Structural Design And Electronic Modulation Of Highly Efficient Electrocatalysts For Water Splitting Reaction

Water electrolysis is the key technology for sustainable and clean hydrogen production.However,due to the high cost,at present only 4%of hydrogen is produced from water electrolysis.One of the major costs in water electrolysis comes from the energy consumption caused by overpotentials originated from two half-reactions:Hydrogen evolution reaction?HER?and Oxygen evolution reaction?OER?.Fortunately,appropriate catalytic materials can change corresponding reaction energy barrier,further effectively reducing the reaction overpotential and improving the energy conversion efficiency.At present,the state-of-art materials for HER and OER are noble metal materials,which are platinum-based catalysts and ruthenium/Iridium-based catalysts,respectively,while their extremely low abundance and high price are another source of cost consumption for water electrolysis.Therefore,developing highly active non-noble metal catalytic materials is crucial for the scalable and sustainable development of water electrolysis.The ideal electrocatalysts should have following desired features:?1?high catalytic activity,which need to be close to or even exceed noble metal catalysts;?2?high catalytic stability,which need to have a stable catalytic structure to prevent deactivation,even maintain its catalytic activity for several decades;?3?the material composition should be cheap and easy to obtain for sustainable development;?4?the material synthesis method should be simple and easy to scale up for industrial conditions.At present,no such materials can meet all of the above requirements.This requires extensive development of catalytic materials with different composition,structure and catalytic properties,further guiding the design and synthesis strategies of catalysts with more efficient performances to satisfy the practical application.In this paper,we designed and synthesized a series of high-performance non-noble metal catalytic materials by optimizing the electronic structure of corresponding materials,such as modulating the crystal phase,introducing heterogeneous structures and constructing defect edges.We further used DFT calculations as auxiliary tool to investigated the internal relationship between the structure of the material and corresponding catalytic properties.The main contents of this paper are as follows:1.Due to the platinum-like electronic structure and relative high conductivity,transition metal carbides are considered to serve as potential catalytic materials for HER.Different from metals and alloy materials which can only survive under alkaline conditions,carbides can maintain their structural stability under a wide range of pH conditions,therefore they are more advantageous in the field of catalytic reactions.However,the catalytic activity of single metal carbides is not good enough based on previous report,therefore we try to introduce another metal into the carbide to modulate the electronic structure of the material,further optimizing corresponding catalytic activity.Based on the above ideas,we synthesized a carbon-coated tungsten-cobalt carbide material which has excellent catalytic performance under acidic,neutral and alkaline conditions.The experimental results show that the high catalytic performance of the material is originated from the synergistic effect of bimetal structure and the carbon coating effect.This work confirms that the bimetallic carbide can efficiently promote the HER reaction,further providing a new route for the design and synthesis of related catalytic materials.2.For the role of the carbon layer on carbides,it is generally accepted that the carbon layer generated during preparation should be avoided because it might block the active sites of metal carbide,further reduce the catalytic performance of materials.However,according to our previous work,we found that although the carbide material is completely encapsulated by the carbon layer,it exhibits superior catalytic performance for HER,which even better than carbon-free carbide materials.Based on the above contradictions,we simplified the synthetic system,further synthesized a series of carbon-coated molybdenum carbide materials,and used them as model materials to study the internal effect of carbon-carbide heterostructures for HER.From the experimental results we found that the catalytic activity of the material is related to the crystal structure and grain size of the carbide,together with the nitrogen content in the carbon layer.Further theoretical calculations concluded that the introduction of the carbon layer on the surface of the molybdenum carbide can effectively modulate the electronic structure of the overall material.Consequently,the hydrogen adsorption on corresponding material surface become moderate,further promoting the entire catalytic reaction.This work demonstrates a kind of carbon-coated molybdenum carbide material with high catalytic performance,confirms the interaction between the carbon layer and the carbide,further points out that the carbon coating strategy is an effective way to improve the catalytic activity of the carbide-based materials.3.Conductive sulfide materials generally have excellent electrical conductivities and structural stabilities,therefore they have great advantages in catalyzing HER reactions.Moreover,several sulfide materials have been reported to exhibit efficient catalytic activity for OER,which provides a reliable assumption that we can use sulfide materials to catalyze both HER and OER,further improving the total efficiency of water electrolysis.Based on the above assumptions,we grew quasi-amorphous CoS_x film on copper foam substrate using chemical deposition method.According to structural characterization,we found that some copper atoms belong to substrate material self-diffused into the CoS_x film during deposition reaction in the form of sub-nanometric copper cluster,eventually forming a unique copper foam-supported,copper cluster-coupled amorphous cobalt sulfide material?denoted as Cu@CoS_x/CF?.Corresponding experimental results and theoretical calculations demonstrate that the synergy between CoS_x and copper clusters is mainly responsible for Cu@CoS_x/CF's excellent catalytic performance for HER.Besides CoS_x's intrinsic catalytic activity and copper clusters'internal high conductivities,the heterostructure of Cu@CoS_x has been proved can dramatically promote the water dissociation step,further accelerating the surface catalytic reaction kinetics.Moreover,Cu@CoS_x/CF also exhibit promising catalytic performance for OER,which make it function effectively as both the cathode and anode of a single alkaline electrolyzer.It can reach a current density of 10 mA cm^-2 at a potential of 1.50 V,whose performance is comparable with the one obtained from an electrolyzer based on the Pt/C-IrO₂catalytic couple.4.Compared with HER,OER is a more energy-intensive process due to intrinsically more complex,multiple proton/electron-coupled steps involved in this half-reaction.Moreover,it turns out that a substantial amount of chemical structures can not keep intact under OER condition due to such harsh oxidizing condition,so it is also very difficult to maintain the catalytic stability of the material during OER reaction.LDH materials,especially NiFe-LDH materials,are a kind of promising OER catalysts with excellent activities.Their chemical structures are active for OER,further can remain stable under corresponding catalytic conditions.The deactivation of related materials generally arises from the weak combination between catalytic materials and electrode,which the active materials may peel off from the electrode under ultra large current density condition.Based on the above research and inspired by the iron corrosion reaction,we developed a corrosion engineering method for transforming inexpensive iron substrates into highly active and ultrastable electrodes for OER.This process results in the growth on iron substrates of thin film nanosheet arrays that consist of NiFe-LDH materials instead of rust.Moreover,we can further control the degree of crystallization of the NiFe-LDH materials by turning the concentration of metal ions in corrosion solution.Related experimental studies confirmed that NiFe-LDH nanosheet array materials with rich grain boundary characteristics have the highest catalytic activity due to the exposure of a large number of highly active unsaturated edge-sites.We further grew the grain-boundary riched NiFe-LDH material on iron foam substrate with a three-dimensional structure.The material can reach a current density of 1000 mA cm^-2 under 340 mV,further can be continuously stabilized at this current density for more than 5,000 hours.In addition to the synthesis of NiFe-LDH materials,this method can further expand to synthesize CoFe,MnFe and MgFe-LDH materials.Moreover,because our corrosion engineering reaction can occur spontaneously without additional energy input,the method is easy to scale up with low cost,which offers a great chance for possible industrial production.