Robust catalyst assessment for the electrocatalytic nitrate reduction reaction

The electrocatalytic nitrate reduction reaction (NO₃RR) can enable distributed conversion of waste to ammonia. Unlike the electrocatalytic dinitrogen reduction reaction, measurements of NO₃RR often consider a fixed quantity of the nitrate anion in a batch system, presenting unique concerns for measurements of catalytic activity and selectivity. In addition, the sensitivity of kinetics and transport to electrolyte composition—where a diverse range of waste feedstocks are of interest—can have a notable impact on catalyst performance, hindering catalyst comparison. We highlight reaction complexities and advocate best practices for robust measurement of catalyst activity, selectivity, and Faradaic efficiency in this burgeoning field.

Fertilizer use, fossil-fuel combustion, and industrial processes have increased nitrate concentrations in many wastewaters and watersheds to levels that threaten environmental and human health^1,2,3. This disruption to the nitrogen cycle primarily originates from energy-intensive production of ammonia by the Haber–Bosch process, which concomitantly emits more CO₂ as a byproduct than any other chemical production process^4,5,6,7,8. Interest in closing this portion of the nitrogen cycle motivates the nitrate electroreduction reaction (NO₃RR), using water and electrons as reducing agents to produce ammonia/ammonium (${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$, depending on the pH), with O₂ produced as a byproduct at the anode.

Although reducing N⁵⁺ in nitrate (${{{{\rm{NO}}}}}_{3}^{-}$) requires more electrons than reducing N⁰ in the dinitrogen reduction reaction (N₂RR), NO₃RR circumvents the stability of the N ≡ N bond, substantially lowering the energetic input⁹. Further benefitting from the ability to investigate high concentrations (>1 M) of reactant ${{{{\rm{NO}}}}}_{3}^{-}$ (in comparison to dissolved N₂), the investigation of NO₃RR targeting ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ has seen rapid growth over relatively short timescales¹⁰. This apparent benefit in solubility comes in concert with a caveat, however: catalytic investigations employing batch cells (where ${{{{\rm{NO}}}}}_{3}^{-}$ is not continually replenished or products removed) can lead to reports convoluting catalyst performance with reactor performance. Although many parallels can be drawn with CO₂ electroreduction (CO₂RR) and N₂RR, the finite nature of the ${{{{\rm{NO}}}}}_{3}^{-}$ reactant and its negative charge also indicate unique differences in best practices and sensitivity to aspects of the electrochemical system. Thus, robust measurement of intrinsic NO₃RR catalyst performance requires not only an understanding of electrochemical systems, but also an understanding of transport¹¹ and reactor-level phenomena often overlooked to date¹².

This comment advocates for community standards in the assessment of intrinsic NO₃RR catalyst performance that would enable fair catalyst comparison while limiting convolution with reactor performance. This is rooted in a brief summary of the important mechanistic aspects that necessitate specific practices for NO₃RR—some of which are distinct compared to other popular electrocatalytic reactions—and explain the limitations in comparing material performance^13,14. We highlight the diversity of possible feedstocks, their inherent complexities, and suggest common platforms for testing. This testing requires specific needs in reactor configuration and product analysis. We conclude with a critical assessment of what is required for accurate comparison of catalyst performance.

Controlling NO₃RR driving force and charge passed to assess catalyst performance without reactor-level effects

On many catalysts and in many experimental conditions, NO₃RR proceeds at appreciable rates below 0 V vs the reversible hydrogen electrode (RHE), where, thermodynamically, the hydrogen evolution reaction (HER) can also occur, resulting in a competition between reactions for protons in solution (H⁺) and on the surface (H*)^15,16. The NO₃RR is a complex reaction network¹⁷, with a diverse range of pH-dependent reaction pathways¹⁸ and products that range from dissolved molecules to ions to gases¹⁹. While we refer the reader to comprehensive reviews for greater detail on the proposed mechanisms^20,21,22, we highlight aspects with implications in measurement practices here. One important metric frequently discussed is the Faradaic efficiency (FE), for example, considering ${{{{\rm{NH}}}}}_{4}^{+}$:

$${{\mbox{FE}}}_{{{NH}}_{4}^{+}}=\frac{{q}_{N{H}_{4}^{+}}}{{q}_{{total}}}=\frac{{C}_{N{H}_{4}^{+}}\,\times \,{V}_{{catholyte}}\,\times \,8{e}^{-}/{{NH}}_{4}^{+}\,\times \,F}{{q}_{{total}}}$$

(1)

Where ${q}_{{{NH}}_{4}^{+}}$ is the charge passed in generating ${{{{\rm{NH}}}}}_{4}^{+}$—typically assessed via the product of a quantified concentration ${C}_{{{NH}}_{4}^{+}}$, catholyte volume V_catholyte, Faraday's number F, and eight electrons passed for production of ${{{{\rm{NH}}}}}_{4}^{+}$ from ${{{{\rm{NO}}}}}_{3}^{-}$—and ${q}_{{total}}$ is the total charge passed.

The driving force for the NO₃RR is the difference in electrochemical potential of the electrocatalyst relative to the reversible potential for a given reactant (referred to as an “overpotential”). As such, it is only appropriate to compare catalyst rates or FE when this driving force is fixed; i.e., in measurements at constant applied potential, also referred to as chronoamperometry. We present three best practices to help enable comparison across materials: (1) defining the electrochemical potential at the catalyst on the RHE scale, (2) adopting a common initial concentration of nitrate in solution, and (3) considering only low conversions, targeting this by controlling (and keeping fixed) the amount of charge passed.

The reason for these practices can be understood in part from the formal potential, E, for NO₃RR to a given product. Considering ${{{{\rm{NO}}}}}_{2}^{-}$ as an example product, NO₃RR consumes an equal number of protons and electrons, resulting in the same pH dependence of ${E}_{{{{{\rm{NO}}}}}_{3}^{-}/{{{{\rm{NO}}}}}_{2}^{-}}$ as E_RHE of 59 mV/pH (Fig. 1) on the standard hydrogen electrode, SHE, scale:

$${E}_{{{{{\rm{NO}}}}}_{3}^{-}/{{{{\rm{NO}}}}}_{2}^{-}}\left({{{\rm{vs\; SHE}}}}\right)=0.85-\frac{{RT}}{2F}{{\mathrm{ln}}}\frac{{a}_{{{NO}}_{2}^{-}}{a}_{{H}_{2}O}}{{a}_{{{NO}}_{3}^{-}}{\left({a}_{{H}^{+}}\right)}^{2}}$$

(2)

Where$\,{a}_{i}$ is the activity of species i; R, T, F are the conventional variables, and 0.85 is the standard potential at standard conditions obtained from literature^23,24. We note that products ${{{{\rm{NH}}}}}_{4}^{+}$ and ${{{{\rm{NH}}}}}_{3}$ consume more protons than electrons (1.25 H⁺/e^−, and 1.12 H⁺/e^−) making their formal potential more sensitive to pH:

$${E}_{{{{{\rm{NO}}}}}_{3}^{-}/{{NH}}_{4}^{+}}\left({{{\rm{vs\; SHE}}}}\right)=0.88-\frac{{RT}}{8F}{{\mathrm{ln}}}\frac{{a}_{{{NH}}_{4}^{+}}{\left({a}_{{H}_{2}O}\right)}^{3}}{{a}_{{{NO}}_{3}^{-}}{\left({a}_{{H}^{+}}\right)}^{10}}$$

(3)

Where 0.88 is the standard potential at standard conditions^23,24. Thus, comparison of catalyst activity at the same electrochemical potential on the RHE scale but at different pH is still fundamentally limited as the driving force (overpotential) for possible products may be slightly different (Fig. S1). Further mechanistic implications of pH are discussed in the following section as well, and we caution that commercial RHEs with an H₂ cartridge can be non-innocent²⁵. The formal potential also depends on the concentration of products and reactants, where ${{{{\rm{NO}}}}}_{3}^{-}$ is consumed in batch cells²⁶—unlike CO₂ or N₂ that are constantly sparged. As shown in Fig. 2a, the formal potential calculated using Eq. 2 for ${E}_{{{{{\rm{NO}}}}}_{3}^{-}/{{{{\rm{NO}}}}}_{2}^{-}}$ shifts by over 50 mV upon reaching a fractional conversion (the amount of ${{{{\rm{NO}}}}}_{3}^{-}$ consumed divided by the initial amount) of 0.5, but this shift in potential is 4× less for ${E}_{{{{{\rm{NO}}}}}_{3}^{-}/{{NH}}_{4}^{+}}$ (calculated using Eq. 3). As such, catalyst performance should be compared only at low conversions in order to accurately capture intrinsic performance at expected conditions and avoid convolution with reactor performance.

Fig. 1: Pourbaix diagram including different NO₃RR species.

Standard potentials obtained from refs. ^23,24.

Fig. 2: Changes in formal potential and product distribution with nitrate conversion.

a Calculated formal potential at pH 7 with fractional ${{{{\rm{NO}}}}}_{3}^{-}$ conversion (|Δ[NO₃^−]|/[NO₃^−]), assuming 100% selectivity to the noted product. b Concentration of remaining ${{{{\rm{NO}}}}}_{3}^{-}$ and products at a series of electrons passed per initial ${{{{\rm{NO}}}}}_{3}^{-}$ ion at −0.4 V vs RHE in pH 7 observed in ref. ²⁶ with a Cu-based catalyst in a batch cell. The inset shows the cumulative FE toward ${{{{\rm{NO}}}}}_{2}^{-}$ (green) and ${{{NH}}_{4}^{+}}$ (purple) calculated from their concentration in solution at the noted charge passed (see Eq. 1 with an example for ${{FE}}_{{{NH}}_{4}^{+}}$).

Additional constraints stem from the reaction mechanism and reactor dynamics of often used batch systems. On some catalysts (e.g., Cu^26,27,28, Ag²⁹, Pd³⁰, Sn³¹, and M-N-Cs^32,33), ${{{{\rm{NO}}}}}_{2}^{-}$ is an intermediate³⁴ that can dissolve into solution and re-reduce on the surface, yielding an eventual product of ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ (Figs. 2b and S2)²⁶. As such, the catalyst FE calculated from cumulative products depends on the amount of ${{{{\rm{NO}}}}}_{3}^{-}$ reduced, or converted. These concepts are illustrated for different kinetic profiles in the Supplemental Information (Fig. S3). Taking the example of Cu, we have observed a 4× increase in the cumulative ${{{\rm{NH}}}_{4}^{+}}$ FE (using Eq. 1) by passing 10× greater charge at constant applied potential, corresponding to approximately 5% and 50% conversion (Fig. 2b²⁶). Because the product distribution in electrochemical reactions is controlled by the charge passed, charge (not time) is important to control for mechanistic and kinetic insight (Figs. S3 and S4). To illustrate the possible pitfalls of comparing catalysts at constant time during chronoamperometry (Table S1): a 10× difference in current (whether from differences in loading, surface area, or intrinsic rates) could yield appreciable differences in product distribution (comparable to Fig. 2b) that are not inherent to the catalyst but instead reflect overall reactor performance, further shown in Fig. S5. Conclusions drawn from existing literature comparing catalysts at constant total time during chronoamperometry should be considered with this caveat in mind.

Conversion should be reported explicitly, however the most transparent and controllable parameter in electrochemical measurements is the cumulative charge passed. Normalizing this variable to the initial amount of nitrate in solution (a product of the catholyte volume and nitrate concentration) gives a transparent variable that is independent of reactor size, reaction conditions (e.g., applied voltage, electrolyte composition), and catalyst surface area, and reporting as a ratio of the moles of electrons passed per mole of initial reactant nitrate maps directly onto conversion in the case of 100% FE (see Supplemental Information for further discussion, Fig. S6). We suggest passing not more than 0.1 ${e}^{-}/{{{{\rm{NO}}}}}_{3}^{-}$, which corresponds to 5% conversion in the case of 100% FE to ${{{{\rm{NO}}}}}_{2}^{-}$ and 1.25% conversion for 100% FE to ${{{{\rm{NH}}}}}_{3}$. However, in cases where FE is low, particularly in conjuncture with low ${{{{\rm{NO}}}}}_{3}^{-}$ concentrations in ground/surface water, a higher number of electrons may be required to have sufficient concentration for reliable quantification via the chosen analytical methods.

Recommended electrolytes reflecting the complexities of NO₃RR feedstocks

Similar to other electroreduction reactions like CO₂RR, the composition of the electrolyte—namely pH and the identity and concentration of cations and anions—can greatly affect NO₃RR reaction rates, FE, and product selectivity. However, the negative charge of the nitrate anion gives rise to unique implications of electrolyte composition, emphasized here below.

Electrolyte pH can impact the NO₃RR mechanism and rate via changing the proton donor, ranging from H₃O⁺ to buffering species, and H₂O (producing OH^–)³⁵. The rates of proton transfer from each species can vary by ~two orders of magnitude^36,37, with proton transfer from H₂O at high pH being the slowest. These distinctions in the molecular origin of H⁺ and rates of its transfer may also influence whether individual steps in NO₃RR proceed via proton-coupled electron transfer or hydrogen addition (via H adsorbed to the surface), with distinctions in reaction pathway likely impacting product distribution. While pH can influence the coverage of adsorbed H (H*) at a given potential, the driving force for hydrogen addition is not inherently pH dependent^38,39,40.

The local charge of the surface can also be impacted by pH, thus influencing nitrate adsorption. At the electrochemical potentials of relevance to NO₃RR, most metals are below their potential of zero charge (PZC), resulting in a negatively charged surface^28,30. Electrostatically, this acts against the adsorption of the nitrate anion⁴¹ (and nitrite), with implications on FE and selectivity. Considering Cu as an example, its PZC has a greater pH dependence than RHE, leading to a more negative PZC in alkaline conditions. The resulting surface, less negative in charge at high pH, may favor anion adsorption, although adsorbed nitrate is less negative than in its ionic form (to an extent dependent on catalyst composition)⁴².

In line with the challenge of bringing a reactive anion to a negatively charged surface, the nature of cations in solution can further impact NO₃RR performance. Such effects have been attributed to (1) manipulating nitrate adsorption via ion pairing⁴³ or electrostatic effects⁴⁴, (2) stabilization of reaction intermediates with large dipole moments via modification of the electric field⁴⁵, and (3) the rate of proton transfer⁴⁶. The negative charge of the nitrate anion makes the first point unique in comparison with other electroreduction reactions, whereas, e.g., the oxygen reduction reaction (ORR) also exhibits notable cation effects for metals below the potential of zero total charge due to the large dipole moment of rate-determining intermediates⁴⁷. Larger cations such as K⁺ have higher ion pairing constants with ${{{{\rm{NO}}}}}_{3}^{-}$ compared to smaller cations, e.g., Li⁺⁴⁸, leading to higher rates of NO₃RR and generally shifting product distribution towards more reduced species (though reports are mixed for Cs⁺)^43,46. Other anions can compete with nitrate in this regard, lowering rates^43,45, although specific adsorption may further convolve these effects. We note, however, that some of these limited investigations are best described as reporting reactor rather than catalyst performance, given the convolution between cation effects and total charge passed and/or local pH^44,46.

Because the kinetics and mechanism of NO₃RR are influenced by pH and the presence of other anions and cations, robust comparison of catalyst performance across labs requires the identification/establishment of standard electrolytes for catalyst testing. However, the applications of interest for NO₃RR span a wide range of reaction environments, and one catalyst may not represent the best performance in every scenario. Here we introduce three model systems for representative catalyst assessment and comparison, and note particular complexities inherent to them for further investigation (Table 1).

Table 1 Model electrolyte systems for NO₃RR

These three model systems illustrate the wide range of NO₃RR reactant concentrations (${{{{\rm{NO}}}}}_{3}^{-}$ and H⁺ equivalents) in potential feedstocks, which influence competitive binding on catalyst surfaces^45,49. Beyond these bulk descriptions, the consumption of 9/10 H⁺ to produce ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ can locally increase the pH^50,51, particularly in circumneutral media with limited buffering^11,52. While we suggest a 0.1 M phosphate buffer to help maintain the local pH for studies in neutral media, its ability to do so is tied to the rate of reaction and limited by mass transfer^53,54.

Important considerations in reactor configuration and product quantification

For important metrics of NO₃RR, including FE and product-specific rates, direct identification and quantification of products requires a combination of analytical chemistry tools^55,56. We refer the reader to other works for a more detailed discussion of the complexities associated with possible solution processes and pH-dependent decomposition processes that may occur^55,57,58. Identification requires separating the anode and cathode via a membrane that allows for ion transport but prevents NO₃RR products from re-oxidizing at the anode^59,60. In acidic and neutral electrolytes, use of an anion exchange membrane prevents crossover of the product ${{{{\rm{NH}}}}}_{4}^{+}$; PiperION-A80 is recommended with the additional consideration of no ${{{{\rm{NH}}}}}_{4}^{+}$ release⁶⁰. However, care should be taken when using alkaline exchange membranes in considering reactant ${{{{\rm{NO}}}}}_{3}^{-}$ and product ${{{{\rm{NO}}}}}_{2}^{-}$ crossover when calculating conversion and FE. The electrochemical potential at the counter electrode should be explicitly measured to ensure the reactions taking place do not influence catalytic performance. For example, Pt counter electrodes should not be used in acidic media due to possible oxidative dissolution^61,62. We note that oxygen, either from ambient or produced at a counter electrode, reduces more readily than ${{{{\rm{NO}}}}}_{3}^{-}$ and should be removed from the solution by sparging with an inert gas. In place of nitrogen, sparging with argon can enable possible N₂ detection from NO₃RR and avoid any manipulation of N-related equilibria (e.g., ${{{{\rm{NO}}}}}_{3}^{-}/{{\rm{N}}_{2}}$) or N₂ reduction at the cathode (e.g., to NH₃). Care should also be taken to ensure species from the reference electrode, such as Cl^− or H₂, are isolated, such as via a secondary frit, to prevent catalyst surface poisoning⁶³ or chemical ${{{{\rm{NO}}}}}_{3}^{-}$ reduction²⁵.

Although ${{{{\rm{NO}}}}}_{3}^{-}$ is highly soluble in comparison to gaseous N₂, it’s transport to the catalyst surface, as well as that of proton sources, can appreciably impact measured rates and product distribution⁵⁰. The regime under which kinetic effects dominate will be a balance between catalyst surface area (dispersion), reaction rate of NO₃RR, rate of competing HER (also consuming protons), convection, migration, and diffusion. Kinetic measurements for the ORR have been corrected for mass transfer effects via forced convection approaches with rotating disk electrodes. This approach may provide similar utility in assessing NO₃RR electrocatalysts. Reactors employed to test NO₃RR catalytic performance can generally be classified as batch or flow. In batch systems (sometimes referred to as H-cells), conversion of ${{{{\rm{NO}}}}}_{3}^{-}$ is primarily controlled via time, with mass transport often enhanced via mechanical agitation like stirring. Flow systems control the conversion of ${{{{\rm{NO}}}}}_{3}^{-}$ via the time nitrate spends in the catalyst-containing chamber (via flow rate), and the means by which the liquid contacts the catalyst has further implications on mass transport (e.g., turbulent vs. laminar flow). Given the possibility of dissolved intermediates such as nitrite for NO₃RR, there are likely unique implications for flow path and catalyst form factor (e.g., supported nanoparticles vs. foil). Care should be taken in either batch or flow configuration to assess catalyst performance at a low number of electrons passed/nitrate in solution. Researchers should consider the impact of mass transport by e.g., comparing performance at different stir or flow rates. In cases where mass transport effects might not be avoided, e.g., considering low ${{{{\rm{NO}}}}}_{3}^{-}$ concentrations at high overpotentials, it is essential to thoroughly report all necessary details, including the exposed catalyst surface area and catholyte volume to ensure reproducibility and enable accurate catalyst comparison.

Need for accurate catalyst characterization to compare catalyst rates

As discussed earlier, catalyst performance^64,65—including reaction rate—should be assessed at low conversion in systems with known (facile) mass transport, at a well-defined electrochemical potential and driving force. Numerous literature reports at high conversions^66,67 result in disparate driving forces even when the voltage relative to a reference electrode is held constant, precluding robust comparison of NO₃RR catalytic performance^68,69. We offered suggestions for the composition of model electrolytes and controlled variables (${e}^{-}{{{\rm{passed}}}}/{{{{\rm{NO}}}}}_{3}^{-}$), as well as required information (catalyst surface area, electrolyte volume) in cases where mass transport effects cannot be mitigated.

Ideally, catalyst rates are normalized to the number of active sites. For systems like single metal atoms in a carbon matrix³³, the active site identity is reasonably well-known and controllable via metal precursor loading. For other systems, such as metal nanoparticles, films, and alloys, assumptions and approximations must be made in choosing an appropriate strategy to normalize rate. The exposed surface area is typically used in the absence of a more detailed understanding of the system (e.g., that defect sites or a specific element is most active). Quantifying this area is challenging, and approaches can be system-specific. For example, the surface area of Pt and Pd nanoparticles can be assessed via the underpotential deposited hydrogen (H_upd) charge⁷⁰. The stripping of deposited metals, such as Cu or Pb, can also be used to estimate surface area in some cases (though care should be taken to avoid contamination resulting from this)^71,72. Measurement of the electrochemical surface area via double layer capacitance is typically reproducible in comparison across research groups, and others recommend a specific capacitance of 0.035 mF cm^−2 in 1 M H₂SO₄ and 0.040 mF cm^−2 in 1 M NaOH for comparison across metallic systems^73,74. We caution, however, that reported specific capacitances for different materials span a range of ~7× these values⁷⁵. The surface atomic composition⁶⁶ and exposed crystallographic orientation²² are two factors known to impact reaction efficiency and selectivity for NH₃ production in NO₃RR. While any high surface area catalyst system will undoubtedly contain a distribution of surface compositions and facets, catalyst characterization should include X-ray photoelectron spectroscopy and X-ray diffraction. This, in addition to information regarding any catalyst conditioning (e.g., holding at reductive potential or acid-treatment), can enable comparison across labs.

We caution, however, that the observation of multiple reaction products—such as nitrite and ammonia—suggest that some catalysts (like Cu, Fig. 2b) can reduce dissolved intermediates. This pathway may occur on dissimilar sites in a cascade-like reaction^33,76, or on comparable sites via local accumulation of species due to poor mass transfer at high reaction rates⁷⁷. In such cases, product distribution (e.g., NH₃ selectivity) would depend on catalyst morphology (dispersion of distinct sites, roughness) in unique ways, highly coupled to mass-transfer. In the absence of characterizing these details of the catalyst and reactor’s mass transfer, understanding the intricacies of NO₃RR and robust characterization of catalyst rates is best enabled by the study of low-roughness catalysts, keeping the amount of charge passed per nitrate ion low. Such catalyst assessment would enable predictive understanding of more complex catalysts and reactor environments, leveraging these unique phenomena to achieve high rates of ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ production.

Conclusion

NO₃RR targeting ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ production is a dynamic area of research, where the performance of electrocatalysts in a wide range of conditions is of interest. We offer suggestions for model systems of study, parameters to report and control, and approaches to take into account to better enable comparison across studies and ultimately drive understanding of this complex reaction network. These include:

• Reporting performance at a fixed electrochemical potential on the RHE scale, which is the best approximation to the driving force, enabling comparison across pH.

• Keeping the amount of nitrate consumed (and products produced) low to avoid conversion of dissolved intermediate species or shifting of reversible potential, where fixing the number of e^− passed/${{{{\rm{NO}}}}}_{3}^{-}$ in solution is the best metric to control.

• Including a benchmark electrolyte, which keeps the ionic strength and concentration of cation and anion species constant, in reporting catalyst performance.

• Characterizing the catalyst surface composition and crystallographic texture, and reporting rates relative to the exposed surface area.

We note, however, that many unanswered questions regarding the mechanisms of NO₃RR, the complexities of the active site, and the role of the electrical double layer still necessitate fundamental studies that investigate beyond these suggestions. These studies may include unique electrolyte compositions and mass transfer limitations or consider higher conversions to assess the application of electrocatalysts to real-world systems. Together with rigorous assessment of catalyst performance in well-defined conditions, the field is poised for rapid progress in our ability to transform waste ${{{{\rm{NO}}}}}_{3}^{-}$ into value-added ${{{{\rm{NH}}}}}_{3}/{{{{\rm{NH}}}}}_{4}^{+}$ in distributed electrochemical systems, employing renewable electricity to help close the nitrogen cycle.