In the present work palladium nanoparticles were deposited on nitrogen-doped nanocarbons and these catalysts were used for electrochemical oxygen reduction reaction in acidic and alkaline solutions.¹ Three mesoporous N-doped carbon materials were procured from Pajarito Powder, LLC and employed as engineered catalyst supports (ECS) for Pd nanoparticles, which were compared to Vulcan carbon XC-72R. All the Pd nanoparticles were prepared in a single pot, and later dispersed on the four nanocarbon materials at three different loadings of 20-40%. From TEM images the average particle size was determined to be 3.9 ± 0.6 nm and from the XRD analysis the crystallite size was found to be 2.1 nm. Raman spectra showed that all the ECS materials had similar Raman characteristics, while Vulcan carbon showed more disordered graphitic structure for comparison. The unique porosity of the ECS materials was studied by BET and all these displayed similar microporosity, however the mesoporous area of these catalyst materials varied greatly. The BET surface area of these ECS materials was provided by the producer ECS-003604 (745 m² g^‒1), ECS-004201 (706 m² g^‒1) and ECS-004601 (853 m² g^‒1) and matched our measurements. The EDX analysis showed nitrogen content of 4.6 wt%, 3.7 wt% and 3.2 wt% for Pd40/ECS-003604, Pd40/ECS-004201 and Pd40/ECS-004601, respectively. By increasing the Pd loading on the support material, we could note the agglomeration of Pd particles, however this effect was more severe on Pd/Vulcan in comparison to the mesoporous Pd/ECS catalysts. The agglomeration of Pd nanoparticles could also be confirmed by CV curves, where positive shift in the PdO reduction peak was observed. This improvement of Pd nanoparticle dispersion could be from either N-doping or the mesoporous structure. During the CO-stripping experiments shifts in the CO-oxidation peak location were observed in alkaline solution, which could not be observed in the acidic media, suggesting that CO-stripping might be sensitive to porosity in the alkaline conditions. These catalyst materials were studied using the RDE method in 0.5 M H₂SO₄ and 0.1 M KOH solutions. The difference between catalyst materials in alkaline media did not prove to be significant (Figure 1a), however in acidic the bimodal mesoporous Pd/C catalyst gave highest specific activity for ORR. Furthermore, a single-cell anion exchange membrane fuel cell (AEMFC) test was conducted with the Pd/ECS and Pd/C materials of 40% Pd loading (Figure 1b). 30% increase in the peak power density could be observed after switching the support material from Vulcan carbon to Pajarito Powder variant. Pd-based catalyst materials prepared in this work showed excellent electrocatalytic activity for ORR displaying Tafel slope values of -60 mV, which corresponds to oxidized Pd and the rate-determining step for ORR is the transfer of the first electron to the O₂ molecule.

Reference:

1. M. Lüsi, H. Erikson, K. Tammeveski, A. Treshchalov, A. Kikas, H.-M. Piirsoo, V. Kisand, A. Tamm, J. Aruväli, J. Solla-Gullón, and J. M. Feliu, Oxygen reduction reaction on Pd nanoparticles supported on novel mesoporous carbon materials, Electrochim. Acta 394, 139132 (2021).