Etching-Chemistry-Driven Ruthenium Doping on Ti ₃ C ₂ T _<i>x</i> MXene for Optimizing Electrochemical Performance

We demonstrate that the etching chemistry used during MXene synthesis from Ti₃AlC₂ MAX phase significantly influences surface functionalization and structural vacancies, which in turn affect ruthenium (Ru) ion interactions. Using hydrofluoric acid (HF) and ammonium bifluoride (NH₄HF₂) as etchants, we obtained MXene surfaces with distinct functional groups and Ti vacancies that impact Ru ion interactions and electrochemical performance. Both MXene variants (labeled MX-(H) and MX-(N), respectively) exhibited negative zeta potentials in their pristine state, but upon the addition of Ru the zeta potential for MX-(H) reached 12.9 mV while that for MX-(N) remained negative at -6.4 mV. This adsorption resulted in a 14.4-fold increase in the specific capacitance of MX-(H)/Ru compared to pristine MX-(H), whereas MX-(N)/Ru exhibited only a 4.4-fold increase over its pristine counterpart. X-ray diffraction analysis identified the formation of ammonium titanium oxide fluoride, (NH₄)₃TiOF₅, on MX-(N), which likely contributed to its reduced Ru adsorption. X-ray photoelectron spectroscopy suggested the presence of Ti vacancies in both MXene variants; however, their behavior toward Ru accommodation differed markedly, with MX-(H) showing the most obvious shift in the Ti 2p peak in the XPS survey spectrum, while MX-(N) showed the most obvious shift in the C 1s peak. Electron paramagnetic resonance spectroscopy further demonstrated a distinct alteration in the spectral signatures of MX-(H) upon Ru addition, in contrast to the negligible changes in MX-(N), indicating effective passivation of the Ti defect sites in MX-(H) via vacancy-assisted Ru doping. Cyclic voltammetry showed that Ru-incorporated MX-(H) nanocomposites exhibit more efficient redox-active sites, as reflected in their higher capacitance values. These findings highlight the pivotal role of MXene surface chemistry in controlling cation adsorption, providing valuable insights for the rational design of high-performance electrodes.