Influence of Mesoporous Carbon Support on Pt Stability for PEMFC Cathode

Introduction of the fuel cell technology into the market requires substantial enhancement of Pt long-term stability. It is meanwhile accepted that electrode efficiency is strongly influenced by the nature of the catalyst support that should allow optimal distribution and stabilization of the catalyst nanoparticles, anchorage of functional groups and facile mass transport of reactants and products. One promising strategy for improving long-term stability of nano-dispersed catalyst based on using mesoporous carbon as hosting sites [1].

In this work, Pt was deposited on different commercial carbon materials by chemical reduction of hexachloro-platinic acid in formaldehyde at 80°C for 1 h. Following carbon materials were selected for preliminary screening step: Vulcan XC-72R (Cabot), High Surface Graphite Timrex HSAG300 (Timcal), Ordered Mesoporous Carbon OMC (ACS Materials), Graphite Nanoparticles GNP10 (ACS Materials) and mesoporous Graphite Nanoparticles GNP500 (Sigma Aldrich). The Pt loading on carbon support was fixed to 20 and 40 wt% for RRDE and GDE experiments, respectively. Commercial 40 wt% Pt/C catalysts from Johnson Matthey (JM) and Heraeus were investigated for comparison.

The structural characterization of the as-prepared catalysts was performed by means of XRD, TGA, and TEM. Best results in terms of Pt particle size distribution were obtained on OMC and GNP500 [see Fig. 1].

Electrochemical activity of Pt/C catalysts was studied in O₂-saturated 0.5 M H₂SO₄ electrolyte by means of rotating ring disk electrode (RRDE) and gas diffusion electrode (GDE) cell experiments. The total catalyst loading on glassy carbon and gas diffusion layer was about 50 μg_Pt cm^-2 and 1 mg_Pt cm^-2, respectively. Electrochemical surface area (ECSA) was calculated by integrating hydrogen desorption region in cyclic voltammogramms at 40 mV s^-1. GDE were prepared by ultrasonic assisted spraying a Pt/C + 10 wt% Nafion + 20 wt% PTFE suspension in 1:1 H₂O:iso-Propanol onto a carbon paper (Freudenberg, H2315 IX91). GDE experiments were performed in an acryl glass cell, in which gas diffusion and reaction layer was exposed to the gas and liquid phase, respectively. GDE geometric active electrode area was 0.5 cm^-2. A Pt wire and reverse hydrogen electrode (RHE) was used as counter and reference electrode, respectively. The accelerated degradation tests (ADT) consisted of 10,000 cycles between 0.4 and 1.4 V at dE/dt=1 V s^-1 and room temperature.

Since self-adherence of some carbon materials such as OMC was insufficient on glassy carbon RRDE, preliminary experiments were devoted to optimization of binder concentration. Best results in terms of both adherence and overpotential towards ORR were obtained with 1 wt% Nafion. Activity of Pt/C for ORR @ 800 mV decreased in following order:

- Pt/GNP500 < Pt/Vulcan < Pt/HSAG < Pt/OMC < Pt/GNP10 for specific activity,  

- Pt/OMC < Pt/GNP10 < Pt/HSAG < Pt/GNP500 < Pt/Vulcan for mass activity.

Typical Tafel slope regions of 60 and 120 mV dec^-1were observed for all systems. The formation of hydrogen peroxide at the ring was estimated to be smaller than 0.1%. The highest stability during ADT was achieved by Pt/OMC with about 30% ECSA retention (see Fig. 2).

Activity and stability evaluation of 40 wt% Pt/Vulcan, Pt/ GNP500 and Pt/OMC under GDE conditions will be also presented.

Acknowledgements

„Bundesministerium für Wirtschaft und Technologie (BMWi) via AiF“ (17955 BG/3) is greatly acknowledged for financial support.

References

[1]   C. Galeano, J.C. Meier, V. Peinecke, H. Bongard, I. Katsounaros, A. A. Topalov, A. Lu, K. J. J. Mayrhofer, F. Schüth, J. Am. Chem. Soc. 134, 20457−20465, (2012).