Relatively large O 2 transport resistance at the ionomer and Pt interface has been thought to be responsible for the large performance loss at high power for a low Pt loading proton-exchange-membrane fuel cell. A facile method to characterize the interface in the fuel cell electrode is needed. In this study, the CO displacement method was explored on polycrystalline Pt and carbon-supported Pt nanoparticles. The displacement charge coverages were used to quantify the adsorption of perchlorate, sulfate, and perfluorosulfonic acid ionomer. Heavy use of platinum in the electrodes of proton-exchange membrane (PEM) fuel cells is a key challenge preventing automotive manufacturers from bringing fuel cell electric vehicles to mass market. Current state-of-the-art fuel cell vehicles use >20 g of Pt per vehicle, which is significantly higher than the internal-combustion engine (ICE) incumbent (<5 g of precious metal per vehicle).1,2 Because heavy use of Pt is needed to obtain high energy conversion efficiency in the fuel cell, improving the activity of Pt-based catalysts has continued to be a high-priority research topic for many years.On the other hand, it was found that at high power density of a low-loaded electrode (<0.10 mg Pt /cm 2 or ∼11 g Pt /vehicle), significant performance losses are observed.3-7 These large performance losses are likely due to the need to deliver more O 2 to a small area of the Pt surface. It was also found that the bulk of the observed O 2 transport resistance occurs at the interface of Pt and electrolyte, 1,4,6 which is surprising because it has generally been seen that the thickness of the ionomer coated on a Pt surface in a well optimized electrode is only a few nanometers. If one calculates the O 2 transport resistance of the thin film using known O 2 permeability of a thick ionomer membrane, 8 it would require an ionomer film with unreasonable thickness (>20 nm) in order to explain the performance loss. Ex-situ measurements on thin-film ionomer performed by several groups have shown that the ionomer nanostructure and its properties such as water uptake, proton conduction, and O 2 permeability can vary substantially depending on its thickness, treatment history, and substrate interaction.9-17 Furthermore, sulfonate groups in the ionomer can adsorb on a Pt surface and reduce the oxygen reduction reaction (ORR) activity. 18,19 Because the adsorption of the acid group immobilizes the ionomer to the Pt surface, [20][21][22] it is surmised that it will also increase O 2 transport resistance. Recent molecular dynamics and density functional theory (DFT) calculations show that ionomers fold onto the Pt surface, leading to a highly dense layer which in turn can reduce the O 2 concentration close to the Pt surface to nearly zero. 23It is also shown that the type of ionomer and operational history can affect the observed performance.1,24 Unfortunately, there is still no characterization method available that will evaluate the ionomer/Pt interface in a fuel cell electrode and in a way that can be re...

The use of hydrogen adsorption/desorption (HAD) is a convenient method to measure the Pt surface area of a catalyst. However, it was shown that electrochemical charges measured by this technique can underestimate the Pt surface area by up to a factor of two for small Pt nanoparticles or Pt alloy nanoparticles. Electrooxidation of CO, so-called CO stripping, has been shown to be more accurate. Yet measurements of CO stripping in MEAs are scarce, especially on high activity alloy catalysts. In this study we investigated CO stripping and the ratio between Pt surface areas measured by CO and by HAD on several Pt and Pt alloy catalysts. The effects on these measurements of temperature and catalyst aging by voltage cycling are discussed.

Reducing the use of precious metal in a proton exchange membrane fuel cell is a key challenge in mass commercialization of fuel cell electric vehicles. However, as Pt loadings are reduced, fuel cell performance losses due to mass transport phenomena become more localized to the Pt and ionomer interface. In this paper, we provide an overview of how we use in-situ electrochemical diagnostics and modeling to understand the performance of low-Pt electrodes, identify their key limiting factors, and guide our catalyst layer development. In particular, diagnostics used to quantify the local-Pt oxygen transport resistance and the ionomer adsorption on the Pt surface will be discussed.