Engineered Nanochannels in MXene Heterogeneous Proton Exchange Membranes Mediated by Cellulose Nanofiber/Sodium Alginate Dual Crosslinked Networks

2D architectures and superior physiochemical properties of MXene offer an exciting opportunity to develop a new class of polymer electrolyte membranes by controlling the stacking behavior of MXene nanosheets. However, assembling MXene nanosheets into macroscopic stable and high‐performance proton conductors is challenging. Here, a general strategy is reported for achieving stable and high‐performance MXene‐based heterogeneous proton conductors via crosslinked cellulose nanofiber/sodium alginate (CNF/SA). Through the coordination of calcium ions with 1D CNF/SA, MXene nanosheets with abundant hydrogen‐bonding networks are firmly locked into the heterogeneous polymer network, and meanwhile, the heterogeneous polymer chains are transformed from a randomly arranged state to a long‐range ordered arrangement, and such arranged polymer molecular channels collaborate with the tightly‐stacked MXene nanosheets jointly guide the stable and efficient proton conduction. Thus, the as‐built CNF/SA/MXene (CSM) composite membrane exhibits superior mechanical properties (164.7 MPa), proton conductivity (45.4 mS cm −1 ), power density (49.5 mW cm −2 ), and low open circuit voltage (OCV) decay rate (0.4 mV h −1 ). The design principle of 2D material anchoring through ionic‐cross‐linking and mixed‐dimensional assembly can inspire the synthesis of various ion exchange membranes for ion filtration, ion transport, ion sieving, and more.