Binder-Free Nickel Borate/Nickel Hydroxide Bifunctional Catalyst for Coupled Nitrate Reduction and Glycerol Oxidation Toward Sustainable Ammonia Production

Abstract

The accumulation of nitrate (NO₃ ^–) from agricultural runoff poses a growing threat to ecosystems and public health. Converting nitrate into ammonia (NH₃) through the electrochemical nitrate reduction reaction (NO₃RR) offers a promising strategy to mitigate environmental contamination while creating a sustainable circular route to fertilizer production. However, achieving high NH₃ production and energy efficiency remains challenging. Here, we report a binder-free mixed-phase nickel borate/nickel hydroxide bifunctional electrocatalyst directly grown on Ni foam that enables efficient NO₃RR without using an external Ni precursor. The optimized catalyst achieves a high NH₃ yield rate of 1.49 ± 0.04 mmol h^–1 cm^–2 and a Faradaic efficiency for ammonia (FE_NH3 ) of 81 ± 4.8% at −0.55 V vs RHE. To further reduce the energy input, we couple NO₃RR with glycerol oxidation reaction at the anode, replacing the energy-intensive oxygen evolution reaction. The integrated NO₃RR||GOR system operates at a low cell voltage of 1.47 V (10 mA cm^–2), producing a NH₃ yield rate of ∼0.029 mmol h^–1 cm^–2 (∼63% FE_NH3 ), and ∼0.101 mmol h^–1 cm^–2 of HCOO^– (∼73% Faradaic efficiency of formate). The hybrid electrolyzer enables a potential energy-saving of ∼13% compared to NO₃RR||OER. This work advances the development of electrochemical platforms for sustainable ammonia synthesis by highlighting the cost-effective, energy-saving, and environmental benefits of coupling reaction.

1. Introduction

The growing population puts pressure on the food supply and the food chain. To increase food production, a nitrogen fertilizer is used for increasing the production of crops. − The primary source for a nitrogen fertilizer is ammonia (NH₃), with the current global consumption of over 170 million tons annually, which is anticipated to rise at a yearly rate of 2.3%. Conventional production of NH₃ involves the Haber–Bosch process and the combination reaction of nitrogen (N₂) and hydrogen (H₂) under high pressure (100–200 atm) and temperature (400–500 °C) in the presence of an iron catalyst. Although the NH₃ synthesis does not directly emit CO₂, most of the associated CO₂ emissions stem from upstream H₂ productionprimarily through steam methane reforming or coal gasification. These fossil-based processes account for approximately 2–3% of global greenhouse gas emissions, making NH₃ production a significant environmental concern. , It is crucial to accelerate the development of green NH₃ synthesis methods to eliminate the emissions associated with the conventional NH₃ production. Among various emerging approaches, electrochemical NH₃ synthesis shows great potential by enabling direct reduction of N₂ to NH₃ at the cathode without generating a direct carbon footprint. , Unfortunately, the sluggish reactivity of N₂, caused by its high dissociation energy and low solubility in electrolytes, along with the competition from the hydrogen evolution reaction (HER), poses significant challenges compared to the Haber–Bosch process. , On the other hand, nitrate (NO₃ ^–) is abundant in the environment due to the nitrification process and excessive agricultural activities. When ingested in substantial amounts, NO₃ ^– ions pose serious health risks to humans and other living organisms, including conditions such as cancer and methemoglobinemia. ,, In addition to public health concerns, NO₃ ^– ion contamination contributes to serious environmental issues such as global acidification, climate change, and water pollution from excessive algae growth. ,

Electrochemical nitrate reduction reaction (NO₃RR) has emerged as a promising strategy within the power-to-X framework, offering a unique opportunity to repurpose NO₃ ^– ions as a feedstock for NH₃ productionthereby contributing to a circular nitrogen economy. Noble metals such as Pt, Pd, Rh, Ru, Ir, Au, and Ag and non-noble metals like Cu have been widely explored as electrocatalysts for NO₃RR. − To date, research on NO₃RR has primarily focused on the development of advanced electrocatalysts capable of delivering cost-effective, high current density, high Faradaic efficiency (FE_NH3 ), and high NH₃ yield rate. Cu-based catalysts have been reported to achieve FE_NH3 values exceeding 90%, but their long-term stability remains a challenge due to corrosion and dissolution, particularly under acidic conditions. In some cases, catalysts have even demonstrated near 100% FE_NH3 for NO₃RR. − However, beyond maximizing FE_NH3 , increasing the NH₃ yield rate and developing low-cost, durable catalysts remain critical goals for advancing the practical application of the NO₃RR.

In electrochemical applications, nanostructured materials are typically incorporated into electrodes by mixing them with a binder, with or without carbon black, and pressing the mixture onto a conductive substrate. However, this method inevitably introduces additional, unwanted interfaces within the electrode, which can hinder performance. To address this issue and simplify the fabrication process, various techniques have been developed to directly grow active electrode materials on conductive substrates, including electrodeposition, , chemical bath deposition, and plasma-assisted oxidation. Binder-free electrodes provide more exposed active sites, strong substrate adhesion, high conductivity, inherent intrinsic catalytic activity, and tunable nanostructure modification for electronic fine-tuning, all of which enhanced electrocatalytic performance. Ni foam is widely employed as a substrate in energy storage and conversion devices owing to its excellent electrical conductivity, high surface area, and good mechanical strength. − Direct growth of nanostructured active materials on Ni foam enhances their adhesion to the substrate, improving their structural stability during operation. This approach also streamlines subsequent modifications and electrochemical measurements by eliminating the need for complex electrode assembly.

Ni foam can serve either as a structural substrate or as a Ni source owing to its 3D crisscrossed structure. However, due to the relatively high work function of Ni foam, it exhibits significant proton adsorption, which predominantly favors HER. To broaden the reaction scope of Ni foam, various Ni-based composites, such as nickel hydroxide (Ni­(OH)₂), nickel sulfide, nickel phosphide, and nickel borate (Ni₃(BO₃)₂), , have been explored as electrode materials. Both Ni­(OH)₂ and Ni₃(BO₃)₂ have recently gained significant research attention due to their promising electrochemical performance, as well as their emerging catalytic, , magnetic, and phosphorescent properties.

Here, we present a simple and efficient hydrothermal method for the direct growth of mixed-phase nickel borate/nickel hydroxide (Ni₃(BO₃)₂/Ni­(OH)₂) nanostructures on Ni foam using only urea and boric acid (designated as Ni_U:B@Ni foam; U:B represents the molar ratio of urea to boric acid). This binder-free approach simplifies electrode fabrication by directly etching Ni ions from the Ni foam substrate to form active catalyst layers. Controlled experiments confirmed that urea acts as the primary reagent activating Ni dissolution, while boric acid promotes Ni₃(BO₃)₂ crystal formation via a borate-driven mechanism. Electrochemical evaluation of Ni_U:B@Ni foam demonstrated excellent nitrate reduction performance, achieving a high NH₃ yield rate up to 2.17 ± 0.75 mmol h^–1 cm^–2 and a FE_NH3 of 87 ± 1.29% under a 1 M potassium hydroxide (KOH) electrolyte containing 0.1 M potassium nitrate (KNO₃). Additionally, we coupled the NO₃RR with glycerol oxidation (glycerol oxidation reaction (GOR)) to form a two-electrode electrolyzer system. By substitution of the energy-intensive oxygen evolution reaction (OER) with GOR, this approach significantly lowers the overall energy requirement for NH₃ synthesis. Furthermore, converting glycerola readily available byproduct from biodiesel productioninto valuable chemical intermediates enhances the economic feasibility of the overall process. For instance, in alkaline environments, the theoretical oxidation potential of glycerol to formic acid is only 0.69 V, which is significantly lower than the 1.23 V needed for the OER, facilitating an electrosynthesis process that simultaneously generates formic acid (or C₁–C₃ product) , and NH₃ (Figure a). Although promising, dual-substrate nitrate–glycerol electrolyzers are rarely studied, for example, NiCu alloys, CoP; CNs@CoP hybrids, and Cu-doped NiCo systems. Distinct from previous studies, we utilize mixed-phase nickel borate/nickel hydroxide (Ni₃(BO₃)₂/Ni­(OH)₂) nanostructures on Ni foam, establishing a bifunctional catalyst architecture tailored for nitrate–glycerol coupling. In this study, the integrated NO₃RR||GOR cell successfully operated at ∼1.47 V, providing FE_NH3 of ∼63% (∼0.029 mmol h^–1 cm^–2) along with a glycerol conversion rate of ∼23% (∼73% Faradaic efficiency of formate (FE_HCOO ^–); 0.101 mmol h^–1 cm^–2), at 10 mA cm^–2, the device reveals a ∼13% energy saving vs OER.

1.

(a) Schematic representation of a renewable electricity-driven electrolyzer for ammonia and green chemical production via nitrate reduction coupled with glycerol oxidation. (b) Schematic of the hydrothermal synthesis process for binder-free Ni_U:B@Ni foam electrodes (U:B represents the molar ratio of urea to boric acid).

2. Results and Discussion

2.1. Catalyst Formation and Structural Characterization

A binder-free electrode was hydrothermally prepared at 150 °C for 24 h using boric acid and urea as precursors on a Ni foam substrate, as shown in Figure b. To validate the distinct roles of urea and boric acid in the hydrothermal growth process, a series of control samples was synthesized with varying urea-to-boric acid molar ratios.

X-ray diffraction (XRD) was employed to identify the crystalline phases formed on Ni_U:B@Ni foam samples (Figure a,b). The Ni_0:8@Ni foam sample (boric acid only) exhibited a distinct (003) plane corresponding to the alpha-phase Ni­(OH)₂ (JCPDS no. 38-0715), but no significant peak for Ni₃(BO₃)₂ crystals (Figure b). However, a shift in the (003) peak to 12.7° suggests the incorporation of borate species within the Ni­(OH)₂ layers, leading to a reduced interlayer d-spacing. This indicates that boric acid alone facilitates the hydrolysis of the Ni foam substrate. In contrast, urea has previously been reported to enhance hydrolysis reactions of Ni foam effectively in the presence of external Ni ions. However, our findings demonstrate that using urea alone (Ni_8:0@Ni foam), without external Ni ions, resulted in amorphous nickel oxide (NiO) formation. This was indicated by the absence of clear diffraction peaks and supported by scanning electron microscopy (SEM) observations, as discussed later in the SEM section. Interestingly, when both urea and boric acid were combined at an equimolar ratio (Ni_8:8@Ni foam), clear crystalline reflections for Ni₃(BO₃)₂including a prominent (101) peak (JCPDS no. 75-1809)alongside Ni­(OH)₂ were observed. This highlights the critical role of urea in activating nickel from the foam substrate to form crystalline Ni₃(BO₃)₂ phases beneficial for subsequent NO₃RR. Increasing the boric acid concentration further (Ni_8:16@Ni foam) unexpectedly reduced the intensity of both Ni­(OH)₂ and Ni₃(BO₃)₂ reflections. Additionally, raising both precursor concentrations (Ni_16:16@Ni foam) resulted in further suppression of the crystallinity. Multiple independent batches of Ni_8:8@Ni foam consistently produced similar XRD patterns (Figure S1), confirming the robust reproducibility of the synthesis method.

2.

Structural and morphological characterization of Ni_U:B@Ni foam samples. (a) XRD patterns of Ni foam, Ni_8:0@Ni foam, Ni_0:8@Ni foam, Ni_8:8@Ni foam, Ni_8:16@Ni foam, and Ni_16:16@Ni foam. (b) Enlarged view of the selected region from (a). (c) Digital photographs of Ni foam before hydrothermal treatment (1) and after reaction (2–6). (d–g) SEM image, TEM images, and the corresponding EDX elemental mapping of Ni_8:8@Ni foam. (h–k) SEM image, TEM images, and corresponding EDX elemental mapping of Ni_8:16@Ni foam.

Following hydrothermal treatment, the Ni_U:B@Ni foam exhibited noticeable color changes [Figure c, transforming from metallic gray (bare Ni foam) to light gray (Ni_0:8@Ni foam-(2)], to dark gray [Ni_8:0@Ni foam-(3) and Ni_16:16@Ni foam-(6)], to greenish gray [Ni_8:8@Ni foam-(4)], and black [Ni_8:16@Ni foam-(5)]. SEM analysis confirmed that the pristine Ni foam possessed a typical porous framework morphology with pore sizes ranging from 200 to 300 μm (Figure S2).

When both urea and boric acid were present in equimolar ratios (Ni_8:8@Ni), SEM imaging (Figures d and S3) revealed well-defined stacked, layered structures on the foam surface. A slight charging effect was observed at the edges of the layers (white arrow, Figure d), suggesting a nonuniform planar alignment or partial amorphization of Ni₃(BO₃)₂, consistent with the XRD identification of mixed crystalline phases. Transmission electron microscopy (TEM) images (Figure e,f) further reveal that Ni₃(BO₃)₂ particles are randomly distributed across the Ni­(OH)₂ nanosheets. Notably, no distinct borate nanoparticles are observed, implying that Ni₃(BO₃)₂ species have integrated into the Ni­(OH)₂ matrix to form nanoflake-like composite structures. Correspondingly, energy-dispersive X-ray spectroscopy (EDX) elemental mapping (Figure g) confirmed a uniform elemental distribution of oxygen, nickel, and boron across the surface, further validating the successful in situ growth of Ni­(OH)₂ and Ni₃(BO₃)₂ crystals without the need for an external Ni precursor.

In the absence of urea, the Ni_0:8@Ni foam (boric acid only) showed no changes on the surface of the Ni foam. Instead, small corrosion pits were observed on the surface (Figure S4), indicating that boric acid alone is insufficient for promoting structured surface modification. Nonetheless, EDX elemental mapping confirmed the random distribution of boron across the surface (Figure S4), suggesting adsorption without crystallization. In contrast, Ni_8:0@Ni foam (urea only) displayed an interconnected sheet-like layer of amorphous NiO, as revealed by SEM (Figure S5). This morphology likely resulted from an elevated pH caused by urea hydrolysis, which promotes Ni surface activation and the rapid nucleation of NiO. The absence of crystalline peaks in the XRD pattern, alongside the homogeneous distribution of oxygen (Figure S5), supports the amorphous nature of this NiO layer.

When the concentration of boric acid was increased beyond the equimolar condition, as in Ni_8:16@Ni foam, SEM images (Figures h and S6) revealed disordered flake-like layers extending in multiple directions, contrasting with the well-aligned stacking observed in Ni_8:8@Ni foam. This disrupted growth pattern suggests that excess boric acid interferes with proper layer formation, possibly by inhibiting agglomeration. Nevertheless, TEM images (Figure i–k) indicated that Ni₃(BO₃)₂ species remained homogeneously dispersed within the Ni­(OH)₂ matrix, suggesting that the Ni₃(BO₃)₂ species is integrated within Ni­(OH)₂. In the sample with the highest precursor loading, Ni_16:16@Ni foam, a distinctly different morphology was observedcharacterized by bubble-like, densely interconnected nanosheets (Figure S7). This morphology closely resembles that of Ni_8:0@Ni foam, indicating that at high concentrations, urea dominates the reaction environment by significantly increasing local alkalinity and triggering rapid, disordered nucleation. Despite the elevated precursor levels, the formation of ordered layered structures was suppressed, suggesting that the reaction pathway is primarily governed by urea-driven hydrolysis under these conditions. These results highlight that variations in precursor ratios lead to distinct structural morphologies, likely due to differing reaction pathways and kinetics for nucleation and growth.

To gain deeper insights into the surface transformation and Ni etching behavior during hydrothermal treatment, Ni K-edge X-ray absorption spectroscopy (XAS) was performed in the fluorescence mode. Figure a presents the Ni K-edge X-ray absorption near edge structure (XANES) spectra of the as-synthesized Ni_U:B@Ni foam samples, compared with reference samples of bare Ni foam and standard NiO. The normalized absorption edges for the Ni_U:B@Ni foam samples were similar to those of the bare Ni foam, indicating that the bulk oxidation state remained largely unchanged. This is attributed to the dominance of metallic Ni (Ni⁰) in the foam structure relative to the surface Ni²⁺ species formed during the reaction. A closer examination of the XANES spectra (Figure b) revealed a slight increase in absorption intensity around 8350 eV for the treated samples, which is consistent with the formation of Ni–O–X bonds, where X could be H, B, or Ni. Among the samples, Ni_8:8@Ni foam exhibited a slightly higher intensity, suggesting a thicker surface layer of Ni₃(BO₃)₂/Ni­(OH)_2. These results are in agreement with the XRD and SEM analyses, which indicated dense crystalline growth in this sample. The variation in intensity correlates with surface oxide content, which is primarily governed by differences in microstructural coverage across the Ni foam substrates. Accordingly, for the urea-containing system, the surface oxide abundance follows the order: Ni_8:8@Ni foam > Ni_8:16@Ni foam > Ni_16:16@Ni foam > Ni_8:0@Ni foam. Extended X-ray absorption fine structure (EXAFS) analysis (Figure c) further supported these observations. A prominent peak at approximately 2.16 Å, corresponding to Ni–Ni coordination, was observed in both the untreated and treated samples. The Ni_8:0@Ni foam and Ni_0:8@Ni foam exhibited only minor reductions in this peak’s intensity, indicating limited surface modification. In contrast, a noticeable decrease in the Ni–Ni peak intensity was observed for Ni_8:8@Ni foam, Ni_8:16@Ni foam, and Ni_16:16@Ni foam, suggesting partial conversion of Ni–Ni bonds into Ni–O–X coordination environments. No distinct peaks were detected at 1.57 Å or 2.54 Å, possibly due to the subtlety of the structural changes or overlapping signals. Nevertheless, the combination of XANES and EXAFS analyses confirms that the Ni substrate undergoes an in situ surface transformation during hydrothermal treatment, forming Ni₃(BO₃)₂ and Ni­(OH)₂ directly from the Ni foam itself without the addition of external Ni precursors. These findings provide strong evidence for a heterogeneous growth facilitated by the hydrothermal environment.

3.

X-ray absorption and photoelectron spectroscopy studies of Ni_U:B@Ni foam samples. (a) XANES spectra, (b) enlarged view of the XANES region in (a), and (c) Fourier-transformed EXAFS spectra of NiO, Ni foam, Ni_0:8@Ni foam, Ni_8:0@Ni foam, Ni_8:8@Ni foam, Ni_8:16@Ni foam, and Ni_16:16 @Ni foam recorded at the Ni K-edge. (d–f) XPS spectra of B 1s, Ni 2p, and O 1s regions for Ni_8:8@Ni foam, and (g–i) corresponding XPS spectra for Ni_8:16@Ni foam. Gray lines indicate experimental data points, while colored lines represent fitted results, background, and peak sum.

The chemical valence state of the growth moieties on the surface of Ni foam, particularly on the Ni_8:8@Ni foam and Ni_8:16@Ni foam samples, was investigated using X-ray photoelectron spectroscopy (XPS) analysis. Figure S8 displays the XPS survey pattern for the Ni_8:8@Ni foam and Ni_8:16@Ni foam samples. It shows that B, Ni, and O elements are present. The binding energies for the B 1s, Ni 2p, and O 1s peaks are 191.74, 855.42, and 531.07 eV, respectively. The B 1s spectra of both the samples (Figure d,g) were fitted to a single peak with binding energies of 191.74 eV, ascribed to the core level of B³⁺ atoms in the borate species. , Furthermore, a spin orbit doublet, specifically Ni 2p_3/2 at 855.45 eV and Ni 2p_1/2 at 873.03 eV, is discernible in the deconvoluted peak of Ni 2p (Figure e,h). These peaks were accompanied by two shake-up satellite peaks at 861.25 and 879.18 eV, which indicate the presence of high oxidation state, i.e., Ni²⁺/Ni³⁺ ions. The Ni 2p_3/2 peak was deconvoluted into a pair of peaks at 855.15 and 856.17 eV assigned for Ni²⁺ and Ni³⁺ ions. The presence of Ni³⁺ ions is due to the metastable structure, inherently abundant defects of amorphous state borate (probably at the side of every layer as seen in SEM and TEM images), and disordered long-range atomic arrangement. Therefore, Ni³⁺ ions existed in both samples, which will be electrochemically reduced to a lower oxidation state (Ni²⁺) at more negative electrode potentials as the catalytically active species for the NO₃RR. Moreover, Figure f,i demonstrates the four different types of O 1s peaks, the binding energy at 529.40, 530.70, 531.80, and 533.04 eV, which are typically designated to the band of lattice oxygen in B–O bond, Ni–O/Ni–O–H bond, Ni–O–B bond, and adsorbed O bond, respectively. , Figure S9 shows that the Ni–O–B bonding ratio is significantly higher in Ni_8:8@Ni foam (50.13%) compared to Ni_8:16@Ni foam (27.61%). Based on XPS results, Ni_8:8@Ni foam is identified as having a higher proportion of active Ni–O–B sites, making it more suitable for a variety of electrochemical reactions.

Based on these analyses, we propose a two-stage process for the hydrothermal in situ nucleation mechanism of mixed-phases Ni₃(BO₃)₂/Ni­(OH)₂. In the first stage, Ni atoms on the surface of the Ni foam are first oxidized into amorphous NiO by urea (Stage 1). Clear evidence for the formation of amorphous NiO was provided in the Supporting Information (Figure S10), including XPS and Raman analyses. This oxidation is likely facilitated by the decomposition of urea into NH₃ and CO₂, which alters the pH of the solution. In the second stage, the presence of boric acid increases the hydrolysis reaction of the Ni foam, leading to the formation of α-Ni­(OH)₂. Simultaneously, boric acid could interact with NiO to form crystalline Ni₃(BO₃)₂ (stage 2).

$$\begin{array}{l}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e}1:{({\mathrm{H}}_2\mathrm{N})}_2\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{N}\mathrm{H}}_3+{\mathrm{C}\mathrm{O}}_2\\ {\mathrm{N}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}\mathrm{H}}_4\mathrm{O}\mathrm{H}\\ \mathrm{N}\mathrm{i}+{\mathrm{N}\mathrm{H}}_4\mathrm{O}\mathrm{H}\to \mathrm{N}\mathrm{i}\mathrm{O}+{\mathrm{N}\mathrm{H}}_3+{\mathrm{H}}_2\\ \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e}2:\mathrm{N}\mathrm{i}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}\mathrm{i}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{H}}_2\\ 3\mathrm{N}\mathrm{i}\mathrm{O}+2\mathrm{B}{(\mathrm{O}\mathrm{H})}_3\to {\mathrm{N}\mathrm{i}}_3{({\mathrm{B}\mathrm{O}}_3)}_2+3{\mathrm{H}}_2\mathrm{O}\end{array}$$

2.2. Electrochemical NH₃ Production from NO₃RR

Electrochemical approaches are particularly valuable for chemical synthesis, as the applied voltage or current can be finely tuned to control the reaction pathway and product selectivity. The selection of a highly alkaline electrolyte (1 M KOH) for NO₃RR is due to its strong conductivity and lower likelihood of forming toxic intermediates. Linear sweep voltammetry (LSV) measurements revealed that all Ni_U:B@Ni foam samples exhibited higher current density and more positive onset potential compared to bare Ni foam samples (Figure a), indicating that introducing the moieties by the reaction of boric acid and/or urea enhances the NO₃RR activities. This enhancement in electrocatalytic performance can be attributed to the improved surface properties and increased active sites. Among the samples, Ni_8:8@Ni foam displayed the lowest onset potential (−0.09 V vs RHE, Figure b) and the highest current density at 0.60 V vs RHE (Figure a). These results confirm that optimizing the molar ratio of boric acid and urea plays a critical role in enhancing the NO₃RR capabilities of the Ni foam-based catalyst. The inset in Figure a compares the LSV curve of Ni_8:8@Ni foam before and after the addition of 0.1 M KNO₃. In 0.1 M KOH alone, Ni_8:8@Ni foam exhibited a current density of ∼230 mA cm^–2 at 0.60 V vs RHE, which is significantly increased to ∼535 mA cm^–2 following the introduction of NO₃ ^– into the cathode compartment. Notably, to reach a current density of 10 mA cm^–2, the Ni_8:8@Ni foam required 120 mV less overpotential for NO₃ ^– reduction compared with HER (261 mV). The substantial enhancement in current density and the reduced overpotential confirm the occurrence of efficient NO₃RR. This could be due to the synergistic effect between mixed-phases Ni₃(BO₃)₂/Ni­(OH)₂, where electron transfer significantly increases, enhancing NO₃RR. The change in electronic structure due to the mixed phases creates abundant active sites for effective NO₃ adsorption, leading to enhanced NO₃RR as seen in XAS analysis. These results indicate that the formation of Ni₃(BO₃)₂ and Ni­(OH)₂ moieties on the Ni foam surface plays a crucial role in boosting the NO₃RR performance.

4.

Electrocatalytic performance of Ni_U:B@Ni foam for nitrate reduction. (a) LSV curves of Ni foam and Ni_U:B@Ni foam samples in 1 M KOH containing 0.1 M KNO₃ under a three-electrode setup. Line (a–f) correspond to Ni foam, Ni_8:0@Ni foam, Ni_0:8@Ni foam, Ni_16:16@Ni foam, Ni_8:16@Ni foam, and Ni_8:8@Ni foam, respectively. Inset: LSV curves of the Ni_8:8@Ni foam with and without the addition of 0.1 M KNO₃. (b) Onset potentials for NO₃RR on different Ni_U:B@Ni foam. (c) FE_NH3 and (d) NH₃ yield rate of Ni foam, Ni_16:16@Ni foam, Ni_8:16@Ni foam, and Ni_8:8@Ni foam [assigned as (a,d,e,f), respectively in the panel (c)] after CPE at different applied potentials. (e) Consecutive cycling stability test of Ni_8:8@Ni foam at −0.55 V vs RHE over 18 cycles.

The controlled potential electrolysis (CPE) was carried out for 15 min across a potential window from −0.40 V to −0.60 V vs RHE (Figure c) to evaluate the NO₃RR performance at different applied potentials (Figure S11). Following electrolysis, the electrolytes were collected for subsequent product analysis. Accordingly, the analysis focused on determining the FE_NH3 and the NH₃ yield rate as the primary performance metrics for the NO₃RR. We found that the NH₃ yield rate increased with increasing KNO₃ concentration (Figure S12), while no detectable NH₃ was found in the blank 1 M KOH solution, confirming that NH₃ generation originated exclusively from the NO₃RR. This result validates the reliability of the product analysis.

The NH₃ yields and FE_NH3 of Ni_U:B@Ni foam samples, along with bare Ni foam, were evaluated by a calorimetric indophenol blue method (see Experimental Methods, Figure S13). As the CPE potential was varied from −0.40 V to −0.60 V vs RHE, the NH₃ yield rate steadily increased, while the FE_NH3 initially rose and then declined beyond its optimal potential (Figures c and S14). Detailed NH₃ yield rate calculations (see Experimental Methods) showed that Ni_8:8@Ni foam achieved NH₃ production rates of 0.41 ± 0.04, ∼0.73 ± 0.03, ∼0.89 ± 0.34, ∼1.49 ± 0.04, and ∼2.17 ± 0.75 mmol h^–1 cm^–2 at −0.40 V, −0.45 V, −0.50 V, −0.55 V, and −0.60 V vs RHE, respectively (Figure d). In contrast, Ni_8:16@Ni foam exhibited significantly lower production rate of ∼0.13, ∼0.63, ∼0.69, ∼0.77, and ∼0.68 mmol h^–1 cm^–2 at the corresponding potentials. This reduced performance is likely due to the competing HER. Among all samples, Ni_8:8@Ni foam demonstrated the highest FE_NH3 of 87% at −0.50 V vs RHE. Based on hydrogen quantification for the Ni_8:8@Ni foam sample, HER activity is more strongly suppressed at −0.6 V (2.5% FE_H2 ) compared to −0.4 V vs RHE (35% FE_H2 ) (Figure S15). This suppression is consistent with the increased favorable nitrate adsorption at higher potentials, which accelerates the consumption of electrons and protons toward NO₃RR, thereby limiting the competing HER process. Notably, at −0.55 V vs RHE, Ni_8:8@Ni foam sample achieved an NH₃ yield rate of 1.49 ± 0.04 mmol h^–1 cm^–2 (81 ± 4.8% FE_NH3 ), approximately ∼2.7 times higher than that of bare Ni foam (∼0.55 mmol h^–1 cm^–2, ∼49% FE_NH3 ). This superior performance is attributed to the optimized ratio of urea and boric acid during synthesis, which promoted the formation of dense, active mixed-phases Ni₃(BO₃)₂/Ni­(OH)₂, as evidenced in Figure b.

To highlight the significance of mixed phases in the NO₃RR, we evaluated the NO₃RR catalytic activity of single phase Ni₃(BO₃)₂ and Ni­(OH)₂. The single phase Ni₃(BO₃)₂ was synthesized by immersing the nickel foam in borate-rich sodium borohydride for 48 h. On the other hand, Ni_0:8@Ni foam showed the formation of single-phase Ni­(OH)₂ on the surface without any detectable of Ni₃(BO₃)₂, as confirmed by XRD analysis (Figure S16a). At −0.55 V vs RHE, both single-phase Ni₃(BO₃)₂@Ni foam and Ni­(OH)₂@Ni foam exhibited lower current densities compared to Ni_8:8@Ni foam (Figure S16b). The distinct electronic properties of the dual Ni sites contribute to efficient charge transfer and polarization, facilitating critical steps such as bond activation. Their spatial proximity can enable dual-site mechanisms and cooperative electron or proton transfers, especially when arranged in specific patterns. The mixed-phase Ni_8:8@Ni foam highlights the synergistic effect between Ni₃(BO₃)₂ and Ni­(OH)₂, demonstrating superior catalytic performance for nitrate reduction.

The long-term catalytic stability of Ni_8:8@Ni foam was further assessed by CPE at −0.55 V vs RHE over 18 consecutive cycles, each lasting 15 min (Figures e, and S17). Throughout these cycles, the catalyst maintained a consistent FE_NH3 , indicating stable selectivity. While minor fluctuations in the NH₃ yield rate were observed, particularly during cycles 11 and 12, the overall catalytic activity remained highly reproducible. These variations are attributed to slight experimental handling differences between cycles, such as changes in electrode positioning or local electrolyte environment, which can influence current density and thus affect yield. The overall catalytic activity remained highly reproducible, confirming the strong durability of Ni_8:8@Ni foam for sustained electrochemical NO₃RR. The mixed phases deliver superior stability, preventing electrocatalyst degradation and sustaining high NH₃ production over 18 cycles. Notably, the performance metrics of Ni_8:8@Ni foamincluding FE_NH3 , onset potential, and NH₃ yield ratecomparable to those of more expensive noble metal catalysts such as Pd- and Ru-based systems (see Table S1).

2.3. Electrochemical Glycerol Oxidation

Ni_8:8@Ni foam was selected for GOR studies based on its optimal morphology, stable electrochemical performance, and well-defined crystalline of Ni₃(BO₃)₂ and Ni­(OH)₂ observed during NO₃RR. Among all tested compositions, the Ni_8:8@Ni foam exhibited the most uniform layered structure and consistent catalytic activity, making it a promising candidate for investigating anodic oxidation reactions such as GOR. To evaluate its electrochemical activity for GOR, LSV was performed over a potential range from 0.90 to 1.60 V vs RHE in 1 M KOH at a scan rate of 10 mV s^–1 in a three-electrode configuration (Figure a). In the alkaline electrolyte, an anodic peak at ∼1.42 V vs RHE corresponds to the oxidation of Ni-based moieties. Upon the addition of 0.1 M glycerol, a sharp increase in current density was observed at ∼1.20 V vs RHE, indicating glycerol electro-oxidation without significant oxidation of Ni moieties, suggesting that Ni species serve as the primary active sites for GOR. In the absence of glycerol, a potential of 1.35 V vs RHE was required to drive OER at 10 mA cm^–2, whereas only ∼1.22 V vs RHE was needed in the presence of 0.1 M glycerol (Figure a). These results show that the Ni₃(BO₃)₂/Ni­(OH)₂ composite exhibits high electrochemical activity, achieving a current density of 100 mA cm^–2 at 1.36 V vs RHE.

5.

Electrocatalytic glycerol oxidation and coupled NO₃RR||GOR performance of Ni_8:8@Ni foam. (a) LSV curves of Ni_8:8@Ni foam in 1 M KOH with and without 0.1 M glycerol in a three-electrode configuration. (b) Glycerol conversion rates of Ni_8:8@Ni foam at different potentials. Inset: FE_HCOO ^– and HCOO^– yield rate of Ni_8:8@Ni foam. (c) Schematic illustration of the NO₃RR||GOR coelectrolysis system with Ni_8:8@Ni foam as both the cathode and anode in two different systems. (d) Comparison of the overall cell voltages for NO₃RR||GOR and NO₃RR||OER at 10 mA cm^–2 in a two-electrode configuration. (e) FE and product yield for NO₃RR and GOR after 24 h electrolysis at 10 mA cm^–2 in an H-type cell.

CPE tests were conducted in 1 M KOH containing 0.1 M glycerol over a potential range of 1.30–1.45 V vs RHE for 4 h (Figure S18b). Postelectrochemical electrolytes were analyzed by high-performance liquid chromatography (HPLC) to determine the glycerol conversion rate and identify reaction products. After 4 h of electrolysis, the Ni_8:8@Ni foam catalyst selectively produced formate with no apparent byproducts. HPLC analysis revealed peaks at retention times of 16.6 and 17.5 min, corresponding to glycerol and formic acid, respectively, with no additional peaks detected (Figure S18a). The product concentration was quantified using calibration curves generated from standard solutions of known concentrations (Figure S18c,d). The glycerol conversion rate increased with increasing applied potential (Figure b), reaching approximately 35% at 1.45 V vs RHE after 4 h. The FE_HCOO– of 97% and 89% were achieved at 1.4 and 1.45 V vs RHE, respectively, with the corresponding formate yield rate of 0.67 and 1.41 mmol h^–1 cm^–2 (inset of Figure b).

2.4. Integrated NO₃RR||GOR Performance and Scale-Up Potential

Following the individual evaluation of the NO₃RR and GOR performance on the Ni_8:8@Ni foam catalyst, the feasibility of coupling these two half-reactions in a single electrochemical system was further explored. This strategy aimed to replace OER (1.23 V vs RHE), which is typically energy-intensive, with GOR (0.69 V vs RHE; theoretical value for glycerol-to-formate conversion) as the anodic process to significantly reduce the overall energy consumption required for the redox system. ,

We first constructed a symmetric NO₃RR||GOR electrolyzer using Ni_8:8@Ni foam (0.25 cm²) as a bifunctional electrocatalyst in a two-electrode configuration within an H-type cell (Figure c). The system consisted of two compartments, each containing 15 mL of the corresponding electrolyte, separated by a proton exchange membrane to ensure ionic conductivity while maintaining product separation. Electrolysis was carried out at a constant current density of 10 mA cm^–2 over 24 h (Figure d). The H-type electrolyzer achieved a stable cell voltage of approximately 1.47 V, resulting in an NH₃ yield rate of 0.029 mmol h^–1 cm^–2 with a FE_NH3 of ∼63%, and a formate yield rate of 0.101 mmol h^–1 cm^–2 with FE_HCOO- of ∼73% (Figures e and S19). The glycerol conversion rate of 22.8% was achieved (Figure S19b), with HPLC analysis confirming selective formate formation (inset of Figure S19b). The reaction at the anode and cathode is described in eqs and

$$\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{d}\mathrm{e}:{\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}_3+8{\mathrm{O}\mathrm{H}}^{-}\to 3{\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+5{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}$$ 1

$$\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}:{{\mathrm{N}\mathrm{O}}_3}^{-}+9{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_3+9{\mathrm{O}\mathrm{H}}^{-}$$ 2

The molar ratio of formate to NH₃ was 3.5:1, deviating from the theoretical value of 3:1. This deviation is attributed to the competing HER at the cathode, as shown in Figure S15.

To evaluate the energy-saving benefits of replacing the OER with GOR, the same Ni_8:8@Ni foam electrode was tested under identical conditions, except the anolyte was used without glycerol. The H-type cell achieved a stable cell voltage of approximately 1.47 V for NO₃RR||GOR, compared to 1.61 V for NO₃RR||OER. This results in an energy-saving efficiency of ∼13% (Figure d). Microstructural analysis by SEM revealed no significant changes in the morphology of the Ni_8:8@Ni foam after 24 h of electrolysis, aside from minor surface oxidation and slight etching of boron species (Figure S20). Additionally, XPS of postcatalysis reveals evidence of boron etching in the case of glycerol oxidation (Figure S21d–f) but not in the nitrate reduction side (Figure S21a–c), indicating notable surface change for Ni₃(BO₃)_2, whereas Ni­(OH)₂ remains stable. However, the absence of the Ni³⁺ oxidation state in both the samples (Figure S21b,e) indicated the leaching of borate on the surface of the Ni­(OH)₂ as supported by SEM analysis. These findings suggest structural evolution under electrochemical conditions, which likely affects catalytic behaviors and long-term stability, especially at high current. This limitation highlights the need for improved pH control and electrolyte management to enable more stable operation at higher current densities. Future work should focus on optimizing membrane selection and flow system design to support long-term performance under practical conditions. −

To assess the scalability of the coupled NO₃RR||GOR system, a flow cell configuration was implemented using a Ni_8:8@Ni foam electrode with an enlarged electrode area (4 cm², Figure S22). The electrolytes (40 mL per compartment) were circulated at a controlled flow rate of 12.5 mL min^–1. At a current density of 10 mA cm^–2, the flow cell operated at a slightly higher voltage of 1.75 V, likely due to differences in mass transport behavior, increased electrical resistance associated with the larger electrode area, and fluid dynamics compared to the H-type cell. Notably, the NH₃ yield rate and FE_NH3 remained comparable to those observed under static conditions, demonstrating the feasibility of translating the reaction system to a continuous-flow platform. These results highlight the potential of the NO₃RR||GOR pairing for scale-up, with opportunities for further optimization of flow dynamics and electrode design. Importantly, the system employs a simply fabricated, bifunctional catalyst that delivers performance comparable to more complex systems reported in the literature (Table S2).

3. Conclusion

We report a self-templated strategy for the direct growth of Ni₃(BO₃)₂/Ni­(OH)₂ nanostructures on Ni foam without the addition of external Ni precursors. This in situ etching–growth process yields a mechanically robust and chemically stable catalyst with active phases intimately integrated into the conductive substrate. Structural and spectroscopic analyses confirm the formation of mixed phases of Ni₃(BO₃)₂ and Ni­(OH)₂ architecture that enables efficient NO₃RR, achieving an NH₃ yield rate of 1.49 ± 0.04 mmol h^–1 cm^–2 at −0.55 V vs RHE with 81 ± 4.8% FE_NH3 . This study also elucidates the distinct roles of urea and boric acid in governing the hydrothermal phase formation and morphology. To further reduce energy input and enhance system sustainability, NO₃RR was coupled with GOR in a two-electrode configuration. The integrated NO₃RR||GOR system operates at a low cell voltage of ∼1.47 V at 10 mA cm^–2, delivering ∼63% FE_NH3 and achieving 22.8% glycerol conversion with ∼73% FE_HCOO ^–. Compared with the NO₃RR||OER electrolyzer, this coupling strategy demonstrated an energy-saving efficiency of ∼13%. Overall, this work highlights the potential of simply fabricated, bifunctional Ni-based electrodes for circular nitrogen utilization and coelectrolysis platforms, offering a scalable pathway toward sustainable NH₃ production.

4. Experimental Methods

4.1. Chemicals and Materials

Boric acid, salicylic acid, and sodium nitroferrocyanide were procured from Sigma-Aldrich; urea and glycerol were obtained from QReC (grade AR); sodium hypochlorite was acquired from Carlo Erba Reagents; potassium nitrate, potassium hydroxide, sodium hydroxide, and trisodium citrate dihydrate were sourced from Kemaus; Nafion 117 membrane (DuPont) and Ni foam (thickness 0.6 mm) were all utilized without further purification unless specified otherwise.

4.2. Preparation of the Binder-Free Electrode via the Hydrothermal Process

Ni foam was first cleaned by sonication for 10 min in acetone, followed by 0.1 M HCl to remove surface impurities. The foam was then sonicated in distilled water for 10 min in four consecutive cycles. Four pieces of the cleaned Ni foam (2 cm × 1 cm each) were placed in a 100 mL Teflon-lined autoclave, and aqueous solutions containing urea and boric acid were placed at designated molar ratios (urea/boric acid = U:B). The autoclave was filled to ∼75% of its volume with distilled water, sealed, and heated in an oil bath at 150 °C for 24 h, during which Ni²⁺ ions were generated from the surface oxidation of Ni⁰. After the reaction, the autoclave was gradually cooled to room temperature within the oil bath, and the Ni foam samples were washed thoroughly with distilled water and ethanol, followed by drying at 70 °C for 6 h. The resulting material was designated as Ni_U:B@Ni foam, where U:B refers to the molar ratio of added urea to boric acid.

4.3. Material Characterization

The prepared Ni_U:B@Ni foam electrodes were characterized by the following instruments. XRD patterns were used for structural analysis of Ni_U:B@Ni foam using PANanalytical, Cu Kα radiation, 1.5418 Å, 40 kV, 15 mA (soller slits- 0.02 rad, divergence slit- 1/8°) and were recorded at a scan rate of 2°/min between 10° and 90° (2θ range). XPS was performed in the VersaProbe 4 model with an Al Kα monochromatic source (X-ray radiation, 1254 eV at 24.6 W; beam diameter, 100 μm; neutralizer, 1.0 V; 3.0 μA). Binding energy values were corrected with reference to C 1s B.E. 284.9 eV. XPSPEAK41 software was used for the data analysis and curve fitting. Sherley-type background and Gaussian data were used for data processing. A morphological study of the Ni_U:B@Ni foams was performed by FESEM on the “Quanta 200 FEG FE-SEM” instrument and TEM for Ni_8:8@Ni foam, and Ni_8:16@Ni foam was performed on a FEI Talos F200X at an acceleration voltage of 200 kV. Ni foam was sonicated in 0.2–0.4 mL ethanol for 6 h, and a few drops of the dispersion were deposited onto Cu mesh grids for TEM analysis. Before testing, the Cu mesh grids were dried at room temperature overnight.

4.4. Electrochemical Measurements

4.4.1. Three-Electrode Configuration

The Squidstat instrument was used for the detailed electrochemical NO₃ reduction reaction. Electrochemical analysis was carried out using two-compartment electrochemical cells (H-type cell) separated by the Nafion membrane, in a three-electrode setup. Ni_U:B@Ni foam (0.25 cm²), Hg/HgO, and platinum sheet employed as a working electrode, reference electrode, and counter electrode, respectively. Nafion 117 was pretreated in a 5% H₂O₂ solution and then in 1 M H₂SO₄ at 80 °C for 1 h each and rinsed with deionized water every 15 min for 1 h. The experimental potentials were changed with respect to RHE by applying E _RHE = E _Hg/HgO + 0.098 V + 0.059 pH. For electrochemical NO₃ ^– reduction, 1 M KOH was used as a blank electrolyte in the anode compartment, while 1 M KOH containing 0.1 M KNO₃ was used as electrolyte solution in the cathode compartment. The counter electrode, a platinum sheet, was placed in the anode compartment. Working and reference electrodes were placed in a cathode compartment. The electrochemical performance of the as-synthesized Ni_U:B@Ni foam electrodes was systematically evaluated for the NO₃RR for NH₃ production through LSV and CPE. The measurements were conducted in a 1 M KOH electrolyte containing 0.1 M KNO_3. The LSV analysis was carried out at 10 mV/s in the potential range from 0.20 to −0.60 V vs RHE. To ensure NO₃ ^– reduction reaction, CPE measurements were done at various potentials for 15 min. The stability of the catalyst for NO₃ ^– reduction was analyzed by repeating 18 cycles at optimized potential.

4.4.2. Two-Electrode Configuration

The overall electrolysis used Ni_8:8@Ni foam as the anode and cathode, with a pretreated Nafion 117 membrane separating the electrolyte of 1 M KOH with 0.1 M KNO₃ in the cathodic chamber and the electrolyte of 1 M KOH with 0.1 M glycerol in the anodic chamber. The stability was compared between the H-type cell and the flow cell by testing at a current density of 10 mA cm^–2, and the electrolytes were changed every 24 h.

4.5. Product Quantification

4.5.1. NH₃ Quantification

NH₃ quantification was executed using UV–vis spectroscopy (SP-UV 300, Spectrum Instruments). The electrolyte collected after 15 min of electrocatalysis was diluted to fall within the UV–vis detection range. NH₃ concentrations were determined by referencing a standard calibration curve prepared using the indophenol blue method with standard solutions of ammonium chloride in 1 M KOH. In brief, ammonium hydroxide was utilized as a reference solution for calibrating ammonia concentration. A known concentration of ammonium hydroxide was prepared in 1 M KOH, which also served as the electrolyte for nitrate reduction. For UV–vis spectroscopic analysis, 2 mL of standard ammonium chloride solution was combined with 2 mL of a 1 M NaOH mixture containing 5 wt % salicylic acid and 5 wt % sodium citrate, followed by the addition of 1 mL of 0.05 M NaClO and 0.2 mL of 1 wt % sodium nitroferrocyanide­(III) dihydrate. The mixture was gently agitated for 1 min and allowed to rest undisturbed for 2 h. UV–vis absorbance measurements (500–800 nm) were recorded, with the absorbance peak at 650 nm used for calibration. The detection and estimation of ammonia in the electrolyte were analyzed by following a similar method. Faradaic efficiency (FE_NH3 )) and the NH₃ yield rate were calculated using the following formulas

$$\mathrm{F}{\mathrm{E}}_{\mathrm{N}{\mathrm{H}}_3}=\frac{8\times F\times C_{\mathrm{N}{\mathrm{H}}_3}\times V}{Q}\times 100\%$$

where F = Faraday constant, C _NH3 = ammonia concentration, V = electrolyte volume, Q = total charge.

$$\mathrm{N}{\mathrm{H}}_3\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}=C_{\mathrm{N}{\mathrm{H}}_3}\times V/(t\times A)$$

where t = electrolysis time and A = electrode area.

4.5.2. H₂ Quantification

H₂ gas evolved during the nitrate reduction test was measured in a Shimadzu GC-2030A Plus gas chromatograph (GC) equipped with Barrier discharge ionization detector (BID) and using helium as the carrier gas at a flow rate of 570 kPa. After electrolysis, a 50 μL sample of the headspace gas was injected directly onto the GC. Faradaic efficiency (FE_H2 ) was calculated using the following formulas

$$\mathrm{F}{\mathrm{E}}_{{\mathrm{H}}_2}=\frac{2\times F\times C_{{\mathrm{H}}_2}\times V}{Q}\times 100\%$$

where F = Faraday constant, C _H2 = hydrogen concentration, V = electrolyte volume, Q = total charge.

4.5.3. Glycerol Conversion Rate

The conversion of glycerol after 4 h of oxidation was measured using HPLC (Shimadzu UFLC HPLC System) equipped with a differential refractive index detector and an organic acid column (7.8 × 300 mm, 10 μm). The column temperature was 55 °C. The eluent was 5 mM H₂SO₄, with a flow rate of 0.5 mL min^–1. The reaction was carried out in 1.5 mL of electrolyte containing 1 M KOH and 0.1 M glycerol. The glycerol conversion rate (η_0.1 M GLY) was estimated using the formulas below

$${\eta }_{0.1\mathrm{M}\mathrm{G}\mathrm{L}\mathrm{Y}}=\frac{C_{0.1\mathrm{M}\mathrm{G}\mathrm{L}\mathrm{Y}}-C_{x\mathrm{M}\mathrm{G}\mathrm{L}\mathrm{Y}}}{C_{0.1\mathrm{M}\mathrm{G}\mathrm{L}\mathrm{Y}}}\times 100\%$$

The faradaic efficiency and yield rate of formate at various potentials were quantified using the formulas below

$$\mathrm{F}{\mathrm{E}}_{\mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}}=\frac{8\times F\times C_{\mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}}\times V}{3\times Q}\times 100\%$$

where F = Faraday constant, C _HCOO ^– = formate concentration, V = electrolyte volume, Q = total charge.

$$\mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}=C_{\mathrm{H}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}}\times V/(t\times A)$$

where t = electrolysis time and A = electrode area.

4.5.4. Energy-Saving Efficiency

The number was calculated using the following equation

$$\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}‐\mathrm{s}\mathrm{a}\mathrm{v}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{(E_{\mathrm{O}\mathrm{E}\mathrm{R}}-E_{\mathrm{N}{\mathrm{O}}_3\mathrm{R}\mathrm{R}})}{E_{\mathrm{O}\mathrm{E}\mathrm{R}}}\times 100\%$$

where E _NO3RR||OER = potential of NO₃RR||OER reaction cell driven by apply 10 mA cm^–2, E _NO3RR||GOR = potential of the NO₃RR||GOR reaction cell. The potentials were averaged over pseudosteady-state values measured between 1 and 4 h.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.5c09453.

• Comparative XRD analysis of Ni_8:8@Ni foam for the two-preparation batch, SEM image shows the porous nature of Ni foam, different resolution SEM images and EDS element mapping of Ni_8:8@Ni foam, SEM and EDS element mapping of Ni_0:8@Ni foam, different resolution SEM images and EDS element mapping of Ni_8:0@Ni foam, different resolution SEM images and EDS element mapping of Ni_8:16@Ni foam, different resolution SEM images and EDS element mapping of Ni_16:16@Ni foam, comparative XPS survey spectra of Ni_8:8@Ni foam with Ni_8:16@Ni foam, comparative O 1s bonding area in Ni_8:8@Ni foam with Ni_8:16@Ni foam, Ni 2p and O 1s XPS spectra collected from Ni_8:0@Ni foam, Raman spectra of Ni_8:0@Ni foam, CPE traces of Ni foam, Ni_16:16@Ni foam, Ni_8:16@Ni foam, and Ni_8:8@Ni foam, comparative LSV curves of Ni_8:8@Ni foam in 1 M KOH with 100, 10, 1, 0.1, and 0 mM KNO₃, CPE traces, and absorbance spectra for 100, 10, 1, 0.1, and 0 mM KNO₃ at −0.55 V, and Faradaic efficiency and NH₃ yield rate in different concentrations, UV–vis spectra for indo-phenol determination of different known concentrations of NH₄ standards, standard curves for NH₄ using the indophenol blue method, absorbance spectra of (a) Ni foam, (b) Ni_16:16@Ni foam, (c) Ni_8:16@Ni foam, and (d) Ni_8:8@Ni foam, comparative NH₄ concentration of Ni foam, Ni_16:16@Ni foam, Ni_8:16@Ni foam, and Ni_8:8@Ni foam using the indophenol blue method, Faradaic efficiency of Ni_8:8@Ni foam toward NH₃ and H₂ at various potentials, comparison: XRD pattern and LSV curve at 10 mVs-1 in 1 M KOH containing 0.1 M KNO₃ for Ni₃(BO₃)₂@Ni foam, Ni­(OH)₂@Ni foam (Ni_0:8@Ni foam), and Ni_8:8@Ni foam, CPE curves and corresponding UV–vis absorption spectra of Ni_8:8@Ni foam for electrochemical catalytic production of NH₃ during cycling tests in 1 M KOH with 0.1 M KNO₃ at −0.55 V, HPLC chromatograms after 4 h CPE test in 1 M KOH containing 0.1 M glycerol for Ni_8:8@Ni foam, HPLC chromatograms of formic acid at various concentrations, corresponding calibration curves for quantitative analysis, absorption UV spectra and glycerol conversion rate of Ni_8:8@Ni foam after 24 h electrolysis at 10 mA cm^–2 in an H-type cell, SEM image of Ni_8:8@Ni foam before electrolysis and after electrolysis for 24 h in a NO₃RR||GOR electrolyzer (H-type cell), XPS spectra of B 1s, Ni 2p, and O 1s regions for post NO₃RR-Ni_8:8@Ni foam, corresponding XPS spectra for post GOR-Ni_8:16@Ni foam, gray lines indicate experimental data points, while colored lines represent fitted results, background, and peak sum, digital photographs of different sizes of electrodes used in the H-type cell (0.25 cm²) and flow cell (4 cm²), NO₃RR||GOR coelectrolysis system with Ni_8:8@Ni foam as both the cathode and anode in a flow cell (PDF)

C.P. and P.M. designed the project. P.M and S.I. synthesized and conduct the electrochemical performance of the Ni_U:B@Ni foam. P.M. and W.Y. (Warunyoo) developed and conducted the catalytic performance of the electrolyzer cell. P.M., W.L., J.P. W.H., Q. W., and S.K. conduct the physical characterization of Ni_U:B@Ni foam and postelectrocatalysis charcterization. P.M., W. Y. (Wijak), and W. T. quantify the formation products. P.M. and S.I. drafted the manuscript with C.P. All authors have given approval to the final version of the manuscript. C.P. supervised the project.

The authors declare no competing financial interest.