Humans have disrupted the global nitrogen cycle by discharging roughly ten times more waterborne nitrogen than we remove, leading to widespread disruptions to aquatic ecosystems and human health. Electrochemical conversion of nitrogen pollutants (e.g., ammonia, nitrate) to valuable products can incentivize wastewater treatment and distributed chemical manufacturing that lead to a circular nitrogen economy. Electrochemical techniques are particularly well-suited to this challenge because of their modularity, tunability, and limited chemical inputs; however, fundamental understanding of interfacial redox reactions and molecularly precise separations is required to produce a broad portfolio of products from various wastewaters (which we call wastewater refining). Thus, we advance the molecular-scale understanding of both sides of the electrode-electrolyte interface during nitrate reduction and ammonia oxidation to various high-purity products such as hydroxylamine, nitrite, and ammonium hydroxide. With particular focus on nitrate reduction to ammonia, we show the effects of surface reconstruction during reaction, interfacial pH and mass transport, and co-occurring cations and anions on observed catalytic selectivity, activity, and stability. Systematic investigation of ideal solutions, simulated wastewaters, and real wastewaters enables use-informed process design and evaluation, as well as comparison to existing nitrogen removal and nitrogen manufacturing processes. Ultimately, we underscore the importance of improved understanding and control of electrochemical interfaces for catalyzing nitrogen redox reactions to valorize wastewater pollutants.

Elucidating and controlling electrochemical interfaces for wastewater nitrogen refining

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