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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10072-10083

High-Performance Oxygen Evolution Anode from Stainless Steel via Controlled Surface Oxidation and Cr Removal Sengeni Anantharaj,†,‡ Murugadoss Venkatesh,∥ Ashish S. Salunke,∥ Tangella V. S. V. Simha,∥ Vijayakumar Prabu,# and Subrata Kundu*,†,‡,§

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Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute (CSIR-CECRI), New Delhi, Delhi 110001, India ‡ Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India ∥ Centre for Education (CFE), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India # Central Instrumentation Facility (CIF), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India § Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Improving the water oxidation performance of abundantly available materials, such as stainless steel (SS), with notable intrinsic electrocatalytic oxygen evolution reaction (OER) activity due to the presence of Ni and Fe is highly anticipated in water splitting. A new method for promoting the corrosion of stainless steel (304) was found which assisted the uniform formation of oxygen evolution reaction (OER) enhancing NiO incorporated Fe2O3 nanocrystals with the simultaneous reduction in the surface distribution of OER inactive Cr. An equimolar combination of KOH and hypochlorite was used as the corroding agent at 180 °C. The effect of corrosion time on the OER activity was studied and found that better water oxidation performance was observed when the corrosion time was 12 h (SS-12). The SS-12 showed an abnormal enhancement in OER activity compared to the untreated SS and other optimized versions of the same by requiring very low overpotentials of 260, 302, and 340 mV at the current densities of 10, 100, and 500 mA cm−2 along with a very low Tafel slope in the range of 35.6 to 43.5 mV dec−1. These numbers have certainly shown the high-performance electrocatalytic water oxidizing ability of SS-12. The comparative study revealed that the state-of-the-art IrO2 had failed to compete with our performance improved catalytic water oxidation anode “the SS-12”. This fruitful finding indicates that the SS-12 has the potential to be an alternate anode material to precious IrO2/RuO2 for alkaline water electrolyzers in future. KEYWORDS: Water oxidation, Oxygen evolution, Overpotential, Tafel analysis, Voltammetry, Stainless steel, Controlled corrosion

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INTRODUCTION Electrochemical water splitting with appropriate high-performance electrocatalysts of nonprecious and abundant metals has widely been accepted as an efficient mean of energy storage by producing the purest hydrogen than the conventional steam reforming of fossil fuels.1,2 In recent days, nonprecious highperformance catalysts are the frequently studied ones to ensure minimum energy loss due to the overpotentials required by the half-cell reactions, namely, the OER at anode and the hydrogen evolution reaction (HER) at cathode.3 Previously employed precious- and noble-metal-based catalysts, such as Pt4 (for HER) and IrO25 and RuO26 (for OER), are disadvantageous though they are better active catalysts than many others when the large scale and economically affordable hydrogen production in considered.3 Hence, they need to be replaced © 2017 American Chemical Society

with the nonprecious ones. Fortunately, the recent evolution of metal chalcogenides,7 hydroxides,8 phosphides,9 and their alloyed versions, such as phosphosulfides10,11 and phosphoselenides,1,3,12 had made it realizable. Among them, the sulfides, selenides, and phosphides are better bifunctional water splitting catalysts than the oxides and hydroxides of the same.1,3 Between HER and OER, OER is the one with the sluggish kinetics as it involves in four electron and four proton transfer process with an oxygen−oxygen bond formation and always requires huge overpotential than HER from its reversible overpotential (1.23 V vs RHE).3 The oxides and hydroxides of Received: June 26, 2017 Revised: August 27, 2017 Published: September 25, 2017 10072

DOI: 10.1021/acssuschemeng.7b02090 ACS Sustainable Chem. Eng. 2017, 5, 10072−10083

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ACS Sustainable Chemistry & Engineering

alkaline conditions.17,18 However, it is important to note here that, in one report of Schafer et al.,36 they have observed highly enhanced activity with the Cr free SS and the one with very low Cr atomic %. In another concomitant work, they have checked the effect of surface oxidation of various SS alloys with Cl2 that had huge Cr content which varied from 29.2% to 53.3%.37 However, the activity is not as the same as the one observed with the surface oxidation of SS of no or less Cr content.36 This implies that the presence of Cr on the corroded or oxidized surface of SS significantly reduces its activity. On the other hand, in the report of Zhong et al.35 the role of presence of Cr as its oxide on the corroded surface of SS was unaccounted, which could have also restricted some of the active sites of FeOOH and Ni(OH)2. Hence, any method to completely leach out or at least to significantly reduce the distribution of Cr on the corroded surfaces of SS would highly be useful to further enhance the OER activity of the same. Moreover, Cr-free SS are comparatively less stable when considered for large scale bulkelectrolysis as they will have poor corrosion resistance because of the absence of Cr oxides on its surface. Hence, it is essential to have some Cr but in lower atomic percentage. With these encouraging perspectives, we have developed a new method of corroding SS-304 hydrothermally in the presence of an equimolar mixture of KOH and NaOCl at 180 °C. The equimolar mixture of KOH and NaOCl is the one used to selectively leach the Cr ions from SS sludge40−42 and the use of the same had also driven the corrosion of SS under proposed conditions which led to the formation of NiO incorporated Fe2O3 nanocrystals with the significantly reduced Cr distribution on the surface. The same was then characterized and screened for electrochemical water oxidation in alkaline conditions and discussed below in subsequent sections.

3d iron group metals are well-known for catalyzing OER in alkaline conditions.13 Among Fe, Co, and Ni (hydr)oxide systems, the oxides and hydroxides of Ni and Co are well-documented ones in literature.14−16 Fe alone was reported as its oxyhydroxide, where Fe is in 3+ oxidation state for efficiently catalyzing OER in alkaline solutions.17,18 When these metals are considered alone as hydroxides in its divalent state, the Ni2+ is the highly active catalyst than the others in the same series due to its OER favoring d-electron configuration as per the reports of Subbaraman et al.19 However, when a combination of more than one of these metals are considered, there are many catalyst systems that had been reported to be showing better activity than Ni(OH) 2 .20−22 Among them, the cobaltates 23−25 (MCo2O4, M = Ni and Fe) and ferrites26−28 (MFe2O4, M = Ni and Co) are the frequently reported ones. Between these two, the ferrites are better than cobaltates.13 Abnormally enhanced OER catalytic activity was observed when the Fe and Ni are composed in single catalyst systems, such as NiFe2O4, Ni2+−Fe3+ (hydr)oxide composite, and NiFe layered double hydroxides (LDH).13 There are many reports debating the role of Fe in enhanced OER activity, when it is combined with Ni. Among them, the systematic study carried out by Trotochaud et al.20,29,30 was the significant one which brought out the role of increased conductivity of the catalytic interface when Fe is present even at trace level. In some other reports on NiFe systems, the formation of NiFe2O4 on the catalytic surface was attributed for the enhanced OER activity.13 It was essentially reported that the Ni must be in divalent state and Fe must be in a trivalent state.31−34 However, the real mechanism behind such an abnormal enhancement is still not established clearly and being kept under debate.13 The design of NiFe catalysts system was done by many methods earlier such as wet-chemical synthesis from respective metal precursors and the surface modification of various commercially available Ni and Fe containing alloy materials (e.g., stainless steels of various grades).13 The second method is advantageous over the first method as the first one requires specialized electrode fabrication step with the use of additive materials, such as binders due to which it almost always suffers with poor stability upon prolonged electrolysis. The second method of surface modification of Ni and Fe containing alloys is advantageous when long-term stability and high-performance with minimum overpotential are considered.3 Stainless steel (SS) is an easily available, a cheap alloys that essentially contains both Ni and Fe. Because of this, the SS of various grades, such as 304, 316, and 916, were chemically, hydrothermally and electrochemically modified to improve its activity toward OER electrocatalysis in alkaline conditions.35−39 SS itself possesses an appreciable intrinsic OER activity. However, the passive oxide layer of Cr on SS surface, which originally protects it from staining and corrosion restricts the OER performance of the same.35−39 Recently, some brilliant works by Schäfer et al.36,37 and Zhong et al.35 on the surface modification of SS for harvesting the improved OER activity were reported in which the SS surface was corroded with chlorine gas assisted oxidation, electrochemical anodization and hydrothermal treatment with ammonia, respectively. In all those three reports, the significant formation of oxides of Fe3+ and Cr3+ along with some Ni2+ oxides was observed and the same were attributed to the enhanced OER performance.35−37 Oxidation of Fe to Fe3+ oxide will easily lead to the formation of FeOOH, which is one of the highly active OER catalysts in

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EXPERIMENTAL SECTION

Controlled Corrosion of SS-304. Pieces of SS-304 of dimension 3 × 1 cm were taken for controlled corrosion. In a clean Teflon lined autoclave vessel of volume 50 mL, about 30 mL of an equimolar mixture (0.01 M) of KOH and NaOCl was added. Four pieces of SS304 with the foresaid dimension were submerged in the above solution and heated at 180 °C. The time of hydrothermal treatment was varied, 6, 12, and 24 h, and the change in the surface nature, such as appearance and brittleness, was monitored. The one treated for 6 h (SS-06) was found to have the same gray color without any significant change in its appearance as that of bare SS. When the time of hydrothermal treatment was increased to 12 h, the whole area of SS pieces turned to olive in color that indicated the substantial corrosion of the same. Meanwhile, the SS-12 was not susceptible to ruptures and cracks. However, when the total time of hydrothermal treatment was increased up to 24 h, the whole SS turned to black in color and was breakable even with the gentle pressure applied with two fingers. This indicated that the extended and over corrosion of SS and the same had rendered us from studying the OER performance of the same. To ascertain the role of the chemical composition taken for the controlled corrosion and selective leaching of Cr from SS at least at the surfaces, a controlled study was carried out only with KOH and found that the SS was merely unaffected even after 24 h of hydrothermal treatment. This indicates the necessity of hypochlorite in our study of controlled corrosion and selective leaching of Cr from SS. Similarly, there are reports earlier saying that the hypochlorite may oxidize and induce corrosion of SS but could not leach Cr from SS surfaces unless a suitable chemical environment is provided. Therefore, it is clear that the presence of both KOH and NaOCl are essential to successfully corrode and significantly remove Cr from the surface of SS. Materials used in this study along with the technical details on the instrumentations employed are provided in Supporting Information (SI). 10073


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ACS Sustainable Chemistry & Engineering Electrochemical Characterizations. The electrocatalytic OER performance of bare SS in comparison with the controllably corroded SS-06 and SS-12 along with IrO2 coated SS and SS treated with KOH alone under identical conditions for 24 h was studied here in 1 M KOH. Fabrication of IrO2 coated SS electrode is elaborated in SI. All polarization curves were recorded between 0 to 1 V vs Hg/HgO reference electrode with the lower scan speed of 5 mV s−1 along with the parallel iR compensation in electrochemical workstation. All potential scales were converted to reversible hydrogen electrode (RHE) scale as per earlier reports for interpretation of the performance and comparison purposes.43,44 To check the repeatability in the OER activity of the best active SS-12, the polarization curves were recorded for five consecutive times and found that the SS-12 was having consistent OER performance in 1 M KOH with acceptable errors. Nyquist plots were recorded at the onset overpotentials of OER for all the studied anode materials with the polarizing potential amplitude of 50 mV from 100 kHz to 1 Hz. The relative electrochemical surface area (ECSA) was determined for all anode materials studied here opting the well-known double layer capacitance method (Cdl). The endurance of SS-12 was examined by two competitive methods such as cycling and potentiostatic prolonged electrolysis. The cycling study was carried out with cyclic voltammetry at a very scan speed of 200 mV s−1 due to which it is also known as accelerated degradation test. The potentiostatic electrolysis was carried out without iR compensation at a potential of 1.515 V vs RHE that corresponds to 25 mA cm−2. Results of the comparative electrocatalytic study are discussed in subsequent sections.

the observed peaks of maghemite phase at 32.15, 33.94, and 35.61 are (221), (310), and (311), respectively. Other than this, there were no peaks observed for the formation other metal oxides and mixed metal oxides of more than one or two metals. The primary diffraction study with XRD diffraction has indicated significant corrosion of the SS surfaces. Moreover, it can also be noticed that the metallic peak intensities of SS are relatively reduced with SS-06 and SS-12, which is once again indicates the surface corrosion of SS as expected. It was earlier mentioned that with the increasing time of hydrothermal treatment, the characteristic gray color of SS slowly turned into olive in color.35−37 Hence, to get a closer view on the surface modifications occurred during such controlled corrosion, a detailed and comparative Field Emission Scanning Electron Microscopic (FESEM) Analysis was carried on bare SS, SS-KOH-24, SS-06 and SS-12. The high magnification FESEM micrographs of bare SS, SS-KOH-24 and SS-06 are given as Figure S1a−c. From Figure S1a and b, it is clear that the hydrothermal treatment of SS with KOH alone did not lead to any significant surface modifications in terms of corrosion. However, it can be noticed from Figure S1c that there are some nanocrystals anchored over the surface of SS but not with maximum coverage. This implies that the corrosion had begun already in SS-06 which is treated with an equimolar mixture of KOH and NaOCl for 6 h at 180 °C. Interestingly, the FESEM micrographs of SS-12 (Figure 2a, b) with increasing magnification revealed that the formation of nanocrystals on SS surface was uniform and the coverage is almost full. The average size of the nanocrystals seen over the surface SS-12 was calculated to be 140 ± 20 nm. Some of these nanocrystals were dispersed in DI water through rigorous ultrasonic treatment for 40 min and the same dispersion was then used to fabricate the transmission electron microscopy (TEM) specimen of the same. The high-angle annular dark filed (HAADF) micrograph of the oxide nanocrystals are shown as Figure 2c from which the brighter nanocrystals of cubic, rectangular, triangular and anisotropic shapes of various size can be seen. This implies that the formation of oxide nanocrystals was not shape or size selective as observed earlier also in SS corrosion.35 The inset of Figure 2c is the bright field TEM micrograph of the same which also shows similar highly contrast cubic, rectangular, triangular, and anisotropic nanocrystals. This is in accordance with the HAADF micrograph. Figure 2d is the high resolution TEM micrograph of a single oxide crystal at a very high magnification which shows clear lattice fringes. The measured distance between two adjacent lines observed in Figure 2d was 0.294 nm, which is well matching with the hkl index of (220) of maghemite phase of iron oxide and also with the corresponding ICDD reference card no. of 89-5892. The inset of Figure 2d is the corresponding selected area electron diffraction (SAED) pattern of the same single oxide nanocrystals which is showing a perfect cubic dot pattern. This implies that though the oxide nanocrystals were formed without any shape and size selectivity as a consequence of controlled corrosion on SS surface, they are sufficiently larger in size to efficiently diffract the projected electron beam and single crystalline in nature. Figure 2e−j shows the elemental color maps of K shells of C, O, Cr, Mn, Fe, and Ni, respectively, acquired in low magnifications ranges to cover the distribution of all elements in a very larger area of SS-12. At first look, it is evident that all the above elements are uniformly distributed on the surface of SS-12 including Cr. However, the corresponding percentage

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RESULTS AND DISCUSSION Material Characterizations. X-ray diffraction (XRD) patterns of bare SS, SS-KOH-24, SS-06, and SS-12 are provided as Figure 1. In all of them three predominant peaks positioned

Figure 1. XRD patterns of bare SS, SS-KOH-24, SS-06, and SS-12, respectively.

at 43.5° (111), 50.79° (200), and 74.7° (220) are due to the characteristic metallic alloy of Fe, Ni, Cr, and Mn in SS-304 with the cubic crystal system of space group Fm3̅m and the space group number of 225. These results are well matching with the earlier reports,45,46 as well as to ICDD reference card number of 33-0397. Interestingly with SS-12, some low intense peaks of considerable counts were noticed between the diffraction angle ranges from 30° to 40°. Those peaks are then calibrated and found to be matching with the cubic maghemite phase (Fe21.16O31.92, which would correspond to general empirical formula of Fe2O3) of iron oxide (ICDD reference No. 89-5892) with the space group and number of P4332 and 212 respectively. The corresponding hkl indices of 10074


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Figure 2. (a, b) FESEM micrographs of SS-12 with increasing magnification. (c) HADDF micrographs of NiO incorporated Fe2O3 nanocrystals, along with the respective bright field TEM micrograph of the same as inset image. (d) High-resolution HRTEM micrograph of a single oxide nanocrystal with the corresponding SAED pattern as its inset image. (e, j) Elemental color maps of K shells of C, O, Cr, Mn, Fe, and Ni with their corresponding surface concentration in percentage acquired on SS-12.

285.9 eV indicating the substantial oxidation of lattice carbon due to the controlled corrosion on the surface. Other than these in Figure 3a and b, two other peaks at 290 and 294.2 eV were observed between which the first one located at 290 eV must be due to the C coordinated to the electronegative metal among all and the one located at 294.2 eV is just the satellite peak of C 1s. These observations are nicely matching with the earlier reports.35−37 Similarly, the O 1s spectra of both bare SS and SS-12 surfaces were acquired and provided as Figure 3c and d from which two distinct peaks at 530 ± 0.2 and 532.1 ± 0.1 eV were observed due to the presence of passive metal oxides of mixed oxidation states on the surfaces of both bare SS and SS-12. Interestingly, with SS-12 the intensity of the peak at 532.1 eV is relatively increased than that of bare SS. This primarily indicates the addition surface oxidation of metals and C present in it. These observations are also matching with the earlier reports.35−37,48,49 Similarly, the Cr 2p3/2 spectra of bare SS and SS-12 were acquired on their surfaces and provided as Figure 3e and f. The peak at 574.1 eV in both Cr 2p3/2 spectra of bare SS and SS-12 are due to the presence of some metallic Cr behind the passive oxide layer of the same on their surfaces. However, it is essential to note here that the characteristic metallic Cr peak intensity is significantly reduced in case of SS12. This is indicating the substantial oxidation of Cr to its higher oxidation states. Likely, 577.1 ± 0.1 eV in both Cr 2p3/2 spectra are due to the presence of Cr 2 O 3 on their surfaces.35−37,50 This Cr2O3 is insoluble in water but when reacts with Cl2 produced in situ from NaOCl, it forms slightly soluble CrCl3, which is believed to be the reaction that selectively leaches Cr from SS surfaces during the controlled corrosion in this study. However, the complete leaching of Cr from SS surface may not be possible as the oxidation of metallic Cr to corresponding Cr2O3 will always project fresh metallic Cr sites on the surface. Hence, significant reduction in the surface distribution of Cr had only been achieved in our study. These results are also matching with earlier XPS studies on many Cr

distribution differed significantly than the bare SS. The percentage composition of C, O, Si, Cr, Mn, Fe, and Ni of bare SS was 6.3, 0.8, 0.84, 19.39, 1.48, 65.0, and 6.2, respectively. However, the measured percentage composition of the same elements in SS-12 was 6.8, 11.3, 0.84, 16.09, 1.15, 56.11, and 6.85, respectively. Three significant changes to be noticed in the percent composition are the ∼11-fold increase in O distribution, ∼3.3% reduction in Cr distribution and retention of Ni distribution on SS-12 surface. It is highly desired to have such a chromium distribution reduced SS surface with unaltered Ni distribution when high-performance OER is considered. Moreover, the ∼11-fold increase in O distribution on the surface also implies that there had been a substantial corrosion on the surface of SS-12 which primarily led to the formation of iron oxide in major proportion as indicated by the color maps. Other than these, the changes in the percent composition of other elements are negligible. The above detailed microstructural characterization had clearly revealed the substantial corrosion of SS, significant removal of Cr distribution from the corroded SS surface and the uniform coverage of NiO incorporated Fe2O3 nanocrystals over the corroded SS surface. With this information, the chemical nature of all the essential elements of SS-12 surface was comparatively studied by XPS analysis and discussed below. X-ray Photoelectron Spectroscopic (XPS) Studies. To obtain information on the changes in chemical nature of all essential elements present is SS during the controlled corrosion, XPS spectra of C 1s, O 1s, Cr 2p3/2, Mn 2p3/2, Fe 2p3/2, and Ni 2p3/2 states were acquired on both bare untreated SS and SS-12 and given as Figure3a−l with alternative alphabets. Figure 3a and b are the high-resolution XPS spectra of C 1s states of C present in bare SS and SS-12. In bare SS, the predominant C 1s peak observed was located at 284.6 eV, which is due to the lattice C atoms of SS and this is in accordance with the earlier report.47 In contrast, the major intense peak of C 1s spectrum acquired on SS-12 is observed at 10075


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Figure 3. continued

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Figure 3. (a, b) High-resolution C 1s spectra of bare SS and SS-12, respectively. (c, d) High-resolution O 1s spectra of bare SS and SS-12, respectively. (e, f) High-resolution Cr 2p3/2 spectra of bare SS and SS-12, respectively. (g, h) High-resolution Fe 2p3/2 spectra of bare SS and SS-12, respectively. (i, j) High-resolution Mn 2p3/2 spectra of bare SS and SS-12, respectively. (k, l) High-resolution Ni 2p3/2 spectra of bare SS and SS-12, respectively.

corresponding Mn3O4 was not seen with bare SS, whereas it is the high intense peak in case of SS-12 and located at 641.9 eV. This observation is also in good agreement with the earlier reports.35−37,53,54 Finally, the Ni 2p3/2 spectra were acquired on both bare SS and SS-12 surfaces and given as Figure 3k−l. The metallic Ni peak of bare SS is observed predominantly at 853 eV, along with two less intense peaks at a much lower binding energy of 849.6 eV, due to the presence of alloyed Ni with other electropositive metals, namely, Cr, Mn, and Fe and at 854.6 eV eV due to the presence of some surface oxidized NiO. Interestingly, with SS-12, the metallic Ni peak at 853 eV is still observed with comparable intensity with broadened peak of NiO at 854.8 eV. This is indicating the surface oxidation of Ni to NiO. The peak observed at 861.5 eV is the satellite peak of NiO formed on the surface of SS-12.35−37,55,56 The above detailed and comparative XPS study has proven the oxidation of all essential elements of SS and the substantial reduction of Cr distribution on the surface of SS-12. With such lowered Cr distribution and with the NiO incorporated Fe2O3 nanocrystals, we proceeded further to evaluate its electrocatalytic OER performance. Comparative Electrocatalytic Water Oxidation. Linear sweep voltammograms (LSVs) of bare SS, SS-KOH-24, SS-06, SS-12, and IrO2/SS were recorded at a slower scan rate of 5 mV s−1 in 1 M KOH. The resultant iR free polarization lines of the same are provided here as Figure 4a. The superior performance of SS-12 over others is clearly evident from Figure 4a. Moreover, within the experimental potential window, the SS12 is the only catalytic anode which could drive an OER current

containing compounds. The Fe 2p3/2 spectra acquired on both bare SS and SS-12 surfaces are provided as Figure 3g and h. From Figure 3g, the peaks corresponding to metallic Fe at 706.6 and 708.1 eV are observed as expected on bare SS surface. In addition to the metallic Fe peaks, two more peaks corresponding to FeO and Fe2O3 are observed at 709.9 and 710.9 eV respectively indicating the presence of passive oxide layer of Fe in both 2+ and 3+ oxidation states. In sharp contrast, the Fe 2p3/2 spectrum obtained on the surface of SS12 had shown only two peaks upon deconvolution at 706.2 and 710.9 eV which are characteristic to the metallic Fe and Fe2O3. This indicates the controlled corrosion done in our study have substantially oxidized all Fe2+ to Fe3+ and also maximum metallic Fe that were present before corrosion on the surface to Fe3+ state as expected. The same is also evident from the significantly reduced intensity of the peak located at 706.2 eV of metallic Fe when compared with the bare SS. These observations are in good resonance with earlier reports.35−37,51,52 The Mn 2p3/2 spectra acquired on the surfaces of bare SS and SS-12 are provided as Figure 3i and j. The noisy background observed with Mn 2p3/2 spectra in both cases could be due to the comparatively lower percentage composition of Mn. Like Cr 2p3/2 and Fe 2p3/2 spectra, the Mn 2p3/2 spectra of both bare SS and SS-12 had shown the presence of metallic Mn at 641.5 and 639.7 eV, respectively. However, the corresponding peak intensity with SS-12 is relatively lower when compared to that of bare SS. This indicates that like Cr and Fe, Mn had also undergone significant surface oxidation during hydrothermal treatment with KOH and NaOCl. Interestingly, peaks 10077


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Figure 4. (a) LSVs of bare SS, SS-KOH-24, SS-06, SS-12, and IrO2/SS, respectively. (b) Plot of current density vs overpotential with the corresponding error bars. (c) Tafel plots of bare SS, SS-KOH-24, SS-06, SS-12, and IrO2/SS, respectively. (d) Nyquist plots of LSVs of bare SS, SSKOH-24, SS-06, and SS-12, respectively. (e) Plot of scan rate vs the difference in double layer charging currents of bare SS, SS-KOH-24, SS-06, and SS-12, respectively. (f) iR free LSVs acquired before and after 5000 cycles of CV cycling on SS-12 surfaces.

density of 500 mA cm−2 at a minimum overpotential of 340 mV. The overpotential required to start the electrolysis which is otherwise known as onset overpotential and the overpotential required to deliver a current density of 50 mA cm−2 by bare SS, SS-KOH-24, SS-06, SS-12, and IrO2/SS are 340 and 410 mV, 340 and 430 mV, 310 and 363 mV, 230 and 288 mV, and 240 and 370 mV, respectively. The observed trend clearly indicates that the surface modification of SS-304 via controlled corrosion associated with the partial reduction of Cr distribution on SS surface is the reason for the observed enhanced OER activity. It can be noticed that when the SS was treated alone with KOH, the onset overpotential was not changed and the overpotential at 50 mA cm−2 was increased by 20 mV. This indicates that the KOH alone cannot help us to improve the OER performance of the SS-304 by inducing the expected controlled corrosion and the use of NaOCl along with KOH is essential. Similarly,

the SS-06 had shown significantly improved OER performance than that of both bare SS and SS-KOH-24 by requiring 20 and 60 mV lower than the same to start the electrolysis and to drive a defined current density of 50 mA cm−2. This is due to the partially corroded SS-06 surface with some NiO incorporated Fe2O3 nanocrystals. The SS-12 required a much lower onset potential of 230 mV and a overpotential of 288 mV to drive a current density of 50 mAcm−2. This is even better than the state-of-the-art IrO2/SS as seen in Figure 4a, where the IrO2 loading was kept 3-fold higher than the generally studied ones in electrocatalytic water oxidation. The observed enhanced activity of SS-12 is mainly attributed to two factors. The first one is the uniform formation of highly OER active NiO incorporated Fe2O3 nanocrystals everywhere on its surface and the second one is the significant reduction in the surface distribution of Cr as a result of controlled corrosion and 10078


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overpotential region and indicates that the kinetics for OER on SS-12 is better than other interfaces. To get a clear view on the same, the Tafel slopes value can be related to the mechanism by which the OER occurs at an electrocatalytic interface and the same has been discussed below. As per the Tafel equation which is obtained by the high overpotential approximation of Butler−Volmer equation, the number of electron transferred in an electrocatalytic process across an interface can be predicted from its Tafel slope assuming that the system’s charge transfer coefficient (α) is 0.5, which is true for most of the electrochemical systems.1−3

selective leaching action of NaOCl. It is well-documented in literature that NiFe based alloys, oxides, sulfides, selenides, phosphides, and layered double hydroxides are highly active OER catalysts than any other composition other transitionmetal-based catalysts including Ir and Ru in alkaline conditions. Enhanced OER performance of NiFe systems were observed earlier by many which mainly varies depending on the ratio of Ni and Fe and their oxidation states.13 Among plenty of such reports, an interesting works reported by Trotochaud et al. on the incidental and intentional incorporation Fe impurities in Ni(OH)2 electrodeposited films is the significant one who proved that the conductivity of Ni(OH)2 films were increased drastically even with a trace amount of Fe impurities.20,29,30 We also believe that such synergistically enhancing mechanism between the oxides of Ni and Fe which present as nanocrystals on the surface of SS-12 must be operating behind such abnormally enhanced high-performance with minimum overpotential. Moreover, the XPS study revealed that almost all Fe on the oxide nanocrystals were 3+ oxidation states which would had easily formed the highly active FeOOH phases upon exposure to highly alkaline and anodic overpotential conditions. A good review including the earlier discovery, mechanism and applications of such highly active Ni- and Fe-based OER electrocatalysts was recently reported by Gong et al.,13 where they stated that though the increased conductivity and formation of FeOOH along with NiOOH were attributed as the factors that causes such abnormal high-performance OER activity, the actual mechanism is still under debate. As stated earlier, in our case, along with the NiO incorporated Fe2O3 nanocrystals formation, the significant reduction of Cr distribution on SS-12 surface can also be additional factor governing the enhanced OER performance of SS-12. It is known that Cr and its oxides are inactive catalysts for water oxidation and the removal of the same from SS surface will leave relatively larger space for the OER active Ni and Fe sites.20,29,30 Moreover, we have also checked the repeatability in the high-performance OER activity of SS-12 for five consecutive times and found that the polarization results were highly reproducible with acceptable errors as shown in Figure 4b. The Tafel plots of bare SS, SS-KOH-24, SS-06, SS-12, and IrO2/SS extracted from their respective LSVs are given as Figure 4c. At first look on Figure 4c, it is clear that there is a dramatic change in the Tafel slopes of electrocatalytic interfaces other than SS-12 from the decade lower overpotential region to the one of higher overpotential region. This indicates that there is a change in the pathway of OER when transition occurs from low overpotential region to high overpotential region. At lower overpotential region, for the decade of 1−10, bare SS, SS-KOH24, SS-06, SS-12, and IrO2/SS were found to have the Tafel slopes of 35.2, 35.1, 35.8, 35.6, and 38.2 mV dec−1. However, at higher overpotential region, the Tafel slopes of bare SS, SSKOH-24, SS-06, SS-12, and IrO2/SS were 119.2, 118.1, 119.8, 43.5, and 117.2 mV dec−1. From these Tafel slopes values at low and high overpotential regions, it is clear that except SS-12, other electrocatalytic interfaces experienced a significant kinetic transformation by showing very low Tafel slopes (∼36 mV dec−1) in lower overpotential region and high Tafel slopes (∼119 mV dec−1) in higher overpotential region. Interestingly, the SS-12 had shown only a slight increase (∼8 mV dec−1) with transition from one decade (of lower overpotential) to another (of higher overpotential). This emphasizes that the SS-12 is able to retain it kinetically efficient OER pathway even in high

d log j/dη = 2.303RT /αnF

(1)

In the above equation, R stands for gas constant, T represents absolute temperature in K, α represents the charge transfer coefficient, which is assumed to be 0.5 here, n denotes the number of electrons transferred, and F stands for the Faraday constant (96485 C). At standard conditions, for a singleelectron transfer reaction, the above equation will yield 120 mV dec−1 as the Tafel slope. Using this, we have interpreted the behavior of our electrocatalytic interfaces. In lower overpotential region, all the studied interfaces have shown Tafel slopes in the range of 35−38 mV dec−1. From the Tafel equation, it is clear that if the slope is 30, the relative number of electrons transferred across the electrocatalytic interface would be 4. Since OER is a four electron and four proton coupled O− O bond formation reaction, such a lower Tafel slopes closer to 30 mV dec−1 in lower overpotential region for all studied interfaces indicated that the OER kinetics is facile, and they followed four electron transfer pathway. However, in higher overpotential region, except SS-12 other interfaces showed Tafel slopes closer to 120 mV dec−1 which would correspond to one electron transfer pathway of OER. Interestingly, the SS12 was still able to maintain Tafel slope closer to 30 mV dec−1 with a small difference (13 mV dec−1) and retained its OER favoring four electron transfer pathway.57−60 Such a disparity in the Tafel slopes between SS-12 and other studied interfaces could have arisen due to number of reasons, such as change in the pathway of OER, varying surface coverage, change in the rate-determining step as per the report of Bockris et al.,61 and could also be due to the change in the conductivity across the electrocatalytic interface as per the report of Cahan et al.62 In our case, we believe that such a dramatic difference in the Tafel slopes must mainly be due to the increased conductivity of SS12. In addition, it is also attributed that the formation of NiO incorporated Fe2O3 nanocrystals are the OER facilitating entity which is not present in bare SS and SS-KOH-24 and the coverage of such NiO incorporated Fe2O3 nanocrystals is very poor in case of SS-06. This suggests that such 0% coverage in bare SS and SS-KOH-24 and poor coverage in SS-06 could also be the reason for the observed anomaly. From the above discussion, the facile OER kinetics on SS-12 is attributed to nearly 100% coverage of OER active NiO incorporated Fe2O3 nanocrystals, increased conductivity due to the presence of metallic Fe behind the NiO incorporated Fe2O3 nanocrystals and its ability to maintain the four electron transfer pathway of OER in all overpotential regions. As observed in LSV and Tafel analyses, the corresponding Nyquist plot of SS-12 had shown a comparatively lower charge transfer resistance (RCT) than others. The charge transfer resistance calculated from the Nyquist plots of bare SS, SS-KOH-24, SS-06, and SS interfaces are 10.5, 12.5, 12, and 4 ohm, respectively. The lower RCT observed with SS-12 interface is the reason for the predicted 10079


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continuous electrolysis at the foresaid constant potential is highly negligible and the same had once again proven the robustness of SS-12 interface in addition to the accelerated degradation test. The overall comparative electrocatalytic water oxidation study had clearly revealed that the controlled corrosion of SS304 with an equimolar mixture of KOH and NaOCl was highly fruitful in converting the electrocatalytically active normal SS into a high-performance surface engineered SS interface decorated with NiO incorporated Fe2O3 nanocrystals and with the reduced OER inactive Cr surface distribution. The enhanced performance observed with SS-12 is mainly by the two synergistic surface modifications, namely, the decoration with NiO incorporated Fe2O3 nanocrystals and selective removal of Cr from SS surface which was brought out by controlled corrosion. The results of the comparative electrocatalytic study are provided as Table 1. The observed performance of SS-12 is better than other related reports of Schäfer et al.36,37 and Zhong et al.35 when the selective leaching of Cr, NiO incorporated Fe2O3 nanocrystals coverage and highperformance with minimum overpotential is considered. After all the robustness of SS-12 surface after such harsh alkaline electrochemical treatments were analyzed again with FESEM and the results are discussed below. Beyond everything, as far as the cost of an electrode of 1 cm2 area with the comparable thickness is concerned, our material (SS-304) is much cheaper (US$ 3.55) than both pure Ni foil (US$ 26.61) and pure IrO2 foil (US$ 237.93), which ensures that our SS-12 derived from SS-304 is certainly a cheaper alternate to the precious (IrO2) and costly (Ni) water oxidation electrodes for large scale water electrolysis. Microstructural Robustness after Electrocatalytic Water Oxidation Studies. The controllably corroded SS-12 with the uniform decoration of NiO incorporated Fe2O3 nanocrystals along with the reduced Cr surface distribution was subjected FESEM analysis again after polarization studies to get information on the microstructural robustness of the same. The FESEM micrographs obtained on the surfaces of SS12 after OER studies are provided as Figure 6a−d. Figure 6a is the low magnified large area image, which shows the uniform array of NiFe oxide nanocrystals array whereas Figure 6b−d are the FESEM micrographs of SS-12 with the same magnification acquired at different region. Figure 6a shows the uniform array of NiO-incorporated Fe2O3 nanocrystals in which the calculated average particle size was 140 ± 30 nm. This resembles closely to the average nanocrystals size measured before OER polarization studies. Similarly, the comparatively magnified micrographs provided as Figure 6b−d are also revealing the same inferences. Moreover, it also imply that the surface morphology and the nature of nanocrystals of NiFe oxide arrangements were unaffected by the electrochemical

facile kinetics from the lower Tafel slope and the highperformance OER activity within minimum overpotentials. The increased conductivity of the catalytic interface as result of uniform formation of NiO incorporated Fe2O3 nanocrystals on SS-12 surface could be the reason for the observed lower RCT and the same is in accordance with the earlier report of Trotochaud et al.20 ECSA calculation from the difference in double layer charging currents and the associated 2Cdl is a widely accepted way of predicting activity trends in water splitting electrocatalysis.3,7 Hence, we have also determined the relative ECSA of bare SS, SS-KOH-24, SS-06, and SS-12 and the corresponding double layer charging current versus scan rate plot is provided as Figure 4e. The calculated relative ECSA of bare SS, SS-KOH-24, SS-06, and SS-12 are 1, 1.05, 1.26, and 5.03, respectively. From Figure 4e, it is again evident from the higher slope of SS-12 that the relative ECSA of SS-12 is far higher than bare SS, SS-KOH-24, and SS-06, respectively. Moreover, the observed trend in the relative ECSA is well matching with the results of polarization, steady-state-polarization and EIS studies on the same. Beyond, the parameters determining the activity trend among bare SS, SS-KOH-24, SS06, SS-12, and IrO2/SS interfaces, the robustness of SS-12 was also checked by accelerated degradation tests and prolonged potentiostatic electrolysis. The iR free LSVs acquired before and 5000 cycles of rapid cycling with cyclic voltammogram (CV) are pictured as Figure 4f. The increase overpotential even at 500 mA cm−2 was just 12 mV. This is certainly testifying the superior robustness of SS-12 catalytic interface and will be highly useful for long-term alkaline water electrolyzers. Similarly, the endurance of SS-12 interface for constant potential of 1.515 V vs RHE for more than 10 h was tested. The resultant chronoamperometric feature is pictured as Figure 5. The loss observed in current density even after 10 h of

Figure 5. Chronoamperometric response of SS-12 upon prolonged potentiostatic electrolysis at 1.515 V vs RHE in 1 M KOH.

Table 1. Results of the Comparative Electrocatalytic Water Splitting Studies catalyst

η@10 mA cm−2 (mV)

η@50 mA cm−2 (mV)

Tafel slope (mV dec−1)

2Cdl (μF cm−2)

relative ECSA (cm2)

bare SS SS-KOH-24 SS-06 SS-12 IrO2/SS

365 365 336 260 296

410 430 363 288 370

141 131 151 41 150

0.0058 0.0061 0.0073 0.0292

1 1.05 1.26 5.03

a

a

a

The corresponding values are not provided for IrO2/SS because it was not evaluated for ECSA determination due to the completely different electrode fabrication method used. 10080


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Figure 6. (a−d) FESEM micrographs of SS-12 after OER polarization studies at different magnifications and at various regions.

studies. With such advantageous and fruitful performance, the SS-12 could be opted as an alternate anode to precious and expensive IrO2 and RuO2. Moreover, the same protocol can also be extended to engineer the surface of other types SS alloys of various percentage compositions of Ni, Fe, and Cr to improve their OER performance.

polarization studies which in turn testifying the microstructural robustness of our high-performance electrocatalytic anode “the SS-12”. Having such a high-performance activity observed due to the engineered corrosion of SS surface, very high stability and an excellent microstructural robustness, our high-performance anode, that is, SS-12 firmly ensures its place instead of precious and expensive anode catalysts, such as IrO2 and RuO2 in bulk-electrolysis for large scale prolonged hydrogen production.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02090. Details on the materials used in the controlled corrosion of SS, technical details of the characterization techniques used in both material and electrochemical characterization along with the IrO2/SS fabrication are provided, and FESEM micrographs of bare SS, KOH-treated SS, and SS-06 (PDF)

CONCLUSION The controlled corrosion promoted simultaneous reduction of surface distribution of Cr was achieved via a hydrothermal treatment of SS-304 with an equimolar mixture of KOH and NaOCl at 180 °C, which led to the formation of firmly anchored OER enhancing NiO incorporated Fe2O3 nanocrystals. The KOH and NaOCl mixture performed a dual role, namely, the surface oxidation of SS by the in situ release of highly oxidizing Cl2 and the selective leaching of Cr2O3 from the surface of oxidized SS by making the Cr2O3 to react with Cl2 to form water-soluble CrCl3. Such OER facilitating NiO incorporated Fe2O3 nanocrystals formation and the partial removal of OER inactive Cr from the SS surface turned it into a high-performance water oxidation anode. The SS-12 electrocatalytic anode required 260, 302, and 340 mV of overpotentials to drive the operationally equivalent current densities of 10, 100, and 500 mA cm−2, respectively, along with the minimum Tafel slope in the range of 35.6−43.5 mV dec−1, which indicated the facileness of OER kinetics on SS-12 surfaces. The comparative electrocatalytic study carried out with the state-of-the-art IrO2/SS (0.6 mg cm−2) and other optimized corroded SS, along with the untreated SS revealed that the SS-12 is a far better and high-performance catalyst. Additionally, the SS-12 showed consistent OER performance even after 5000 cycles of CV and 10 h of chronoamperometric

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: (+91) 4565-241486; (+91) 4565-241487. ORCID

Sengeni Anantharaj: 0000-0002-3265-2455 Murugadoss Venkatesh: 0000-0002-9496-338X Tangella V. S. V. Simha: 0000-0003-3787-5508 Subrata Kundu: 0000-0002-1992-9659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the unfading support and encouragements of Dr. V. K. Pillai, The Director, CSIR-CECRI. S.A. 10081


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acknowledges CSIR, New Delhi for the award of Senior Research Fellowship (SRF). Support from Dr. B. Subramanian, Sr. Scientist, ECMS Division, CSIR-CECRI, Mr. A. Rathishkumar, Mr. J. Kennedy, Mr. P. Nagesh Reddy, Mr. Ranjith of CIF, CSIR-CECRI, Karaikudi, Tamil Nadu, India is thankfully acknowledged.

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