Proton-exchange membrane fuel cell (PEMFC) is a clean technology for automobile and stationary applications that converts a chemical energy of H₂ into electricity producing environmentally benign water as the only product.^[1] The main bottlenecks of PEMFC has been considered to be a sluggish oxygen reduction reaction (ORR) at cathode side, and the use of costly Pt metal catalysts. To circumvent this problem, various Pt-based alloys,^[2-4] core-shell structures^[5-6] and nanostructure catalysts^[7-9] have been extensively investigated to enhance Pt utilization and to improve an intrinsic catalytic activity.

Recent catalyst developments have been focusing on dispersing single metal atoms on supports to maximize the metal usage and to introduce the unique catalytic properties compared to the bulk counterparts.^[10-11] For instance, theoretical investigation suggested that Au doped graphene is highly active and stable ORR catalysts in contrast to Au (111).^[12] Furthermore, comparative experiments demonstrated that the support materials affect the catalytic activity and selectivity significantly^[13], indicating that choosing an appropriate combination of metal and support is crucial to design the single atom catalysts with desired catalytic properties.

In this work, we focus on two-dimensional materials as supports for ORR single atom catalysts because of their high surface area and capability of stabilizing metal atoms.^[14-17] We first investigate the stability of single metal atom on various two-dimensional materials, and then analyze ORR activity of stable metal-support combinations. We find that *OH and *OOH binding free energies are linearly correlated similar to the previous metal-based scaling relation, but we also show that rather smaller slope of the single atom catalysts scaling allows to design catalysts beyond the conventional volcano. We further investigated their possibility as hydrogen evolution reaction catalysts.

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[11] J. Liu, M. Jiao, L. Lu, H. M. Barkholtz, Y. Li, Y. Wang, L. Jiang, Z. Wu, D.-j. Liu, L. Zhuang, Nature communications 2017, 8, ncomms15938.

[12] S. Stolbov, M. Alcántara Ortigoza, The Journal of chemical physics 2015, 142, 154703.

[13] S. Yang, Y. J. Tak, J. Kim, A. Soon, H. Lee, ACS Catalysis 2017, 7, 1301-1307.

[14] C. Zhang, J. Sha, H. Fei, M. Liu, S. Yazdi, J. Zhang, Q. Zhong, X. Zou, N. Zhao, H. Yu, ACS nano 2017, 11, 6930-6941.

[15] S. Siahrostami, C. Tsai, M. Karamad, R. Koitz, M. García-Melchor, M. Bajdich, A. Vojvodic, F. Abild-Pedersen, J. K. Nørskov, F. Studt, Catalysis Letters 2016, 146, 1917-1921.

[16] X. Zhang, J. Guo, P. Guan, C. Liu, H. Huang, F. Xue, X. Dong, S. J. Pennycook, M. F. Chisholm, Nature communications 2013, 4, ncomms2929.

[17] G. Elumalai, H. Noguchi, H. C. Dinh, K. Uosaki, Journal of Electroanalytical Chemistry 2017.