Stratification of cathode catalyst layers for low-temperature proton exchange membrane fuel cells (PEMFC)

Abstract

Over the past few years, significant progress has been made to commercialize proton exchange membrane fuel cells (PEMFC) however, they are still being manufactured at a high cost. PEMFCs currently use platinum as a catalyst for both anode and cathode which vastly contributes to the high raw material cost. The great value of platinum as a catalyst in PEMFC applications is that it outperforms all other catalysts in its activity, selectivity as well as stability (Holton & Stevenson, 2013). The rate of the oxidation reduction reaction (ORR) occurring in the cathode catalyst layer primarily determines the cell voltage, as the rate of the anodic reaction is considerably faster by comparison. The characteristics of the cathode catalyst layer, therefore, have a huge impact on the overall performance of PEMFC. The existing PEMFC cathode catalyst layers (CCLs) are sub-optimally designed, leading to significant mass transport limitations that result in inefficient catalyst utilization. Many works have been done to improve the CCL characteristics which concluded that porosity variation in the catalyst layer improved membrane electrode assembly (MEA) performance. Designing an electrode with a distinctive inner and outer catalyst layer to improve the ORR was proposed by Yoon et al., (2003). This technique is called stratification and was used in this study to optimize the cathode catalyst (CL). In the current study Ionomer gradient MEAs were designed to increase Pt utilization in the catalyst layer enabling Pt loading reduction, without sacrificing performance and lifetime. To study the MEA performance, the electrochemical surface area (ECSA), polarization curves, and electrochemical impedance spectroscopy (EIS) responses were evaluated under various relative humidity (RH) conditions. The integrity of stratified MEAs was tested by conducting an electrochemical carbon corrosion test and was compared to that of a monolayer MEAs. The optimal two-layer design was achieved when the Pt loading ratio between the layers was 1:6. An MEA containing this ratio presented the highest electrochemical surface area (ECSA) and highest performance at 0.65 V with reduced mass transport losses. Using ionomer stratification to decrease the Pt loading in an MEA yielded better performance compared to the traditional monolayer MEA. Stratified MEAs were shown to be more durable than monolayer MEAs both at high (0.4 mgPt/cm2) and lower (0.35 mgPt/cm2) Pt loadings. The high ionomer loading adjacent to the membrane and the bi-layer interface of the stratified CCL increased moisture in the CL, decreasing the degradation rate of the stratified CCLs. This study, therefore, contributes to the development of more durable MEAs for low-temperature PEMFCs.