Improved Near-Neutral Hydrogen Evolution Reaction Kinetics through Electrolyte Engineering with the Efficient Hydrogen Source NH₄⁺

A near-neutral HER process suffers from sluggish kinetics. Many efforts have been focused on the design of advanced electrocatalysts. However, the field of electrolyte engineering has rarely been investigated. Considering the complicated ion composition of electrolytes in a near-neutral environment, this work investigated the HER performance of several buffer electrolytes composed of different charged hydrogen sources. The results indicated that a positively charged hydrogen source, namely, NH4+, possessed a superior HER performance to other buffer electrolytes. Under the condition of high concentration, Tafel slopes at 10 and 31 mA·cm–2 were 61 and 84 mV·dec–1, respectively, on the Pt/C catalyst. At an overpotential of 530 mV, the current density of the NH4+ electrolyte was 1000 mA·cm–2 in contrast to only 240 mA·cm–2 for the phosphate buffer solution (PBS) electrolyte. Furthermore, to take a deep perspective into the HER mechanism under a near-neutral environment, based on the experimental values and grand canonical DFT, this work designed a two-step thermodynamic circle to calculate the formation energy of ionic hydrogen sources needed to be transferred from a bulk electrolyte solution to the vicinity of a charged electrode. The result clearly demonstrated that the negatively charged hydrogen sources could not spontaneously approach the Pt electrode surface under certain cathode overpotentials. This work further implemented ab initio molecule dynamics (AIMD) to investigate solvated NH4+ and found that the desolvation process was facilitated by the cathode potential. The proton dissociation process was studied through constrained AIMD. The results clearly showed that the proton dissociated from NH4+ would be directly transferred to the electrode surface, while the proton dissociated from other hydrogen sources would be captured by a hydrogen bond network of water. This discrepancy demonstrated a possibility that NH4+ could directly participate in HER under a near-neutral environment or that the proton dissociation efficiency of NH4+ near the cathode was superior to other hydrogen sources.