Preparation And Electrochemical Performance Of 3D Carbon Fiber-based Composite Cathodes For Seawater Dissolved Oxygen Battery

Seawater oxygen-dissolved battery has some advantages such as high specific power,low cost,good safety,long preservation lifetime,excellent low-tempareture performance,and so on,meanwhile,its distinct open architecture benefits to solve the design problem of heavy pressure shell for the deep-sea application of chemical power sources.As one of novel high specific power batteries,seawater oxygen-dissolved battery possesses higher theoretical energy density（810 Wh/kg）than traditional batteries and actually obtain high energy density results（about 350 Wh/kg）,thus obtains good potential applications of underwater glider,marine resource exploitation equipment,submarine earthquake monitor,marine life saving lamp,deep sea rescue capsule,and so on.Therefore,seawater oxygen-dissolved battery has become attracting more and more attentions from worldwide researchers.Carbon fibers have shown excellent performance as a cathode material matrix for seawater batteries because of their positive potential,good electrical conductivity and large specific surface area.However,two-dimensional carbon fibers have poor resistance to polarisation,low oxygen reduction catalytic activity,and are not ideal when used singularly as a cathode material for seawater batteries.In this work,three-dimensional carbon fiber brushes were selected as the substrate and three-dimensional carbon fiber-based graphene-platinum composite cathode materials were prepared by a one-step hydrothermal reduction method,and further three-dimensional carbon fiber-based conducting polymer-graphene-platinum composite cathode materials were prepared by a stepwise in-situ polymerisation method.Matrial performance containging microscopic morphology,structural composition,specific surface area oxygen adsorption capacity,thermal stability and so on were investigated by SEM,TEM,Raman,IR,XPS,BET,O₂-TPD,TGA and so on.The electrocatalytic oxygen reduction performance and stability performance of the electrode materials were investigated by CV,LSV,EIS and constant current polarization tests in 3.5 wt.%Na Cl solution.The performance of the single cell was evaluated by constant current discharge test,dynamic potential polarization test.Some experimental results are shown as below.The carbon fiber-graphene-platinum composite cathode materials with different platinum contents were prepared by a one-step hydrothermal reduction method using three-dimensional carbon fiber brushes as the substrate.It is found that the combination of graphene and Pt effectively increases defects,specific surface area and oxygen adsorption capacity of the composite electrode materials.The prepared CFB/（2）Pt-G composite cathode has the most uniform distribution of Pt particles,the largest specific surface area,the best oxygen adsorption capacity,and the highest ultimate diffusion current density of up to 5 m A/cm² in3.5 wt.%Na Cl solution.The electrocatalytic oxygen reduction process is dominated by a fast4-electron process and the stability of CFB/（2）Pt-G composite cathode is the best.The best performance of CFB/（2）Pt-G composite is demonstrated.Two composite electrodes,CFB@PANI and CFB@PPy,were prepared by a stepwise in-situ polymerization method,and further CFB@PPy-G-Pt and CFB@PPy-Pt composite electrodes were prepared.It is found that CFB@PPy has better performance than CFB@PANI composite electrode under the same preparation conditions.The Pt particle size in CFB@PPy-G-Pt composite cathode is the smaller,the specific surface area is the larger and the oxygen adsorption performance is the better.The ultimate diffusion current density in 3.5 wt.%Na Cl solution can reach 4.9 m A/cm²,which can mainly be ascribed to a 4-electron transfer process,and the CFB@PPy-G-Pt composite cathode has the better stability.The performance of CFB@PPy-G-Pt is similar to that of CFB/（2）Pt-G,indicating that the long-term discharge stability of the composite cathode can be improved by loading the carbon fiber brushes with polypyrrole and reducing the amount of graphene oxide.CFB/（2）Pt-G and CFB@PPy-G-Pt composite cathodes were selected to form batteries with Al alloys and Mg alloys respectively for discharge tests.It is found that the optimum cathode to anode area ratio is 6:1.The battery performance is better when Mg alloy is used as the anode than Al alloy as the anode,and the voltages of both CFB/（2）Pt-G-Al/Mg and CFB@PPy-G-Pt-Al/Mg are easy to reach smoothly at a low current discharge.