In addition, the volcano-curve has been extensively reported as a forceful tool for screening and developing the electrocatalysts with the high catalytic performance. As depicted in Figure , showing the η OER with a descriptor of (∆𝐺 𝑂 * -∆𝐺 𝑂𝐻 * ) and the η ORR with a descriptor of ∆𝐺 𝑂𝐻 * , respectively. Obviously, a strong correlation can be found between η OER and (∆𝐺 𝑂 * -∆𝐺 𝑂𝐻 * ), implying that the second proton-coupled electron transfer step is the key step for the OER over most TMP4 catalysts.