Nanostructured Nickel Nitride with Reduced Graphene Oxide Composite Bifunctional Electrocatalysts for an Efficient Water-Urea Splitting.

A three-dimensional nickel nitride with reduced graphene oxide composite on nickel foam (s-X, where s represents Ni3N/rGO@NF and the annealing temperature X can be 320, 350, or 380) electrode has been fabricated through a facile method. We demonstrate that s-350 has excellent urea oxidation reaction (UOR) activity, with a demanded potential of 1.342 V to reach 10 mA/cm2 and bears high hydrogen evolution reaction (HER) activity. It provides a low overpotential of 124 mV at 10 mA/cm2, which enables the successful construction of its two-electrode alkaline electrolyzer (s-350||s-350) for water-urea splitting. It merely requires a voltage of 1.518 V to obtain 100 mA/cm2 and is 0.145 V lower than that of pure water splitting. This noble metal-free bifunctional electrode is regarded as an inexpensive and effective water-urea electrolysis assisted hydrogen production technology, which is commercially viable.

1. Introduction

The limited consumption of fossil fuels and the consequent envn class="Chemical">ironmental pollution problems make the development of clean and renewable energy systems and high-efficiency catalysts rich in the earth imminent [1,2,3,4]. It is significant to explore new clean energies to replace mineral energy. Hydrogen energy serves as a green energy with abundant resources, high energy and no secondary pollution, and it has received extensive attention [5,6]. Hydrogen has turned into the ideal energy to deal with the energy crisis in the oil age. At present, water splitting has been considered of the most effective hydrogen production methods. However, the anode of water splitting, oxygen evolution reaction (OER), enjoys a high theoretical oxidation potential (Et = 1.23 V vs. RHE) [7,8,9,10]. This lowers the efficiency of the entire hydrogen production electrolyzer, which hinders the marketization of hydrogen energy to a large extent. Thus, it is crucial to select hydrogen carriers with low theoretical oxidation potentials. The Et of the urea oxidation reaction (UOR) as an anode is 0.37 V vs. RHE, which is much lower than that of the OER, which implies that the UOR is more efficient than the OER and can take the place of it to produce hydrogen better and the Et of water–urea splitting is 1.198 V vs. RHE. It is well known that urea is contained in urine. Since urea wastewater can be naturally converted into toxic environmental pollutants, such as ammonia, and exposed to the air, it is considered to be one of the most serious environmental pollutants [11,12,13,14]. Therefore, through the electrolysis of water–urea waste water, we are able to significantly reduce environmental pollution and help efficiently produce hydrogen. The reactions of overall water–urea splitting to produce non-toxic nitrogen, carbon dioxide, and hydrogen are as follows [3,6,15,16,17,18,19,20]:CO(NH

Water–n class="Chemical">urea splitting requires active UOR and hydrogen evolution reaction (HER) catalysts. In recent years, transition metal-based catalytic materials have attracted much attention thanks to their high electrochemical activity and stability. Non-noble metal-based Ni materials have been used as low-cost materials for water–urea splitting [20,21,22]. These exert an impressive catalytic effect in water–urea splitting as a result of multi-component synergistic effects, such as Ni–Co hydroxide [23], nickel hydroxide [24], Ni/WC [25], Ni–Rh [26] and nickel oxide [27]. Though these combinations of Ni-based catalysts exhibit excellent catalytic activity compared to noble metals, such as platinum, iridium and rhodium [20], most catalysts have poor electrical conductivity and an insufficient specific surface area. Therefore, appropriate substrates must be selected with the intention of improving conductivity and increasing specific surface area [26]. Since nickel foam (NF) with 3D structure bears all these characteristics, we combined an electrocatalyst with NF to obtain the uniform and regular growth of 3D porous nanostructures, thus promoting electrolyte movement without adding a binder in water–urea splitting [27,28,29]. In addition, it also enhances more reactive sites, such as NiO/NF [30] and FeCo2S4/NF [31]. Currently, reduced graphene oxide (RGO) is widely used in composite materials thanks to its low mass density, excellent electron conductivity, large specific surface area, and the like [32]. Furthermore, while trying to improve the activity of the electrocatalyst, a combination of a heteroatom, such as nitrogen (N), could be adopted to increase the electron mobility [33].

In this paper, a new bifunctional flower-like nickel nitride and n class="Chemical">reduced graphene oxide composite (s-350) was synthesized. Firstly, Ni(OH)2 and a reduced graphene oxide composite (Ni(OH)2/rGO@NF) was synthesized by hydrothermal reaction. Secondly, s-350 (s-X, where s represents Ni3N/rGO@NF and the annealing temperature X can be 320, 350, or 380) was obtained from Ni(OH)2/rGO@NF by calcination. Thanks to the unique porous nanostructure, the high activity of the UOR with potentials of only 1.381 V and 50 mA/cm2 was obtained. More importantly, the s-350 can be used as a bifunctional catalyst for the UOR and the HER for large-scale hydrogen evolution, and only the voltage of 1.405 V is required to realize 20 mA/cm2.

2. Materials and Methods

2.1. Materials

Graphene oxide (n class="Chemical">GO) sheets were purchased by XFNANO Materials Tech Co., Ltd. (Nanjing, China). Ni(NO3)2·6H2O was purchased from Aladdin Ltd. in Shanghai, China. Ammonium fluoride (NH4F), urea was purchased from Beijing Chemical Works (Beijing, China). Nickel foam (NF) was provided by Hongshan District, Wuhan Instrument Surgical Instruments business (Wuhan, China). 20 wt% Pt/C and 20 wt% IrO2 were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China).

2.2. Synthesis of Ni3N and Reduced Graphene Oxide Composite on Ni Foam

In a typical procedure, 5 mmol Ni(n class="Chemical">NO3)2·6H2O, 8 mmol NH4F and 15 mmol urea were dissolved in 60 mL distilled water (mechanism: Ni2+ + F− → NiF+; CO(NH2)2 + H2O → 2NH3 + CO2; NH3 + H2O → NH4+ + OH−; NiF+ + OH− → Ni(OH)F) and stirred to form a clear solution, and then 50 mg GO was added to above and ultrasonicated for 2 h to form a suspension. Then, the above solution and a piece of cleaned NF (2 × 3 cm) were transferred to a 50 mL Teflon-lined stainless-steel autoclave and maintained at 120 °C for 12 h. After the autoclave cooled down naturally, the surplus NF was taken out and washed with distilled water and ethanol, followed by drying 2 h at 60 °C to obtain Ni(OH)2/rGO@NF. To prepare s-320, s-350 and s-380, Ni(OH)2/rGO@NF was placed in a tube furnace and heated at 320, 350 and 380 °C, respectively, for 3 h with a heating speed of 2 °C min−1 in NH3 atmosphere.

2.3. Apparatus

Scanning electron microscope (SEM) images were taken from JEOL JSM-7001F SEM instruments, Tokyo, Japan at 3 kV. Transmission electron microsn class="Gene">cope (TEM) images were obtained from FEI Tecnai g2t20 at 200 kV. Raman spectra were recorded on an INVIA Raman microprobe (Renishaw Instruments, London, UK) with a 514 nm laser excitation. X-ray photoelectron (XPS) characterizations were carried out on a Thermo ESCALAB 250i X-ray photoelectron spectrometer. X-ray diffraction (XRD) data were measured on a Rigaku D-MAX 2500/PC using Cu Kα radiation (λ = 1.54056 Å).

2.4. Electrochemical Measurements

Electrochemical measurements of cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were performed in a typical three-electrode electrochemical cell at scan rate of 5 mV/s under IR compensation. The loading of n class="Chemical">s-350 was 0.31 mg/cm2. An NF (or s-350) electrode, a graphite rod, and HgO/Hg (MOE) were used as the working, counter, and reference electrodes, respectively. All onset potentials reported in this study were converted to reversible hydrogen electrode (RHE, Equation: ERHE = EHg/HgO + 0.098 + 0.059 × pH). The impedance was conducted from the frequencies of (Hz) 1 to 105.

3. Results and Discussion

Figure 1A displays the X-ray diffraction (XRD) pattern of s-350. The diffraction peaks at 38.9°, 42.1°, 58.7°, 70.3° and 78.4° can be indexed to the (110), (002), (112), (300) and (113) lattices of n class="Chemical">Ni3N (JCPDS No. 10-0280) [8]. In addition, the peaks at 44.4°, 51.7° and 76.7° from Ni foam correspond to Ni (JCPDS No. 04-0850). s-320 and s-380 are also in line with standard cards, and no additional peaks were discovered. The peak values of Ni(OH)2/rGO@NF can also be well indexed by the standard card in Figure S1. The surface concentration and valence bond or interactions for the surface layer can be obtained by X-ray photoelectron spectroscopy (XPS), and s-350 characteristic peaks of Ni 2p and N 1s are shown in Figure 1B,C. The peaks at about 853.1 and 870.9 eV corresponded to Ni (I) [34]. The peaks at 879.1 and 861.4 eV belong to the satellite peak [35]. The four peaks at 398.1, 398.7, and 403.1 eV are in accordance with the pyridine-like structures N, pyrrolic-N, and weak oxidized N, respectively [36]. Figure 1D shows the XPS of C1s for s-350, the C–C=C, C–O, C–N, and O–C=O can be indexed to 284.6, 285.4, 285.9, and 288.1 eV. Figure 2 displays Raman spectra of s-320, s-350, and s-380. There are two characteristic peaks at 1355 and 1584 cm−1 corresponding to rGO [37]. These results well prove rGO’s successful participation in s-320, s-350, and s-380. The occurrence of the G-peak splitting in s-350 was due to the fact that even the single layer of the G-peak is caused by the superposition of several peaks, which may become apparent with the decrease of the rGO dimension [38].

Figure 1

(A) XRD patterns of s-320, s-350, and s-380. XPS (X-ray photoelectron spectroscopy) spectra of (B) Ni 2p, (C) N 1s, and (D) C 1s for s-350.

Figure 2

Raman spectra of s-320, s-350, and s-380.

The morphology of the catalyst was observed SEM, of which s-350 demonstrated that the bare n class="Chemical">nickel foam (Figure S2A) was completely covered by the Ni3N flower-shaped nanosheet array (Figure 3A). Obviously, s-350 was distributed in a uniform nanoarray with a porous structure (Figure 3B). Figure 3C displays the TEM of s-350. It is evident that the boundary of Ni3N was clear and showed a porous sheet-like structure (Figure 3D). The SEM of Ni(OH)2/rGO@NF (Figure S2B) revealed that the gracile nanosheet arrays grew uniformly on NF. The distribution of s-320 was fluffy and retained the sheet structure (Figure S2C); furthermore, the sheet structure of s-380 changed and was irregular (Figure S2D).

Figure 3

SEM (A,B) of s-350 and TEM (C,D) of s-350.

Figure 4A shows a fast Fourier transform (FFT) of s-350. It could be observed that n class="Chemical">s-350 dot matrix was quite bright; that is to say, s-350 had a high quality crystallization performance. A high-resolution transmission electron microscopy (HRTEM) of s-350 displayed lattice fringes with a good resolution (Figure 4B). In addition, the distance between the planes of 0.23 nm was consistent with Ni3N (110), and 0.21 nm could be indexed to Ni3N (002). This conclusion conforms to XRD analysis results. The distribution of s-350 was checked by energy dispersive X-Ray spectrometer (EDX) analysis (Figure 4C–F), from which we were allowed to tell the uniform distribution of nickel, nitrogen and carbon elements in the composite.

Figure 4

The FFT (fast Fourier transform) (A), HRTEM (high-resolution transmission electron microscopy) (B), and EDX analysis (C–F) of s-350.

To study it further, we focused on the catalytic UOR activity of the prepared materials through electrochemical experiments. The commercial IrO2 (20 wt%) was also evaluated for comparison [8,39], and the most suitable n class="Chemical">urea concentration was screened as 0.5 M (Figure S3). Figure 5A shows linear sweep voltammetry (LSV) in different electrolytes. The UOR occurred in a 1 M KOH solution (1) containing urea, and the OER occurred in (2) 1 M KOH without urea. It was clearly proved that there was no anode current density in (3) pure urea, indicating that the UOR process cannot be carried out without potassium hydroxide on s-350. At 100 mA/cm2, the potential of the UOR was 1.409 V, while the potential of the OER was 1.675 V. Since the potential of the UOR is lower than that of the OER, the UOR is more efficient than the OER.

Figure 5

(A) LSVs of s-350 in (1) 1 M KOH with 0.5 M urea, (2) 1 M KOH, and (3) 0.5 M urea. (B) LSVs of (1) s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF, (5) IrO2 and (6) bare NF (nickel foam). (C) Tafel plots of (1) s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF and (5) IrO2. (D) LSVs of s-350 at the different scan rates (in the inset, the data are redrawn for different scan rates). (E) Multi-voltage process of s-350. (F) Potentiostatic measurements of s-350 samples (inset: Perform LSV before and after 3000 cyclic voltammetry (CV) cycles).

With the purpose of exploring the electrochemical properties of the prepared materials, LSVs of the prepared catalysts were performed (Figure 5B). At a current density of 10 mA/cm2, (1) n class="Chemical">s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF, (5) IrO2 and (6) NF had potentials of 1.342, 1.352, 1.367, 1.371, 1.433 and 1.514 V, respectively. Evidently, s-350 showed the lowest potential, which means that it exhibited the best UOR activity. In addition, at 100 mA/cm2, s-350 asked for only 1.409 V, which was 0.228 V lower than Pt/C. This implies that s-350 had a much better catalytic activity than IrO2. Tafel plots derived from LSVs helped evaluate the dynamic response of the UOR [40,41]. (1) s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF, and (5) IrO2 corresponded to 32.4, 45.4, 52.1, 61.1 and 79.7 mV/dec, respectively (Figure 5C). s-350 showed a lower Tafel slope compared with other prepared catalysts, which means that it enjoyed fast kinetics and de-electron UOR catalytic activity. In addition, the surface transport mechanism of s-350 is under discussion. As is shown in Figure 5D, LSVs of s-350 at the scan rates from 5 to 50 mV·s−1 were obtained, and the current density was proportional to sweep speed at a potential of 1.508 V (inset of Figure 5D). This result proves an efficient transfer of charge and mass in the catalytic process [42,43].

The durability of the catalyst is of great importance to the UOR. Figure 5E displays the multi-step chronopotentiometric analysis of s-350, showing that the potential increased from 1.342 to 1.382 V (4 mV per 2 h). The current density can be ken class="Chemical">pt steady immediately and will last for two hours till the next potential step at the initial potential. Similar results were received in other steps; all of them showed the excellent conductivity, mass transport properties, and good mechanical robustness of s-350 [35,44]. Meanwhile, the chronoamperometric response of s-350 at a constant potential (1.409 V) was also conducted (Figure 5F). Obviously, the current density curve with time shows that its stability remained unchanged for at least 30 h. Additionally, the current density maintained the original current density of 95.8% after 3000 cycles (inset of Figure 5F). The results show that s-350 enjoyed excellent stability and durability.

To further study catalytic activity, the LSVs of n class="Chemical">s-350 for different electrolytes are shown in Figure 6A. Obviously, at 300 mA/cm2, (1) there was no urea, (2) there was only a 12 millivolt deviation between urea, and (3) there was still no response in pure urea. This means that urea slightly exerted its effect on the HER reaction of potassium hydroxide. Figure 6B shows the LSVs of the prepared catalysts. There current density at 10 mA/cm2 for (1) Pt/C, (2) s-350, (3) s-380, (4) s-320, (5) Ni(OH)2/rGO@NF, and (6) bare NF were 42, 124, 171, 200, 319, and 366 mV, respectively. The overpotential of bare NF was deemed as the worst, and the overpotential of s-350 was around Pt/C, indicating that the catalytic activity of s-350 was around that of Pt/C and the electrochemical impedance spectroscopy (EIS) of prepared catalysts also reflected the same phenomenon (Figure S4) Figure 6C displays the Tafel plots of the prepared catalysts. (1) s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF, (5) NF and (6) Pt/C corresponded to 73.5, 159.5, 165.2, 196.9, 260.2 and 41.4 mV/dec, respectively. Clearly, the Tafel slope of s-350 ranked the closest to Pt/C, which indicates its efficient performance for the HER. Generally speaking, the hydrogen evolution reaction was achieved by transition metal composite materials following the Tafel–Volmer–Heyrosky mechanism in the alkaline medium (Equations (4)–(6)) [45]. The crucial role of the formation of transition (M–H*) is the rate determining step (Tafel–Volmer step, Equation (6)) [46]. The s-350 is equipped with excellent HER performance, which can be attributed to the advantageous Tafel–Volmer step, which facilitates the initiation of low overpotential hydrogen evolution.

Figure 6

(A) LSVs of s-350 in (1) 1 M KOH, (2) 1 M KOH with 0.5 M urea, and (3) 0.5 M urea. (B) LSVs of (1) Pt/C, (2) s-350, (3) s-380, (4) s-320, (5) Ni(OH)2/rGO@NF, and (6) bare NF. (C) Tafel plots of (1) s-350, (2) s-380, (3) s-320, (4) Ni(OH)2/rGO@NF, (5) NF and (6) Pt/C. (D) Potentiostatic measurements of s-350 samples.

Furthermore, chronoamperometry (i-t) of s-350 was performed at −0.234 and −0.282 V, which con class="Chemical">nfirmed that the flower-like s-350 catalyst had a high stability for 45 h without any changes (Figure 6D). Additionally, XRD, XPS, SEM and TEM of s-350 after 3000 CV cycles were also evaluated. Results were almost the same as what we got before in 3000 CV cycles (Figure S5). These results imply s-350 has a prominent stability. Double layer capacitance (Cdl) can be obtained from cyclic voltammetry (CV), which is linearly and directly related to electrochemical active surface area (EASA) and scanning rate (Cdl ∝ v × EASA) [47]. Therefore, the values of Cdl can demonstrate the EASA. Figure S6 shows that the Cdl of s-350 is 34.7 mF/cm2, which ranks higher than that of s-380 (30.1 mF/cm2) and s-320 (16.3 mF/cm2). That means s-350 has the largest ECSA and is able to expose more active sites, thus improving electrocatalytic HER performance.

Inspired by the outstanding UOR and HER catalytic performances of the porous flower-like s-350, the overall n class="Chemical">water–urea splitting was established in a two-electrode system, in which s-350 was employed as both an anode and a cathode, (s-350||s-350), as seen in Figure 7A. Similarly, s-380 and s-320 are represented as s-380||s-380 and s-320||s-320. Iridium dioxide as a cathode and platinum carbon as an anode (Pt/C||IrO2) were taken to make a comparison. Figure 7B displays the LSV of s-350||s-350. The voltage of (1) water–urea splitting was 1.518 V and achieved 100 mA/cm2. The voltage of (2) water splitting was 1.663 V. Obviously, the voltage of water–urea splitting was far lower than that of the water splitting (1.663 V); that is to say, compared with water electrolysis, the electrochemical performance was significantly improved. (1) s-350||s-350, (2) s-380||s-380, (3) s-320||s-320, and (4) Pt/C||IrO2 required voltages of 1.405, 1.426, 1.461 and 1.558 V at 10 mA/cm2, respectively, as shown in Figure 7C. The results prove that the performance of the s-350||s-350 electrolyzer is much higher than that of other electrolyzers. By the same token, the electrochemical activity of the catalytic materials in other literatures were used for comparison. We could see that s-350 is equipped with better catalytic activity than other catalysts (Table S1). In addition, the timing current measurement under 1.518 V in water–urea splitting indicated that s-350||s-350 is able to maintain high current density within 25 h of operation (Figure 7D). Its high stability was confirmed, and the inset of Figure 7D shows the fluctuation of current density around the 20th hour, which can be attributed to the accumulation and release of the remaining bubbles on the electrode surface [48].

Figure 7

(A) Schematic of s-350 as electrodes for two-electrode water–urea splitting system. (B) LSVs of (1) water–urea splitting and (2) water electrolysis. (C) LSVs of (1) s-350||s-350, (2) s-380||s-380, and (3) s-320||s-320, and (4) Pt/C||IrO2. (D) Potentiostatic measurements of s-350||s-350 samples (inset is an expansion of area represented by a rectangle).

4. Conclusions

To sum up, a simple hydrothermal and calcination route can be adopted to synthesize a non-precious n class="Chemical">metal flower-like nickel nitride and a reduced graphene oxide composite on Ni foam for high-efficiency water–urea splitting to produce hydrogen energy. When s-350 is taken as the anode and cathode for water–urea splitting, it bears remarkable stability for more than 25 h for the HER and the UOR. Therefore, the two-electrode electrolyzer was conducted to further study catalytic characteristics. To drive 10 mA/cm2, only 1.362 V was required, which is 0.174 V lower than the electrolysis of water and helps continue electrolysis for at least 25 h, at a higher current density, at 100 mA·cm−2. The simple preparation method of s-350 and its excellent properties give it potential for mass production, and it is expected to be a dual-function electrocatalyst when it comes to separating urea wastewater. Meanwhile, s-350 performs quite well in a complete water–urea electrolyzer, which can be deemed as promising for the industrial technology when it comes to an extensive energy storage and conversion applications.