Stainless steel as an efficient electrocatalyst for water oxidation in alkaline solution

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Short Communication

Stainless steel as an efficient electrocatalyst for water oxidation in alkaline solution Fengshou Yu a, Fei Li a,*, Licheng Sun a,b a State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024, Dalian, China b Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 10044, Stockholm, Sweden

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abstract

Article history:

Commercially available 316L stainless steel was found to be a highly efficient material for

Received 16 July 2015

catalytic water oxidation. By directly using the stainless steel as an anode without any

Received in revised form

substrate support, a 10 mA/cm2 current density with 96% Faradaic efficiency was obtained

30 December 2015

at s ¼ 0.37 V in alkaline solution (1 M KOH). The stainless steel film exhibits excellent

Accepted 19 January 2016

longevity and a small Tafel slope of 30 mV/decade in water oxidation, making it an ideal

Available online 24 February 2016

anodic electrocatalyst. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Keywords: Water splitting Stainless steel Electrocatalysis

Collecting and storing solar energy in chemical bonds, as nature accomplishes through photosynthesis, is a highly desirable approach to solving the energy challenge [1]. Photoelectrochemical (PEC) water splitting as an promising way to transform solar energy into hydrogen has received considerable attention [2]. As for the two half reactions involved in water splitting, oxygen evolution reaction (OER) is more demanded due to multiple electron and proton transfer processes. Until now, efficient catalyst for OER remains a key challenge in pursuit of the solar production. Since precious metal electrocatalysts such as IrO2 and RuO2 suffer from the scarcity and high cost [3,4], there is tremendous endeavor in developing inexpensive and earth-abundant OER catalysts. CoOx, NiOx and CuOx have been found to be active catalysts for

electrocatalytic OER but large overpotentials are usually required in electrolysis [5e9]. In order to lower the overpotential, extensive efforts have been made to develop the mixed metal oxides. For example, FexNiyOz, CuxCoyO4, CoFeOx, NiCeOx and NiCoOx have been proven to be highly active electrocatalysts in alkaline media [10e17]. In this regards, the stainless steel as the alloy of iron, nickel, chromium and other metals could be an suitable candidate for efficient water oxidation catalysts. Furthermore, a layer of austenite structure on the surface of stainless steel composed of compact mixture of iron, nickel, chromium oxides is expected to provide high corrosion resistance in harsh reaction conditions. Due to the unique combination of high corrosion resistance and excellent mechanical strength, stainless steel

* Corresponding author. Tel.: þ86 41184986247; fax: þ86 41184986245. E-mail address: [email protected] (F. Li). http://dx.doi.org/10.1016/j.ijhydene.2016.01.108 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 5 2 3 0 e5 2 3 3

has been applied in a wide range of fields such as petrochemical, construction, maritime, aviation and catalysis [18]. Stainless steel was also used as electrode for aqueous lithiumair batteries in saturated LiOH solution because of its stability and low price [19]. Although stainless steel is employed as catalyst for water reduction [20e22], its intrinsic ability towards catalytic water oxidation is seldom studied [23e28]. Until recently, 304L stainless steel as water oxidation catalyst € fer et al. [29,30] Since 316L stainless has been reported by Scha steel is a more promising material that possesses relatively superior resistance to chloride erosion with respect to 304L, we investigate its catalytic properties towards water oxidation in this study. Particularly, the Tafel slope and the overpotential for performing 10 mA/cm2 current density for the stainless steel are reasonably compared with the state-of-theart electrocatalysts. The catalytic activity of stainless steel towards water oxidation was evaluated in 1 M KOH alkaline solution with a standard three electrode setup, including a piece of 316L stainless steel (1 cm  2 cm  0.1 cm) as working electrode, a Pt mesh counter electrode and a Ag/AgCl reference electrode. Cyclic voltammogram (CV) in Fig. 1 displays an anodic feature centered at Epa ¼ 1.47 V versus the reversible hydrogen electrode (RHE) and the subsequent return scan shows a cathodic peak at Epc ¼ 1.37 V. In order to assign these peaks, the electrochemical behavior of stainless steel was compared with those of NiOx and Cr2O3 under the same conditions (Fig. S1). In 1 M KOH solution, NiOx and 316L stainless steel showed similar redox couples in peak shape and position, while no obvious redox peak was found for Cr2O3, indicating the observed redox couple for stainless steel was due to Ni(II)/ Ni(III). By passing 1.52 V, an abrupt enhancement in current due to the catalytic water oxidation is observed, corresponding to an overpotential of 0.29 V. Iron is known to undergo corrosion under highly oxidizing conditions. The stability of the steel anode against corrosion is investigated by linear sweep voltammetry (LSV) in comparison with an iron sheet electrode. As shown in Fig. 2A, a sharp onset of OER at 1.52 V was observed for stainless steel, which is consistent with the behavior observed by CV measurement and no corrosion current was detected. By contrast, a 8 mA/

Fig. 1 e Cyclic voltammogram (CV) of stainless steel conducted with standard three electrode system in alkaline solution (1 M KOH) at a scanning rate of 100 mV/s. The inset shows magnified views of the redox peaks.

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Fig. 2 e (A) LSV curves of stainless steel and iron sheet in 1 M KOH solution with Ag/AgCl as reference electrode and Pt as counter electrode. (B) Tafel plot of stainless steel in 1 M KOH aqueous solution with iR compensation.

cm2 background current was observed for iron electrode across the scan range from 0.9 to 1.65 V, which is attributed to the corrosion of iron electrode. With stainless steel as the electrode, the Faradaic efficiency of bulk electrolysis at 1.6 V for 10 h was determined to be 96% (Fig. S2) and the ratio between the evolved hydrogen and oxygen was 2:1. These results highlight the crucial role of Ni and Cr components on resisting corrosion. Since 10 mA/cm2 current density is expected for a solar-tofuels conversion device with 10% efficiency, the overpotential for 10 mA/cm2 has been envisioned as a standard to evaluate the activity of electrocatalyst [13]. As for the stainless steel, 10 mA/cm2 current density was achieved at s ~ 0.37 V. This potential is comparable to those for the mixed metal oxide electrocatalysts such as NiFeOx, (0.35 V), CuxCoyO4 (0.39 V), CoFeOx (0.37 V), and NiCoOx (0.38 V) under similar experimental conditions (1 M NaOH or KOH) [13,31] and lower than those for Co3O4 (0.43 V) and NiCeOx, (0.43 V) [32]. Tafel slope is another vital parameter for OER catalysts. Under the same conditions as above mentioned, the steadystate current density (j) was recorded as a function of overpotential ranging from 250 to 360 mV, which exhibits a Tafel slope of 30 mV/decade (Fig. 2B). It is worthy to mention that this value is remarkable in comparison with the reported data for mixed metal oxide films. It is comparable to 24e33 mV/ decade for a-Fe100-y-zCoyNizOx produced by photochemical meta-organic deposition [10] and superior to other metal

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oxide and mixed metal oxide composites such as cobaltmanganese layered double hydroxide (CoMn LDH) (43 mV/decade) and Co3O4-carbon (70 mV/decade) [12,33]. Taking CoMn DLH as an example, in spite of lower onset potential (0.3 V) exhibited by CoMn DLH, stainless steel is proven to be more efficient at higher current densities due to the smaller Tafel slope. Bell has studied the effect of Fe content on the catalytic activity of mixed metal oxides and concluded that the NieFe films with moderately high content of Fe (0.3 < x < 0.85) are optimal to give smaller Tafel slopes [17]. The composition of 316L stainless steel was analyzed by X-ray photoelectron spectroscopy (XPS). The main species on electrode surface were found to be NiOx, Cr2O3 and FeOx with a ratio of 17.1: 30.0: 50.6 for Ni, Cr and Fe (Fig. S3, Table S1). Thus the Fe content on the surface of 316L stainless steel is around 0.5, which is exactly among the optimal values as Bell suggested. Stainless steel exhibited high durability in alkaline solution. Fig. 3A depicts chronopotentiometry curve of the stainless steel at 10 mA/cm2, the overpotential to maintain this current density essentially keeps constant for a period of 20 h. The stability was further demonstrated by constant potential electrolysis at s ¼ 0.40 V, a stable current density of 17 mA/cm2 was found to persist over 20 h (Fig. 3B). The LSV curve of stainless steel after long-term electrolysis was found to be unchanged (Fig. S4). Energy dispersive X-ray spectroscopy (EDS, Fig. S5) and XPS (Fig. S3) performed after 50 h electrolysis at 10 mA/cm2 was consistent with the fresh one, also suggesting no obvious change during the OER. In another experiment, the composition of the steel was analyzed by XPS after bulk electrolysis for 50 h. The ratio for Ni, Cr and Fe was found to change from 17.1: 30.0: 50.6 to 16.8: 33.0: 47.8. Further analysis by inductively coupled plasma mass spectrometry (ICP-MS) showed that there are 0.9809 ppm Ni, 0.0266 ppm Cr and 0.0261 ppm Fe dissolving in the resulting electrolysis solution (20 mL). In addition, the possible deposition on the counter electrode was evaluated by washing Pt mesh with 10 mL dilute HCl solution after bulk electrolysis. The dissolved Fe, Cr and Ni were found to be 1.0551, 0.1352, 0.0175 ppm, respectively. Taken together, these results are consistent with that obtained from XPS analysis, indicating only a slightly loss of Fe and Ni from the electrode surface.

Based on the above findings, the low onset potential, small Tafel plot, and high stability exhibited by stainless steel makes it an ideal material for alkaline water electrolysis. OER catalysts based on metal oxide normally need to be supported by substrates such as indium tin oxide (ITO) and fluorine doped tin oxide (FTO) [13]. However, the instability of ITO and FTO at anodic potentials required for water oxidation may lead to the loss of conductivity [34,35]. In addition, indium is a scarce element, and the vapor-phase sputtering process used to make ITO films further contributes to their high cost. The lowcost stainless steel, on the contrary, can stand by itself as an anode, which offers substantial simplicity to device manufacture. According to the previous studies on NixFeyOz and CoxFeyOz, the FeOx, NiOx composites on the steel electrode surface was proposed to be the active species for oxygen evolution [17,36]. Fig. S6A showed that a larger onset overpotential and a reduced current at 1.8 V are obtained by polished 316L stainless steel compared with those of unpolished sample. The decrease in catalytic activity was attributed to the removal of oxide layer on the surface of electrode. This assumption was supported by the fact that the catalytic performance of polished electrode could be improved by anodization with consecutive CV scans, resulting in a similar CV curve to that of unpolished electrode (Fig. S6B). These results suggest the oxidized surface is more active for water oxidation. Although we are not able to specify the function of the individual component of stainless steel at the moment, Fe cation in mixed metal oxides has been suggested to be the catalytic center for water oxidation by different research groups [17]. The incorporation with Ni was proposed to improve the electronic properties of Fe, leading to a lower overpotential for OER. Although no evidence showing Cr is a catalytic active component, it is considered to be indispensable for corrosion resistance. In summary, a commercially available product 316L stainless steel was investigated as an electrocatalytic water oxidation catalyst without supported substrate. Besides the low price and excellent stability, stainless steel exhibited high activity with an extraordinary Tafel slope of 30 mV/decade, offering a promising large-scale application potential for water splitting.

Acknowledgment This work was supported by the National Basic Research Program of China (2014CB239402), the National Natural Science Foundation of China (21476043, 21120102036, 21361130020), the Natural Science Foundation of Liaoning Province, China (2014020010), the Swedish Energy Agency and the K&A Wallenberg Foundation.

Fig. 3 e (A) Chronopotentiometry curves of stainless steel anode in 1 M KOH aqueous solution at a current density of 10 mA/cm2 for 20 h. (B) Currentetime curve obtained for water oxidation reaction with stainless steel anode at 1.63 V vs. RHE.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.01.108.

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