Structure And Preformance Optimization Of SO₂ Depolarized Electrolysis Cells

Hybrid sulfur cycle（Hy S-cycle）is one of the simplest thermochemical cycles for hydrogen production by water splitting.As a hydrogen production step,the efficient operation of SO₂-depolarized electrolysis（SDE）is very important to improve the hydrogen efficiency of Hy S-Cycle system.Based on the basic principle of SDE,this paper carried out an in-depth study on the structure and key materials of the electrolytic cell,providing theoretical reference and technical support for optimizing the structure of SDE cells,developing new electrode materials,and improving the overall efficiency of Hy S-cycle.In this work,graphite plates with different flow channels,together with porous graphite felts and carbon papers as diffusion layers are adopted to fabricate SDE cells with different structures.Evaluation of the cell structures is carried out,by comparing the SDE performance and taking into account the fluid resistance（pressure drop）of anode side.The effects of graphite felt compression ratio,hydrophilicity or hydrophobicity of carbon papers,anodic fluid flow rate,and operating temperature on the SDE performance are investigated.Square porous flow fields provided by graphite felts show excellent performance.Serpentine channel covered by hydrophobic carbon paper reveals advantages when adopted on the cathode side.Combination of above two flow fields and using them on anode and cathode sides respectively,could achieve excellent SDE performance.Under the condition of 40℃and360 m L/min anolyte flow rate,the current density could reach 760 m A/cm²at the cell voltage of 1.19 V.The computational fluid dynamics（CFD）simulation of different flow fields was carried out to compare the characteristics of the fluid flow and verify the excellent performance of the square flow field.The influence of catalyst Pt loading and proton exchange membrane（PEM）thickness on the performance was further investigated,for the SDE cells applying square porous flow fields.When Nafion^?117 PEM is used,considering SDE performance and catalyst cost,the optimal Pt loading capacity is 0.42 mg Pt/cm²sprayed on both anode and anode sides.When different thicknesses of PEM are used,the performance of the SDE cell using Nafion^?212（thickness 50μm）as PEM is better than that using Nafion^?115（thickness 125μm）and Nafion^?117（thickness 175μm）.While the difference between Nafion^?115 and Nafion^?117 is not significant.Electrochemical impedance spectroscopy（EIS）verifies and explains the above results.The impedance of the SDE cell can be significantly reduced by a large thickness reduction of PEM,but the thickness of Nafion^?115 and Nafion^?117 is similar,and the influence on the cell performance is relatively small.The three-dimensional anodes for SDE cells are prepared by loading Pt/C on high void content graphite felts,with the method of ultrasonic spray and vacuum suction.SEM results confirm the three-dimensional space distribution of Pt in graphite felts,which ensures sufficient contact between Pt and SO₂in anolyte.Comparing with the two-dimensional anodic catalyst layer loaded on the proton exchange membrane in a conventional SDE cell,the application of the three-dimensional anode decreases cell impedance greatly and improves the SDE performance significantly.In this study,0.63 mg/cm²Pt loading amount shows the best performance when Pt/C is double-side-sprayed on graphite felt,and the current density reaches 1.24 A/cm²at cell voltage of 1.2V,as operating at 60℃and anolyte flow rate of 360 m L/min.