Non-Noble-Metal Catalysts For Electrochemical Hydrogen Evolution Reaction

As a high-density and pollution-free energy carrier,hydrogen is considered as one of the best sustainable resources to solve the energy crisis and environmental deterioration problems.Recent hydrogen generation via electrochemical water splitting for scalable applications has impeded by the high cost of the state-of-the-art platinum-based hydrogen evolution reaction(HER)electrocatalysts.Thus,it is of great importance to explore low-cost and efficient HER materials with earth-abundant elements to replace Pt-based electrocatalysts.Although plenty of non-noble-metal HER electrocatalysts have been developed,their HER activities remain a certain issue as compared with Pt-based ones.There are two main reasons accounting for poor electrocatalytic performance of these non-noble-metal HER electrocatalysts:one is the relatively low intrinsic activity of these materials,and the other is the limited exposure of associated active sites within the structure.Here,this doctoral dissertation first reviews the development of HER,mainly introduces the HER mechanism,summarizes the research situation of various non-noble-metal electrocatalysts and concludes the feasible strategies to optimize the HER performance.Then,it focuses on developing different doping methods and structural designs for improved HER activity.Molybdenum sulfide(MoS2)and molybdenum carbide(Mo2C)are selected as two representative research objects.Various strategies have been developed to enhance their HER behaviors by means of heterogeneous dopings and structural fabrications.Furthermore,a three-dimensional porous structure is achieved by using a self-foaming method,which is applied to the synthesis of electrocatalyst materials in order to maximize their active site exposure toward HER.The main contents are as follows:1.MoS2 is widely considered as a promising HER electrocatalyst owing to its low free energy of hydrogen absorption,electrochemical stability and cost-effectiveness.However,limited active edge sites and low intrinsic electronic conductivity of MoS2 material lead to poor HER performance.Here,the HER performance of MoS2 is considerably boosted through substitution of Mo with controllable isovalent W element.The first-principle calculations predict that 50%substitution of Mo by W shows the best catalytic activity,because of the reduction of H adsorption free energy and facilitation of charge transfer,which has been demonstrated by electrochemical properties of Mo1-xWxS2 samples in experimental studies.Accordingly,Mo0.5W0.5S2 shows significant enhanced HER performance of MoS2-based compounds,having an onset potential of-37 mV vs.reversible hydrogen electrode(RHE)and requiring an overpotential of 138 mV at a current density of 10 mA cm-2.Furthermore,a self-template method is developed to carry out the structure and doping engineering of MoS2 at the same time.Ultrafine N-doped MoS2 nanocrystals(-3 nm)embedded in a three-dimensional(3D)porous N-doped carbon network with interconnected internal channels(N-MoS2/PNCN)is synthesized by one-pot pyrolysis of the mixture composed of ammonium tetrathiomolybdate and dicyanamide.The intermediate graphitic carbon nitride derived from dicyanamide simultaneously serves as carbon and nitrogen source as well as the template.The N dopants can activate the basal plane of MoS2,and small-size particles lead to more exposed edge sites of MoS2,meanwhile the 3D hierarchical porous nanostructure of carbon delivers high accessibility by electrolyte to active sites of MoS2 nanocrystals and facilitates charge transfer at the catalyst/electrolyte interface.Consequently,N-MoS2/PNCN achieves a superior HER activity in 0.5 M H2SO4 with a low onset potential of-30 mV vs.RHE and a small overpotential of 114 mV at a current density of 10 mA cm-2,better than most reported MoS2-based catalysts in the literatures.Overall,this study modifies the MoS2 by co-doping of anion and cation and structure engineering to optimize the intrinsic activity and together expose its active edge sites,which provides feasible routes to boost MoS2-based HER electrocatalysts.2.As compared with MoS2 that limits active sites only at the edges,the basal planes of Mo2C are catalytically active for HER.Thus,in order to obtain Mo2C-based electrocatalysts with more exposed active sites and enhanced electronic conductivity,works on rational structural design and introduction of conductive supporter are both carried out.Here,an efficient HER catalyst of Mo2C nanoparticles embedded in nitrogen-doped porous carbon nanofibers(Mo2C/NPCNFS)has been prepared by electrospinning a mixture of ammonium molybdate and polyvinylpyrrolidone(PVP).The well-aligned nanopores with uniform pore size distribution in the desirable hybrid M02C/NPCNFS results in superior conductivity and rich active sites,and thus M02C/NPCNFS reveals excellent HER catalytic performance with an onset potential of-85 mV vs.RHE in 0.5 M H2SO4.In addition,the cycling stability of this core-shell 1D nanostructure is also boosted by exhibiting the durable catalytic activity over 10 h.For the further optimization,multi-channel structure is introduced by adding the polymethyl methacrylate(PMMA)into the precursor.This structure contributes to extra exposed active sites in Mo2C nanoparticles,and thus boosts associated HER performance.Moreover,two-dimensional carbon sheets accommodating Mo2C nanoparticles(Mo2C/CS)is prepared by one-pot pyrolysis of the mixture of ammonium molybdate and glucose,and effects of different reactant ratio on the morphology is carefully studied.The optimized Mo2C/CS shows superior HER activity with an onset potential of-60 mV vs.RHE in 1M KOH,owing to collaborative effects from the highly active catalytic nature of Mo2C nanoparticles,the high surface area of carbon sheets and the efficient charge transfer between the strongly coupled composite.It is noticeable that Mo2C/CS also exhibits unexpected activity towards oxygen evolution reaction,which can be served as both anode and cathode electrodes in the electrolysis cell.Only a low cell voltage of 1.73 V is required to drive the current density of 10 mA cm-2 with an excellent stability over 100 h.Overall,the section provides two strategies to develop different dimensional Mo2C/C composites as efficient HER electrocatalysts,which could be extended to design and synthesis of other carbide/carbon composites for electrocatalysis and beyond.3.Three-dimensional interconnected porous architecture could achieve high accessibility of active sites and facilitate the charge transfer during HER processes.Here,a self-foaming method is utilized to synthesize metal-free hierarchical interconnected N-doped carbon nanosheets(NCNS).The doping N species within the 3D interconnected carbon network affords rich active sites for the HER and facilitates fast charge transfer.As a result the NCNS exhibit excellent catalytic activity with an onset potential of-65 mV,and a Tafel slope of 81 mV dec-1 with robust stability over 10 h in acidic media.Further analyses suggest that the graphitic N species in the NCNS contribute to enhanced catalytic activity.Moreover,this structure can be extended to carbide/carbon composites.Typically,nickel carbide(Ni3C)nanoparticles embedded in a porous carbon network(Ni3C@PCN)is obtained by rationally incorporating a small amount of Ni salt precursor into the carbon source.As a novel hydrogen evolution reaction(HER)catalyst,the Ni3C@PCN shows superior catalytic activity with an onset potential of-65 mV,an overpotential of 262 mV to achieve the current density of 50 mA cm-2,and durability over 12 h in acidic media.Overall,this section introduces 3D interconnected porous structure into HER electrocatalysts by a proposed self-foaming method,which could be applied to the design of more carbon-based structured materials for energy conversion and storage.