Understanding the Increase in H2/Air PEM Fuel Cell Performance of Pt Based Cathode Catalysts with the Tailored Pore Structure of Mesoporous Graphitic Spheres

Abstract

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 H2/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 H2/air performance, single cell tests were performed using 5 cm2 membrane electrode assemblies (MEAs) with cathode loadings of 0.1 mgPt cmMEA -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 H2/air performance at different operating conditions (high and low relative humidity and high pressure), the H2/O2 performance, and the O2 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 O2 transport, resulting in an improved H2/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 (imass ≈ 400 A/gPt) which is similar to the one of Pt/KB cathode catalysts (with higher internal microporous structure), leading to the good H2/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 O2 and H+ transport through larger mesopores while still shielding the Pt surface from ionomer contact by incorporation of Pt nanoparticles into smaller micropores.