Abstract
Optimization of the ionomer materials in catalyst layers (CLs) which sometimes is overlooked has been equally crucial as selection of the membranes in membrane electrode assembly (MEA) for achieving a superior performance in proton exchange membrane fuel cells (PEMFCs). Four combinations of the MEAs composed of short-side-chain (SSC) and long-side-chain (LSC) perfluorosulfonic acid (PFSA) polymers as membrane and ionomer materials have been prepared and tested under various temperatures and humidity conditions, aiming to investigate the effects of different side chain polymer in membranes and CLs on fuel cell performance. It is discovered that SSC PFSA polymer used as membrane and ionomer in CL yields better fuel cell performance than LSC PFSA polymer, especially at high temperature and low RH conditions. The MEA with the SSC PFSA employed both as a membrane and as an ionomer in cathode CL demonstrates the best cell performance amongst the investigated MEAs. Furthermore, various electrochemical diagnoses have been applied to fundamentally understand the contributions of the different resistances to the overall cell performance. It is illustrated that the charge transfer resistance (Rct) made the greatest contribution to the overall cell resistance and then membrane resistance (Rm), implying that the use of the advanced ionomer in CL could lead to more noticeable improvement in cell performance than only the substitution as the membrane.
Highlights
Proton exchange membrane fuel cells (PEMFCs) have been widely considered to be a critical conversion technology in a hydrogen-based energy infrastructure due to their high theoretical energy efficiency and zero-emission [1,2]
The premiere commercial Perfluorosulfonic acid (PFSA) used in PEMFCs is Nafion®, a long-side-chain (LSC) ionomer, produced previously by DuPont and Chemours, which is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer discovered in the late 1960s by Walther Grot of DuPont [5]
The Electrochemical surface area (ECSA) of Pt catalyst in the cathode was evaluated by cyclic voltammetry (CV)
Summary
Proton exchange membrane fuel cells (PEMFCs) have been widely considered to be a critical conversion technology in a hydrogen-based energy infrastructure due to their high theoretical energy efficiency and zero-emission [1,2]. The proton exchange membrane (PEM) functioning as a proton conductor as well as a separator for electrodes and reactant gas was recognized as one of the most expensive stack component and a key component to determine the cell performance [3]. The proton exchange polymers employed in PEMFC as membrane and as binder/proton conductor in the catalyst layer (CLs) are crucial to the entire cost and performance of a PEMFC. The short-side-chain (SSC) PFSA polymer with a similar structure as Nafion, but bearing a shorter -OCF2 CF2 SO3 H pendant chain, has been considered as a promising candidate for PEMFC applications due to the higher crystallinity, the higher thermal transition temperature, and the higher ion exchange capacity (IEC) compared to LSC ionomer [8]. In 2010, Solvay-Solexis developed a simpler approach to synthesize SSC
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