The structural dynamics of the solid-liquid interfaces (SLEIs) determines the chemistry in all electrochemical processes. Here, by combining multiple operando synchrotron spectroscopies, we identify at the atomic level a general evolution of single-atom Ni at SLEIs into a near-free atom state in the electrochemical oxygen reduction reaction (ORR). We uncover that the single-atom Ni at SLEIs tends to be dynamically released from the nitrogen-carbon substrate and then forms a near-free, isolated-zigzag active site (Ni1(2-delta)+N2) during the reaction. This isolated-zigzag Ni1(2-delta)+N2 active site facilitates the adsorption and dissociation of O-2 into a crucial *O intermediate within the electrical double layers, realizing a highly efficient single-atom catalyst with the best ORR performance in alkaline solutions reported thus far. These findings may pave a general way for dissecting other important structural dynamic processes at SLEIs, such as hydrogen evolution, oxygen evolution, and CO2 reduction reactions.