Synthesis Of Bifunction Transition Metal Based Electrocatalysts For Hydrogen Generation

Hydrogen has been emerged as promising alternative power sources due to the benefits of renewable and clean fuel.Electrochemical splitting of water into H₂ and O₂ represents an emerging key technology for the long-term storage of electricity from renewable sources to mitigate their intermittent availability.Efficient electrocatalysts must be applied to reduce the overpotentials of the anodic oxygen evolution reaction?OER?and the cathodic hydrogen evolution reaction?HER?.Moreover,the use of bifunctional catalysts for both reactions has the advantages of simplifying the system and lowering the cost.Alkaline water splitting has emerged as a strong candidate for commercialization towards the largescale production of pure hydrogen.Accordingly,efficient bifunctional catalysts for overall water splitting are highly desired.Compared to water reduction,oxygen evolution involves multiproton-coupled electron-transfer steps,suffering from the limitation of high activation energy barrier for the O-O bond formation.The sluggish OER kinetics remains the major bottleneck in the overall water splitting process and the hydrogen generation efficiency is largely dependent on this anodic process.A viable way for more energy-efficient electrolytic hydrogen production is the replacement of oxygen evolution with the oxidation of more readily oxidizable species,such as methanol,urea.The dissertation mainly focuses on preparation and electrocatalytic application of transition metal nanomaterials,the points are addressed as follows:1.Herein,we describe our recent finding that NiCo₂S₄ nanowires array on carbon cloth?NiCo₂S₄NA/CC?topotactically converted from its NiCo₂O₄ NA/CC precursor behaves as an active bifunctional HER and OER catalysts with good durability in strongly alkaline electrolytes?see ESI for preparation details?.The NiCo₂S₄NA/CC affords 100 mA cm-2 at HER overpotential of 305 mV and 100 mA cm-2 at OER overpotential of 340 mV,with much superior activity to NiCo₂O₄ NA/CC.We also show the use of NiCo₂S₄ NA/CC to make a stable two-electrode alkaline water electrolyzer with 10 mA cm-2 water-splitting current at a cell voltage of 1.68 V.2.In this communication,we describe that CoP nanowall array in situ grown on carbon cloth?CoP NA/CC?can be used as an efficient 3D anode for methanol electro-oxidation in alkaline media.This electrode shows high catalytic activity and strong long-term stability.It achieves a current density of 96 mA cm-2 toward 0.5 M methanol at 0.5 V in 1 M KOH.Besides,it maintains 79%of its initial current density after 1000 cyclic voltammetry cycles while the current density can return to 92%of the original value when remeasured in new electrolyte.3.In this communication,we demonstrate our recent finding that nickel phosphide nanoflakes array on carbon cloth?Ni₂P NF/CC?behaves as a high-active 3D catalyst electrode for UOR with the need of potential of 0.447 V to achieve a geometrical catalytic current density of 100 mA cm-2 in 1.0 M KOH with 0.5 M urea,outperforming most reported UOR catalysts.Remarkably,the high hydrogen-evolving activity of Ni₂P NF/CC enables it as a bifunctional catalyst for both UOR and HER toward less energy-intensive electrochemical hydrogen production,and its two-electrode alkaline electrolyzer demands a cell voltage of only 1.35 V to attain 50 mA cm-2,which is 0.58 V less compared with pure water splitting to achieve the same current density.