(Invited) Uncovering the Role of Alkaline Pretreatment for Hydroxide Exchange Membrane Fuel Cells

Abstract

As an emerging alternative to proton exchange membrane fuel cells, hydroxide exchange membrane fuel cells (HEMFCs) can use more cost-effective platinum-group-metal-free (PGM-free) catalysts, bipolar plates, and other components.1 Taking advantage of the recent significant progress in hydroxide exchange membranes (HEMs) and optimized electrodes based on hydroxide exchange membrane ionomers (HEIs), HEMFCs achieved unprecedented performance and the record of peak power density was updated frequently in the past few years. Interestingly, nearly all membrane electrode assemblies (MEAs) with satisfactory performances reported in the literature were pretreated with alkaline solutions such as KOH before cell assembly.Here we show that the most crucial role of the alkaline soak is to remove carboxylates that are generated from the oxidation of primary alcohols, used as solvents for ionomers and/or catalyst inks, by platinum/palladium catalysts during membrane electrode assembly fabrication. These carboxylates are difficult or impossible to eliminate by self-purging processes. We also show that bicarbonates are an adequate replacement for the most frequently used strong bases for the pretreatment. As shown in Figure 1, the pretreatment effect can be seen in the shape of polarization curves. And Figure 2 depicts the electrochemical purging process in HEMFCs for acetate and carbonate/bicarbonate, respectively. As a result, alkaline pretreatment is unnecessary for catalysts that do not oxidize primary alcohols. In fact, membrane electrode assemblies with Pt/Pd-free catalysts deliver a better performance without the alkaline pretreatment: a voltage of 0.64 V at 1.0 A/cm2 and a peak power density of 0.69 W/cm2 in H2/O2. References B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat Nanotechnol, 11 (12), 1020-1025 (2016). Figure 1