(485e) Achieving High PEFC Performance at Rated Power through Modifications to Electrode Microstructure

Improvements in polymer electrolyte fuel cell (PEFC) electrode performance have primarily focused on catalyst and ionomer development without fully considering the importance of process variables such as ink formulation or electrode fabrication. Differences in catalyst ink formulations can lead to significant differences in kinetic activity, ionic and gas transport properties.^1–3 The catalyst ink solvent (water:alcohol) ratio is an important parameter that has been shown to dramatically affect ionomer/electrocatalyst interactions at the catalyst interface and throughout the electrode microstructure.⁴ The difficulty of characterizing electrodes (especially the ionomer/catalyst interface) has limited our understanding of how the catalyst ink formulation influences electrode structure and performance. In this work, we comprehensively examine the effect of water:nPA ratio across a series of PEFC cathodes comprised of state-of-the-art commercial platinum alloy catalysts supported on high surface area carbon (PtCo/HSC) with Nafion (D2020) ionomer.⁵

Using a myriad of in situ electrochemical and ex situ characterization techniques to probe electrode properties across multiple length scales, we relate performance differences (namely O₂ transport) with changes in electrode microstructure. CO displacement chronoamperometry and electrochemical impedance spectroscopy (EIS) experiments indicate that ink solvent ratio had a minimal effect on ionomer coverage unlike earlier work on Pt/Vu electrodes. Nano-computed tomography (nano-CT) and ultra-small angle X-ray scattering (USAXS) establish that the optimal ink formulation (3:7 nPA:H₂O) coincides with smaller ionomer/catalyst aggregates and improved local O₂ transport. Using commercially available materials (catalyst and ionomer), a state-of-the-art membrane electrode assembly with 0.03/0.08 mg_Pt/cm² loading on the anode and cathode, respectively, achieves i) a mass activity exceeding 1 A/mg_Pt (0.9 V_iR-free, 150 kPa, 80^oC, 100% RH, H₂/O₂), ii) 320 mA/cm² at 0.8 V, 150 kPa, 80^oC, 100% RH, H₂/Air), and iii) > 1 W/cm²_elec at rated power (0.67 V, 250 kPa, 94^oC, 65% RH, H₂/Air) or < 0.11 g_Pt/kW_rated, all of which meet or exceed current DOE targets.^5,6

References

1. Orfanidi, A., Rheinländer, P. J., Schulte, N. & Gasteiger, H. A. Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. J. Electrochem. Soc. (2018). doi:10.1149/2.1251814jes
2. Wang, X. X., Sokolowski, J., Liu, H. & Wu, G. Pt alloy oxygen-reduction electrocatalysts: Synthesis, structure, and property. Chinese Journal of Catalysis 41, 739–755 (2020).
3. Takahashi, S., Mashio, T., Horibe, N., Akizuki, K. & Ohma, A. Analysis of the Microstructure Formation Process and Its Influence on the Performance of Polymer Electrolyte Fuel-Cell Catalyst Layers. ChemElectroChem 2, 1560–1567 (2015).
4. Van Cleve, T. et al. Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application. ACS Appl. Mater. Interfaces 11, 46953–46964 (2019).
5. Van Cleve, T. et al. Tailoring Electrode Microstructure via Ink Content to Enable Improved Rated Power Performance for PtCo/HSC Based Polymer Electrolyte Fuel Cells. J Power Sources [submitted]
6. Fuel Cell Multi-Year Research, Development and Demonstration Plan. (2012).