Intrinsic Proton Conduction in Metallic Ti3C2Tx MXene Membranes Revealed by Direct-Current Decoupling.

MXenes are metallic two-dimensional conductors with highly anisotropic electronic conductivity, yet their intrinsic ability to support proton transport has remained experimentally unresolved because dominant electron conduction obscures ionic contributions in conventional measurements. Here, we resolve this long-standing limitation by establishing a hydrogen-pumping-inspired direct-current methodology with electronic blocking layers, enabling unambiguous decoupling of ionic and electronic transport in metallic solids. Ti3C2Tx membranes exhibit intrinsic proton conductivity of 9 × 10-4 S cm-1 at 90 °C with a low activation energy of 0.26 eV, consistent with water-mediated hopping in confined nanochannels. The successful operation of fuel cells employing Ti3C2Tx membranes as solid electrolytes provides strong evidence of bulk proton conduction. This unusual coexistence originates from intrinsically decoupled transport pathways, in which delocalized electrons are confined to the conductive Ti3C2 layers, whereas protons are transported through hydrogen-bonded water networks in the interlayer galleries. Density functional theory (DFT) calculations further support this picture by showing that the metallic band structure of Ti3C2Tx is preserved across different surface terminations, consistent with the decoupled nature of proton and electron transport. Leveraging this coupled transport behavior, hydrogen separation is achieved at 80 °C through a mechanism fundamentally distinct from conventional molecular sieving. These findings redefine the functional classification of MXenes, establishing them as a new class of proton/electron mixed-conducting layered solids.