Preparation And Electrocatalytic Performance Of Self-supporting Manganese And Molybdenum Based Electrodes

Since the 19^th century,fossil fuels such as coal,natural gas,and oil have become the primary energy source to sustain human activities.The large-scale consumption of fossil fuels caused by unprecedented industrialization and rapid population growth,however,has resulted in detrimental environmental consequences and prompted the quest for more sustainable energy sources.Among the candidates,hydrogen（H₂）is at the forefront,because it has a high gravimetric energy density and is environmentally benign.But hydrogen is currently produced from steam reforming or partial oxidation of methane,or coal gasification.All these processes are fossil fuel based,with the added drawback that the purity of H₂ produced is also not high.H₂ production through electrochemical water splitting has recently become an elegant and eco-friendly way to generate high purity H₂ at room temperature.Electrochemical water splitting is a thermodynamically uphill reaction,which consists of two half-cell reactions:the cathodic hydrogen evolution reaction（HER）and the anodic oxygen evolution reaction（OER）,respectively.In ideal reaction conditions,a thermodynamic potential of 1.23 V is needed to drive electrochemical water splitting.A practical water electrolyzer requires a much higher potential due to the overpotentials on both of the electrodes（anode and cathode）.Therefore,highly efficient electrocatalysts are urgently needed.Over the past decade,numerous electrode materials have been developed and studied as electrocatalysts for HER/OER.So far,the state-of-the-art Pt/C and Ir O₂,Ru O₂ exhibit the best catalytic HER and OER activity,but their low terrestrial availability and high cost limit their use in large scale applications.Therefore,significant research has been directed toward developing earth-abundant transition-metal-based electrocatalysts including sulfides,phosphide,selenides and alloy materials.The transition-metal-based catalysts proved to have lower cost,higher natural abundance,unique electronic and mechanical properties,and good HER/OER performances.Transition metal powder electrocatalysts have problems such as small active area,poor electrical conductivity and tendency to fall off.In this thesis,one-piece self-supporting manganese and molybdenum（Mn,Mo）based electrocatalysts were prepared by constructing heterojunctions,heteroatomic doping and alloying as follows:（1）The development of transition metal-based materials to replace precious metal electrocatalysts in a facile and efficient approach is of great importance for water splitting.The Ni S/Mo S₂ complex grown in situ on carbon paper（Ni S/Mo S₂/CP）by a one-step hydrothermal method was successfully constructed as an efficient electrocatalyst.The synergetic effects of the superhydrophilic/aerophobic surface,hierarchical nanostructure and strong mechanical adhesion to a highly conductive substrate give the Ni S/Mo S₂/CP complex outstanding catalytic activity and durability.Specifically,it exhibits relatively low overpotentials of 119 m V(at a current density of 10 m A cm^-2)and 314 m V(at a current density of 100 m A cm^-2)for the hydrogen evolution reaction and oxygen evolution reaction,respectively.In addition,the Ni S/Mo S₂/CP complex can also be used as an electrolyzer that reaches a current density of 10 m A cm^-2 at a cell voltage of 1.48 V without decay after 30 h of durability testing,making it an ideal electrocatalyst for water splitting towards practical application.（2）Transition metal-based nanomaterials exhibit promising potential as highly active and low-cost electrocatalyst for alkaline water splitting,which can be achieved via elaborating compositional modulation and structural manipulation.This,however,normally involves multiple or even complex synthetic procedures.Herein,we report a simple one-step sulfidation of Fe Mn Zn multi–metal skeletons for the preparation of highly efficient electrocatalysts.The incorporation of Mn and Zn induced hierarchical nano/micro sheet–to–sheet supported on open porous skeleton（Fe Mn Zn/Mn-Fe S,FMZS2）,which not only facilitates electron/ion transport but also expands the accessible surface.Meanwhile,the Mn is introduced to optimize the adsorption/desorption ability of intermediates on the S sites in Fe S.The resultant effect leads to remarkable electrocatalytic performance with good durability.The optimized FMZS2 delivers a 20 m A cm^–2 at low overpotential of 118 m V for HER and a 100 m A cm^–2 at overpotential of 390 m V for OER,outperfoming Pt/C and Ir O₂catalyst,respectively.Moreover,the assembled FMZS2||FMZS2 alkaline electrolytic cell has excellent overall water splitting superior to that of the noble metal Pt/C||Ir O₂.（3）Highly efficient electrocatalysts for water splitting generally involve noble metals（Pt,Ir,Ru,etc.）or expensive transition metals（Ni,Co,Cu,etc.）,which has hindered their widespread application.Here,we report a brand-new,low-cost phosphated Zn-doped bimetallic（Fe/Mn）skeleton（Zn-Fe/Mn@Mn-Fe P,FMZP4）with genuine potential as a highly effective water-splitting electrocatalyst.Benefiting from heterogeneous atom doping as well as a self-supported electrode composed of a porous Zn-Fe/Mn skeleton and in-situ grown phosphides（Mn-Fe P）with a hierarchical ultrathin nanosheet structure,which provide rapid electron transport and efficient mass transport channels.In addition,Mn doping Fe P enhances the adsorption/desorption of intermediates at the FMZP4_{bri Fe-Fe} site.The optimized FMZP4 exhibits low overpotentials of 53 m V and 184 m V to reach current densities of 10 m A cm^-2 and 20 m A cm^-2 for the hydrogen and oxygen evolution reactions,respectively,revealing good stability in a long potential cycling test（1000 cycles）and remaining completely stable over an 80 h galvanostatic measurement at 10/50 m A cm^-2.It needs just 1.79 V to achieve 50 m A cm^-2 for full water splitting in an alkaline electrolyte and exhibits superior electrochemical durability.The excellent electrocatalytic activity makes it a candidate material for low-cost electrocatalyst with broad applicability in water splitting.（4）In order to meet industrial requirements,the design of low-cost,short synthetic routes for self-supported,highly active electrocatalysts for hydrogen evolution reactions is essential.Here,a novel intermetallic Ni Mn alloy has been developed for the first time as an efficient electrocatalyst for HER by adjusting the content of metallic Ni.The electrode has a high intrinsic activity as well as a very small nanoparticle morphology,which facilitates an increased electrochemically active surface area.The electrode exhibits excellent electrocatalytic activity and remarkable durability in alkaline electrolytes,i.e.the Ni₂₀Mn₈₀electrode requires overvoltages of 31.9 m V and 165 m V at current densities of 10 m A cm^-2and 400 m A cm^-2,respectively,and possesses an extremely small Tafel slope(41.2 m V dec^-1)and charge transfer resistance（1.6Ω）.The test is maintained for 100 h at 50 m A cm^-2without significant decay.These excellent electrocatalytic properties enable a wide range of applications in industrial production.