Electrodeposited CoP₂ on CO₂-laser-modified graphite felt: a robust electrocatalyst for nitrite reduction to ammonia

Abstract

The conversion of nitrite-based pollutants to value-added ammonia (NH₃) via sustainable electrocatalysis represents a remarkable advancement in waste management research. Herein, a two-step strategy was developed to synthesize well-dispersed cobalt phosphide (CoP₂) on graphene oxide (GO)–graphite felt (GF), termed CoP₂/GO–GF. The electrodeposited CoP₂ exhibited exceptional performance in the electrocatalytic NO₂⁻ to NH₃ reduction reaction (NO₂RR), achieving a maximum NH₃ yield rate of 10.6 mg h⁻¹ cm⁻² with a faradaic efficiency of 80% at −0.4 V vs. the reversible hydrogen electrode (RHE). The high efficiency of CoP₂/GO–GF is attributed to its improved surface-active site density, enhanced electrochemical double-layer capacitance (3.37 mF cm⁻²), and optimized electron transfer resistance (13.31 Ω). Furthermore, a turnover frequency analysis of the NO₂RR indicated the abundance of active sites, facilitating smooth charge tunneling from CoP₂ to CO₂ laser-developed GO on GF in CoP₂/GO–GF. In situ FTIR analysis confirmed the sequential reduction pathway from NO₂⁻ to NH₃, identifying NO as a key intermediate. Additionally, density functional theory (DFT) calculations revealed a moderate free energy barrier (0.26 eV) for the rate-limiting step, thus validating the thermodynamic feasibility of the reaction. Furthermore, durability tests demonstrated stable performance over 10 reuse cycles, confirming the efficiency and robustness of CoP₂/GO–GF as an electrocatalyst in the NO₂RR.

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1. Introduction

Nitrogenous waste generated by modern agricultural practices, sewage discharge, and industrial processes has become a major concern for groundwater quality. Reports indicate that elevated levels of nitrate/nitrite (NO₃⁻/NO₂⁻) contribute to various health-related issues, including reproductive problems and methemoglobinemia, while also damaging aquatic life by creating oxygen (O)-deficient “dead zones” in aquatic ecosystems.¹ To facilitate NO₂⁻ removal from water, methods such as catalytic reduction,^2–4 adsorption,⁵ membrane filtration,⁶ and ion exchange⁷ are increasingly being used. Compared with conventional methods, NO₂⁻-to-ammonia (NH₃) conversion is a more efficient and sustainable approach for NO₂⁻ removal.⁸ Thus, the electrocatalytic conversion of NO₂⁻ to NH₃ is a promising strategy. Consequently, optimizing this process could substantially enhance agricultural efficiency and environmental sustainability. In NO₂⁻ reduction, the considerably low N=O bond strength makes NO₂⁻ an effective precursor for NH₃ production through N–H bond formation.^9–11 However, efficiency and selectivity are major issues in the case of the NO₂⁻ to NH₃ reduction reaction (NO₂RR), as competing reactions forming N₂ and H₂ hinder the NH₃ yield.¹² Thus, developing materials capable of simultaneously mitigating pollution and producing value-added chemicals remains a critical challenge.

Metal phosphides, particularly transition metal phosphides (TMPs), have recently attracted attention owing to their unique electronic and chemical structure. These properties render them promising candidates for diverse electrocatalytic applications, such as water splitting reactions,¹³ carbon dioxide (CO₂) reduction reactions (CO₂RR),^14,15 and NO₂⁻/NO₃⁻ reduction reactions (NO₂RR/NO₃RR).^16,17 The combination of metals such as cobalt (Co), nickel, molybdate, and iron with phosphorus (P) promotes a delocalized electron system, resulting in high electrical conductivity, which is crucial for electrochemical reactions.^18–20 Among TMPs, cobalt phosphide (CoP₂) is a particularly attractive material owing to its excellent electrical conductivity, strong metal–support interactions, and robust catalytic activity.^21,22 Recently, Hou et al. demonstrated that ultrafine deposition of CoP₂ on carbon nanosheets markedly enhanced water oxidation reaction performance by improving mass and electron transport properties of CoP₂.²³ Furthermore, Song et al. revealed that CoP₂ facilitates the hydrogenation of carbon monoxide (CO), with P incorporation altering the electronic and adsorption properties of metallic Co. Notably, Co^δ+ sites were identified as active centers for the formation of higher oxygenates responsible for CO hydrogenation.²⁴

However, enhancing the overall catalyst reactivity in catalytic reactions requires tuning the material for highly active surfaces, which is of utmost importance. In this case, graphite felt (GF) largely fulfills these requirements by providing large surfaces with high chemical stability. However, the low wettability of GF restricts charge transfer during electrochemical processes.²⁵ The surface engineering of GF with graphene oxide (GO) or the introduction of surface functionalities, such as oxidative modification, has proven to be an effective strategy for increasing the electrode–electrolyte interaction.^26–28 Furthermore, to optimize the graphitic surface and enhance its surface characteristics, CO₂ laser treatment has gained traction due to its rapid, controllable, and localized energy input. This technique effectively introduces surface micro-defects and increases surface roughness, thereby enhancing wettability, which is advantageous for electrodeposition and electrochemical performance.^29–31

Thus, this study aims to establish strong interactions between GO–GF and metal phosphides via coupled CO₂-laser-based thermal treatment and electrodeposition to boost NO₂RR performance. Furthermore, defect-modulated CoP₂ minimizes the charge transfer resistance, enhances the surface redox activity, and boosts NH₃ yield at −0.4 V vs. RHE. The efficient NO₂RR results further demonstrate the consistency between density functional theory predictions and experimental findings.

2. Experimental section

2.1. Preparation of CoP₂-loaded GF

CoP₂ was decorated on GF using a two-step method. First, the GF surface was converted to GO by treating it with a CO₂ laser at 26.5 W (95% power) for 15 min. Thereafter, electrodeposition was performed on the treated GF using a platinum wire as the counter electrode and Ag/AgCl as the reference electrode at −1.3 V (vs. Ag/AgCl) for 25 min in a 40 mL electrolyte solution containing 0.001 M CoCl₂·6H₂O, 0.5 M NaH₂PO₂·H₂O, and 0.1 M NaCl. Following electrodeposition, the resulting CoP₂-loaded GF was rinsed with deionized water and ethanol, followed by drying at 60 °C. For simplicity, the untreated GF, CO₂-treated GF, and CoP₂-decorated CO₂-treated GF were termed bare GF, GO–GF, and CoP₂/GO–GF, respectively.

3. Results and discussion

3.1. Surface transformation and morphological evolution of the graphite felt

The electrodeposition of CoP₂ onto GO-engineered GF was performed in a two-step process, as illustrated in Fig. 1a. Owing to the poor wettability of bare GF hindering the electrolytic process, a two-step modification was employed to electrodeposit CoP₂ onto the GF carbon (C) surface. First, the GF was thermally etched using a CO₂ laser, thereby introducing a GO structure to enhance the wettability and interaction between the GF and CoP₂. This tailored GF surface provided a highly receptive platform for the subsequent deposition of CoP₂. In the second step, CoP₂ was electrochemically deposited at −1.3 V (vs. Ag/AgCl) onto the GO-engineered GF, forming CoP₂/GO–GF. This surface modification induced noticeable morphological and compositional changes, altering the elemental ratio of the material (Fig. 1b and c). The initial C content of the bare GF (Fig. 1b) was 99.68 wt%. This decreased to 97.68% after CO₂ laser irradiation (Fig. 1c), with an increase in the O content to 2.32%. Finally, electrodeposition introduced Co (2.3%) and P (5.7%), thereby reducing the C content to 82.5%. This confirmed successful electrodeposition, as shown in Fig. 1d. In addition, energy dispersive spectroscopy (EDS) spectra (Fig. S1a and b†) confirmed the formation of CoP₂, with compositional analysis of Co and P revealing a stoichiometry consistent with CoP₂. This result is further supported by the TEM studies, as depicted in Fig. S2.† The TEM image shows the nanoscale characteristics of the synthesized CoP₂/GO–GF, demonstrating a uniform distribution of Co, P, O, and C.

Fig. 1 (a) Schematic of the CoP₂/GO–GF synthesis process and FESEM-EDS analysis of (b) bare GF, (c) GO–GF, and (d and e) CoP₂/GO–GF.

3.2. Structural and chemical characterization

The crystalline, molecular, and chemical characteristics of the prepared electrocatalytic materials were analyzed through X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), as presented in Fig. 2a–e. The Raman spectra of the GF, GO–GF, and CoP₂/GO–GF samples (Fig. 2a) exhibited characteristic D and G bands at 1347 and 1595 cm⁻¹, respectively, indicating the presence of C-based structures. The ratio of D to G band intensities (I_D/I_G) provided insights into the structural order of the C surface, with bare GF exhibiting a high I_D/I_G ratio (1.22), indicating a highly disordered C structure. However, the functionalization of GF with GO reduced the I_D/I_G ratio to 1.18, suggesting an increased structural order owing to the layered structure of GO. In contrast, the electrodeposition of CoP₂ onto the GO-modified surface increased the I_D/I_G ratio to 1.31, implying a high defect density. This increase in the I_D/I_G ratio likely resulted from CoP₂-induced disruptions in the C framework, altering the electronic structure of the CoP₂/GO–GF and introducing localized disorder within the matrix catalyst. The diffraction pattern presented in Fig. 2b revealed a prominent peak at 10.68° upon CO₂ laser irradiation, thereby confirming the presence of GO on GF, along with C reflections at 26.6°, 44°, and 54.7° (JCPDS No. 00-026-1080).³² However, the low concentration of CoP₂ yielded weak diffraction signals. To address this, Fig. 2c presents an expanded diffraction view of CoP₂/GO–GF, showing two additional peaks at 2θ values of 33° and 35°, which are attributed to electrodeposited CoP₂ (JCPDS No. 00-026-0481).

Fig. 2 Structural and compositional analysis of bare GF, GO–GF, and CoP₂/GO–GF: (a) Raman spectra, (b) XRD patterns, (c) expanded XRD view, and XPS spectra of (d) C 1s, (e) O 1s, (f) Co 2p, and (g) P 2p.

The XPS survey spectra of GF, GO–GF, and CoP₂/GO–GF showed the elemental composition view of C, O, Co, and P, as presented in Fig. S3.† As expected, the survey spectra exhibited an increase in O content from bare GF to GO–GF, providing clear evidence of GF surface transformation. Fig. 2f presents the core C 1s spectra, which display multiple deconvoluted peaks at binding energies (BEs) of 283.8, 284.6, 285.1, 286.1, and 286.6 eV, corresponding to various C bonding states. The peaks at 283.8 and 284.6 eV were attributed to sp² hybridized C=C and sp³ hybridized C–C bonds, respectively, originating from the graphitic framework of GF. Meanwhile, the peaks at 285.1, 286.1, and 286.6 eV were attributed to O-functionalized C species, specifically the –C–OH, –C–O, and O=C–O groups, respectively.³³ In addition, a peak at 291.5 eV observed in GO–GF corresponded to the π–π* transition, which is characteristic of the conjugated electronic structure in GO.³⁴ Furthermore, the deconvoluted O 1s core-level spectra (Fig. 2g) revealed three distinct peaks corresponding to the C=O, C–O, and H–O–H functional groups, with BEs of 531.78, 532.81, and 534.5 eV, respectively, in GF, GO–GF, and CoP₂/GO–GF.³⁵ Interestingly, an additional peak at 532.41 eV, observed in the GO–GF and CoP₂/GO–GF spectra, corresponded to C–O–H,³³ indicating the surface functionalization of GF via GO incorporation. Thereafter, the high-resolution Co spectrum (Co 2p_3/2 and Co 2p_1/2), depicted in Fig. 2f, exhibited distinct Co states upon deconvolution. The Co 2p signals at BEs of 782.5 and 798.7 eV were attributed to Co–P species, whereas those centered at 779.3 and 794.3 eV corresponded to the Co²⁺ species, accompanied by satellite peaks at 789.5 and 805.72 eV.^36,37 Furthermore, the P 2p signals at low BEs of 129.1 eV (P 2p_3/2) and 130 eV (P 2p_1/2), were attributed to Co–P linkage.³⁸ However, the peak at 133.2 eV indicated the oxidized P species, a common feature in metal phosphide-based materials.^39,40

3.3. Electrocatalytic NO₂⁻ reduction reaction (NO₂RR)

The electrocatalytic conversion of NO₂⁻ to NH₃ was investigated using the as-synthesized GF, GO–GF, and CoP₂/GO–GF electrocatalysts in an H-type electrochemical cell setup (Fig. S3†), with the results presented in Fig. 3a–h. Following 200 activation cycles (Fig. S4†), linear sweep voltammetry (LSV) was conducted in the presence and absence of NO₂⁻. It revealed notable changes in surface reactions across all studied electrocatalysts in the NO₂⁻-containing environment (Fig. 3a). Furthermore, a substantial enhancement in the current density was observed for CoP₂/GO–GF compared to that for bare GF and GO–GF (Fig. 3b), which is attributed to the strong interaction between the electroactive CoP₂ and GO–GF.⁴¹ This observation was further supported by the Tafel slope analysis (Fig. 3c), which indicated the trend CoP₂/GO–GF (148.8 mV dec⁻¹) < GO–GF (220.6 mV dec⁻¹) < GF (335.8 mV dec⁻¹). The small Tafel slope of the CoP₂/GO–GF catalyst demonstrated fast charge transfer kinetics, highlighting its superior intrinsic catalytic performance. Similarly, the enhanced charge transfer capability of the catalyst was analyzed using the Nyquist plot measured at −0.13 V vs. RHE (Fig. S5a†). The charge transfer resistance (R_ct) was calculated from the Nyquist plot. It was 13.31 Ω for CoP₂/GO–GF, which was lower than the values of 49.9 and 49.04 Ω for GF and GO–GF, respectively (Fig. S5b†). The remarkably lower R_ct value of CoP₂/GO–GF indicates improved electron transport kinetics compared to GO–GF and GF, thereby highlighting its superior conductivity, which facilitates the efficient NO₂RR. In addition, notable phase shifts at higher frequencies, as shown in Fig. S6,† reflect the optimized charge transfer dynamics of CoP₂/GO–GF, enhancing the rate of the electrochemical NO₂RR. Furthermore, the CV curves recorded at different scan rates (10–100 mV s⁻¹, Fig. S7†) provided insights into the surface capacitance and active surface area of the electrocatalysts.

Fig. 3 Electrocatalytic NO₂RR measurements: (a) LSV measurements; (b) comparative current density (mA cm⁻²) vs. potential (V vs. RHE) trend; (c) Tafel slopes; (d) NH₃ partial current densities of GF, GO–GF, and CoP₂/GO–GF; (e) NH₃ yield via UV-vis spectroscopy; (f) FE of NH₃ production; (g) NH₄⁺ yield via NMR spectroscopy; (h) reaction stability over 10 h for CoP₂/GO–GF; and (i) comparison of ammonia yield with reported studies.

Thus, the considerably higher electrochemical double-layer capacitance (C_dl) of CoP₂/GO–GF (3.37 mF cm⁻²) compared to that of GO–GF (2.84 mF cm⁻²) and bare GF (1.38 mF cm⁻²), as depicted in Fig. S8a†, suggests that CoP₂/GO–GF possesses a greater density of accessible active sites. A high C_dl indicates the increased ability of the electrocatalyst to store and release charge, thereby increasing its interaction with the electrolyte and yielding an enhanced catalytic performance.⁴² Electrochemically active surface area calculations were conducted (Fig. S8b†) and revealed a higher density of catalytically active sites in CoP₂/GO–GF (0.038 cm²), outperforming GF (0.015 cm²) and GO–GF (0.032 cm²). This resulted in the enhancement of surface–reactant interactions and consequently boosted the electrocatalytic efficiency.

The partial current density is a key metric for evaluating the efficiency of the electrocatalysts in producing the reaction products. In Fig. 3d, the highest partial current density for NH₃ production was achieved with CoP₂/GO–GF at −0.4 V vs. RHE, surpassing both GO–GF and bare GF. This NO₂RR trend (CoP₂/GO–GF > GO–GF > GF) was also reflected in the ultraviolet-visible (UV-vis) absorbance (Fig. S9a–c†), which exhibited a higher absorption peak for CoP₂/GO–GF compared to that for GO–GF and GF. A similar trend was observed in the proton nuclear magnetic resonance (¹H NMR) spectra (Fig. S10†), showing an increased peak area for CoP₂/GO–GF. To quantify the NH₃ yield, a standard calibration curve was plotted using the indophenol blue method with known NH₃ concentrations (Fig. S11a and b†). Bulk electrolysis on CoP₂/GO–GF (Fig. S12†) was performed at 0 to −0.4 V vs. RHE, with NH₃ yield rates determined from the UV-vis absorption spectra shown in Fig. 3e. A linear increase in NH₃ production was observed, reaching a maximum production capacity of 10.6 mg h⁻¹ cm⁻² at an optimal potential of −0.4 V vs. RHE. Faradaic efficiency (FE) is a crucial parameter for determining the effectiveness of electrochemical reactions toward target products. However, CoP₂/GO–GF demonstrated an exceptional FE of 80% for NH₃ measured at −0.4 V vs. RHE, thereby confirming its outstanding NO₂RR performance (Fig. 3f). The accuracy of the NH₃ yield rate for CoP₄/GO–GF was evaluated through three repetitive cycles at each tested potential (−0.1 to −0.4 V vs. RHE), as shown in Fig. S13–S16,† revealing high reproducibility with negligible variations. Meanwhile, a gradual drop in NH₃ production and FE at elevated potentials was observed in all the studied materials (Fig. S17a and b†). This suggests that increased competing hydrogen evolution reaction (HER) becomes predominant at high negative potentials.⁴³

Furthermore, ¹H NMR was employed to understand the accuracy of the NH₃ yield, with known NH₃ concentrations (Fig. S18a†) used to construct a calibration curve for comparison with NH₃ produced during the NO₂RR. Thereafter, the ¹H NMR spectra (Fig. S18b†) of the electrolyte confirmed an NH₃ yield of approximately 10.35 mg h⁻¹ cm⁻² over CoP₂/GO–GF at −0.4 V vs. RHE (Fig. 3g), with an FE of ∼78%. This validated the precision and consistency of the reaction, as corroborated by the indophenol blue method (Fig. S19†). The improved performance of CoP₂/GO–GF is largely attributed to its high active site density, which enhances its NO₂⁻ adsorption and activation capabilities. Active site density calculations for CoP₂/GO–GF, GO–GF, and GF yielded values of 17.32, 16.02, and 10.91 μmol cm⁻², respectively (Fig. S20†). Consistent with the results reported by Pan et al.,⁴⁴ who demonstrated that strong active site interactions enhanced the electrochemical performance in Co–P-based HER catalysts, our turnover frequency analysis (Fig. S21†) revealed that CoP₂/GO–GF possessed higher intrinsic catalytic activity than GF and GO–GF, thus aligning with its remarkable performance in the NO₂RR.

Extended studies on the reusability of the CoP₂/GO–GF electrocatalyst were conducted over 10 cycles, with the electrolyte replaced every 1 h. The material exhibited minimal variations in the current density, FE, and NH₃ yield, thereby confirming its stable catalytic performance (Fig. 3h and S22a–c†). Furthermore, the comparative analysis presented in Fig. 3i and Table S1† illustrates the superior performance of CoP₂/GO–GF relative to recently reported catalysts for NO₂⁻ reduction. Following 10 repetitive cycles, the compositional stability of the material was assessed using SEM-EDS color mapping (Fig. S23†) on CoP₂/GO–GF. Apart from slight variations, no profound changes in elemental distribution were observed. This finding was further supported by XPS analysis of CoP₂/GO–GF (Fig. S24†), exhibiting consistent peak splitting in the C 1s, O 1s, Co 2p, and P 2p spectra before and after the reaction. Notably, the Co 2p and P 2p peaks at BEs of approximately 782 eV (2p_3/2) and 130/129 eV (2p_1/2/2p_3/2), respectively, indicated no structural deformation in CoP₂. The material characterization of CoP₂/GO–GF following the stability study aligned with the results from the multiple cycle analysis (Fig. 3h), as demonstrated earlier. Additionally, an extended 50 h chronoamperometric stability test was conducted (see Fig. S25†), during which the catalyst retained ∼80% of its initial current density (from −125.5 to −100.97 mA cm⁻²), thereby underscoring the prolonged durability of CoP₂/GO–GF.

3.4. In situ NO₂RR and DFT studies

The ex situ FTIR analysis was conducted to investigate the detailed NO₂⁻ reduction reaction (NO₂RR) pathway under varying applied potentials, ranging from open circuit potential (OCP) to −0.5 V vs. RHE. The results, presented in Fig. 4a–c, provide insights into the sequential transformation of nitrite into ammonium. At OCP, the IR spectra exhibit a characteristic peak corresponding to NO₂⁻ vibrations at 1226 cm⁻¹, along with a broad peak at 1637 cm⁻¹ attributed to H₂O. However, due to the overlap between the N–O stretching vibration (∼1600–1700 cm⁻¹) and the H–O–H bending mode of water, distinguishing NO as an intermediate is challenging.⁴⁵ Despite this spectral overlap, as the applied potential becomes more negative, the progressive evolution of the spectra reveals the formation of NH₄⁺ (∼1400 cm⁻¹), confirming a stepwise electrochemical reduction process. This spectral trend suggests that NO appears as a key intermediate in the NO₂RR pathway, undergoing further reduction to NH → NH₂ before its ultimate conversion to NH₄⁺. Fig. 4b and c provide a colormap representation of potential (V vs. RHE) versus wavenumber, enabling a clearer visualization of intermediate species and product formation. The color gradient represents IR absorbance intensity, where red-shifted regions at more negative potentials indicate an increased presence of NH₄⁺ alongside residual NO₂⁻. A noticeable shift in peak intensity in Fig. 4c, particularly at −0.3 V and −0.4 V, correlates with enhanced NH₄⁺ production, suggesting that these potentials favor the highest ammonium yield. These results confirm that NO acts as an active intermediate, undergoing further reduction (NO → NH → NH₂ → NH₃) in the NO₂RR pathway.^46,47

Fig. 4 Ex situ FTIR spectra (a)–(c) and DFT studies of the CoP₂/GO structure: (d) optimized structure, (e) band and DOS plot, (f) PDOS plot of the CoP₂/GO structure, and (g) reaction profile of the NO₂RR pathway over CoP₂/GO–GF.

To gain deeper insights into the structural and electronic interactions governing NO₂RR efficiency, density functional theory (DFT) calculations were performed. The CoP₂/GO–GF structure was modeled using the monoclinic CoP₂ crystal system (P2₁/c) with the (200) plane, as observed in the XRD pattern. Graphene oxide (GO) was incorporated into the crystal system with CoP₂ as the active surface, and the structure was subsequently optimized. The optimized structure is shown in Fig. 4d, where the C–C bond lengths are approximately 1.49 Å. This length results from the heterostructure formation and strain induced in GO by the monoclinic crystal structure, aligning with previous studies.^48–50 The Co–P, P–P, C–O, and O–H bond lengths are measured to be ∼2.33 Å, 2.27 Å, 1.44 Å, and 0.99 Å, respectively, closely matching previously reported values of 2.32 Å, 2.26 Å, 1.46 Å, and 0.99 Å.^51–54 To evaluate the structural stability of the system, the cohesive energy (E_C) was calculated, yielding a value of −1.85 eV per atom, which confirms the structural integrity of the CoP₂/GO heterostructure. Additionally, the formation energy (E_F) was computed to evaluate the thermodynamic feasibility of the structure, revealing a negative E_F value of approximately −1.23 eV, indicating that the formation is energetically favorable. To examine the electronic properties of the system, the band structure and density of states (DOS) were analyzed, with the corresponding plots displayed in Fig. 4e. The band structure of the CoP₂/GO heterostructure shows metallic characteristics, featuring overlapping energy states near the Fermi level, which is consistent with the DOS analysis. The DOS plots reveal symmetric energy states for both spin-up and spin-down channels, indicating a non-magnetic nature with no net spin polarization. The atomic contributions to the energy states were further investigated using the projected DOS plot in Fig. 4f. The Co atom shows significant contributions to energy states in the valence band region and near the Fermi level, extending into the conduction bands. Notably, there is a strong overlap of C atoms with CoP₂ in the valence band region. At the Fermi level, the overlap between C and Co atoms suggests strong interactions between GO and Co atoms, facilitating charge transfer and enhancing electronic coupling.

To evaluate NO₂RR performance, the NO₂ molecule was initially adsorbed onto the CoP₂/GO structure. The calculated adsorption energy of approximately −1.55 eV indicates that NO₂ adsorption is highly favorable. The catalytic activity of the system primarily depends on the adsorption of reaction intermediates and the reaction energies of elementary steps.⁵⁵ The intermediates were placed at an initial distance of 3 Å, optimized, and the corresponding structures are presented in Fig. S26.† The free energy changes (ΔG) for each reaction step were calculated, and the reaction energy profile is shown in Fig. 4g. The free energy of the initial adsorption was determined to be −1.55 eV, suggesting that the reaction proceeds spontaneously. In the fifth step of the NO₂RR pathway, two possible reaction mechanisms can occur: (i) NOH* → N*, involving the elimination of H₂O, and (ii) NOH* → NHOH*, where an H* atom interacts with the N atom before the H₂O elimination. Based on the calculated ΔG values, the NOH* → N* pathway exhibits a lower reaction free energy, indicating that it is more thermodynamically favorable than the NOH* → NHOH* pathway. This suggests that the direct elimination of H₂O from NOH* to form N* is the preferred route in the NO₂RR process. Finally, for NH₄⁺ formation, the ΔG value for the step must be sufficiently low to facilitate the reaction. The observed ΔG of −0.13 eV confirms the feasibility of NH₄⁺ formation. The highest reaction energy among all elementary steps is considered the rate-limiting step (RLS), which determines the catalytic performance of the material.^56,57 In this study, the third step (NO₂H* → NO*) is identified as the RLS, with a reaction energy barrier of 0.26 eV, suggesting that the material must overcome a moderate energy barrier. However, this energy barrier remains within a feasible range for the reaction to proceed. Furthermore, the faradaic efficiency (FE) for the NO₂RR process was calculated, yielding a value of ∼100% under ambient conditions for the RLS of 0.26 eV. This result indicates that the material exhibits outstanding catalytic performance.⁵⁸ These findings suggest that the CoP₂/GO heterostructure demonstrates remarkable energetic stability and high selectivity toward NO₂, with an RLS of 0.26 eV and an FE of 100%, making it a highly promising candidate for NO₂ reduction applications.

4. Conclusion

This study demonstrated the fabrication of a highly active CoP₂/GO–GF electrocatalyst, developed through the stepwise modification of GF with a CO₂ laser followed by electrodeposition of CoP₂. Comprehensive structural and compositional analysis revealed prominent surface transformations, indicating the importance of enhanced active site density in the NO₂RR. The CoP₂/GO–GF electrocatalyst exhibited remarkable NO₂RR performance, achieving a high NH₃ yield rate of 10.6 mg h⁻¹ cm⁻² and an FE of 80% at −0.4 V vs. RHE. Interestingly, enhanced charge transfer kinetics, lower resistance, and a high density of accessible active sites were key to its superior catalytic activity. Ex situ FTIR and DFT calculations confirmed NO* as a key intermediate in the NO₂RR pathway, with the NO₂H* → NO* step identified as the rate-limiting step (0.26 eV), ensuring a feasible and selective conversion to NH₃. Furthermore, the material demonstrated excellent durability and reusability in NH₃ formation, retaining consistent performance over 10 extended cycles. Its high stability and selectivity highlight its potential for practical applications in sustainable nitrogen conversion. This study showcases the rapid design of CoP₂/GO–GF and its promising performance for sustainable NO₂⁻ to NH₃ conversion, thereby demonstrating its applicability in waste-to-value-added product processes.

Data availability

Data are available within the article and its ESI.†

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This research was supported by the Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (RS-2024-00434932). The authors express their gratitude for the financial assistance of the National Research Foundation of Korea (NRF) (2022R1A2C2010686 and RS-2024-00405324). This work was supported by the Glocal University 30 Project Fund of Gyeongsang National University in 2024.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02053k

‡ These authors contributed equally to this work.

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