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Understanding the Increase in H 2 /Air PEM Fuel Cell Performance of Pt Based Cathode Catalysts with the Tailored Pore Structure of Mesoporous Graphitic Spheres

2023; Institute of Physics; Volume: MA2023-02; Issue: 37 Linguagem: Inglês

10.1149/ma2023-02371746mtgabs

2152-8365

Roberta Karla Francesca Della Bella, Alexander Gunnarson, Hannaneh Hosseini, Paul Paciok, Marc Heggen, Carlos Cuadrado‐Collados, Matthias Thommes, Ferdi Schüth, Hubert A. Gasteiger,

Electrocatalysts for Energy Conversion

To reach its climate neutrality goal by 2050, the European Union has agreed on ceasing the sale of new petrol and diesel-driven vehicles by the end of 2035. Therefore, interest in carbon-neutral transport options, such as battery-electric vehicles (BEV) or fuel-cell-electric vehicles (FCEV) has tremendously increased. Especially for heavy-duty applications, proton exchange membrane fuel cells (PEMFCs) can be a viable solution, as they exhibit a higher gravimetric power density compared to BEVs. [1] However, cost and supply constraints of the catalyst active material, i.e. platinum, are still a major hurdle to make PEMFCs competitive with incumbent technologies. While this has been addressed by a reduction of the Pt loading, this induces some major drawbacks regarding the overall performance due to the decrease in catalyst activity towards the oxygen reduction reaction (ORR) and the increase in proton and oxygen mass transport resistances to the Pt surface. [2] It has been shown that those issues can be mitigated by using cathode catalyst materials with a mesoporous carbon support, as they exhibit a high ORR activity while additionally facilitating the transport of protons and gaseous oxygen to Pt nanoparticles that are located inside the pores of the carbon support particles. [3] Previous studies have demonstrated that by using a multi-step catalyst synthesis procedure where Pt nanoparticles are incorporated into a mesoporous graphitic sphere (MGS) support and subjected to a pore-confined growth by a subsequent high-temperature treatment, a precisely defined catalyst structure is achieved with a good H 2 /air performance as well as high stability towards catalyst degradation. [4] In order to correlate the impact of a precisely defined mesoporous carbon structure on the overall H 2 /air performance, single cell tests were performed using 5 cm 2 membrane electrode assemblies (MEAs) with cathode loadings of 0.1 mg Pt cm MEA -2 using different commercially available 20 wt.% Pt/C catalysts as well as a 28 wt.% Pt/C catalyst based on lab-scale synthesized carbon supports: i) Ketjen black (KB, TEC10E20E, TKK); ii) Vulcan (Vu, TEC10V20E, TKK); and, iii) mesoporous graphitic spheres (MGS). The latter was produced following the procedure by Knosalla et al. [5] A full MEA characterization was included to measure the electrochemically active surface area ( ECSA ) by CO-stripping, the H 2 /air performance at different operating conditions (high and low relative humidity and high pressure), the H 2 /O 2 performance, and the O 2 and H + transport resistance in the cathode by limiting current measurements and electrochemical impedance spectroscopy (EIS), respectively. Finally, the catalyst morphology has been studied ex/situ using three/dimensional reconstruction by electron tomography and by gas adsorption measurements (argon at 87 K and water at 298 K). It was found, that the mesoporous carbon support significantly improves the H + and O 2 transport, resulting in an improved H 2 /air performance compared to both commercial catalyst materials (see. Fig. 1 MGS vs. KB or Vu). Interestingly, the MGS retains also a high ORR catalyst mass activity (i mass ≈ 400 A/g Pt ) which is similar to the one of Pt/KB cathode catalysts (with higher internal microporous structure), leading to the good H 2 /air performance also at low current densities. The overall optimal performance of Pt/MGS suggests that these materials have a defined porous internal network structure which combines the beneficial effect of a facilitated O 2 and H + transport through larger mesopores while still shielding the Pt surface from ionomer contact by incorporation of Pt nanoparticles into smaller micropores. References: D. A. Cullen, K. C. Neyerlin, R. K. Ahluwalia, R. Mukundan, K. L. More, R. L. Borup, A. Z. Weber, D. J. Myers, and A. Kusoglu, Nat. Energy, 6 (5), 462-474 (2021). A. Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7 , 1127-1137 (2016). T. Lazaridis and H. A. Gasteiger, J. Electrochem. Soc., 168 (11), 114517 (2021). J. C. Meier, I. Katsounaros, C. Galeano, H. J. Bongard, A. A. Topalov, A. Kostka, A. Karschin, F. Schüth and K. J. J. Mayrhofer, Energy Environ. Sci., 5 , 9319-9330 (2012). J. Knosalla, J. Mielby, D. Göhl, F.R. Wang, D. Jahlapoor, A. Hopf, K. J. J. Mayrhofer and F. Schüth, ACS. Appl. Energy Mater., 4 , 5840-5847 (2021). Acknowledgement: We gratefully acknowledge funding from the German Federal Ministry for Economic Affairs and Energy (BMWi) under the funding scheme POREForm (funding number 03ET B027C). Figure 1: Measured single-cell H 2 /air differential flow PEMFC performance at 80°C, 95% RH, and 170 kPa abs using MEAs with Pt/C cathodes (0.1 mg Pt /cm 2 MEA ) based on different carbon supports: solid Vulcan carbon (Vu), porous Ketjen black (KB), or mesoporous graphitic spheres (MGS). Figure 1