Ammonium Cations for Enhancing Hydrogen Evolution Reaction: A GC-AIMD study*

Hydrogen evolution reaction (HER) holds great promises for sustainable production of hydrogen as a clean fuel and energy carrier. In HER, the Volmer step (H₂O + e⁻ + * → OH⁻ + *H) is regarded as the rate-limiting step under neutral pH. Furthermore, cations are expected to play a pivotal role in HER. We present here results of Grand Canonical ab initio molecular dynamics (GC-AIMD) simulations in which we have examined the dissociation of H₂O on the Pt(111) electrode at a constant potential of -0.5 V vs RHE, with both metal and nonmetal cations. We found that in the presence of Na⁺ the grand canonical activation free energy (ΔΩ^†) of the Volmer step is 1.07 eV. The resulting OH⁻ is stabilized by Na⁺ forming NaOH and is transported away from the electrode via proton transfer within the H₂O network. On the other hand, with the nonmetal cation NH₄⁺, ΔΩ^† of the Volmer step reduces to 0.66 eV. We trace this reduction of the activation barrier with NH₄⁺ to the strong electrostatic interaction between the cation and H₂O and OH⁻, thanks to the directional electric field created by the cation which leads to proton transfer from NH₄⁺ to OH⁻ to form NH₃…H₂O complex. NH₄⁺ is also found to act as a proton donor for *H formation on Pt electrodes (ΔΩ^† = 0.81 eV). The lower activation energy of the Volmer step and proton donation to the electrode by NH₄⁺ makes it a better cation than Na⁺ for enhancing HER. Ongoing experiments [1] validate our findings and provide valuable input in our quest for descriptors for HER.

[1] K. Shi and X. Feng, Private Communications.