Ammonia (NH₃) is vital for synthesizing fertilizers and has gained great attention as a carbon-free hydrogen carrier and a hydrogen-rich fuel. Electrochemical ammonia synthesis from nitrate in a water-fed polymer electrolyte membrane electrolyzer is an innovative approach to wastewater treatment. However, the major hurdles to practical implementation are competing hydrogen evolution reactions (HERs) and constrained catalytic efficiency. Herein, we demonstrate the use of polyvinylpyrrolidone (PVP)-modified ruthenium (Ru) nanoparticles as a strategy to drive the desired reaction of nitrate to ammonia. The particle size of Ru was controlled by PVP, enhancing the metal-utilization efficiency and the electrochemical active surface area. PVP modification was found to alter the electron density on Ru, suppressing the HER by increasing the energy barrier of hydrogen coupling to form H₂, while promoting absorbed hydrogen (H*) formation, facilitating the hydrogenation of intermediates to ammonia. Benefiting from the combined effects, PVP-10 wt % Ru/C achieved an ammonia production rate of 3800 μg·mg_Ru^–1·h^–1, compared to 590 μg·mg_Ru^–1·h^–1 for 40 wt % Ru/C at 2 V.

Electrochemical conversion of NO from gaseous pollutants into ammonia using abundant and cost-effective catalyst materials holds great promise for pollutant abatement and for advancing a more closed, sustainable nitrogen cycle. However, regulating product selectivity remains challenging because NO reduction involves complex multielectron/proton pathways. Here, we report two different crystal phases of MoS2 (2H and 1T′) exhibiting prominent activity in the electrochemical NO reduction reaction (NORR), but showing different selectivities. The faradaic efficiency of ammonia reaches 86% over 2H-MoS2, outperforming 1T′-MoS2 (31%) at 2.1 V. In contrast, 1T′-MoS2 displays higher selectivity towards N2, especially at a lower cell voltage (50% at 1.7 V). Kinetic and spectroscopic analyses further suggest phase-dependent rate-control characteristics, consistent with distinct pathway preferences on 1T′ versus 2H. Overall, these results demonstrate that NORR activity and selectivity can be efficiently tuned by choosing the appropriate MoS2 phase, providing a simple strategy to tune product selectivity in complex multistep reactions.

The transition to zero-carbon fertilizers challenges conventional ammonia production via the Haber-Bosch process. Electrochemical ammonia synthesis offers a sustainable alternative using only water, electricity, and nitrogen from waste streams such as nitrate and NOx. This dissertation employs a polymer electrolyte membrane (PEM) electrolyzer and addresses key cost and efficiency drivers through catalyst design, mechanistic analysis, and cell-level engineering.

Ru/C catalysts were optimized with polyvinylpyrrolidone (PVP), reducing Ru loading from 40 to 10 wt.% while enhancing NH₃ faradaic efficiency, electrochemical surface area, hydrogen binding, and wettability. Earth-abundant MoS₂ catalysts were phase-engineered to steer proton-electron transfer pathways for selective NO reduction. A CO-mediated poisoning strategy was developed to suppress the competing hydrogen evolution reaction, improving NH₃ selectivity.

Proof-of-concept C–N coupling for urea synthesis from bicarbonate and nitrate was demonstrated using gas-diffusion electrodes. Finally, a novel operando ATR-IR cell was designed to probe reaction mechanisms under realistic conditions, bridging the gap between batch-cell studies and PEM electrolyzer operation.

Carbon-supported nickel and nitrogen co-doped (Ni-N-C) catalysts have been extensively studied as selective and active catalysts for CO2 electroreduction to CO. Most studies have focused on adjusting the coordination structure of Ni-Nx active sites, while the impact of the carbon supports has often been overlooked. In this study, a series of Ni-N-C catalysts on different carbon supports, including carbon black (CB), multi-walled carbon nanotubes (CNT), and activated nitrogen-doped biochar (ANBC), were synthesized using a ligand-mediated method. The effect of the carbon support on the electrocatalytic performance for CO2 reduction was investigated at both low current densities, in a H-cell, and high current densities, in a MEA electrolyzer. All of the prepared Ni-N-C catalysts show good faradaic efficiencies (FE) toward CO production (up to ∼90 %), however, the onset potentials and partial current densities for CO production vary greatly. The textural properties of the carbon support and the distribution of Ni-Nx active sites on the carbon support are demonstrated as the main factors behind the performance differences. In particular, hierarchical porous structures with a large specific surface area are helpful to facilitate mass transport and improve the dispersion of active sites, which allows for a better CO2 reduction performance of Ni-N-ANBC compared to Ni-N-CB and Ni-N-CNT. This study demonstrates the importance of the carbon support for Ni-N-C catalysts and provides new insights into the design of efficient Ni-N-C catalysts for the CO2RR.

Electrochemical ammonia (NH₃) synthesis from nitrate (NO₃⁻) offers a promising greener alternative to the fossil-fuel-based Haber-Bosch process to support the increasing demand for nitrogen fertilizers while removing environmental waste. Previous studies have mainly focused on designing catalysts to promote the direct conversion (NO₃⁻ → NH₃) while suppressing the two-step pathway (NO₃⁻ → NO₂⁻ → NH₃). We hypothesize that efficient nitrate reduction is possible on simple catalysts by instead promoting the two-step reaction and using chemical reactor principles in a membrane electrode assembly, despite NO₂⁻ intermediates. Here, we use an unmodified copper catalyst and control reactivity through current density, flow rate, and electrolyte recycling. Balancing the electrolyte flow rate with current density results in ideal residence times for NO₂⁻, allowing for 91% FE_NH3 in a 5 cm² electrolyzer with a NO₃⁻ to NH₃ partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO₃ reduction reaction, even for simple catalysts.

The influence of nanostructures and interaction of Sn and Ir in oxygen evolution catalysts in a polymer electrolyte membrane electrolyzer were investigated. For this aim, two synthesis methods, namely, the one-step solution combustion method and the precipitation-deposition method with sodium borohydride reduction, were evaluated to prepare distinct nanostructures. Sn addition to Ir-based oxygen evolution reaction catalysts has been reported to yield materials with higher activity; however, in our case, this was observed only for Sn/Ir catalysts prepared by the precipitation-deposition method. The nanolayer of Sn/SnO₂ deposited over metallic Ir particles was identified to enhance the interfacial contacts, resulting in synergistic interactions. By deconvolution of the polarization curves into constituting contributions, the performance improvement was attributed to the higher exchange current density of the Sn/Ir powder as a consequence of a higher number of surface reaction sites created by the Sn-Ir interactions.

Direct electroreduction of nitric oxide offers a promising avenue to produce valuable chemicals, such as ammonia, which is an essential chemical to produce fertilizers. Direct ammonia synthesis from NO in a polymer electrolyte membrane (PEM) electrolyzer is advantageous for its continuous operation and excellent mass transport characteristics. However, at a high current density, the faradaic efficiency of NO electroreduction reaction is limited by the competing hydrogen evolution reaction (HER). Herein, we report a CO-mediated selective poisoning strategy to enhance the faradaic efficiency (FE) towards ammonia by suppressing the HER. In the presence of only NO at the cathode, Pt/C and Pd/C catalysts showed a lower FE towards NH₃ than to H₂ due to the dominating HER. Cu/C catalyst showed a 78 % FE towards NH₃ at 2.0 V due to the stronger binding affinity to NO* compared to H*. By co-feeding CO, the FE of Cu/C catalyst towards NH₃ was improved by 12 %. More strikingly, for Pd/C, the FE towards NH₃ was enhanced by 95 % with CO co-feeding, by effectively suppressing HER. This is attributed to the change of the favorable surface coverage resulting from the selective and competitive binding of CO* to H* binding sites, thereby improving NH₃ selectivity.