution of Moin the CoP_x/P-NiMoO_x catalyst and hydroxyls in the electrolyte combined with nickel sites to form amorphous hydroxyl-layers during HER.The reconstructed H-CoP_x/P-NiMoO_x catalyst shows higher HER activity than Pt/C,it only needs overpotential of 67 m V to afford 100 m A·cm^-2.The overall water splitting system assembled by the reconstructed catalyst only needs voltage of 1.55 V to drive with excellent stability.（4）In order to solve challenge of the lack of bifunctional catalyst for hydrazine-oxidation-assisted electrolyzer,the bifunctional self-supported CoNiMo/CoNiMoO_x catalyst composed of monocrystalline CoNiMoO_xnanorods and Co/MoNiheterostructure is synthesized on NF by hydrothermal combined with calcination under H₂/Ar.The hybrid seawater electrolyzer assembled by such catalyst only needs 0.059 V to drive current density of 100 m A·cm^-2,saving 90%energy consumption compared with traditional electrolyzer.DFT calculations reveal the Co/MoNiheterostructure reduces energy barriers of N₂H₄*to N₂H₃*rate-determining dehydrogenation step during hydrazine oxidation,water dissociation and hydrogen adsorption during HER.This work highlights the new design of hybrid seawater electrolyzer to produce hydrogen with low power consumption and efficiently dispose hydrazine-rich wastewater.（5）In order to solve the challenge of low nitrogen cycling efficiency in urea oxidation and the formation of NiOOH is unfavorable to CO₂desorption,self-supported Ni₃S₂ nanosheet catalyst is synthesized on NF by two-step hydrothermal reaction.Experimental results reveal that Ni₃S₂initially involves reconstruction to hydroxyl-ligands,and then urea decomposition to ammonia and urea oxidation to N₂ occur at hydroxy-modified nickel sites,in which urea oxidation maintains low potential region to avoid formation of NiOOH.DFT calculations reveal hydroxy-modified Ni₃S₂ owns metal-like conductivity,favorable thermodynamic CO₂ desorption ability for urea oxidation,and strong adsorption ability of urea derivatives for urea decomposition.Due to the faster kinetics of such unique reaction path,the reconstructed Ni₃S₂ catalyst only needs potential of 1.339 V vs.RHE to drive 100 m A·cm^-2 with long-term stability.（6）In order to solve challenges of insufficient understanding of the mechanism of sulfion oxidation and catalyst deactivation caused by sulfur deposition,the bifunctional self-supported Co₃S₄ nanowire catalyst is synthesized on NF by two-step hydrothermal reaction.The energy consumption of the sulfion-oxidation-assisted seawater electrolyzer assembled by Co₃S₄ is only 1.185 k Wh·m^-3H₂ at current density of 100m A·cm^-2,which cuts over 70%power consumption compared with the traditional water splitting electrolyzer.Different from the oxidation of water to O₂ at high potential during OER,experimental results reveal that S^2-ions are gradually oxidized to short chain polysulfides and then to S₈.DFT calculations show that Co₃S₄ owns lower energy barriers at S₃^2-to S^4-rate-determining step and S₈ desorption step,which is conducive to the efficient conversion of short-chain polysulfides and desorption of S₈.