Co₃O₄-nanoparticle-entrapped nitrogen and boron codoped mesoporous carbon as an efficient electrocatalyst for hydrogen evolution†

Duihai Tang ^a, Xue Sun ^a, Huan Yu ^a, Wenting Zhang ^a, Ling Zhang ^c, Xuefeng Li ^d, Zhen-An Qiao *^b, Junjiang Zhu ^a and Zhen Zhao *^a
aInstitute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, P. R. China. E-mail: zhaozhen1586@163.com; zhaozhen@synu.edu.cn; zhenzhen@cup.edu.cn
^bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: qiaozhenan@jlu.edu.cn
^cCollege of Chemistry, Jilin University, Changchun 130012, P. R. China
^dAlan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, P. R. China

First published on 7th February 2019

---

---

1 Introduction

Hydrogen has been considered an ideal energy carrier, which is environmentally friendly and renewable.^1,2 Nowadays, most H₂ is produced via one of two methods. One is partial oxidation of gasified oil and coal, and the other is the steam reforming method from fossil resources.³ However, neither of these processes is environmentally friendly, and they cause serious environmental pollution.⁴ To achieve clean hydrogen production, electrochemical water splitting has been considered an effective method.^5,6 Pt-Based materials have been considered one of the most effective electrocatalysts for the hydrogen evolution reaction (HER). However, the large-scale applications of these Pt-based electrocatalysts have been hindered due to their expense and scarcity.^7,8 To tackle these issues, much research focuses on the development of non-Pt catalysts for HER.^9,10 Among these catalysts, transition metal-based materials are already regarded as one of the promising candidates.^11,12 Co₃O₄-based materials have been investigated as electrocatalysts for HER. However, Co₃O₄ could not show good electrical conductivity. The main motivation to combine Co₃O₄ with a carbon matrix is to improve the electrical conductivity.^13–16

Mesoporous carbons are receiving more and more attention, due to their high surface area, pore volume, and pore size.^17–20 The template is one of the most important factors in fabricating mesoporous carbons, and it should be non-toxic and environmentally friendly. Besides the soft template method and the hard template method, a number of efforts have been focused on the molten salt method.^21–23 The salts can be used as the templates for the formation of mesopores, and can be easily removed and recovered. Various kinds of salts have been used as templates, such as ZnCl₂,²⁴ Na₂CO₃-K₂CO₃,²⁵ and NaCl-KCl.²⁶ However, when used as electrocatalysts for HER, traditional mesoporous carbon could not show good electrocatalytic activities, because of its chemical inertness.^27,28 To tackle this issue, heteroatoms, such as boron (B), nitrogen (N), sulfur (S), and phosphorus (P), have been introduced into the carbon matrix, which can improve the electronic features and change the surface chemistry.^29,30 However, the syntheses of N,B-codoped mesoporous carbon via the molten salt method have rarely been reported and are thus desirable.

Herein, we synthesized a series of N,B-codoped mesoporous carbon-armored Co₃O₄ nanoparticles, with high boron and nitrogen content. CoCl₂ was used as the molten salt template. Moreover, the amount of CoCl₂ and the carbonation temperature determined the mesoporous structures of the final samples, and then affected the electrocatalytic activities for HER. The amount of boric acid scarcely affected the porosities of the catalysts. However, the doped amount of boron decreased with an increase in the amount of boric acid. When used as electrocatalysts for HER in alkaline media, the optimal catalyst Co_0.65B_0.3NC₈₀₀ showed superior catalytic performance, including high current density, low overpotential, and good stability.

2 Experimental

2.1. Synthesis of the electrocatalysts

In a typical process, melamine (4.71 g) was mixed with formaldehyde solution (10 ml) and deionized water (10 ml). The pH was adjusted to 10 by adding 1 M NaOH solution. This white suspension was heated to 75 °C and a clear solution was formed. After 40 min, the pH was adjusted to 4 by adding 1 M HCl solution. This resultant solution (2.5 g) was blended with ethanol (10 ml). After stirring for 10 min, the desired amount of CoCl₂ was added. The resultant pink solution was poured onto dishes, which were placed in an oven at 40 °C. When H₂O and ethanol had totally evaporated, the temperature was raised to 180 °C for further thermopolymerization for 6 h. The as-obtained film was carbonated under Ar at 350 °C for 5 h at a heating rate of 1 °C min⁻¹. This intermediate was dispersed in 6 M HCl solution to dissolve cobalt oxide on the surface of the samples, and dried at 50 °C under vacuum for 12 h. This black powder was mixed with the desired amount of boric acid, and hand-milled for 30 min. The as-made gray powder was further carbonated under Ar at the desired temperature for 2 h with a heating rate of 1 °C min⁻¹. After cooling down to room temperature, the annealed samples were washed with deionized water to remove the excess boric acid. The final products were obtained by drying under vacuum at 50 °C for 10 h. These samples were designated as CoxByNCz, where x means the amount of CoCl₂, y means the amount of boric acid, and z means the calcination temperature. The amounts of CoCl₂ and boric acid are both in grams (Scheme 1).

2.2. Sample characterization

The crystalline structures of the electrocatalysts were determined on a Rigaku Ultima IV using Cu Kα radiation (λ = 1.5418 Å). The Raman spectra were measured by a HORIBA HR800 Confocal spectrometer with a 532 nm laser. The porosities of the samples were carried out by a Micromeritics Tristar II 3020 physisorption analyzer. The XPS spectra were performed on a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectroscope. The microstructures were characterized on a JEOL-1200 transmission electron microscope.

2.3. Electrochemical measurements

A CHI 660E electrochemistry workstation was utilized to test the electrochemical performance via the three-electrode system. The counter electrode was a carbon rod, and the reference electrode was a saturated calomel electrode (SCE). A catalyst-loaded glassy carbon electrode (GCE) was used as the working electrode. The catalyst ink was prepared as follows: 5 mg of the catalyst was dispersed in 100 μl of isopropanol. After being sonicated for 30 min, the as-formed ink (5 μl) was dropped onto a GCE. When isopropanol was totally evaporated, 0.3 wt% Nafion solution in isopropanol (2 μl) was dropped onto the film. Linear sweep voltammetry (LSV) was measured in 1 M KOH (pH = 14). The potentials (E) were converted to the reversible hydrogen electrode (RHE) scale, E_{(versus RHE)} = E_{(versus SCE)} + E_0(SCE) + 0.059pH.

3 Results and discussion

3.1 Sample characterization

The electrocatalysts were synthesized by a facile molten salt method. The crystalline structures of the as-synthesized catalysts were investigated. As shown in Fig. 1a, the carbonation temperature could determine the crystalline structures of the final samples. When the carbonation temperature is low, such as 600 or 700 °C, a broad peak located at 26.5° can be observed, which correspond to the (002) facet of carbon.³¹ However, there are no other peaks which can be assigned to crystalline Co₃O₄. When the carbonation temperature goes up to 800 or 900 °C, besides the broad peak at 26.5°, peaks located at 37.2° and 44.0° can clearly be observed, which can be assigned to the (311) and (400) facets of Co₃O₄, respectively.³² As shown in Fig. 1b and c, the amounts of CoCl₂ and boric acid scarcely affect the crystalline structures of the final products. The XRD patterns of Co_0.49B_0.3NC₈₀₀, Co_0.82B_0.3NC₈₀₀, Co_0.65B_0.1NC₈₀₀, and Co_0.65B_0.5NC₈₀₀ are all similar to that of Co_0.65B_0.3NC₈₀₀, indicating that these electrocatalysts consist of carbon and crystalline Co₃O₄. The above results reveal that the annealing temperature can affect the crystalline structures. However, neither the amount of CoCl₂ nor the amount of boric acid determined the crystalline structures of the catalysts. The Raman spectra are shown in Fig. 1d–f to further determine the structures of the catalysts. A D band and G band are detected, which are located at 1350 and 1585 cm⁻¹, respectively.³³ These results show that these electrocatalysts possess disordered and graphitic structures. Among these electrocatalysts, Co_0.65B_0.3NC₈₀₀ shows the lowest I_D/I_G value of 0.96, indicating that this material possesses the highest graphitic degree (Table 1).

---

The porous structures of the as-made catalysts are revealed in Fig. 2. The BET surface areas of the samples were determined by the carbonation temperature and the amount of CoCl₂. As shown in Fig. 2a, Co_0.65B_0.3NC₇₀₀, Co_0.65B_0.3NC₈₀₀, and Co_0.65B_0.3NC₉₀₀ all show type IV adsorption isotherms, with H3 hysteresis loops. When the carbonation temperature is 600 °C, the BET surface area of Co_0.65B_0.3NC₆₀₀ is as low as 6.7 m² g⁻¹, indicating nearly no porosity. When the temperature goes up to 700 °C, the BET surface area of Co_0.65B_0.3NC₇₀₀ is 73.2 m² g⁻¹. When the temperature rises to 800 °C, the sample of Co_0.65B_0.3NC₈₀₀ shows the highest BET surface area of 181.3 m² g⁻¹. Furthermore, when the temperature goes up further to 900 °C, the BET surface area of Co_0.65B_0.3NC₉₀₀ is 153.6 m² g⁻¹. Co_0.65B_0.3NC₈₀₀ has the highest BET surface area, while Co_0.65B_0.3NC₆₀₀ possesses the lowest BET surface area. Besides the annealing temperature, the amount of salt template can also affect the porosities of the final products. The BET surface areas of Co_0.49B_0.3NC₈₀₀, Co_0.65B_0.3NC₈₀₀, and Co_0.82B_0.3NC₈₀₀ are 161.4, 181.3, and 292.8 m² g⁻¹, respectively, indicating that the BET surface area increases with an increase in the amount of CoCl₂. It is worth mentioning that the amount of boric acid can scarcely determine the porous structures of the final samples. The BET surface areas of Co_0.65B_0.1NC₈₀₀, Co_0.65B_0.3NC₈₀₀, and Co_0.65B_0.5NC₈₀₀ are 171.7, 181.3, and 178.6 m² g⁻¹, respectively.

The surface composition of Co_0.65B_0.3NC₈₀₀ was determined from the XPS spectra. As shown in the XPS survey spectrum of Co_0.65B_0.3NC₈₀₀, the elements of cobalt, oxygen, nitrogen, carbon, and boron can be observed in Fig. 3a, indicating that these elements exist on the surface of Co_0.65B_0.3NC₈₀₀. The high-resolution B 1s spectrum can be fitted into two peaks located at 192.1 and 190.3 eV, which correspond to BC₂O and BC₃, respectively (Fig. 3b).³⁴ As shown in Fig. 3c, the N 1s peak can be fitted into two peaks at 399.9 and 398.3 eV, corresponding to pyrrolic N and pyridinic N, respectively.³⁵ However, the N 1s peak of the control sample, Co_0.65B_0.0NC₈₀₀, can be fitted into three peaks at 400.9, 399.9 and 398.3 eV, corresponding to graphitic N, pyrrolic N and pyridinic N, respectively (Fig. S1†). These results reveal that the introduction of boron into the carbon matrix can affect the species of nitrogen. As shown in Fig. 3d, the high-resolution Co 2p spectrum can be fitted into six peaks. In detail, the peaks located at 796.7 and 781.0 eV can be assigned to Co²⁺. The peaks located at 795.0 and 779.5 eV can be indexed to Co³⁺. Moreover, the peaks located at 803.8 and 788.9 eV correspond to satellite peaks.³⁵

The crystalline and porous structures were further investigated with the TEM and HRTEM images. As shown in Fig. 4a, Co_0.65B_0.3NC₈₀₀ consists of carbon and Co₃O₄ nanoparticles. The Co₃O₄ nanoparticles disperse evenly in the carbon matrix, with a size of 10–50 nm. Moreover, the mesopores can also be clearly observed, consistent with the N₂ adsorption–desorption isotherm. Furthermore, the HRTEM image reveals that the Co₃O₄ nanoparticle is covered by carbon layers. The [311] crystal plane of Co₃O₄ can clearly be observed with a lattice fringe of 0.244 nm, while the lattice fringe of 0.334 nm corresponds to the [002] crystal plane of carbon.³⁶

3.2 Electrocatalytic performance of the catalysts

The electrocatalytic activities for HER were measured with a three-electrode system in 1 M KOH. Commercially available 20 wt% Pt on graphitized carbon was used as a comparison. First, the carbonation temperature was controlled to investigate the electrocatalytic activities. As shown in Fig. 5a, Pt/C shows the best electrocatalytic performance. Besides this electrocatalyst, Co_0.65B_0.3NC₈₀₀ exhibits a small onset overpotential of 74 mV (Fig. 5a). However, the other electrocatalysts show larger onset overpotentials than Co_0.65B_0.3NC₈₀₀. The onset overpotentials of Co_0.65B_0.3NC₆₀₀, Co_0.65B_0.3NC₇₀₀, and Co_0.65B_0.3NC₉₀₀ are 456, 183, and 112 mV, respectively. Moreover, when the current density is 10 mA cm⁻², Co_0.65B_0.3NC₈₀₀ shows a low overpotential of 178 mV, which is lower than those of Co_0.65B_0.3NC₇₀₀ (346 mV) or Co_0.65B_0.3NC₉₀₀ (246 mV). However, Co_0.65B_0.3NC₆₀₀ would not reach this current density when the overpotential was 460 mV. Co_0.65B_0.3NC₈₀₀ reveals a smaller Tafel slope of 100.3 mV dec⁻¹ than Co_0.65B_0.3NC₇₀₀ (189.2 mV dec⁻¹) or Co_0.65B_0.3NC₉₀₀ (139.2 mV dec⁻¹) (Fig. 5d). Second, the amounts of CoCl₂ were controlled to synthesize the electrocatalysts. As compared to Co_0.65B_0.3NC₈₀₀, Co_0.48B_0.3NC₈₀₀ and Co_0.82B_0.3NC₈₀₀ both show worse electrocatalytic HER performance (Fig. 5b). Co_0.48B_0.3NC₈₀₀ reveals an onset overpotential of 75 mV, which is similar to that of Co_0.65B_0.3NC₈₀₀. However, Co_0.48B_0.3NC₈₀₀ delivers a current density of 10 mA cm⁻² at an overpotential of 190 mV. Co_0.82B_0.3NC₈₀₀ reveals an onset overpotential of 167 mV and a current density of 10 mA cm⁻² at an overpotential of 315 mV. Furthermore, the Tafel slopes for Co_0.48B_0.3NC₈₀₀, Co_0.65B_0.3NC₈₀₀, and Co_0.82B_0.3NC₈₀₀ are 124.9, 100.3, and 147.4 mV dec⁻¹ (Fig. 5e). Third, the amount of boric acid used to prepare the electrocatalysts was also controlled. Among these samples, Co_0.65B_0.3NC₈₀₀ shows the best HER activities. For Co_0.65B_0.1NC₈₀₀, the onset overpotential and the overpotential at a current density of 10 mA cm⁻² are 88 and 196 mV, respectively. Moreover, for Co_0.65B_0.5NC₈₀₀, the onset overpotential and the overpotential at a current density of 10 mA cm⁻² are 100 and 249 mV, respectively (Fig. 5c). To evaluate the effect of boron doping, a control sample was synthesized, without adding boric acid. As shown in Fig. S2,† the as-made catalyst, Co_0.65B_0.0NC₈₀₀, could not show better electrocatalytic performance than Co_0.65B_0.5NC₈₀₀. According to the above results, Co_0.65B_0.5NC₈₀₀ reveals the best electrocatalytic performance among all the as-obtained samples.

The electrocatalytic stability is shown in Fig. 6a, as revealed by the current–time plots, suggesting good long-term stability. Furthermore, the LSV curves of Co_0.65B_0.3NC₈₀₀ remain overlapped before and after 1000 cycles, which also indicates superior stability. The electrocatalytic HER kinetics of these catalysts were investigated by electrochemical impedance spectroscopy (EIS). According to the N₂ adsorption–desorption isotherms, Co_0.82B_0.3BC₈₀₀ has the highest BET surface area. However, this catalyst could not show better electrocatalytic HER performance than Co_0.65B_0.3BC₈₀₀. As shown in Fig. 7a, the charge transfer resistance of Co_0.65B_0.3BC₈₀₀ is much lower than those of Co_0.65B_0.3BC₆₀₀, Co_0.65B_0.3BC₇₀₀, or Co_0.65B_0.3BC₉₀₀. As shown in Fig. 7b and c, when the amounts of CoCl₂ and baric acid are different, Co_0.65B_0.3BC₈₀₀ reveals the lowest charge transfer resistance, which is the reason why this catalyst shows the best electrocatalytic performance in 1 M KOH. As shown in Table 2, the electrocatalytic performance of Co_0.65B_0.3BC₈₀₀ is among the highest values for Co-based materials. The C_dl values of the catalysts are shown in Fig. 8. The C_dl value of Co_0.65B_0.3BC₈₀₀ is 43.71 mF cm⁻², which is much higher than those of the other samples. According to the results above, there might be three reasons why Co_0.65B_0.3BC₈₀₀ shows the best electrocatalytic activity. First, Co_0.65B_0.3BC₈₀₀ reveals the lowest I_D/I_G value of 0.96, indicating Co_0.65B_0.3BC₈₀₀ possesses the highest graphitic degree. Second, Co_0.65B_0.3BC₈₀₀ exhibits the lowest charge transfer among the as-obtained catalysts, which indicates that this sample has the best electrical conductivity. Third, Co_0.65B_0.3BC₈₀₀ shows the highest C_dl value.

---

4 Conclusions

We developed Co₃O₄-entrapped N,B-codoped mesoporous carbon via the molten salt method. MF resin was the carbon and nitrogen precursor. CoCl₂ was used as the template and the cobalt precursor. Boric acid was the boron precursor. The amounts of CoCl₂ and the annealing temperature can determine the crystalline and porous structures of the final products. However, the porosity was not affected by the amount of boric acid. The optimized sample, Co_0.65B_0.3BC₈₀₀, exhibits a high BET surface area of 181.3 m² g⁻¹, with high nitrogen and boron content. When utilized as an electrocatalyst for the HER, Co_0.65B_0.3BC₈₀₀ exhibited superior performance for the HER, achieving a current density of 10 mA cm⁻² at 178 mV in basic media, in addition to having good stability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Science Foundation of China (No. 21601128), the Support Plan for Innovative Talents in Colleges and Universities of Liaoning Province, High-level Innovative Talents Project of Shenyang City, the Natural Science Foundation of Liaoning Province of China (Materials Joint Foundation, No. 20180510031), Liaoning Provincial Instrument and Equipment Sharing Service Platform Building Project, the Program for Excellent Talents in Shenyang Normal University (No. 054-51600210, BS201621), the Engineering Technology Research Center of Catalysis for Energy and Environment, the Major Platform for Science and Technology of the Universities in Liaoning Province, Liaoning Province Key Laboratory for Highly Efficient Conversion and Clean Utilization of Oil and Gas Resources, and the Engineering Research Center for Highly Efficient Conversion and Clean Use of Oil and Gas Resources of Liaoning Province, the Young Thousand Talented Program and the National Natural Science Foundation of China (21671073, 21621001, 21671074, and 21604030), the “111” Project of the Ministry of Education of China (B17020), the Program for JLU Science and Technology Innovative Research Team, and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201829).

References

1. Y. Chen, K. Yang, B. Jiang, J. Li, M. Zeng and L. Fu, J. Mater. Chem. A, 2017, 5, 8187–8208 RSC .
2. Y. Zheng, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2015, 54, 52–65 CrossRef CAS PubMed .
3. Y. Shi and B. Zhang, Chem. Soc. Rev., 2016, 45, 1529–1541 RSC .
4. X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148–5180 RSC .
5. J. Xie and Y. Xie, ChemCatChem, 2015, 7, 2568–2580 CrossRef CAS .
6. C. G. Morales-Guio, L.-A. Stern and X. Hu, Chem. Soc. Rev., 2014, 43, 6555–6569 RSC .
7. J. F. Callejas, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak, Chem. Mater., 2016, 28, 6017–6044 CrossRef CAS .
8. P. Xiao, W. Chen and X. Wang, Adv. Energy Mater., 2015, 5, 1500985–1500997 CrossRef .
9. J. D. Benck, T. R. Hellstern, J. Kibsgaard, P. Chakthranont and T. F. Jaramillo, ACS Catal., 2014, 4, 3957–3971 CrossRef CAS .
10. B. You and Y. Sun, ChemPlusChem, 2016, 81, 1045–1055 CrossRef CAS .
11. X. Chia, A. Y. S. Eng, A. Ambrosi, S. M. Tan and M. Pumera, Chem. Rev., 2015, 115, 11941–11966 CrossRef CAS PubMed .
12. Y. C. Kimmel, X. Xu, W. Yu, X. Yang and J. G. Chen, ACS Catal., 2014, 4, 1558–1562 CrossRef CAS .
13. X. Liu, W. Liu, M. Ko, M. Park, M. G. Kim, P. Oh, S. Chae, S. Park, A. Casimir, G. Wu and J. Cho, Adv. Funct. Mater., 2015, 25, 5799–5808 CrossRef CAS .
14. H. Jin, J. Wang, D. Su, Z. Wei, Z. Pang and Y. Wang, J. Am. Chem. Soc., 2015, 137, 2688–2694 CrossRef CAS PubMed .
15. B. Wang, L. Xu, G. Liu, P. Zhang, W. Zhu, J. Xia and H. Li, J. Mater. Chem. A, 2017, 5, 20170–20179 RSC .
16. X. Wen, X. Yang, M. Li, L. Bai and J. Guan, Electrochim. Acta, 2019, 296, 830–841 CrossRef CAS .
17. Y. Wan, Y. Shi and D. Zhao, Chem. Mater., 2008, 20, 932–945 CrossRef CAS .
18. R. Zhang, A. A. Elzatahry, S. S. Al-Deyab and D. Zhao, Nano Today, 2012, 7, 344–366 CrossRef CAS .
19. Y. Ren, Z. Ma and P. G. Bruce, Chem. Soc. Rev., 2012, 41, 4909–4927 RSC .
20. D. Yang, Z. Lu, X. Rui, X. Huang, H. Li, J. Zhu, W. Zhang, Y. M. Lam, H. H. Hng, H. Zhang and Q. Yan, Angew. Chem., Int. Ed., 2014, 53, 9352–9355 CrossRef CAS PubMed .
21. J. Fu, Y. Hou, M. Zheng, Q. Wei, M. Zhu and H. Yan, ACS Appl. Mater. Interfaces, 2015, 7, 24480–24491 CrossRef CAS PubMed .
22. J. Wang, B. Ding, X. Hao, Y. Xu, Y. Wang, L. Shen, H. Dou and X. Zhang, Carbon, 2016, 102, 255–261 CrossRef CAS .
23. P. Zhang, Z.-A. Qiao, Z. Zhang, S. Wan and S. Dai, J. Mater. Chem. A, 2014, 2, 12262–12269 RSC .
24. X. Deng, B. Zhao, L. Zhu and Z. Shao, Carbon, 2015, 93, 48–58 CrossRef CAS .
25. H. Yin, B. Lu, Y. Xu, D. Tang, X. Mao, W. Xiao, D. Wang and A. N. Alshawabkehju, Environ. Sci. Technol., 2014, 48, 8101–8108 CrossRef CAS PubMed .
26. Z. Yu, X. Wang, X. Song, Y. Liu and J. Qiu, Carbon, 2015, 95, 852–860 CrossRef CAS .
27. M. Zhang and L. Dai, Nano Energy, 2012, 1, 514–517 CrossRef CAS .
28. Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi and K. Hashimoto, Nat. Commun., 2013, 4, 2390–2396 CrossRef PubMed .
29. J. P. Paraknowitsch and A. Thomas, Energy Environ. Sci., 2013, 6, 2839–2855 RSC .
30. Y. Cao, H. Yu, J. Tan, F. Peng, H. Wang, J. Li, W. Zheng and N.-B. Wong, Carbon, 2013, 57, 433–442 CrossRef CAS .
31. H. Wang, L. Shi, T. Yan, J. Zhang, Q. Zhong and D. Zhang, J. Mater. Chem. A, 2014, 2, 4739–4750 RSC .
32. L. Yang, H. Zhou, X. Qin, X. Guo, G. Cui, A. M. Asiri and X. Sun, Chem. Commun., 2018, 54, 2150–2153 RSC .
33. Q. Wei, H. Fan, F. Qin, Q. Ma and W. Shen, Carbon, 2018, 133, 6–13 CrossRef CAS .
34. J. Zhang, A. Byeon and J. W. Lee, Chem. Commun., 2014, 50, 6349–6352 RSC .
35. B. Wang, L. Xu, G. Liu, P. Zhang, W. Zhu, J. Xia and H. Li, J. Mater. Chem. A, 2017, 5, 20170–20179 RSC .
36. H. Wang, N. Mao, J. Shi, Q. Wang, W. Yu and X. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 2882–2890 CrossRef CAS PubMed .
37. T. Huang, Y. Chen and J. M. Lee, ACS Sustainable Chem. Eng., 2017, 5, 5646–5650 CrossRef CAS .
38. Z. Xing, Q. Liu, W. Xing, A. M. Asiri and X. Sun, ChemSusChem, 2015, 8, 1850–1855 CrossRef CAS PubMed .
39. H. Zhang, Z. Ma, J. Duan, H. Liu, G. Liu, T. Wang, K. Chang, M. Li, L. Shi, X. Meng, K. Wu and J. Ye, ACS Nano, 2016, 10, 684–694 CrossRef CAS PubMed .
40. H. Su, H. H. Wang, B. Zhang, K. X. Wang, X. H. Li and J. S. Chen, Nano Energy, 2016, 22, 79–86 CrossRef CAS .
41. J. Wang, D. Gao, G. Wang, S. Miao, H. Wu, J. Lia and X. Bao, J. Mater. Chem. A, 2014, 2, 20067–20074 RSC .
42. S. Gao, G. D. Li, Y. Liu, H. Chen, L. L. Feng, Y. Wang, M. Yang, D. Wang and S. Wang, Nanoscale, 2015, 7, 2306–2316 RSC .

---

Footnote

† Electronic supplementary information (ESI) available: Figures show the XPS spectra and LSV curves of the resultant samples. See DOI: 10.1039/c8dt05033c

This journal is © The Royal Society of Chemistry 2019