Optimizing Electrodeposited Manganese Oxide at Carbon Cloth Electrodes for Harvesting Salinity-Gradient Energy

Electrical power can be generated from the controlled mixing of salinity gradients when solutions of differing salt concentration bathe opposing electrodes in a flow-cell configuration. The power density of such salinity-gradient cells is enhanced by moving beyond the double-layer capacitance of high-surface-area carbons to electrode materials that store charge via pseudocapacitance (e.g., manganese oxides; MnOx). We investigate the effects of MnOx loading on salinity-gradient power production, using anodic electrodeposition to coat carbon cloth (CC) electrodes with nanostructured Akhtenskite-type MnOx at controlled incremental mass loadings. Uniform MnOx deposition at exposed CC surfaces is confirmed by scanning electron microscopy with elemental mapping. Power-density measurements with the resulting MnOx@CC electrodes in a salinity-gradient cell fed by low (0.02 M NaCl) and high (0.5 M NaCl) concentration solutions show a positive correlation with MnOx loading, reaching a competitive peak average power density of 0.221 ± 0.001 mW cm-2 at 1.90 mg cm-2 MnOx. By comparing flow-cell data with results from half-cell electroanalytical characterization of individual electrodes, we show that salinity-gradient performance is ultimately limited by resistive losses in the modestly conductive MnOx coating at higher mass loading/thickness.