Cathodic activated stainless steel mesh as a highly active electrocatalyst for the oxygen evolution reaction with self-healing possibility

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Cathodic activated stainless steel mesh as a highly active electrocatalyst for the oxygen evolution reaction with self-healing possibility Gui-Rong Zhang , Liu-Liu Shen , Patrick Schmatz , Konrad Krois , Bastian J.M. Etzold PII: DOI: Reference:

S2095-4956(20)30040-1 https://doi.org/10.1016/j.jechem.2020.01.025 JECHEM 1078

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Journal of Energy Chemistry

Received date: Revised date: Accepted date:

24 December 2019 10 January 2020 13 January 2020

Please cite this article as: Gui-Rong Zhang , Liu-Liu Shen , Patrick Schmatz , Konrad Krois , Bastian J.M. Etzold , Cathodic activated stainless steel mesh as a highly active electrocatalyst for the oxygen evolution reaction with self-healing possibility, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.025

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Cathodic activated stainless steel mesh as a highly active electrocatalyst for the oxygen evolution reaction with selfhealing possibility

Gui-Rong Zhang*, Liu-Liu Shen, Patrick Schmatz, Konrad Krois, Bastian J. M. Etzold* Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt, 64287 Darmstadt, Germany

*Corresponding authors: Dr. Gui-Rong Zhang; Prof. Bastian J. M. Etzold Ernst-Berl-Institut für Technische und Makromolekulare Chemie Technische Universität Darmstadt 64287 Darmstadt, Germany Tel.: +49 6151 1629984 Fax: +49 6151 1629982 E-mail addresses: [email protected] (G.-R. Zhang); [email protected] (B. J. M. Etzold).

1

Abstract The oxygen evolution reaction (OER) represents one of the major bottlenecks for broad-based applications of many clean energy storage/conversion technologies. The key to solving this problem lies in developing high-performing, cost effective and stable catalysts for the OER. Herein, we demonstrate that ubiquitous stainless steel mesh (SSM) materials activated by a facile cathodization treatment can be employed as a high performing OER catalyst, as showcased by the impressively low overpotentials of 275 and 319 mV to reach the benchmark current densities of 10 and 100 mA cm-2 (1.0 M KOH), respectively. Cathodized SSM also exhibits excellent performance in a two-electrode water electrolyzer, which requires a low cell voltage of 1.58 at 10 mA cm-2 and outperforms many of water electrolyzers using earth-abundant OER catalysts. Moreover, cathodized SSM with minor performance degradation after the stability test can also be readily healed by subjecting it to an additional cathodization treatment. It is disclosed that the superior performance of cathodized SSMs stems from the surface enrichment of OER active Ni (oxy)hydroxide, facile gas-bubble removal and transportation over the unique mesh-structured surfaces, while the abundant reservoir of nickel in the bulk allows healing of the catalyst by a facile cathodization. Keywords: Oxygen evolution reaction; Water splitting; Stainless steel mesh; Cathodization; Mass transfer limitation; Self-healing

2

1. Introduction The oxygen evolution reaction (OER), or water oxidation, plays an essential role in enabling many key energy conversion/storage processes, such as electrochemical water splitting, CO 2 electrolysis, rechargeable metal-air battery and unitized regenerative fuel cell systems, which represent excellent future options to store the intermittent renewable electricity (e.g., solar, wind energy) [1,2]. However, the OER is a multiple proton coupled electron transfer process suffering from notorious slow reaction kinetics, and therefore usually requires a rather high overpotential to proceed [3–5]. The resultant low energy efficiency of the OER process thus imposes a major barrier for achieving economic viability of those aforementioned clean energy conversion/storage technologies. Intensive efforts have been made to improve the OER efficiency by utilizing various electrocatalysts, among which noble metal oxides such as IrO2 and RuO2 set the benchmark for the most efficient OER catalysts in both acidic and alkaline electrolytes [6,7], but their scarcity/high cost along with limited durability (especially of RuO2) have greatly restricted their broad-based practical applications [8]. In view of this, a variety of non-precious metal OER catalysts (e.g., metal oxides/sulfide/phosphide, perovskites, layered double hydroxides) have been developed [2,9–12]. One particularly promising category amongst them is 3d transition metal oxide/hydroxide catalysts [1,3,11]. Their earth-abundant nature along with superior OER performance makes them a highly appealing option for practical applications. Nevertheless, a major drawback comes from the poor electrical conductivity of the OER active metal oxide (or hydroxide/oxyhydroxide) species [13,14]. A conventional measure to partially mitigate this issue is to immobilize metal oxide catalysts onto other conductive substrate such as glassy carbon, Au, Ti plates, Ni foams [15,16], while as a matter of fact, it is found that only the fraction of catalyst film that is in direct contact with conductive substrate can be electron accessible and active for OER [17], resulting in a limited catalyst utilization. Moreover, the catalyst layer is inclined to detach from the conductive substrate especially under vigorous gas evolution conditions [13,18], representing a major cause for the system degradation. Besides mechanical damage by gas bubble formation, stability issues of metal oxide OER catalysts stemming from surface blockage/poisoning by electrolyte impurities, dissolution of active species [19–21], catalyst particle growth [22], also need to be properly tackled before their practical applications. Therefore, finding 3

new approaches to cost-effective, free-standing, high-performing and stable or regenerable metal oxide OER catalysts is still highly desirable. Stainless steel is an inexpensive and ubiquitous material, made up of Fe alloyed with other transition metals such as Ni, Cr and Mo, the combination of which is well documented to be active OER catalysts or the sources to form OER active species [23–25]. Meanwhile, the stainless steel itself can act as an excellent current collector and free-standing scaffold for carrying those OER active species. Not surprisingly, stainless steel has been widely studied as an active OER catalysts, whose performance can usually be further improved upon surface modification (e.g., oxidation [26], sulfidation [27], corrosion [13]) treatment. Nevertheless, conventional water electrolyzers or rechargeable metal-air batteries would usually adopt a gas-diffusion electrode (GDE) configuration at their oxygen/air electrode, on which the OER and its reverse reaction oxygen reduction take place [28]. The feasibility of directly using stainless steel plates/foils in those devices as anodic OER catalysts is therefore largely compromised. To this end, stainless steel with open mesh structure (stainless steel mesh, SSM) would be an ideal candidate as a low cost, free-standing and GDE compatible OER catalyst. Directly utilizing SSM as OER catalyst can also bypass the tedious catalyst coating process in constructing GDEs, which is usually implemented by drop-casting or spray-coating of OER active materials on GDEs and requires the usage of bothersome materials such as carbon black and polymer binders [28]. There are numerous reports using SSM as a structure scaffold to grow OER active nanostructures such as Co3O4 nanowires [28], Ni(Fe)OxHy nanosheets [29], Ni(Fe)OxHy nanocones [30], and graphene encapsulated Fe3C [31], while the potential of directly exploring the underlying SSMs as an active OER catalyst have long been overlooked. Here in the current work, we demonstrate that a facile electrochemical cathodization treatment can activate a SSM into an excellent OER catalyst. The cathodized SSM exhibits a low OER overpotential of 340 mV at 10 mA cm-2 in 0.1 M KOH electrolyte, which is 70 mV more negative than the pristine SSM and even comparable to the state-of-the-art Ir-based catalyst. The cathodized SSM is also found to be highly robust, as evidenced by little degradation over 15 h continuous electrolysis operation at a current density of 10 mA cm-2. In the case of 1.0 M KOH electrolyte, even more impressive OER performance can be observed on cathodized SSM, which displays overpotentials of 275 and 319 mV to achieve current densities of 10 4

and 100 mA cm-2, respectively, being comparable or even superior to many of those reported earthabundant metal OER catalysts. It is also disclosed that superior OER performance of the cathodized SSM would benefit largely from both the surface enrichment of nickel hydroxide/oxyhydroxide species and the facilitated removal of O2 gas bubbles during the OER. Furthermore, although minor degradation can be observed on cathodized SSM after the long term water electrolysis test, its superior initial performance can be easily regained by performing another cathodization treatment, owning to the abundant sources for regenerating catalytically active species in the backbone of SSM. These results demonstrate the great promise of SSM to be a new extremely cheap, high performing and selfhealing OER catalyst to be used in practical applications of those clean energy storage/conversion technologies. 2. Experimental 2.1.Materials Stainless steel mesh (316L) was purchased from Beissermetall GmbH. KOH (> 85%) in pellet form was purchased from Carl Roth. Nafion solution (5 wt%) was purchased from Sigma-Aldrich. Pt/C (20 wt%, HiSPEC 3000) was purchased from Alfa Aesar. Ir/C (20 wt%) was purchased from Premetek. Platinum plated electrode was purchased from IKA-Werke GmbH. Deionized water purchased from VWR Chemicals was used for all of solution preparations. 2.2.Synthesis of cathodized SSM SSMs were first cut into rectangular pieces with dimensions of 20 mm × 10 mm. Prior to use, these SSM pieces were intensively cleaned by ultrasonic agitation in ethanol for 30 min, followed by additional surface rinsing with deionized water for 10 s before drying. The projected active geometric surface area of SSMs is controlled at 1 cm2 by properly masking the SSMs with heat-softened Parafilm. The cathodization of SSMs was achieved by applying a repetitive potential cycling using a conventional three-electrode setup which consists of SSM working electrode, graphite counter electrode and Hg/HgO reference electrode. The cathodization process was carried out in the potential region of -1.5 − -0.4 V vs. Hg/HgO at a scan rate of 10 mV s-1 in 0.1 M KOH electrolyte for up to 10

5

cycles. The SSMs before and after the cathodization treatment are denoted as SSM-Pristine and SSMCathodization, respectively. 2.3.Structural analysis The morphology and elemental mapping of SSMs were characterized using a scanning electron microscope (SEM, Philips XL30 FEG) equipped with an EDAX X-ray detector (CDU Leap) operated at an accelerating voltage of 20 kV. Raman spectra were captured using a Raman microscope spectrometer (Bruker Senterra) with 532 nm excitation. X-ray photoelectron spectroscopy (XPS) measurements were performed on an XPS spectrometer (SSX 100 ESCA) using monochromatic Al K radiation as the excitation source. The binding energies (BEs) were calibrated against the adventitious C 1s signal at 284.8 eV. 2.4.Electrochemical measurements Electrochemical

measurements

were

carried

out

using

a

PARSTAT

Multichannel

Potentiostat/Galvanostat (PMC-1000, AMETEK). All electrode potentials in the current work were reported against the reversible hydrogen electrode (RHE) using hydrogen evolution-oxidation reaction on a Pt electrode. For the reference Pt/C and Ir/C catalysts, the measurements were carried out using the thin film rotating disk electrode (RDE) technique. A glassy carbon RDE (5 mm diameter, PINE) was used as the working electrode, which was polished to a mirror finish prior to use. To prepare the working electrode, the catalyst suspension was prepared by dispersing 5 mg of a solid catalyst in 725 µL of deionized water, 225 µL of isopropanol and 50 µL of Nafion solution (5 wt%), followed by a ultrasonication treatment for 5 min using an ultrasonic vialTweeter (Hielscher). Then 10 µL of catalyst suspension was drop-casted onto the RDE to achieve a catalyst loading of 0.255 mg cm-2. The electrochemistry measurements were conducted at room temperature in O2-saturated 0.1 M (or 1.0 M) KOH solution. Before the OER measurements, the electrolyte was saturated with high purity O2 for at least 30 min, followed by catalyst pretreatment using repetitive potential cycling for up to 20 cycles at a scan rate of 100 mV s-1. The OER polarization curves were then recorded using the linear scan voltammetry (LSV) technique at a scan rate of 10 mV s-1. To study the electrochemically accessible surface area (EASA) of the SSMs before and after cathodization, CV measurements at scan rates 6

ranging from 5 to 100 mV s-1 were recorded. The EASA values were determined by plotting the capacitance current corresponding to the double layer charging against the scan rates, which can then be calculated according to the following equation: (1) where Cdl stands for the double layer capacitance which can be extracted by the slope of the currentscan rate plots; Cs stands for the capacitance value for a flat standard with 1 cm2 of real surface area, and a typically adopted value is 40 F [32]. The ohmic drop was compensated for all the half-cell OER measurements. The solution resistance was determined by carrying out electrochemical impedance spectroscopy (EIS) measurement with an AC amplitude of 5 mV. The overall water splitting experiment was carried out using SSM-Cathodization and platinum plated electrode as anode and cathode, respectively, in 1.0 M KOH electrolyte. The regeneration of degraded SSM electrodes was implemented by subjecting the electrode to a cathodization treatment, i.e., potential cycling in the range of −1.5–−0.4 V vs. Hg/HgO at a scan rate of 10 mV s-1 in 0.1 M KOH electrolyte for 10 cycles. 3. Results and discussion The cathodization was carried out by subjecting SSMs to an electrode potential excursion down to −0.6 V vs. RHE. An abrupt increase in the reduction current at the low potential region along with the formation of copious gas bubbles at the electrode surfaces suggests that hydrogen evolution reaction is taking place (Fig. S1). After the cathodization treatment, the color of the SSMs changes from metallic gray to brownish, as shown in Figs. 1 and S2. To probe the detailed morphology of the SSMs before and after cathodization, we performed SEM analyses on pristine and cathodized SSMs. As shown in Fig. 1(b), SSM-Pristine exhibits a typical twill wire weaving structure. Both the diameter of stainless steel wires and the aperture width of SSMs are around 25 m, and the open screening area is calculated to be 25%. The SEM image with a higher magnification (Fig. 1c) shows that the stainless steel wire has a smooth surface despite that mechanical scratches along the wire can also be observed. Fig. 1(e and f) displays the SEM images of SSM-Cathodization. It can be observed that some parts of the stainless steel wire get a darker contrast (Fig. 1e), indicating that surface corrosion takes place 7

during the electrochemical cathodization process, which might partially arise from the hydrogen embrittlement of stainless steel materials [33]. A zoom-in image reveals the pitting corrosion occurring on the surface of SSM wires (Fig. 1f). Interestingly, the pitting corrosion can only be selectively observed at the bridge areas where large local strain is to be expected [34], which can be partially rationalized by the selective stress corrosion phenomenon. Attempts were also made to probe possible changes in the elemental distribution for the corroded areas using EDS mapping technique. Fig. 2 shows the EDS elemental mapping images of SSM-Cathodization, where three major elements including Fe, Cr and Ni are found homogeneously distributed over the mesh wires. However, the limited sensitivity of the EDS technique would have prevented the identification of any pronounced difference in the elemental distribution between corroded and corrosion-free areas (Fig. 2).

Fig. 1. Digital photos (a, d) and SEM images (b, c, e, f) of SSM-Pristine (a-c) and SSM-Cathodization (d-f).

8

Fig. 2. SEM (a) and EDS elemental mapping images (b-d) of SSM-Cathodization.

To better probe the cathodization-induced changes in surface composition and chemical state of SSMs, XPS analyses were performed on both SSMs. Fig. 3 shows the XPS signals for the typical elements of 316L-grade stainless steels, consisting mainly of Fe, Cr, Ni and Mo. Clear signals of Fe 2p and Cr 2p can be identified on both samples, from which it can be seen that both elements are in an oxidation state, implying that the surface of SSM is susceptible to air oxidation. At the same time, no pronounced signal of Mo can be seen on neither sample, probably due to the low content of Mo in SSMs (< 3 wt%). The most intriguing change is that the Ni 2p signal, which is barely visible on SSMPristine, becomes much more well-defined after the cathodization, which provides a solid piece of evidence that the Ni species (identified mainly as NiOOH/Ni(OH) 2 [7]) is enriched at the SSM surfaces. Similar results were also reported for other stainless steel materials, on which Ni species would diffuse from substrate to surface after the electrochemical potential cycling treatment in alkaline solution [35,36]. As mentioned above, the cathodization treatment is always accompanied by the generation of large amount of hydrogen on SSM. In view of the much higher bond dissociation energy of Ni-H (240 kJ mol-1) than that of either Fe-H (118 kJ mol-1) or Cr-H (189 kJ mol-1) [37], we tend to believe that the segregation of Ni to the surface of SSM in response to the cathodization treatment might originate from the stronger bonding strength of H on Ni than on the other components. Specifically, at the cathodic vertex of the potential cycling, the hydrogen evolution reaction is taking 9

place, which is accompanied by hydrogen adsorption on metal surfaces. The diffusion of Ni to SSM surfaces may then occur through the adsorption induced surface segregation mechanism [38,39], as a result of the stronger adsorption of H on metallic Ni than that on Fe or Cr. On the consequent anodic scan, metallic Ni is being oxidized to α-Ni(OH)2 along with the oxidation and evolution of adsorbed hydrogen [40, 41]: (

)

(2)

Under positive potential conditions, α-Ni(OH)2 phase tends to irreversibly transform into β-Ni(OH)2 phase which cannot be reduced on the cathodic scans [42]. With further increasing the electrode potential, Ni(OH)2 is being oxidized into NiOOH, which is well documented to be an OER active phase, via the following reaction: (

)

(3)

This reaction involves expulsion of one H+ from the Ni(OH)2 layer to produce water in alkaline solution [42]. It is also suggested that the Ni-Fe spinel (nickel ferrite, NiFe2O4) which is documented to be active toward OER [43, 44], may be present on the outer oxide layer of stainless steel materials [36, 45]. To clarify this possibility, we perform Raman analyses on both SSM-pristine and SSM-Cathodization, and the spectra of two possible spinel phases, i.e., NiFe2O4 and NiCr2O4, were also included for comparison (Fig. S3). Both SSMs exhibit comparable Raman spectra, and at the same time no spinel structure can be identified on SSMs regardless of cathodization treatment. To investigate the electrochemically accessible surface areas (EASA) of SSMs before and after the cathodization treatment, we performed CV measurements at different scan rates, from which the EASA can be evaluated by the double layer charging capacitance (Fig. S4). It is found that the EASA value is slightly higher on SSM-Cathodization (3.9 cm2) than that on SSM-pristine (3.5 cm2), which might arise from the observed surface corrosion and associated surface restructuring process during the cathodization treatment. Based on these results, we can see that the most dramatic change induced by the cathodization is the surface enrichment of Ni (oxy)hydroxide species, while the overall 10

morphology, chemical state and EASA (or roughness factor) of SSM is more or less comparable for both samples.

Fig. 3. X-ray photoelectron spectra of the SSMs before and after the cathodization. (a) Fe 2p, (b) Ni 2p, (c) Cr 2p and (d) Mo 3d.

The electrocatalytic OER performance of SSMs before and after the cathodization was first evaluated in 0.1 M KOH electrolyte. The free-standing SSMs were directly employed as working electrode. For comparison, the catalytic activity of commercial Ir/C and Pt/C was also measured using RDE system at 1600 rpm rotation rate. Representative voltammograms of these samples are shown in Fig. 4(a). Here in the current study, the overpotential (η) required to achieve the benchmark current density of 10 mA cm-2 (based on projected geometric area) is employed as the primary figure of merit for comparing the OER activity of different catalysts. It is found that SSM-Pristine requires an 11

overpotential of 410 mV to reach a current density of 10 mA cm-2, which is quite comparable to that of the commercial Ir/C (407 mV). Meanwhile, Pt/C exhibits rather poor performance toward OER, which is not surprisingly, due to the formation of inactive surface oxide species [46,47]. Intriguingly, the cathodized SSM (SSM-Cathodization) only requires an overpotential of 340 mV to reach 10 mA cm-2, which is 70 mV lower than that of the pristine counterpart and even slightly superior to that of those reported state-of-the-art IrO2/C catalysts (360–370 mV) [48,49]. Moreover, as shown in Fig. S5, a minor reduction peak at around 1.4 V can be observed after the cathodization treatment. This peak is absent on SSM-Pristine and usually assigned to the redox couple of NiOOH/Ni(OH)2 [36, 50]. The emergence of this addition peak provides another piece of evidence that the cathodization treatment could lead to surface enrichment of Ni species, which is in line with the XPS measurements. To better quantify the difference in OER activity, the specific activity of SSMs is calculated by normalizing the OER current at η = 0.35 V to the measured EASA. To be noted, due to the inherent inaccuracy in determining EASA, the calculated specific activity should be viewed as an approximate guide in assessing intrinsic activity [51]. It turns out that the specific activity of SSM-Cathodization (3.04 mA cm-2) is ~3.5 times that of SSM-Pristine (0.86 mA cm-2), demonstrating again that a readily cathodization treatment can significantly boost the OER activity of SSMs. The turnover frequency (TOF) of the OER on SSM-Cathodization is also calculated via the equation [47]: (

) (

)

(

)

(4)

where F is the Faraday constant (96485.3 C/mol), and the number of active sites (in mol) is estimated by integrating the redox wave at around 1.4 V according to the following equation [47]: (

)

(5)

where Q is the charge associated with the above redox process; e is the elementary charge (1.602 × 1019

C); and NA is the Avogadro number (6.02 × 1023). As summarized in Table S1, SSM-Cathodization

shows an impressively high TOF value of 4.8 s-1 at η = 0.35 V, which is superior to many of those reported OER catalysts [52–54].

12

Attempts were also made to further improve the OER performance of SSM by repeating the cathodization treatment, while it turns out that there is not significant difference in the OER polarization curves between the SSMs after the first and second cathodization treatment. This result gives us a hint that the surface enrichment of Ni species would likely occur by an equilibrium segregation mechanism [55], or in other words, the cathodization treatment may bring a constant number of OER active Ni species to the surface of SSM. This allows on the one hand to achieve reproducible catalysts and on the other hand gives hints, how the number of active sites could be increased through the segregation mechanism. In the current study, repetitive potential cycling method is used for the cathodization treatment. Then an intriguing question is about the cycle number of cathodization that is necessary to reach the segregation equilibrium on SSM electrodes. To answer this question, we carefully inspected the OER performance of SSM by recording the OER polarization curve after each cathodic potential cycle, as shown in Fig. S6(a). It turns out that SSM electrode has been almost fully activated after the first potential cycle, and the resultant polarization curve is already dramatically shifted to negative potentials, which gets more or less stabilized after 4-5 cycles (Fig. S6). Therefore, it seems that one cathodic potential cycle is sufficient to make the SSM electrode almost reach the segregation equilibrium. Little influence on either the structure or the final OER performance of SSM is to be expected later on with further increasing the potential cycle number, which is in line with the previous finding that the second cathodization treatment cannot bring about any further improvement on the OER performance of cathodized SSM. Tafel analyses were conducted to better understand the cathodization treatment induced difference in the OER activity of SSMs, by comparing their Tafel slopes, which indicate the overpotential necessary to reach a 10× increase in OER current [47]. It is found that SSM-Cathodization exhibits the smallest Tafel slope of 70 mV dec-1 among all four catalysts (Fig. 4b), implying a fast charge transfer at the catalyst-electrolyte interface [56]. To gain further insight into the OER kinetics, we performed 13

electrochemical impedance spectroscopy (EIS) measurements on both SSMs. A Randles-type model (inset of Fig. 4c) is used as the equivalent circuit for data fitting, and capacitor elements were replaced by constant phase elements (CPE) to account for the inhomogeneity of electrode surfaces [57]. Fig. 4(c) displays the Nyquist plots of both SSMs, which feature well-defined semi-circle shape. The smaller diameter of the semi-circle for SSM-Cathodization indicates its lower charge transfer resistance (6.2 Ω cm2) relative to that for SSM-Pristine (12.9 Ω cm2), verifying that the cathodization treatment would significantly enhance the OER kinetics on SSM materials. To assess the electrocatalytic stability of cathodized SSM toward OER, chronopotentiometry experiment was carried out at 10 mA cm-2. As shown in Fig. 4(d), SSM-Cathodization is a rather robust OER catalyst, which suffers from little degradation in potential (i.e., < 10 mV) during over 15 h electrolysis at 10 mA cm-2. In contrast, the reference Ir/C catalyst exhibits severe degradation during the chronopotentiometry experiment. Specifically, the electrode potential increases from 1.67 to 1.99 V after 3000 s, and thereafter the detachment of the catalyst layer from the RDE starts being noticed, which is companied by a sudden increase in the electrode potential as shown in the inset of Fig. 4(d). The catalyst layer detachment might originate from the generated gas bubbles on/within the catalyst layer which can hardly be fully removed even by rotating the RDE, while the rotation of the RDE might cause additional mechanical damage on the catalyst layer, which further compromises the intactness of the catalyst layer. These results again demonstrate the great advantage of using freestanding SSM electrode as a robust catalyst for the OER.

14

Fig. 4. (a) OER polarization curves and (b) Tafel plots of different catalysts in O 2-saturated 0.1 M KOH solution. (c) Nyquist plots of SSM-Pristine and –Cathodization at 1.6 V. The inset displays the corresponding equivalent circuit model, where CPE is constant phase element, Ru and Rct are Ohmic (solution) resistance and charge transfer resistance, respectively. (d) Stability test of SSMCathodization and Ir/C (shown in the inset) using chronopotentiometry technique at 10 mA cm-2.

We also tested the OER performance of SSM-Cathodization in 1.0 M KOH electrolyte, as shown in Fig. 5. It is found that the overpotential necessary to achieve 10 mA cm-2 is around 275 mV, which is 65 mV lower than that obtained in 0.1 M KOH electrolyte. Moreover, it only requires an impressively low overpotential of 319 mV to deliver a current density of 100 mA cm-2, making SSM-Cathodization among one of the highly active catalysts for the OER (Table 1). Intrigued by the superior OER activity of cathodized SSM, we further tested its performance in a 2-electrode water electrolyzer with 1.0 M KOH electrolyte. To minimize the overpotential caused by the cathodic hydrogen evolution reaction, a 15

commercial platinum plated electrode is employed as the cathode. Fig. 5(b) shows the polarization curve for the overall water electrolysis process. It can be seen that a cell voltage of 1.588 V is required to achieve a current density of 10 mA cm-2, which corresponds to a low overall overpotential of 0.358 V. SSM-Cathodization can also exhibit satisfying performance at high current densities, and a cell voltage of 1.857 V is sufficient to afford a current density of 100 mA cm-2. To be noted, herein the cell voltages for overall water electrolysis are reported without any iR-compensation. Table 1 compares the OER performance of some representative earth-abundant catalysts in 3-electrode half-cell and/or 2-electrode water electrolyzer configurations, from which we can see that the OER activity of SSMCathodization is comparable or even superior to many of those reported OER catalysts including the benchmark RuO2 and IrO2. Electrochemical stability of SSM-Cathodization for water electrolysis is also studied by performing chronoamperometry measurement at 10 mA cm-2 for 8000 s (Fig. 5c). It turns out that the cell suffers from a minor performance degradation, as reflected by the increase in the cell voltage from 1.59 to 1.65 V. It can be noticed that the degradation of SSM-Cathodization in the 2electrode electrolyzer is more pronounced than that in the 3-electrode setup, which would mainly originate from their difference in KOH concentration (i.e., 1.0 M vs. 0.1 M). It is documented that the OER active Ni species (Ni(OH)2 or NiOOH) tends to dissolve in alkaline solution and form water soluble Ni2+ species (identified mainly as Ni(OH)3−) [58]. (

)

(

)

(6)

These Ni2+ species is reported to be stable in aqueous solutions and cannot be re-oxidized to form OER active phase, the presence of which in electrolyte/cell is therefore less likely to be an activity booster for follow-up measurements. By inspecting the polarization curves before and after the chronoamperometry measurement, it can be found that the peak featuring the redox couple of NiOOH/Ni(OH)2 has significantly vanished (Fig. S7), indicating that the dissolution of OER active Ni species is taking place and likely responsible for the overall performance degradation. Considering that SSM-Cathodization electrode displays little degradation over the long term operation in 0.1 M KOH (Fig. 4d), it leads us to hypothesize that the OER active species is more prone to dissolution in more concentrated KOH electrolyte. Nevertheless, 16

since the backbone of SSM can in principle act as abundant sources for generating those OER active Ni species, we made attempt to recover the degraded SSM-Cathodization electrode by subjecting it to an additional cathodization treatment. Intriguingly, we found that the reduction signal of NiOOH/Ni(OH)2 reappeared after the second cathodization (Fig. S7). The follow-up water electrolysis test using the recovered SSM-Cathodization electrode indicates that the cell voltage drops back to ~1.60 V (Fig. 5d), based on which we can confirm first that the surface Ni species is of significance to the OER performance of SSM, and second, the performance loss during the long term operation can be facilely recovered by regenerating the OER active Ni species through the cathodization treatment. The intriguing self-healing capability of SSM-Cathodization electrode is further studied by subjecting the same electrode to up to 10 regeneration cycles (Fig. S8), it can be seen that the initial performance can always be recovered by performing the cathodization treatment. Nevertheless, it also is noticed that the degradation rate starts increasing from the 6th regeneration cycle, especially at the initial stage of the chronoamperometry measurement, as reflected by the increasing slope of cell voltage vs. time (Fig. S8). It appears that newly regenerated OER active Ni species might suffer from a more rapid dissolution with increasing the regeneration cycle number. Further studies and characterization are needed to identify the exact reason for this observation and to identify a suitable regeneration protocol and operation conditions to fully leverage the regeneration possibilities of SSM materials. Nonetheless, these results have demonstrated the unique feasibility of SSM acting as a self-healing material for the OER applications, which is impracticable on most of the other conventional OER catalysts while highly desirable for practical applications.

17

Fig. 5. (a) OER polarization curve of SSM-Cathodization using 3-electrode cell configuration in 1.0 M KOH electrolyte. The potential is shown after iR-compensation. (b) Polarization curve (without iRcompensation) of water electrolysis using SSM-Cathodization||Pt electrode pair in a water electrolyzer using 1.0 M KOH electrolyte. The inset shows a picture of water electrolysis process where O2 and H2 bubbles can be observed on SSM and Pt electrodes, respectively. (c) Stability test of water electrolysis on SSM-Cathodization at 10 mA cm-2. (d) The overpotential of initial, degraded and recovered SSMCathodization electrodes. Table 1. Comparison of the OER overpotential and cell voltage during water electrolysis.a OER Sample

ηOER (V)

Water electrolysis b

Cell voltage (V)c

100 mA cm-2 -

Cathode

NiFeOx

10 mA cm-2 0.350

NiFeLDH-Ti-La

0.260

-

NiCoLDH

0.367

-

Year Reference

-

10 mA cm-2 -

100 mA cm-2 -

2013

[59]

-

-

-

2014

[60]

-

-

-

2015

[61]

2015

[62]

CoP film

0.345

~0.410

CoP film

-

1.74

d

NiFe/C

0.330

-

NiFe/C

1.58

-

2016

[63]

SS-316 L

0.370

-

-

-

-

2016

[25]

SS-SPN50

0.290

-

-

-

-

2016

[13]

18

Sulfurized SS foil

0.262

0.306

Sulfurized SS foil

~1.62d

~1.77d

2017

[27]

Ni3S2/Ni Foam

0.222

0.306

Ni3S2/Ni foam

1.59

~1.88

2017

[64]

CoMnCH

-

0.349

CoMnCH

1.68

~1.98

2017

[65]

Ni2P4O12

0.270

-

Pt

1.6

> 2.0

2018

[66]

NiFe LDH@NiCoP

0.220

-

1.57

~1.9

2018

[67]

Nickel phosphite

0.232

0.364

NiFe LDH@NiCoP Nickel phosphite

1.60

-

2018

[68]

IrO2 on Ni Foam

0.310

0.445

-

-

-

2018

[68]

RuO2 on Ni Foam

0.292

0.420

-

-

-

2018

[68]

Fe3C on SSM

0.290

-

-

-

-

2019

[31]

SSM-Cathodization

0.275

0.319

Pt

1.59

1.86

a. b. c. d.

2019 This work

The electrolyte is 1.0 M KOH or NaOH. OER overpotentials measured in half-cell configuration after iR-compensation. Cell voltage measured in water electrolyzer without iR-compensation unless otherwise specified. Potential reported after iR-compensation.

The last puzzle is about the origin of excellent performance of cathodized SSM especially under high current density conditions. The most dramatic change induced by the cathodization treatment is the surface enrichment of Ni (oxy)hydroxide, which is well documented to be OER active species in alkaline electrolyte [3, 69] and therefore may endow SSMs with enhanced intrinsic activity toward the OER. It is also noteworthy to observe that the reference Ir/C is intrinsically more active toward the OER than both SSM catalysts, as reflected by the lower OER onset potential on Ir/C (~1.4 V) than that on SSMs (> 1.5 V). Nevertheless, as shown in Fig. 4(a), the OER performance of Ir/C is dramatically inferior to those SSMs in the high current density regime where the mass transfer limitation starts setting in. Moreover, the OER on Ir/C exhibits an exceptionally high Tafel slope (157 mV dec-1, Fig. 4b), and this is usually a feature of the increased mass transport resistance of reactants (e.g., OH-) [70]. Therefore, it appears that the inferior performance of It/C in high current regime stems from the limited mass transport. On an OER catalyst, the mass transfer limitation is usually caused by O2 bubbles produced and sticking on the electrode, which reduce the available reactive interface and prevent adsorption/activation of OH- in the electrochemical double layer [70-72]. In fact, gas-bubble detachment efficiency for a gas evolving reaction is believed to play at least equally important role as the electrocatalysis of the reaction [73]. Mass transfer resistance caused by O2 bubbles is also widely observed on some other catalytic systems, and this is especially the case for those planar electrodes operated at high current density [74]. A common solution is to use a rotating disk electrode (RDE) and to increase rotation rate of the RDE, however, as demonstrated in the current work, the mass transfer resistance caused by O2 bubbles seems not to be fully eliminated by rotating the working electrode and 19

may cause additional mechanical damage to the catalyst layer. In contrast, SSMs exhibit excellent OER performance in high current density regime, and the increase in Tafel slope with overpotential is less pronounced on SSMs that that on Ir/C (Fig. 4b). These results imply that the OER performance is not severely affected by the generated O2 bubbles on SSMs as that on Ir/C or, in other words, SSMs could be highly efficient in dissipating evolved O2 bubbles and minimizing the bubble accumulation on the electrode. It is well documented that surface morphology of electrodes is crucial for bubble removal in a gas evolving reaction [75]. For instance, Zhao et al. observed that Ni foams with open macroporous structure could help rapidly dissipate the gas bubble which favors the OER kinetics at high current density [74]. Mayrhofer et al also reported that surface cracks on a RuO2 electrode could efficiently promote the gas bubble detachment and therefore brings about improved OER performance [73]. They suggested that surface cracks would give birth to smaller gas bubbles which are more prone to detach due to their higher thermal oscillation frequency compared to larger bubbles [73]. Nevertheless, this is not the case for smooth or crack-free surfaces, on which the evolved gas bubbles have free spaces to grow into large bubbles and tend to stay on electrode surfaces, as predicted by the theory that gas bubble detachment frequency is inversely proportional to the radius of gas bubbles [76]. Lewerenz et al reported that nanostructured electrode could efficiently prevent the gas bubble coalescence and favor the gas bubble detachment compared to planar electrode [77]. There are also report suggesting that the smaller diameter of wire electrodes would lead to faster bubble nucleation rate and also smaller diameter of detached gas bubbles [78]. These inspiring reports lead us to hypothesize that the superior OER kinetics on SSM especially at high current densities would benefit largely from its unique open mesh structure. Specifically, SSM with small wire diameter (~25 µm) can act as an array of metal-wire electrodes, which in principle favor fast bubble nucleation and easy detachment of gas bubbles at a small radius. The openings in the SSM could prevent the coalescence of gas bubbles to form larger ones that feature slow detachment rate. Moreover, the inherent open mesh structure of SSM provides many available pathways within the electrode, which would allow a good gas transport without rewetting SSM surfaces. Based on these above results, we can see that the superiority of SSM-Cathodization as an OER catalyst could originate from the following aspects: 1) surface enrichment of Ni oxy(hydroxide) 20

species brings more active sites toward the surface and makes the SSM materials intrinsically more active toward OER, as evidenced by the dramatically lowered overpotential and 3.5 times higher specific activity than that of the pristine SSM material; 2) The backbone of SSM provides abundant sources for regenerating OER active Ni species, and the performance degradation can be readily repaired by an additional cathodization treatment, making SSM a promising self-healing catalyst for the OER. 3) SSMs with open mesh structure greatly favor a facile removal and transport of gas bubble at electrode surfaces, the process of which plays a key role in determining the overall OER kinetics especially under high current density conditions. These results also remind us that there is still room left to further improve the OER performance of SSM materials, and this can be achieved by either optimizing their surface composition and or geometric structure (e.g., aperture width, wire diameter, types of weave), which will bring SSM materials closer to one of the simplest and cheapest eco sound options as the OER catalyst for practical applications. 4. Conclusions In summary, we present herein the activation of ubiquitous SSM materials toward an active, stable and self-healing OER electrode, which is implemented by a simple in situ electrochemical cathodization procedure. Despite its ubiquitous nature and extremely low cost, SSM-Cathodization exhibits excellent performance toward the OER, as reflected by the low overpotential (340 mV) at 10 mA cm-2 and minor performance loss during over 15 h stability test in 0.1 M KOH electrolyte. The OER activity of SSM-Cathodization can be further improved in 1.0 M KOH electrolyte, and impressively low overpotentials of 275 and 319 mV are sufficient to deliver the benchmark current densities of 10 and 100 mA cm-2, respectively, outperforming many of those reported earth-abundant OER catalysts. The superior OER performance of SSM-Cathodization can also be transferred to a two-electrode water electrolyzer, and a low cell voltage (1.59 V@10 mA cm-2) and steady performance can be observed during the water electrolysis experiment. The high intrinsic activity of SSM-Cathodization would stem from the surface enrichment of Ni (oxy)hydroxide after the cathodization treatment while its excellent OER performance at high current density would benefit largely from its unique open mesh structure, which is supposed to promote the dissipation of oxygen bubbles into electrolyte. Moreover, SSMCathodization electrode shows the unique capability of self-healing, and minor performance loss can 21

be easily recovered/healed by subjecting the electrode to an additional cathodization treatment. These results demonstrate that cathodized SSM material can be employed as an exciting new option of highperforming OER catalyst, which is very cheap in price, robust in operation and capable of self-healing, and therefore holds great potential to be a new benchmark OER catalyst for future clean energy storage/conversion technologies. Acknowledgments The authors acknowledge the funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No.681719). L.-L.S. acknowledges the funding from the China Scholarship Council (No. 201506210077). We thank Mr. Karl Kopp for his kind help on the XPS measurements. References [1] B.M. Hunter, H.B. Gray, A.M. Müller, Chem. Rev. 116 (2016) 14120-14136. [2] B.-Q. Li, C. Tang, H.-F. Wang, X.-L. Zhu, Q. Zhang, Science Advances 2 (2016) e1600495. [3] B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. García-Melchor, L. Han, J. Xu, M. Liu, L. Zheng, F.P. García de Arquer, C.T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, A. Janmohamed, H.L. Xin, H. Yang, A. Vojvodic, E.H. Sargent, Science 352 (2016) 333-337. [4] Y. Lin, K.-H. Wu, Q. Lu, Q. Gu, L. Zhang, B. Zhang, D. Su, M. Plodinec, R. Schlögl, S. Heumann, J. Am. Chem. Soc. 140 (2018) 14717-14724. [5] G.-R. Zhang, B.J.M. Etzold, J. Energy Chem. 25 (2016) 199-207. [6] B. Wang, C. Tang, H.-F. Wang, X. Chen, R. Cao, Q. Zhang, J. Energy Chem. 38 (2019) 8-14. [7] H. Liang, A.N. Gandi, C. Xia, M.N. Hedhili, D.H. Anjum, U. Schwingenschlögl, H.N. Alshareef, ACS Energy Lett. 2 (2017) 1035-1042. [8] H. Zhong, T. Liu, S. Zhang, D. Li, P. Tang, N. Alonso-Vante, Y. Feng, J. Energy Chem. 33 (2019) 130-137. [9] J. Suntivich, K.J. May, H.A. Gasteiger, J.B. Goodenough, Y. Shao-Horn, Science 334 (2011) 1383-1385.

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26

Graphical Abstract

A facile cathodization treatment can activate ubiquitous stainless steel mesh into a new extremely cheap and high performing OER catalyst, which shows great promise for practical applications in clean energy storage/conversion technologies.

27

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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