Development of high-performance ultra-low Pt-loaded cathode catalyst layers for PEM fuel cell based on tailored catalysts and advanced coating engineering

Kurzfassung

Polymer electrolyte membrane fuel cells (PEMFCs) are promising energy converters to realize the transition to a carbon-free society, especially for mobile and stationary applications. The two main barriers hindering widespread commercialization are high cost and limited durability. One aim of this work is to reduce the amount of the platinum, while maintaining constant or higher electrochemical performance in the cathode catalyst layers (CLs). The optimization of the catalyst layer microstructure, which can be determined by the coating parameters and ink formulation, is one of the approaches to mitigate the cost issue. At present, piezoelectric inkjet printing (IJP) technology can be used to fabricate CLs with low or ultralow Pt loading by depositing very small amounts of the material at once and design CL microstructure by drop modulation to reduce the catalyst waste and maintain high performance. At present, significant advancements have been achieved in printed electrodes for PEMFCs, but the optimal printing conditions of IJP technique and development of inkjet-compatible inks are not known. In addition, reducing Pt loading at cathode side may enhance the degradation of the catalyst. Thus, developing highly stable cathode catalysts to mitigate performance loss in durability test is also an important topic in this thesis. The fundamental goal of this dissertation is to understand the relationships between structural properties and performance of catalytic layers, and to derive strategies for a goal-oriented development. A detailed investigation into the effect of IJP operating parameters on the catalyst layer microstructure is provided, focusing on tailoring drop spacing and drop size by varying pattern resolution and pulse voltage. And the effect of varied microstructures on electrochemical properties are also included. To be specific, drop spacing controlled by pattern resolution results in different thickness and porosity of the CLs. Electrochemical performance results show that the modified thickness and porosity can impact the mass transfer resistance (Rmt) of the electrode, but the influence on ECSA and charge transfer resistance (Rct) is minimal. Differently, the size of drops can be controlled by changing pulse voltage, influencing the distribution of aggregates inside the dried drop while the maintaining the quite similar thickness and porosity of CLs. Based on electrochemical characterization, it is found that uneven distribution of aggregates ascribed from low pulse voltage leads to an increase in Rct, while higher voltage causes the increased Rmt because of the reduced proportion of macropores inside the CL. Finally, the optimal performance of electrode was obtained with medium pattern resolution and pulse voltage. This work is able to provide theoretical support to finely design the microstructure of the CLs when the Pt loading and ink composition is the same, enabling enhanced performance of CL in the next step. Furthermore, PeakForce TUNA mode was used to distinguish the adsorbed ionomers, deposited ionomers and catalysts in the dried drop prepared by inkjet printing according to the height and conductivity difference. The volume of ionomer and catalyst can also be quantified by MATLAB based on the collected AFM images. Our results demonstrate that the increase in IPA content within the catalyst ink results in a higher concentration of free ionomers and reduced agglomeration, creating a more dispersed free ionomer configuration in the dispersion medium, while higher water content in the solvent promotes ionomer adsorption driven by the hydrophobic interaction between the ionomer backbone and carbon support and increase ionomer agglomeration. Finally, a catalyst ink formulation containing 54 wt% IPA appears to be a compromise that provides best cell performance due to its proper ionomer distribution. These findings provide new insights into CL designs with optimal performance in PEM fuel cells. The next study focuses on the development of inks by adding high-boiling point solvents (propylene glycol (PG) or ethylene glycol (EG)) to improve printing efficiency and investigating the effect of PG on CL structure and performance to allow optimization of inkjet-printed electrodes. Specifically, the addition of PG or EG can prevent clogging of the nozzles in printhead by prolonging the evaporation rate of the inks, increasing by up to 66.67% compared to ink without high-boiling point solvent. Subsequently, the relationship between catalyst ink properties, CL microstructure, and electrochemical performance of the MEA is nvestigated to optimize the PG content. Results show that an increased amount of PG is detrimental in terms of electrochemical performance due to the formation of larger agglomerates, lower porosity of the catalyst layer, and reduced ECSA. This work offers guidance for the development of inkjet-printed inks for industrial coating processes in future. Finally, a commercial Pt/C catalyst was encapsulated by few-layer h-BN shells to improve the PEMFC durability under aggressive AST conditions. The Pt@h-BN/C core–shell nano catalysts present similar electrochemical surface area and higher ORR mass activity in comparison with the Pt/C catalyst at RDE level, which suggests that h-BN shells do not block Pt-catalyzed electrocatalytic reactions but enhance them. Additionally, its behavior in terms of stability was characterized by rotation disc electrode (RDE). Results showed the Pt@h-BN/C catalyst exhibits a slight ECSA degradation (14.5 %) in contrast to the commercial Pt/C catalyst (27.1 %), indicating the possibility of h-BN shell as a protective barrier to prevent the catalyst degradation after 20k AST cycles. In addition, durability test based on the EU harmonized drive cycle testing procedures was adopted to test the degradation of the catalyst. The Pt@BN/C-based CL runs for 9.91 days before the cell voltage drops to threshold of 0.4 V, whereas the Pt/C-based CCM reaches the voltage threshold in 5.22 days, indicating the Pt@BN/C-based CCM has a more stable behavior in the durability test. Eventually, this work lays a solid foundation for further exploring the application of inkjet printing technique in fabrication of catalyst layers in PEMFCs and makes an essential contribution for developing electrocatalyst to hinder the degradation.