Stainless Steel Scrubber: A Cost Efficient Catalytic Electrode for Full

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Stainless Steel Scrubber: A Cost Efficient Catalytic Electrode for Full Water Splitting in Alkaline Medium Sengeni Anantharaj, Shubham Chatterjee, Karukkampalayam Chinnusamy Swaathini, Thangavel Sivagurunathan Amarnath, Elangovan Subhashini, Deepak Kumar Pattanayak, and Subrata Kundu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03964 • Publication Date (Web): 23 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Sustainable Chemistry & Engineering

Stainless Steel Scrubber: A Cost Efficient Catalytic Electrode for Full Water Splitting in Alkaline Medium Sengeni Anantharaj†‡, Shubham Chatterjee‖₴, Karukkampalayam C. Swaathini‖₴, Thangavel S. Amarnath‖₴, Elangovan Subhashini‖₴, Deepak Kumar Pattanayak# and Subrata Kundu†‡* †Academy

of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical

Research Institute (CSIR-CECRI) Campus, New Delhi, 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 #Chlor-Alkali

Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-

630006, Tamil Nadu, India ₴These

authors have contributed equally.

* To whom correspondence should be addressed, Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), College Road, Karaikudi630006, Tamil Nadu, India E-mail: [email protected]; [email protected], Phone: (+ 91) 4565-241486 and (+ 91) 4565-241487.

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ABSTRACT Sometimes, search for a cost efficient bi-functional catalytic material for water splitting can be accomplished from a very unlikely place. In this work, we are reporting such a discovery of utilizing the stainless steel (SS) scrubber directly as a catalytic electrode for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of water electrolysis in 1 M KOH. The iR corrected overpotential calculated at an areal current density of 10 mA cm-2 for SS scrubber in HER is 315 mV which is 273 mV higher than Pt/C. Similarly, SS scrubber required 418 mV at 10 mA cm -2 which is just 37 and 98 mV higher than Ni(OH)2 and RuO2. Interestingly, the kinetic analysis revealed that SS scrubber had facile kinetics for both HER and OER in 1 M KOH as reflected by their corresponding Tafel slope values viz., 121 mV dec-1 and 63 mV dec-1 respectively. In addition, the two electrode cell fabricated using the same SS scrubber electrode delivered 10 mA cm-2 at 1.98 V. Beyond everything, the SS scrubber had shown ultra-high stability in both half-cell and full-cell studies for total water splitting. In addition, as far as the cost of an electrode material per gram is concerned, the SS scrubber defeats all the best electrocatalysts of water splitting by having a price of just 0.012 US$ which is 2.228 US$ lower than pure Ni, 59.658 US$ lower than RuO2 and 158.028 US$ lower than Pt/C 20 wt% catalyst. The overall study specified that SS scrubber can be adapted for cost-efficient large scale water electrolysis for bulk hydrogen production. Keywords: stainless steel scrubber, catalytic electrode, oxygen evolution, hydrogen evolution, water splitting, voltammetry INTRODUCTION Electrocatalytic water splitting is one among the most aspired fields of applied electrochemistry in recent days. The two major goals of researchers with electrocatalytic water splitting are the large scale production of high pure hydrogen and indirect large-scale storage of electrical energy derived from sustainable but seasonal sources like wind, solar tidal and geothermal energies as chemical fuels.1–3 Hydrogen has recently been accepted as the fuel of the future due to its attractive characteristics such as the highest specific chemical energy, zero carbon emission and humongous abundance of raw material (water). Though there are other methods like steam reformation of hydrocarbons and dissolution of metal hydrides in water for 2 ACS Paragon Plus Environment

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hydrogen production, water electrolysis is superior to all as it does not require high pressure and temperature, operates at ambient conditions, purity of hydrogen produced is the highest of all and can readily be opted for large-scale hydrogen production.4–8 Besides, as far as energy storage is considered, water electrolysis is the only eco-friendly and efficient way that can store electrical energy in large scale as chemical fuel which can be utilized by fuel cell to generate back the electricity on demand. Electrochemistry of water splitting is relatively simpler than many known reactions which include two major half-cell reactions viz., OER at anode and HER at cathode. 9 Nonetheless, the one and only but a serious issue with water electrolysis is huge energy loss in terms of overpotential. Since formation enthalpy of water is highly positive, breaking of O-H bond requires huge energy in terms of potential. OER is comparatively a more complex reaction that involves in the formation of four-electron and four-proton coupled oxygen-oxygen bond formation reaction than the simple HER which has a relative simpler mechanism. 7,8 Due to this reason OER always requires huge overpotential than HER. In the past decades, OER and HER were catalyzed by precious and noble metals like Ir/IrO 2,10 Ru/RuO211,12 and Pt13,14 and these were found to be kinetically efficient only in acidic medium. Fortunately, recent advances made in the field of water electrolysis in alkaline conditions with non-precious 3d transition metals have taken this field of applied electrochemistry to newer heights.1,3,7,8 Recently, various 3d transition metals based electrocatalysts as nano-powder catalysts, as self-supported arrays and as thin films have been reported in literature. 1,7,15 Among them, catalysts designed out of iron group metals are in higher numbers. In old days, metals and alloys of 3d series were directly employed for water electrolysis.3,4,9 However, recent researches have shown that the compound of these metals such as oxides, hydroxides, 16 layered double hydroxides (LDHs),8 chalcogenides,17,18 pnictogenides19 and their mixed versions are better electrocatalysts for the same. All of these catalysts were prepared/made/fabricated using various chemical and physical synthetic routes which require time, energy and above all huge economical support. Often, it is also possible to see that some outstanding catalysts and catalytic materials are discovered from very unlikely source materials. 20 Some examples are as follows: Sun et al. turned the well-known substrate material carbon cloth (CC) which has poor activity for OER into a better performing OER electrode by simple acidic oxidation.21 Schäfer et al. turned the moderately active SS foils of varying composition by surface oxidation with Cl 2 and electrochemical anodization into a high-performance OER electrode22,23 and the same group is 3 ACS Paragon Plus Environment


pioneering the field of converting moderately active various type of steels into high performance electrocatalytic materials for total water splitting. 24–26 Very recently, we suggested another method of modifying SS surface with KOH and NaOCl to make it as a high-performance OER electrode.27 Luckily, sometimes the known substrate materials itself can be effective catalytic electrodes for OER and HER. One such example is the Ni foam which is good active for both OER and HER as reported earlier.28 Another such interesting and equally active support material is SS. 29–31 Almost all types of SS contains Fe, Ni, Mn along with Cr, Si, C and some other traces. The first metals mentioned in the composition of SS and their compounds are well-known electrocatalysts in water electrolysis. Hence, there is no wonder that people have studied these materials for both OER and HER earlier.29–31 For instance, various surface modifications including electrochemical oxidation and high temperature annealing with desired hetero atom sources such as sulfur, hypophosphite and ammonia were demonstrated very recently.32,33 To further enhance the intrinsic catalytic activity of SS, people have also used hydrothermal surface oxidation and in situ growth of other active catalyst such as NiS on its surface.34 However, as far as the pure and unmodified alloys that come under the category of SS are concerned, the studies on those materials for OER and HER are relatively minimum and are as follows: Olivares-Ramírez et al. have studied the HER and its mechanism of various SS alloys of types 304, 316 and 430 in 1 M KOH and 1 M NaOH respectively.31 Herraiz-Cardona et al. studied the HER at Ni/Zn and NiCo/Zn SS based electrodes with impedance technique. 29 Moureaux et al. studied the OER properties of SS 316L directly without any surface modification for its use in aqueous Li-air batteries.35 Huang et al. very recently reported that 3D printed cellular SS are better active OER catalytic electrodes than foils of SS.30 Though these reports are important ones as far as the direct use of SS alloys in OER and HER are considered, people have studied the SS alloys that are procured from global vendors at great expenses and especially the report of Huang et al. dealt with the time and energy consuming relatively difficult 3D printing method. 30 It is wondered and at the same time we are fortunate that why there is no single study on the OER and HER activities of the well-known household product of SS i.e., SS scrubber which is several fold cheaper than many SS alloys that are available in market. Though SS scrubber was used as the HER electrode in microbial fuel cells in recent days, 36 we were wondered to not seeing any such evaluation on SS scrubber for water electrolysis directly. Hence, with the view that we got from 4 ACS Paragon Plus Environment

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the literature review, we have procured SS scrubbers from local grocery shops and evaluated their properties with various advanced characterizations and studied its activity for both OER and HER in 1 M KOH. The results are discussed below. EXPERIMENTAL SECTION Making Working Electrodes from SS Scrubber As purchased SS scrubber (Figure S1a in Supporting Information (SI)) looked like a bundle of finely curled springs of SS stripes with blackish grey color and lustrous surface. The SS scrubber in whole was washed several times alternatively with detergent liquid, de-ionized water and ethanol then dried at 80 °C for 1 h in a hot-air oven. Fine stripes of approximate length of 20 cm were cut out of the bundle for making working electrodes. Each stripe was masked by paraffin film leaving exactly a length of 10 cm (of mass ~ 10 ± 0.1 mg) which was to be immersed in electrolyte for its evaluation in water splitting electrocatalysis. In another end also sufficient length SS scrubber stripes was left unmasked to enable electrical connectivity to the electrochemical workstation. Figure S1b and Figure S1c in SI are the optical images of one such working electrode made from SS scrubber. In particular, Figure S1b in SI shows the exact length of the unmasked stripe which was 10 cm. These SS stripes were then used as working electrodes directly. Fabricating Ni(OH)2/CFP, RuO2/CFP and Pt/C/CFP Electrodes for Comparative Studies As it is always advised to study the performance of a water splitting electrocatalyst in comparison with the state-of-the-art electrocatalysts, we have fabricated three other working electrodes with the best OER electrocatalysts (Ni(OH)2 and RuO2) and the best HER electrocatalyst (Pt/C 20 wt.%) taking carbon fiber paper (CFP) as the substrate electrode. Pieces of CFP of dimension 1 cm × 5 cm were washed with ethanol and water and dried before casting the catalyst ink with a hot air drier gently. For preparing the ink of the catalysts to be studied comparatively, 3 mg of the same were dispersed by sonication in 1 mL solution of 2-propanol, water and Nafion 5% solution in 2-propanal of ratio 2.5:7.0:0.5 respectively.37–39 Sonication was sustained for a period of 10 min within which the catalysts were well-homogenized. About 68.3 µL of those inks were carefully drop-casted over an area of 1 cm2 on CFP leaving which remaining parts of CFP were masked with a scotch tap except the part destined for electrical contact at the other end after drying the coated catalyst ink at ambient conditions under dark. 5 ACS Paragon Plus Environment


Loading of all these catalysts was 0.205 mg cm-2. Soon after drying was completed, electrodes Ni(OH)2/CFP, RuO2/CFP and Pt/C/CFP were ready for their evaluation in water splitting to compare the activity of SS scrubber. Additional information on materials and reagents used along with the instrumental specifications of the characterization techniques is provided in SI. Electrochemical Characterizations All electrochemical characterizations were done in a solution of 1 M KOH at room temperature and at atmospheric pressure. Polarization curves such as the linear sweep voltammogram (LSV) and cyclic voltammogram (CV) were obtained at a polarization rate of 5 mV s-1 wherever used to extract Tafel plots. The CV obtained for wide potential window covering both OER and HER regions was done at a scan rate of 50 mV s -1. Endurance test by CV cycling was done at a very high scan rate of 200 mV s -1. Electrochemical impedance spectroscopic (EIS) analysis was done at the onset overpotentials with oscillation amplitude of 50 mV before and after each cycling and chronoamperometric studies to get insights on the changes of the electrochemical properties of SS scrubber. Chronoamperometry was done at 1.8 V for the two electrode system consisted of only SS scrubber as electrodes. All potential scales of all polarization curves except the ones obtained with two electrode systems of SS scrubber were converted into reversible hydrogen electrode (RHE) scale as per earlier literature.40–46 Results of the material characterization studies and the comparative electrocatalytic water splitting studies are discussed coherently in subsequent sections. RESULTS AND DISCUSSION Perceiving the Properties of SS Scrubber via Detailed Material Characterization As we began to work with SS scrubber without knowing to which kind of SS alloys it belonged to, the most obvious first study was ascertaining the elemental composition of the SS scrubber used. To do so, energy dispersive analysis of X-rays (EDS) spectrum for the SS scrubber on a large area was acquired. Figure S2 in SI is the corresponding field emission electron microscopy (FESEM) micrograph which reveals that the surface of SS scrubber is nearly flat with few noticeable stripy patterns running in one direction. The rectangular pinklined space denotes the area in which the EDS spectrum was acquired. Figure 1 is the EDS spectrum of the SS scrubber taken for the study which shows the presence of C, O, Si, Cr, Mn, Fe and Ni as expected for a SS alloy. The corresponding weight% and atomic% composition of 6 ACS Paragon Plus Environment

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the same is provided as Table S1 in SI. The atomic% & weight% of C, O, Si, Cr, Mn, Fe and Ni are 14.49 ± 0.2 & 39.4 ± 0.543, 6.84 ± 0.13 & 13.96 ± 0.265, 0.29 ± 0.02 & 0.34 ± 0.023, 10.56 ± 0.18 & 6.63 ± 0.113, 0.41 ± 0.01 & 0.24 ± 0.005, 67.37 ± 1.22 & 39.4 ± 0.713 and 0.05 ± 0.001 & 0.03 ± 0.0006 respectively. This characteristics composition of elements in SS scrubber closely resemble to SS-434L type except that there is no detectable Mo in SS scrubber. X-ray diffraction (XRD) analysis on SS scrubber was also performed to support the results of EDAX analysis. XRD pattern of SS scrubber (Figure 2) was acquired in two different modes such as by taking the SS scrubber roles as such and the SS scrubber disc made out SS scrubber roles by applying hydraulic pressure. Optical images of SS scrubber disc and SS scrubber roles used to acquire XRD patterns are provided as insets of Figure 2 just over the corresponding diffraction pattern. As expected, both of them have shown exactly similar patterns having three distinct and characteristics peak that correspond to the diffraction planes of (111), (220) and (321) respectively. Interestingly, the relative intensity of (111) plane is slightly higher in case of SS scrubber disc than the SS scrubber roles which could be due to the fact that the relative area of (111) plane exposed to incident X-ray is lower in case of SS scrubber roles than that of SS scrubber disc. However, the positions of the diffraction peaks were not changed at all. The same has been compared with the standard ICDD patterns of three most closely related elemental compositions namely martenstic SS (ICDD card No. 50-1296, red stick pattern), 434-L (ICDD card No. 34-0396, green stick pattern) and 304 (ICDD card No. 33-0397, blue stick pattern). From this, it is clear that the SS scrubber taken for the study is perfectly matching with the SS alloy of type 434-L which is a high Cr and low Ni alloy of Fe and other elements. 26,28,31 Having confirmed the composition and the type of SS alloy used to manufacture the SS scrubber taken for the study, it was mandatory to know the chemical nature by getting information on the oxidation states of elements present in it. To do this, X-ray photoelectron spectroscopic (XPS) analysis was done. Figure 3, a-g are the high resolution XPS spectra of C 1s, O 1s, Si 2p3/2, Cr 2p3/2, Mn 2p3/2, Fe 2p3/2 and Ni 2p3/2 respectively. The high resolution XPS spectrum of C 1s (Figure 3a) had shown three different peaks upon deconvolution that are located at 284.9, 285.9 and 288.8 eV respectively where the first two are of sp3 hybridized interstitial carbon atoms in SS alloys and of oxides of C on the surface. The last one is the satellite peak of C 1s state. This finds good resonance with literatures too. 26,28,31–34 Figure 3b, the 7 ACS Paragon Plus Environment


O 1s spectrum shoed two distinct peaks at 530 eV for M-O where M could be Ni, Fe, Cr and Mn and at 532.1 eV for the oxides of C and Si respectively. The high resolution Si 2p 3/2 spectrum (Figure 3c) revealed the presence of three different Si atoms in the studied SS scrubber viz., Si coordinated to C as SiC, elemental Si and Si oxides respective at 98.4, 100.3 and 102.1 eV respectively.26,28,31–34 The Cr 2p3/2 spectrum (Figure 3d) also revealed that there are metallic Cr which gave a corresponding peak at 576.1 eV and oxidized Cr which gave a peak at 577.9 eV that closely resembles the Cr2O3 which is a characteristic oxide of Cr that present over the surfaces of all SS alloys to prevent rust formation or corrosion. 26,28,48,49,36 Unlike the Cr 2p3/2 spectrum (Figure 3d), the high resolution spectrum of Mn 2p3/2 (Figure 3e) is slightly noisy which is attributed to the very low percentage of Mn in the studied SS scrubber.23,31 Upon deconvolution, Mn 2p3/2 had also revealed the presence of both metallic Mn (a peak at 641 eV) and Mn oxides (a peak at 645.7 eV) respectively as expected for a typical SS alloy. 29–31,51,52 The High resolution XPS spectrum of Fe 2p3/2 (Figure 3f) also gave similar information to that of Cr and Mn by revealing the presence of both metallic Fe and Fe oxides by having two deconvoluted peaks at 710.1 and 711.9 eV respectively. Another low intense and broad located at slightly higher binder energy is its corresponding satellite peak. These values are well matching with the earlier reports.29,53 At last, the high resolution spectrum of Ni 2p3/2 (Figure 3g) which looks as noisy as Mn 2p3/2 due to its low abundance in the taken SS scrubber. However, careful deconvolution of the same revealed that there are three chemically different Ni moieties such as Ni-C, Ni0 and Ni-O as indicated by their corresponding deconvoluted peaks located at 849.8 eV, 851.3 and 853.5 eV respectively. Another peak seen at a relatively higher binding energy must be the satellite peak of Ni 2p3/2 as the formation of hydroxide of Ni on SS surface is very unlikely to occur. These observations are also matching well with earlier reports. 30,51,54 The detailed XPS study on SS scrubber revealed that the almost all elements are in their native state (except O) with some surface oxidized species covering over them which is the characteristic of SS alloys. To further prove the conclusion drawn out of EDS, XRD and XPS analyses, microscopic studied coupled with elemental color mapping was also done with FESEM analysis. Figure 4a and Figure 4b are the low magnified FESEM micrographs of stripes of SS scrubber taken for the study. The measured width of the SS stripe taken from SS scrubber was in the range of 450 to 460 µm as indicated. In addition, the measured thickness of the SS stripe (Figure S3 in SI) was 8 ACS Paragon Plus Environment

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in the range of 23 to 24 µm. Moreover, it can also be seen that the surface of the SS stripe looks almost smooth except some line patterns running above the SS surface along the length of the same. To get insights on the distribution of various elements present in these stripes, elemental color mapping with EDS was done. Figure S4a in SI is the electron micrograph which shows the overlapped maps of all elements that present within the area marked by the pink rectangle. The smart map of C K shell is provided along with the same as Figure S4b in SI which shows that C is almost equally distributed with some dominance in the line patterns which indicated that the line pattern observed must be accompanying some C species. Smart maps of O K shell (Figure 4c), Si K shell (Figure 4d), Cr K shell (Figure 4e), Mn K shell (Figure 4f), Fe K shell (Figure 4g) and Ni K shell (Figure 4h) are also provided to learn comparative information on their distribution. These smart maps fundamentally implied one common thing that all the elements are equally distributed all over the studied surface of the SS scrubber. However, the major difference was noted with O K shell map (Figure 4c) which showed the dominant presence of the same only in the line patterns seen in the electron micrograph. This indicated that the oxidized species are mainly at the line patterns seen over the stripes of SS scrubber. Interestingly, the Si K map (Figure 4d) does not have any oxygenated species on those stripes which indicates that it is in its elemental state. Moreover, the maps of Cr K shell (Figure 4e) and Fe K shell (Figure 4g) also showed similar absence for oxygenated species over these line patterns which indicate that the lines are containing oxides of C only. In contrast, Si K shell (Figure 4d), Mn K shell (Figure 4f) and Ni K shell (Figure 4h) showed weak signal owing to their smaller percentage composition. The overall mapping study also confirmed the conclusion drawn out of other characterizations that the taken SS have all of its elements equally distributed over its surface except for the oxides of C. After acquiring sufficient information on the material properties of the SS scrubber taken for the study. A set of detailed electrochemical characterization studies was carried out in order to evaluate its ability in catalyzing the OER and HER in 1 M KOH electrolyte and the results of which are discussed below orderly. Cyclic Voltammetric (CV) Studies with SS Scrubber in 1 M KOH Before rushing into the evaluation of SS scrubber for HER and OER activities in 1 M KOH, primary information on the electrochemical properties of the SS scrubber electrode was studied with CV measurements in 1 M KOH at standard conditions taking carbon cloth (CC) counter electrode and Hg/HgO reference electrode. Ten consecutive CV cycles were run at a 9 ACS Paragon Plus Environment


scan rate of 50 mV s-1 and the CV of 1 st, 2nd, 3rd and 10th run are pictured as Figure 5a. From Figure 5a, the distinct formation and reduction α-Ni(OH)2 at 0.32 V and -0.072 V vs. RHE were witnessed which is exactly matching with the earlier observations. The same is clearly shown as enlarged image of the same as Figure 5b. In addition, a hump just before the oxygen evolution corresponding to M-OOH formation is observed which is also in agreement with the earlier reports if SS alloys in KOH electrolyte. The region beyond (anodic) 1.59 V vs. RHE is the region of OER on taken SS scrubber interface. Similarly, the region beyond (cathodic) -0.200 V vs. RHE is the region of HER on taken SS scrubber interface. This implies that the total water splitting can be initiated using SS scrubber within the cell voltage of just 1.8 V which would be better than many reported catalysts in literature and certainly comparable to the state-of-the-art electrocatalysts with minimum excess overvoltage. Therefore, SS scrubber suggested can be adapted for cost and energy efficient hydrogen production by water electrolysis in alkaline medium. Having found the encouraging electrochemical properties of SS scrubber, the specific characterizations for OER and HER evaluation on SS scrubber were done as discussed below. Electrocatalytic HER and OER Studies with SS Scrubber As the preliminary CV study indicated that SS scrubber was an appropriate bi-functional water splitting electrocatalytic material in 1 M KOH, a set of detailed essential electrochemical characterizations that include LSV, Tafel analysis and the stability studies was carried out to elucidate the ability of SS scrubber to catalyze both HER and OER in the same solution of 1 M KOH. Figure 6a is the iR-uncorrected CV of SS scrubber in the potential range of -1.1 to 0.42 V vs. RHE which includes the regions of formation and reduction of α-Ni(OH)2 and the so anticipated HER region. The obtained current was normalized by the area of SS scrubber electrode. From this CV, it is clear that activity of 10 mA cm-2 can be achieved with an iR uncorrected overpotential of 373 mV in HER taking the SS scrubber as the electrocatalytic cathode. Figure 6b is the plot of various area normalized activities vs. their corresponding overpotentials in 1 M KOH with corresponding standard deviation as indicated by the error bars. Smaller deviations witnessed with each area normalized activity indicates that the HER activity is highly reproducible on SS scrubber electrode. To have a comparison with the state-of-the-art Pt/C catalyst, the iR corrected LSVs acquired with both SS scrubber and Pt/C/CFP are pictured together as Figure 6c. From Figure 6c, the measured difference in the overpotential between SS scrubber and Pt/C to drive a area normalized activity of 50 mA cm-2 was just 260 mV which is 10 ACS Paragon Plus Environment

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highly significant as far as the cost of these electrocatalytic materials are considered. Despite being a very cheap material, SS scrubber portrayed almost parallel activity to that of the highly expensive and noble Pt just by requiring slightly excess overpotential. This is important when it comes to prolonged, large scale bulk electrolysis. To get insights on the nature of HER kinetics on both SS scrubber and Pt/C/CFP interfaces, Tafel curves of both were extracted from their respective iR corrected LSVs of areal current density (Figure 6c). The Tafel curves of SS scrubber and Pt/C/CFP are provided as Figure 6d from which we can see that the measured slope for SS scrubber is 121 mV dec-1 and for Pt/C/CFP is 59 mV dec-1. It is mandatory to emphasize here that though Pt/C is the best electrocatalyst for HER, it cannot follow Tafel mechanism (the facile way for HER) in alkaline conditions due to poor the availability of protons for simultaneous adsorption, discharge and displacement via hydridic bond formation. The same has been evidenced once again here in our study too from the measured Tafel slope of Pt/C/CFP which is 59 mV dec-1. Moreover, the Tafel slope of SS scrubber in alkaline condition is 121 mV dec-1. These observations indicated that both Pt/C/CFP and SS scrubber had followed VolmerHeyrowsky pathway which is comparatively a slower pathway than Tafel pathway. Similarly for OER, a set of polarization by CV and LSV analysis and Tafel analysis was carried out. Figure 7a is the CV of SS scrubber in the potential window of 0.924 to 1.924 V vs. RHE without iR compensation. The measured iR uncorrected overpotential for a area normalized activity of 10 mA cm-2 from this CV for SS scrubber is 439 mV. Figure 7b is the plot of various area normalized activities vs. their corresponding overpotentials with error bars. The smaller magnitude error bars in overpotentials implied that like HER, OER is also highly reproducible on SS scrubber electrode. In addition, to have a meaningful comparison with other OER reports in literature as we did for HER, we normalized the OER activity also in the same way we did in HER study. The iR corrected LSVs of SS scrubber, Ni(OH)2/CFP and RuO2/CFP are comparatively pictured as Figure 7c from which it was measured that the iR corrected overpotentials required for a current density of 10 mA cm -2 by SS scrubber, Ni(OH)2/CFP and RuO2/CFP were 418, 384 and 320 mV respectively. It can be noticed here that despite being a non-precious material, SS scrubber had shown comparable activities at higher areal current densities beyond 1.7 V vs. RHE. These observations from OER characterizations implied that the SS scrubber is certainly a cost efficient electrode material for OER in alkaline conditions. The nature of OER kinetics on the surfaces of SS scrubber, Ni(OH) 2/CFP and RuO2/CFP were 11 ACS Paragon Plus Environment


examined by Tafel analysis for which the corresponding Tafel plots were extracted from their respective iR corrected LSVs with activity of areal normalization (Figure 7c). Figure 7d is the resultant Tafel plots of SS scrubber, Ni(OH)2/CFP and RuO2/CFP which had shown Tafel slopes of 63, 61 and 80 mV dec-1 respectively. This trend in Tafel slopes indicates that though RuO 2 is one of the state-of-the-art electrocatalyst for OER in acidic medium, the kinetics of OER in alkaline medium is relatively poor when compared to the systems that contain 3d iron group metals and the same has been witnessed here too from the observed Tafel slopes of SS scrubber and Ni(OH)2 which had shown 17 and 19 mV dec -1 relatively lesser slope than RuO2. From the overall OER characterization, it is concluded here that the activity of SS scrubber is parallel to Ni(OH)2 and comparable to RuO2 in alkaline conditions. Now, it is also mandatory to subject the SS scrubber electrodes for endurance test in both HER and OER conditions. To do this, rapid CV cycling at a higher scan rate of 200 mV s -1 was performed for both HER and OER under identical experimental conditions with SS scrubber electrode. Figure 8, a and b are the HER and OER CVs acquired at 1 st and 5000th cycles respectively for SS scrubber electrode. The observed degradation even after 5000 cycles of harsh cycling in highly corrosive medium like 1 M KOH is very less as witnessed from Figure 8a and Figure 8b. This study had testified the appropriateness of SS scrubber to be used as a bi-functional water splitting electrode for long-term bulk electrolysis. Total Water Splitting with SS Scrubber Being excited by the interesting results of the half-cell characterizations, we moved on to study the total water splitting taking the same SS scrubber electrode as both anode (for OER) and cathode (for HER) in 1 M KOH as a two electrode system (SS scrubber||SS scrubber). Figure 9a shows the CV of the system SS scrubber||SS scrubber recorded at a scan rate of 5 mV S -1. The two electrode system of SS scrubber||SS scrubber had shown a significantly high activity of 10 mA cm-2 at the cell voltage of 1.98 V without any iR compensation. This is comparable to the activities observed so far with non-precious metals based electrocatalytic materials in total water splitting. The stability of the system SS scrubber||SS scrubber was checked by subjecting it for rapid CV cycling at a high scan rate (200 mV s-1) and for chronoamperometry for more than 55k s at the cell voltage of 2 V. Figure 9b shows the LSVs of SS scrubber||SS scrubber system recorded at 5 mV s-1 before cycling, after 100 cycles, after 1000 cycles and after 2500 cycles respectively. It is evidenced from these LSVs that there is no change at all in the activity of SS 12 ACS Paragon Plus Environment

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scrubber||SS scrubber system even after such rapid cycling for 2500 cycles. This is highly significant result as far as its application to real-time large scale electrolysis for longer period of time is considered. Moreover, the stability test under potentiostatic mode at 2 V revealed an interestingly enhanced activity upon prolonged exposure. As per the CV and LSVs of SS scrubber||SS scrubber, the same must deliver a current density of 9 to 13 mA cm-2 at 2 V. However, the observed activity in real case is significantly higher as seen in Figure 9c where it is seen that for initial few hundred seconds, the activity is close to 10 mA cm-2. Interestingly, it was increasing with the increasing time of experiment till 30k s. Within this time the maximum activity achieved with 2 V was 35 mA cm-1. Thereafter, activity had again begun decreasing and settled at a steady state after 45k s. At the final stages of potentiostatic electrolysis, no further change in activity was noted which indicated that after significant surface modification induced by the applied potential the SS scrubber||SS scrubber interfaces has now achieved a steady state in which the performance is constant. To get more insights on the electrochemical changes that occurred with SS scrubber||SS scrubber system by this potentiostatic electrolysis, EIS measurements were carried out before and after CA at the same cell voltage of 2 V. The resultant Nyquist plots are pictured as Figure 9d which clearly showed that there is significant reduction in the charger transfer resistance (Rct) of SS scrubber||SS scrubber system after CA at 1.75 V. The measured Rct before and after CA with SS scrubber||SS scrubber are 165 and 80 Ω respectively. Reduction in Rct after CA is 85 Ω which is 53.03% lower than the initial Rct. This observation made from EIS studies had actually clued us that the activity of scrubber||SS scrubber had been increased. Hence, one LSV at a sweep rate of 5 mV s -1 after CA was acquired and pictured as Figure 10 along with the LSV acquired before CA. The measured activities at the voltage of 2 V before and after CA form the respective LSVs are 10 and 25 mA cm-2 respectively. This is well matching with the activity trend witnessed with Figure 9c.

This observation had further

confirmed the activation of SS scrubber||SS scrubber system under potentiostatic anodic polarization which is in accordance with the earlier reports where such activation was observed by electrochemical anodization with other different SS alloys. To find out such predicted activation, the anode of the SS scrubber||SS scrubber system has been back-characterized with XRD, Raman, SEM, EDS and XPS studies. It is needed to be annotated here that in an electrolyser, anode is the one which undergoes a restless cycles of consecutive oxidation and reduction unlike the cathode that catalyzes the HER. The same was 13 ACS Paragon Plus Environment


also evidenced from the physical appearance of the electrode that the cathode SS scrubber retained its lustrous look whereas the anode SS scrubber stripe turned to light brown in color indicating the possible formation of iron oxides. This is why we have chosen the anode for the post-electrochemical characterization studies. Figure S5, a-b in SI are showing the XRD pattern and the Raman spectra of the SS scrubber anode used in SS scrubber||SS scrubber. The XRD pattern (Figure S5a) did not show any significant change in its feature and no sign of oxides of Fe, Cr, Mn and other metals were observed. This has puzzled us on the prediction that we made on the surface activation of the anode by oxidation. Hence, the same was subjected for Raman analysis and the respective Raman spectra acquired at various places (Figure S5b) have confirmed that there was significant oxidation of the SS surface. Various peaks for the presence of the Fe3O4, α-Fe2O3, Cr2O3 and γ-Fe2O3 were observed which are in good agreement with the earlier reports.55–57 The reason for not observing peaks for these oxides may be due the facts that the oxide entities were not crystalline or the formed grains of the same might have been much smaller so that the X-ray was not diffracted to the detectable extent. Figure S6, a-c in SI are the SEM micrographs of SS scrubber stripes before electrochemical activation that shows similar surface features that we have witnessed with FESEM studies. However, the same had undergone a rigorous surface oxidation (corrosion) during the period of 55k s of potentiostatic electrolysis as seen in Figure S6, e-f in SI. This is in accordance with the Raman studies and also backed up our prediction of surface oxidation. To further know the change in the elemental composition at the surface of SS scrubber, a comparative EDS study was done on SS scrubber stripes before and after electrochemical activation. The EDS spectra of pristine SS scrubber stripe are shown as Figure S7, a-c in SI with the corresponding elemental composition Tables as Table S2, a-c in SI. The measured weight and atomic percentage of each element is in close agreement with the ones listed in Table S1 in SI. Interestingly, the elemental composition measured at the surface of the activated SS scrubber stripe had shown that the percentage of O was dramatically increased as evidenced from the corresponding EDS spectra (Figure S8, a-c in SI) and the Tables containing the details of elemental composition (Table S3, a-c, in SI). This is also a convincing proof that there had been a significant oxidation at the anode surface during potentiostatic electrolysis with SS scrubber||SS scrubber cell. Further, the XPS spectra of C 1s, O 1s, Si 2p3/2, Cr 2p3/2, Mn 2p3/2, Fe 2p3/2 and Ni 2p3/2 states of SS scrubber stripes (Figure S9, a-g in SI) after 55k s of chronoamperometry have also revealed that there were more exposed metal oxides than the zero14 ACS Paragon Plus Environment

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valent metals29,31,32,53 on the surface which had once again confirmed the surface oxidation of SS scrubber stripes. Moreover, it was also found from Figure S9e and Figure S9g that the dominant oxidation of Fe and Cr to their oxides on the surface did not give any space for the Ni and Mn species on the surfaces. This could also be due to the fact that both Mn and Ni were in negligible compositions when compared to Fe, C and Cr. From the above post-electrochemical studies on the material properties of the SS scrubber anode, the reason behind such activation on the electrochemical performance of SS scrubber||SS scrubber cell after 55k s has been revealed. The results of this study are tabulated as Table 1. The overall half-cell and full-cell electrochemical characterizations of SS scrubber have indicated that the SS scrubber is a promising cost efficient material for total water splitting in 1 M KOH. The observed activity in OER was parallel to the relatively costlier Ni(OH)2 and comparable to the highly expensive and precious RuO2. Moreover, the energy lost in HER in terms of excess overpotential from that of Pt/C is negligible and the beauty of SS scrubber is that its availability and ultra-low price. Cost of 1 g of electrode materials used in this study are 0.012 US$ for SS scrubber, 2.23 US$ for pure Ni, 59.66 US$ for RuO2 and 158.04 US$ for Pt/C 20 wt% catalyst. This is sufficient to show the superiority of SS scrubber over other electrode materials for total water splitting in high alkaline conditions for cost efficiency. CONCLUSION For the first time ever the readily available SS scrubber has been screened for total water splitting in alkaline conditions. Fortunately sometimes, it is possible that need for highly active electrocatalytic materials are met from unexpected sources. Such a source is SS scrubber in this study which has been proved to be an extremely stable and cost efficient material for total water splitting. The SS scrubber have required a lower overpotential of 380 mV to drive a HER current density of 50 mA cm-2 which is just 260 mV higher than the state-of-the-art Pt/C. Similarly, for a area normalized current density of 10 mA cm-2 in OER, the SS scrubber have required a lower overpotential of 418 mV which is just 37 and 98 mV higher than relatively costlier Ni(OH)2 and the highly expensive and precious state-of-the-art RuO2 under identical conditions. Moreover, the SS scrubber have shown facile kinetics for both HER and OER in 1 M KOH by showing the Tafel slopes of 121 and 63 mV dec-1 respectively for HER and OER which is highly significant. The total water splitting studies carried out with scrubber||SS scrubber system showed an exceptional activity of 10 mA cm-2 at the cell voltage of 1.98 V. In addition, the stability studies 15 ACS Paragon Plus Environment


carried out in both half-cell and full cell characterizations showed that the SS scrubber is an extremely stable electrocatalytic electrode material for both HER and OER in high alkaline conditions. This is mainly attributed to the high corrosion resistance of Cr rich SS alloys to which the studied SS scrubber belonged to. The encouraging and highly positive results of the overall study implied that the SS scrubbers can be chosen as the cost-efficient and stable electrocatalytic electrode material for bulk water electrolysis in future hydrogen production and the same can be swapped with the precious and expensive materials like, Pt, Ir and Ru used in current water electrolysers to reduce the cost of hydrogen production. ASSOCIATED CONTENT Supporting Information (SI) Available Details on instruments and materials used for the study are provided along with the figures related to optical images of SS scrubber and the prepared electrodes, FESEM images, overlapped maps of elemental color mapping study and the smart map of C K shell. In addition the Table S1 showing the composition of elements present in the used SS scrubber is provided. XRD, Raman, SEM and EDS data with related figures that are used to characterize the material after electrochemical studies are also provided. This material is available at free of cost at http://www.pubs.acs.org ACKNOWLEDGEMENTS All authors wish to acknowledge Dr. V. K. Pillai, Director, CSIR–CECRI. S.A wishes to thank CSIR, New Delhi for the award of Senior Research Fellowship (SRF). All-time help and support from the faculties of CIF, CSIR-CECRI is also kindly acknowledged by all authors.


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Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an Improved Transition Metal Phosphide Catalyst for Hydrogen Evolution Using Experimental and Theoretical Trends. Energy Environ. Sci. 2015, 8 (10), 3022–3029. (DOI: 10.1039/C5EE02179K)

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Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612–13614. (DOI: 10.1021/ja104587v)

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Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araújo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and the Catalytically Active State of Crystalline Co3O4 during Oxygen Evolution. Nat. Commun. 2015, 6, 8625–8633. (DOI: 10.1038/ncomms9625)

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Forgie, R.; Bugosh, G.; Neyerlin, K. C.; Liu, Z.; Strasser, P. Bimetallic Ru Electrocatalysts for the OER and Electrolytic Water Splitting in Acidic Media. Electrochem. Solid-State Lett. 2010, 13 (4), B36–B39. (DOI: 10.1149/1.3290735)

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Sha, C.-H.; Lee, C. C. Microstructure and Surface Treatment of 304 Stainless Steel for Electronic Packaging. J. Electron. Packag. 2011, 133, 021005–021009. (DOI: 10.1115/1.4003990)


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Atta, N. F.; Fekry, A. M.; Hassaneen, H. M. Corrosion Inhibition, Hydrogen Evolution and Antibacterial Properties of Newly Synthesized Organic Inhibitors on 316L Stainless Steel Alloy in Acid Medium. Int. J. Hydrogen Energy 2011, 36 (11), 6462–6471. (DOI: 10.1016/j.ijhydene.2011.02.134)

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Call, D.; Merrill, M. D.; Logan, B. E. High Surface Area Stainless Steel Brushes as Cathodes in Microbial Electrolysis Cells (MECs). Environ. Sci. Technol. 2009, 43, 2179– 2183. (DOI: 10.1021/es803074x)

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Dinamani, M.; Kamath, P. V. Electrocatalysis of Oxygen Evolution at Stainless Steel Anodes by Electrosynthesized Cobalt Hydroxide Coatings. J. Appl. Electrochem. 2000, 30 (10), 1157–1161. (DOI: 10.1023/A:1004020825529)

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De Silva Muñoz, L.; Bergel, A.; Féron, D.; Basséguy, R. Hydrogen Production by Electrolysis of a Phosphate Solution on a Stainless Steel Cathode. Int. J. Hydrogen Energy 2010, 35 (16), 8561–8568. (DOI: 10.1016/j.ijhydene.2010.05.101)

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Pärna, R.; Nõmmiste, E.; Kikas, A.; Jussila, P.; Hirsimäki, M.; Valden, M.; Kisand, V. Electron Spectroscopic Study of Passive Oxide Layer Formation on Fe-19Cr-18Ni-1AlTiC Austenitic Stainless Steel. J. Electron Spectros. Relat. Phenomena 2010, 182, 108– 114. (DOI: 10.1016/j.elspec.2010.09.002)

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Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: EDAX spectrum of SS scrubber showing the presence of Ni, Fe, Mn, Cr, Si, C and O atoms respectively.



Figure 2: XRD pattern of SS scrubber acquired after making a disc and as such.


Page 26 of 37

Page 27 of 37


(a)

(b)

C 1s

O 1s

18.0k

30.0k

16.0k

cps

25.0k

cps

22.0k 20.0k

35.0k

20.0k

14.0k 12.0k

15.0k

10.0k

10.0k

8.0k

5.0k

6.0k

292

290

288

286

284

282

280

536

Binding Energy / eV (c)

534

532

530

528

526

Binding Energy / eV

(d)

3.2k

Si 2p3/2

9.4k

Cr 2p3/2

9.2k

3.0k

2.8k

8.8k

cps

cps

9.0k

2.6k

8.6k 8.4k 8.2k

2.4k

8.0k 104

102

100

98

582

Binding Energy / eV (e)

9k

(f)

Mn 2p3/2

578

576

574

572

Fe 2p3/2

14k

cps

9k 9k 9k 9k

580

Binding Energy / eV

16k 15k

9k

cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40.0k

13k 12k 11k 10k

648

646

644

642

640

638

716

Binding Energy / eV

714

712

710

Binding Energy / eV

Figure 3: (a-f) High resolution XPS spectra of C 1s, O 1s, Si 2p 3/2, Cr 2p3/2, Mn 2p3/2 and Fe 2p3/2 respectively


708


(g)

22.8k Ni 2p3/2 22.6k 22.4k

cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

22.2k 22.0k 21.8k 856

855

854

853

852

851

850

Binding Energy / eV

Figure 3g: XPS high resolution spectrum of Ni 2p3/2 state in SS scrubber


849

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: (a) and (b) are FESEM micrographs of SS scrubber stripes showing the measured width at various places. (c-h) EDAX elemental smart maps of K shells of elements O, Si, Cr, Mn, Fe and Ni observed in SS scrubber respectively.



(a)

60 st

1 run nd 2 run rd 3 run th 10 run

40

i / mA

20

OER -Ni(OH)2

formation

M-OOH formation

0 -20

-Ni(OH)2

reduction

-40

HER -60

-0.6 -0.3

0.0

0.3

0.6

0.9

1.2

1.5

1.8

E / V vs. RHE

(b)

st

-Ni(OH)2

1 run nd 2 run rd 3 run th 10 run

5

i / mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

OER

formation M-OOH formation

0

-5

-Ni(OH)2

HER

-0.3

reduction

0.0

0.3

0.6

0.9

1.2

1.5

E / V vs. RHE Figure 5: (a) Full range CV measured with SS scrubber electrode in 1 M KOH with carbon cloth counter electrode at room temperature showing the regions of HER, OER, α-Ni(OH)2 formation and reduction along with an hump for M-OOH formation as indicated. (b) is showing the closer view of the redox active peaks of SS scrubber.


Page 31 of 37

(b)

0

600

-10 -20

HER / mV

-30 -40 -50

400

-80

300

SS-Scrubber in 1M KOH

-70 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0

0.4

10

20

30

40

-2

E / V vs. RHE

jgeo / mA cm

0

(d)

SS-Scrubber Pt/C/SS

SS-Scrubber Pt/C/CFP

0.4

-20

-1

0.3

iR / V

jgeo / mA cm

500

-60

-2

jgeo / mA cm

-2

1 2 3 4 5 6 7 8 (a) 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26(c) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-40

-60

mV 121

0.2

-0.8

-0.6

-0.4

-0.2

0.0

0.0

-1

ec

59 m V d

0.1

-80 -1.0

dec

1

E - EiR / V vs. RHE

10 -2

log j / mA cm

Figure 6: (a) iR uncompensated CV of SS scrubber for HER polarization in 1 M KOH. (b) Plot of current density vs. overpotential (iR uncompensated) for SS Scrubber. (c) iR corrected LSVs of SS scrubber and Pt/C/CFP for HER polarization in 1 M KOH acquired at 5 mV s -1 respectively. (d) Tafel plots of the same.



60

(b)

SS-Scrubber in 1M KOH

700

50

OER / mV

40 30 20

600

500

10 0

400

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

0

10

20

50 40

30

40

-2

E / V vs. RHE

jgeo / mA cm (d)

SS-Scrubber Ni(OH)2/CFP

0.5

RuO2/CFP

iR / V

jgeo / mA cm

-2

jgeo / mA cm

-2

1 2 3 4 5 6 7 (a) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 (c) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

30 20

SS-Scrubber Ni(OH)2/CFP RuO2/CFP

-1

63 mV 0.4 -1

61 m V 0.3

10

dec

dec

-1

c

de 86 mV

0 1.3

1.4

1.5

1.6

1.7

1.8

1

E - EiR / V vs. RHE

10 -2

log j / mA cm

Figure 7: (a) iR uncompensated CV of SS scrubber for OER polarization in 1 M KOH. (b) Plot of current density vs. overpotential (iR uncompensated) for SS Scrubber. (c) iR corrected LSVs of SS scrubber, Ni(OH)2/CFP and RuO2/CFP for OER polarization in 1 M KOH acquired at 5 mV s-1 respectively. (d) Tafel plots of the same.


Page 33 of 37

(a)

10

st

1 Cycle

jgeo / mA cm

-2

0

th

5000 cycle

-10 -20 -30 -40 -50 -60 -70 -80

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E / V vs. RHE (b)

60

-2

50

jgeo / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


st

1 Cycle th

5000 cycle

40 30 20 10 0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

E / V vs. RHE Figure 8: (a) and (b) CV acquired at 1 st and 5000th cycles of accelerated degradation test with SS scrubber for HER and OER polarizations respectively in 1 M KOH at 200 mV s -1.



50

(b)

SS-Scrubber||SS-Scrubber

50 Before cycling After 100 cycles After 1000 cycles After 2500 cycles

40

40 -2

jgeo / mA cm

30 20 10 1 M KOH

0 1.4

1.6

1.8

2.0

2.2

20 10 1 M KOH 1.4

2.4

Voltage / V 60

(d) SS-Scrubber||SS-Scrubber at 2 V

50

30

0 1.6

1.8

2.0

2.2

2.4

Voltage / V

180

Before AD and CA After CA

160 140 120

40

-Z" / 

jgeo / mA cm

-2

jgeo / mA cm

-2

1 2 3 4 5 6 7 (a) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24(c) 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

30 20

100 80 60 40

10 0

20 0 0

10k

20k

30k

40k

0

50k

20

40

60

80

100 120 140 160 180

Z' / 

Time / s

Figure 9: (a) iR uncompensated CV of SS scrubber||SS scrubber system in 1 M KOH. (b) iR uncompensated LSVs acquired at various cycles of SS scrubber||SS scrubber system in 1 M KOH. (c) j-t curve for SS scrubber||SS scrubber system in 1 M KOH at 2 V for more than 55 K s. (d) Nyquist plots of the same before and after CA.


Page 35 of 37

-2

100

jgeo / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

Before CA After CA

60 40 20 0 1.4

1.6

1.8

2.0

2.2

2.4

Voltage / V Figure 10: LSVs of SS scrubber||SS scrubber system acquired before and after CA study for more than 55k s in 1 M KOH showing enhanced water splitting ability after CA.



Page 36 of 37

Table 1: Results of electrocatalytic water splitting studies with SS scrubber. Process

Catalytic

Loading

Relative cost of

Overpotential (iR

Tafel

material

(mg cm-2)

the electrode

free) at area

Slope

material

normalized activity

(mV dec-1)

(US$)

( mV)

10 ± 0.1

0.00012

380 @ 50 mA cm-2

121

Pt/C

0.205

32.52

120 @ 50 mA cm-2

59

SS

10 ± 0.1

0.00012

418 @ 10 mA cm-2

63

Ni(OH)2

0.205

0.58

385 @ 10 mA cm-2

61

RuO2

0.205

12.35

320 @ 10 mA cm-2

80

SS HER

OER

Scrubber

Scrubber


Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only

Electrocatalytic total water splitting ability of readily available SS scrubber has been thoroughly studied in 1 M KOH in which it has been found that SS scrubber has comparable activity to the state-of-the-art electrocatalysts such as Pt/C and RuO2 with minimum excess overpotentials.


Stainless Steel Scrubber: A Cost Efficient Catalytic Electrode for Full

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