Hairy sphere-like Ni9S8/CuS/Cu2O composites grown on nickel foam as bifunctional electrocatalysts for hydrogen evolution and urea electrooxidation
Graphical abstract
Introduction
Facing the global energy crisis and the associated environmental problem caused by the severe depletion of fossil fuels, scientific researchers have accelerated the search for sustainable and renewable energy [[1], [2], [3]]. As a green, renewable and efficient energy carrier, hydrogen is considered to be the most potential substitute for fossil fuels [4,5]. Water electrolysis is the most effective and environmentally friendly technology for hydrogen production. It consists of two half reactions including oxygen generation (OER) at the anode and hydrogen evolution (HER) at the cathode [6,7]. However, the slow kinetics of reactions at both electrodes limit the development of water electrolysis. Especially, the anodic OER with a high thermodynamic potential is more sluggish than the cathodic HER due to the 4e− transfer process and has greatly reduced the total energy conversion efficiency [8]. Therefore, replacing anodic OER with an oxidation reaction with a lower thermodynamic potential is an effective way to improve the total efficiency of hydrogen production.
Electro-oxidation of urea [9], methanol [10], ethanol [11] and hydrazine hydrate [12] are all promising reactions to replace OER. Among them, urea is most favored because of its non-toxicity, reproducibility, and extremely low theoretical oxidation potential (0.37 V vs. RHE) [13,14]. Compared with water splitting using OER as the anodic half reaction, urea aqueous solution electrolysis with urea oxidation reaction (UOR) as the anodic half reaction is more attractive. The first reason is that the cell voltage for hydrogen production can be reduced due to the low theoretical oxidation potential of UOR. In addition, urea-rich sewage can be purified during the urea solution electrolysis process, which is beneficial for alleviating environmental pollution [15]. However, the UOR also faces the similar challenge to that of OER. Because UOR is a typical 6e− transfer process, and the reaction kinetics is also slow [16,17]. Therefore, it is significant to find cheap and efficient catalysts for UOR. Moreover, the UOR catalysts can be applied into the ureal fuel cells. The investigation on UOR electrocatalysts has attracted the attention of more and more researchers [18].
Although urea aqueous solution electrolysis has more advantages for hydrogen production than water splitting, highly efficient and precious metal-free electrocatalysts for HER and UOR are also strongly desired. Moreover, bifunctional catalysts for both HER and UOR are more preferable from the perspective of practical application. Thus, the electrocatalysts of both electrodes can be produced by the same equipment and process, which could reduce the cost of production [19]. In addition, the bifunctional electrocatalysts may improve the total efficiency of electrolysis due to the same requirement of electrocatalysts to the electrolyte [20]. Some bifunctional electrocatalysts for HER and UOR such as transition metal phosphides [21], sulfides [15] and nitrides [22] have been reported, however, the development of catalysts with low overpotential, good stability and low cost still remains challenging.
To construct bifunctional electrocatalysts for HER and UOR, the catalysts for single reaction have been reviewed. For UOR, the catalysts are mainly Ni based catalysts such as Ni(OH)2 [23], NiO [24], and Ni-MOF [25]. The doping of transition metal (e.g., Co [26], Mn [27] and Cr [28]) and forming compounds with non-metal (e.g., Se [29], P [30] and S [31]) have proven to be an effective method to improve the UOR catalytic activity of Ni based catalysts. Especially, nickel sulfides display high UOR catalytic activity [32,33]. In addition, the Cu element has the advantages of good redox properties and abundant reserves and has been widely used in electrocatalysts [34,35]. However, it is seldomly used in UOR catalysts [36]. Recently, it was reported that single copper oxides also exhibited good UOR performance [37]. Therefore, Ni/Cu bimetallic sulfides may be good UOR catalysts due to the simultaneous introduction of S and Cu elements. On the other hand, transition metal sulfides are also good electrocatalysts catalysts for HER [38,39]. Among them, Ni [40,41] and Cu [42,43] sulfides both show HER catalytic activity in the alkaline medium. Therefore, Ni/Cu bimetallic sulfides are possible bifunctional catalysts for HER and UOR. In order to further improve the activity of catalysts, Ni/Cu bimetallic sulfides can be grown in-situ on the support with porous structure, large specific area and good conductivity [[44], [45], [46]].
In this study, Ni/Cu bimetallic sulfides were prepared on nickel foam (NF) by a two-step hydrothermal method. In the first step, hydroxides (oxides) of Ni and Cu were synthesized, which suffered from a sulfuration process in the second step. The final product is composed of Ni9S8, CuS and Cu2O and shows a morphology of hairy sphere. The Ni9S8/CuS/Cu2O/NF displays good UOR and HER catalytic activities due to the synergistic effect among the multiple components. Moreover, the UOR mechanism on Ni9S8/CuS/Cu2O/NF was studied in detail. Finally, the two-electrode urea solution electrolytic cell composed of Ni9S8/CuS/Cu2O/NF only needs 1.47 V to achieve the current density of 10 mA cm−2 and shows a good stability.
Section snippets
Materials
The nickel foam (NF) was purchased from Shanxi Taiyuan source of power battery sales department. Nickel nitrate hexahydrate, urea, ammonium fluoride, absolute ethanol and potassium hydroxide were bought from Sinopharm Chemical Reagent Co., Ltd. Copper nitrate trihydrate was purchased from Shanghai Xinbao Fine Chemical Factory and thioacetamide was obtained from Beijing Bailingwei Technology Co., Ltd. All reagents were used directly without further purification.
Preparation of NiCu–O(OH)/NF precursor
NiCu–O(OH)/NF precursor was
Synthesis and characterization of S–NiCu–O(OH)/NF
Ni/Cu bimetallic sulfides are possible bifunctional electrocatalysts for HER and UOR which were prepared by a two-step hydrothermal method and the synthesis diagram is shown in Fig. 1. First, urea was used as the source of OH− to synthesize the hydroxide (or oxide) of Ni and Cu as the precursor named as NiCu–O(OH)/NF and then thioacetamide (TAA) was used as the sulfur source to vulcanize the precursor to obtain the target sample named as S–NiCu–O(OH)/NF.
The morphology of the synthesized
Conclusions
To sum up, Ni9S8/CuS/Cu2O composites have been grown in situ on nickel foam by a two-step hydrothermal method and the Ni9S8/CuS/Cu2O/NF (or S–NiCu–O(OH)/NF) have proved to be a bifunctional electrocatalyst with excellent UOR (10 mA cm−2 at 1.357 V vs. RHE) and HER (10 mA cm−2 at −0.146 V vs. RHE) catalytic activity and good long-term stability. Based on these results, a symmetrical two-electrode urea solution electrolytic cell was constructed, which reached 10 mA cm−2 at a low voltage of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by the Bioanalytical Chemistry Innovation Foundation team. Thanks to the staff of Scientific Compass for their technical support for the sample detection. Dr. Y.L. Wang thanks the Natural Science Foundation of Anhui Province (2008085MB38), China for financial support.
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