Editors' Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography

Abstract

Further reduction of Pt in hydrogen fuel cells is hampered by reactant transport losses near the catalyst surface, especially for degraded catalysts. Strategically mitigating these performance losses requires an improved understanding of the catalyst nanostructure, which controls local transport and catalyst durability. We apply cryo-tomography in a scanning transmission electron microscope (STEM) to quantify the three-dimensional structure of carbon-supported Pt catalysts and correlate to their electrochemical accessibility. We present results for two carbon supports: Vulcan, a compact support with a large majority of Pt observed on the exterior, and HSC, a porous support with a majority of Pt observed within interior carbon pores, which have relatively constrictive openings. Increasing Pt content shifts the Pt distribution to the exterior on both carbon supports. By correlating to the electrochemical surface area, we find that all Pt surface area is accessible to protons in liquid. However, the interior Pt fraction quantitatively tracks Pt utilization losses at low humidity, indicating that the interior Pt is inaccessible to the proton-conducting ionomer, likely because narrow carbon pore openings block ionomer infiltration. These results imply different proton transport mechanisms for interior and exterior Pt, and quantitatively describe the catalyst structure, supporting development of transport and durability models.

Along-the-channel modeling and analysis of PEFCs at low stoichiometry: Development of a 1+2D model

“…The sharp change between 80% and 100% RH can also poses a challenge, as the trend between these points is not known and more data is required. However, the fit values are within those reported in the literature [72].…”

“…A minor increase is observed in ECSA with RH at unsaturated conditions; however, a significant increase is observed for 100% RH. This is understandable as the interior pores of high surface carbon are fully flooded at 100% RH making them accessible to protons and thereby increasing the available Pt area significantly[72]. While the ECSA would change locally in the CL depending on local water content, this is currently still being investigated experimentally.…”

Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements

“…Changing the primary-particle loading varies the distribution of Pt nanoparticles on the external agglomerate surface compared to the interior of the primary particles, with the fraction of Pt particles on the external surface increasing at high primary particle loadings. 60 Thus, both η and γ change because the number of Pt par- ticles on the agglomerate external surface changes. η decreases with increasing primary particle loading, due to lower ionomer/gas interface area per Pt particle, while γ increases due to the higher fraction of Pt particles on the agglomerate external surface.…”

“…η decreases with increasing primary particle loading, due to lower ionomer/gas interface area per Pt particle, while γ increases due to the higher fraction of Pt particles on the agglomerate external surface. 60 Different primary-particle loadings (20,30, and 50 wt-%) were studied while holding the total Pt loading constant at 0.05 mg/cm 2 . HAADF-STEM images of the samples shown in Figure 11 demonstrate the heterogeneous CL structure with separate agglomerates formed by Pt-loaded HSC and bare KB particles and nonuniform ionomer distributions.…”

A Mathematical Model toward Real-Time Monitoring of Automotive PEM Fuel Cells

“…Finally, the ORR kinetics are assumed to be RH dependent based on the experimental observations in the literature [69,70]. Although the significance of this effect has been questioned [71,72], the approach adopted herein is still beneficial in capturing the impact of lowered RH on accessibility of Pt particles deposited inside the inner catalyst pores of high surface area carbon support [73,74].…”