The Mechanism Of Crack Evolution In Catalyst Layer Of Fuel Cell And The Design Of New Electrode Structure

Fuel Cell is an energy conversion device that converts the chemical energy stored in the reactants into electrical energy directly,which can be utilized with high efficiency and no pollution.Especially,proton exchange membrane fuel cell is most widely used for the advantages of small size,low operating temperature,high energy density,fast start-up speed and rapid response.The catalyst layer(CL)consists of carbon-supported platinum particles,ionomer,and catalyst voids,which comprehensively determines the performance of fuel cell and accounts for most of the cost of fuel cells.In actual operation,catalyst layer will experience frequent start/stop switches,during which the temperature,gas humidity,and local gas composition change constantly and accelerate the cracking of catalyst layer.Therefore,studying the microstructural changes of catalyst layer under dynamic operating conditions will be beneficial to understanding the degradation mechanism of CL,thus providing insights for the design of CL with stable performance and improving the durability of fuel cells.At the same time,in order to reduce the cost and accelerate the promotion of fuel cells,researchers gradually realize the importance of the improvement and optimization of catalysts and catalyst layer.A lot of work have been studied on improving the durability of catalyst layer.Therefore,the design and preparation of new electrode structures and materials are of great significance for the development of fuel cells.The accelerated stress test is used to detect the degradation of the microstructural changes of the catalyst layer under the condition of temperature and humidity cycles.Different temperature and humidity cycle experiments were designed,ranging from the starting condition of 25 ~oC,45%RH to different working conditions 85 ~oC,(20%,45%,99%)RHs.Based on the results of polarization curves,electrochemical tests,SEM and AFM adhesion analysis,it is found that there are three typical crack changes on the surface of the catalyst layer:the generation of new crack,the growth of primary cracks and the interaction between cracks.The crack propagation will be accelerated by inceasing humidity cycle amplitude.During the humidity and temperature cycles,especially with the increase of absolute humidity,the migration of ionomer reconstructs the catalyst layer,which has a significant impact on both the microstructural changes of catalyst layer and the fuel cell performance.In order to inhibit the cracking of the catalyst layer,a new structure of gas diffusion electrode is proposed,that is,the structure stability of catalyst layer is improved by directly designing and growing a vertical nanocrystals(fixed layer)between the gas diffusion layer and the catalyst layer.Cyclic voltammetry method is used to deposit a vertical staggered CoS nanosheet structure on microporous layer(MPL).The results shows that the microstructural changes of catalyst layer decrease when applying the CoS nanosheet between CL and MPL after the same temperature and humidity cycle experiment,which indicates that CoS nanosheet mitigates the microstructural changes of catalyst layer.The electrochemical test results prove that this new type of electrode has good oxygen reduction activity,and can effectively reduce the impedance of the oxygen reduction reaction,which is expected to further study the application on the fuel cell.In summary,through the accelerated stress tests on the catalyst layer of proton exchange membrane fuel cell,this research discovered three propagation modes of the crack evolution in CL.The observation by atomic force microscope indicates that the fundamental reason of the microstructural changes lies in the reconstruction of the catalyst layer induced by ionomer migration.In addition,the fixed layer with vertical nanosheet structure prepared by electrochemical deposition improves the structural stability of the catalyst layer.Furthermore,the new gas diffusion electrode prepared by electrochemical deposition shows good oxygen reduction performance under alkaline condition,which providing a new design method and electrode structure for the gas diffusion electrode of fuel cell.