Boosting the oxygen evolution activity of copper foam containing trace Ni by intentionally supplementing Fe and forming nanowires in anodization

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Boosting the Oxygen Evolution Activity of Copper Foam Containing Trace Ni by Intentionally Supplementing Fe and Forming Nanowires in Anodization Sengeni Anantharaj , Hisashi Sugime , Bozhi Chen , Natsuho Akagi , Suguru Noda PII: DOI: Reference:

S0013-4686(20)31563-2 https://doi.org/10.1016/j.electacta.2020.137170 EA 137170

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

24 April 2020 6 July 2020 21 September 2020

Please cite this article as: Sengeni Anantharaj , Hisashi Sugime , Bozhi Chen , Natsuho Akagi , Suguru Noda , Boosting the Oxygen Evolution Activity of Copper Foam Containing Trace Ni by Intentionally Supplementing Fe and Forming Nanowires in Anodization, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.137170

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Highlights: 

Poorly OER active Cu foam with trace Ni was made into a highly active catalyst by anodization.



Intentionally supplementing Fe during anodization further improved the OER activity.



Pure Cu containing no Ni did not show such enhancement.



Cu foam containing Ni formed structurally interesting Cu(OH)2-CuO nanowires while anodizing in the presence of Fe.



As a combined effect, η50 was lowered by 153 mV after anodization in Fe added KOH.

1

Boosting the Oxygen Evolution Activity of Copper Foam Containing Trace Ni by Intentionally Supplementing Fe and Forming Nanowires in Anodization Sengeni Anantharaj*1, Hisashi Sugime1, Bozhi Chen1, Natsuho Akagi1, and Suguru Noda*1,2 1

Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda

University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan 2

Waseda Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo,

Shinjuku-ku, Tokyo 169-8555, Japan

* Correspondence should be addressed to: [email protected] and [email protected]

ABSTRACT Oxygen evolution reaction (OER) is the bottleneck for realizing energy-efficient hydrogen production through water electrolysis in both acid and alkali. Alkaline OER electrocatalyzed by Ni and Co hydroxides are well known which showed unexpected enhancement with the addition of Fe. We found that the commercially procured Cu foam containing trace amount of Ni (~1.5 wt.%) upon anodization formed Cu(OH)2-CuO nanowires with conceivable formation of Ni(OH)2 and experienced a notable enhancement in its OER activity. When sufficient amount of Fe was intentionally supplemented during anodization, OER activity of the same was further improved. Specifically, as a combined result of anodization in KOH and in Fe supplemented KOH, overpotential at 50 mA cm-2 was lowered by 153 mV. Such an activation also improved the kinetics of OER by lowering the Tafel slope by 100 mV dec -1. With these, it has been shown here that a moderately active OER catalyst i.e., Cu(OH) 2-CuO/Cu formed upon the anodization of Cu foam can be turned into a highly active catalyst just by 2

utilizing the trace Ni that it already contains and intentionally supplementing sufficient amount of Fe. Keywords: Oxygen evolution reaction; Hydrogen generation; Anodization; Cu(OH)2-CuO, Water electrolysis; Electrocatalysis. INTRODUCTION Electrocatalytic water oxidation or the otherwise known oxygen evolution reaction (OER) is the anodic half-cell reaction of water electrolysis that determines the energy efficiency of hydrogen production in major part [1–5]. The complexity of coupled four electron – four proton transfer reaction with the formation of O–O bond accounts for the high overpotential witnessed in water electrolysis [6]. Despite being the handy, quicker, and safer way of producing hydrogen in its purest form than all other methods, cost of electrode materials and the prementioned energy loss as overpotential has been forbidding this technology from successful commercialization [7,8]. On the other hand, water electrolysis is also regarded as one of the indirect means of large-scale energy storage that stores intermittent surplus energy in the form of H2 fuel [9–13]. In general, the half-cell reactions responsible for the electrochemical splitting of water, the hydrogen evolution reaction (HER) occurring on cathode and the OER occurring on anode are catalyzed by the materials that possess optimal energy of interactions of intermediates. Conventionally, currently scaled-up electrolysers employ Pt and Pt/C as HER electrocatalysts and IrO2 and RuO2 as OER electrocatalysts [14–17]. These materials are noble and expensive which account for the higher cost of production of hydrogen. To overcome this, many nonprecious metals have recently been screened for both the half-cell reactions of water electrolysis and shown to be efficient. Some catalysts have even been shown to perform better than Pt/C in HER (particularly in operations at high current densities) and IrO2 and RuO2 in OER (albeit only 3

in alkali in this case). A few examples include Co-Fe-P [18], NiTe2 [19], Cu-Se [20], Ni-Se [21], Fe-P [22–25], NiFe-layered double hydroxide (LDH) [26,27], NiFe-P [28,29], and NiFe-Se [30,31]. Some of these stated examples benefitted from materials’ tailoring in the engineering aspects (nanostructuring, increased mass loading, high specific surface area, etc.,) and others benefitted from materials’ tailoring in the electronic aspects. In most of the recent reports, tailoring the materials in both the aspects have been shown to be more effective in realizing energy-efficient water electrolysis. As stated earlier, OER is the bottleneck in realizing energy-efficient production of hydrogen from water in both acid and alkali. Alkaline water electrolysis is superior in a way that it could use cheaper first-row transition metals (Ni, Co, Fe, and Mn) [32] and their oxides/hydroxides as OER electrocatalysts among which the OER activity of Co and Ni in the presence of certain quantity of Fe is always higher than others [26,27,33]. On the other hand, these materials when employed as a pristine oxide/hydroxide tend to exhibit relatively poor OER activity. The order of OER activity of pristine monometallic oxide/hydroxide catalysts of these metals in alkali can be arranged in the order of Ni > Co > Fe > Mn [32]. It is not surprising to not seeing Cu in this list as it does not have the optimal energy of interaction of intermediates in alkaline medium. Nonetheless, a few material engineering strategies had been proven to bring out appreciable OER activity with Cu oxides. Examples are; growing CuO nanostructures on Cu foams/foils by different means [34,35], making heterostructured catalysts of CuO with other Cu chalcogenides [34], wrapping CuO with conductive carbon shells [36], and others [37–45]. Here in, we report an easier way of boosting the OER activity of Cu(OH)2-CuO nanowires grown through electrochemical galvanostatic anodization in KOH with an assistance of trace Ni (~1.5 wt.%) that it contained and by supplementing with sufficient Fe during

4

anodization. Such a programmed anodization of Cu foam containing trace Ni in Fe added KOH resulted in a significant enhancement in the OER Activity. EXPERIMENTAL Anodization of Ni Containing Cu Foam in KOH Cu foam procured from Alfa Aesar was found to contain trace amount of Ni and the same was anodized as follows. A piece of Cu foam with a dimension of 1 cm × 5 cm was dipped into a solution of 1.0 M KOH without any acid-pretreatment as followed by others previously. Along with it, a large surface area carbon cloth (CC) was used as a counter electrode while a Hg/HgO electrode filled with 1.0 M KOH was used as a reference electrode. For anodization, a current density of 50 mA cm-2 was applied for 30 min. During the course of the anodization, the typical color of metallic copper turned into black indicating the formation CuO. Then, the anodized electrode was used directly for electrocatalytic OER studies. Anodization of Ni Containing Cu Foam in KOH with Intentionally Supplemented Fe Another piece of the same Cu foam was anodized in a similar way with an exception that the 1.0 M KOH solution used in this case was supplemented with 1 mmol Fe2+ ions (as FeCl2) and the solution was stirred during the anodization to maintain homogenous population of Fe2+ in the electrolyte. The remaining anodization conditions were kept constant. At the end of anodization, this electrode had also turned into black indicating the exclusive formation of CuO. This electrode was then directly used for the comparative electrocatalytic OER studies. For showing that the observed OER enhancement was indeed due to the trace Ni present in Cu foam, we have procured a Cu foil (a foam of this level purity is not available in market) of purity 99.96% from NILACO and anodized it in similar ways and studied its OER activity under identical conditions. 5

Electrochemical Characterizations All electrochemical characterizations were done in 1.0 M KOH solution in a conventional three electrode assembly. All the potential scales were converted to reversible hydrogen electrode (RHE) scale and corrected for iR drop unless otherwise mentioned. Linear sweep voltammetric (LSV) responses were obtained with a scan rate of 5 mV s-1. Cyclic voltammetric (CV) responses were obtained at 100 mV s-1 for Cu foam containing ~1.5 wt.% Ni and for a pure Cu foil (99.96%) to show the presence of Ni in the Cu foam. All electrochemical impedance spectroscopic (EIS) measurements were taken at potentials that are 50 mV anodic to the OER onset potential with a sinusoidal AC potential amplitude of 10 mV. The frequency range for the EIS measurements was from 100 kHz to 0.1 Hz. Stability of the Cu foam anodized in Fe supplemented KOH was tested by chronoamperometry at 1.77 V vs. RHE. RESUTLS AND DISCUSSION Presence of Trace Ni in Cu Foam Upon CV and energy dispersive X-ray analysis (EDS) interrogations, the Cu foam procured from Alfa Aesar was found to have a trace (1.5 wt.%) level of Ni in it. The first evidence for the existence of Ni in Cu foam was obtained through CV acquired at 100 mV s -1 in KOH within the potential window of -1.5 to 1.0 V vs. Hg/HgO (Figure S1). From Figure S1, peaks corresponding to the formation of Cu(OH)2- at -0.556 V (I), formation of Cu2O at -0.304 V (II), formation of Cu(OH)42- at -0.007 V (III), formation of Cu(OH)2/CuO at 0.134 V (IV), and the formation of CuOOH at 0.724 V vs. Hg/HgO (VI) were observed. Similarly, in the cathodic curve, peaks for the reduction of CuOOH to Cu(OH)2 at 0.594 V (VII), reduction of CuO to Cu at -0.546 V (IX), reduction of Cu(OH)2 to Cu at -0.832 V (X), and the reduction of Cu(OH)42- at -0.982 V vs. Hg/HgO (XI) were witnessed. All these peak positions resemble closely to the 6

earlier reports in literature.[46,47] In this CV response, the presence of Ni was indicated by the presence of a relatively weaker redox pair located at 0.57 and 0.43 V vs. Hg/HgO (denoted as (V) and VII respectively) in the anodic and cathodic sweeps respectively. These peaks were never observed for pure metallic copper in KOH.[47] To confirm that the redox pair observed at 0.57 and 0.43 V vs. Hg/HgO in Figure S1 is of Ni, a CV of pure Cu foil (99.96%) was also acquired under identical electrochemical conditions and analyzed (Figure S2). All the peaks characteristic to that of Cu were witnessed in Figure S2 but no redox pair at 0.57 and 0.43 V vs. Hg/HgO was observed. This has clearly indicated that the Cu foam that was procured for this study contains a certain amount of Ni. Hence, to determine the amount of Ni in it, elemental composition of as procured Cu foam was analyzed by EDS. Table S1 shows the elemental composition of Cu foam from which the dominant presence of Cu can be noted. Significantly, it also contained Ni and Al with the wt.% of 1.53 and 5.14 respectively. Presence of Al is accountable as Cu is usually recovered through aluminothermic process from its ore.[48] These results have clearly indicated the presence of Ni in the Cu foam. This has fascinated us to study its effect on the OER performance of anodized Cu which was in the recent past was shown to possess appreciable activity. Anodization and Electrocatalytic OER Studies Being fascinated with the presence of trace level Ni in Cu foam, we performed anodization of the same in KOH and in KOH supplemented with 1 mmol Fe2+ ions. Figure 1a illustrates the E vs. t responses of the galvanostatic anodization performed with Cu foam containing trace level Ni in KOH (red) and in 1 mmol Fe2+ supplemented KOH (green) at 50 mA cm-2. Clearly, there is a difference in the pattern of change in potential during anodization between in KOH and in Fe2+ supplemented KOH. For the same current density (50 mA cm-2) of 7

polarization, Ni containing Cu foam required 0.79 V vs. Hg/HgO initially in KOH which kept on increasing throughout the anodization period and ended at 0.87 V vs. Hg/HgO.

Figure 1: (a) CP responses of Ni containing Cu foam in KOH and in Fe2+ supplemented KOH. (b) LSVs of Ni containing Cu foam before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH (1 M KOH, 5 mV s-1). (c) Nyquist plots of Ni containing Cu foam before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH acquired at 1.65 V vs. RHE (100 kHz to 0.1 Hz, 10 mV AC sinusoidal perturbation). (d) The corresponding Tafel plots. On the other side in Fe2+ supplemented KOH, initial potential was 0.755 V vs. Hg/HgO and was maintained throughout the anodization period. This observation indicates that the 8

charge-transfer characteristics of Cu foam was keep on changing while anodizing in KOH that led it to increase its potential gradually to maintain the constant polarization current density of 50 mA cm-2. On the other hand, the charge transfer characteristics of the same Cu foam was not greatly altered while anodizing it in Fe2+ supplemented KOH which means that a current density of 50 mA cm-2 was sufficient to form a stable phase on the Cu foam that performed chargetransfer consistently across the electrode – electrolyte interface during anodization. This is also indicated by the increasing difference in potential of anodization between these two methods. At this stage, it was believed that the trace Ni that was present along in Cu foam got oxidized and formed a NixFe1-xOOH catalytic sites on anodized Cu foam that helped it to realize the stable potential during anodization. To see the effects of anodization of this Ni containing Cu foam, a standard LSV experiment was performed with as procured Cu foam, Cu foam anodized in KOH and Cu foam anodized in Fe2+ supplemented KOH and the responses are given as Figure 1b (corrected for iR drop taking their respective uncompensated resistance values viz., 0.92, 0.65, and 0.35 ohms from Figure 1c). Clearly, the OER current density at an indicative potential of 1.8 V got increased to 83.3 and 268.2 mA cm-2 from 42 mA cm-2 after anodization in KOH and in Fe2+ supplemented KOH. Similarly, for a current density of 50 mA cm-2, as procured Cu foam required 587 mV. On the other hand, the Cu foam anodized in KOH and Fe2+ supplemented KOH necessitated 532 and 434 mV which 55 and 153 mV respectively lower than that of as procured Cu foam. These observations clearly advocate the advantage of having trace Ni in Cu which led OER enhancement after anodization in KOH and the same had exhibited further enhancement when supplemented with Fe2+. This is indicating the possible formation of NixFe1xOOH

sites on the anodized Cu foam which must also obviously contain moderately OER active

Cu(OH)2/CuO. To further know how such an enhancement materializes, EIS measurements were

9

taken at catalytic turnover conditions for all the three electrodes and pictured as Figure 1c. These EIS measurements revealed that as procured Cu foam exhibited a charge transfer resistance (Rct) of 2.5 Ω which got lowered to 0.8 Ω after anodization in KOH and again lowered to 0.29 Ω after anodization in Fe2+ supplemented KOH. This clarifies the observed activity difference among the studied electrodes in alkaline OER and indicates that there has been a formation of NixFe1-xOOH sites along with Cu(OH)2-CuO on Cu foam. OER enhancing effect of anodizing Ni containing Cu foam with purposeful of supplementation Fe2+ was also reflected in its Tafel slope (Figure 1d). As procured Cu foam exhibited a Tafel slope of 257 mV dec-1 and that got lowered to 206 mV dec-1 after anodization in KOH. Interestingly, the Tafel slope of the Cu foam anodized in Fe2+ supplemented KOH was just 157 mV dce-1 which fetches a difference of 100 mV dec-1 before and after anodization. This observation implies that the kinetics of the rate determining step was improved with anodization in KOH and in Fe2+ supplemented KOH. We attribute this enhancement to the existence of 1.5 wt.% Ni in the Cu foam and the possible formation of NixFe1-xOOH sites upon anodization in Fe2+ supplemented KOH. However, it cannot be concluded here so. As far as the mechanism of OER in alkali is concerned, there have been more than five different paths that were detailed in our earlier perspective[49] of which the electrochemical oxide path and oxide path are widely accepted [50]. In both of these paths, the catalyst is required to undergo a cycle of oxidation and reduction throughout OER which is true for our catalyst too. However, as we have more than one metal (Cu, Ni, and intentionally supplemented Fe) in our case, there is a possibility for the combination of mechanisms being followed concurrently. Mechanism of OER with NixFe1-xOOH systems has been intensively investigated by many[51–58] and the recent comprehension is that NixFe1-xOOH follows the adsorbate

10

evolution mechanism (AEM) (equations 1 – 4) in which oxygen from the catalyst’s very own lattice do not participate in OER [59]. * + H2O(l)  OH* + H+ + e−

(1)

OH*  O* + H+ + e−

(2)

O* + H2O(l)  OOH* + H+ + e−

(3)

OOH*  O2(g) + H+ + e−

(4)

Besides, many other transition metals have been found to follow the lattice oxygen participation mechanism (LOM) described with equations 5 – 8 in which Vo indicates the lattice oxygen vacancy. OH*  (Vo + OO*) + H+ + e−

(5)

(Vo + OO*) + H2O(l)  O2(g) + (Vo + OH*) + H+ + e−

(6)

(Vo + OH*) + H2O(l)  (HO-site* + OH*) + H+ + e−

(7)

(HO-site* + OH*)  OH* + H+ + e−

(8)

We also believe that NixFe1-xOOH follows AEM path and compositionally dominating Cu(OH)2CuO could follow either or both AEM and LOM. To ensure that observed activity enhancement was mainly due to trace level Ni in Cu foam that forms active NixFe1-xOOH sites during anodization and also to learn the effect of anodization in Fe2+ supplemented KOH on the OER activity of pure Cu, we purchased a high purity Cu foil (99.96%) from NILACO, Japan and anodized following the same protocols in KOH and in Fe2+ supplemented KOH. Figure 2a depicts the E – t responses of anodization of Cu

11

foil (99.96%) in KOH and in 1 mmol Fe2+ supplemented KOH. Unlike the Cu foam containing trace Ni, this Cu foil (99.96%) showed a steady response throughout the anodization in KOH except a gradual increase in the first four minutes.

Figure 2: (a) CP responses of Cu foil (99.96%) in KOH and in Fe2+ supplemented KOH. (b) LSVs of Ni containing Cu foil (99.96%) before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH (1 M KOH, 5 mV s-1). (c) Nyquist plots of Ni containing Cu foil (99.96%) before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH acquired at 1.65 V vs. RHE (100 kHz to 0.1 Hz, 10 mV AC sinusoidal perturbation). (d) The corresponding Tafel plots.

12

Similarly, it had also shown the stable potential when anodized in 1 mmol Fe2+ supplemented KOH except that there was a 11 mV lowering which is indicating that the change in the charge transfer characteristics was not so different. Figure 2b shows the LSVs of Cu foil (99.96%) before anodization, after anodization in KOH and after anodization in 1 mmol Fe2+ supplemented KOH (corrected for iR drop taking their respective uncompensated resistance values viz., 1.10, 1.05 and 1.20 ohms from Figure 2c). This shows that the OER activity of Cu foil (99.96%) was actually lowered notably after anodization with an increase in overpotential at 10 mA cm-2 by 25 mV. This observed difference before and after anodization could also be due to the contribution of anodization in pure Cu foil when subjected to LSV. In contrast, anodization in KOH supplemented with Fe2+ resulted in the lowering of overpotential at 10 mA cm-2 by 50 mV. However, the total OER current density delivered remains the same before and after anodization in Fe2+ supplemented KOH. These results indicate that a simple anodization of pure Cu cannot lead to enhancement in OER activity. Previous studies also suggested that the Cu(OH)2 formed during anodization were inactive or poorly active for OER. Hence, it is clear that the trace level Ni that was present in the Cu foam was behind the OER enhancement after anodization in KOH. However, a notable enhancement observed with the Cu foil (99.96%) anodized in Fe2+ supplemented KOH could only be attributed to the FeOOH deposited anodically on it as there is no way that Fe2+ could enhance the OER activity of Cu(OH)2 as it does for Ni(OH)2 and Co(OH)2. Nyquist plots of as procured Cu foil (99.96%), after anodization in KOH and after anodization in Fe2+ supplemented KOH are presented together as Figure 2c. From this, it can be evidenced that the Rct got increased (from 14.5 Ω to 21.5 Ω) at catalytic turnover conditions of OER after anodization which explains the lowering in OER activity. In contrast to this, Cu foil anodized in Fe2+ supplemented KOH exhibited a much lower Rct of 5.5 Ω which is in parallel

13

trend with the observed OER activity. The Tafel analysis (Figure 2d) of these catalytic electrodes revealed that the kinetics of the rate determining step was actually lowered after anodization in KOH and after anodization in Fe2+ supplemented KOH. This is again stressing the advantages of having trace Ni in Cu foam.

Specifically, the Tafel slopes of as procured Cu foil, after

anodization in KOH and after anodization in Fe2+ supplemented KOH are 117, 122 and 265 mV dce-1. Despite having lower overpotential at 10 mA cm-2 as revealed by LSV analysis, Cu foil anodized in Fe2+ supplemented KOH exhibited a large Tafel slope which is due the poor OER kinetics FeOOH. Tafel slope values are used to predict the mechanism of OER assuming the charge transfer coefficient is ~0.5 so that the slopes 120, 90, 60, and 30 mV dec-1 would correspond to one, two, three, and four electron transfer in the rate determining step (RDS), respectively. In the case of Cu foil, before and after anodization remained closer to 120 mV dec1

, it is clear that the first electron transfer remained the RDS during OER which corresponds to

the oxidation of adsorbed hydroxide anion on the catalytic site. However, the Tafel slope became abnormal when Fe is supplemented during anodization with Cu foil which must essentially due to the formation of OER inactive FeOOH phase on the anodized Cu foil. Such abnormal Tafel slopes were commonly observed earlier when the catalyst’s activity is limited by the adsorption of hydroxide anion even before the first electron transfer [60–63]. In the case of Cu foam, the Tafel slopes remained always higher than 120 mV dec-1 indicating that OER is controlled by the adsorption of hydroxide prior to the first electron transfer. However, unlike Cu foil, Cu foam containing trace Ni exhibited a gradual lowering in Tafel slope after anodization and after anodization in Fe supplemented KOH. This implies that the anodization and Fe supplemented anodization must have resulted in the highly OER active NiOOH and NixFe1-xOOH phases that improved the kinetics of OER in the RDS.

14

The overall comparative electrocatalytic OER studies clearly advocated the advantages of having trace level (1.5 wt.%) Ni in the Cu foam for realizing enhanced OER activity and also showed that pure Cu will get benefitted neither by anodization in KOH nor by supplementing Fe2+. Such an OER activity enhanced Cu foam containing trace Ni was subsequently tested for its endurance by chronoamperometry (CA) at 1.77 V vs. RHE for more than 14 h under identical electrochemical conditions (Figure S3a). After 14 h of CA testing, Cu foam anodized in Fe2+ supplemented KOH showed a very low activity loss (7.6%). Post-CA LSV (Figure S3b) acquired at 5 mV s-1 showed an increase in overpotential by 26 mV at 50 mA cm-2 accounting for the loss observed with CA. An increase by 0.32 Ω in Rct after CA was noticed which indicates the overoxidation of Cu that inhibited the OER activity of NixFe1-xOOH sites possibly by masking it with a relatively poor Cu(OH)2/CuO moieties on the catalyst surface. With this, it has now been shown that utilizing trace level Ni in Cu foam, the OER activity of the same could be enhanced with a simple 30 min anodization in Fe2+ supplemented KOH. Besides, the OER activity of our catalyst is also compared with other related catalysts in KOH at the potential of 1.65 V vs. RHE in Table 1, which once again advocates the superiority of having trace Ni and anodizing it simultaneously with Cu while accompanying concurrent Fe incorporation. The only catalyst which showed better activity was CuS0.55 [64] which was apparently due to a very high catalyst loading of 5.5 mg cm-2. Table 1: Activity comparison of Fe supplemented anodized Cu foam containing Ni trace with other related catalysts in 1.0 M KOH at 1.65 V vs. RHE. Catalyst

Loading /

Current density @ 1.65

mg cm-2

V vs. RHE / mA cm-2

1.3

52

Reference

Cu foam with trace Ni after anodization in Fe added KOH

15

This work

Cu foil after anodization in Fe added KOH

1.1

5

CuS0.55

5.5

150

Zhang et al [64]

Cu2OxS1-x/Cu

N/A

50

Zhang et al [65]

CuO/C

N/A

50

Zhang et al [66]

Cu0.3Co2.7O4

0.2

38

Karmakar et al [40]

CuO/Ni

N/A

25

Roy et al [67]

2

25

Masud et al [38]

N/A

20

Li et al [68]

20

Ahmed et al [69]

Cu2Se CuO-NiO Cu cubes

This work

CuO/AgCuZnS

N/A

20

Jin et al [70]

Cu spheres

N/A

18

Ahmed et al [69]

Cu delafossite

20*

15

Hinogami et al [71]

12

Ahmed et al [69]

Cu rods CuO

0.7

11

Qian et al [72]

Cu/(Cu(OH)2-CuO)

2.5

10

Cheng et al [73]

CuO

N/A

9

Liu et al [74]

Cu3P

1.2

8

Han et al [39]

CuO

N/A

6

Joya et al [41]

1

5

Zhang et al [75]

Cu-Co oxide

Note: N/A indicates that the corresponding data were not available in the cited report. *This loading value has a unit of µmol cm-2. Results of Material Characterizations Before and After Anodization in KOH and Fe 2+ Supplemented KOH Having shown that a simple anodization of Ni containing Cu foam could lead to enormous enhancement in OER activity of the same, a set of pre- and post-anodization material characterizations were carried out and discussed along below. Firstly, Raman spectra of all three catalytic electrodes analyzed in this study viz., Cu foam, Cu foam after anodization in KOH and 16

Cu foam after anodization in Fe2+ supplemented KOH were acquired in the range of 800 to 200 cm-1 and pictured as Figure 3a. Cu foam (black) showed a couple of peaks at 210 and 642 cm-1 corresponding to CuO which is indicating that as procured Cu foil was significantly surface oxidized [76,77]. After anodization in KOH (red) revealed almost the same feature indicating that during such extended anodization, the same CuO was formed on the surface in addition to the existing CuO. Interestingly, Cu foam anodized in Fe2+ supplemented KOH showed an additional peak at 490 cm-1 for the stretching vibration of Cu–OH bond in Cu(OH)2.[78]

Figure 3: (a) Raman spectra of Ni containing Cu foam before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH. (b) XRD patterns of Ni containing Cu foam before anodization, after anodization in KOH and after anodization in Fe2+ supplemented KOH taken using Co Kα radiation and 2θ values were converted to Cu Kα radiation. The Miller indices in green and black show the peaks for CuO and Cu, respectively. This indicates that added Fe2+ somehow forbade the direct formation of CuO indicated by the Raman peak at 300 cm-1. To further gain knowledge on the surface structures of Cu foam, Cu foam after anodization in KOH and Cu foam after anodization in Fe2+ supplemented KOH, X-ray diffraction (XRD) patterns were acquired in the range of 10 to 100º (Figure 3b). These patterns 17

showed no apparent difference among them but the dominant the presence of CuO (indicated by the Miller indices in green of (200) and (112)) along with that of Cu (indicated by the Miller indices in black of (111), (200) and (220)). These observations match well with the powder diffraction file (PDF) nos. of 45-0937 and 02-1225 respectively. Even though there was an indication for the formation of Cu(OH)2 after anodization in Fe2+ supplemented KOH via Raman spectrum, XRD patterns did not show any sign of it which indicates that the formed Cu(OH)2 must be X-ray amorphous and could be nanocrystalline. To gain knowledge on the surface structural changes of Cu foam, Cu foam after anodization in KOH and Cu foam after anodization in Fe2+ supplemented KOH were screened through field emission scanning electron microscopy (FESEM) and the resultant micrographs are presented as Figure 4, a-f.

Figure 4: (a) FESEM micrograph of pristine Cu foam surface. (b – c) FESEM micrographs of Ni containing Cu foam after anodization in KOH. (d – f) FESEM micrographs of Ni containing Cu foam after anodization in Fe2+ supplemented KOH with increasing magnification.

18

Figure 4a shows the smoother surface of as procured Cu foam without any acid pretreatment. After anodization in KOH (Figure 4b and Figure 4c), it was roughened and an assembly of fine sheets were observed which could be due to the formation of CuO as indicated by Raman and XRD analyses. Interesting structural outcomes were witnessed after anodizing this Ni containing Cu foam in Fe2+ supplemented KOH. Figure 4d-f shows a forest of wire-like nano-structures with sharp tips at different magnifications.

Figure 5: (a) FESEM micrograph of Ni containing Cu foam after anodization in Fe2+ supplemented KOH showing the region where elemental FESEM-EDS mapping was done. (b-f) EDS elemental maps of K, O, Fe, Ni, and Cu respectively. We believe that the observed structure is Cu(OH)2-CuO hybrid nanowire as indicated by Raman spectrum. To further get information on the change in the elemental composition after anodization in KOH and in Fe2+ supplemented KOH, EDS spectra were acquired. Their respective elemental composition tables (Table S2 and Table S3) showed that the quantity of Ni remained the same while there was a significant increase in wt.% of O. In addition, it was also

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noted that a major part of Al was etched out by the alkali during anodization and a significant amount of K (6%) was present. This is expected as we did not wash the electrode after anodization to prevent any possible structural damage to emerged nanostructures. Increase in the wt.% of O was nearly tripled when anodized in Fe2+ supplemented KOH which also accompanied the inclusion of negligible amount of Fe (0.24 wt.%) albeit sufficient to form the highly active composition of Ni-Fe catalyst when we consider the amount of Ni present in it.

Figure 6: (a - f) TEM images of Cu(OH)2-CuO nanowires with increasing magnification. To find out whether the expected elements are uniformly dispersed or not, EDS elemental color mapping was performed in the chosen area of the specimen (Figure 5a). The corresponding elemental maps of K shell of K, K shell of O, K shell of Fe, K shell of Ni and K shell of Cu are provided as Figure 5b-f, respectively. Despite having very low wt.% of Fe and Ni, the corresponding elemental maps showed that they present together at almost all over the 20

sample indicating the possible formation of Ni-Fe oxyhydroxide. To interrogate further specifically the wire-like structure formed upon anodizing the Cu foam in Fe2+ supplemented KOH, the grown nano-wires were dispersed into water with a rigorous ultrasonication for 5 min. This solution was then drop-casted on carbon coated Mo grids and dried in room temperature for over a night before analysis.

Figure 7: (a) High-angle Annular Dark Field (HAADF)-STEM image of Ni and Fe containing Cu(OH)2-CuO nanowires. (b – f) STEM-EDS elemental smart maps showing the presence of O, Cu, Fe, Ni and K in Cu(OH)2-CuO nanowires. Figure 6, a-c shows the distinct morphology of the wire-like assembly witnessed in FESEM analysis. The length and width of the longer wire appears to be as long as ~2.5 µm while smaller nanowires could also be witnessed along. A relatively higher magnification image (Figure 6d) revealed that the nanowires are actually consisting of nanocrystalline grains and an 21

amorphous continuum that extends throughout the structure and binding all the nanocrystalline grains together. Further magnified images (Figure 6e and Figure 6f) show clear lattice fringes and the calibration of which resulted in the measured distance (between two adjacent lattice fringes) of 0.252 nm that matches exactly with the (002) plane of Cu(OH)2 as PDF no. 35-0505. This is in accordance with the Raman spectroscopic result of the Cu foam anodized in Fe2+ supplemented KOH. To show that these nanowires consists of O, Cu, Fe, Ni and K, EDS elemental mapping was carried out in STEM mode. Figure 7a is the high angle annular dark field (HAADF) image of the nanowires to be mapped. The elemental maps of O, Cu, Fe, Ni and K are provided as Figure 7b-d, respectively. Figure 7b and Figure 7c clearly show that the nanowires are mainly composed of Cu and O while Figure 7d-f shows that Fe, Ni and K are randomly dispersed within the structural boundaries of the nanowires. These results have once again stressed that there was an incorporation of Fe into these nanowires upon anodization in Fe2+ supplemented KOH that enhanced the OER activity. Besides, to know the morphological outcome of pure Cu foil anodized under identical conditions, FESEM analysis was carried out and the corresponding images are provided as Figure S4a-f. These images show that similar nanowire morphology was formed not only when anodizing in Fe added KOH but also in just KOH solution. Even then, no significant improvement in the OER activity of pure Cu foil was not witnessed as the one witnessed with Ni containing Cu foam. This has once again confirmed that having trace Ni is essential to witness such an highly enhanced OER activity. To perceive the changes in the oxidation states of Cu, Ni, O and Fe after anodization in KOH and in Fe2+ supplemented KOH, X-ray photoelectron spectroscopic analysis was done. Figure 8a-c are the high-resolution spectra of Cu 2p3/2, Ni 2p3/2 and O 1s states of Cu foam anodized in KOH respectively.

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Figure 8: (a – c) XPS spectra of Cu 2p3/2, Ni 2p3/2 and O 1s states of Ni containing Cu foam after anodization in KOH. (d – f) XPS spectra of Cu 2p3/2, Ni 2p3/2 and O 1s states of Ni containing Cu foam after anodization in Fe2+ supplemented KOH. 23

The Cu 2p3/2 spectrum (Figure 8a) showed three distinct peaks upon deconvolution at 931.9, 933.2, and 934.8 eV for the presence of Cu0, Cu2+ and Cu3+ species as expected.[36,41,42,76,79] Similarly, Ni 2p3/2 (Figure 8b) also showed two different peaks at 854.5 and 856 eV respectively for Ni2+ and Ni3+ species [80–82]. These observations have clearly suggested that anodization in KOH led to the oxidation of both Cu and Ni. The O 1s spectrum of the same showed two peaks at 530.2 and 532.1 eV upon deconvolution respectively for the presence of M-OOH and M-OH species [83,84]. This is in accordance with the indications made by Cu2p3/2 and Ni2p3/2 spectra of Cu foam anodized in KOH. The Cu 2p3/2 (Figure 8d) and Ni 2p3/2 (Figure 8e) spectra of the Cu foam anodized in Fe2+ supplemented KOH also showed the presence of Cu0, Cu2+ and Cu3+ at 932.1, 933.8, and 935.3 eV and the presence of Ni0, Ni2+ and Ni3+ at 853, 854.2, and 855.4 eV respectively [36,41,42,76,79]. Unfortunately, Fe 2p3/2 scan (Figure S5) did not show any peak due to the infinitesimally lower Fe content in the anodized Cu foam. The O 1s spectrum (Figure 8f) Cu foam anodized in Fe2+ supplemented KOH showed four different peaks upon deconvolution that are assigned to M–O (at 529 eV), M– OH (at 530.6 eV), M–OOH (at 531.6 eV), and adsorbed moisture (at 532.7 eV) [74,85]. Peaks assigned to M–OOH species in O 1s spectra (Figure 8c and Figure 8f) imply the possible formation of NixFe1-xOOH in anodized Cu foam. The overall pre- and post-anodization material characterizations showed the extensive oxidation of Cu0 and Ni0 to 2+ and 3+ states. When intentionally supplemented with Fe, anodization under identical conditions incorporated infinitesimally low but sufficient quantity of Fe that led to enormous enhancement in the OER activity. CONCLUSIONS

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In summary, a commercially purchased Cu foam from Alfa Aesar was found to have a trace level (1.5 wt.%) Ni. Being fascinated by this, anodization of the same was carried out in KOH and in Fe2+ (1 mmol) supplemented KOH to learn the effects on its OER activity and for achieving possible enhancement by enabling the formation of highly active NixFe1-xOOH species from the trace Ni and intentionally supplemented Fe2+. As expected, such an anodization with Fe2+ supplemented KOH led to OER enhancement which was indicated by the lowering of overpotential (at 50 mA cm-2) by 153 mV and Tafel slope by 100 mV dec-1. It was also revealed that anodization of this Ni containing Cu foam in Fe2+ supplemented KOH led to the formation of nanowire forest made mainly of Cu(OH)2-CuO whereas the same when anodized in just KOH, it resulted in a roughened surface of randomly assembled nanosheets. This strategy showed how a trace level Ni could influence the OER performance of a moderately active Cu and its oxides/hydroxides. Hence, by intentionally incorporating more and more Ni into Cu while simultaneously supplementing sufficient Fe, one can achieve better active anode for OER in alkaline medium. ASSOCIATED CONTENT Supporting Information Available Details of materials used, graphs related to CVs of Cu foam and Cu foil (99.96%), CA, post-CA LSV, post-CA Nyquist plot and Fe 2p3/2 XPS spectrum are provided. This material is available at the journals home page. ACKNOWLEDGEMENTS This work is supported by the grant-in-aid (JP19F18346) from the Japan Society for the Promotion of Science (JSPS). S.A is thankful to the JSPS for the award of standard postdoctoral

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fellowship (Fellowship ID: P18346). Authors thank Mr. Shoei Yamaoka of Noda-Hanada Lab for the help in acquiring XRD data. REFERENCES [1]

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CRediT author statement Sengeni Anantharaj: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization Funding acquisition Hisashi Sugime.: Investigation Bozhi Chen.: Investigation Natsuho Akagi.: Investigation Suguru Noda.: Supervision, Project administration, Funding acquisition, Writing - Review & Editing.

Graphical Abstract

A Cu foam that contains a trace amount of Ni upon anodization in alkali with sufficiently supplemented Fe resulted in humongous enhancement in OER activity.

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