Letter www.acsami.org
Subnanometer Cobalt-Hydroxide-Anchored N‑Doped Carbon Nanotube Forest for Bifunctional Oxygen Catalyst Ji Eun Kim,† Joonwon Lim,† Gil Yong Lee,† Sun Hee Choi,‡ Uday Narayan Maiti,†,§ Won Jun Lee,† Ho Jin Lee,† and Sang Ouk Kim*,† †
National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Material Science and Engineering, KAIST, Daejeon 34141, Republic of Korea ‡ Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea S Supporting Information *
ABSTRACT: Electrochemical oxygen redox reactions are the crucial elements for energy conversion and storage including fuel cells and metal air batteries. Despite tremendous research efforts, developing high-efficient, low-cost, and durable bifunctional oxygen catalysts remains a major challenge. We report a new class of hybrid material consisting of subnanometer thick amorphous cobalt hydroxide anchored on NCNT as a durable ORR/OER bifunctional catalyst. Although amorphous cobalt species-based catalysts are known as good OER catalysts, hybridizing with NCNT successfully enhanced ORR activity by promoting a 4e reduction pathway. Abundant charge carriers in amorphous cobalt hydroxide are found to trigger the superior OER activity with high current density and low Tafel slope as low as 36 mV/decade. A remarkably high OER turnover frequency (TOF) of 2.3 s−1 at an overpotential of 300 mV was obtained, one of the highest values reported so far. Moreover, the catalytic activity was maintained over 120 h of cycling. The unique subnanometer scale morphology of amorphous hydroxide cobalt species along with intimate cobalt species−NCNT interaction minimizes the deactivation of catalyst during prolonged repeated cycles. KEYWORDS: amorphous metal oxide, cobalt hydroxide, bifunctional oxygen catalyst, carbon nanotubes, N-doping
A
rechargeable metal-air batteries, also suffer from multistep electron transfer with retarded kinetics.2 To date, a great deal of research efforts have been devoted to establish efficient and cost-effective catalysts for the oxygen involved redox reactions.2 Precious-metal-based catalysts, such as platinum,3,4 ruthenium, and iridium5 are well-known to exhibit the highest catalytic efficiencies for ORR and OER, respectively. Cost effective alternatives, such as nonprecious metals,6 their oxides7 and metal-free materials8 have also been widely exploited.9 Recently, bifunctional catalysts for ORR and OER attract a great deal of research attention, particularly for the applications in regenerative fuel cells and rechargeable metal-air batteries. Conventional approaches for the bifunctional catalysts have employed the simple physical mixing of ORR and OER catalysts to form a composite catalyst, which commonly have suffered from significant electron transfer resistance and low cycling stability.10 To address this issue, noticeable reports about one-step fabrication to hybridize OER and ORR electrocatalysts into one catalyst have been
long with the increasing demands for sustainable energy supply, regenerative, environmentally benign, and costeffective energy storage and conversion are becoming even more significant. Electrochemical systems may offer promising solution to this end, based on their high thermodynamic efficiency and environmentally benign process. Nonetheless, as electrochemical reactions are commonly limited by high activation barriers, electrocatalyst is a crucial element to determine the overall performance of an electrochemical system, including energy efficiency, conversion rate, durability, and cost effectiveness. Typical electrochemical reactions for energy conversion/ storage system consist of the redox reactions involved with hydrogen and oxygen. Hydrogen-involved reactions, such as hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER), generally undergo fast reaction kinetics involving a two-electron-transfer process. By contrast, oxygen reduction reaction (ORR), which is the cathodic reaction for fuel cells, metal-air batteries, and chlor-alkali electrolysis, requires a 4e process to modify the strong OO bond and thereby exhibit sluggish kinetics.1 Oxygen evolution reaction (OER), the reverse reaction of ORR widely employed for water splitting, solar fuel synthesis, reversible alkaline fuel cell, and © XXXX American Chemical Society
Received: October 27, 2015 Accepted: January 8, 2016
A
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. Cobalt-species-anchored NCNT hybrid catalyst. (a) Schematic illustration of hybrid catalysts synthesis. (b) TEM image of bare NCNT. (c) TEM image of as-synthesized Co(OH)x-NCNT. (d) TEM image of hybrid catalyst after a short heat treatment at 400 °C (CoOx-NCNT). (e) TEM image of hybrid catalyst after a short heat treatment at 700 °C (CoO-NCNT). (f) Scanning transmission electron microscopy high-angle annular dark-field (STEM-HADDF) image and energy-dispersive X-ray spectroscopy (EDS) mapping Co(OH)x-NCNT.
published.11−13 Meanwhile, durable single bifunctional catalyst for ORR and OER is a major challenge, as the catalyst should endure the alternate different reaction conditions for ORR and OER repeatedly. Alternatively, durable bifunctional catalysts with moderate catalytic activity were investigated recently.14,15 Herein, we report that subnanoscale amorphous cobalthydroxide (Co(OH)x) anchored N-doped carbon nanotubes (NCNTs) exhibits superior cycling stability with a moderate ORR activity and excellent OER activity in an alkaline condition (representative published bifunctional oxygen catalysts are listed as Table S1). The typical OER catalytic activity of amorphous metal oxides16 and metal hydroxides17 can be effectively supplemented with additional ORR activity by the hybridization with N-doped carbon species. Significantly, the lowered work function of CNTs via N-doping enhances the 4e reaction pathway for ORR. Moreover, N-dopants also greatly strengthen the coupling between subnanometer scale Co(OH)x and NCNT for a better reliability in the harsh condition of repeated OER and ORR cycles. Vertical NCNT forests were grown by plasma enhanced chemical vapor deposition.18 Amorphous Co(OH)x anchored NCNT forests were prepared by simple solution immersion process (Figure 1a, see the Supporting Information for synthetic details). Noteworthy that cobalt ions in the precursor solution are stable at room temperature but spontaneously
deposited at NCNT surface as cobalt (II, III) hydroxides, as temperature was increased above ∼45 °C. It is well-known that the pH value, where MxOy and Mz+ are in equilibrium, depends on the activity of metallic ion and temperature.19 The deposition was adjusted to proceed very slowly for the fine control of the deposition thickness and surface coverage. The residual water trapped in the hybrid catalyst forest was removed by rapid vacuum drying20 to maintain the vertical forest morphology (Supporting Information, Figure S1a, b). We prepared a set of hybrid catalysts, including Co(OH)x anchored CNT and NCNT (Co(OH)x-CNT and Co(OH)x-NCNT), amorphous cobalt oxide anchored NCNT (CoOx-NCNT) by annealing Co(OH)x-NCNT at 400 °C, and cobalt oxide nanocrystal anchored NCNT (CoO-NCNT) by annealing Co(OH)x-NCNT at 700 °C (Figures 1b−e). The thermal annealing was conducted under vacuum. The amorphous nature of Co(OH)x on NCNT/CNT was confirmed by transmission electron microscopy (TEM) and Xray diffraction (XRD) spectra (Figure S2a). Due to the dense nucleation and slowly controlled deposition rate, NCNTs were well-covered with amorphous Co(OH)x nano patches with trace of overstacked Co(OH)x nano patches after a sufficient deposition time (Figure 1c, Figure S2). As the Co(OH)xNCNT was rapidly heat-treated at 400 °C, coated morphology was maintained, and crystallization started within coated B
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
In the high binding energy (BE) region, presence of the multiple electron excitation, commonly known as the shakeup satellites, can be observed near 785 eV. This is frequently used to distinguish the elemental compositions between Co(II) oxides and Co(III) oxides, as the latter do not have the multiple electron excitations. As the hybrid catalyst was rapidly heat treated at 400 and 700 °C, Co(III) peak (779.5 eV) gradually decreased, and Co(II) peak (781.0 eV) increased together with its shakeup satellites. Meanwhile, Co−Nx peak (781.8 eV)22 is detected in the Co 2p spectra of the three hybrid catalysts with NCNT, but not in Co(OH)x-CNT. We could confirm the existence of Co−Nx interaction from N 1s spectra (Figure S6a). From the comparison between bare NCNT and Co(OH)xNCNT, N-dopant peaks for pyridinic-N (398.3 eV), pyrrolic-N (399.9 eV), graphitic-N (401.0 eV), and oxidized N (403.3 eV) were detected in both cases, whereas only Co(OH)x-NCNT showed the N−Co peak (399.3 eV).22 In Figure 2b, the O 1s spectra of the Co(OH)x-NCNT exhibit two major peaks at 531.3 and 529.5 eV, which correspond to hydroxide (or defective oxide) and oxide, respectively. A minor peak is also detected around 532.5 eV, which is typically associated with water and oxygen from adsorbed organic compounds. As Co(OH)x-NCNT is was rapidly heat-treated at 400 and 700 °C, the hydroxide and defective oxide peak decreased while oxide peak increased. The ratios of curve-fitted Co 2p and O 1s peaks for Co(OH)x-CNT, Co(OH)x-NCNT, CoOx-NCNT, and CoO-NCNT are summarized in Table S2. For Co(OH)x-CNT, Co 2p and O 1s spectra (Figure S6b ) show similar features with Co(OH)xNCNT spectra, but Co-Nx peak is missing in the Co 2p spectra. As XPS is surface-sensitive spectroscopy, we also carried out Co K-edge X-ray absorption near edge structure (XANES) spectroscopy to clarify the ratio of Co(III) hydroxide to Co(II) hydroxide in Co(OH)x-NCNT, because XANES is an element specific, crystallinity-independent, and local bulk structuredetermining probe. In Figure 2c, two reference samples of Co(OH)2 and Co(OH) 3 along with Co(OH) x -NCNT exhibited a characteristic pre-edge peak denoting a quadrupole transition of 1s → 3d at 7110 eV. The Co K-edge XANES spectrum of Co(OH)x-NCNT is close to the reference Co(OH)3, but its peak position is slightly lower than that of Co(OH)3, and the intensity of the peak for dipole transition at 7730 eV is weaker than that of Co(OH)3. Linear combination fitting was performed in order to determine its ratio of Co(III) hydroxide to Co(II) hydroxide by adopting two reference spectra of Co(OH)2 and Co(OH)3, resulting in deposited amorphous cobalt hydroxide consisting of 78% Co (III) hydroxide and 22% Co (II) hydroxide with 1% of uncertainty. From the above spectroscopy results, Co(OH)x as-deposited on NCNT is confirmed to be amorphous (Figure S3b) cobalt hydroxide consisting of 76−78% Co (III) hydroxide and 22− 24% Co (II) hydroxide, which is denoted by Co(OH)x-NCNT (x is ∼2.8). As we rapidly heat-treated the as-prepared catalyst at 400 °C, cobalt hydroxide is decomposed into oxide to form amorphous CoOx-NCNT (x is ∼1.3) consisting of 59% Co (III) oxide and 41% Co(II) oxide. After a higher temperature rapid heat treatment at 700 °C, amorphous cobalt oxide species transform into nanoscale crystalline CoO particles. Although minor trace of amorphous CoOx remain at the surface of nanoparticles. (Figure 1e), 700 °C rapid heat treated sample is simply denoted as CoOx-NCNT. The ORR catalytic activity of hybrid catalyst was evaluated by cyclic voltammetry (CV). Co(OH)x-NCNT shows (Figure 3a)
amorphous nano patches. (Figure S4) However, when the Co(OH)x-NCNT was rapidly heat-treated at 700 °C, the deposited layer no longer shows amorphous structure but becomes sparse distributed nanoscale crystalline particles to minimize the surface energy (Figure S5). In the TEM analysis (Figure 1e), the lattice spacing of 0.21 and 0.24 nm was detected, which is consistent with the typical interlayer spacing of crystalline CoO (Figure S3d). X-ray photoelectron spectroscopy (XPS) was utilized to characterize the chemical nature and bonding state of hybrid catalysts (Figure 2). High-resolution Co 2p spectra show 2p1/2
Figure 2. Chemical composition analysis (a) XPS spectra of Co 2p3/2 in cobalt species anchored NCNT hybrid, as synthesized (top), 400 °C thermal annealed (middle), and 700 °C thermal annealed (bottom) catalysts. (b) XPS spectra of O 1s in cobalt-speciesanchored NCNT hybrid, as synthesized (top), 400 °C heat-treated (middle), 700 °C heat treated (bottom) catalysts. (c) Co K-edge XANES spectra of Co(OH)x-NCNT.
and 2p3/2 components due to spin−orbit splitting, which qualitatively contains the same chemical information. The high intensity peak for Co 2p3/2 including the shakeup satellites of the Co2+ ions was curve-fitted (Figure 2a).21 In the Co 2p spectra of the as-prepared Co(OH)x-NCNT, 400 °C annealed catalyst (CoOx-NCNT), and 700 °C annealed catalyst (CoONCNT) show two major peaks at 779.5 and 781.0 eV, which correspond to Co(III) and Co(II) oxidation states, respectively. C
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. Co(OH)x-NCNT hybrid as ORR catalyst. (a) CV curves of Co(OH)x-CNT, Co(OH)x-NCNT, CoOx-NCNT, CoO-NCNT, (b) Rotating-disk voltammogram of Co(OH)x-NCNT hybrid at various rotation speeds indicated. The inset in (b) shows corresponding Koutecky− Levich plots (J−1 vs ω−0.5) at different potentials. (c) ORR polarization curves of hybrid catalysts at a rotation speed of 1600 rpm. (d) Tafel plot was derived from (c). Catalyst loading was 0.1 mg cm−2 on glassy carbon electrodes in O2 saturated 1 M KOH with sweep rate of 5 mV s−1 for all samples.
NCNT, and CoOx-NCNT and a limiting current of Co(OH)xNCNT was the highest among all four samples. Direct comparison of the ORR catalytic properties among Co(OH)x-CNT, Co(OH)x-NCNT and CoOx-NCNT clarifies that N-dopant leads to the improvement of ORR kinetics to a nearly 4e process. For more information on the reaction kinetics, the Tafel plot was derived from previous polarization curves (Figure 3d). Tafel slope of 54 mV/decade for Co(OH)xNCNT and that of 66 mV/decade for CoOx-NCNT signifies the rate-determining step is associated with the adsorption of oxygen or adsorbed leading to a coverage-dependent activation barrier for electrochemical reactions. By contrast, Tafel slope of 98 mV/decade for Co(OH)x-CNT proposes that the ratedetermining step is associated with the first electron transfer step (from O2 to adsorbed OOH*).23 Because the thickness of anchored cobalt oxide and hydroxide species is less than 1 nm, electron transfer from CNT/NCNT to the reacting oxygen species should be based on tunneling effect.24 N-doping is known to effectively lower the work function of CNT (work function of CNT: ∼4.9 eV, NCNT: ∼4.6 eV vs. vacuum level)18 and thereby facilitates the electron tunneling to the reacting oxygen species such that the first reaction step is no longer the rate-determining step of the overall reaction. This mechanism eventually enhances a 4e pathway.25 The typical OER catalytic activities of our hybrid catalyst was characterized by simple modification of the electrochemical
an onset potential of 0.87 V and a peak potential of 0.76 V vs. the reversible hydrogen electrode (RHE) in 1 M KOH, which are higher than those of Co(OH)x-CNT (without N-doping) and CoO-NCNT, while smaller than those of the CoOxNCNT. Nonetheless, Co(OH)x-NCNT shows the peak current density of −0.48 mA cm−2, which is the highest among all four samples. Rotating disk electrode (RDE) measurements were performed to investigate the electrochemical kinetics of Co(OH)x-NCNT for ORR (Figure 3b). The polarization curves with different rotation rates were presented with the corresponding Koutecky−Levich (K-L) plots at different potentials (inset). The linear and nearly parallel fitted lines in the K-L plots suggest the first order reaction kinetics toward the concentration of dissolved oxygen and the similar electron transfer number for the ORR at different potentials. From the slope in K-L plot, the electron transfer number (n) was calculated as 3.85 at 0.45 V (RHE) for Co(OH)x-NCNT. The electron transfer numbers of Co(OH)x-CNT, CoOx-NCNT, and CoO-NCNT were also calculated from polarization curves (Figure S5). The obtained values suggest that Co(OH)xNCNT and CoOx-NCNT (n = 3.92) favor direct 4e oxygen process like commercial Pt/C, whereas Co(OH)x-CNT (n = 3.33) exhibits mixed 4e and two-step 2e reduction process. In Figure 3c, polarization curves for Co(OH)x-CNT, Co(OH)xNCNT, CoOx-NCNT, and CoO-NCNT are compared at 1600 rpm. A half wave potential of Co(OH)x-NCNT at 1600 rpm was noticeably higher than those of Co(OH)x-CNT, CoOD
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 4. Co(OH)x-NCNT hybrid as OER catalyst (a) Oxygen evolution currents of the hybrid catalysts in a 1 M KOH with a sweep rate of 5 mV s−1. (b) Tafel plots of OER currents in (a). Catalyst loading was 0.25 mg cm−2 on carbon fiber paper electrodes for all samples.
explain the change of Tafel slope after the thermal annealing, we conducted a Mott−Schottky analysis.28 When semiconductor is immersed in an electrolyte, space charge carriers move to adjust the Fermi-level of semiconductor and electrolyte to the same level, while external bias to make depletion or accumulation layer to be zero is flat-band potential. We can get some insights from the Mott−Schottky plot (1/C2 vs. electrochemical potential), where the capacitance C is typically measured by impedance measurement. Positive slope value of the fitted line indicates n-type semiconductor character, while the negative value indicates a p-type character. Extrapolated intercept is the flat band potential, and the slope is proportional to 1/Nsc (density of space charge carrier). Figure S8 shows the Mott−Schottky plots in dark. Noteworthy that in our catalyst system, measured flat-band potential gives neither the exact value of Fermi level nor band edge potential. The flat band potential is usually given by the calculation based on the assumptions of bulk and flat geometry. Our catalysts with nanoscale curvature and subnanometer scale thickness do not meet these assumptions. Nevertheless, still some important conclusions can be drawn from the comparison among different samples. Although the flat band potential of Co(OH)x-NCNT have value of ∼1.6 V vs. NHE (pH 13.6), and that of Co(OH)x-CNT with undoped CNT shows the potential of ∼2.5 V vs. NHE (pH 13.6). This implies that the work function difference between CNT and NCNT caused an apparent flat band potential difference of hybrid catalysts. As all four samples have negative slopes, catalysts show p-type behaviors. In the comparison of the slopes for fitted lines, Co(OH)x-NCNT and Co(OH)x-CNT show similar low slopes, whereas CoOxNCNT and CoO-NCNT (heat treated catalysts) show similar high slopes. In general, defect-rich oxide materials are found to exhibit n-type or p-type conductivity. Oxygen vacancy defects normally act as electron donors, developing n-type conductivity, whereas metal vacancy defects are found to produce delocalized holes near the top of valence band, leading to ptype conductivity.29 Upon vacuum heat treatment, metal vacancy defects annihilate. The reduction of defect density during vacuum annealing results in the high slopes in M-S plot and also results in the lower activation sites, which causes increment of Tafel slope from ∼30 to ∼60 mV/decade.27 For practical applications, high catalytic activity is not the sole issue for consideration. Another equivalent important issue is cycling stability. For rechargeable batteries or fuel cells, charge/discharge cycle stability of bifunctional catalyst is crucial.7 Nonetheless, such a stability is challenging, as the catalyst should endure the alternate reaction conditions for
potential range (1.3−1.7 V vs. RHE) in the similar electrolyte system. During the OER measurement, vigorously generated oxygen bubbles were accumulated at the surface of catalyst forest severely hindering the electrolyte contact, and peeled the catalyst off from the glassy carbon electrode. To address such problems, hybrid catalyst strands were dispersed in the Nafion/ ethanol solution under mild sonication, which was subsequently loaded onto Teflon-treated carbon fiber paper (CFP) (loading amount: 0.25 mg cm−2) by drop casting. Figure 4a shows the measured steady-state iR-compensated (i: cell current, R: nuisance resistor) polarization curves of the samples. As the hydrophobic Teflon-treated CFP provides a gas diffusion layer,26 the catalyst layer maintained the stable contact with bottom CFP electrodes and electrolyte solution upon the generation of oxygen bubble. As seen in the polarization curves, Co(OH)x-NCNT showed a current density of 10 mA cm−2 at a small overpotential of 0.35 V in 1 M KOH electrolyte, which is comparable to the performance of the best Co-based OER catalysts ever reported thus far (Table S1). Likewise, Co(OH)xCNT, CoOx-NCNT, CoO-NCNT, and Co(OH)x-CB (carbon black, detailed synthesis information is described in the Supporting Information) show the over potentials of 0.39, 0.38, 0.41, and 0.38 V at a current density of 10 mA cm−2. On the basis of the mass content of cobalt in the hybrid catalyst, we calculated a turnover frequency (TOF) of Co(OH)x-NCNT. The amount of deposited cobalt, which was determined by the inductively coupled plasma mass spectrometry (ICP-MS), was 17 wt %. The hybrid catalyst demonstrated a remarkably high TOF of 2.3 s−1 at an overpotential of 300 mV in 1 M KOH, which is one of the highest values reported thus among OER catalysts (Table S1). Beneficial effect from the maximized active surface of subnanometer scale cobalt species coating is demonstrated by the high TOF values. The OER kinetics was further examined with Tafel plots (Figure 4b). In the low overpotential region, Tafel slopes of 36 and 34 mV/decade are obtained for Co(OH)x-NCNT and Co(OH)x-CNT, respectively. These values are lower than the values reported for the cobalt based oxides.17 By contrast, CoOx-NCNT and CoONCNT exhibited significantly higher Tafel slopes of 47 and 56 mV/decade, respectively. Tafel slope of Co(OH)x-NCNT and Co(OH)x-CNT (∼30) means their rate-determining step is recombination step, caused by high density of active site, whereas the Tafel slope of CoOx-NCNT (∼60) means its ratedetermining step is associated with rearrangement step, caused by less active sites.27 Surface states and morphology of hybrid nanostructure may significantly influence the energetics for catalytic behaviors. To E
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
tunnelling to the oxygen reactant with the lowered work function of CNTs. Also, abundant charge carriers in amorphous Co(OH)x species are found to trigger the superior OER activity with high current density and low Tafel slope. The unique subnanometer scale morphology along with strong cobalt species-NCNT interaction minimizes the deactivation of catalyst during prolonged repeated cycles. For further research, we are working on preparing Zn-air battery to utilize our unique bifunctional catalyst. Our approach offer a novel route to N-doped graphitic carbon-based nanohybrid catalyst structures, which may effectively transform typical OER unifunctional catalysts into high-performance bifunctional catalysts.
ORR and OER repeatedly, which generally causes mechanical or chemical degradation of the catalysts. Unfortunately, among many previous reports on bifunctional oxygen catalyst, only a few have reported the long-term cycling stability.30,31 We investigated the cycling stability in the typical alternating oxidation and reduction conditions. Co(OH)x-NCNT and Co(OH)x-CNT loaded (0.25 mg cm−2 for each) CFP exhibited cycling stability under galvanostatic charging/discharging at 10 mA cm−2 of current density and 2 h per cycle in 1 M KOH electrolyte (Figure 5a). Co(OH)x-NCNT did not show
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10297. Experimental methods, additional SEM and TEM images, and additional measurements; Figures S1−S8 and Tables S1 and S2 (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address §
U.N.M. is currently at Department of Physics, Indian Institute of Technology Guwahati (IITG), Guwahati 781039, Assam, India Notes
The authors declare no competing financial interest.
■
Figure 5. Cycling stability tests. (a) Galvanostatic charging and discharging test of Co(OH)x-NCNT and Co(OH)x-CNT at 10 mA cm−2 of current density with 2 h of cycle time in 1 M KOH electrolyte. (b) The extended cycling test of Co(OH)x-NCNT at 10 mA cm−2 in 6 M KOH electrolyte. Catalysts were (loading amount of 0.25 mg cm−2 each) loaded on carbon fiber paper electrodes for all samples.
ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061) and the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project (CISS-20110031848) of the National Research Foundation of Korea (MSIP).
significant overpotential change during 12 h of cycling. By contrast, Co(OH)x-CNT exhibited ∼0.03 V increments of discharge overpotential (ORR side) in the same period. This implies that subnanometer thick Co(OH)x is strongly coupled to CNT/NCNT, and N-dopants in NCNTs even further strengthen the coupling. In general, the durability of supported catalyst depends on the catalyst−support interaction. Previous reports have clarified that a strong interaction of d-orbital of transition metal with the p-orbital of N-dopants may offer a robust binding.32 We conducted a cycling test under an extended cycle period of 20 h, Co(OH)x-NCNT sample was tested at 10 mA cm−2 with 0.25 mg cm−2 in 6 M KOH electrolyte (Figure 5b). The catalytic activity was maintained over ∼120 h of cycling. This confirms that the interaction between subnanometer scale Co(OH)x and NCNT is strong enough to endure the harsh reaction conditions for long-term cycling. In summary, we have designed and prepared amorphous Co(OH)x anchored NCNT as a stable ORR/OER bifunctional catalyst. The hybrid catalyst exhibits moderate ORR activity and excellent OER activity with superior cycling stability. Direct comparison of the catalytic behaviors with and without Ndopants reveals that N-dopants successfully promoted a 4e reduction pathway during ORR by facilitating electron
■
REFERENCES
(1) Koper, M. T. M. Thermodynamic Theory of Multi-Electron Transfer Reactions: Implications for Electrocatalysis. J. Electroanal. Chem. 2011, 660, 254−260. (2) Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800−3821. (3) Wu, J.; Yang, H. Platinum-Based Oxygen Reduction Electrocatalysts. Acc. Chem. Res. 2013, 46, 1848−1857. (4) Stephens, I. E. L.; Bondarenko, A. S.; Gronbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the Electrocatalysis of Oxygen Reduction on Platinum and Its Alloys. Energy Environ. Sci. 2012, 5, 6744−6762. (5) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile Iro2 and Ruo2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399−404. (6) Wang, B. Recent Development of Non-Platinum Catalysts for Oxygen Reduction Reaction. J. Power Sources 2005, 152, 1−15. (7) Liang, Y.; Li, Y.; Wang, H.; Dai, H. Strongly Coupled Inorganic/ Nanocarbon Hybrid Materials for Advanced Electrocatalysis. J. Am. Chem. Soc. 2013, 135, 2013−36.
F
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780− 786. (27) Lyons, M. E. G.; Brandon, M. P. Redox Switching and Oxygen Evolution Electrocatalysis in Polymeric Iron Oxyhydroxide Films. Phys. Chem. Chem. Phys. 2009, 11, 2203−2217. (28) Sato, N. Electric Double Layer at Electrode Interfaces. In Electrochemistry at Metal and Semiconductor Electrodes; Sato, N., Ed.; Elsevier Science: Amsterdam, 1998; Chapter 5, pp 119−199. (29) Sudakar, C.; Singh, S.; Rao, M. S. R.; Lawes, G. The Role of Defects in Functional Oxide Nanostructures. In Functional Metal Oxide Nanostructures; Wu, J., Cao, J., Han, W.-Q., Janotti, A., Kim, H.-C., Eds.; Springer: New York, 2012; Chapter 3, pp 37−68. (30) Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J. E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H. Advanced Zinc-Air Batteries Based on HighPerformance Hybrid Electrocatalysts. Nat. Commun. 2013, 4, 1805. (31) Lee, D. U.; Choi, J. Y.; Feng, K.; Park, H. W.; Chen, Z. W. Advanced Extremely Durable 3D Bifunctional Air Electrodes for Rechargeable Zinc-Air Batteries. Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201301389. (32) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Molybdenum Sulfide/N-Doped Cnt Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction. Nano Lett. 2014, 14, 1228−33.
(8) Dai, L.; Xue, Y.; Qu, L.; Choi, H. J.; Baek, J. B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823−92. (9) Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T. J. Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction. Catal. Sci. Technol. 2014, 4, 3800−3821. (10) Wang, Z.; Xiao, S.; Zhu, Z.; Long, X.; Zheng, X.; Lu, X.; Yang, S. Cobalt-Embedded Nitrogen Doped Carbon Nanotubes: A Bifunctional Catalyst for Oxygen Electrode Reactions in a Wide Ph Range. ACS Appl. Mater. Interfaces 2015, 7, 4048−4055. (11) Tian, G. L.; Zhao, M. Q.; Yu, D.; Kong, X. Y.; Huang, J. Q.; Zhang, Q.; Wei, F. Nitrogen-Doped Graphene/Carbon Nanotube Hybrids: In Situ Formation on Bifunctional Catalysts and Their Superior Electrocatalytic Activity for Oxygen Evolution/Reduction Reaction. Small 2014, 10, 2251−9. (12) Hou, Y.; Wen, Z. H.; Cui, S. M.; Ci, S. Q.; Mao, S.; Chen, J. H. An Advanced Nitrogen-Doped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting. Adv. Funct. Mater. 2015, 25, 872−882. (13) Liu, S.; Li, L.; Ahn, H. S.; Manthiram, A. Delineating the Roles of Co 3 O 4 and N-Doped Carbon Nanoweb (Cnw) in Bifunctional Co 3 O 4/Cnw Catalysts for Oxygen Reduction and Oxygen Evolution Reactions. J. Mater. Chem. A 2015, 3, 11615−11623. (14) Zhan, Y.; Du, G.; Yang, S.; Xu, C.; Lu, M.; Liu, Z.; Lee, J. Y. The Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution. ACS Appl. Mater. Interfaces 2015, 7, 12930. (15) Lee, D. U.; Choi, J.-Y.; Feng, K.; Park, H. W.; Chen, Z. Advanced Extremely Durable 3d Bifunctional Air Electrodes for Rechargeable Zinc-Air Batteries. Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201301389. (16) Smith, R. D.; Prevot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K.; Trudel, S.; Berlinguette, C. P. Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340, 60−3. (17) Gong, M.; Dai, H. J. A Mini Review of Nife-Based Materials as Highly Active Oxygen Evolution Reaction Electrocatalysts. Nano Res. 2015, 8, 23−39. (18) Lee, D. H.; Lee, W. J.; Lee, W. J.; Kim, S. O.; Kim, Y. H. Theory, Synthesis, and Oxygen Reduction Catalysis of Fe-Porphyrin-Like Carbon Nanotube. Phys. Rev. Lett. 2011, 106, 175502. (19) Kaye, M. H.; Thompson, W. T. Computation of Pourbaix Diagrams at Elevated Temperature. In Uhlig’s Corrosion Handbook; John Wiley & Sons: New York, 2011; pp 111−122. (20) Lee, W. J.; Lee, J. M.; Kochuveedu, S. T.; Han, T. H.; Jeong, H. Y.; Park, M.; Yun, J. M.; Kwon, J.; No, K.; Kim, D. H.; Kim, S. O. Biomineralized N-Doped Cnt/Tio2 Core/Shell Nanowires for Visible Light Photocatalysis. ACS Nano 2012, 6, 935−943. (21) Miller, C. R.; Wang, D. K.; Smart, S.; Diniz da Costa, J. C. Reversible Redox Effect on Gas Permeation of Cobalt Doped Ethoxy Polysiloxane (Es40) Membranes. Sci. Rep. 2013, 3, 1648. (22) Arechederra, R. L.; Artyushkova, K.; Atanassov, P.; Minteer, S. D. Growth of Phthalocyanine Doped and Undoped Nanotubes Using Mild Synthesis Conditions for Development of Novel Oxygen Reduction Catalysts. ACS Appl. Mater. Interfaces 2010, 2, 3295−3302. (23) Wu, G.; Chung, H. T.; Nelson, M.; Artyushkova, K.; More, K. L.; Johnston, C. M.; Zelenay, P. Graphene-Riched Co9s8-N-C NonPrecious Metal Catalyst for Oxygen Reduction in Alkaline Media. ECS Trans. 2011, 41, 1709−1717. (24) Sato, N. Electrode Reactions in Electron Transfer. In Electrochemistry at Metal and Semiconductor Electrodes; Sato, N., Ed.; Elsevier Science: Amsterdam, 1998; Chapter 8, pp 235−288. (25) Cheon, J. Y.; Kim, J. H.; Kim, J. H.; Goddeti, K. C.; Park, J. Y.; Joo, S. H. Intrinsic Relationship between Enhanced Oxygen Reduction Reaction Activity and Nanoscale Work Function of Doped Carbons. J. Am. Chem. Soc. 2014, 136, 8875−8. (26) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Co3o4 Nanocrystals on Graphene as a Synergistic G
DOI: 10.1021/acsami.5b10297 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
