Supporting Information Enhanced Pseudocapacitance in Multicomponent Transition-Metal Oxides by Local Distortion of Oxygen Octahedra Hyeon Jeong Lee +, Ji Hoon Lee +, Sung-Yoon Chung,* and Jang Wook Choi* anie_201511452_sm_miscellaneous_information.pdf
Supporting Information S1
Experimental Section 1. Preparation of GO powder. Graphene oxide (GO) was first obtained by a modified Hummer s method. Briefly, 2 g of graphite (Alfa Aesar) and 1 g of sodium nitrate (NaNO3, Aldrich) were dissolved in concentrated sulfuric acid (H2SO4, 46 ml, Aldrich) by vigorous stirring for 1 h in an ice bath. 8 g of potassium permanganate (KMnO4, Aldrich) was then slowly added to the suspension until its color became dark gray. Next, deionized (DI) water (96 ml) was added to the suspension drop by drop to obtain graphite oxide solution. Finally, warm DI water (280 ml) and hydrogen peroxide solution (20 ml, 50 wt%, Aldrich) were added to eliminate inorganic residues. This solution was filtrated and washed with DI water several times until its ph value reached neutral. The solution was then freeze-dried to obtain graphite oxide powder. 5 g of the graphite oxide power was dissolved in 300 ml of DI water, and this solution was tip-sonicated (80 W) for 2 h in an ice bath. The sonicated solution was centrifuged for 30 min at 9500 rpm, and the supernatant solution was freeze-dried for 5 days to obtain GO powder. 2. Preparation of MO-rGO samples. The series of metal oxide (MO; M=Ni, Co, Mn and Mix) nanoparticles were grown on reduced graphene oxide (rgo) through a simple chemical synthetic step followed by a heat treatment; First, GO powder (0.15 g) was thoroughly dispersed in DI water (100 ml) inside a bath sonicator (200 W). Another solution (50 ml) containing metal precursors (1 mmol) with designated stoichiometric ratios was added to the GO solution and sonicated again for 30 min while the temperature is carefully monitored (<30 o C) to prevent unwanted precipitation. The metal precursors were Ni(CH3COO)2 4H2O, Co(CH3COO)2 4H2O, and Mn(CH3COO)2 4H2O (Aldrich). The S2
solution was then heated at 72 o C, and hydrazine hydrate solution (N2H4 H2O, 50 wt%, 1.5 ml, Aldrich) was added. After 1 h of the reduction step, black powder was filtrated and washed with DI water and acetone several times by using an alumina membrane (pore size = 200 nm, Whatman). After vacuum drying (70 o C, 12 h), except for MnO-rGO, the dried powder was heated at 180 o C for 1 h under Ar flow (300 sccm) and then at 180 o C in air for another 1 h to complete the synthesis. For MnOrGO, the dried powder was heated at 800 o C for 2 h under vacuum (<0.05 Torr) to produce the Mn with bivalency. The heat treatment at a lower temperature produces Mn with higher oxidation states. 3. Characterization. X-ray diffraction (XRD, Rigaku Smart Lab) analyses were performed to characterize the crystal structures of the samples. To observe the morphologies of the samples, field emission-transmission electron microscopy (FE-TEM, TECNAI) was used. X- ray absorption near edge spectroscopy (XANES) measurement was performed in 7D beam line at Pohang Accelerating Laboratory (PAL) in Republic of Korea. The K- edge spectra were recorded in the fluorescence mode at room temperature under He flow. A calibration for each K-edge spectrum was achieved by using the reference spectrum from the corresponding metal foil. The metal oxide contents in the samples were determined using thermogravimetric analyses (TGA, Setsys 16/18, Setaram, France, see Figure S1). The porosity data were collected from N2 adsorptiondesorption measurements by using Micromeritics 3FLEX (USA) at 77 K. The specific surface area (SSA) was calculated based on the Brunauer-Emmett-Teller (BET) method [S1] and the specific pore volume (Vpore) was obtatined by the amount of N2 S3
adsorbed at relative pressure of ~0.99. Pore size distribution curves were attained from the Barrett-Joyner-Halenda (BJH) model. [S2] 4. Electrochemical Measurements. Each electrode was fabricated by first dispersing active material, super-p (TIMCAL), and poly(vinylidene difluoride) (PVDF, Aldrich) in 1-methyl-2-pyrrolidine (NMP, Sigma Aldrich) at a mass ratio of 8:1:1. The slurry was then cast on a conductive carbon paper (Toray, Waterproofed, TGP-H-090, see Figure S3), followed by a vacuum dry step at 70 o C for 12 h. Ni plate used as a control current collector was purchased from Aldrich (>99.9 %). The mass loadings of the active materials were 0.5~0.6 mg cm -2. 1 M sodium hydroxide (NaOH, Aldrich) aqueous solution was used for electrolyte. Platinum (Pt) mesh and Hg/HgO were used for the counter and reference electrodes, respectively. For electrochemical tests, galvanostatic charge/discharge curves and cyclic voltammetry (CV) curves were recorded using a battery cycler (VMP3, Biologic, France). The specific capacitances (Cs, F g -1 ) were obtained from the galvanostatic profiles through the following equation: C s = I t m V Where I is the applied current (A), t is the discharge time (sec), m is the mass of electrode active material (g) and V is the potential change (V) during discharge. 5. DFT Calculations. Ab initio DFT calculations were carried out using the spin-polarized local density approximation (LDA) functional for exchange correlation along with the ultrasoft pseudopotentials for ionic cores, as implemented in the CASTEP code (Materials S4
Studio ver. 8.0, Biovia). To account for the electron localization around transition metal ions, the LDA + U method with the Hubbard U parameters was employed for all the geometry optimizations. The values of U for 3d electrons were set to be 5.1 ev for Ni, 5.0 ev for Co, and 5.04 ev for Mn. [S3,S4] All the trasition metal ions were assumed to be high-spin. The plane-wave basis set for a kinetic energy cutoff was 600 ev. A full optimization of the internal coordinates for each 2a 2b 2c or 3a 3b 3c NiO supercell was performed using the BFGS algorithm with convergence tolerances of 0.1 ev/å for the maximum ionic force, 5 10 5 ev/atom for the total energy, and 0.005 Å for the maximum ionic displacement. S5
Figure S1. Thermogravimetric analyses (TGA) of MO-rGO composites: (a) MixOrGO, (b) NiO-rGO, (c) CoO-rGO, and (d) MnO-rGO. S6
Figure S2. CV curve of a Pt plate in the voltage range from -0.05 V to 1 V (vs. Hg/HgO) at a scan rate of 10 mv s -1. Oxygen evolution reaction (OER) is detected above 0.6 V (vs. Hg/HgO) in 1 M NaOH. S7
Figure S3. CV curves of the conductive carbon paper (black and red lines for the 1 st and 100 th cycles, respectively) and Ni plate (blue and green lines for the 1 st and 40 th cycles, respectively) at a scan rate of 10 mv s -1. Throughout the cycling, Ni plate undergoes the repeated phase transition between Ni and NiO and NiOOH. S8
Figure S4. The galvanostatic charge/discharge curves of (a) NiO-rGO and (b) CoOrGO at various current densities from 2 A g -1 to 10 A g -1. S9
Figure S5. CV curves at a scan rate of 10 mv s -1 in the voltage range from -0.05 V to 0.55 V (vs. Hg/HgO): (a) MixO-rGO, (b) NiO-rGO, (c) CoO-rGO, and (d) MnO-rGO. The equilibrium redox potentials are denoted with gray dotted lines. S10
Figure S6. The electrochemical tests of reduced graphene oxide (rgo) in the voltage range of 0~0.55 V (vs. Hg/HgO). (a) CV curve at 10 mv s -1. (b) Galvanostatic charge/discharge curve at 2 A g -1. S11
Figure S7. The electrochemical performance of Ni0.5Co0.5O-rGO. (a) Comparative galvanostatic charge/discharge curves of MixO-rGO and Ni0.5Co0.5O-rGO at 2 A g -1. (b) Capacitance changes at different current densities. (c) CV curves at various scan rates ranging from 10 mv s -1 to 60 mv s -1. (d) The power law relation between the anodic/cathodic peak current and the scan rate for Ni0.5Co0.5O-rGO. S12
Figure S8. CV curves for (a) MixO-rGO, (b) NiO-rGO, and (c) CoO-rGO at various scan rates ranging from 10 mv s -1 to 60 mv s -1. The power law relation between the anodic/cathodic peak current and the scan rate for (d) MixO-rGO, (e) NiO-rGO, and (f) CoO-rGO. The enhanced electrochemical performance of MixO-rGO was elucidated by monitoring the current density (A g -1 ) at various scan rates (Figure S8) in CV measurements. First, the higher specific capacitances of MixO-rGO were verified by its larger integrated areas in the CV curves compared with those of NiO-rGO and CoO-rGO. It was also consistently observed that at all the scan rates, the redox reactions of MixO-rGO occurred within the stable electrochemical window below the OER regime (Figure 8a) owing to the sufficiently small overpotentials, in contrast with the other two individual MO-rGO cases. In electrochemical processes, the response of the current density to the scan rate variation can be described by a power law: i = av b, where i is the current (A) from redox reaction, a and b represent arbitrary coefficients, and V is scan rate (mv s -1 ). [S5-S7] From the coefficient b, the limiting electrochemical step can be identified [S8-S9] between the charge transfer on the S13
electrode surface and the carrier diffusion in the solid phase; if b is close to 1, the former is the case, and if b is close to 0.5, the latter is the case. As shown in Figure S8(d)-(f), the coefficient b values of MixO-rGO, obtained from the slopes in log (peak current density) vs. log (scan rate) curves, are closer to 1 than those of NiO-rGO and CoO-rGO. This result indicates that compared to the MO-rGO control cases, MixOrGO is less limited by the carrier ion diffusion in the oxide host for its overall kinetics, which is once again associated with the facilitated redox flip by the reduced Jahn Teller effect on [NiO6] octahedra. By contrast, the carrier ion diffusion in the MO framework is more limiting than the surface charge transfer for the other two MO-rGO cases, implying that the carrier ion diffusion can be hindered even for the small size particles, if the physicochemical environment surrounding the redox species is not favorable. This trend is also commensurate with the XANES results in the main text that reveal the greater oxidation state change of MixO-rGO. S14
Figure S9. (a) Nitrogen adsorption-desorption isotherms of rgo (black) and MixOrGO (red) at 77 K. (b) BJH pore size distribution curves from the corresponding desorption branches in (a). To reveal the effect of the interface between metal oxide NPs and rgo support, N2 adsorption/desorption isotherm curves were obtained for MixO-rGO and rgo. Both samples give the type 4 isotherms with a capillary condensation at P/P0 = ~0.45, indicating that MixO NPs do not affect the nitrogen adsorption/desorption isotherm profiles. Also, in considering their BET surface areas (bottom inset in figure below) and the content (~45 wt%) of MixO NPs, it can be deduced that SSA of MixO-rGO originates mainly from that of rgo (419.2 m 2 g -1 x 0.55= 230 m 2 g -1 ). Thus, the effect of the interface, if any, is very negligible. S15
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