(Photo)Electrochemical Performance Predictions and Electrocatalytic Measurements for Wastewater Nutrient Recovery and Solar Hydrogen Production

Abstract

Increasing food demand due to the growing global population underlines the surging dependence on the Haber-Bosch process to produce ammonia (NH3), which is a vital fertilizer for high crop yields. This process is energy and carbon intensive due to H2 feedstock production and high temperature and pressure requirements. Motivated by these challenges, this doctoral dissertation evaluates the performance of: (a) (photo)electrochemical systems for nitrogen nutrient recovery from wastewater to enable renewable NH3 production and (b) solar hydrogen production pathways to mitigate H2 production energy requirements. Wastewater nitrates (NO3-) represent an untapped source for nutrient and energy recovery, with pH and NO3- concentrations varying substantially depending on the source. We investigated the effects of NO3- concentration (0.1M–1M) and pH (8–14) on the catalytic performance of polycrystalline Cu electrodes. Cyclic voltammograms indicate pH and concentration-dependent reaction kinetics and chronoamperometry experiments achieved maximum NH3 Faradaic efficiencies of 46% ± 11% for 1M NaNO3 at pH 14 at -0.55V vs RHE, with a minimum of 25% ± 6% for 1M NaNO3 at pH 8. Large trial-to-trial uncertainties motivate the application of in situ electrochemical impedance spectroscopy, where trial-to-trial changes in the electrochemically active surface area are more dominant for 0.1M NaNO3 solutions, whereas, for 1M NaNO3, variations in the facet composition of the Cu catalyst surface play a significant role. High energy requirements needed to drive NO3- reduction to NH3 hinder its competitiveness with other treatment technologies. To probe solar-driven wastewater nutrient recovery, we developed a numerical model analogous to detailed-balance calculations for photovoltaic cells, with additional electrochemical loads. This model quantifies the dependence of solar-to-chemical efficiencies on light-absorber band gaps, electrocatalytic kinetic parameters, competing oxygen reduction and hydrogen reduction, and NO3- concentrations. With a single light-absorber and state-of-the-art catalysts, optimal solar-to-chemical efficiencies of 7% and 10% are predicted for NO3- reduction to NH3 and N2O respectively. Equivalent circuit modeling was also applied for solar water splitting to produce hydrogen and oxygen. We studied Z-scheme photocatalytic suspension reactors in the presence of aqueous redox shuttles to facilitate 3-D photoelectrochemistry while avoiding the co-production of hydrogen and oxygen on the same photocatalyst. The modeling framework was used to understand the role of competing reactions and mass-transfer effects on solar-to-hydrogen efficiencies for individual and ensembles of light absorbers mimicking a photocatalytic particle suspension. Parameters of interest are the electrocatalytic reaction kinetics, the limiting current densities, the redox shuttle thermodynamic potentials, the presence of a selective coating and the number of light absorbers considered in the ensemble. The extent of the competing reactions effect was found to be dependent on the redox shuttle thermodynamic potentials: for small potentials, the efficiencies did not depend on the hydrogen oxidation implemented; for large potentials, both hydrogen oxidation and redox shuttle reduction affected the solar-to-hydrogen efficiency. In addition, increasing the number of light absorbers for severely mass-transport limited cases resulted in an optimum efficiency due to the additive gains of having multiple absorbers competing with the downside of light-limited reduction in operating potentials. Overall, electrocatalytic measurements quantify sensitivity of nitrate reduction to pH, concentration, and surface composition on copper electrodes, and motivate future investigations of ammonia recovery in wastewater streams with more complex compositions. Equivalent-circuit based models that account for competing reactions and mass-transfer limitations provide a powerful framework to predict performance limits for photocatalytic and photoelectrochemical systems.