Fig. 2. In situ NH₄⁺ formation at the catalyst surface.

a In situ SEIRAS spectra of different metal nitrides at −0.05 V in Ar-saturated natural seawater. b The proportion of NH₄⁺ species in the EDL versus N 2p band center for different metal nitrides. c The proportion of s-HB-H₂O versus the proportion of NH₄⁺ species in the EDL for different metal nitrides. d Online electrochemical DEMS measurements over Mo₂N catalyst in natural seawater. e The proportion of NH₄⁺ species in the EDL of the Mo₂N catalysts as a function of applied potentials. The proportion of NH₄⁺ in the EDL were extracted from the in situ SEIRAS spectra in Supplementary Fig. 23. f Schematic of the NH₄⁺ formation on the Mo₂N surface. g Chronopotentiometry curves of Mo operated at −10 mA cm⁻² in natural seawater with different concentration of NH₄Cl and NaCl, respectively. The pH of the bulk seawater was fixed at 7.84 by adding NaOH. h The proportion of s-HB-H₂O versus the proportion of NH₄⁺ species in the EDL for the Mo catalyst operated at −0.05 V in Ar-saturated natural seawater with different concentrations of NH₄Cl. The proportion of s-HB-H₂O and the proportion of NH₄⁺ were extracted from the in situ SEIRAS spectra in Supplementary Fig. 24.