Micromodification of the Catalyst Layer by CO to Increase Pt Utilization for Proton-Exchange Membrane Fuel Cells

Abstract

Improving the utilization of platinum in proton-exchange membrane (PEM) fuel cells is critical to reducing their cost. In the past decade, numerous Pt-based oxygen reduction reaction catalysts with high specific and mass activities have been developed. However, the high activities are mostly achieved in rotating disk electrode (RDE) measurement and have rarely been accomplished at the membrane electrode assembly (MEA) level. The failure of these direct translations from RDE to MEA has been well documented with several key reasons having been previously identified. One of them is the resistance caused by complex mass transport pathways in the MEA. Herein, we improve the proton and oxygen transportations in the MEA by building a thin and uniform distribution of ionomer on the catalyst surface. As a result, a PEM fuel cell design is capable of showing a current density improvement of 38% at the same voltage (0.6 V) under the H 2 /air operation.

Water Condensation in the Nanoscale Pores of Pt/C Catalyst Particles and Its Impact on Catalyst Utilization: A Simulation Based on a Reconstructed Structure from Nanoimaging

“…Sample images taken by transmission electron microscopy (TEM) of the region of interest are shown in Figure (a,b). The use of an ionomer-free sample simplifies the image processing, as distinguishing ionomer from carbon and pores becomes challenging in samples containing ionomers. ,, However, given the well-established knowledge that ionomers can significantly cover the carbon support’s exterior surface, − we can reasonably assume that Pt particles on the catalyst powder’s exterior surface are ionomer-covered.…”

“…COSV has been the routinely used method to measure catalyst utilization. ,,− , However, to measure the RH-dependent ECSA through COSV, it requires setting up the membrane electrode assembly (MEA). This means that the RH-dependent catalyst utilization of the catalyst powder alone (ionomer-free), cannot be directly measured unless it is combined with an ionomer to create a catalyst layer.…”

“…However, this can also lead to the disruption of proton pathways due to disconnection of the interior catalyst particles from the ionomer network. 11,[15][16][17][18]26 Despite the trade-offs, HSCs have been recently adopted in PEMFCs as opposed to LSCs. In fact, the Toyota fuel cell vehicle (FCV) system "Mirai", a leading commercial FCV in Japan, has shifted from using LSC in its first-generation FCV to using HSCs for their second-generation FCV.…”

“…Unlike low surface carbon (LSC) supports, HSCs are highly porous which allows exterior and interior deposition of Pt catalyst particles. − ,− The nanopore openings of HSCs inhibit the penetration of the long ionomer polymer chain into the interior pores of HSCs, hence, avoiding ionomer poisoning. However, this can also lead to the disruption of proton pathways due to disconnection of the interior catalyst particles from the ionomer network. ,− , Despite the trade-offs, HSCs have been recently adopted in PEMFCs as opposed to LSCs. In fact, the Toyota fuel cell vehicle (FCV) system “Mirai”, a leading commercial FCV in Japan, has shifted from using LSC in its first-generation FCV to using HSCs for their second-generation FCV .…”

“…Additionally, Pt/C catalyst particles are still too large for molecular dynamics simulations. Fortunately, in terms of quantitative analysis, experimental measurement through carbon monoxide stripping voltammetry (COSV) can be used to evaluate Pt catalyst utilization. ,,− , However, since COSV measurement requires the setting-up of the membrane electrode assembly (MEA), there could be several other factors that can affect the measurement. This makes it difficult to focus on the role of water condensation in the Pt catalyst utilization.…”