Unsupported Pt3ni Aerogels As High Stability Catalysts for PEFC Anodes Under Fuel Starvation Conditions
Authors: Shimizu Ryo , Henning Sebastian , Herranz Juan , Schmidt Thomas J. , Kühn Laura , Eychmüller Alexander , Kakinuma Katsuyoshi , Uchida Makoto 
Published: February 2020

In polymer electrolyte fuel cells (PEFCs), both anodic and cathodic Pt/C catalysts suffer from significant corrosion of the carbon support during the high potential excursions (> 1 V vs. the reversible hydrogen electrode) concomitant to PEFC operation, which limit the device’s service life. Therefore, further cost reductions could be achieved using catalysts with higher durability, also allowing lower Pt loadings. In this respect, anode catalysts with high stability in cases of gross hydrogen starvation events constitute a pressing need. The latter fuel starvation events are caused by the blockage of the hydrogen gas flow in an anode flow field by water droplets, leading to anode potential increases from ≈ 0 V to ≥ 1.5 V.1 At these potentials, significant corrosion of the carbon support in commercial Pt on carbon black (Pt/CB) catalysts and concomitant performance decreases have been reported.2 Motivated by our previous study, in which an unsupported Pt3Ni aerogel demonstrated remarkable corrosion stability as a PEFC cathode catalyst,3 in this work, the applicability of the former material on the anode side was investigated.
Pt3Ni aerogel catalyst was synthesized according to the steps described in reference 4, and commercial Pt/CB (47 wt% Pt, TKK, TEC10E50E) and Pt on graphitized carbon black (Pt/GCB, 30 wt% Pt, TKK, TEC10EA30E) catalysts were used as benchmarks. The hydrogen starvation events were simulated by an accelerated stress test (AST) performed at 80°C, 100% RH, ambient pressure, and H2 / N2 flows of 100 ml/min at cathode and anode, respectively. Under these conditions, 250 potential cycles in a square wave voltammetry pattern between 0 and 1.5 V with a holding time of 10 s at each potential were applied to the anode side (Figure 1).
Figure 2 shows polarization curves of membrane electrode assemblies (MEAs) utilizing Pt3Ni aerogel, Pt/CB or Pt/GCB as anode catalysts (with loadings of ≈ 0.05 mgPt/cm2
anode) at beginning and end-of-life (BOL, EOL). The agreement of the BOL data indicates that the performance attained with the aerogel CLs is identical to that obtained with the commercial benchmark catalyst. Interestingly, the AST caused a large performance deterioration for the MEA using Pt/CB or Pt/GCB at the anode, whereas the cell using Pt3Ni was not affected at all.
The reasons for the former degradation were investigated using electrochemical impedance spectroscopy and hydrogen pump tests to decouple the overpotential contributions to the materials’ BOL and EOL performance. Figure 2 displays the hydrogen oxidation/evolution polarization curves recorded in these H2-pump experiments, whereby the measured potential was corrected for the cell resistance (R). H2-pump polarization curves in Figure 3A recorded on the Pt3Ni aerogel at BOL and EOL almost completely overlay, indicating minimal kinetic and/or mass transport losses. On the other hand, the overall potential increased for Pt/CB and Pt/GCB (Figures 3B and 3C). The EOL polarization curves for Pt/CB and Pt/GCB deviate from the purely kinetic response derived from the fitting to the Butler-Volmer equation at current densities above 0.5 A/cm2 (see insets in Figures 3B and 3C), pointing at significant mass transport limitations by the end of the AST.
To understand the reason for the profound increases in ηtx,anode of Pt3Ni, Pt/CB and Pt/GCB anodes, their catalyst layer structure was investigated by scanning ion microscopy and compared to that of the initial state (Figure 4). For the images of Pt3Ni catalyst surfaces, no change was observed. On the other hand, the Pt/CB and Pt/GCB catalysts at EOL were collapsed and became dense compared to their initial structure, thereby explaining the poor gas transport in the catalyst layers.
Ultimately, this approach leads to the conclusion that the majority of the performance losses are related to changes in the anodic CL mass transport properties. It also illustrates the superiority of the Pt3Ni aerogel vs. Pt/CB and Pt/GCB as a PEFC anode catalyst.

Acknowledgement

This work was carried out in the Electrochemistry Laboratory of Paul Scherrer Institute and funded by the Swiss National Science Foundation (20001E_151122/1), the German Research Foundation (EY 16/18-1) and the European Research Council (ERC AdG 2013 AEROCAT).

References

1) P. Rodriguez, T.J. Schmidt, Encyclopedia of Applied Electrochemistry, Springer New York, New York, NY, 1606(2014).
2) J. Speder, A. Zana, I. Spanos, J.J.K. Kirkensgaard, K. Mortensen, M. Hanzlik, M. Arenz, J. Power Sources 261, 14(2014).
3) S. Henning, J. Herranz, H. Ishikawa, B.J. Kim, D. Abbott, L. Kühn, A. Eychmüller, T.J. Schmidt, J. Electrochem. Soc. 164, F1136(2017).
4) S. Henning, L. Kühn, J. Herranz, J. Durst, T. Binninger, M. Nachtegaal, M. Werheid, W. Liu, M. Adam, S. Kaskel, A. Eychmüller, T.J. Schmidt, J. Electrochem. Soc. 163, F998(2016).

Figure 1