(309h) Reactor and Microkinetic Modeling to Understand the Impact of Alkali Cations on Electrocatalytic Nitrate Reduction Kinetics in Laminar Flow

Remediating anthropogenic nitrate pollution in wastewater streams simultaneously attenuates environmental damage and presents an opportunity for closed loop recovery for nitrogen resources, namely by converting nitrate into value-added products such as ammonia. The electrocatalytic nitrate reduction (ENO₃RR) is a promising method for achieving this goal as it can use renewably sourced electricity to drive the transformation and operate at standard temperature and pressure, but like many electrocatalytic reactions it is sensitive to co-contaminants. Since many real-world nitrate containing waste streams contain a diverse range of dissolved species, it is important to understand the effects that these species have on reaction kinetics.

Alkali cations such as Na⁺, K⁺, and Cs⁺ have been identified to significantly impact the kinetics in a number of reactions including electrochemical CO₂ reduction and hydrogen evolution (HER).¹ Cations modify ENO₃RR total Faradaic efficiency toward N-products as well as their distribution, but it is not well understood which kinetic steps these species impact.^2-4 As HER competes with ENO₃RR, a relative increase in ENO₃RR Faradaic efficiency with certain cations can come from those cations slowing down HER, increasing ENO₃RR, or both.

In this talk we will present the use of a lab scale flow electrolyzer to deduce a combined microkinetic and reactor model that describes ENO₃RR on a copper electrode in the presence of Na⁺, K⁺, and Cs⁺. We will use this model to estimate rate constants for nitrate reduction, nitrite reduction, and hydrogen evolution based on our kinetic measurements, and compare the impacts that each cation has on these rate constants. Additionally, we will discuss how these implications can inform better design of large scale flow electrolyzers.

(1)

Jiang, T.-W.; Wang, S.-S.; Qin, X.; Zhang, W.-Y.; Li, H.; Ma, X.-Y.; Jiang, K.; Zou, S.; Cai, W.-B. Uncovering the Cation Effects on the Electroreduction of CO2 on Pd/C Catalysts − an SEIRAS Study. Journal of Catalysis 2024, 434, 115520. https://doi.org/10.1016/j.jcat.2024.115520.

(2)

Fajardo, A. S.; Westerhoff, P.; Garcia-Segura, S.; Sánchez-Sánchez, C. M. Selectivity Modulation during Electrochemical Reduction of Nitrate by Electrolyte Engineering. Separation and Purification Technology 2023, 321, 124233. https://doi.org/10.1016/j.seppur.2023.124233.

(3)

Zhang, Y.; Ma, Z.; Yang, S.; Wang, Q.; Liu, L.; Bai, Y.; Rao, D.; Wang, G.; Li, H.; Zheng, X. Element-Dependent Effects of Alkali Cations on Nitrate Reduction to Ammonia. Science Bulletin 2024, 69 (8), 1100–1108. https://doi.org/10.1016/j.scib.2024.02.011.

(4)

Fan, J.; Arrazolo, L. K.; Du, J.; Xu, H.; Fang, S.; Liu, Y.; Wu, Z.; Kim, J.-H.; Wu, X. Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight. Environ. Sci. Technol. 2024. https://doi.org/10.1021/acs.est.4c03949.