Investigation On Dealloying-derived Fabrication And Electrocatalytic CO₂ Reduction Of Nanoporous Sb-based Alloys

Electrocatalytic CO2 reduction(ECR)could convert the CO2 to high value-added chemical products utilizing the electricity generated by renewable resources such as solar energy and wind energy,which can effectively reduce the concentration of CO2 and store intermittent renewable energy into chemicals to realize the closed loop of carbon cycle simoutaneously,exhibiting great potential for development.Unfortunately,the stable thermodynamic properties and high activation energy barrier of CO2 molecule result in high overpotential for electrocatalytic reduction;moreover,the competitive hydrogen evolution reaction(HER)also leads to poor selectivities towards ECR.Hence,improving the efficiency of ECR depends largely on the development of high-efficiency catalysts.Up to now,relevant research about Sbbased alloy catalysts used for ECR is still in its infancy,and the catalytic activity,products selectivity and stability need to be further improved.Specially,exploring the influences of element composition and microstructure on the ECR performance is of great importance to the development of efficient Sb-based electrocatalysts.In this work,a series of nanoporous SbBi(np-SbBi),SbSn(np-SbSn)and SbBiSn(np-SbBiSn)alloy catalysts with different composition ratios were prepared by vacuum melting,melt-spinning and dealloying method,and their phase compositions,microstructures and ECR properties were further investigated.Firstly,the as-fabricated np-Sb100-xBix(x=0,0.5,1,2,5)alloy catalysts with a low Bi content consist only of(Sb,Bi)solid solution phase,and their microstructures show similar three-dimensional bi-continuous ligament-channel network.Compared with nanoporous Sb(np-Sb),the electrochemical test results show that the catalytic activity of Sb is changed from HER to ECR after adding a trace amount of Bi.At-1.0(V vs.RHE),the Faraday efficiency(FE)of formate on np-Sb99.5Bi0.5 increases to 52.2%,about 4 times than that of np-Sb(12.7%).Moreover,with the increase of Bi content,at-1.2(V vs.RHE),the FE of formate on np-Sb98Bi2 reaches up to 88.8%(only about 13.7%on np-Sb),the corresponding partial current density(jformate)reaches 29.1 mA cm-2(about 2 mA cm-2 on np-Sb),and the reaction stability is greatly improved.The improvement of properties is attributed to the alloying effect of Bi and Sb,which activates the ECR intrinsic activity of Sb.Secondly,compared with Bi,Sn has more abundant reserves and similar ECR activity.Therefore,in this work,series np-Sb100-xSnx(x=5,25,50,75)alloy catalysts were prepared by replacing Bi with Sn.The microstructures also present three-dimensional bi-continuous ligament-channel network.Specially,different from SbBi alloy,the ligaments become rough and irregular,and the alloy phases for different compositions are(Sb,Sn)solid solution(npSb95Sn5),SbSn alloy and(Sb,Sn)solid solution(np-Sb75Sn25),SbSn alloy phase(np-Sb50Sn50)and SbSn alloy and Sn phase(np-Sb25Sn75)respectively.Compared with np-Sb,ECR results show that although the performance of np-Sb95Sn5,np-Sb75Sn25 and np-Sb50Sn50 has been improved,the products selectivity is not particularly satisfactory(the FE of H2 close to 50.0%),especially that of the np-SbsoSn50 which is only composed of intermetallic compound SbSn.Moreover,the poor reaction stability is also obtained,especially at-1.1(V vs.RHE)and more negative potential.In contrast,at-1.2(V vs.RHE),the np-Sb25Sn75 shows excellent ECR performance with the FE of formate reaching 80.7%and the corresponding j formate reaching 47.1 mA cm-2(the total current density is close to that of np-Sb50Sn50,about 56.0 mA cm-2),which is correlated with the synergism of rhombohedral SbSn phase and tetragonal Sn phase in the alloy.Finally,the above two works demonstrate that the binary alloying strategies could effectively improve the ECR properties of Sb,especially when forming solid solution alloys.On these basics,in this work,np-Sb95Bi2Sn3 and np-Sb90Bi5Sn5 ternary solid solution alloys,which also exhibit a three-dimensional bi-continuous ligament-channel network,were prepared by the same process through composition design to further investigate the effect of ternary alloys on the ECR properties of Sb-based alloy catalysts.The electrochemical results show that at-1.2(V vs.RHE),the FE of CO2-to-formate conversion on np-Sb95Bi2Sn3 and np-Sb90Bi5Sn5 reaches 54.4%and 56.5%respectively,which is about 4 times than that of np-Sb(13.7%).The above results demonstrate that the ECR performance of Sb is improved definitely by the ternary alloying strategy,exhibiting better product selectivity and reaction stability.In addition,the reaction current density increases with the increasing of Bi and Sn content,but the product selectivity shows no significant improvement.