Electrochemical Capacitance of Ni-doped Metal. Organic Framework & Reduced Graphene. Oxide Composites: More than the Sum of its

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1 SUPPORTING INFORMATION Electrochemical Capacitance of Ni-doped Metal Organic Framework & Reduced Graphene Oxide Composites: More than the Sum of its Parts Parama Chakraborty Banerjee 1, Derrek E. Lobo 1, Rick Middag 1, Woo Kan Ng 1, Mahdokht E. Shaibani 1, Mainak Majumder 1* 1 Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia * mainak.majumder@monash.edu TABLE OF CONTENTS S1. Graphene oxide (GO) synthesis S2. Electrical conductivity measurements S3. Parametric calculations and data analysis S4. SEM micrographs of MOF-5 doped with 10% and 30% Ni S5. TEM and EDX of MOF-5 Ni50% and MOF-5 Ni50% rgo50% S6. Surface area and pore textural properties of MOF-5 Ni50% and MOF-5 Ni50% rgo50% S7. Conductivity of MOF-5 Ni50% rgo as a function of rgo concentration S8. Electrochemical response of the titanium (Ti) current collector 1

2 S9. Electrochemical response of rgo S10. Electrochemical response of the MOF- 5 electrodes due to Ni addition S11. Electrochemical response of MOF-5 Ni50% electrodes due to rgo addition S12. EIS data analysis using an electrical equivalent circuit S1. Graphene oxide (GO) synthesis Graphene oxide (GO), the precursor for reduced graphene oxide (rgo) was synthesized from natural graphite powder (provided by SER Pty Ltd) using a modified Hummers method 1. Briefly, 50 ml of concentrated sulphuric acid (H 2 SO 4 ) was heated at 90ºC in a 300 ml beaker with 5 g of potassium persulphate (K 2 S 2 O 8 ) and 5 g of potassium pentoxide (P 2 O 5 ). This mixture was stirred till the salts completely dissolved, and then it was cooled to 80ºC. 10 g of natural graphite powder was added to this solution for approximately 30 min. This mixture was kept at 80ºC for 4.5 h and was diluted with 1 L of deionized (DI) water, which was left overnight. The next day it was filtered and washed using a 0.2 μm nylon filter, the solid residue was placed in a Petri dish and allowed to air dry overnight. For the oxidation, 230 ml of concentrated H 2 SO 4 was placed into a 2 L Erlenmeyer flask and chilled to 0ºC via an ice bath. The pre-treated graphite was added to this along with 30 g of potassium permanganate (KMnO 4 ). The mixture was slowly stirred while the temperature was maintained below 10ºC. This solution was then allowed to react at 35ºC for 2 h, after which 460 ml of DI water was added in ml aliquots. This procedure was performed in an ice bath with the temp not exceeding 50ºC. This is then stirred for 2 h after which 1.4 L of DI water is added followed by 25 ml of 30% H 2 O 2. After 24 h the clear supernatant was decanted. The remaining solution is centrifuged and washed with 2.5 L of 10% HCl solution followed by 2.5 L DI water and then put through dialysis for 2 weeks. This was then air dried and diluted to the desired concentration. 2

3 S2. Electrical conductivity measurements Conductivity was measured by pressing a small amount of MOF powders between two electrodes (Figure S1).. Compression (P) amounting to 2.76 MPA was applied for each test. The total length of the electrodes was measured with and without the material present to obtain the length of material added ded (L). An Agilent B2902A source measuring unit (SMU) with Agilent Quick IV software was used. The current was measured as the electrode potential was swept from V to V. The scan rate used in the measurements was V/s with a measurement taken every V. The conductivity was calculated using the following formulae, Where, ρ is the resistivity, R is the electrical resistance, A is the cross sectional area and l is the length of material. A was a constant, the area of the 3 mm inner diameter of the rubber tube. L was measured for each test with Vernier calipers. The conductivity of the material is simply 1/. Figure S1. Schematic of the conductivity measurement setup. Material was compressed between two conducting electrodes in a tube with inner diameter of 3 mm 3

4 S3. Parametric calculations and data analysis S3.1 Parametric calculations The average specific capacitance ( ) was calculated from CV curves using the following formulae 2 :. Here, is the oxidation or reduction current (in ma), is the time differential, is the mass of the active material in the electrode and indicates the voltage range of one sweep segment. Energy density (E) was derived from the CV plots using the following equation 2, 1 2 The power density (P) was evaluated using the following equation 2, Here, Δt represents the time required for a sweep segment in the CV plot. S3.2 Fitting procedure to analyse the impedance data using an electrical equivalent circuit (EEC) In the present study, complex nonlinear least squares fitting (CNLS) was used to analyse the impedance data. In this method, all the data are simultaneously fitted to a given electrical equivalent circuit (EEC) containing a set of unknown parameters (e.g., circuit elements), which may enter nonlinearly in the formula for the measured function of frequency and impedance. CNLS also provides uncertainty estimates for all the estimated parameters in a given EEC. CNLS procedure applies minimization of the sum of square functions (S) 3, s k = w i = 1 a i [ a a 2 f ( ω; p) ] + w b f b b( ω; p) ei f t i ei f t 2 4

5 f ( p) Where, t ω; is a function of both the angular frequency (ω) and a set of EEC parameters p. i = 1,., k represents the data points associated with ω i ( ). f t ω; p can be divided into two parts, f a ( ω ; p) t and f b ( ω ; p) t, which both depend on the same set of parameters and can either represent Z real and Z imaginary, or Z and θ (phase angle of impedance) respectively. a ω i and b ω i are the weights related to the i th data point. a f ei and b f ei are the experimentally obtained data values. When f b ( ω ; p) t =0, CNLS reduces to ordinary nonlinear least squares 3. In the present study, since a f ei and b f ei values do not vary over several orders of magnitude, a b the fitting procedure was chosen to be unity weighted (i.e., ω i = ω i =1). The circuit description code (CDC) used by Boukamp 4, has been followed in the present study. S4. SEM micrographs of MOF-5 doped with 10% and 30% Ni (a) (b) 1 μm 1 μm Figure S2. SEM micrographs of MOF-5 Ni10% and MOF-5 Ni30% The SEM micrographs show that the flake like morphology of MOF-5 (Figure 1a) did not change significantly due to Ni doping. 5

6 S5. TEM and EDX of MOF-5 Ni50% and MOF-5 Ni50% rgo50% (a) (b) 5 nm 5 nm C O Cu (c) Ni Zn Ni Zn Figure S3. High resolution TEM micrographs of (a) MOF-5 Ni50% and (b) MOF-5 Ni50% rgo50%. (c) EDX analysis of MOF-5 Ni50% High resolution TEM of MOF-5 Ni50% rgo50% shows presence of MOF-5 Ni50% crystals distributed on thin interconnected sheets of rgo. EDX analysis of MOF-5 Ni50% (Figure S3c) showed presence of Ni and Zn along with C, O and Cu. Cu and the majority of C signal appears due to the Cu- holey carbon grid used for sample loading, whereas it is very difficult to delineate the source of O peak as it can appear due to the presence of terephthalic acid ligand or contamination. 6

7 S6. Surface area and pore textural properties of MOF-5 Ni50% and MOF-5 Ni50% rgo50% Table S1 summarizes the surface area, average pore diameter and total pore volume of MOF-5 Ni50% and MOF-5 Ni50% rgo50%. The BET and the Langmuir surface areas of MOF- 5 Ni50% are in the similar order of magnitude reported by Huang et al 5, Saha et al 6 and Petit et al 7. The difference in surface areas of MOF-5 Ni50% compared to other reports 5-7 on MOF-5 can be attributed to the temperature at which MOF was synthesized and also the way of outgassing during sample preparation 7. Consistent with the literature 7, the BET and Langmuir surface areas decreased due to rgo addition to MOF-5 Ni50%, which is not surprising since the quantity of porous MOF per unit gram of the composite is lower (50% of the total composite). The average pore diameter of the MOF and the composite are 63 Å and 195 Å respectively indicating that both these materials were mesoporous, which is consistent with the literature 6. In fact, the nitrogen adsorption isotherms (Figure S4) clearly indicate that both these materials were mainly consisted of pores with diameters larger than micropores 8. The pore volume of the composite is marginally higher than MOF, which can be attributed to the formation of new pore spaces at the interface of MOF and rgo 9. Table S1 Surface area and pore textural properties of MOF-5 Ni50% and MOF-5 Ni50% rgo50% Material BET specific surface area (m 2 /g) Langmuir specific surface area (m 2 /g) Adsorption average pore diameter (4V/A) (Å) Single point adsorption total pore volume at P/P0 (cm 3 /g) MOF-5 Ni50% MOF-5 Ni50% rgo50%

8 Figure S4. Nitrogen adsorption isotherm of MOF-5 Ni50% and MOF-5 Ni50% r rgo50% S7. Conductivity of MOF-5 Ni50% rgo as a function of rgo concentration Figure S5. Conductivity of MOF-5 Ni50% with different concentrations of rgo MOF-5 and MOF-5 Ni50% samples were insulators. The conductivity (Figure S5) data shows a slow increase with concentration, but beyond 10% rgo the increase is rapid. This is not surprising since rgo nanosheets are initially scattered and develops interconnectivity at higher concentration of rgo (Figures 1c-f and 2b). 8

9 S8. Electrochemical response of the titanium (Ti) current collector (a) (b) Figure S6. (a) CV and (b) Nyquist plot of Ti current collector The CV of the Ti current collector is shown in Figure S6a. The current response of the Ti current collector was orders of magnitude lower than the MOF-5 Ni and MOF-5 Ni50% rgo electrodes. The specific capacitance for the Ti current collector was 5 mf/cm 2, which is at least two orders of magnitude smaller than MOF-5 Ni50% (607 mf/cm 2 ) and MOF-5 Ni50% rgo50% (1.28 F/cm 2 ). The capacitance arising due to the Ti current collector was subtracted from the MOF electrodes in order to eliminate any contribution of Ti current collector on the overall capacitance of the MOF electrodes. The semicircular nature of the Nyquist plot (Figure S6b) confirms that the Ti current collector behaves as a resistive material and the total resistance of the current collector is orders of magnitude higher than the total resistance of the MOF electrodes. 9

10 S9. Electrochemical response of rgo (a) (b) Figure S7. (a) CV and (b) Nyquist plot of rgo The CV (Figure S7a) and Nyquist (Figure S7b) plots are representative of the predominance of the charge transfer processes associated with the electrostatic charging of the electrochemical double layer with some faradaic contributions from surface functional groups. S10. Electrochemical response of the MOF- 5 electrodes due to Ni addition (a) (b) Figure S8. (a) CV and (b) Nyquist plots of MOF-5 as a function of Ni doping concentration 10

11 Distinct redox peaks were visible in the CV plots (Figure S8a) of MOF-5 electrodes due to Ni addition. The current densities increased with increasing Ni concentration. This is further confirmed by the Nyquist plot (Figure S8b), which suggests that increase in the Faradaic redox activity due to increase in Ni concentration decreases the charge transfer resistance of the MOF-5 electrodes. The redox reactions at the MOF electrodes were highly reversible (even after 500 cycles of CV) as is shown in Figure S9a. However, the redox peaks gradually disappear with an increase in the voltage scan rate (Figure S9b) from mass transport limitations at higher scan rates 10. (a) (b) Figure S9. (a) Reversibility of the redox reactions in case of MOF-5 Ni50% over 500 cycles of CV and (b) diminution of the redox current density as a function of scan rate 11

12 S11. Electrochemical response of MOF-5 Ni50% electrodes due to rgo addition (a) (b) Figure S10. (a) CV and (b) Nyquist plots of MOF-5 Ni50% as a function of rgo concentration Specific capacitance of an electrode is directly proportional to the area under the CV plot. It is evident from Figure S10a that the maximum specific capacitance was achieved in case of the highest rgo concentration. In fact, the broad CV response at the lower over potential range (especially prominent in the reduction segment of the plot) in case of MOF-5 Ni50% rgo50% was not prominent at low rgo concentration. This is further confirmed by the Nyquist plots of the MOF-5 Ni50% rgo composites (Figure S10b). The shape of the Nyquist plots at lower concentrations of rgo resembles MOF-5 Ni50% but starts resembling the capacitive response of rgo at higher concentration of rgo in the composite. S12. EIS data analysis using an electrical equivalent circuit Q 1 Q 2 R s R 1 R 2 12

13 Figure S11. Proposed EEC to analyse the impedance data of the MOF electrodes In a composite system, where both Faradaic redox reactions and electrochemical double layer charging processes coexist, the capacitance, C p, arising due to the Faradaic redox reactions is coupled in parallel with the double layer capacitance, C dl, via a Faradaic reaction resistance (R p ). Such a system then has two regimes of dispersion of impedance with frequency; (i) one at relatively high frequencies due to the charge transfer processes associated with the double layer formation and (ii) another at relatively lower frequencies due to the charge transfer processes associated with the redox reactions 11. Contributions of the charge transfer processes due to Faradaic redox reactions and electrochemical double layer formation can be distinguished if the dispersions of impedance can be easily resolved in a complex plane plot of the real and imaginary components. However, it may not be possible to decouple the double layer and Faradaic processes from each other if; (i) R p, which decouples these two processes, is small 11 and/or (ii) the charge transfer due to electrochemical double layer charging and Faradaic redox reactions occur simultaneously 12. Particularly in case of a porous electrode, such as MOF, when C p >C dl, the distinction between these two processes is significantly difficult. This is attributed to the distributed nature of both C p and C dl throughout the porous matrix with a gradually increasing electrolyte resistance down the pore 11. The electrical equivalent circuit (EEC) used to analyze the impedance data for the different MOF electrodes are shown in Figure S11 and consists of an electrolyte resistance (R s ) and two time constants, each connected in series with the other. Each time constant consists of a constant phase element (Q) and a resistance (R) connected in parallel. The incorporation of constant phase element in the EEC provided a much better agreement of the experimental data with the simulated one. Use of constant phase element instead of a capacitance (C) is generally attributed to the electrode porosity, surface reactivity, roughness, current and 13

14 potential distributions associated with electrode geometry and anion adsorption kinetics 13. The first time constant (represented by Q 1 and R 1 ) represents the charge transfer processes and the second time constant (represented by Q 2 and R 2 ) represents the mass transfer processes through the pores. Decoupling of Faradaic from the non-faradaic charge transfer processes in case of a porous electrode is exceedingly difficult and often not possible 11. Hence, the time constant representing the charge transfer processes accounts for both the Faradaic and non-faradaic charge transfer processes. Hence, R 1 represents the total charge transfer resistance of the system and is the most important parameter for this study. The simulated impedance data were in good agreement with the experimentally obtained one for the MOF electrodes. Representative simulated data and the observed Bode impedance plots of MOF-5 Ni50% rgo50% are shown in Figure S12a. Error plots for all the MOF electrodes show that the maximum error in the simulated data was less than 4% for both the modulus of impedance ( Z ) and the phase angle. A representative error plot is shown in Figure S12b. (a) (b) Figure S12. (a) A comparison of the simulated data and the experimentally obtained impedance data in case of MOF-5 Ni50% rgo50% and (b) representative error plot for the experimental and simulated impedance data. 14

15 The calculated parameters (Rs, Q 1, R 1, Q 2 and R 2 ) are shown in Table S2. Table S2. Simulated values for the parameters associated with the proposed EEC. All the parameters are normalised with respect to 1 g of electro-active (MOF or MOF/rGO) material Sample R s (Ω) Q 1 (Ω s n ) Q 1 _n R 1 (Ω) Q 2 (Ω s n ) Q 2 _n R 2 (Ω) MOF MOF-5 Ni50% MOF-5 Ni50% rgo50% REFERENCES 1. Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D., Layer-by-Layer Assembly of Ultrathin Composite Films from Micron-Sized Graphite Oxide Sheets and Polycations. Chem. Mater. 1999, 11, Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H., Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132, Macdonald, J. R., Impedance Spectroscopy Theory, Experiment, and Applications. In DATA ANALYSIS [Online] Second Edition ed.; Barsoukov, E.; Macdonald, J. R., Eds. John Wiley & Sons, Inc.: Hoboken, New Jersey, Boukamp, B. A., Electrochemical Impedance Spectroscopy in Solid State Ionics: Recent Advances. Solid State Ionics 2004, 169,

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