In-situ evolution of active layers on commercial stainless steel for stable water splitting
Graphical abstract
Introduction
Electrochemical water splitting as a promising friendly environmental technology for the future hydrogen-based economy has received considerable attentions [[1], [2], [3], [4], [5]]. As for the two half reactions involving in water splitting: oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), OER is a determinant step due to the multiple electron and proton transfer process [6,7]. Although noble metals or their compounds, such as Pt and RuO2, are famous for their comparatively low overpotential as HER and OER electrocatalysts, the high cost and scarcity of noble metals severely limit their wide used in the electrochemical water splitting [8,9]. Many efforts have been made to develop low-cost and earth abundant water-splitting electrocatalyst [[10], [11], [12], [13], [14]].
Iron is one of most abundant elements on earth, and recently there is an obvious excess capacity in the global steel industry, especially in China. Applying the commercial stainless steels as the efficient electrocatalysts for water splitting is a challenging and interesting way to solve the issues [[15], [16], [17], [18], [19]]. The stainless steel contains rich transition metal (TM) elements like iron and nickel, which usually are active centers in the electrochemical reactions. In fact, the original stainless steel still shows sluggish water-splitting performance, in particular OER. Currently, the 304-type stainless steels are employed as the electrocatalyst for OER because of low cost. In the previous work, some groups have used the original or treated 304 type stainless steels as the electrocatalysts to obtain a good performance for water splitting [[15], [16], [17], [18]]. In comparison to 304-type stainless steels, 316L-type stainless steels (denotes as SS) has more stable and corrosion-resistant structure, even under positive applied bias, implying the potential application for the electrocatalyst. But, the surface of SS usually shows relatively chemical inertness, and thus is difficult to be modified as an efficient electrocatalyst for water splitting. Apparently, it is highly desirable to develop a facile and feasible route to achieve an efficient SS-based electrocatalyst for water splitting.
Due to surface chemical inertness, the water-splitting performance of modified SS by single traditional method, such as annealing, is seldom successful. In addition, in the previous reports, the new treatment technologies like plasma treatment were just employed to modified the mechanical properties of SS [20,21], but the effect of plasma treatment on the electrocatalytic performance of SS is absent. However, developing an efficient route to improve the electrocatalytic performance of SS and understanding its working mechanism are necessary and significant. In this respect, we utilize a facile route including three steps to construct an excellent SS-based electrocatalyst: (i) etched by hydrochloric acid, (ii) oxidized by thermal treatment, and (iii) carbonized by CH4 plasma treatment. Among them, the etching process gives rise to the formation of rough surface for SS and the elimination of passivation materials like chromium oxide, which can be regarded as the activation step for developing the next steps. Subsequently, the particles of transition metal oxide (TMO) (e.g. Fe2O3) are created from the surface of SS via thermal treatment in air and are employed as the source of TMs to facilitate the carbonizing process. The CH4 plasma treatment is the dominant step in the total process of surface modification for SS, which could produce the transition metal carbides (TMCs) on the surface of SS. In addition, the TMs encapsulated by carbon can prevent the corrosion of TMs materials in the electrolyte and efficiently promote the catalytic reaction [[22], [23], [24], [25]].
In this study, we successfully realized the in-situ evolution of active layers from the SS surface, and demonstrated a novel and facile route to achieve the SS-based electrodes with the amorphous carbon (a-C) layer and graphene encapsulated Fe3C nanoparticles (Fe3C@G). The modified SS electrodes showed a good water-splitting performance, especially OER, which achieves a Tafel slope of 38 mV dec−1 and overpotential of 290 mV for OER. We investigated the effect of CH4 plasma treatment on the composition and structure of the SS-based surface and understood the relationship between these changes and water-splitting behaviors. Our understanding of the SS-based electrode with a-C layer and Fe3C@G resulted in the development of a new strategy to optimize both the structure and composition of the SS surface, enabling efficient utilization of surface-forming active sites (such as Fe3C) for electrocatalytic water splitting.
Section snippets
Preparation of etched SS (ESS)
Firstly, the commercial 316L type SS (2.0 cm * 4.0 cm, Tianhong Stainless Steel Co., Ltd) were cleaned ultrasonically in distilled water and absolute ethanol for 15 min, respectively. Then, the SS were etched ultrasonically in 3 M HCl for 30 min. After erosion process, the samples were thoroughly washed with distilled water to remove the impurities from the source solution, and then were dried in 24 h at 60 ℃ in vacuum. The obtained samples were denoted as ESS.
Preparation of oxidized ESS (OESS)
The as-prepared ESS samples were
Materials characterization
Three fabrication steps of SS-based anodes are illustrated in Fig. 1, including chemical etching, thermal oxidation and plasma carbonizing. The composition of original commercial SS was Fe0.7Cr0.17Ni0.11Mo0.02 (Fig. S1). The fresh SS with smooth surface (Fig. S2a) was used as both a current collector and precursor. To create the more activities, the SS samples successively underwent the acid etching (denoted as ESS) and oxidizing progress (denoted as OESS), and a wrinkle and porous surface on
Conclusion
In summary, we developed a simple and efficient strategy to in-situ evolved the active layer, consisting of amorphous carbon and graphene encapsulated Fe3C nanoparticles, on the surface of 316L-type stainless steel as highly efficient electrocatalyst for OER. The CH4 plasma treatment was used to realize the edges and defects, and carbide (Fe3C) on the surface of 316L-type stainless steel. After treatment, the increase of active sites and the tuning of electronic properties made a significant
Acknowledgements
The authors are grateful to the National Nature Science Foundation of China (51402100, 21573066, 21825201, 21573066 and 21805080), the Provincial Natural Science Foundation of Hunan (2016JJ1006 and 2016TP1009), and China Postdoctoral Science Foundation.
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