Design,Preparation And Electrocatalytic Water Splitting Performances Studies Of Transition Metal-based Materials

With the development of the global economy and society,there are two major problems that humans are facing:energy and environment.At present,traditional fossil fuels are still the primary energy source in the whole world,while their reserves are limited and their utilization also bring tremendous environment deterioration.In order to achieve the sustainable development of human society,it is urgent to accelerate the development and utilization of renewable energy like sun,wind,and water energy,etc.Compared with other intermittent energy sources,hydrogen energy is an important secondary energy,which has the advantages of light weight,high combustion value,rich source,clean pollution-free,and is usually regarded as the Holy Grail of clean energy.Compared with that of the mainstream hydrogen production methods of fossil energy and industrial by-products,electrochemical water splitting provides a new hydrogen production approach with simple equipment,green environmental protection,high hydrogen purity,flexible production and large-scale distributed production.In recent years,the development of wind and solar power generation results in the surplus electric energy,which can be absorbed through water electrolysis to ruduce wind and light abandonment and realize peak cutting and valley filling of power grid.However,at present,hydrogen production through water electrolysis accounts for less than 5%of the total hydrogen production in the world,and the industrial application still faces many challenges:(1)the electric power cost is much too high,especially the high theoretical potential of anodic oxygen evolution reaction(OER),leads to that the actual voltage to achieve the ideal hydrogen production level is much higher than that of the thermodynamic equilibrium potential(1.23 V);(2)the state-of-the-art electrocatalysts are mainly the expensive noble-metal-based materials(such as Ru/Ir-based and Pt-group);(3)the oxygen generated at the anode could result in safety risks caused by the mixing with cathodic hydrogen through gas crossover.In view of this,the dissertation plans to solve above problems existing in electrocatalytic hydrogen production from two aspects:(1)design and exploitation of high-efficiency and low-cost non-noble metalbased OER catalysts,further revealing the real active species and the possible structure-activity relationship;(2)the combination of functional strategies and theoretical calculation to design highly active transition metal based catalysts for hydrazine-assisted water splitting system,thus providing guidances for the development of transition metal based catalysts.The specific research contents as follows:1.We developed a mild one-pot room temperature solution phase method for the large-scale synthesis of hierarchical trimetallic NiCoFe-based MOF nanofoam(denoted as(Ni2Co1)1-xFex-MOF-NF),which can be directly used as efficient and robust OER catalysts in alkaline condition.By controlling the Ni/Co/Fe molar ratios,the obtained optimal(Ni2Co1)0.925Fe0.075-MOF-NF can deliver an overpotential of 257 mV and a small Tafel slope of 41.3 mV dec-1 in 1.0 M KOH.More importantly,the underlying origination of the high activity is deciphered through the investigation on the intermediates during the OER process,where it is found that the metal hydroxide and oxyhydroxide evolved from pristine MOF structure could be the active species while the hierarchical foam-like MOF architecture provides an advanced platform to reach the morphology and composition optimization.2.To further verify whether this kind of hydroxylated phenomenon could be observed in other multi-transition metal based OER catalysts,we reported a versatile strategy based on anion-exchange reaction to prepare a series of amorphous mixed metal oxide hollow nanoprisms using solution phase method under mild conditions,including FeCoWOx,FeCoVOx,FeCoPOx,FeCoBOxand FeCoSeOx.Benefiting from the simutaneous modification on the morphology and composition,the optimal OER activity can be achieved in amorphous FeCoSeOx hollow nanoprisms(denoted as AFeCoSeOx-HoNPrs),where a lower overpotential of 294 mV at 10 mA cm-2 with a much smaller Tafel slope of 45.1 mV dec-1 than that of pristine FeCo-MIL-88B can be reached.The underlying origin is unraveled using the in-situ Raman spectroscopy measurement combined with the ex-situ anlysis,which unravels that the electrochemically transformed metal hydroxide and oxyhydroxide species could be the active species while the role of the incorporated anion could be the electron modulator for Fe3+/Co2+.3.Based on the research work regarding OER catalyst materials in previous chapters,it is found that the electrocatalytic properties of transition-metal based compounds could be effectively regulated through the synergy of polymetallic.In this chapter,we designed a hierarchical multi-component nanosheet arrays grown on Ni foam composed of abundant NiCo/MoNi4 heterostructure interfaces on the amorphous MoOx substrate(denoted as NiCo-MoNi4 HMNAs/NF),which sucessfully integrates the HER active MoNi4 and HzOR active NiCo phases into a unified system and thus achieves outstanding bifunctionality.The two-electrode overall hydrazine splitting(OHzS)electrolyzer with NiCo-MoNi4 HMNAs/NF as both the anode and cathode can drive a large current density of 250 mA cm-2 at a cell voltage as low as 0.63 V,significantly lower than that of 2.87 V required for OWS.The post-catalysis characterizations further disclose that the good stabilization of NiCo/MoNi4 active phases,while the partial formation of transition metal hydroxide species probably provide favorable active sites for water dissociation and hydroxyl adsorption,thereby enabling fast reaction kinetics.4.On the basis of previous work on Ni-based alloy towards HER and HzOR,the material system is further expanded to nickel-based nitrides.We present the insitu growth of hierarchical porous nanosheet arrays with abundant Ni3N-Co3N heterointerfaces on Ni foam(denoted as Ni3N-Co3N PNAs/NF),which can exhibit superior bifunctionality towards,both hydrogen evolution and hydrazine oxidation catalysis in alkaline electrolyte.Compared to the pure Ni3N or Co3N counterpart,the introduction of Ni3N-Co3N heterointerface could effectively modulate the electronic structure of the nitrides,thus facilitating the thermodynamic behavior of both hydrogen adsorption(ΔGH*≈0)for HER and dehydrogenation process for HzOR.In the two-electrode electrolyzer for OHzS utilizing the Ni3N-Co3N PNAs/NF electrodes as bifunctional efectrocatalysts,the cell voltages of 0.071 and 0.76 V can drive 10 and 400 mA cm-2,respectively,which suggests the great potential for energy-saving H2 production compared to OWS system.Furthermore,a selfpowered H2 production system was integrated by using a home-made direct hydrazine fuel cell(DHzFC)to drive OHzS system,where a decent H2 evolution rate of 0.65 mmol h-1 can be achieved.Additionally,when powered by a commercial solar cell with an open circuit voltage of 1.0 V,this hybrid water electrolyzer presents a splendid OHzS performance enabling a current density of 214 mA cm-2.