Abstract:
A new highly graphitized N-doped carbon (HGNC) material combines high pyrrolic-N content with high porosity. As an electrochemical catalyst support [1,2], the support was found to have exceptional performance in Pt fuel cell catalysis and Ru Hydrogen Evolution Reaction (HER) applications. Supporting Pt on HGNC exhibits excellent oxygen reduction reaction activity for PEMFC with greatly improved electrocatalyst and support durability, the first to surpass both the DOE 2020-2025 mass activity durability targets for both catalyst and support [3]. For HER, Ru/HGNC catalyst shows a remarkably low overpotential of 9.6 mV (vs RHE) at 10 mA/cm2, which is far better than state-of-the-art Pt/C and also illustrates superb stability.
In order to understand the atomistic origins of HGNC support, we carried out a series of Quantum Mechanics (QM) calculations at the density functional theory (DFT) level. The binding energy of Pt nanoparticles on HGNC was calculated. Strong binding in HGNC was attributed to its high pyrrolic-N content when compared to other types of N-doped graphene support. The formation energy of different N-dopants was calculated. Pyridinic-N and graphitic-N have low formation energies while pyrrolic-N has higher energy of formation. The low synthesis temperature (650oC) of HGNC promotes the formation of less thermodynamically stable pyrrolic-N groups.
QM calculations were also performed to understand the improved catalytic activity of Ru on HGNC for HER [4]. For potential dependent free energies, we used the grand-canonical potential kinetics (GCP-K) method [5]. Compared to Ru/graphene, the pyrrolic groups of the Ru/HGNC lowers the barrier for the rate-determining Tafel step of HER. A low Tafel slope of 25-30 mV/dec was found experimentally and theoretically.
References:
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2) Fang, B., Kim, J.H., Kim, M.S., and Yu, J.S. (2013). Hierarchical nanostructured carbons with meso-macroporosity: design, characterization, and applications. Acc Chem Res 46, 1397-1406.
3) New Highly Graphitized N-Doped Carbon for Durable Pt Fuel Cell Supports, Satisfying DOE Durability Standards for Both Catalyst and Support, Ha-Young Lee; Cheol-Hwan Shin; Ted H. Yu; Alessandro Fortunelli; Tong-Hyun Kang; Byong-June Lee; Boris V. Merinov; William A. Goddard III; Jong-Sung Yu Submitted
4) “Ru-loaded pyrrolic-N-doped extensively graphitized porous carbon for high performance electrochemical hydrogen evolution”, Cheol-Hwan Shin, Ted H. Yu, Ha-Young Lee, Byeong-June Lee, Keong-jin Lee, Soonho Kwon, William A. Goddard III, and Jong-Sung Yu Submitted
5) “The Reaction Mechanism with Free Energy Barriers for Electrochemical Dihydrogen Evolution on MoS2” Y. Huang, R. J. Nielsen and W. A. Goddard III, J. Am. Chem. Soc., 2018, 140, 16773–16782.