Interfacial pH Dynamics in Lattice-Nitrogen Protonation to Ammonia

Abstract We recently discovered a new class of nitride MXenes (MNenes) that undergo lattice-nitrogen protonation to form ammonia under cathodic potentials. Unexpectedly, neutral electrolytes outperform both acidic and alkaline media, revealing the critical role of the interfacial proton environment in the formation of ammonia. However, how dynamic interfacial pH governs this mechanism remains poorly understood, because conventional ex situ characterization cannot probe local pH under operating conditions. Here, we develop an in-house in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy with phosphate speciation as a vibrational pH reporter to monitor in real time the interfacial pH near the Ti2NTx MNene catalyst during cathodic polarization. On one hand, we found that in a nominally neutral phosphate buffer, cathodic operation drives the Ti2NTx interface strongly alkaline despite the fact that the bulk remains neutral, whereas a mildly acidic electrolyte partially moderates this pH rise by increasing proton availability. On the other hand, a strongly acidic electrolyte minimizes interfacial pH changes but can also promote non-catalytic lattice-nitrogen protonation and release of NH3. Time-dependent measurements show that interfacial pH is governed by the balance between current-driven alkalinity generation and electrolyte-mediated neutralization. Overall, this in situ study shows that electrolyte pH and current density must be tuned together to maintain a fairly neutral interfacial environment favorable for lattice-nitrogen protonation to ammonia.