Role of\nMass Transfer Phenomena in\nElectrochemical Nitrate Reduction: A Case\nStudy Using Ti and Ag-Modified Ti-Hollow Fiber Electrodes

Decentralized electrochemical\nreduction of nitrate into\nammonium\nis explored as a viable approach to mitigate nitrate accumulation\nin groundwater. In this study, tubular porous electrodes made of titanium\n(termed hollow fiber electrodes or HFEs) were successfully modified\nwith silver (Ag) nanoparticles through electrodeposition. Under galvanostatic\ncontrol and in acidic electrolyte, Ag deposition on Ti HFE resulted\nin an increase in the Faradaic efficiency for ammonium formation from\nlow concentrations of nitrate (50 mM), but only under reaction conditions\nof restricted mass transport. For conditions of favorable transport,\nfacilitated by an inert gas flow (Ar) exiting the pores, a higher\nnitrate conversion but an increase in hydroxylamine selectivity at\nthe expense of the ammonium selectivity are observed for Ti/Ag hollow\nfiber electrodes. For Ti/Ag electrodes, it is concluded that ammonium\nformation is prevented by effective removal of surface intermediates.\nRemarkably, for unmodified Ti hollow fiber electrodes, the Faradaic\nefficiency to ammonium is significantly improved when operated at\nhigh current densities and in conditions of high mass transport. The\nselectivity to liquid products even surpasses the selectivity of Ti/Ag\nelectrodes. These findings indicate that nitrate reduction to ammonium\nat Ti and Ti/Ag hollow fiber electrodes can be achieved at comparable\nrates but under distinctly different process conditions. In fact,\nfor Ti electrodes, operation at a lower applied potential compared\nto Ti/Ag electrodes is feasible, ultimately resulting in reduced energy\nconsumption. This study thus highlights the importance of controlling\nthe interfacial electrode environment, particularly when comparing\nand evaluating the effectiveness of electrode materials in electrochemical\nnitrate reduction. The study also reveals that transport phenomena\naffect electrode material-dependent activity–selectivity correlations\nand must be considered in ongoing material development efforts.