Controllable Synthesis Of Molybdenum-based Catalysts And Their Performance For Electrocatalytic Water Splitting

Efficient electrocatalytic water splitting is considered as one of the most promising technologies to realize hydrogen production and oxygen production.However,no matter for hydrogen evolution reaction（HER）or oxygen evolution reaction（OER）,noble metal-based materials are the state-of-art catalysts with highest catalytic activity.Their scarce resources and expensive price are one of the reasons,which result in the high cost of water electrolysis technology,and thus hindering water electrolysis technology from large-scale application.Notably,molybdenum-based nanomaterials are a class of water electrolysis materials with great application potential owing to their advantages,such as easy access and low price.However,there are still some challenges in how to obtain molybdenum-based catalysts with excellent comprehensive properties such as high activity,high conductivity and high stability.In addition,there are relatively few studies on the surface reconstruction of molybdenum-based catalysts during the catalytic reaction process.Therefore,in this paper,molybdenum-based catalysts were synthesized and their catalytic performances were optimized by morphology regulation,heteroatom doping,phase regulation or component recombination strategies.Moreover,the surface reconstruction of as-prepared catalysts was studied.Molybdenum carbide/nitrogen-doped carbon（β-Mo2C/NC）hybrid nanocomposite catalysts were synthesized and their morphologies were regulated via an acid-induced method.That is,the self-assembly of melamine and molybdate ions was induced by using acid to obtain the precursors,and thenβ-Mo2C/NC catalysts were prepared through the carbonization process of precursors.The morphologies ofβ-Mo2C/NC were regulated via introducing different acid during the synthesis process of precursor.The results indicated that oxalic acid（OA）would decompose in the carbonization process and result in the formation of porous structure,contributing to the exposure of more active sites and mass transfer process during catalytic process.Therefore,the as-preparedβ-Mo2C/NC（OA）obtained by using oxalic acid showed excellent HER catalytic performances both in acidic and alkaline electrolyte,affording a geometric catalytic current density of 10 m A cm^-2 at a low overpotential of 152 and 135 m V,respectively.To further promote the HER and OER catalytic performances of molybdenum-based oxides,strategies of formation of bimetallic oxide and heteroatom doping were adopted.As a result,P-doped cobalt molybdate microrod arrays were constructed on the nickel foam（P-CMO/NF）via hydrothermal and subsequent low-temperature phosphorylation process.It found that the partial Mo O4^2-on the surface of catalyst were replaced by PO4^3-group.Meantime,the abundant oxygen vacancies also formed.The introduction of phosphate ions,the formation of rich-valance molybdenum oxides and oxygen vacancies can effectively improve the catalytic activity and facilitate the surface reconstruction to transform into more active components and morphologies during HER and OER process,where Co Mo O4-Co（OH）2 nanosheets and hollow Co OOH/P-Co Mo O4 microrods were formed,respectively.Therefore,the prepared P-CMO/NF-400 presented excellent both HER and OER catalytic performances.It only needs a low potential of 44 m V to achieve a current density of10 m A cm^-2 for HER and 260 m V to achieve a current density of 20 m A cm^-2 for OER.Moreover,by applying P-CMO/NF-400 as both cathodic and anodic electrocatalysts of electrolyzer for overall water splitting in 1.0 mol L^-1 KOH solution,it only needs a low cell voltage of 1.54 V to deliver a current density of 10 m A cm^-2.It also can offer the outstanding stability over 72 h under a current density of 50 m A cm^-2.By adopting the composition and phase coordination strategies,amorphous molybdenum oxides supported crystalline nickel metaphosphate nanowire arrays with pine-like morphology were successfully constructed on the nickel foam（c-Ni2P4O12/a-NiMoOx/NF）through hydrothermal and subsequent low-temperature phosphorylation process.The prepared c-Ni2P4O12/a-NiMoOx/NF demonstrated outstanding catalytic performances profited from the close integration between c-Ni2P4O12 and a-NiMoOx,and the synergistic effect of components.Furthermore,the surface reconstruction of c-Ni2P4O12/a-NiMoOx/NF during HER and OER process,where c-Ni2P4O12/a-NiMoO4 and c-Ni2P4O12/a-Ni OOH was formed,respectively,further improving the catalytic activities.Hence,c-Ni2P4O12/a-NiMoOx/NF can afford a geometric catalytic current density of 10 m A cm^-2 at a low overpotential of78 m V for alkaline HER,and a geometric catalytic current density of 20 m A cm^-2 at an overpotential of 250 m V for alkaline OER.In addition,it possessed an excellent overall water-splitting performance comparable to noble-based catalysts and also offered outstanding long-term durability under a current density of 10 m A cm^-2 over84 h at room temperature and then continued to operate under a current density of 250m A cm^-2 for at least 48 h at the temperature of 56 ^oC,showing a promising practical application potential.