Since the reduction of platinum-loadings in polymer electrolyte fuel cells (PEFCs) is essential for the further spread of their technology, many researchers have been working on this subject. Among those studies, the importance of controlling and optimizing structures of catalyst layers including the carbon frame has been reported for designing highly efficient PEFC with the low platinum-loading.1 In this study, membrane electrode assemblies (MEAs) with various platinum-loadings in the cathode are made and evaluated as a fundamental approach to reduce the platinum-loading. Then, we are trying to understand minimum requirement of platinum and to correlate resulting overvoltages with structures of cathode layers, and we are accordingly aiming to design highly efficient PEFC with the low platinum-loading.
Experimental
MEAs with various platinum-loadings in the cathode were prepared with 46% Pt deposited Ketjen black (TEC10E50E) and Nafion® by a spray printing method.2 Then, MEAs were evaluated in terms of impedance, IV performance, and ECSA. The thickness of catalyst layers were also measured by FIB-SEM. For further designing ideal electrocatalysts, electrocatalysts with different amounts of Pt deposition were made using Vulcan carbon as a carbon support and platinum(II) acetylacetonate as a platinum precursor through thermal reduction.3
Results and discussion
MEAs with the fixed anode (0.30 mgPt/cm2) and the varied cathodes (0.05 – 0.40 mgPt/cm2) have been prepared and evaluated in our study. Then, the minimum requirement of platinum in the cathode has been determined to 0.15 – 0.20 mgPt/cm2 through correlating the platinum-loading with activation overvoltage. Furthermore, the requirement of sufficient thickness of cathode layers has been found for reducing concentration overvoltage.4
In this report, the correlation between carbon amounts and concentration overvoltage was further evaluated by preparing MEAs with 0.05 – 0.60 mgPt/cm2 cathodes. Since decrease in the gas diffusion pathway by decreasing the cathode layer also leads to increase in the concentration of water produced by fuel cell reactions in the cathode layer, there should be the optimum thickness in terms of sufficient fuel diffusion. As a result, the optimum thickness was found to be 8.24 µm in this study. Since the importance of cathode thickness was realized for designing PEFC with the low platinum-loading, electrocatalysts with reduced Pt deposition on carbon were prepared in order to increase the cathode thickness even with reduced platinum-loadings. Consequently, successful reduction of concentration overvoltage was suggested from our experimental results. Obtained results will be explained more in details.
(1) M. Lee, et al., Electrochimica Acta, 56 (2011) 4783-4790.
(2) M. Okumura, et al., J. Electrochem. Soc., 164 (9), F928 (2017).
(3) A. Hayashi, et al., Electrochemica Acta, 53, 6117 (2008).
(4) K. Uda, et al., ECS Trans., 80 (8), 789 (2017).