Coupling of NiFe Layered Double Hydroxides with Sulfides for Highly Efficient Urea Electrolysis and Hydrogen Evolution

Abstract

Urea electrolysis is regarded as a prospective method for energy-saving hydrogen production. However, the practical application of this technology is limited by the lack of high-performance bifunctional catalysts for hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Herein, a heterostructure catalyst composed of NiFe layered double hydroxide (LDH) and sulfides (NiFe LDH-NiFeS_x/NF) catalysts is prepared via a simple one-step hydrothermal approach. Remarkably, the prepared NiFe LDH-NiFeS_x/NF required 138 mV and 1.34 V to achieve 10 mA cm⁻² for HER and UOR in 1 M KOH and 0.33 M urea, respectively. Furthermore, when NiFe LDH-NiFeS_x/NF is used as a cathode for urea electrolysis, only 1.44 V is required at 10 mA cm⁻², which is much lower than the 1.53 V needed for overall water splitting.

1. Introduction

Electrochemical water splitting, consisting of anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), is considered a potential technology for large-scale production of hydrogen (H₂) because of its environmental friendliness [1,2,3,4,5]. However, the kinetically sluggish OER requires a high theoretical potential of 1.23 V, which restricts the practical application of electrochemical water splitting [6,7,8,9]. In addition, H₂ produced in the cathode chamber and O₂ produced in the anode chamber can easily form dangerous explosive mixtures [10,11,12]. Utilizing urea oxidation reaction (UOR, CO(NH₂)₂ + 6OH⁻ → N₂ + CO₂ + 5H₂O + 6e⁻) with low thermodynamic potential of 0.37 V instead of the OER can effectively solve the above problems and is capable of purifying urea containing wastewater while producing H₂ [13,14,15]. Unfortunately, UOR suffers from slow reaction kinetics owing to the complex 6e⁻ transfer process, which needs efficient electrocatalysts to facilitate this process [16,17,18]. Precious metal-based catalysts such as IrO₂ and RuO₂ have excellent UOR activities, but the poor stability, high price, and low reserves limit their large-scale production [19,20,21,22,23,24]. Moreover, these catalysts usually exhibit poor HER performance and need to be paired with catalysts with HER activity such as Pt/C for overall urea electrolysis [25,26,27,28]. The development of low-cost non-precious-metal-based bifunctional catalysts with high UOR and HER activity is of great significance for the sustainable production of green H₂ and the treatment of urea-containing wastewater; however, great challenges remain.

Recently, the abundant reserves and high electrocatalytic activity of NiFe layered double hydroxide (NiFe LDH) has caused it to demonstrate significant promise as a UOR electrocatalyst [29,30,31]. For example, Xie et al. synthesized NiFe LDH nanosheets on NiFe alloy foam, which required a potential of 1.459 V to reach 100 mA cm⁻² for UOR [30]. Wang et al. synthesized partially amorphous fluorine-modified NiFe LDH for UOR [31]. However, the weak binding energy of H* intermediates on Fe³⁺ sites generally results in unsatisfactory HER activity of NiFe LDH [32]. Coupling multiple active sites to form heterogeneous interfaces can optimize the electronic structure and result in improved electron transfer, which has been proven to be an effective way to enhance the multifunctional catalytic activity of electrocatalysts [33,34,35]. Ni/Fe-based sulfides have excellent HER electrocatalytic properties due to their abundant H* adsorption active sites [36,37]. Pan et al. prepared Ni-Fe sulfide which displayed an overpotential of 262 mV at 100 mA cm⁻² for HER [38]. Coupling LDHs with high UOR activity and transition metal sulfides with high HER activity to form heterostructure catalysts is expected to reduce the overall overpotential of urea electrolysis, thereby realizing energy-saving hydrogen preparation. Recent work has demonstrated that Ni/Fe(OH)₂ and Ni/FeOOH formed by surface reconstitution of Ni/Fe based catalysts under applied voltage act as potential HER and UOR active sites, respectively [39,40]. The presence of S accelerates the decomposition of intermediate CO₃^2- into CO₂ [41]. Recently, many heterogeneous catalysts have been demonstrated to have enhanced bifunctional catalytic activity, such as CoO−Co₄N@NiFe-LDH [42], MoP@NiCo-LDH [43], CoN/Ni(OH)₂ [44], and Ni₃S₂−Ni₃P [45]. However, at present, the preparation of high performance heterostructure catalysts usually requires tedious multi-step synthesis processes. Therefore, manufacturing high-performance bifunctional LDH/sulfide heterostructure electrocatalysts through simple and facile method is of great significance.

Herein, we report the in-situ growth of NiFe LDH-Ni₃S₂-FeS heterostructure nanosheets on nickel foam (NiFe LDH-NiFeS_x/NF) by a simple one-step solvothermal approach. The abundant heterogeneous interface and the accelerated mass transfer allowed by the nanosheet array structure endow NiFe LDH-NiFeS_x/NF excellent HER and UOR activities. Specifically, the NiFe LDH-NiFeS_x/NF required 138 mV and 1.34 V to achieve 10 mA cm⁻² for HER and UOR in 1 M KOH + 0.33 M urea, respectively. Furthermore, when the NiFe LDH-NiFeS_x/NF electrocatalyst was used as both anode and cathode materials for urea electrolysis, a cell voltage of 1.44 V was required at 10 mA cm⁻², lower than many recently reported urea electrolysis electrolyzers, and 1.53 V for overall water splitting.

2. Materials and Methods

2.1. Materials

Nickel chloride hexahydrate (NiCl₂·6H₂O), ferric chloride (FeCl₃), thiourea (CH₄N₂S), hydrochloric acid (HCl), urea (CO(NH₂)₂), methanol, ethanol, and potassium hydroxide (KOH) are all analytical grade and can be used directly without further purification. Nickel foam (NF) was sequentially sonicated with 1.0 M HCl, ethanol, and deionized water (DI) for five minutes to remove organic matter and oxides.

2.2. Synthesis of NiFe LDH-NiFeS_x/NF

To obtain a light-yellow solution, 6 mmol CH₄N₂S, 0.4 mmol NiCl₂·6H₂O, and 0.2 mmol FeCl₃ were added in 30 mL of methanol and 5 mL of DI with magnetic stirring for 10 min. The solution and NF (1 cm × 2 cm) were heated at 100 °C for 8 h in a 100 mL Teflon-lined stainless-steel autoclave. The reacted product was cooled to room temperature, taken out, cleaned with DI, and dried at 60 °C for 12 h under vacuum to obtain NiFe LDH-NiFeS_x/NF. Similarly, Ni₃S₂/NF and NiFe LDH/NF were synthesized in the absence of FeCl₃ and CH₄N₂S, respectively.

3. Results and Discussion

Figure 1a shows the preparation process of the NiFe LDH-NiFeS_x/NF heterostructure catalyst. First, a piece of pretreated nickel foam was immersed in a homogeneous solution of Ni²⁺, Fe³⁺, and CH₄N₂S. During the hydrothermal process, Fe³⁺ reacts with NF to form Fe²⁺ and Ni²⁺ (Equation (1)) [46,47]. Meanwhile, CH₄N₂S hydrolyzes and releases HS⁻, which reacts with NF and Fe²⁺ to form Ni₃S₂ and FeS (Equations (2)–(4)) [48,49]. NiFe LDH was formed by the reaction of metal ions (Fe³⁺, Fe²⁺, and Ni²⁺) and OH⁻ in solution (Equation (5)) [50,51,52].

2Fe³⁺ + Ni → Ni²⁺ + 2Fe²⁺(1)

CH₄N₂S + 3H₂O → HS⁻ + HCO₃⁻ + 2NH₄⁺(2)

3Ni + 2HS⁻+ H2O → Ni₃S₂ + 2OH⁻ + 2H₂(3)

Fe²⁺ + HS⁻ → FeS(4)

Fe³⁺ + Ni²⁺ + Fe²⁺ + OH⁻ → NiFe LDH(5)

The surface morphologies of Ni₃S₂/NF, NiFe LDH/NF, and NiFe LDH-NiFeS_x/NF were characterized by SEM in Figure 1b and Figure S1 (Supporting Information). The NiFe LDH-NiFeS_x/NF 3D nanosheets array are evenly grown on the NF with an average size of 30 nm. The nanosheet array structure can provide plentiful reaction sites which accelerate the permeation of electrolytes and the release of bubbles during urea electrolysis [37,53]. The TEM image further affirms the ultrathin structure of NiFe LDH-NiFeS_x/NF (Figure 1c). The high-resolution TEM (HRTEM) image shows three sets of different lattice fringes with interplanar spacings of 0.21, 0.23, and 0.26 nm, which belong to the (114) plane of FeS, the (003) plane of Ni₃S₂, and the (101) plane of NiFe LDH, respectively (Figure 1d,e). The corresponding selected area electron diffraction (SAED) pattern further proves the intergrowth of NiFe LDH, Ni₃S₂, and FeS in NiFe LDH-NiFeS_x/NF nanosheet (Figure 1f), suggesting the successful formation of metal hydroxide/sulfide heterostructure. In Figure 1g, the EDS elemental mapping images of a NiFe LDH-NiFeS_x/NF nanosheet confirm the even distribution of Ni, Fe, O, and S. The XRD patterns of NiFe LDH-NiFeS_x/NF (Figure S2a) are consistent with the standard XRD patterns of NiFe LDH (JCPDS No. 40-0215), Ni₃S₂ (JCPDS No. 44-1418), and FeS (JCPDS No. 37-0477), suggesting the successful preparation of NiFe LDH-NiFeS_x/NF. The peak of metal Ni comes from NF, which is introduced during the ultrasonic during the preparation of XRD samples.

The chemical and element valence states of the materials were studied by the XPS technique. The XPS survey spectrum demonstrated the presence of Ni, Fe, S, and O elements in NiFe LDH-NiFeS_x/NF, which coincided with the EDS characterization results (Figure 2a). The Ni 2p spectrum of NiFe LDH-NiFeS_x/NF displayed two typical peaks at 855.4 eV and 873.07 eV, which are attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively (Figure 2b). Two satellite peaks (Sat.) were observed at 861.1 and 878.93 eV [36,38,54,55]. The extra Ni⁰ peak at 852.3 eV in Ni₃S₂ represents the exposed NF substrate [39]. The binding energy of Ni 2p_3/2 in NiFe LDH-NiFeS_x/NF is negatively shifted by about 0.2 eV and 0.75 eV compared to NiFe LDH/NF and Ni₃S₂/NF, respectively. In addition, the peak position of Fe 2p_1/2 and Fe 2p_3/2 for NiFe LDH-NiFeS_x/NF exhibited a 0.3 eV positive shift compared with NiFe LDH/NF (Figure 2c). These results suggest that the heterogeneous interface between metal sulfides and hydroxides induces charge transfer and electron rearrangement among the components, which may enhance the catalytic activity of the material [56,57]. The high-resolution S 2p spectrum of NiFe LDH-NiFeS_x/NF can be indexed to S 2p_1/2 (163.54 eV), S 2p_3/2 (162 eV), and S-O (167.7 eV), respectively (Figure 2d) [16,58]. High-resolution XPS spectra of O 1s in NiFe LDH-NiFeS_x/NF is shown in Figure S3a, and the peaks at 529.1, 530.5 and 531.6 eV are attributed to metal-O (M-O), hydroxyl oxygen tethered with metal (M-OH), and absorbed molecular water (M-H₂O), respectively [36,52].

The OER and UOR catalytic activities of the NiFe LDH-NiFeS_x/NF electrocatalyst were tested in 1 M KOH and 1 M KOH + 0.33 M urea, respectively. The linear sweep voltammetry (LSV) curves were shown in Figure 3a. The OER curve observed an oxidation peak at ~1.4 V due to the oxidation of Ni²⁺ to Ni³⁺ [19,59]. Obviously, the UOR curve current density increased rapidly after adding 0.33 M urea to 1.0 M KOH. At 100 mA cm⁻², the UOR electrode potential of NiFe LDH-NiFeS_x/NF was only 1.368 V, which is far below 1.521 V for OER. This result shows that the addition of urea can significantly decrease the anodic oxidation potential and thus pave the way for energy-efficient hydrogen production. As a comparison, the UOR catalytic activity of Ni₃S₂/NF and NiFe LDH/NF, RuO₂/NF, and blank NF were measured under identical conditions. The NiFe LDH-NiFeS_x/NF electrode requires only 1.368 V at 100 mA cm⁻², which is smaller than Ni₃S₂/NF (1.439 V) and NiFe LDH/NF (1.379 V), further demonstrating that the metal sulfide and hydroxide heterostructure are beneficial to the UOR performance (Figure 3b). Simultaneously, NiFe LDH-NiFeS_x/NF requires a smaller potential than the RuO₂/NF (1.457 V) to reach 100 mA cm⁻², which suggests their potential to replace noble metal catalysts. The kinetics of the UOR catalyzed reaction was subsequently investigated by the Tafel slope obtained from the LSV polarization curves. In Figure 3c, the Tafel slope of NiFe LDH-NiFeS_x/NF is only 22 mV dec⁻¹, which is much lower than that of Ni₃S₂ (43 mV dec⁻¹), NiFe LDH/NF (40 mV dec⁻¹), RuO₂ (50 mV dec⁻¹), and the substrate NF (197 mV dec⁻¹), demonstrating boost UOR kinetics. Moreover, the UOR performance of NiFe LDH-NiFeS_x/NF was superior to some non-precious-metal-based electrocatalysts reported recently (Table S1).

Similarly, the HER catalytic performance of the catalyst was tested under the same conditions. The HER curves of NiFe LDH-NiFeS_x/NF in KOH solutions with and without urea differ only by 6 mV at 100 mA cm⁻² (Figure 3d), which indicates that the introduction of urea has a negligible effect on the HER performance. According to Figure 3e, NiFe LDH-NiFeS_x/NF reached 100 mA cm⁻² with an overpotential of 244 mV, which was higher than that of Ni₃S₂/NF, NiFe-LDH/NF, and the substrate NF at 323 mV, 414 mV, and 468 mV, but still lags behind the 20% commercial Pt/C/NF (64 mV). In addition, the HER reaction kinetics was investigated by Tafel slope. The NiFe LDH-NiFeS_x/NF had a lower Tafel slope of 103 mV dec⁻¹ compared to those of Ni₃S₂ (158 mV dec⁻¹) and NiFe LDH (178 mV dec⁻¹), indicating a faster kinetic rate of NiFe LDH-NiFeS_x/NF.

The electrochemically active surface area (ECSA) of the catalyst was evaluated by the double-layer capacitance (C_dl) calculated from CV at different scan rates to further explain the enhanced electrocatalytic performance. Figure S4 shows the CV curves of different catalysts at different scan rates (10–50 mV s⁻¹). The C_dl value of 45.63 mF cm⁻² for NiFe LDH-NiFeS_x/NF is much larger than that of the comparison sample (Figure 3g). This also confirms that this ultrathin 3D nanoarray with heterostructure structure can provide more active sites. The charge transfer resistance (R_ct) between electrolyte and electrode was estimated by electrochemical impedance spectroscopy (EIS) tests. The R_ct of NiFe LDH-NiFeS_x/NF is 1.1 Ω, which is lower than NiFe LDH/NF (1.6 Ω), Ni₃S₂/NF (2.1 Ω), and NF (2.8 Ω). The presence of NiFe LDH-NiFeS_x/NF heterostructure can significantly improve the charge transfer efficiency and therefore achieve better UOR properties (Figure 3h). Stability is also an important indicator in evaluating the electrocatalytic performance. The UOR and HER stability of NiFe LDH-NiFeS_x/NF catalysts were tested in 1 M KOH with 0.33 M urea by chronopotentiometry at a current density of |j| = 10 mA cm⁻², which both can operate stably for 24 h without obvious degradation (Figure 3i and Figure S5).

Based on the outstanding HER and UOR properties of NiFe LDH-NiFeS_x/NF, a two-electrode system with it as cathode and anode was further investigated for electrochemical H₂ production in 1.0 M KOH with 0.33 M urea (Figure 4a). The electrolyzer requires 1.44, 1.55, and 1.60 V to achieve 10, 50, and 100 mA cm⁻², respectively, in the presence of urea, which is 90, 160, and 170 mV lower than those of water splitting (1.53 V @ 10 mA cm⁻², 1.71 V @ 50 mA cm⁻², and 1.77 V @ 100 mA cm⁻²), respectively (Figure 4b,c). Therefore, this result further demonstrates that the introduction of urea can effectively reduce the energy consumption for H₂ production. Importantly, the urea electrolyzer showed good stability for 12 h at 10 mA cm⁻² (Figure 4d). Moreover, the urea electrolysis performance of NiFe LDH-NiFeS_x/NF surpasses most non-precious-metal-based bifunctional electrocatalysts previously reported (Figure 4e and Table S2).

4. Conclusions

In summary, a NiFe LDH-NiFeS_x/NF heterostructured nanosheet electrocatalyst was synthesized via a simple one-step hydrothermal approach. Importantly, the NiFe LDH-NiFeS_x/NF exhibits outstanding electrocatalytic performance, requiring only 1.34 V and 138 mV to achieve ± 10 mA cm⁻² for UOR and HER, respectively. Moreover, an overall urea splitting electrolyzer using NiFe LDH-NiFeS_x/NF as both anode and cathode needs a low cell voltage of 1.44 V to achieve 10 mA cm⁻², which is lower than many recently reports, and 1.53 V for overall water splitting. The excellent urea electrolysis performance of NiFe LDH-NiFeS_x/NF can be explained by the following: (1) The heterostructure structure formed between the metal sulfide and hydroxide can optimize the electronic structure of metal site, thus improving the intrinsic catalytic activity. (2) The nanosheet array structure can provide plentiful reaction sites while accelerate the permeation of electrolytes and the release of bubbles. (3) In-situ growth of active material on conductive substrate NF improves electrical conductivity and avoids dead volume caused by binder introduction.

Author Contributions

Conceptualization, W.L., Z.Q. and X.C.; methodology, W.L. and Z.Q.; software, Z.Q. and X.D.; validation S.M.; formal analysis, F.W.; investigation, W.S.; resources, X.D.; data curation, X.N.; writing—original draft preparation, W.L. and Z.Q.; writing—review and editing, W.L., Z.Q. and X.C.; visualization, Z.Q.; supervision, W.L. and X.C.; project administration, W.L., Z.Q. and X.C.; funding acquisition, W.L. and X.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 1. (a) Schematic illustration for the fabrication process of NiFe LDH-NiFeSx/NF. (b) SEM, (c) TEM, (d,e) HRTEM, (f) SAED pattern, and (g) EDS elemental mapping images of NiFe LDH-NiFeSx/NF.

Figure 2. (a) XPS spectrum of NiFe LDH-NiFeSx/NF. (b–d) XPS spectra of NiFe LDH/NF, Ni3S2/NF, and NiFe LDH-NiFeSx/NF: (b) Ni 2p, (c) Fe 2p, and (d) S 2p.

Figure 3. (a) UOR and OER LSV curves of NiFe LDH-NiFeSx/NF in the electrolytes 1 M KOH with 0.33 M Urea, and 1 M KOH. (b) UOR LSV curves and (c) the corresponding Tafel slopes. (d) HER LSV curves of NiFe LDH-NiFeSx/NF in different electrolytes. (e) HER LSV curves, (f) the corresponding Tafel slopes, (g) the Cdl values, (h) Nyquist plots, and (i) HER and UOR chronopotentiometry curves in 1 M KOH and 0.33 M Urea.

Figure 4. (a) Schematic illustration of a urea electrolytic electrolyzer using NiFe LDH-NiFeSx/NF as both the anode and cathode. (b) LSV curve of NiFe LDH-NiFeSx/NF for overall urea electrolysis and water splitting. (c) Comparison of potentials at different current densities for overall urea electrolysis and water splitting. (d) Long-term stability test of urea electrolysis. (e) Comparison of cell voltages of the bifunctional urea electrocatalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en16031092/s1, Figure S1: Photograph and SEM images of Ni₃S₂/NF, NiFe LDH/NF, and NiFe LDH-NiFeS_x/NF; Figure S2: XRD patterns of NiFe LDH-NiFeS_x/NF, Ni₃S₂/NF, and NiFe LDH/NF; Figure S3: XPS spectra of O 1s for NiFe LDH/NF and NiFe LDH-NiFeS_x/NF. Figure S4: CV curves of NiFe LDH-NiFeS_x/NF, Ni₃S₂/NF, NiFe LDH/NF, and NF in 1 M KOH + 0.33 M urea at different scan rates; Figure S5: Polarization curves for HER and UOR of NiFe LDH-NiFeS_x/NF before and after long-term stability tests. Figure S6: Polarization curves for overall urea electrolysis; Table S1: Comparison of UOR performances of NiFe LDH-NiFeS_x/NF and other reported electrocatalysts; Table S2: Comparison of recent bifunctional electrocatalysts for overall urea electrolysis.

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