fig2

Figure 2. (A) Top and side views of M-N₄-CNT(6, 6); (B) The 28 transition metal atoms considered in this study; (C) Comparison of the ∆G_H* values for M-N₄-CNT(4, 4), M-N₄-CNT(5, 5) and M-N₄-CNT(6, 6); (D) Gibbs free energy diagram of the HER on M-N₄-CNT(4, 4); (E) Volcano plot of the exchange current density as a function of the ∆G_H* on M-N₄-CNT(4, 4). ∆G_H*: Gibbs free energy of hydrogen adsorption; CNT: carbon nanotube; HER: hydrogen evolution reaction.