To understand how K + further suppressed HER on SnO2, Au and Cu in strongly acidic solutions, we measured linear sweeping voltammetry (LSV) curves of a polycrystalline Au rotating disk electrode (RDE) in different electrolyte solutions (fig. ). The onset potential of HER in 0.1 M HOTf + 0.4 M KOTf is close to that in HOTf solution (0.1 M or 0.5 M), while in the K + -containing solution, a plateau of current density appears from -0.6 V vs SHE. The current density increases again at -1.3 V vs SHE, close to the onset potential of HER in 0.4 M KOTf, indicating reduction of water molecules starts at this potential. The plateau is about 6% higher than the limiting diffusion current density of hydronium reduction calculated according to the Levich equation. In sharp contrast, in 0.1 M HOTf, no plateau corresponding to the diffusion limitation of hydronium reduction was observed. LSV curves of SnO2/C on glassy carbon RDE (fig. ) and CV curves of SnO2/C on GDE in N2 atmosphere (fig. ) show similar trends to curves in fig. , indicating the suppression of HER by K + in strongly acidic medium is a universal effect.