Defect engineering of zirconium-based metal-organic frameworks via heterovalent indium(III) substitution for enhanced proton conductivity.

Aliovalent metal substitution provides a precise means of engineering defects in Zr-based metal-organic frameworks (Zr-MOFs). However, previous studies have focused largely on In-based frameworks, and sulfur-containing conjugated Zr-MOFs remain underexplored. Herein, a series of gradient In3+-substituted Zr-DTD MOFs (denoted In@1-X, X = 5-40 mol%) (H2DTD = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid) was solvothermally prepared; valence mismatch between In3+ and Zr4+ spontaneously generates tunable oxygen vacancies on Zr₆ clusters. Comprehensive characterization confirmed that moderate In3+ doping (10 mol%) optimized both porosity and hydrophilicity, yielding a Brunauer-Emmett-Teller (BET) surface area of 1032 m2⋅g-1, a water contact angle of 56.8°, and a water uptake of 358 mg⋅g-1 at P/P0 = 0.9. Electrochemical measurements revealed a volcano-shaped relationship between defect density and proton conductivity. At 100 °C and 98% relative humidity (RH), In@1-10% exhibited a conductivity of 1.4 × 10-2 S⋅cm-1, approximately twice that of pristine Zr-DTD and maintained conductivities on the order of 10-2 S⋅cm-1 at 100 °C over 68-93% RH. Arrhenius analysis indicated a transition from Grotthuss to Vehicle transport upon heating in the moderately substituted samples, whereas excessive In loading (30 or 40 mol%) disrupted the H-bonding network and reduced conductivity. All samples retained good water, acid-base, thermal, and electrochemical stability. These findings establish aliovalent substitution as a straightforward strategy for tuning pore structure, surface affinity, and proton transport in Zr-MOFs through controlled node-defect engineering.