Shell

Letter pubs.acs.org/NanoLett

Three-Dimensional Crystalline/Amorphous Co/Co3O4 Core/Shell Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction Xiaodong Yan,† Lihong Tian,†,‡ Min He,†,§ and Xiaobo Chen*,† †

Department of Chemistry, University of MissouriKansas City, Kansas City, Missouri 64110, United States Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Hubei University, Wuhan 430062, China § Wuhan University of Science and Technology, Wuhan, Hubei 430081, China Downloaded via KAOHSIUNG MEDICAL UNIV on September 30, 2018 at 08:01:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Earth-abundant, low-cost electrocatalysts with outstanding catalytic activity in the electrochemical hydrogen evolution reaction (HER) are critical in realizing the hydrogen economy to lift our future welfare and civilization. Here we report that excellent HER activity has been achieved with three-dimensional core/shell Co/Co3O4 nanosheets composed of a metallic cobalt core and an amorphous cobalt oxide shell. A benchmark HER current density of 10 mA cm−2 has been achieved at an overpotential of ∼90 mV in 1 M KOH. The excellent activity is enabled with the unique metal/oxide core/shell structure, which allows high electrical conductivity in the core and high catalytic activity on the shell. This finding may open a door to the design and fabrication of earth-abundant, low-cost metal oxide electrocatalysts with satisfactory hydrogen evolution reaction activities. KEYWORDS: Cobalt/cobalt oxide, crystalline/amorphous core/shell, three-dimensional nanosheets, hydrogen evolution reaction, electrocatalysis

C

Although earth-abundant, low-cost transition metal oxides, such as nickel oxide and cobalt oxide, have been studied as HER catalysts in alkaline media for many years, these oxides showed poor HER activities. Recently, metal−metal oxide/ carbon hybrids (Ni−NiO/carbon nanotube and Co−CoOx/Ndoped carbon) created by thermal decomposition showed excellent HER activities in alkaline electrolytes as a result of the synergetic effect of the metal, metal oxide, and functional carbons,24,25 reviving these metal oxides as promising HER catalysts and triggering new research interest in these compounds. Despite these great achievements, new strategies still must be developed to advance these metal oxides in practical applications. Inspired by the amorphous molybdenum sulfide that showed enhanced HER activity over the crystalline counterpart,26−28 we anticipate that amorphous metal oxides covered on metallic cores may be of high promise in achieving high HER activities under alkaline conditions. In this study, we report the realization of such a novel catalytic system, that is, three-dimensional (3D) crystalline/ amorphous Co/Co3O4 core/shell nanosheets, with excellent HER performance. The 3D Co3O4 nanosheets are grown on Ni foam using a highly scalable solution growth method, and then

ost-effective, large-scale production of hydrogen is one of the keys to realizing the hydrogen economy in the decades-long dream of a future with clean and renewable energy and environment.1−4 Hydrogen evolution from water by electrolysis has been suggested as one of the viable ways to produce hydrogen on a large scale.4 Noble metals, especially platinum, are still regarded as the best electrocatalysts for efficient hydrogen production.5−8 However, their limited abundance and high cost have inevitably prevented the practical realization of the dreamed hydrogen economy. Thus, developing earth-abundant, low-cost electrocatalysts with high catalytic activity in the electrochemical hydrogen evolution reaction (HER) is critical in realizing the hydrogen economy to lift our future welfare and civilization. Over the past years, tremendous effort has been devoted to the exploration of noble-metal-free HER catalysts, and great progress has been achieved with discoveries of highly active earth-abundant transition-metal-based materials such as phosphides,9−15 dichalcogenides,16−20 and Ni−Mo alloys.21−23 Most of these catalysts are stable and/or have high activities under acidic conditions, and only very few of them possess high and stable activities in alkaline media. However, hydrogen produced by electrolysis under alkaline conditions is widely employed in industries at the cost of a moderate amount of energy.24 Therefore, the importance of developing HER catalysts that are highly efficient in alkaline electrolytes has always been emphasized. © 2015 American Chemical Society

Received: June 4, 2015 Revised: August 20, 2015 Published: August 21, 2015 6015

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Figure 1. (A, B) SEM and (C, D) TEM images of Co3O4 nanosheets. (E, F) SEM and (G, H) TEM images of Co/Co3O4 nanosheets formed after heating of Co3O4 nanosheets in hydrogen at 200 °C for 3 h.

overpotential of ∼90 mV in 1 M KOH. More importantly, the Co/Co3O4 nanohybrids exhibit a very small onset potential (∼30 mV). The excellent HER activity is enabled by the unique structure of the 3D crystalline/amorphous Co/Co3O4 core/ shell nanosheets, which allow high electrical conductivity in the core and high HER activity on the surface. Thus, this study may open a new avenue for the development of earth-abundant, low-cost electrocatalysts with high HER activities.

the 3D Co/Co3O4 core/shell nanosheets are achieved by a lowtemperature hydrogen reduction. The hydrogen reduction leads to the formation of a 3D metallic main frame for higher electrical conductivity, and the thin amorphous cobalt oxide surface layer enriched with hydroxyl groups induces high HER activity. Excellent HER activity in alkaline media is obtained by adjusting the composition and structure of the Co/Co3O4 nanohybrid with the reduction conditions. A benchmark HER current density of 10 mA cm−2 has been achieved at an 6016

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Figure 2. (A) XRD patterns [(○) Co3O4 (JCPDS no. 43-1003); (#) Co (JCPDS no. 05-0727)] and (B−D) XPS spectra [(B) survey; (C) Co 2p; (D) O 1s] of (a) Co3O4 and (b) Co/Co3O4 nanosheets.

The 3D Co3O4 nanosheets were first grown on nickel foams by reacting cobalt nitrate with hexamethylenetetramine at 90 °C in a water/ethanol solution with immersed nickel foams, followed by calcination at 300 °C in air. Their morphologies were checked with scanning electron microscopy (SEM). The surface of the nickel foam was largely covered with nanosheets (Figure 1A), and the nanosheets self-assembled into flowerlike structures (Figure 1B). On the other hand, the bare Ni foam (Figure S2 in the Supporting Information) and the Ni foam treated in a water/ethanol solution containing sodium nitrate and hexamethylenetetramine (Figure S3) displayed smooth surfaces, indicating that the nanosheets were not grown from the Ni foam substrates but were deposited by the reaction of cobalt nitrate with hexamethylenetetramine. The powders precipitated from the solution were annealed under the same conditions for transmission electron microscopy (TEM) studies. The TEM images showed that the nanosheets were composed of many small (5−8 nm in diameter) nanoparticles (Figure 1C shows a corner of one Co3O4 nanosheet; more images are shown in Figure S4). These nanoparticles were wellcrystallized with clearly resolved lattice fringes of Co3O4 (Figures 1D and S3). The adjacent-plane distance of 4.65 Å corresponds well to the (111) plane of Co3O4 (JCPDS no. 431003) (Figure 1D). The 3D Co/Co3O4 core/shell nanosheets were obtained by chemical reduction of the above Co3O4 nanosheets at 200 °C for 3 h in a hydrogen atmosphere. After hydrogen reduction, the Ni foam was still well-covered (Figure 1E), and the nanosheet morphology and self-assembled flowerlike structures were maintained (Figure 1F). However, the nanosheets were apparently composed of some round and large (20−50 nm) nanoparticles that were covered with a thin layer of amorphous material, displaying a core/shell structure (Figures 1G and S5). The high-resolution TEM (HRTEM) image further showed

that the primary nanoparticles had a crystalline core and an amorphous shell (Figure 1H). The core had clearly observable lattice fringes with adjacent-plane distance of 2.17 Å, which matched well with the (100) plane of Co crystal (JCPDS no. 05-0727). Thus, the core was Co metal. The Co nanoparticles likely had crystalline domains around 5−7 nm in diameter (Figure S5C), although the size of aggregated crystalline particles could extend to 20−30 nm in diameter (Figure 1H). The shell had a thickness of about 2−5 nm and an amorphous characteristic, and it was likely Co3O4 and/or CoO, as indicated by the results of X-ray photoelectron spectroscopy (XPS). Thus, the Co/Co3O4 nanosheets were likely made of small Co/ Co3O4 nanoparticles with a crystalline Co core and an amorphous Co3O4 and/or CoO shell. The X-ray diffraction (XRD) pattern of the Co 3 O 4 nanosheets (curve a in Figure 2A) matched well with the standard XRD pattern (JCPDS no. 43-1003) (Figure S6A). The apparent diffraction peaks indicated well-crystallized Co3O4 nanosheets. The grain size was calculated using the Scherrer equation: τ = kλ/(β cos θ), where τ, k, λ, β, and θ are the mean size of the ordered (crystalline) domains, the shape factor (with a typical value of 0.9), the X-ray wavelength, the line broadening full width at half-maximum (fwhm) in radians, and the Bragg angle, respectively.29 The calculated grain size was 6.6 nm, consistent with the TEM observations. For the Co/Co3O4 nanosheets (curve b in Figure 2A), only one peak corresponding to the (311) plane of Co3O4 was observed, and other diffraction peaks were attributed to Co (JCPDS no. 050727). The decreased (311) peak of Co3O4 and the disappearance of other peaks suggested that the crystallinity of Co3O4 was largely reduced while some crystalline Co3O4 particles might still exist due to the partial reduction under these conditions. The calculated grain size of the crystalline Co was 4.8 nm. Combined with the information from the TEM 6017

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Figure 3. Electrochemical characterization of Co/Co3O4 nanosheets obtained at 200 °C in hydrogen toward the HER. (A) Polarization curves of bare Ni foam, Co3O4 nanosheets, Co/Co3O4 nanosheets, and Pt wire. (B) Tafel plots derived from (A). (C) Current−time characteristics of Co/ Co3O4 nanosheets at an overpotential of 120 mV. (D) Polarization curves of Co/Co3O4 nanosheets before and after the current stability test.

crystalline phase to the amorphous phase. Meanwhile, the intensity of the hydroxyl groups near 531.1 eV increased dramatically, indicating that many more hydroxyl groups and likely more oxygen vacancies were created on the amorphous Co3O4 surface layers, consistent with the observations for hydrogenated metal oxides according to previous studies.37−39 The catalytic properties of the Co3O4 and Co/Co3O4 nanosheets grown on Ni foams were investigated in a 1.0 M KOH solution using a typical three-electrode system. Bare Ni foam and commercial Pt/C were also tested for comparison. Potentials are reported versus the reversible hydrogen electrode (RHE). The polarization curves are shown in Figure 3A. All of the polarization data are iR-corrected. The Co/Co3O4 nanosheets exhibited a remarkably high activity with an onset potential of ∼30 mV. To achieve an HER current density of 20 mA cm−2, an overpotantial of as small as 129 mV was needed. In comparison, Co3O4 nanosheets had an overpotential of 302 mV, and bare Ni required 390 mV at 20 mA cm−2. The catalytic activity of Co/Co3O4 nanosheets was also much better than that of the cobalt−cobalt oxide/N-doped carbon hybrids (CoOx@CN), which were the best cobalt oxide-based catalysts reported to date.24 Although their activity was still slightly lower than that of Pt/C, the Co/Co3O4 nanosheets reported here were among the best alkali-based Pt-free catalysts studied to date and even comparable to the best Pt-free acid-based catalysts, as shown in Table 1. The linear regions of the Tafel plots (Figure 3B) were fitted to the Tafel equation (η = b log(j) + a, where b is the Tafel slope). The values of the Tafel slope for Pt/C, Co/Co3O4 nanosheets, Co3O4 nanosheets, and Ni foam were 28, 44, 49, and 83 mV/decade, respectively. This confirmed that the Co/ Co3O4 nanosheets had a high HER activity. Generally, there are

observations, this suggested that one Co/Co3O4 particle contained several smaller crystalline Co nanograins/nanoparticles (Figure 1F). XPS survey spectra of both Co 3 O 4 and Co/Co 3 O 4 nanosheets are shown in Figure 2B. The spectra were similar: signals from Co and O were observed with C deposition from the atmosphere. All of the spectra were calibrated with the C 1s peak at 284.6 eV. Figure 2C displays the Co 2p core-level XPS spectra. Both the Co3O4 and Co/Co3O4 nanosheets had two typical primary peaks from Co in Co3O4: Co 2p1/2 (795.0 eV)30 and Co 2p3/2 (779.8 eV).31 However, the corresponding satellites at 803.7 and 789.2 eV,32 respectively, for Co3O4 nanosheets shifted to 802.1 and 785.6 eV for the Co/Co3O4 nanosheets. These shifts were likely due to the influence of the Co metal on the Co3O4 surface layer, which showed some CoO features.33 The existence of the metallic Co core in the Co/ Co3O4 nanosheets was seen from the weak shoulder peaks around Co 2p1/2 at 793.5 eV and Co 2p3/2 at 778.5 eV.30,34 Since XPS can only probe chemical information within a few atomic layers near the surface, these results thus confirmed that the Co/Co3O4 nanosheets had a crystalline Co core and an amorphous Co3O4 shell. The O 1s XPS spectra are shown in Figure 2D. The Co3O4 nanosheets had two peaks near 529.2 and 530.2 eV, attributed to the lattice O2− in Co3O4,35,36 and one small shoulder near 531.1 eV from the hydroxyl groups on the surface.37−39 For the Co/Co3O4 nanosheets, the O2− peak shifted to 529.6 eV. This was explained as follows: the O2− ions face two different environments in Co3O4, being bonded to Co2+ and Co3+ (Co3O4 = CoO + Co2O3); these two bonding environments are merged into one in the amorphous Co3O4 shell in the Co/Co3O4 nanosheets, likely as a result of the apparent alteration in lattice structure in going from the 6018

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possible structural changes that caused the slight degradation in the polarization curves before and after stability test. As suggested from the XRD patterns (Figure S7), the content of the metallic cobalt in Co/Co3O4 nanosheets slightly increased after the stability test, but the diffraction peaks corresponding to Co3O4 did not show an apparent change after the stability test. As the crystalline Co3O4 is likely located in the bulk on the basis of previous analysis, this indicated that the increased metallic cobalt was more likely derived from electrochemical reduction of the amorphous domain near the surface. Thus, the slight reduction of the amorphous Co3O4 shell to Co metal in the shell likely led to the gradual degradation in performance. A slight difference was observed in the polarization curves of the Co/Co3O4 electrode before and after the stability test (Figure 3D). After the stability test, the HER current slightly decreased at overpotentials below 200 mV but increased at overpotentials above 200 mV. This was likely a result of the above gradual composition change due to the emergence of more Co from the amorphous Co3O4 layer. On the other hand, these results suggested that the metallic cobalt contributed to a higher current at higher overpotentials, while the amorphous Co3O4 lowered the overpotential for the HER process. In other words, the metallic Co could act as a current reservoir while the amorphous Co3O4 shell could reduce the energy barrier for the HER. The reduction temperature in hydrogen on Co3O4 nanosheets had a significant influence on the HER activity of the formed Co/Co3O4 nanosheets. All of the Co3O4 nanosheets reduced in hydrogen had better HER performances than pristine Co3O4 nanosheets. The sample treated at 200 °C for 3 h (Co/Co3O4-200) displayed a better HER performance than

Table 1. Comparison of the HER Performances of the Electrodes Reported Recently catalyst

mass loading (mg cm−2)

electrolyte

η20 (mV)a

ref

NiFe LDH FeP NiP CoP CoOx/CNb NiO/Ni-CNTc Co/Co3O4

− − 1 0.92 0.42 0.28 0.85

1 M NaOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 1 M KOH

∼250 ∼320 130 100 ∼270 ∼120 129

2 10 11 12 24 25 this work

η20 = overpotential at an HER current density of 20 mA cm−2. bCN = N-doped carbon. cCNT = carbon nanotube.

a

two mechanisms involving the HER process in alkaline media, namely, the Volmer process (electrochemical hydrogen adsorption (Hads): H2O + e− → Hads + OH−) followed by either the Heyrovsky process (electrochemical desorption: Had + H2O + e− → H2 + OH−) or Tafel process (chemical desorption: Hads + Hads → H2).40,41 A Tafel slope of 120, 40, or 30 mV/decade would be expected if the Volmer, Heyrovsky, or Tafel step, respectively, is the rate-determining step.40−42 Thus, the electrochemical hydrogen desorption (Heyrovsky process) was the rate-determining step for the HER on both Co/Co3O4 and Co3O4 nanosheets. The stability of Co/Co3O4 nanosheets was evaluated using a constant-voltage technique, and the results are shown in Figure 3C. At a constant overpotential of 120 mV, the current showed a slight degradation during a long period of 6000 s. We performed XRD measurements on the Co/Co3O4 nanosheets after the stability test to reveal the

Figure 4. Electrochemical characterization of various Co/Co3O4 nanosheets obtained by annealing of Co3O4 nanosheets in hydrogen at 150 °C (Co/Co3O4-150), 200 °C (Co/Co3O4-200), and 300 °C (Co/Co3O4-300). (A) Polarization curves. (B, C) Nyquist plots at overpotentials of (B) 20 mV and (C) 220 mV. (D) Relationship between the HER current density and the charge-transfer resistance (Rct) at an overpotential of 220 mV. 6019

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Nano Letters those reduced at 150 °C (Co/Co3O4-150) and 300 °C (Co/ Co3O4-300) (Figure 4A). No obvious metallic cobalt was detected in Co/Co3O4-150, while only cobalt XRD peaks were observed in Co/Co3O4-300 (Figure S6A). A thin layer of amorphous phase was also observed for Co/Co3O4-300 with HRTEM (Figure S6B). Apparently, increasing the hydrogen treatment temperature increased the percentage of crystalline Co3O4 being reduced. The remaining crystalline Co3O4 was likely located in the bulk, as only a thin layer of amorphous phase (HRTEM) with Co3O4 chemical features (XPS, surface chemical information) was observed. This indicated that a suitable balance between Co and Co3O4 in the catalyst is crucial for high catalytic activity, likely from a synergistic effect of Co and amorphous Co3O4. The remarkably enhanced HER activity of the nanocomposites was explained with the synergistic effect between the metal core and the amorphous metal oxide shell, where the Co core could lower the internal resistance and act as an electron reservoir to increase the HER current density, and the amorphous Co3O4 shell could lower the gas desorption energy barrier for higher HER activity (based on Tafel analysis). The sample reduced at 200 °C for 3 h might have the optimal core/shell configuration to take full advantage of this synergistic effect. It is worth noting that a reduction peak was clearly observed prior to hydrogen evolution for all of the Co/Co3O4 nanosheets (Figure S8). This reduction peak was dependent on the reduction temperature, that is, Co/Co3O4150 had a negligible reduction peak, Co/Co3O4-200 had a very obvious reduction peak, and then the reduction peak became smaller as the temperature increased. That reduction peak could be from the reduction of Co2+ to Co in the amorphous Co3O4 shell, as the standard Co2+/Co redox potential is −0.28 V vs NHE, but it might also be caused by the hydroxyl groups and/or oxygen vacancies as suggested previously, which showed a pseudocapacitive behavior generated by the hydrogen reduction.43,44 To reveal the HER kinetics on the surface of the catalysts, electrochemical impedance spectroscopy (EIS) analyses were performed. At an overpotential of 20 mV, two semicircles were observed in the Nyquist plots (Figure 4B), indicative of twotime-constant behavior. The low-frequency semicircle was correlated with hydrogen adsorption onto the electrodes to form hydride-type species, while the high-frequency semicircle was associated with the charge-transfer resistance.41,45 Both the adsorption impedance (Rad) and charge-transfer resistance (Rct), especially the former, experienced a tremendous decrease for Co/Co3O4 compared with Co3O4. The onset potential displayed a substantial correlation with Rad, that is, the smaller the Rad, the more positive the onset potential was (Figure S9). This explained that the highly positive-shifted onset potential of Co/Co3O4 was due to lowering of the H2 adsorption energy barrier, consistent with previous Tafel analysis results. At a high overpotential of 220 mV, Bode plots suggested a one-timeconstant process for all of the electrodes (Figure S10), and Nyquist plots showed only one semicircle (Figure 4C). This suggested that the kinetic impedance played a critical role in determining the HER kinetics. To clearly reveal the relationship between the HER current density (j) and Rct, the j−Rct plot was made (Figure 4D). The smaller the Rct, the larger the HER current density was. Therefore, the excellent HER performance of Co/Co3O4-200 was likely due to the low Rad and the fast charge-transfer kinetics on the surface of the electrode caused by the hydroxyl-enriched amorphous cobalt oxide. The diminished Rad was favorable for the Volmer process, while

the decreased Rct facilitated both the Volmer and Heyrovsky processes. It is expected that the Volmer−Heyrovsky process was greatly accelerated at the Co/Co3O4 interface. On the other hand, it was reported that hydroxide clusters facilitated the dissociation of water by weakening the O−H bond of the absorbed water46,47 and that more Lewis acidic groups promoted the activation of Lewis basic H2O through Lewis acid−base interactions, thus leading to improved HER activity, as the HER process involves the adsorption of water onto the surface of the catalyst and breaking of the O−H bond of the absorbed water.42 We also noticed that the possible oxygen vacancy alone was unlikely to facilitate the HER process because the Co3O4 nanosheets that were vacuum-treated at 200 °C, which likely had more oxygen vacancy defects on the basis of previous studies,48,49 had a lower catalytic activity than the Co3O4 nanosheets (Figure S11). However, it cannot be neglected that the oxygen vacancies may also play a role in enhancing the adsorption of water through the O atom, as oxygen-vacancy-bearing oxide enables a more positive potential on the surface.48,50 Thus, it can be concluded that the highly enhanced charge-transfer kinetics originated from the synergetic effect of the metallic cobalt and the amorphous cobalt oxide in the unique 3D core/shell nanosheet structure. The metallic core enhanced the electrical conductivity and acted as an electron reservoir to ensure a high current density, and the hydroxyl-enriched amorphous surface layer (along with some possible oxygen vacancies) favored the adsorption of water molecules and lowered the HER energy barrier. In summary, we have demonstrated a high-performance HER electrocatalyst in alkaline media with 3D Co/Co3O4 metal/ metal oxide core/shell nanosheets. Co3O4 nanosheets are first grown directly on Ni foam. Low-temperature treatment in hydrogen effectively induces the structural evolution from Co3O4 to Co nanosheets, leading to well-tuned cobalt/ amorphous cobalt oxide hybrids. The best catalytic performance was achieved with a near-zero onset potential and an HER current density of 20 mA cm−2 at an overpotential of 129 mV. The high HER activity is attributed to the synergetic effect of the metallic core and the amorphous oxide shell in providing both good bulk conductivity and surface activity. Compared with Pt, cobalt and cobalt oxide possess the advantages of large abundance and low cost, and thus, these 3D crystalline/ amorphous Co/Co3O4 core/shell nanosheets hold a promising future as practical catalyst candidates for hydrogen evolution from water electrolysis. Therefore, this study may provide us a new strategy for the design of new earth-abundant, low-cost catalysts toward the realization of a hydrogen economy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02205. Materials and Methods and Figures S1−S11 (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6020

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ACKNOWLEDGMENTS X.C. acknowledges the support from the College of Arts and Sciences, University of MissouriKansas City, the University of Missouri Research Board (UMRB), and the University of Missouri Interdisciplinary Intercampus (IDIC) Program. M.H. appreciates the financial support from the China Scholarship Council for overseas research. L.T. thanks the National Natural Science Foundation of China (51302072) and the China Scholarship Council for financial support.

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DOI: 10.1021/acs.nanolett.5b02205 Nano Lett. 2015, 15, 6015−6021