Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 7735−7742
pubs.acs.org/journal/ascecg
Bifunctional Electrodeposited 3D NiCoSe2/Nickle Foam Electrocatalysts for Its Applications in Enhanced Oxygen Evolution Reaction and for Hydrazine Oxidation Kamran Akbar,†,‡ Jae Ho Jeon,† Minsoo Kim,§ Junkyeong Jeong,§ Yeonjin Yi,§ and Seung-Hyun Chun*,† †
Department of Physics, Sejong University, Seoul 05006, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea § Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea
ACS Sustainable Chem. Eng. 2018.6:7735-7742. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/30/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: The development of stable and efficient oxygen evolutional electrocatalysts is fundamental to the production of hydrogen by water electrolysis. However, so far the majority of electrocatalysts require a substantial overpotential (η) (approximately >250 mV) to catalyze the bottleneck oxygen evolution reaction (OER). To overcome this large overpotential for OER, herein we report the growth of nickel−cobalt− selenide (NiCoSe2) nanosheets over 3D nickel foam (NF) via a facile and scalable electrodeposition method. The resulting 3D NiCoSe2/NF hybrid electrode requires an overpotential of merely 183 mV to reach the current density (J) of 10 mA cm−2. To the best of our knowledge, this is the lowest η value reported so far for any earth-abundant material-based OER electrocatalyst to attain the same current density. Moreover, a significant reduction in Tafel slope (88 mV dec−1) is observed between bare NF and NiCoSe2/NF. Hence, as a result, the 3D hybrid NiCoSe2/NF OER electrode outperforms the previously reported electrocatalysts including the expensive state-of-the-art OER electrocatalysts like RuO2 and IrO2. Such enhancement in the OER catalytic efficiency of NiCoSe2 nanosheets over NF can be attributed to its enormous electrochemical active surface area (ECSA) (108 cm2), large roughness factor (270), highly conductive NF substrate, and the presence of multiple catalytically active OER species (NiOOH and CoOOH) on its surface. In addition, 3D hybrid NiCoSe2/NF electrocatalyst was tested for hydrazine oxidation for its bifunctional utilization. Much lower onset potential values (−0.7 V vs SCE) and high current densities (>200 mA cm−2) are observed for 3D hybrid NiCoSe2/NF when benchmarked against bare NF (−0.4 V and 12 h). In another study, Wang et al. synthesized porous NiCo diselenide nanosheets via similar hydrothermal method on conductive carbon cloth and utilized it for OER, but fairly high overpotential of 258 mV is yet required to attain the current density of 10 mAcm−2. However, for the following reasons, we believe that there is still substantial room for further development in the synthesis and catalytic performance of these selenide OER catalysts. At first, as mentioned earlier, normally the hydrothermal route had been adopted for the synthesis of these metallic/bimetallic selenides materials, which thus require hours to synthesize the material. Thus, it is essential to come up with novel approaches to reduce synthesis time and temperatures to overcome high costs. Second, for many OER electrocatalysts, a conductive agent and a binder is normally needed. However, due to inactivity of binder/conductive agent for OER, the overall activity of electrocatalyst is lowered, owing to the presence of added dead volume.22 Furthermore, at higher current density operations when the oxygen evolution at electrode surface is violent, the glued electrocatalyst peels off from the surface and hence results in its poor stability. To circumvent this issue, one could utilize an inexpensive electrodeposition method and avoid the hassle of using binders or other additional treatments, like high temperature and longer synthesis times.13 Third, for most of the OER catalysts, the η@10 mA cm−2 is still greater than ∼250 mV, which is significantly high enough to hamper the overall energy-harvesting efficiency for a water-splitting reaction, and thus needs to be lowered for the further development of efficient and high-performance OER electrocatalysts.23 In addition to OER, catalysts with dual functionalities are obvious choices instead of the prepared catalyst serving only one purpose. Hydrazine as a fuel stands out among other liquid
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EXPERIMENTAL DETAILS
Nickel foam (NF) (1.6 mm thickness, surface density = 346 g/m2) was sonicated in 3 M HCl solution for 10 min to remove the oxide layer on the surface, rinsed successively with deionized (DI) water and ethanol, and then dried with N2 gas. The electrodeposition was carried out in a standard three-electrode electrochemical cell containing NF as the working electrode, a Ag/AgCl (sat. KCl) as the reference, and Pt wire as counter electrode. The electrodeposition bath contained 10 mM of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and SeO2 each, with pH maintained at 2 by addition of HCl. Chronoamperometry was employed to electrodeposit NiCoSe2 on NF at constant potential of −1 V (vs Ag/AgCl) at 25 °C for 600 s. Shorter deposition time results in insufficient formation of NiCoSe on NF with certain places with no deposition at all, whereas prolonged deposition time leads to thick films that can inhibit the charge transfer between NiCoSe2 and NF. After deposition, the electrode was withdrawn from the deposition bath, rinsed with water and ethanol, and dried with gentle N2 gas pressure. Materials Characterizations. The scanning electron microscopy (SEM) was performed with JEOL JSM-7001F at an acceleration voltage of 10 kV. Energy-dispersive spectroscopy (EDS) was also obtained from the SEM microscope. Transmission electron microscopy (TEM) samples were prepared using Cu grids. The atomic structure was characterized by JEOL-2010F TEM with 200 keV accelerating voltage. X-ray photoelectron spectroscopy (XPS) was performed using a K-alpha (Thermo VG, U.K.) X-ray photoelectron spectrometer with monochromatic Al X-ray source with 12 kV power. X-ray diffraction (XRD) was performed with an X-ray diffractometer (Rigaku RINT 2000) with Cu Kα radiation. For inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements, NiCoSe2/NF was dissolved in 1 M HCL to remove NF substrate and then black powered was ingested in concentrated HNO3 for further analysis. Electrochemical Characterizations. All electrochemical measurements were carried out with Biologic SP-300 workstation. The working electrode was either bare NF or NiCoSe2/NF and was directly used without additional treatment. Linear sweep voltammograms (LSVs) were recorded in 1 M KOH solution saturated with O2 using Ag/AgCl (sat. KCL) and Pt wire as reference and counter electrode, respectively. Scan rate was fixed at 10 mV s−1. All potentials measured were converted to reversible hydrogen electrode (RHE) scale using the following equation: ERHE = EAg/AgCl + E°Ag/AgCl + 0.059 pH. Tafel slopes were derived from LSV curves with 85% iR compensation (after taking impedance spectra at open circuit voltage) based on the following equation. η=a+
2.3RT log(j) αnF
(1)
where η is the overpotential, j is the current density, and the other symbols have their usual meanings. Electrochemical impedance was performed at frequency ranges from 1 MHz to 1 Hz at an overpotential of 400 mV. Calculation of ECSA. The electrochemically active surface area (ECSA) was calculated on the basis of measured double-layer 7736
DOI: 10.1021/acssuschemeng.8b00644 ACS Sustainable Chem. Eng. 2018, 6, 7735−7742
Research Article
ACS Sustainable Chemistry & Engineering capacitance in non-Faradaic regime of cyclic voltammetry (CV).5,10 Non-Faradaic regime was first determined by taking CV of NiCoSe2/ NF in 1 M KOH. Multiple CVs were then conducted at various scan rates for estimation of charging current (ic). The relation between ic, the scan rate (ν), and the double-layer capacitance (Cdl) is given in eq 2. Hence, the slope of ic as a function of scan rate is a straight line and is equal to Cdl.
ic = vCdl
(2)
The measured Cdl for NiCoSe/NF is 4.43 mF, which is converted to ECSA according to eq 3. A specific capacitance (Cs) value Cs = 0.040 mF cm−2 in 1 M NaOH is adopted from previous reports, and hence ECSA of 108 cm2 is obtained.5
ECSA =
Cdl Cs
(3)
Finally, the roughness factor (RF) is calculated on the basis of eq 4; given the geometric surface area (GSA) of NiCoSe2/NF (0.4 cm2), RF of 270 is obtained. RF =
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ECSA GSA
(4) Figure 1. Field-emission SEM (FE-SEM) images of (a) bare NF and (b) NF covered with NiCoSe2. (c) Magnified SEM image of NiCoSe2 nanoparticles. (d) High-resolution TEM (HR-TEM) image of a NiCoSe2 nanoparticles.
RESULTS AND DISCUSSION Herein, for the first time, we report nanostructured NiCoSe2 on Ni foam (NiCoSe2/NF) as a highly active OER catalyst via a facile, one-step, ultrafast electrodeposition method at room temperature. Ni foam acts as a highly conductive 3D scaffold for growth of NiCoSe2 nanoparticles. The resulting NiCoSe2/ NF electrode possesses extremely small overpotential of only 183 mV to attain a J of 10 mA cm−2 for the OER. According to the best of our knowledge, this is the lowest overpotential value reported so far for any OER catalysts in alkaline solutions. In addition, the hybrid electrocatalyst shows remarkable stability for continuous OER reaction. In fact, NiCoSe2/NF performs significantly better than all of the reported OER electrocatalysts including the benchmark IrO2 and RuO2. Furthermore, the 3D NiCoSe2/NF hybrid electrode has further been tested for hydrazine oxidation reaction. Lower onset potential values of −0.7 V vs SCE have been observed for NiCoSe2/NF electrode compared to −0.4 V for bare NF for hydrazine oxidation. The electrodeposition of NiCoSe2 onto NF was undertaken at constant deposition potential of −1.0 V versus Ag/AgCl (see experimental details in Supporting Information). The direct growth of NiCoSe2 on macroporous NF substrates proves to be a better technique for preparation of electrode due to its roomtemperature working conditions along with its binder-free nature. Furthermore, high intrinsic conductivity of NF would result in a lower contact resistance with the deposited electrocatalyst, which ultimately reduces the overall ohmic loses of the system.28 Parts a and b of Figure 1 show the scanning electron microscopy (SEM) images for NF before and after electrodeposition. It can be clearly seen that surface of NF is fully covered with densely packed NiCoSe2 film after electrodeposition (Figure S1). The high-magnification SEM (Figure 1c) shows that the particle size of NiCoSe2 is in the nm range with average diameter of ∼50−100 nm. It is also evident that the nanoparticles are interconnected with each other and offer a hierarchical structural morphology. Furthermore, Ni, Co, and Se are homogeneously distributed within the discrete grains of NiCoSe2 as shown by energy-dispersive X-ray (EDX) mapping analysis (Figure S3). A uniform coverage of NiCoSe2 on NF was attained for 10 min electrodeposition time as shown in
Figure S4. The deposition of shorter time (5 min) resulted in nonuniform coverage of NiCoSe2 onto NF, while the longer deposition time (15 min) resulted in thick films with the presence of wider cracks. The HR-TEM image of NiCoSe2 deposited for 10 min confirms the high crystallinity of prepared nanoparticles with clearly visible lattice fringes as shown in Figure 1d. Figure 2a shows the X-ray diffraction (XRD) patterns of NiCoSe2/NF. The two peaks at 2θ = 44.58 and 51.88 can be indexed to NF (JCPDS no. 65-2865). The diffraction peaks of NiCoSe2 could be assigned to hexagonal NiSe (JCPDS no. 702870) and hexagonal CoSe (JCPDS no. 89-2004), indicating that bimetallic NiCoSe2 possesses the same hexagonal crystal structure as the individual Ni and Co-selenide.29 It is worth noting that peaks are slightly shifted toward a larger diffraction angle (with respect to NiSe), suggesting a minor adjustment within the lattice parameter. This variation in the lattice parameter could be the result of the gradual replacement of Ni ions with Co ions having larger ionization energy in NiSe lattice and hence resulting in the formation of NiCoSe2. Moreover, further estimation of the composition of NiCoSe2 was performed by comparing the lattice spacing of (100) planes (d = 0.316 nm from the HRTEM image in Figure 1d) and applying the Vegard’s law on (100) plane of NiSe (d = 0.317 nm) and on CoSe (d = 0.313 nm) to obtain Ni0.77Co 0.23Se2. Similarly, EDX elemental analysis of NiCoSe2 (Figure S5) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis indicate identical Ni/Co ratios within NiCoSe2. Hence, it is reasonable to say that our NiCoSe2 is rich in Ni content when considering the accuracy. The NiCoSe2/NF was also characterized by X-ray photoelectron spectroscopy (XPS), and the corresponding survey spectrum (Figure S6) demonstrates the presence of Ni, Co, and Se within the sample lattice. Oxygen peaks in the spectra are originated from the exposure of the sample to the ambient air. The high-resolution XPS spectra of Co 2p and Ni 2p can be fitted with two spin−orbit doublets as shown in Figure 2b and 7737
DOI: 10.1021/acssuschemeng.8b00644 ACS Sustainable Chem. Eng. 2018, 6, 7735−7742
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Figure 2. (a) XRD patterns of NiCoSe2 on NF. XPS spectra of the NiCoSe2: (b) Ni 2p, (c) Co 2p, and (d) Se 3d.
Figure 3. (a) Polarization curves for OER on NiCoSe2/NF and Ni foam at a scan rate of 10 mV s−1. (b) Corresponding Tafel plot (overpotential versus log current) derived from (a). (c) Cyclic voltammograms of NiCoSe2/NF at different scan rates. (d) Chronoamperometry curves obtained with the NiCoSe2/NF electrode with the applied potential at 0.4 V (vs Ag/AgCl). All experiments were carried out in O2-saturated 1.0 M KOH.
were detected for the high-resolution spectra of Se 3d (Figure 3d), which indicates the presence of a single crystal lattice with both Co and Ni attached to Se.13 This fact, when corroborated with XRD data, confirms the formation of NiCoSe2 on NF substrate. The OER catalytic activity and stability of the NiCoSe2/NF electrode is presented in Figure 3. A standard three-electrode
c. For Ni 2p, the peak-fitting analysis reveals that the peaks at 874.0 eV in Ni 2p1/2 and 856.1 eV in Ni 2p3/2 arise due to the presence of Ni2+ chemical species.7,29 A similar fitting analysis for Co 2p suggests that the presence of peaks at 797.1 eV in Co 2p1/2 and 781.3 eV in Co 2p3/2 is due to the existence of Co2+ within sample lattice.29,30 Furthermore, two types of metal− selenium bonds (Se 3d 5/2 at 53.92 and 54.7 eV, respectively) 7738
DOI: 10.1021/acssuschemeng.8b00644 ACS Sustainable Chem. Eng. 2018, 6, 7735−7742
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Figure 4. (a) Hydrazine oxidation in the presence of 100 mM hydrazine in 0.5 M KOH. (b) Long-term stability of hydrazine oxidation at constant 0 V vs SCE. (c, d) XPS spectra of the NiCoSe2/NF after 50 h of constant OER. (a) Ni 2p. (b) Co 2p.
electrochemical cell was used where NiCoSe2/NF serves as working electrode. All the potentials were adjusted against the reversible hydrogen electrode (RHE) reference, while the currents were also corrected for ohmic losses. It can be seen from Figure 3a that NiCoSe2/NF clearly demonstrates high performance toward OER compared with bare NF. The appearance of a sharp oxidation peak before the start of oxygen evolution process (Figure 3a and Figure S7) is ascribed to the transformation of NiII to NiIII in the case of NF.31 Peak broadening and the appearance of an additional oxidation peak in the case of NiCoSe2/NF could be assigned to the transformation of CoII to CoIII and CoIII to CoIV.32 In fact, this peak broadening is beneficial because it advocates the presence of additional active sites in NiCoSe2/NF compared with bare NF. Furthermore, the rapid increment in the peak current density of NiCoSe2/NF is indicative of its much larger active surface area along with the presence of numerous catalytically active centers for OER. In general, the OER catalytic efficiency is evaluated on the basis of minimum overpotential required to attain a current density of 10 mAcm−2, which is equivalent to the upper limit of a realistic solar device having 12% solar to hydrogen efficiency.33,34 NiCoSe2/NF requires an overpotential (η) of merely 183 mV to reach J = 10 mA cm−2 (Figure 3a and Figure S8). To the best of our knowledge, this is the lowest η value reported so far to attain J of 10 mA cm−2, indicating significantly diminished OER activation energies. The catalytic activity of NiCoSe2/NF is further assessed against the benchmark OER catalysts on the basic of overpotential required to attain a current density of 10 mA cm−2 (Table S1). The catalytic performance of the NiCoSe2/NF electrode is significantly higher than those of all of the previously reported OER electrocatalysts, including the state-of-the-art IrO2 and RuO2 catalysts.5,6,35 Furthermore, the comparison of RuO2
deposited on NF has been performed for OER performance as shown in Figure S9. 3D NiCoSe2/NF outperformed the RuO2/ NF in terms of OER performance at similar overpotential values. In addition, NiCoSe2/NF also outperforms the reported Ni- and Co-based electrocatalysts including corresponding selenides.9,13,20,36−39 The electrodeposition time strongly affects the OER performance as displayed in Figure S10, with the best performance being observed for 10 min deposition time. This is due to the uniform growth of NiCoSe 2 nanocrystals as shown in the FESEM image (Figure S4c). The deposition of 5 min results in poor coverage of NiCoSe2 and hence displays a degraded OER performance. The OER performance of thick films (Figure S4d) is also poor, because thick films lose the nanocrystalline nature of NiCoSe2 and hence result in reduced active surface area. To gain further insight into OER kinetics, Tafel slopes have been plotted as shown in Figure 3b. The Tafel slope for NiCoSe2/NF is 97 mV dec−1, which in fact is much lower than that for the bare NF (185 mV dec−1), indicating the faster kinetics and more favorable OER at NiCoSe2/NF compared with bare NF. It should be kept in mind that, despite the wide discrepancies in the reported values of Tafel slopes of bare NF, our NiCoSe2/NF system exhibits a much larger reduction of 48% (88 mV/dec) in its Tafel slope value (against bare NF) than NiSe/NF (30%) and Ni3S2/NF (39%).12,23 Beside the lower Tafel value, the NiCoSe2/NF can afford a rapid increment in OER performance at overpotential >200 mV simply because its current density rises more swiftly with an increment in overpotential. In nanostructured catalysts, the enhanced catalytic activity is often the direct consequence of the electrochemically active surface area and the corresponding roughness factor (RF). Larger RF normally provides better OER performance because of the greater electrochemical surface area, providing more 7739
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in EDS mapping of NiCoSe2/NF after 50 h of continuous oxygen evolution (Figure S15) further confirms the stability of the hybrid catalyst. Small contributions from Co3+ and Ni3+ in XPS spectra (Figure 4c and d) may be evidence of CoOOH and NiOOH on the surface of NiCoSe2/NF, which is beneficial for an OER catalyst. The presence of these CoOOH/NiOOH species on the surface of our electrocatalyst is consistent with previously reported bimetallic selenide electrocatalysts carrying similar species on their surfaces while displaying long-term stability.13 Numerous prior studies pointed out that the presence of CoOOH and NiOOH is crucial in achieving high OER activity in Co and Ni based electrocatalysts because they serve as electrocatalytic active sites for an OER.6,8,12,13,22 These data further suggest that NiCoSe2/NF can be a stable electrocatalyst for both OER and hydrazine oxidation. Particularly, the stability of this bimetallic selenide suggests that its surface remains catalytically active and is not prone to surface passivation faced by metallic catalysts under similar oxidizing conditions.25,26 The enhanced activity of NiCoSe2/NF can be explained by the following aspects: (i) intrinsically high activity of the NiCoSe2 nanoparticles, (ii) high ECSA offered by unique nanoarchitectured surface, (iii) extremely rough surface with excellent gas bubble dissipation ability, (iv) possible presence of both electrocatalytically active (NiOOH and CoOOH) species on the surface of catalyst, and (v) firmly bonded NiCoSe2 on highly conductive NF with low electrical resistance for overall water splitting and the absence of dead volume in electrocatalyst arisen from the binder-free electrodeposition approach of NiCoSe2 on NF. Furthermore, the macroporous 3D surface provided by NF permits rapid dissipation of the larger oxygen bubbles into the solution.
catalytically active sites for a particular reaction. The ECSA of NiCoSe2/NF was assessed by measuring the double-layer capacitance (Cdl) of the solid−liquid interface as reported previously (Figure 3c and Figure S12).5 An ECSA of 108 cm2 and the corresponding RF of 270 are obtained for NiCoSe2/NF electrode. It is noteworthy that these values of RF and ECSA are larger than those of the previously reported nanostructured OER catalysts.7,9,10,19,34 The presence of such an enormous ECSA on nanostructured NiCoSe2/NF provides plentiful electrocatalytic active sites for conversion of anions into the molecular oxygen. Moreover, AC impedance spectra show the smaller semicircle for NiCoSe2/NF compared to the larger semicircle for NF at overpotential of 400 mV (Figure S11). This implies an improved electrical conductance of NiCoSe/ NF over NF, in accord with the better catalytic activity toward OER. Electrochemical stability of an OER catalyst is the prime factor to estimate its lifetime in strongly alkaline solutions. The durability of the NiCoSe2/NF electrode is evaluated at a constant current density of 10 mA cm−2 by chronoamperometry, as displayed in Figure 3d. NiCoSe2/NF demonstrates outstanding long-term stability for water electrolysis, over the duration of 50 h. Even though vigorous oxygen evolution is observed under high-current-density operations, gas bubbles leave the electrode surface quite rapidly with virtually no accumulation on the electrode, hence avoiding any reduction in the ECSA (see the Movie clip in the Supporting Information). On the other hand, bare NF exhibits lower performance for OER owing to the formation of the NiOx in alkaline solutions, which ultimately passivates its surface and results in a gradual decrease in its catalytic activity (Figure S13).10,40 The remarkable physical stability of the NiCoSe2/NF is further verified by the macrostructural analysis and SEM of NiCoSe2/ NF electrode after 50 h of constant OER (Figures S14 and S15). The structural morphology of NiCoSe2 remains wellpreserved after electrolysis, and no detachment or dissolution of nanoparticles was observed from the electrode. For bifunctional catalytic applications, NiCoSe2/NF hybrid electrode was further tested for its applications in hydrazine oxidation as displayed in Figure 4a and b. It is evident that NiCoSe2/NF is far superior in its performance toward hydrazine oxidation compared to bare NF. Sharp increase in the current density at equivalent given voltage shows the high catalytic performance of the hybrid electrode. In addition, a low onset potential (−0.7 V) is also observed in the case of NiCoSe2/NF compared to the high onset potential for bare NF (−0.4 V), as displayed in Figure S16. The stability of the prepared electrode is also an integral part for its widespread industrial utilization. To that end, the stability of NiCoSe2/NF for hydrazine oxidation was tested at fixed 0 V as shown in Figure 4c. Relative stable current densities indicate that the hybrid NiCoSe2/NF electrode is capable of catalyzing hydrazine for a fairly prolonged period of time. It is worth noting that these low onset potential values are superior to many metallic electrocatalysts previously used for hydrazine oxidation.41−43 To better evaluate the mechanism of enhanced OER capabilities of NiCoSe2/NF, the XPS spectra after 50 h of continuous water electrolysis were measured and displayed in Figure 4c and d. It can be seen that the majority of Ni and Co in NiCoSe2/NF are present in their divalent electronic states, emphasizing the stability of NiCoSe2/NF in the harsh alkaline environment for a prolonged time period. The presence of Se
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CONCLUSIONS In summary, a NiCoSe2/Ni foam composite electrode is prepared via a facial and scalable electrodeposition route. The resulting NiCoSe2/NF electrode exhibits enhanced electrocatalytic performance for OER and hydrazine oxidation with excellent stability. Moreover, the overpotential to attain a viable current density of 10 mA cm−2 is merely 183 mV for OER. The presence of multiple catalytically active centers, high electrochemically active surface area, and synergistic coupling effects between NiCoSe2 nanoparticles and NF support might be the reason for improved catalytically activity of the NiCoSe2/NF composite electrode. Utilization of earth-abundant raw materials, low manufacturing costs, excellent stability, low overpotential, and large oxidation current make NiCoSe2/NF a viable candidate for its widespread use in various water oxidation technologies.
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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/acssuschemeng.8b00644. Digital photographs of bare NF and NiCoSe2/NF; SEM and corresponding EDS data of the electrodes; effect of electrodeposition time on the growth of prepared electrocatalyst on NF; XPS elemental mapping analysis and XPS survey spectra of hybrid electrode; electrochemical measurements; optical images of electrode before and after 50 h stability testing; EDS mapping of 7740
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electrocatalyst after 50 h stability testing; close-up view of corresponding CV (Figure 4a) curves for hydrazine oxidation; comparison of the OER performance of numerous electrocatalysts with our hybrid electrocatalyst (PDF) Vigorous oxygen evolution observed under high-currentdensity operations, with gas bubbles leaving the electrode surface quite rapidly with virtually no accumulation on the electrode, hence avoiding any reduction in the ECSA (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yeonjin Yi: 0000-0003-4944-8319 Seung-Hyun Chun: 0000-0001-8397-4481 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (nos. 2010-0020207, 2011-0030786, 2016R1E1A1A01942649, and 2017R1A2B4002442).
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DOI: 10.1021/acssuschemeng.8b00644 ACS Sustainable Chem. Eng. 2018, 6, 7735−7742
