Nanoporous Metal-based Hybrid Electrodes As Highly Efficient Electrocatalysts For Alkaline Hydrogen Evolution Reaction

Environmental pollution and climate change caused by excessive use of fossil fuels and its green-house gas emission have raised tremendous demands for new energy sources with renewability and sustainability.Hydrogen,a clean and renewable energy carrier,can be produced by electrochemical water splitting and react with oxygen into water for high-density energy.It is known as one of the most potential new energy sources in the 21^st century,and most hopeful to replace fossil fuel and widely used in various fields of daily life.Electrochemical splitting water powered by electricity from renewable energy sources?e.g.,solar,wind and tidal energy?is an efficient and environmentally-friendly technology to convert electric energy into chemical energy for energy storage.It can not only regulate and control these intermittent energy resources for reasonable utilization,but also achieve no greenhouse gas emissions.Hydrogen production from water splitting based on alkaline water electrolyzer is an inverse process of hydrogen and oxygen reaction to generate water,which is realized by water reduction reaction?hydrogen evolution reaction,HER?at cathode and water oxidation reaction?oxygen evolution reaction,OER?at anode driven by external electric power.However,electrolytic water is heavily dependent on electrocatalysts.At present,the commonly used electrocatalysts are mainly platinum?Pt?,iridium?Ir?,ruthenium?Ru?and other noble metal catalysts with high electrocatalytic activities,but the scarcity and high cost have severely hindered their industrial promotion and practical application.Therefore,it is the key of hydrogen production by water splitting to develop new catalysts with high activity and low cost.In this paper,we designed and prepared nanoporous metal composite electrocatalysts with efficient and stable hydrogen evolution performance under alkaline condition.The main contents and results are as follows:1.The eutectic Cu₁₂Al₈₈ alloy precursor was designed based on Cu-Al equilibrium phase diagram,and the microstructure of?-Al and Al₂Cu intermetallic compound were regulated by changing the cooling rate.Combined with dealloying mechanism,we proposed the strategies of controlling the diffusion path of Cu atoms and adjusting nanoporous structure.Based on the diffusion process of Cu atoms at the interface of electrolyte/Cu/Al₂Cu,bimodal nanoporous Cu with large and small pores was prepared by Cu₁₂Al₈₈ precursor alloy with lamellar eutectic structure,and single mode nanoporous Cu was fabricated by Cu₁₂Al₈₈ precursor alloy with network structure of Al₂Cu.The results show that at normal temperature and pressure,with the extension of dealloying time,the pore size driven by the minimization of free energy tends to be uniform,but the average sizes of large and small pores in bimodal nanoporous Cu remain unchanged,and the bimodal Cu shows better structural stability than single Cu.However,due to the weak intrinsic Cu-H bond energy,even if there are large number of low coordination surface atoms on the surface of nanoporous Cu,the adsorption of H and its transient state are still weak.Therefore,bimodal nanoporous Cu exhibits low catalytic activity for HER.Even so,the self-supporting and interconnected bimodal three-dimensional nanoporous Cu has excellent mass transfer ability and electron transport characteristics,as an ideal collector for HER catalytic materials,which provides the basis for the subsequent construction of high-performance catalytic materials.2.In view of the low intrinsic catalytic activity of nanoporous Cu,Mo-doping Cu_12-xMo_xAl₈₈ precursor?x=0.5,1,3 at%?was further designed,and based on the low solid solubility of Mo in Cu matrix,the self-supporting bimodal nanoporous Cu/Mo@Mo O_x composite electrode material was prepared for hydrogen evolution reaction in alkaline medium.Due to the influence of Mo doping on the microstructure and Cu atoms diffusion of Cu_12-xMo_xAl₈₈ alloy precursor,bimodal nanoporous Cu/Mo@Mo O_x composite electrode is composed of Cu/Mo@Mo O_x composite electrode?200 nm large pores and?20 nm small pores.During the dealloying process,with the dissolution of Al atoms,Cu atoms diffuse on the solid-liquid interface,Mo atoms embedded in the Cu ligaments gradually expose to the surface and form nanoparticles,and in the subsequent spontaneous oxidation process,bimodal nanoporous Cu/Mo@Mo O_x composite electrode is formed.Here,Mo@Mo O_x serves as the catalytic active site,and bimodal nanoporous Cu skeleton as the collector realizes electron transport,whose large channels make for the mass transmission of electrolyte and small pores increase the surface area for exposure of catalytic active site.The integrated structure of this bimodal nanoporous Cu/Mo@Mo O_x composite electrode can not only minimize the interface resistance,but also effectively improve its stability.As a result,the bimodal nanoporous Cu/Mo@Mo O_x composite electrode exhibits excellent performance in 1 M KOH aqueous electrolyte,whose overpotential is 185m V at the current density of 400 m A cm^-2 with 150 m V lower than that of naoporous Cu.Furthermore,in the long-term durability measurement,the NP Cu/Mo@Mo Ox electrodes show exceptional stability.3.Based on the bimodal nanoporous Pt₃Al intermetallic compound model system and sluggish hydrogen evolution reaction kinetics in alkaline medium,we report hierarchical NP(Pt_1-x-TM _x)₃Al?x=1/6?with transitionmetal?TM=Co,Fe?doping as bifunctional catalysts in an alkaline environment.(Pt_1-xTM_x)₁₂Al₈₈ precursor alloy was designed,and then bimodal nanoporous(Pt_1-x-Co_x)₃Al/Pt_1-x-Co_x and(Pt_1-x-Fe_x)₃Al/Pt_1-x-Fe_x core-shell structures were prepared by chemical dealloying in KOH solution.Co and Fe in the shell of Pt_1-x-Co_x and Pt_1-x-Fe_x alloy have high chemical activities in alkaline medium,which can convert into Co?OH?₂ and Fe?OH?₃,thus form Pt-Co?OH?₂and Pt-Fe?OH?₃ composite surfaces.Because the surface Co?OH?₂ and Pt atoms have moderate hydroxyl and hydrogen binding energies,respectively,that boost water dissociation and facilitate H_ad adsorption/desorption in alkaline electrolyte,the(Pt_1-xCo_x)₃Al/Pt-Co?OH?₂ as a bifunctional catalyst exhibits superior HER catalysis in 0.1M KOH aqueous electrolyte,with an ultralow Tafel slope(48 m V dec^-1)and overpotential?43 m V?at 10 m A cm^-2.These electrocatalytic properties outperform state-of-the-art Pd-,Ru-and Pt-based catalysts,which indicated multi-site design as an effective strategy to develop bifunctional catalysts towards the alkaline HER.However,due to the strong hydrogen binding energy of Fe?OH?₃,which deviates from the kinetic equilibrium of OH_ad adsorption/desorption,NP(Pt_1-x-Fe_x)₃Al/Pt-Fe?OH?₃ shows poor HER performance,with an Tafel slope(78 m V dec^-1)and overpotential?108 m V?at 10m A cm^-2.The impressive stability results from the unique nanoporous structure of the(Pt_1-x-TM _x)₃Al intermetallic core with a Pt-Co?OH?₂ or Pt-Fe?OH?₃ composite surface,wherein the strong Pt-Al and Co-Al or Fe-Al covalent bonds essentially depress the change in the surface Pt and Co or Fe atoms and in turn protect the internal Al atoms against further dissolution.