Sloughing a precursor layer to expose active stainless steel catalyst

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Energy, Environmental, and Catalysis Applications

Sloughing a precursor layer to expose active stainless steel catalyst for water oxidation Minoh Lee, Michael Shincheon Jee, Seung Yeon Lee, Min Kyung Cho, JaePyoung Ahn, Hyung-Suk Oh, Woong Kim, Yun Jeong Hwang, and Byoung Koun Min ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04871 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Applied Materials & Interfaces

Sloughing a precursor layer to expose active stainless steel catalyst for water oxidation Minoh Lee,† Michael Shincheon Jee,† Seung Yeon Lee,†,‡ Min Kyung Cho,⊥ Jae-Pyoung Ahn,⊥ Hyung Suk Oh,† Woong Kim,‡ Yun Jeong Hwang,*,† and Byoung Koun Min*,†,∥ †Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbukgu, Seoul 02792, Republic of Korea. ‡Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. ⊥

Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea. ∥

Green School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea.

KEYWORDS: water splitting, oxygen evolution, stainless steel, nanoporous, sulfurization

ABSTRACT: Hydrogen production by water electrolysis has been regarded as a promising approach to wean away from sourcing energy through fossil fuels, as the produced hydrogen gas can be converted to electrical or thermal energy without any harmful byproducts. However, an efficient hydrogen production is restricted by the sluggish oxygen evolution reaction (OER) at the counter anode. Therefore, the development of new OER catalysts with high catalytic activities is crucial for high performance water splitting. Here, we report a novel sloughing method to the fabrication of an efficient OER catalyst on a stainless steel (SS) surface. Chalcogenide (Fe-S) overlayer generated by sulfurization on the SS surface is found to play a critical role as a precursor layer in the creation of active surface during water oxidation. Interestingly, a newly exposed catalytic layer after sloughing off the Fe-S overlayer has a nanoporous structure with changed elemental composition, resulting in a significant improvement in OER performance with an overpotential value of 267 mV at current density of 10 mA cm-2 (in 1M KOH). Our novel method for preparation of OER catalyst provides an important insight to designing an efficient and stable electrocatalyst for the water splitting community.

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18 19 As fossil fuel reserves are depleting and their frequency of 20 discovery is declining, the growing global energy demand 21 is starting to be supplied by resorting to unconventional 22 oil and gas. Dissenters of extraction methods such deep 23 water drilling and hydraulic fracturing argue that 24 sourcing energy from such technology would toll the 25 environment and public health. In response, many 26 researchers are developing clean renewable energy 27 systems that are also economically viable to supplement 28 and even replace the current energy infrastructure. 29 Water splitting by electrolysis in conjunction with 30 renewable electricity is an promising route for the 31 production of renewable hydrogen as an alternative to 32 fossil fuels since no carbon is involved in the combustion 33 to produce CO2 or CO.1-2 Water electrolysis consists of 34 1. INTRODUCTION

two half reactions: oxygen evolution reaction (OER) and

hydrogen evolution reaction (HER).3 While HER is a two electron reaction, OER needs four electrons to be transferred in order to generate one oxygen molecule which generally demands a much larger overpotential. Thus, OER is considered to be the bottleneck process for water electrolysis that requires particular catalyst design in reducing the large overpotential.4 Precious metal compounds like RuO2, IrO2 are known to be efficient electrocatalysts, but the material price point prevents their commercial application. Therefore, usage of low cost and earth abundant materials is another requisite for the future commercialization of the water electrolysis system.5 Stainless steel (SS) is composed of earth-abundant elements that happen to be active for the OER (i.e. Ni, Cr, Fe, etc.), resulting in a potential for mass production.6-7 In addition, surface modification is facile in conjunction

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with the segregation phenomena of added elements from the iron matrix by thermal or electrochemical treatments, which often leads to superior OER activity. Intentional surface modification on SS for OER was firstly introduced by Schäfer group.8 In this report, surface of 304 SS was modified by exposing SS under Cl2 gas environment, chlorination arranged a mixture composed of Fe and Cr. The chlorinated SS showed very efficient and stable OER properties. In addition, Schäfer et al.9 and Lee et al.10 prepared OER catalysts on SS by harsh electro-oxidation in strong alkaline conditions, where significantly enhanced OER properties were observed for both electro-oxidized SS’s in alkaline and neutral electrolyte, respectively. They proposed that electro-oxidation induces SS surface Figure 1. Schematic illustration of active SS catalyst synthesis modification, and the origin of the activated SS catalyst is by sloughing method. related to the dominant formation of a surface NiOOH phase. 59 atmosphere. Anomalous shape of Fe-S overlayer was Recently, Kundu group also modified SS with 60 found to form on the SS surface, which is not stable at the combination of KOH and hypochlorite as the corroding 61 anodic potential in alkaline electrolyte that is common for agent. Enhanced OER properties were observed through 62 water oxidation. Importantly, sloughing of the Fe-S fomation of new layer on the SS surface consisting of NiO 63 overlayer during water oxidation occurred spontaneously incorporated Fe2O3 nanocrystals in conjunction with Cr 64 to form an amorphous Ni-Fe mixed oxide layer with a removal.11 65 nanoporous structure. This new bare surface exhibited a More detailed information in terms of the history of the 66 dramatic decrease in OER overpotential (265 mV @ 10 mA -2 development on SS for OER can be found in recent review 67 cm ) and was found to be in complete absence of any 12 68 sulfur content. Moreover, the high OER performance of article. Despite all different preparation methods and 69 the modified SS catalyst is not limited to the alkaline conditions mentioned above, the substantially enhanced 70 electrolyte but is applicable to a CO2-saturated 71 bicarbonate electrolyte (neutral pH), which is a desirable OER performance was consistently observed when the surface of SS was adequately modified and the 72 condition for CO2 electro-reduction, also known as distribution of constituent metals in SS was rearranged. 73 artificial photosynthesis. Our unique modification Therefore, the surface modification would be the key 74 strategy provides a new insight into the efficient and process of converting a regular SS into a highly active 75 stable OER electrocatalyst design with low-cost transition 76 metals. catalyst for OER. In order to activate a SS catalyst, we developed a totally 77 new synthesis strategy that originates from a strategy 78 mediated via metal chalcogenide catalysts. Recently, 79 metal sulfides, in conjunction with common oxide forms, 80 have also been considered as promising OER catalysts, 81 but the active sites of these catalysts were assumed to be 82 states associated with oxides exclusively.13-17 For instance, 83 Chen et al. demonstrated that neat transition metal (e.g., 84 Co, Ni, Fe) sulfides can be electrochemically turned into their oxide forms in-situ in an alkaline electrolyte. Such 85 metal oxides derived from metal sulfides have high 86 surface areas, which contributes significantly to improved 87 OER activity.15 Mabayoje et al. prepared NixSy film by 88 pulse-electrodeposition method, and it was observed that 89 NixSy is converted into a nickel oxide after OER while the 90 sulfur anion is depleted.16

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Because of the assumption that conversion of transition 92 metal sulfide to oxide form is critical to improve OER 93 activity, we apply similar concept to modify the surface of 94 SS catalyst where a sulfide precursor layer is formed on 95 the surface followed by its removal by an electro- 96 oxidation process (Figure 1). The precursor layer was 97 formed by a simple thermal sulfurization in H2S/Ar 98

2. EXPERIMENTAL SECTION 2.1 Catalyst Preparation. Stainless steel substrate (AISI 304; KISTEC, Republic of Korea) with size specification of 2.5 cm X 2.5 cm was cleaned with ethanol by sonication and rinsed with deionized water. As prepared stainless steel was dried in air, and moved into tube furnace. To sulfurize the stainless steel (H2S_SS), H2S gas (H2S (1%)/N2) was supplied with flow rate of 100 sccm for 1 h at 500 ℃. After sulfurization, the furnace was cooled down to room temperature. Elox_SS was prepared by 10 cycles of cyclic voltammetry (CV) of H2S_SS in alkaline electrolyte (1 M KOH). Air-annealed_SS was prepared as a control sample in which air was used instead of H2S during thermal treatment at 500 ℃. 2.2 Electrochemical Measurements. All of the electrochemical measurements were conducted by a potentiostat (Ivium, Iviumtechnology) employing a conventional three-electrode cell in alkaline electrolyte (1 M KOH), where H2S_SS was used as an anode electrode, Pt and Hg/HgO were used as a counter and reference electrode, respectively. CO2 saturated 0.5M bicarbonate (99.7%, Sigma-Aldrich) were used as a neutral electrolyte,


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where pH was controlled to be pH 7.3. The Ag/AgCl was 25 used as the reference electrode. The active area of 0.5 cm2 26 was exposed in a circular shape by masking with 27 insulating tape (see Figure S1). All of the measured 28 potential values were changed to a reversible hydrogen 29 electrode (RHE) by using following equations; 30

31 32 . . / 0.11 0.059 33 (1) 34 . . / 0.21 0.059 35 (2) 36 37 Impedance analyses were carried out at 10 mAcm-2 in 38 the frequency range from 1 Hz to 100 kHz. Ohmic voltage drop was corrected from the solution resistance value 39 measured by electrochemical impedance spectroscopy 40 (EIS).18 Unless otherwise noted, 100% compensation of iR 41 drop is conducted in this manuscript. 42 2.3 Spectroscopic Characterization. Field emission 43 gun scanning electron microscopy (FE-SEM, FEI Inc., 44 Inspect F) was used to investigate surface morphologies of 45 the prepared samples with an acceleration voltage of 15 46 kV. An X-ray diffraction (XRD, Shimadzu, XRD-6000) 47 patterns of the samples were recorded using a Cu Kα 48

radiation (0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were conducted with Al Kα radiation at 1486.6 eV (PHI 5000 VersaProbe). The depth profiling was carried out by time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis was carried out (ION-TOF, Germany), where a 10-keV beam of Cs+ was used. Raman spectroscopy analysis was conducted with 532 nm Nd;YAG laser (Alpha 300S, WITec). Transmission electron microscopy along with diffraction pattern was conducted by employing a FEI TitanTM 80-300 microscope operated at 300 kV. Energy dispersive spectrometer (EDS) analysis with Super-X EDS was used for an elemental mapping analysis (FEI, Talos F200X at 200kV).

3. RESULTS AND DISCUSSION The synthesis procedure of our active SS OER catalyst is described in Figure 2a. First, the sulfide precursor layer on SS was prepared by introducing H2S gas over SS substrate in tube furnace at 500 ℃ (from here on denoted as H2S_SS). The bare SS is generally shown to have cracked surfaces while polyhedron-like surface structures were formed after sulfurization (Figure 2b,c). Consistently, as seen in cross-sectional SEM images of SS samples obtained by a focused ion beam (FIB) analysis, the

Figure 2. (a) Schematic illustration for the preparation of samples through spontaneous peeling off process on the SS. SEM images at each steps (b) bare, (c) after sulfurization, and (d) after electro-oxidation, and (e-g) the corresponding FIB cross-sectional images with EDS element distribution (inset).



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Figure 3. TEM images of (a) H2S_SS and (b) Elox_SS with element mapping of Fe, Cr, Ni, O and S. Corresponding line-scan TEM-EDS elemental distribution profiles are shown (c) H2S_SS and (d) Elox_SS, respectively.

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relative flat surface of bare SS became very rough after the 29 sulfurization step (Figure 2e,f). This surface layer was 30 determined to be Fe-S compounds structured in feature 31 sizes ranging from a few hundred nanometer to 32 micrometers. This sample was then treated by 10 cycles of 33 cyclic voltammetry (CV) in alkaline electrolyte (1 M 34 KOH). Interestingly, during electro-oxidation (from here 35 on denoted Elox) we observed a certain degree of surface 36 layer detachment. The surface morphology of SS after 37 Elox dramatically changed again showing a thinner layer 38 of nanoporous structure (Figure 2d,g). Another aftereffect 39 of Elox was the disappearance of the Fe-S overlayer based 40 on our elemental analysis (discussed later in detail). 41 Forming and sloughing of the chalcogenized overlayer 42 (considered as a precursor layer) was more clearly 43 revealed by transmission electron microscopy (TEM) and 44 elemental analysis. As seen in annular dark-field imaging 45 (HADDF) STEM images (Figure 3) the Fe-S overlayer was 46 formed with around 300 nm from the surface after 47 sulfurization. Dominant distribution of Fe was clearly 48 seen in both the bulk and overlayer region (Figure 3c) 49 while S is distributed only at the top layer (denoted as Fe- 50 S overlayer). This implies that the composite elements 51 such as Ni or Cr did not diffuse toward the surface during 52 the sulfurization. Notably, no Fe was observed in the 53 border region between Fe-S overlayer and bulk, whereas 54 Cr-S and Ni-S moieties were detected at the boundary 55 which indicates partial segregation of Cr and Ni at the 56

interface between the overlayer and the bulk. After Elox of the sample, no S residue was detected, indicating the complete molting of the Fe-S overlayer. Instead, we found a distinct evolution of O distribution throughout the surface region. Meanwhile, metal composition noticeably varied, exhibiting an increase of Ni and disappearance of Cr near the surface. Both Fe and Ni were detected near the surface indicating the formation of a new overlayer composed of Fe, Ni, and O. In order to further elucidate this surface change phenomenon, several different sulfurization temperatures were applied followed by Elox in the same conditions (Figure S2, Supporting Information). No apparent surface layer was formed for the SS sulfurized at 200 ℃ (Figure S2a,d, Supporting Information) but the surface morphology started to change for the sample prepared at 300 ℃ sulfurization (Figure S2b,e, Supporting Information). More angled structures was then observed for the sample prepared at 400 ℃ sulfurization (Figure S2c,f, Supporting Information). For comparison, we performed the same experiment using other foils (bare Fe and Ni, alloy Ni(45)Fe and Ni(78)Fe) to investigate the identity of the polyhedron shaped surface layer (Figure 3c). After sulfurizing the Ni and Fe foil (Figure S3b, e, Supporting Information), grains with different shapes and sizes were observed though a nanoporous structure was formed after Elox. However, the surface overlayer did not detach during Elox and the nanoporous surface structure


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was noticeably different from that of SS. On the other 38 hand, in the case of Ni-Fe alloy foils (Figure S4b,e, 39 Supporting Information), porous surface layer are formed 40 after sulfurization. We suspect that either Ni-S or Fe-S 41 grains with the different sizes are simultaneously grown 42 during sulfurization, which may induce porous structures 43 due to the different growth kinetics. On the basis of our 44 observation, diffusion of the elements Ni, Fe, and Cr in SS 45 or kinetics of chalcogenide formation correlates with the 46 formation of anomalous shape of Fe-S overlayer on the 47 surface during sulfurization. We should note that the 48 detachment of Fe-S overlayer and formation of a 49 nanostructure seems to be a unique behavior of SS for 50 this process. Elemental analysis by energy dispersive 51 spectrometer (EDS) also showed that after sulfurization, 52 ratio of both Cr and Ni are reduced compared to the 53 untreated SS and elements Fe and S accumulated to the 54 surface of SS (inset of Figure 2e-g). Remarkably, less than 55 expected amount of sulfur and increased amount of 56 oxygen was observed after Elox of the sulfurized sample 57 in alkaline electrolyte (inset of Figure 2g). 58

while significantly less amount of both Cr and Ni were detected throughout the film thickness, consistent with TEM EDS results. Therefore, we are able to conclude that S preferentially interacts with Fe while suppressing the diffusion of other metals (i.e. Cr and Ni) at the SS surface during sulfurization due to its weak reaction with them. As already seen in SEM and TEM data, S did not exist on the Elox_SS surface. No element S was detected throughout the film thickness, indicating Fe-S overlayer was not stable when anodic potential was applied in the alkaline electrolyte conditions. It should be noted that FeS overlayer were not directly converted to oxide analogues but rather detached entirely in the electrolyte. The SS surface before and after sloughing off the Fe-S overlayer was further compared by various spectroscopic analyses as shown in Figure 5. X-ray diffraction (XRD) analysis (Figure 5a) showed beyond the bottom SS substrate related peaks, three distinct peaks at 33.97o, 44.11o, and 53.33o were evolved on H2S_SS which are matched with (101), (102), and (110) planes of hexagonal FeS, respectively.21 However, those peaks were not observed after sloughing by Elox while a new peak at around 50o appeared which corresponds to the Ni-Fe metal alloy. No metal oxide related XRD peak was detected. Combining XRD data with TEM results (Figure 3b,d) we assume that mixture of metal alloy and amorphous phase of Ni-Fe mixed oxide was also formed on the Elox_SS surface. Raman analysis also supports the formation of Fe-S overlayer for the H2S_SS showing vibrational peaks at 216.4, 280.5, and 390.7 cm-1. Consistent to our analysis so far, the Fe-S overlayer related vibrational peaks disappeared after Elox; instead, various metal oxide features were seen.

The compositional distribution was further investigated 59 by X-ray spectroscopy (XPS) depth profiles for four 60 samples: Bare_SS (Figure 4a), air-annealed (500 ℃)_SS 61 (Figure 4b), H2S_SS (Figure 4c), and Elox-SS (Figure 4d). 62 In contrast to the Bare_SS, Fe signal in the air- 63 annealed_SS was found to be suppressed on the surface 64 region (~100 nm) while that of Cr was increased. Oxygen 65 content was also found to be high near the surface, but it 66 gradually decreased towards the bulk. Diffusion of 67 element Ni was also suppressed at the surface region. 68 These trends are consistent with previous studies 69 reporting thermal diffusion of each metal in SS.19-20 On 70 the other hand, thermal treatment under H2S condition 71 XPS was analyzed as seen in Fig. 5c-f for Ni 2p, Fe 2p, S (H2S_SS) showed a completely different behavior in 72 2p, and O 1s, respectively. First, we carefully compared Ni element diffusion. Fe-S overlayer with similar atomic 73 element states at each stage of SS samples, because Ni has ratios of Fe and S were observed up to ~500 nm thickness 74 been reported as the active metal species for OER on the 75 modified SS samples. Ni 2p spectra showed no noticeable 76 Ni signals on the bare_SS, while weak peaks intensity near 77 852.6 eV on the H2S_SS associated with Ni-S or metallic 78 Ni (Ni0) state emerged possibly due to small amount of Ni 79 presence on the surface. These Ni peaks have lower 80 binding energy compared to the peak related to Ni-O 81 bonding at 855 eV (Ni 2p3/2).9 After Elox, no such S 82 bonding or metallic state related peaks were detected 83 while a strong peak intensity at 855 eV was observed, 84 again indicating the real active species of Ni relates to Ni85 O bonding (Ni(Ⅱ) or Ni(Ⅲ)) and not Ni-S. In case of the 86 Fe XPS spectra, the bare SS revealed a peak at 710 eV (Fe 87 2p3/2) which is attributed to Fe-O bonding (Fe(Ⅱ) or Fe( 88 Ⅲ)).22 The apparent peak at 706 eV was evolved after 89 sulfurization, which is assumed to be Fe-S bond.[16,18,19] 90 Reiterating EDX and XPS results, this peak disappeared 91 while more intensified O bonding peak was observed after 92 Elox. For Cr (Figure S5, Supporting Information), the peak Figure 4. Depth profiles for different conditions of SS from 93 around 577 eV (Cr 2p3/2) was shown in spectra of Bare_SS, XPS analysis. (a) Bare_SS, (b) Air annealed_SS, (c) H2S_SS, 94 which is mainly Cr(Ⅲ)0.9, 22 This peak disappeared after and (d) Elox_SS. 95 sulfurization, but appeared again after electro-oxidation,



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suggesting that Cr emerged near the surface after casting 20 Information), and Elox_SS is prepared as the result of off the Fe-S overlayer. 21 these cycles. High current density was observed during Complete removal of S moiety due to the Elox was 22 first positive sweep and some particles were detached clearly evident in S XPS spectra as shown in Figure 5e. 23 from catalyst surface. High current may be related to the Simultaneously, SS surface revealed different oxygen 24 displacement of sulfur with oxygen, but the high current states at each condition as seen in Figure 5f. Three kinds 25 was observed no more from second cycle in the low of states are represented: metal oxygen (~529.5 eV), metal 26 potential range (before water oxidation, below than 1.5 V hydroxide (~531.2 eV), and water molecules that are 27 vs. RHE). Notably, within the CV process, sulfur moieties adsorbed in metal compounds (~533 eV).23-25 For H2S_SS, 28 should be completely removed to avoid any reduction or the intensity of metal oxygen (M-O) bonding decreased 29 oxidation current generation associated with residual compared to SS but was recovered after Elox. Moreover, 30 sulfur dissolution during the oxygen evolution reaction. the metal hydroxide bonding peak was found to be 31 The reason for the detachment of particles (possibly Fe-S slightly blue shifted, suggesting an oxide-like 32 overlayer or generated oxide materials) can be environment in the catalyst.26 Notably, no clear difference 33 hypothesized as follows. The volume and crystal structure in the peaks for water molecule bonding was observed in 34 change would occur at the interface since the 35 displacement of sulfur with oxygen can induce rapid all measured conditions. 36 change in the structure when applying an anodic The electrochemical properties for water oxidation 37 potential in alkaline electrolyte. The induced lattice were investigated in 1 M KOH electrolyte (Figure 6). CV of 38 mismatch might cause a low adhesion between Fe-S 10 cycles on H2S_SS is shown in Figure S6 (Supporting 39 overlayer and bulk substrate. Alternatively, the newly

Figure 5. Spectroscopic investigations on three kinds of the samples (bare, H2S_SS, and Elox_SS). (a) XRD, (b) Raman, and XPS spectra of (c) Ni 2p, (d) Fe 2p, (e) S 2p, and (f) O 1s.


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formed interface may have exhibited poor adhesion, 45 thereby the Fe-S (or converted oxidation compound) 46 overlayer are detached from the surface underneath. Last 47 polarization curves (10th cycle) of Elox_SS, Bare_SS, and 48 air-annealed_SS are plotted in Figure 6a, and air- 49 annealed_SS was prepared as a control sample following 50 the same steps with Elox_SS except air being used instead 51 of H2S during thermal treatment to see the effect of the 52 H2S treatment on the OER activity. Unlike Bare_SS (346 53 mV @ 10 mA cm-2), the air annealed_SS showed rather 54 poor performance (380 mV @ 10 mA cm-2) in water 55 oxidation. On the basis of XPS depth result (Figure 4b), 56 such poor activity would probably be attributed to the Cr 57 diffusion toward the surface on air-annealed_SS because 58 relatively Cr rich environment in addition to Ni and Fe 59 deficiencies in catalysis would negatively affect water 60 oxidation. In contrast, Elox_SS shows a significant 61 enhancement in water oxidation activity (267 mV @ 10 62 mA cm-2). To analyze the interfacial charge-transfer 63 kinetics of SS based OER electrocatalysts, impedance 64 analysis was performed after 10 cycles CV treatment. It is 65 well known that the charge transfer resistance is placed in 66 the inverse proportion with active surface area.27 All 67 impedance experiments were carried out at 10 mAcm-2 68 after 10 times CV cycles in 1 M KOH electrolyte condition. 69 As shown in Figure S7 and Table S1, all ohmic resistances 70 are very similar, but the kinetic charge-transfer resistance 71 of H2S_SS is smaller than those of the other 72 electroctalysts due to the increased active surface area 73 during displacement of sulfur with oxygen. 74

42 43 44

not lead distinct crystal growth on the surface, thereby no 86 area compared to bare counterpart. Finally, amorphous clear difference is observed in OER activities. On the 87 phase of the electrocatalyst can be another reason leading other hand, as the temperature rises, the degree of crystal 88 to the enhanced OER performance. According to previous

We also compared the OER activities of the samples 75 where H2S gas was supplied at varied temperature (Figure 76 S8, Supporting Information). In case of the sample treated 77 at 200 ℃, the OER activity was almost the same as that of 78 bare SS, but it was possible to observe gradual 79 enhancement of OER activities as an increase in 80 temperature range from 300 ℃ to 500 ℃. On the basis of 81 the previous SEM analysis (Figure S2, Supporting 82 Information), it is presumed that it has the similar surface 83 characteristics as bare SS since the surface of SS was not 84 sulfurized well at 200 ℃, meaning this temperature could 85

growth on the SS surface appears to increase, resulting more active species could be exposed after electrooxidation. The origins of the high performance with Elox_SS are proposed on the bases of the previous material characterization results. First, Ni-Fe mixed oxide acts as an excellent active materials in the alkaline electrolyte. Ni and Fe individually are well demonstrated to have poor activity as OER catalysts but once properly mixed, they exhibit a higher performance.28-32 Figure S9 (Supporting Information) demonstrates this phenomena by sulfurizing individual Ni and Fe foils and two Ni-Fe alloys (i.e. Ni(45)Fe and Ni(78)Fe). In the case Ni-Fe alloy, the OER activity is shown to be better than those of individual Ni and Fe. It can be seen that there is not much difference in OER activity between H2S_Ni-Fe alloy and H2S_SS. Referring to the previous analysis results, Cr plays an important role in the crystal growth of abnormally shaped Fe-S; but there is no distinct influence on the OER activity assuming Cr is not exposed on the surface after the electro-oxidation. Although the no difference in OER activities was observed between SS and Ni-Fe alloy, it is suspected that there might be a slight difference in the active species because Ni-Fe alloy shows a small anodic peak near 1.4 V (vs RHE), which is not present in the case of SS. This peak indicates Ni(II)/Ni(III) conversion (i.e. Ni(OH)2 to NiOOH).25, 28-29, 32-33 As the amount of Fe increases in the Ni matrix, the peak shows a tendency to decrease, meaning insertion of Fe suppresses oxidation to NiOOH. In the case of H2S_NiFe alloy, NiOOH could be present on the surface, but in the case of SS, Ni is possibly not segregated to the surface since the Fe amount is comparatively high (Figures 3d and 4d). Comprehensively, there is no clear difference in the OER activities for samples (H2S_SS and H2S_NiFe alloy), but active sites would differ to some extent. Secondly, self-organized nanostructure morphology would provide an increased surface area, implying more active sites are exposed for water oxidation compared to Bare_SS. As shown in Figure S10 (Supporting Information), Elox_SS had around 40% increase in surface

Figure 6. Electrochemical characterization in 1 M KOH electrolyte. (a) Last LSV polarization curves (after 10 CV cycles) of Bare_SS, H2S_SS and Air_SS, (b) Tafel plots of both Bare_SS and H2S_SS, (c) continuous chronopotentiometry test of Bare_SS, -2 H2S_SS and Air_SS at a stationary current density of 100 mA cm for 10 h. For better understanding, the term H2S_SS was used in figures instead of Elox_SS.



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Figure 7. Electrochemical characterization in CO2-saturated 0.5 M bicarbonate electrolyte. (a) CV curves on the H2S_SS sample, (b) comparison CV curves conducted at first and last cycle, (c) comparison LSV polarization curves between Bare_SS and H2S_SS. For better understanding, the term H2S_SS was used in figures instead of Elox_SS. Here we used H2S_SS sample in CO2saturated 0.5 M bicarbonate electrolyte after electro-oxidation in 1 M KOH.

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reports, an amorphous phase can deliver a positive 43 influence on the OER activity since a large number of 44 defect sites and lattice dislocations inherent in 45 amorphous phase increases the probability of being the 46 active sites than that in crystalline form.34-37 47 Tafel plots, η versus log (j), for Elox_SS and Bare_SS are 48 shown in Figure 6b. The value of Tafel slope is found to 49 be 79.2 mV in the low overpotential range for Bare_SS, a 50 value that is not too different from the value obtained for 51 Elox_SS (77.1 mV dec-1). This indicates that the kinetic 52 process for water oxidation is similar on both samples.4 It 53 should be noted that this range (usually from ∼0.01 to 10 54 mA cm−2) can be commonly regarded as the kinetic 55 ndicator, but we cannot solely make a mechanistic 56 57 conclusion with this analysis alone.

58

Stability is also an important OER catalyst property. We 59 tested the long-term stability of Bare_SS, H2S_SS and 60 Air_SS after electrochemical oxidation process through 61 continuous electrolysis at a constant current density of 62 100 mA cm-2. As shown in Figure 6c, the electrode 63 potentials of the Bare_SS and Air_SS started to increase 64 after 4 hours, whereas the H2_SS catalyst showed 65 negligible degradation for up to 10 h, indicating H2_SS 66 after electrochemical oxidation (Elox_SS) has a good 67 durability in alkaline media. XPS analysis after long term 68 chronopotentiometry test is shown in Figure S11 69 (Supporting Information). According to the XPS depth 70 spectra (Figure S11a, Supporting Information), the oxygen 71 content slightly increased near the surface region (~100 72 nm) comparative to the fresh sample (Figure 4d). 73 Additionally, no notable change was observed in terms of 74 other elements (Figure S11b-d, Supporting Information).

75

Highly active OER catalyst in alkaline electrolyte does 76 not guarantee its good performance in a neutral 77 electrolyte such as bicarbonate solution, which is 78 necessarily used in a combinatory system with CO2 electro-reduction (i.e. artificial photosynthesis).38-39 A 79 significantly high overpotential occurred in the neutral 80 electrolyte at OER electrode because of the lower 81 hydroxide content inherent in the CO2 saturated 82 solution.40 This problem can be mitigated with a 83 bicarbonate electrolyte to create a buffering environment 84

to the CO2 gas. To investigate OER performance of our catalysts in a neutral electrolyte, an identical process was carried out using the bare SS in CO2-saturated 0.5 M bicarbonate solution. To put concretely, 10 times cycles of CV were carried out on the sample (Elox_SS at 1 M KOH) and its result is shown in Figure 7a. Note that H2S_SS is not stable in bicarbonate electrolyte, therefore we used it in CO2-saturated 0.5 M bicarbonate solution after electrooxidation in 1 M KOH. A light decrease in performance was observed after Elox, where the polarization curves slightly shifted to the right direction (Figure 7b), indicating relative stability in the neutral electrolyte condition compared to previous study showing instability of Ni-Fe mixed oxide catalyst in phosphate neutral electrolyte.41 The reason for the slight decrease in the performance is probably attributed to the catalyst loss. Ni is most likely lost since notable decrease in the XPS peak intensity of Ni was observed in Figure S12a (Supporting Information) but no significant difference in other elements (Fe and O) was detected(Figure S12c,d, Supporting Information), which is in good agreement with previous reports of Ni’s instability at high currents in neutral electrolyte condition.10, 41 As shown in the last polarization curves (Figure 7c), a considerable enhancement of water oxidation ability was observed compared to Bare_SS, as similar to experiments in alkaline condition. To analyze the O2 selectivity of Elox_SS in different pH electrolyte condition, faradaic efficiency of O2 production was calculated from the total current (10 mAcm-2) and the partial current from the area of GC chromatogram peak for O2 (see Figure S13). The faradaic O2 efficiencies were increased to ~85% and it was not affected by the electrolyte pH condition. Given the absent of any other variable product, this result implies that ~15% faradaic charge was used to oxidize the stainless steel. 4. CONCLUSION In summary, we prepared the amorphous Ni-Fe mixed oxide OER catalyst through a spontaneous sloughing process. Our electrocatalyst showed efficient and stable properties for water oxidation than the bare counterpart. Moreover, we briefly discussed how as-prepared catalyst


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K.; Schneider, L. Surface Oxidation of Stainless Steel: Oxygen works in the CO2-saturated bicarbonate system. Based on 59 Evolution Electrocatalysts with High Catalytic Activity. ACS all of the analysis, mixed binary (Ni-Fe) oxide species, 60 61 Catalysis 2015, 5 (4), 2671-2680. increased active surface area, and amorphous phase of the material could have led to the remarkable OER 62 (9) Schäfer, H.; Sadaf, S.; Walder, L.; Kuepper, K.; Dinklage, S.; 63 Wollschläger, J.; Schneider, L.; Steinhart, M.; Hardege, J.; properties. 64 Daum, D. Stainless steel made to rust: A robust water-

65 66 Supporting Information 67 Optical, SEM and AFM images, XPS spectra and depth pro- 68 files, LSV curves, performance comparisons and O2 Faradaic 69 70 Efficiency This material is available free of charge via the Internet at 71 72 http://pubs.acs.org. 73 74 AUTHOR INFORMATION 75 Corresponding Author 76 77 *E-mail: [email protected] and [email protected] 78 79 Funding Sources 80 Any funds used to support the research of the manuscript 81 should be placed here (per journal style). 82 Notes 83 The authors declare no competing financial interests. 84 85 ACKNOWLEDGMENT 86 This research was supported by the program of the Korea 87 Institute of Science and Technology (KIST) and by the KU- 88 KIST Graduate School Project. This work was also supported 89 by the Korea Center for Artificial Photosynthesis (KCAP) 90 through the National Research Foundation of Korea (No. 91 92 2014M1A2A2070004), funded by the Korean Government. 93 94 REFERENCES 95 (1) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, 96 P. The Mechanism of Water Oxidation: From Electrolysis via 97 Homogeneous to Biological Catalysis. ChemCatChem 2010, 2 98 (7), 724-761. 99 (2) Artero, V.; Chavarot-Kerlidou, M.; Fontecave, M. Splitting 100 Water with Cobalt. Angewandte Chemie International Edition 101 2011, 50 (32), 7238-7266. 102 (3) McKone, J. R.; Lewis, N. S.; Gray, H. B. Will Solar-Driven 103 Water-Splitting Devices See the Light of Day? Chemistry of 104 Materials 2014, 26 (1), 407-414. 105 (4) Stevens, M. B.; Enman, L. J.; Batchellor, A. S.; Cosby, M. R.; 106 Vise, A. E.; Trang, C. D. M.; Boettcher, S. W. Measurement 107 Techniques for the Study of Thin Film Heterogeneous Water 108 Oxidation Electrocatalysts. Chemistry of Materials 2017, 29 (1),109 120-140. 110 (5) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and 111 Fe)-Based Electrocatalysts for the Water Oxidation Reaction. 112 Advanced Materials 2016, 28 (42), 9266-9291. 113 (6) Moureaux, F.; Stevens, P.; Toussaint, G.; Chatenet, M. 114 Development of an oxygen-evolution electrode from 316L 115 stainless steel: Application to the oxygen evolution reaction in116 aqueous lithium–air batteries. Journal of Power Sources 2013, 117 229, 123-132. 118 (7) Anantharaj, S.; Chatterjee, S.; Swaathini, K. C.; Amarnath, T. 119 S.; Subhashini, E.; Pattanayak, D. K.; Kundu, S. Stainless Steel120 Scrubber: A Cost Efficient Catalytic Electrode for Full Water 121 Splitting in Alkaline Medium. ACS Sustainable Chemistry & 122 Engineering 2018, 6 (2), 2498-2509. 123 (8) Schäfer, H.; Beladi-Mousavi, S. M.; Walder, L.; Wollschläger, 124

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ACS Applied Materials & Interfaces Table of Contents Graphic


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Sloughing a precursor layer to expose active stainless steel catalyst

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