Na0.86Co0.95Fe0.05O2 layered oxide as highly efficient water

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Na0.86Co0.95Fe0.05O2 layered oxide as highly efficient water oxidation electrocatalyst in alkaline media Jie Dai, Yinlong Zhu, Yubo Chen, Wei Zhou, and Zongping Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06004 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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ACS Applied Materials & Interfaces

Na0.86Co0.95Fe0.05O2 layered oxide as highly efficient water oxidation electrocatalyst in alkaline media †

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Jie Dai, Yinlong Zhu, Yubo Chen, Wei Zhou, and Zongping Shao*, , †

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key

Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R. China. ‡

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key

Laboratory of Materials-Oriented Chemical Engineering, College of Energy, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R. China. §

Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845,

Australia. KEYWORDS: electrocatalysis, doping, sodium cobalt oxide, oxygen evolution reaction, synergistic effects

ABSTRACT: Electrochemical energy storage and conversion technologies, such as watersplitting devices, rechargeable metal-air batteries, and regenerative fuel cells, are promising alternatives to traditional non-renewable energy systems. Given the sluggish oxygen evolution reaction (OER) in the above renewable-energy technologies, the development of efficient OER

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electrocatalysts with high performance is of great importance. Here, we demonstrate a layerstructured oxide Na0.86Co0.95Fe0.05O2 (NCF0.05) as a novel electrocatalyst for efficient water oxidation in alkaline media. NCF0.05 shows enhanced performance, including lower overpotential, lower Tafel slope and better stability than the parent Na0.86CoO2 (NC). Especially, the OER performance of NCF0.05 is comparable to the state-of-the-art IrO2 catalyst. This enhanced catalytic activity of NCF0.05 may be ascribed to the unusual synergistic interplay between Fe and Co. A possible dual-metal-site mechanism was also proposed for OER on NCF0.05.

Currently, there is serious concern all over the world about the energy crisis and environmental pollution due to the increasing global population and excessive use and extensive depletion of fossil fuels.1 To protect the environment and satisfy the increasing global energy demand, renewable energy conversion and storage systems, such as water-splitting devices, rechargeable metal-air batteries, and regenerative fuel cells, have received considerable attention as promising sustainable alternatives to traditional non-renewable energy systems.2-6 Unfortunately, the rate and efficiency of these systems is often limited by the sluggish oxygen evolution reaction (OER) due to its complex multi-electron intermediate processes.7 Accordingly, the exploitation and utilization of highly active electrocatalysts for OER play an indispensable role in the sustainable energy field. IrO2 and RuO2 are state-of-the-art electrocatalysts for OER with favourable activity,8-9 but their prohibitive cost and poor stability significantly hamper their commercial application.10-11 New efficient OER electrocatalysts that are stable, low-cost and highly-active are in great demand for realizing the wide-spread practical use of such emerging energy systems.


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To date, considerable effort has been devoted to seeking metal oxides (hydroxides),12-14 metal chalcogenides,15 functional carbon materials and various composites7, 16 as alternative precious-metal-free electrocatalysts for OER. In particular, some remarkable advances have been achieved recently with the employment of Li-containing metal oxides with the general formula LiMO2, where M is commonly a transition metal ion.17-19 Through proper composition or surface modification, the electrocatalytic activity may reach or even out-perform that of precious metal oxide-based electrocatalysts. For example, Lu et al. discovered that delithiated Li0.5CoO2 exhibits considerably enhanced catalytic performance compared with the benchmark IrO2 catalyst due to its desirable electronic structure.17 The Shao-Horn group found that efficient morphology modification of layered oxide particles can make a dramatic difference in OER electrocatalysis.18 Zhu et al. reported that the Fe-doped layered oxide LiCo0.8Fe0.2O2 markedly outperforms its parent LiCoO2 in enhancing the oxygen evolution activity, benefiting from a synergetic effect between cobalt (Co) and iron (Fe) cations in its structure.19 Notwithstanding the above progress, large-scale applications of LiMO2 oxides as OER electrocatalysts have been restricted by the availability of lithium resources.20 In addition to the limited resources, the high expense of lithium also becomes a large challenge for commercial processes.21 In light of the fact that sodium (Na) is among the most common elements in the crust and sea,22 the development of Na-containing electrocatalysts is highly attractive. In fact, research activities on layered sodium oxides in rechargeable Na-ion batteries have increased recently.23 Nevertheless, very little information is available on Na-containing layered oxides for OER electrocatalysis in alkaline media. These above considerations highlight the promise of utilizing Na-containing oxides as a new category of low-cost and eco-friendly catalysts towards OER.


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Herein, we demonstrated the layer-structured oxide Na0.86CoxFe1-xO2 (denoted as NCF) as an efficient OER electrocatalyst with high activity as well as good stability in alkaline solution. In particular, the electrocatalyst NCF0.05 (x=0.05) afforded a current density of 10 mA cm-2 at a low overpotential of ~0.45 V, which is comparable to that of the state-of-the-art IrO2 catalyst under identical experimental conditions. Moderate Fe doping was found to significantly improve the electrocatalytic performance in terms of both activity and stability. These results highlight the great potential of NCF0.05 as an alternative noble-metal-free catalyst for OER due to its low cost, high activity and good stability.

Figure 1. (a) XRD patterns of the as-synthesized NC, NCF0.05, and NCF0.1 samples. (b) Schematic illustration of the layered crystal structure of NCF0.05 oxide. (c, d) SEM images of the NC and NCF0.05 powders. Insets are the enlarged images.


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To determine the solubility of Fe in NaCoO2, three different samples with nominal compositions of NaCoO2 (NC), NaCo0.95Fe0.05O2 (NCF0.05) and NaCo0.9Fe0.1O2 (NCF0.1) were synthesized by a sol-gel method. Shown in Figure 1 are the room-temperature X-ray diffraction (XRD) patterns of the three samples calcined at 800 °C. All the diffraction peaks of NC and NCF0.05 matched fairly well with the standard pattern of the layer-structured Na0.74CoO2 phase with a space group of P63/mmc (JCPDS No. 01-087-0274), indicating their layered structural features. No impurity phases were detected, demonstrating high purity of NC and NCF0.05. Compared with the parent NC sample, the corresponding peaks of NCF0.05 slightly shifted to lower angles, suggesting lattice expansion, which further supported the successful doping of larger Fe ions into the Co sites.24 Based on the Rietveld refinements of the XRD patterns, the lattice parameters of a=2.8301 Å and c=10.9937 Å (c/a=3.885) were obtained for NCF0.05 (Table S1), while the corresponding values for NC are 2.8324 Å and 10.9439 Å (c/a=3.864). For the NCF0.1 sample, in addition to the main diffraction peaks for the layer-structured phase, some weak diffraction peaks at 2θ values of 35.3° and 41.7° were also observed, which can be assigned to the FeO impurity phase. This suggests that the solubility of doped Fe in NC is less than 10%. Structural information for the layered NC and NCF0.05 samples is given in Fig. 1b, in which Na cations occupy in one layer and Co/Fe form edge-sharing [MO6] octahedra in a second adjacent layer. Besides, the actual bulk composition of the NC and NCF0.05 samples were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) and turned out to be Na0.86CoO2 and Na0.86Co0.95Fe0.05O2, respectively. The particulate morphology of NC and NCF0.05 were examined using scanning electron microscopy (SEM), with the representative images at different magnifications shown in Figure 1c, d. Both samples show large aggregates with sizes of 4-6 µm. The Brunauer-Emmett-Teller (BET) specific surface area of NC and


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NCF0.05 (Figure S1) was measured by the nitrogen adsorption and desorption method and calculated to be 0.71 and 0.99 m2 g-1, respectively. The similar morphology and surface area suggests that any difference in the electrochemical performance between NC and NCF0.05 should be mainly contributed by other aspects. The electrocatalytic activity of NCF0.05 for OER was first evaluated by linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 in an O2-saturated 0.1 M KOH solution on a rotating disk electrode (RDE) with a rotation speed of 1600 rpm. NC and benchmark IrO2 (Aladdin Industrial Corporation, see Figure S2 for detailed information) catalysts were also studied for comparison. The potentials were calibrated to the reversible hydrogen electrode (RHE, see more details in the Supporting Information, Figure S3). All potential values were iRcorrected during each LSV to compensate for the resistance of the electrolyte solution. From the polarization curves in Figure 2a, NCF0.05 displayed a considerably more negative onset potential (~1.61 V) and a larger current density at the same potential than those of NC. For instance, at 1.65 V, the current density of NCF0.05 is 2.42 mA cm-2, which is larger than that of NC (0.55 mA cm-2). These merits indicate that NCF0.05 possesses much better catalytic activity towards OER than NC. Notably, conductive carbon, which was added in the electrodes to enhance the electrical conductivity, showed negligible catalytic activity. It is useful to compare the overpotential (η) values at 10 mA cm-2 current density, which represent figures-of-merit relevant to solar fuel synthesis.25 The overpotential of NCF0.05 (0.45 V) at a current density of 10 mA cm-2 was much lower than that of NC (0.54 V) and comparable to that of the benchmark IrO2 catalyst (0.45 V, Figure S4a, consistent with the literature reported26-27), suggesting the high electrocatalytic activity of NCF0.05 for OER.


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Figure 2. (a) OER polarization curves of NC and NCF0.05 at a scan rate of 5 mV s-1 in an O2saturated 0.1 M KOH solution on an RDE (1600 rpm). (b) Corresponding Tafel plots. (c) MA and SA of NC and NCF0.05 at an overpotential of 0.45 V. Error bars represent standard deviations from at least three independent measurements. (d) Chronopotentiometry response of NC and NCF0.05 at a constant current density of 5 mA cm-2disk. We further examined the OER kinetics over the three catalysts by Tafel slopes, with the results shown in Figure 2b. The Tafel slope for NCF0.05 (60 mV dec-1) was found to be much smaller than that for NC (77 mV mV dec-1) and the benchmark IrO2 catalyst (89 mV dec-1) (Figure 2b and Figure S4b), implying much faster OER kinetics over the NCF0.05 catalyst. The OER activity of NCF0.05 is comparable to or even better than some well-known precious-metalfree OER catalysts, as listed in Table S3. In addition, the mass activity (MA, normalized to the oxide mass loading) and specific activity (SA, normalized to the real oxide surface area


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estimated from BET measurements) of the three catalysts were further calculated. As seen in Fig. 2c, at η=0.45 V (the value of η needed to deliver a current density of ~10 mA cm-2), NCF0.05 exhibited as high as ~4.8- and ~3.4-fold improvement in MA and SA (43.59 A g−1ox and 4.40 mA cm−2ox) as compared to NC (9.06 A g−1ox and 1.28 mA cm−2ox). In addition to the activity, the stability is another significant factor to evaluate the practical value of an electrocatalyst. To assess this, chronopotentiometry (CP) measurements were adapted for the NCF0.05 sample in 0.1 M KOH electrolyte. As shown in Figure 2d, NCF0.05 maintained a current density of 5 mA cm-2 at a relatively constant potential of ~1.68 V for 3 h, implying favourable stability. In contrast, the parent NC displayed unstable performance, with the overpotential increasing distinctly after ~1 h. Such difference in stability between NCF0.05 and NC suggests that Fe doping in the parent NC also enhances its stability in addition to its electrocatalytic activity. Recently, we found that the layered oxide LiCo0.8Fe0.2O2 markedly outperformed LiCoO2, with the same structure, in catalyzing OER due to the presence of a synergetic effect between the Co and Fe cations in LiCo0.8Fe0.2O2.19 In fact, synergistic effects have also been observed in some other Fe-doped electrocatalysts, such as amorphous CoFeOx,28 mesoporous Co3−xFexO429 and electrodeposited Co-Fe (oxy)hydroxides30. The better OER performance of NCF0.05 relative to un-doped parent NC is explained by the following factors, originating from the unusual synergistic interplay of Co and Fe in NCF0.05. First, NCF0.05 possesses a higher electrochemically active surface area (ECSA). Generally, an electrocatalyst with more exposed active sites has a higher catalytic activity. We approximated the ECSA via cyclic voltammetry (CV) measurements of the electrochemical double-layer capacitance (Cdl), which is determined by the linear slope of the capacitive current versus the scan rate.27 CVs were recorded at different


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scan rates of 10, 20, 40, 60, and 80 mV s-1 with an appropriate potential variation between 1.147 V and 1.197 V (Fig. 3a and Figure S5). As seen in Figure 3b, the Cdl value of NCF0.05 (1.72 mF cm-2) was larger than that of NC (1.30 mF cm-2), suggesting the higher ECSA of NCF0.05. Secondly, NCF0.05 provides better charge transfer ability. A fast charge transfer rate is crucial for efficient OER electrocatalysis.31 Charge transfer resistances (Rct) were obtained by electrochemical impedance spectroscopy (EIS) measurements (Figure 3c). The Rct was indicative of the semicircular diameter in EIS.32 The Rct value of NCF0.05 (~100 Ω) was smaller than that of NC (~200 Ω), indicating more rapid charge transfer for OER. Finally, a high proportion of active oxygen species (O22−/O−) were formed on the surface of NCF0.05. Highly oxidative O22−/O− species on the catalyst surface have been reported in previous studies to be active for OER electrocatalysis.32-33 Figure 3d displays the O 1s spectra of NC and NCF0.05, which can be deconvoluted into four different characteristic peaks, including lattice oxygen (O2−, ~528.6 eV), highly oxidative oxygen (O22−/O−, ~529.9 eV), hydroxyl groups or adsorbed oxygen (−OH/O2, ~531.2 eV), and adsorbed water or carbonate species (H2O/CO32-, ~532.4 eV).34 The molar fractions of various oxygen species were estimated from the relative area of these fitted subpeaks. The corresponding deconvolution results (Table S4) showed a larger amount of O22−/O− species on the NCF0.05 surface, which is responsible for the enhancement in OER activity. The XPS Co 2p spectra were also shown in Figure S7, and no obvious distinction in the surface chemical state of Co between NC and NCF0.05 was found.


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Fig. 3. (a) CV measurements in the non-faradic current region (1.147-1.197 V vs. RHE, no iRcorrection) at scan rates of 10, 20, 40, 60 and 80 mV s-1 for NCF0.05. (b) Linear fitting of the capacitive currents versus CV scan rates at 1.172 V vs. RHE for NC and NCF0.05. Error bars represent standard deviations from at least three independent measurements. (c) EIS of NC and NCF0.05 at 0.7 V vs. Ag|AgCl with an amplitude of 10 mV. (d) XPS spectra of O 1s spices on the surface of the NC and NCF0.05 samples. Understanding the OER electrocatalysis mechanism is helpful for designing new efficient catalysts. Currently, a single-site mechanism, which involves four electrochemical steps with coupled electron-proton transfer for each step, is most widely accepted. A sequence of reaction intermediates, HO*, O*, and HOO*, are formed on a single metal site, followed by O2 generation.35 Recently, reaction mechanisms using two-site models have also been proposed in several studies for OER in alkaline solutions.36-37 Most recently, work by Edward et al. showed


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profound insight into the role of transition metal (Fe, Co, W) coordination and their electron structure in enhancing the energetics for OER in the dual-site mechanism.36 Their experimental and computational studies revealed that a favourable local coordination environment for Co sites can be produced by incorporating metal elements (Fe and W), which ultimately tunes the catalytic performance. Analogously, we proposed a possible dual-metal-site OER mechanism for NCF0.05, as shown in Figure 4, to account for the property modifications and promoted activity after doping Fe into the parent NC. Notably, we found that there was no obvious distinction between the Co 2p spectral peaks for NC and NCF0.05 (Figure S7), implying no evident change in the oxidation state and electron structure of Co. Here, the incorporation of Fe into NC oxides may give birth to a favourable local coordination environment for Co cations.36 Such unique coordination could provide a direct coupling pathway for two metal (Co and Fe) oxo species to form O-O bonds (step 4, Figure 4) and accelerate the entire reaction.37 Therefore, dual metal sites on the surface of NCF0.05 could have more advantages in OER catalysis over single metal sites on the surface of NC (Figure S8). Admittedly, accurate mechanistic insights into OER electrocatalysis of NCF0.05 oxide are currently still limited, thus future in-depth work, e.g., modeling methods and advanced in-situ techniques, are required to confirm this.


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Figure. 4. Schematic illustration of the dual-metal-site mechanism for OER on the surface of NCF0.05, where blue and yellow represent species in the solution and on the catalyst surface, respectively. Inset is the unit cell of NCF0.05. In summary, a layered oxide NCF0.05 prepared by a facile sol-gel method was developed as an efficient OER electrocatalyst with excellent activity and stability in alkaline media. The lowcost and earth-abundant NCF0.05 catalyst demonstrated a small η of ~0.45 V at a current density of 10 mA cm-2 and a Tafel slope of 50 mV decade-1, which is comparable to the benchmark IrO2 catalyst. In addition, NCF0.05 showed better stability than its parent NC. Compared with the parent NC, NCF0.05 possessed a much higher OER activity, which can be attributed to the following factors, originating from the synergistic interplay of Co and Fe: increased ECSA, faster charge transfer rate, and larger amount of active oxygen species (i.e., O22−/O−). The dualmetal-site catalytic mechanism was also proposed to possibly rationalize the synergistic interplay of Co and Fe in NCF0.05, which contributed to the enhanced catalytic performance. This work


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creates opportunities for a new family of Na-containing layered oxides as efficient, low-cost and earth-abundant OER catalysts in alkaline media. ASSOCIATED CONTENT Supporting Information. Experimental details; Figure S1-S8; Table S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. ASSOCIATED CONTENT Corresponding Author *E-mail: [email protected]. Tel: (+86) 25-83172256. Fax: (+86) 25-83172242. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Key Projects in the Nature Science Foundation of Jiangsu Province under contract No. BK2011030, the National Nature Science Foundation of China under contract No. 21576135, the Major Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the CAS Interdisciplinary Innovation Team, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Program for Changjiang Scholars, the Program for Jiangsu Specially-Appointed Professors, and the Youth Fund in Jiangsu Province under contract No. BK20150945. REFERENCES (1) Cox, P. M.; Betts, R. A.; Jones, C. D.; Spall, S. A.; Totterdell, I. J., Acceleration of Global Warming due to Carbon-Cycle Feedbacks in a Coupled Climate Model. Nature 2000, 408, 184187.


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(17) Lu, Z.; Wang, H.; Kong, D.; Yan, K.; Hsu, P.-C.; Zheng, G.; Yao, H.; Liang, Z.; Sun, X.; Cui, Y., Electrochemical Tuning of Layered Lithium Transition Metal Oxides for Improvement of Oxygen Evolution Reaction. Nat. Commun. 2014, 5, 4345. (18) Han, B.; Qian, D.; Risch, M.; Chen, H.; Chi, M.; Meng, Y. S.; Shao-Horn, Y., Role of LiCoO2 Surface Terminations in Oxygen Reduction and Evolution Kinetics. J. Phys. Chem. Lett. 2015, 6, 1357-1362. (19) Zhu, Y.; Zhou, W.; Chen, Y.; Yu, J.; Liu, M.; Shao, Z., A High-Performance Electrocatalyst for Oxygen Evolution Reaction: LiCo0.8Fe0.2O2. Adv. Mater. 2015, 27, 71507155. (20) Grosjean, C.; Miranda, P. H.; Perrin, M.; Poggi, P., Assessment of World Lithium Resources and Consequences of Their Geographic Distribution on the Expected Development of the Electric Vehicle Industry. Renewable and Sustainable Energy Rev. 2012, 16, 1735-1744. (21) Vikström, H.; Davidsson, S.; Höök, M., Lithium Availability and Future Production Outlooks. Appl. Energy 2013, 110, 252-266. (22) Taylor, S. R., Abundance of Chemical Elements in the Continental Crust: a New Table. Geochim. Cosmochim. Acta 1964, 28, 1273-1285. (23) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S., Sodium-Ion Batteries. Adv. Funct. Mater. 2013, 23, 947-958. (24) Shannon, R. D., Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A 1976, 32 (5), 751-767. (25) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F., Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (26) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z., Synthesis of Highly Active and Stable Spinel-Type Oxygen Evolution Electrocatalysts by a Rapid Inorganic Self-Templating Method. Chem. Eur. J. 2014, 20, 12669-12676. (27) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z., Metal–Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136 (39), 13925-13931. (28) Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette, C. P., Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340, 60-63. (29) Xiao, C.; Lu, X.; Zhao, C., Unusual Synergistic Effects upon Incorporation of Fe and/or Ni into Mesoporous Co3O4 for Enhanced Oxygen Evolution. Chem. Commun. 2014, 50, 1012210125. (30) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W., Cobalt–Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. (31) Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X., Hierarchical ZnxCo3–xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889-1895. (32) Liu, R.; Liang, F.; Zhou, W.; Yang, Y.; Zhu, Z., Calcium-Doped Lanthanum Nickelate Layered Perovskite and Nickel Oxide Nano-Hybrid for Highly Efficient Water Oxidation. Nano Energy 2015, 12, 115-122. (33) Chou, N. H.; Ross, P. N.; Bell, A. T.; Tilley, T. D., Comparison of Cobalt-based Nanoparticles as Electrocatalysts for Water Oxidation. ChemSusChem 2011, 4, 1566-1569.


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Table of Contents graphic


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Figure 1. (a) XRD patterns of the as-synthesized NC, NCF0.05, and NCF0.1 samples. (b) Schematic illustration of the layered crystal structure of NCF0.05 oxide. (c, d) SEM images of the NC and NCF0.05 powders. Insets are the enlarged images. 119x99mm (300 x 300 DPI)


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Figure 2. (a) OER polarization curves of NC and NCF0.05 at a scan rate of 5 mV s-1 in an O2-saturated 0.1 M KOH solution on an RDE (1600 rpm). (b) Corresponding Tafel plots. (c) MA and SA of NC and NCF0.05 at an overpotential of 0.45 V. Error bars represent standard deviations from at least three independent measurements. (d) Chronopotentiometry response of NC and NCF0.05 at a constant current density of 5 mA cm-2disk. 119x94mm (300 x 300 DPI)



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Fig. 3. (a) CV measurements in the non-faradic current region (1.147-1.197 V vs. RHE, no iR-correction) at scan rates of 10, 20, 40, 60 and 80 mV s-1 for NCF0.05. (b) Linear fitting of the capacitive currents versus CV scan rates at 1.172 V vs. RHE for NC and NCF0.05. Error bars represent standard deviations from at least three independent measurements. (c) EIS of NC and NCF0.05 at 0.7 V vs. Ag|AgCl with an amplitude of 10 mV. (d) XPS spectra of O 1s spices on the surface of the NC and NCF0.05 samples. 119x94mm (300 x 300 DPI)


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Figure. 4. Schematic illustration of the dual-metal-site mechanism for OER on the surface of NCF0.05, where blue and yellow represent species in the solution and on the catalyst surface, respectively. Inset is the unit cell of NCF0.05. 85x82mm (300 x 300 DPI)


Na0.86Co0.95Fe0.05O2 layered oxide as highly efficient water

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