Dual Modulation via Electrochemical Reduction Activiation on

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Dual Modulation via Electrochemical Reduction Activiation on Electrocatalysts for Enhanced Oxygen Evolution Reaction si liu, Han Cheng, kun xu, Hui Ding, jingyi zhou, bojun liu, Wangsheng Chu, Changzheng Wu, and Yi Xie ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Energy Letters

Dual Modulation via Electrochemical Reduction Activiation on Electrocatalysts for Enhanced Oxygen Evolution Reaction Si Liu,§† Han Cheng,§† Kun Xu, † Hui Ding,† Jingyi Zhou, † Bojun Liu, † Wangsheng Chu,Δ Changzheng Wu,*† and Yi Xie† †

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Δ

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, PR China ABSTRACT: Cobalt based spinel are promising candidates for OER. However, these electrocatalysts usually suffer from the lack of active species transformation during OER due to the high stability and reversibility of spinel structure. Herein, we highlight that electrochemical reduction could serve as general activation process with dual modulation on spinel and enhance the catalytic activity. Taking Co3O4 as an example, both rock salt generation and increased surface roughness could be realized. Moreover, due to the synergistic effect of dual modulations, surface rock salt could be massively generated, which breaks the scale limitation of chemical reduction. With optimal modulation, electrochemical reduced Co3O4 showed lower overpotential and enhanced OER activity. Furthermore, electrochemical reduction activation could be extended to other spinel oxides. We anticipated that electrochemical reduction activation would be a facile route to realize efficient electrocatalytic performance and offers new insights toward the correlation among processing methods, active sites and catalytic performance.

With increasing concern of global warming and energy crisis, it is even more crucial to develop the supply of clean resources such as hydrogen energy1-4. Electrocatalytic water splitting is regarded as a promising way to produce clean energy through two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)5-9. It is well known that oxygen evolution reaction (OER) is kinetically sluggish with O−H bond breaking and O−O bond formation due to a multiple proton coupled electron transfer process10-13. Thus, researchers contributed to develop catalysts with high activity and stability for OER. However, until now, noble metal and their compounds are still considered as the most active electrocatalysts for OER while the high price and element scarcity seriously hinder their practical application for water splitting.14,15 In this regard, it is still crucial to design highly active OER catalysts based on non-noble metals. As an alternative to noble metal catalysts, spinel transition metal oxides have been studied intensely for OER in the past few decades. Cobalt based spinel oxides, especially Co3O4, has been regarded as promising catalyst for the oxygen evolution reaction due to earth abundance and superior catalytic properties16-18. For better understanding the mechanism of the

superior catalyatic performance of Co3O4, many work have been reported19. As is well known that, the real catalytic active species of cobalt oxides in alkaline medium are determined to be hydroxide/oxyhydroxide20. Moreover, the valence state of cobalt was also proved to have great impact on the generation of active species. Based on the mechanism researches, related strategies have been developed to further enhance the catalytic performance of Co3O4 catalysts, such as introducing defects, valence state regulation and so on21-23. Despite many efforts have been made, performance of developed Co3O4 catalysts for OER is still limited due to inadequate generation of active sites during OER process 23-25. Unlike the rock salt compounds, the catalyst with spinel structure shows instrict inertness and inhibits the in-suit transformation to layered hydroxide/oxyhydroxide structure during OER process20. As a method to modify the Co3O4 spinel structure for OER, the chemical reduction strategy has been found to introduce the low valence rock salt structure on the surface of Co3O426. However, the thickness of adapting active surface layer by chemical reduction is limited on scale with 1 nm and larger amount of reductant led to some undesired reactions. Thus, it is still urgent to develop an

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tween Co3O4 and CoO could be seen, representing the different thickness of Co3O4 and CoO (Surficial CoO seems to be thinner). Moreover, the different degree of crystallization was used to tell the interfaces between Co3O4 and CoO. Co3O4 possesses high crystallization while CoO shows poor crystallization with lots of boundaries. As for the ER-Co3O4 NWs-1 (300 segments), a rough surface layer of about 3~5 nm could be clearly observed in Figure 1b, the lattice space of surface layer was reduced to 0.21 nm compared to the 0.24 nm of pristine Co3O4, which could be attributed to the generation of numerous rock salt CoO islands. With increasing electrochemical reduction cycles to 450 segments, more rock salt islands with a 7~10 nm thickness were generated (Figure 1c and 1d) on the surface of Co3O4, while the islands kept stable without fusion. As the surface reduction cycles further extend to 600 segements, the ER-Co3O4 NWs-3 exhibits the surficial islands with a 12~15 nm thickness. The HRTEM results verified the surface roughness evolution of Co3O4 after the electrochemical reduction activation and the controllable generation of rock salt structure with the evolution of treated cycles. Valence and local structure information of cobalt in above catalysts were investigated by X-ray photoelectron spectra (XPS) and X-ray absorption fine structure (XAFS). As shown in Figure 2a, the fine-scanned Co 2p XPS spectra of the pristine Co3O4 NWs and the ER-Co3O4 NWs were fitted with different valences for investigation of the electronic states of Co atoms. Two fitted peaks for Co 2p1/2 are Co3+ (ca.795.2 eV) and Co2+ (ca. 797.0 eV) respectively27,28 and the area that the fitted curve covered shows the relative atomic ratio of surface Co2+/Co3+. The ratios of Co2+/Co3+ of the pristine Co3O4 is 0.55 and significantly increased with the increasing level of electrochemical reduction (0.86, 0.91 and 0.96 for ER-Co3O4 NWs-1, ER-Co3O4 NWs-2 and ER-Co3O4 NWs-3), representing more Co2+ was generated on the surface of ER-Co3O4 during the electrochemical reduction activation, contributed by the creation of low valence rock salt structure. As shown in Figure 2b, despite a slight left shift, the Co K-edge XANES spectrum of the ER-Co3O4 NWs were still similar to pristine Co3O4 NWs, suggesting that the lattice framework of the ERCo3O4 NWs was not changed while the surface was slightly reduced to lower valence during the electrochemical reduction activation. The corresponding Co K-edgek (k) oscillation

effective and facile method to boost a mass of rock salt structure generation breaking the scale limitation on the surface of spinel oxides with controllability. Herein, we highlight that electrochemical reduction could serve as an effective actviation methods, which has dual modulation on spinel and further enhance the catalytic activity. During electrochemical reduction activation process, both active structure transformation and increased surface roughness could be realized. More interestingly, due to the synergistic effect of these two modulations, active rock salt islands could be massively generated on the surface of electrocatalysts, which breaks the scale limitation of chemical reagent processing. Electrochemical results show that the electrochemical reduced Co3O4 nanowires (denoted as ER-Co3O4 NWs) exhibited a 10-fold enhancement of OER activity compared with the pristine Co3O4 nanowires. Moreover, the electrochemical reduction activation strategy could be widely applied to other spinel oxides, such as ZnCo2O4, NiCo2O4 and three dimension electrode. Our work provides a new method for activating spinel oxide catalysts to an optimal OER catalytic performances by electrochemical reduction activation and offers new insights toward the correlation among processing methods, active sites and electrocatalytic performance. To realize the dual modulation of structure and surface on spinel oxide, electrochemical reduction activation was applied on Co3O4 nanowires at voltage from 0V to -0.5V with different cycles, denoted as ER-Co3O4 NWs-1 (300 segments), ERCo3O4 NWs-2 (450 segments), ER-Co3O4 NWs-3 (600 segments), respectively. To verify the successful generation of rock salt CoO structure on ER-Co3O4 NWs surface, HRTEM was performed to study the surface microstructure of Co 3O4 after the electrochemical reduction activation. Figure 1a shows the HRTEM image of pristine Co3O4 nanowires with a lattice spacing value of 0.240 nm which could be ascribed to (311) plane. Meanwhile, the pristine Co3O4 samples show great crystallinity with smooth surface. After the electrochemical reduction activation, an obvious change of contrast be-


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ACS Energy Letters were recorded. As shown in Figure 3a, comparing to the pristine Co3O4 NWs, the ER-Co3O4 NWs-2 shows a greatly decreased overpotential of about 50 mV at 10 mA cm-2, even better than precious metal oxide IrO2 commercial electrocatalyst (1.574 V for ER-Co3O4 NWs-2, 1.626 V for the pristine Co3O4 NWs and 1.624 V for IrO2 electrocatalyst). Figure 3d exhibits that the OER current densities of the ER-Co3O4 NWs2 reaches nearly 10 times of the pristine sample at a constant overpotential of 400 mV in 1M KOH. Meanwhile, OER activity of these electrochemical reduced sample appear as a volcano-like trend with the increase of time, indicating the amount of cobalt oxide species was adjusted by the time control. As shown in Figure 3b, Tafel slope for the pristine sample and the ER-Co3O4 NWs-2 sample are 66.9 and 50.0 mV/dec, respectively, which are similiar with that of commercial IrO2. Tafel slope around 60 mV/dec suggests the step of the recombination of OH species after the first electron transfer step is the rate-determining step. All the samples have Tafel slope in 60mV/dec range, suggesting that the reaction mechanisms of their catalytic behavior are similar. According to previous reports, a smaller Tafel slope usually indicating that the rate-determining step is at the ending step of the multielectron transfer reaction and is considered to be a sign of effective catalysts. In addition, the Electrochemical Impedance Spectroscopy (EIS) was also performed. It is revealed by the Nyquist plots (Figure 3c) that the semicircular diameter of all the electrochemical tuned samples showed the same volcanolike trend as their OER performance, suggesting that the ERCo3O4 NWs-2 possessed the smallest charge transfer impedance. Moreover, electrochemical surface area of all these samples were determined by electrochemical double-layer capacitance. As shown in Figure S6, a continuous increase of electrochemical surface area (from 31.99 F/cm2 to 121.07 F/cm2) could be observed as electrochemical reduction cycles extend, which is corresponding to the increased surface roughness. However, the catalytic performance of all electrochemical reduced sample turn out to be a volcano trend against treated cycles, which could be attributed from excess amorphous CoO with many interface. Excess amorphous phase and interface may not be suitable for the charge transfer inside the electrocatalysts, thus the OER activity would not monotonically increase with ECSA, but show the antagonism effect of both ECSA and charge transfer. As a matter of fact, stability is also a key index to evaluate electrocatalysts. Thus we loaded the ER-Co3O4 NWs-2 onto carbon fiber paper (CFP) to assess its durability with a loading of 5 mg/cm2. The Chronoamperometry measurement (j−t) was performed at a constant voltage of 1.58 V vs RHE in 1 M KOH. As shown in Figure S7, the OER current density of ER-Co3O4 NW-2 could be maintained for 18h with little decay, demonstrating the great stability of our catalyst.

curves of the ER-Co3O4 NWs and pristine Co3O4 NWs were shown in Figure 2c while the ER-Co3O4 NWs showed a reduction in the oscillation amplitude comparing to the pristine Co3O4 NWs, suggesting a changed coordination environment of the Co atoms. The Fourier transformed k (k) functions (Figure 2d and Table S2) further clarified the amplitude reduction. To extract the quantitative local structures, the EXAFS data was fitted in the r space by the IFEFFIT code. The achieved local structure information of the ER-Co3O4 NWs showed a decreased coordination number compared to the pristine Co3O4 NWs (N=5.2) (Table S2). Moreover, the coordination number of the ER-Co3O4 NWs samples decreases with increasing electrochemical reduction time. The results of XAS further provided solid evidence for the generation of CoO species with lower valence via electrochemical reduction activation, which is consistent with the HRTEM results. Therefore, all above results suggested that a texture surface layer composed of numerous rock salt islands was successfully produced by electrochemical reduction activation process and precisely controlled active structure are vital for the following OER process. With the generation of numerous rock salt islands and increased surface roughness after electrochemical reduction, all the pristine and treated Co3O4 NWs are compared as electrocatalysts for alkaline OER. Electrochemical measurements on the pristine and the electrochemical reduced Co3O4 NWs were performed in 1M KOH solution. Dozens of CV cycles were performed initially to make sure the CV curves were stable before all the OER polarization curves



clearly demonstrate that the electrochemical reduction activation on electrocatalysts could significantly enhance the alkaline OER activity. For further introducing our strategy into the three-dimensional electrodes, the Co3O4 NWs were then grown on the Ni foam for OER electrocatalysis. As demonstrated in Figure S9, the X-ray diffraction (XRD) patterns support the formation of Co3O4 NWs/NF. The peaks at 44.5 degree and 51.8 degree could be ascribed to the Ni foam and other peaks match with Co3O4. The scanning electron microscopy (SEM) was performed to investigate the structure of the as prepared Co3O4 NWs/NF. As shown in Figure 4a, the Co3O4 NWs possess a

Rotating ring-disk electrode (RRDE) was finally used for analyzing the content of by-product (peroxide intermediates) generated at the surface of ER-Co3O4 NWs-2 and further understanding the reaction mechanism29,30. As shown in Figure 3e, the ring current (blue curve) possessed a relatively low value comparing to the disk current (grey curve) suggesting the negligible hydrogen peroxide formation. Moreover, the ring current decreased gradually with the increase of the potential, which suggested that fewer peroxide intermediates were formed at high potential region. This result suggests that the desirable four-electron transfer pathway of generating O2 for the OER process could lead to the spanking increase of the current density. Furthermore, to make sure that the observed current was generated from oxygen evolution reaction rather than other side reactions, the Faradaic efficiency was also calculated (Figure 3f). A ring potential of 0.55 V was applied to an RRDE to reduce the produced oxygen, representing a continuous oxygen evolution reaction (disk electrode) to oxygen reduction reaction (ring electrode) process. The oxygen generated from the ER-Co3O4 NWs-2 sweep across the Pt ring electrode under an ORR potential and get reduced rapidly. The disk current was held at 110 μA and consequently, the ring current was detected to be ∼39.96 μA (collection efficiency 0.37). The corresponding Faradaic efficiency of 98.1% indicated the detected oxidation current catalyzed by ER-Co3O4 NWs-2 catalyst could be ascribed to the oxygen evolution reaction process. All the above electrochemical measurements


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ACS Energy Letters to 0.24 nm which is constant with (101) plane of CoO(OH). It suggests that the rock salt CoO transfer into layered CoO(OH), which is regarded as the real active sites during OER. As for the pristine Co3O4 NWs, we could hardly see any CoO(OH) active species cover on the surface after OER (Figure 5a). Xray photoelectron spectra (XPS) were then performed to detect the surface property change of the pristine Co 3O4 NWs after OER and ER-Co3O4 NWs after OER samples. As demonstrated in Figure 5c and d, the ratios of surface Co2+/Co3+ of ERCo3O4 NWs-2 after OER was significantly decreased from 0.91 to 0.42, lower than that of pristine Co3O4, indicating more Co3+ was generated during OER and numerous CoOOH active sites were formed while the ratios of Co2+/Co3+ of pristine Co3O4 after OER is nearly the same. To further investigate local structure and chemical configuration conversion of the surface rock salt species during the OER process, the ex-suit X-ray absorption near-edge structure (XANES) measurements were performed with ER-Co3O4 NWs-2 and ER-Co3O4 NWs-2 after OER. As demonstrated in Figure 5e, the Co K-edge showed a slight shift towards higher energy, supposing an increase in the Co oxidation state. Meanwhile, The Fourier transformed k (k) functions of ER-Co3O4 NWs-2 after OER showed an increasing intensity around 2.4 Å comparing to ERCo3O4 NWs-2 (Figure 5f), which is due to the strengthened Co-Co bond corresponding to the added Co–Co edge-sharing polyhedra31. The EXAFS data was then fitted in the r space by the IFEFFIT code to extract the quantitative local structures. The coordination number of ER-Co3O4 NWs-2 after OER showed no significant change compared to the ER-Co3O4 NWs-2. No change in the coordination number of the first

width of 70 nm and a length of about 1 µm. The electrochemical reduction activation was then performed via the same process as above and the samples were denoted as ER-Co3O4 NWs/NF-1, ER-Co3O4 NWs/NF-2, ER-Co3O4 NWs/NF-3, respectively. As demonstrated in Figure 4b, all the electrochemical reduced samples showed greatly promoted electrocatalytic activity in OER with the same volcano trend as the free standing samples. The ER-Co3O4 NWs/NF-2 showed a significantly decreased overpotential of about 80 mV at 200 mA cm-2 compared with pristine Co3O4 NWs/NF. Furthermore, control experiments on bare Ni foam were performed. As observed in Figure 4b, contribution of Ni foam could nearly be ignored, and the performance of Ni foam showed barely any improvement after electrochemical reduction activation. Moreover, to verify that our electrochemical reduction activation is a general modification strategy for the spinel type materials, we also treated ZnCo2O4 nanowires and NiCo2O4 nanowires with the same activation process and further tested their electrocatalytic activity of oxygen evolution reaction. The electrochemical reduced samples also showed a significantly decreased overpotential compared with that of pristine NiCo2O4 and pristine ZnCo2O4 nanowires (Figure S16a and b). All these results suggest the generality of our reduction activation strategy, which could also be further extended to the current collection application. To understand the function of rock salt structure during the OER process, the ex-suit HRTEM was also performed to investigate the surface microstructure change after OER process. As shown in Figure 5b and Figure S17, the lattice space of the surface layer of ER-Co3O4 NWs after OER was increased



oxygen shell with increased Co oxidation state and the increased intensity around 2.4 Å indicate the development of a CoO(OH) layer31,32. Both XPS and XANES results indicate that the generated sub-nanometer CoO could transfom into CoO(OH) species which is crucial for OER catalysis while barely no CoO(OH) was generated on the surface of pristine Co3O4 NWs. Thus, more active sites originated from rock salt islands with higher intrinsic activity could be provided on ERCo3O4 NWs via electrochemical reduction activation and further enhance the OER performance. Based on the above results, the correlation between processing methods and active sites was first discussed. Different from the compact CoO layer generation by chemical reduction processing, the electrochemical reduction activation leads to the CoO islands with increased surface roughness. The formation of this special structure could be attributed from the cyclic voltammetry from -0.5~0V vs RHE, during which the cobalt atoms in Co3O4 first dissolved at low potential and deposited at high potential. Due to the point discharge effect, the cobalt atoms could concentrate at tip of surface and further grows as islands. With the island structure and reductive electric field, the Co3O4 could keep touching with electrolyte and generate CoO continuously, which could break the scale limitation in chemical processing. In this case, surficial CoO islands promotes the generation of active sites and subsurface spinel oxides facilitate charge transfer during electrochemical process. The spinel structure is stable during OER process, so that the integrated crystallization in core was benefited for charge transfer. On the contrast, if directly using pure CoO, the rock salt mainly transfers into CoOOH or amorphous phase during OER process (Figure S15a). Significantly increased interface and disorder would be unsuitable for charge transfer inside the electrocatalysts resulted in the inhibited catalytic performance (Figure S15b). Based on the above reason, as shown in Figure 2 and 3a, at beginning, the activity of ER-Co3O4 NWs increased with the amount of CoO species. However, after reaching a certain level, excessive CoO species might hinder the charge transfer, leading to an inhibited OER performance and turned out as a volcano trend. These results indicated the electrochemical activity of OER catalysts was contributed from both transformation of rock salt to the active oxyhydroxide phase and suppressed electronic charge transfer. With the controllable regulation of these two factors to a proper proportion, as the ER-Co3O4 NWs-2 sample, the electrochemical activity could reach the optimal value.

performance was disclosed. Electrochemical reduction activation paves a new way to modulate spinel oxide catalysts with superior OER catalytic performances and offers new insights toward the correlation among the processing methods, active sites and electrocatalytic performance.

ASSOCIATED CONTENT XRD patterns, TEM images and additional electrochemical date. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected];

Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2015CB932302), Natural Science Foundation of China (no. U1432133, 21331005, 21601172, U1532265, J1030412), National Program for Support of Topnotch Young Professionals, Chinese Academy of Science (XDB01020300) and the Fundamental Research Funds for the Central Universities (no. WK2060190027, WK2060190061)..

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In summary, we have demonstrated a general electrochemical reduction activation process of spinel oxide to greatly promote alkaline OER performance. Arising from the dual modulation of taxture surface and structure transformation in this process, the generation of numourous CoO islands with rock salt structure which breaks the scale limitation was realized on the surface of spinel Co3O4 for the first time, which is previously considered exceptionally challenging. With controllable reduction process, the optimal ER-Co3O4 NWs showed a 10-fold enhanced electrocatalytic activity. Electrochemical reduction activation strategy was widely applied to other spinel oxides such as ZnCo2O4 and NiCo2O4 and could be further extended to the three-dimensional electrodes. Moreover, based on the ideal material platform built on Co3O4, the relationship among processing methods, active sites and electrocatalytic


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