Graphene metal organic framework composites and their potential applications 6

Chapter 6: Graphene Oxide and Copper-centered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER and ORR Abstract: A composite of graphene oxide and copper-centered metal organic framework shows good performance as a tri-functional catalyst for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Efficient catalysts for HER, OER and ORR are of paramount importance for electrochemical energy applications in fuel cells, batteries and electrochemical water splitting. Oxygen reduction and generation reactions are quite irreversible with large overpotential, and this irreversibility causes serious efficiency losses in electrochemical system involving oxygen electrodes. One of the challenges in the area of electrocatalysis is to find an effective catalyst that will reduce as well as generate oxygen at moderate temperatures. Here we described the synthesis of a composite made from intercalated graphene oxide and Cu (II)-centred MOF which shows enhanced electrocatalytic properties. 6.1 Introduction With rising concerns about energy shortage, there are intense efforts worldwide to find renewable and green energy sources as alternatives to fossil fuels. Efficient catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction 125 reaction (ORR) are of paramount importance for electrochemical energy applications in fuel cells, batteries and electrochemical water splitting.1-4 However, the high cost of Pt catalysts commonly used for such applications restrict widespread applications. The ongoing search is for a non-Pt catalyst that can rival the performance of Pt catalyst.5 Reduced graphene oxide (GO) based materials have been studied extensively as electrode materials and catalyst supports due to its high electrical conductivity, high specific surface area and very efficient charge transfer at the electrodes.6-8 GO is bifunctional in the third dimension because it contains epoxy and hydroxyl functional groups on both sides of its surfaces.9 GO sheets can be used as pillar connectors in metal organic framework (MOF). With the incorporation of GO, it may overcome the poor electron-conductive properties of most MOFs which exclude them from being used as electrode materials or electrocatalysts. Previous attempts to improve the electrical conductivity of MOFs hybrids include mixing them with conductive phases like carbon nanotubes, fuctionalized graphenes, and metal nanocrystals.10-13 Although this can provide electron conduction at a macroscopic level, local charge transport is still limited in the MOF due to size-exclusion effects defined by the pore apertures. Therefore our motivation is to explore whether the incorporation of GO as an integral component of the framework will lower the overpotential and improve the charge transfer properties. Cu compounds exhibit biomimetic chemistry with O2 such as the reductive activation of O2 in enzymes and the protein Laccase. 14-16 It is interesting to construct copper MOF on GO as an alternative to Pt-based electrodes. To date, most copper complexes have been studied in alkaline media17-21 due to their corrosion and instability in acid media. To address the stability issue, we designed a GO/copper–MOF hybrid structure which can coordinate to two strong electronegative ligands based on oxygen and nitrogen functional groups, thus leading to an 126 improvement in the framework stability, especially when it is encapsulated by GO in acid media. GO sheets become an integral component of the MOF framework by acting as struts to link MOF nodes, besides serving as a good electron transfer mediator. 6.2 Experimental Section Graphite Oxide (GO) was prepared using a method mentioned in chapter 3. Cu-MOF:47 Copper nitrate trihydrate (0.740 g), 1,4-benzenedicarboxylic acid (0.680 g) and triethylene-diamine (0.480 g) were dissolved in 150 mL DMF. The final mixture was sealed in a small capped Teflon vial and sonicated to ensure homogeneity. The autoclave vial was heated at 120 °C in an oven for 36 h followed by slow cooling to room temperature. The blue crystals were collected via filtration and washed with DMF several times and dried under vacuum. GO (2, 4, 6, wt %) Cu-MOF: Copper nitrate trihydrate (0.740 g), 1,4-benzene dicarboxylic acid (0.680 g) and triethylenediamine (0.480 g) were dissolved in 150 mL DMF. Different amounts of GO (2, 4, 6, and wt %: based on the total mass of starting solid materials) were added to the mixtures. After sonication the solution, the final mixture was transferred into a small Teflon capped autoclave. The autoclave was heated at 120 °C in an oven for 36 hours followed by slow cooling to room temperature. The blue-grayish crystals were collected filtrated and washed with ample DMF and dried under vacuum. Electrochemistry RDE and RRDE experiments were carried out on a RRDE-3A (ALS Co., Ltd) and the CH instruments electrochemical workstation (CH instrument, Inc. Austin) bipotentiostat. RDE measurements were performed at rotation rates varying from 500 to 3500 rpm and with the scan 127 rate of mV/s. Linear sweep voltametry was performed at glassy carbon disk electrode with a 3mm diameter, Pt electrode, and Ag/AgCl reference electrode. For RRDE measurement, Pt ring / GC disk electrode was used. Prior to use, the working electrode was polished mechanically with diamond down to alumina slurry to obtain a mirror-like surface and then washed with DI water and acetone and allowed to dry. mg of catalyst and 80 μl of wt% Nafion solution were dispersed in ml of 4:1 v/v water/ethanol by at least 30 sonication to form a homogeneous ink. Then μl of the catalyst ink (containing 16 μg of catalyst) was loaded onto glassy carbon electrode. The electrode was allowed to dry at room temperature for 30 in a desiccator before measurement. After drying, a catalyst loading of 226.3 µg/cm2 was obtained. All the potentials reported are against the reversible hydrogen electrode (RHE). In order to measure fuel cell property, the Nafion 1135 membranes was used and cleaned by being immersed in boiling 35% H2O2 for 45 min, and then in boiled M H2SO4 for 45 min. The membrane was then rinsed in boiling deionized water for 30 and the procedure was repeated at least twice to remove the sulfuric acid completely. On completing the purification procedure, the membrane was stored in M H2SO4 at room temperature prior to use. A single stack fuel cell was assembled from a membrane/electrode assembly, two stainless steel plates with flow manifolds on the supply sides for gas and water, and two Teflon gaskets. The anode was a cm2 20% Pt on Carbon black (Alfa Aesar Co., HiSPECTM 3000) electrode with a platinum loading of 0.8 mg/cm2. The cathode was prepared from a suspension containing (70%) of catalyst, (30%) of wt% Nafion (Sigma Aldrich) recast solution, and ethanol had been ultrasonically blended for h to form a uniform and homogenous ink. The suspension ink was spread uniformly across the surface of a carbon paper substrate followed by drying in the vacuum oven at 80 °C for h. For a cathode area of cm2, the amount of catalyst suspension 128 applied to the cathode was 2.7 mg/cm2 Cu wt% and 1.8 mg/cm2 wt% Pt. A single cell assembly was prepared by sandwiching a Nafion 1135 membrane between the cathode and the anode and was pressed by the stainless steel plates. The cell was connected to a conventional gas flow system. The pure hydrogen and oxygen gases were provided to fuel cell through the mass flow controller from storage tank. Pure H2 was introduced to the anode compartment. The gas of O was also introduced to the cathode compartment. Flow rates of the cathode and anode gases were controlled by the mass-flow valves (40 ml/min). The fuel-cell reaction was operated at 80 oC. A current– potential curve was measured by a digital multimeter (GDM-8145) and DC Electronic Load (0-360V/150W). 6.3 Results and Discussion The chemical structures of the subunits in the assembled MOF are illustrated in Scheme 6.1. The MOF is created by linking Copper nitrate trihydrate, 1,4-benzenedicarboxylic acid (bdc) and triethylene-diamine (ted), abbreviated as Cu-MOF, as illustrated in Scheme 6.1(b). The secondary building units (SBU) is a paddle-wheel Cu2(COO)4(ted)2 unit as shown in Scheme 6.1(c). Each paddle-wheel SBU is linked by bdc within the layer to form a 2D net parallel to the xy plane, which is further connected by ted molecules to produce the 3D framework. Generally, the octahedral geometry of SBU with six connections renders the cubic structure. Considering two different ligands (bdc and ted) at the apical and equatorial positions, the paddle-wheel unit is changed from the cubic structure to the tetragonal structure. The composite formed by the combination of GO and starting materials of pure MOF (bdc, ted , and Cu2+) is named as (GO-X wt%) Cu-MOF (Scheme 6.1(d)). The improved catalytic performance of the composite can be expected based on two factors: (1) the electron accepting ability of the nitrogen and oxygen atoms which polarizes the adjacent copper atom in the framework; (2) The high surface area of 129 GO also enhances the electron transfer during electrochemical reactions. Cu-MOF, which is a 3D framework of the paddle-wheel unit, is synthesized by hydrothermal reaction. The structurecomposition relationship was investigated by mixing different weight percentages of GO (2, 4, 6, wt %) with the chemical precursors of Cu-MOF to synthesize (GO-X wt %) Cu-MOF composites where X = 2, 4, or wt %. GO sheets decorated by OH and epoxy groups on either side of the sheets (Scheme 6.1(a)) are analogous to pillar connectors such as 1,4-benzene dicarboxylic acid used in classic MOF synthesis, which serve as bifunctional linkers for the paddle wheel unit. Scheme 6.1 Schematic of the chemical structures of (a) GO, (b) Cu-MOF, (c) The paddle-wheel secondary building units of pure Cu-MOF and (d) (GO X % wt) Cu-MOF which is synthesized via hydrothermal reaction between GO, Copper nitrate trihydrate, 1,4-benzenedicarboxylic acid and triethylene-diamine in DMF. 130 The effects of GO on the crystallization and electrochemical properties of the MOF was studied systematically using optical absorption, vibrational spectroscopy, X-ray crystallography, cyclic voltammetry (CV), linear Sweep Voltammetry (LSV), rotating disk electrode (RDE) and rotating ring disk electrode (RRDE). Figure 6.1 UV-Vis absorption spectra of (i) Cu-MOF (3.5 mg L-1), (ii) (GO wt %) Cu-MOF (3.2 mg. L-1), and (iii) GO (2.5 mg L -1) in methanol. Figure 6.1 shows UV-Vis absorption spectra of GO, Cu-MOF and (GO wt %) CuMOF in methanol. The absorption peak of GO at 221 nm is due to the characteristic -plasmon absorption.22-25 Cu-MOF shows absorption bands at 239 to 296 nm due to π→π* transition.26,27,28 The presence of GO in (GO wt %) Cu-MOF creates a new band at 223 nm and a red shift in absorption bands compared to Cu-MOF. The functional groups present in GO, Cu-MOF and the composite are characterized by FTIR. As shown in Figure 6.2, the vibrational peaks of GO are consistent with the presence of fingerprint groups such as carboxylic species, hydroxyl species and epoxy species (C=O, 1734 cm-1; OH deformation, 1400 cm-1; the C-OH stretching, 1230 131 cm-1; C-O-C (epoxy group) stretching, 1061 cm-1; skeletal ring stretch, 1624 cm-1). The CuMOF is characterized by fingerprint bands at 243 cm-1, 269 cm-1, 305 cm-1, 339 cm-1, 574 cm1, 750 - 933 cm-1, 1053 cm-1, 1294 cm-1, 1400 - 1601 cm-1, 1686 cm-1, which can be assigned to s(Cu-N), as(Cu-N), s(Cu-O), as(Cu-O), Cu-O stretching, C-H benzene ring, C-O vibration, C-N stretching, phenyl C=C ring stretch, and C=O vibration of COOH, respectively. The redshift of the C=O stretch from 1734 cm-1 to 1686 cm-1 compared to the corresponding band for GO is due to the Cu:O coordination. Figure 6.2 FTIR spectra of (i) Cu-MOF, (ii) (GO wt %) Cu-MOF, and (iii) GO. Figure 6.3 shows AC impedance spectroscopy study of (GO wt %) Cu-MOF and pure Cu-MOF. The (GO wt %) Cu-MOF shows smaller faradic impedance compared to Cu-MOF, confirming that GO can act as a good electron relay network. 132 Figure 6.3 AC impedance spectroscopy study of (GO wt %) Cu-MOF and pure Cu-MOF at  =0.14 V. Thermal stability of (GO wt %) Cu-MOF composite was examined by TGA study (Figure 6.4). The composite was highly stable up to 250 °C. Even for ambient-exposed samples, no significant weight loss due to water desorption is observed, which points to the hydrophobic nature of the Cu-MOF channels. The major mass reduction at ~ 250 - 300 °C was caused by pyrolysis of the oxygen-containing functional groups in graphene oxide. The major mass reduction at ~ 400°C was due to the structural collapse of MOF. The (GO wt %) Cu-MOF shows smaller Faradaic impedance, suggesting that the addition of GO improves the conductivity of the composite. Since the conductivity of GO is related to its oxygen content, according to elemental analysis, the C/O ratio of GO is around 2.4 while the C/O ratio of the composite is around 4.5 indicating the C/O ratio increased through the synthetic process under pressure and high temperature. 133 Figure 6.4 (a) TGA plots for freshly synthesized (GO wt %) Cu-MOF, Cu-MOF and GO under N2 at 10 oC /min. (b) TGA and DSC plots for (GO wt %) Cu-MOF. (c) TGA and DSC plots for Cu-MOF. 134 good straight line, as shown in Figure 6.15(b). The slopes of the straight lines in this plot are close to one, 1, which indicates that there is a single electron transfer process to the adsorbed oxygen on the electrode surface, following the reaction (O2)ads + H + + e-  (HO2)ads as the ratedetermining step.35,36 Figure 6.15 (a) Koutecky–Levich plots of (GO wt %) Cu-MOF at different potentials. (b) Plots for the determination of the reaction order with respect to O for ORR on (GO wt %) Cu-MOF electrode . (c) Mass transfer corrected Tafel plots for ORR on (GO wt %) Cu-MOF and Cu-MOF electrodes. Figure 6.15(c) shows the mass transport-corrected Tafel plots for which the oxygen reduction kinetics studies were conducted in 0.5 M H2SO4 O2-saturated at 25 °C. The Tafel plots 149 were obtained after the measured currents were corrected for diffusion to give the kinetic currents in the mixed activation-diffusion region, calculated from equation: jK = j jL / ( jL – j ) where jL / ( jL - j ) is the mass transfer correction term.37 The mass transport correction was made using the limiting current jL = B . ω ½ where ω is the rotation rate . The kinetic parameters deduced for the oxygen reduction are presented in Table 6.3. Figure 6.16 (a) Rotating disk electrode (RDE) linear sweep voltammograms of ORR for (GO wt % ) Cu-MOF in O2-saturated 0.5 M H2SO4 with various rotation rates at a scan rate of mV/s. (b) Koutecky– Levich plots of (GO wt % ) Cu-MOF, Cu-MOF and GO at -0.25V. (c) RDE voltammograms of CuMOF, GO, and (GO wt % ) Cu-MOF at a rotation rate of 3500 rpm. (d) The dependence of the electron transfer number on potential for (GO wt % ) Cu-MOF, Cu-MOF and GO at various potentials. 150 Oxygen reduction occurred by different mechanisms in low and high current density regions. In the low current density region, the Tafel slope for (GO wt %) Cu-MOF is 69 mV/dec , while in high current density regions the slope is 132 mV/dec (Figure 6.15(c)). For O2 reduction by Pt in acid, a Tafel slope of 60 mV/dec can be obtained38 and there is no detectable amount of peroxide measured indicating that O2 reduction occurred by the four-electron pathway.34 The change of Tafel slope from 69 mV/dec to 132 mV/dec suggests a change in the reaction mechanism from Temkin type adsorption at low current to Langmuir adsorption condition at high current.38-41 Rotating disk electrode (RDE) study of ORR for is shown in Figure 6.16. Figure 6.17 Comparison of ORR performance among our compounds. Comparing ORR current density of (1) GO, (2) (Graphene wt %) Cu-MOF (physically mixed graphene and Cu-MOF], (3) CuMOF, (4) (GO wt %) Cu-MOF, (5) (GO wt %) Cu-MOF, (6) (GO wt %) Cu-MOF. The inset shows ORR RDE voltammograms of (1) GO, (2) (Graphene wt %) Cu-MOF, (3) (Cu-MOF), (4) (GO wt %) Cu-MOF, (5) (GO wt %) Cu-MOF, (6) (GO wt %) Cu-MOF at a rotation rate of 3500 rpm (scan rate mV/s). 151 Figure 6.17 compares the ORR kinetics of the various synthesized samples. The onset potential for ORR is the first to be reached in the (GO wt %) Cu-MOF (at 0.29 V vs. RHE) and the oxygen reduction current density of this electrode, at -5.3 mA/cm2, is higher than the rest of the composites and the reduction potential has shifted to more positive potentials. Once again, this suggests that the incorporation of GO into the MOF improves the electrocatalytical behavior. However, increasing the GO content to 20 wt % resulted in a decrease electrocatalytic activity due to dilution effect and decreasing of the crystallinity (Figure 6.18). Figure 6.18 The effect of increasing GO content on the structure of the MOF. XRD pattern of (i) Cu-MOF, (ii) (GO wt %) Cu-MOF , (iii) (GO 10 wt %) Cu-MOF, (iv) (GO 15 wt %) Cu-MOF ,(v) (GO 20 wt %) Cu-MOF, and (vi) GO Increasing the content of GO in the composite from wt % to 20 wt % causes aggregation of GO sheets, as seen in the SEM image (Figure 6.19(a &b)). GO plays the role of an impurity intercalant in the basic framework of MOF (Figure 6.19(c)). The catalytic activity is 152 decreased by increasing the GO content to 20 wt %, due to the dilution effect on the catalytic properties of copper as an active site (Table 6.4). The comparison between RDE voltammograms of ORR for (GO wt %) Cu-MOF and (GO 20 wt %) Cu-MOF shows that the current density is decreasing indicating the lower catalytic activity of (GO 20 wt %) Cu-MOF compared to (GO wt %) Cu-MOF (Figure 6.19(d)). Furthermore, LSV of HER shifted to more negative voltages for (GO 20 wt %) Cu-MOF compared to (GO wt %) Cu-MOF (Figure 6.19(e)). These results reveal that the porosity, the content of active site, and the amount of nitrogen groups in the structure play important roles in the performance of the catalyst. Table 6.4 CHN elemental analysis of MOF and its composites Elemental analysis report No. Sample ID C wt % N wt % Cu wt % Cu-MOF 43.08 5.27 20.03 (GO wt %) Cu-MOF 43.54 5.16 19.84 (GO wt %) Cu-MOF 43.95 5.07 19.49 (GO 10 wt %) Cu-MOF 44.47 4.94 18.87 (GO 15 wt %) Cu-MOF 45.02 4.79 18.58 (GO 20 wt %) Cu-MOF 46.89 4.26 16.07 153 Figure 6.19 The effect of increasing GO content on the morphology and catalytic activity. SEM images for (a) (GO wt %) Cu-MOF and (b) (GO 20 wt %) Cu-MOF. (c) XRD pattern of (i) (GO wt %) Cu-MOF and (ii) (GO 20 wt %) Cu-MOF. (d) RDE voltammograms of ORR for (GO wt % ) CuMOF [blue curve] and (GO 20 wt %) Cu-MOF [red curve] at a rotation rate of 3500 rpm in 0.5 M H2SO4 - O2 saturated. (e) LSV polarisation curves of HER for (GO wt % ) Cu-MOF , and (GO 20 wt % ) CuMOF in 0.5 M H2SO4 - N2 saturated ; scan rate: mV/s at 25oC. Importantly, our catalyst exhibits good stability compared to Pt/C in acid media. The ORR current did not decrease significantly after 100 cycles suggesting the catalyst has good stability in acid media. (Figure 6.20 & 6.21). 154 Figure 6.20 Cyclic voltammograms of (GO wt %) Cu-MOF in 0.5 M H2SO4 at scan rate of 50 mV/s after 50 cycles. Figure 6.21 ORR stability of (GO wt %) Cu-MOF catalyst dispersed on glassy carbon electrode in 0.5 M H2SO4- oxygen saturated. Cycles were swept between V and -0.4 V at mV/s, 25oC, and 3500 rpm. The durability of (GO wt %) Cu-MOF as ORR catalyst for cathode was evaluated as shown in Figure 6.22. An important criterion for a good electrocatalyst is strong durability. To assess this, we cycled our composite continuously for 1000 s. At the end of cycling the 155 composite catalyst in (b) afforded more stable i-V curves compared to Pt/C black. The test was performed using chronoamperometry at a constant voltage of -0.2 V in 0.5 M H2SO4 solution saturated with O2. The corresponding current–time chronoamperometric response of (GO wt %) Cu-MOF exhibits a very slow attenuation after a 49% loss in its initial current density. In contrast, the Pt cathode shows a current loss of 58%. This result suggests good durability of (GO wt %) Cu-MOF catalyst. Figure 6.22 ORR current-time chronoamperometric responses of (a) (GO wt %) Cu-MOF and (b) Pt (20 wt % / C) electrode on a GC electrode at -0.2 V in O2-saturated 0.5 M H2SO4. The percentages are reference to the initial current at time zero. To further examine the ORR catalytic pathways of our composite, we carried out rotating ring-disk electrode (RRDE) measurement in order to monitor the peroxide species formation during the process. Figure 6.23(a) gives the RRDE polarization curves for O2 reduction on the Cu-MOF and (GO wt %) Cu-MOF in 0.5 M H2SO4 saturated with oxygen. The H2O2 yield and the number of electrons transferred during the process can be determined by the followed equations: 156 n = N. ID / (N. ID +IR) % H2O2 = 200 IR / ( N. ID +IR ) N = - IR/ ID Where ID is disk current, IR is ring current and N is current collection efficiency. Figure 6.23(b) shows the H2O2 yield on the catalysts. The measured H2O2 yields are below 50 % and 20 % for Cu-MOF and (GO wt %) Cu-MOF, respectively. The RRDE results confirm our results calculated from the Koutecky - Levinch equation that the ORR electron number is close to from -0.1 V to -0.4 V for the (GO wt %) Cu-MOF, but about between to on the Cu-MOF over the potential range (Figure 6.23(c)). Based on ORR response, the electrocatalytic performance of GO/Cu-MOF composites in a single Polymer Electrolyte Membrane Fuel Cell (PEMFC) was evaluated next. In order to compare the performance, the same mass of Cu (2.7 mg Cu/cm2) was used in all samples and for comparison 1.8 mg/cm2 wt% Pt was loaded at the cathode. A loading of 0.8 mg Pt /cm2 was used in the anode. Figure 6.24(a) shows the current density versus cell voltage of a single cell obtained for membrane electrodes assembly (MEA). The polarization curve of (GO 4, 6, wt %) Cu-MOF is higher than that of Cu-MOF in the low and high current region which reveals the catalytic activity of the composite is higher than pure MOF. The open circuit voltages for (GO wt %) Cu-MOF is 0.73 V while that of Pt is 0.93 V. 157 Figure 6.23 Investigation of peroxide percentage in ORR catalyzed by our catalysts. (a) Rotating ring–disk electrode voltammogrames recorded for Cu-MOF and (GO wt %) Cu-MOF electrodes, respectively, in O2-saturated 0.5 M H2SO4 at 3500 rpm (scan rate mV/s). (b) Peroxide percentage of CuMOF and (GO wt %) Cu-MOF at different potentials calculated from RRDE measurement. (c) The electron transfer number (n) of Cu-MOF and (GO wt %) Cu-MOF at various potentials based on the corresponding RRDE data in (a). Figure 6.24(b) shows power density curves using Cu-MOF, (GO 4, 6, wt %) Cu-MOF and commercial 20 wt % Pt/C as the catalysts for the cathodes. A maximum power density of 145.2 mW/ cm2, 110.5 mW/ cm2 and 40.4 mW/ cm2 was achieved for Pt, (GO wt %) Cu-MOF and Cu-MOF, respectively. Although the power density of the Cu-MOF composite cathodes is only 76 % that of Pt/C catalyst, its performance as a lower cost substitute is quite comparable to previous reports using non-precious catalysts.42-46 158 Figure 6.24 (a) Polarization and (b) Power density curves for the H2/O2 fuel cell MEAs operating at 80 o C with different cathodes catalysts (1) 20 wt. % Pt/C, (2) (GO wt %) Cu-MOF, (3) (GO wt %) CuMOF, (4) (GO wt %) Cu-MOF, and (5) Cu-MOF. The higher catalytic activities of our composites compared to pure MOF or GO alone for all electrochemical reactions (HER, OER, and ORR) should be attributed to the synergistic effects of framework porosity, a larger bond polarity due to oxygen ligand in the GO and the catalytically active copper in the hybrid MOF. Physically mixing the Cu MOF and GO gave relatively poorer response in ORR and OER (Figure 6.25) compared to the GO-intercalated Cu MOF. GO plays the role of a charge sink to allow rapid charge transfer in redox reaction, especially at the covalently bonded interface between the Cu-MOF and GO. Our work has confirmed that the incorporation of GO in MOF helps to improve the catalytic performances in electrochemical catalysis. 159 Figure 6.25 Comparison of the performance between (GO wt % ) Cu-MOF composite and a sample prepared by physical mixing wt % GO and pristine Cu-MOF. (a) LSV polarisation curves of ORR in 0.5 M H2SO4 - N2 saturated; scan rate: mV/s at 25oC. (b) LSV polarisation curves of OER in 0.5 M H2SO4- N2 saturated; scan rate: mV/s at 25oC. 6.4 Conclusion We have investigated GO intercalated Cu-MOF composite as a tri-functional catalyst for ORR, OER and HER. The GO-intercalated composites exhibit smaller overpotentials and higher current for all electrocatalytic reactions and show better stability in acid media compared to pure MOF. 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Energy & Environmental Science 2011, 4, 3167. (47) Yong Lee, J; Olson,D ; Emge, T.J ; Li, J Adv. Funct. Mater. 2007, 17, 1255 164 [...]... (220) o o 18.70 (312) 11.80 (002) 160 0 o 21. 36 (320) Intensity 1400 o o o 26. 46 29.79 (132) (340) 1200 38.58 (114) o 43.52 ( 460 ) 1000 800 60 0 (ii) o 20.91 (213) o 8.28 (101) o 18.04 (004) o 29.20 o (323) 38 .68 o (307) 42.27 (530) o 400 o 26. 17 (231) 16. 53 (211) 200 (i) 0 10 20 30 40 50 60 2 Figure 6. 6 Evolution in the morphology and the crystal structure of the catalysts with different weight % of GO... Cu-MOF, (ii) (GO 8 wt %) Cu-MOF Table 6. 1 X-ray powder diffraction peaks and indices of (i) Cu-MOF, (ii) (GO 8 wt %) Cu-MOF (i) (ii) h k l 1 0 1 Angle (degree) 2 8.28 h k l 1 1 0 Angle (degree) 2 8.29 2 1 1 16. 53 2 0 0 11.8 0 0 4 18.04 2 2 0 16. 71 2 1 3 20.91 3 1 2 18.7 2 3 3 1 26. 17 3 2 0 21. 36 2 3 29.2 1 3 2 26. 46 3 0 7 38 .68 3 4 0 29.79 5 3 0 42.27 1 1 4 38.58 4 6 0 43.52 137 The electrocatalytic... (mV/dec) 89 71 65 61 Tafel slop b2 (mV/dec) 211 182 157 122 1.54 1.42 1.34 1.43 Onset potential (V vs RHE) 1.39 1.22 1.19 1.30 Highest Current density (mA/cm2) 2.1 4.2 5.8 12.5 Tafel slop b1 (mV/dec) 98 79 69 60 Tafel slop b2 (mV/dec) Reactions 1 96 142 132 121 -0.21 -0.09 -0.03 0.82 Onset potential (V vs RHE) 0. 16 0.24 0.29 0.90 - Current density (mA/cm2) at -0.4 V 1.10 3.73 5.32 6. 17 Overpotential (V... of (1) GO, (2) (Graphene 2 wt %) Cu-MOF (physically mixed graphene and Cu-MOF], (3) CuMOF, (4) (GO 4 wt %) Cu-MOF, (5) (GO 6 wt %) Cu-MOF, (6) (GO 8 wt %) Cu-MOF The inset shows ORR RDE voltammograms of (1) GO, (2) (Graphene 2 wt %) Cu-MOF, (3) (Cu-MOF), (4) (GO 4 wt %) Cu-MOF, (5) (GO 6 wt %) Cu-MOF, (6) (GO 8 wt %) Cu-MOF at a rotation rate of 3500 rpm (scan rate 2 mV/s) 151 Figure 6. 17 compares the... H2SO4 nitrogen-saturated solution was investigated for (GO 2, 4, 6, 8 wt %) Cu-MOF composites, pure Cu-MOF and compared with commercial Pt catalyst (20 wt % Pt on carbon black) (Figure 6. 10(a)) The onset potential for HER is observed at -0.202 V for Cu-MOF, and the values decrease steadily from -0.123 V to 0.087 V and approaches the onset potential of Pt/C (0.013 V) as the composition of GO in the hybrid... report No 1 2 3 4 5 6 Sample ID C wt % N wt % Cu wt % Cu-MOF 43.08 5.27 20.03 (GO 4 wt %) Cu-MOF 43.54 5. 16 19.84 (GO 8 wt %) Cu-MOF 43.95 5.07 19.49 (GO 10 wt %) Cu-MOF 44.47 4.94 18.87 (GO 15 wt %) Cu-MOF 45.02 4.79 18.58 (GO 20 wt %) Cu-MOF 46. 89 4. 26 16. 07 153 Figure 6. 19 The effect of increasing GO content on the morphology and catalytic activity SEM images for (a) (GO 8 wt %) Cu-MOF and (b) (GO 20... (Figure 6. 12) Figure 6. 12 LSV polarisation curves of OER for Pt/C and (GO 8 wt % ) Cu-MOF in 0.5 M H2SO4 - N2 saturated ; scan rate: 2 mV/s at 25oC The kinetics of the OER for (GO 2, 4, 6, 8 wt %) Cu-MOF electrodes were determined by recording Tafel polarization (E vs log j) curves at a slow scan rate (2 mV/s) Each curve as shown in Figure 6. 11(b) displays two Tafel slopes, one at low overpotential and. .. monoand di-carboxylates in the paddlewheel structures.30,31 Considering the high density of hydroxyl and epoxy groups on GO planes, the twisting of the bdc ligand arising from covalent bonding on GO planes may result in the phase transformation of MOF observed here, especially when a higher concentration of GO is added 1 36 (g) o o 1800 8.29 (110) 16. 71 (220) o o 18.70 (312) 11.80 (002) 160 0 o 21. 36 (320)... number on potential for (GO 8 wt % ) Cu-MOF, Cu-MOF and GO at various potentials 150 Oxygen reduction occurred by different mechanisms in low and high current density regions In the low current density region, the Tafel slope for (GO 8 wt %) Cu-MOF is 69 mV/dec , while in high current density regions the slope is 132 mV/dec (Figure 6. 15(c)) For O2 reduction by Pt in acid, a Tafel slope of 60 mV/dec... , (c) Raman, and (d) XRD pattern 135 The morphology evolution of GO, Cu-MOF crystal and (GO 2, 8 wt %) Cu-MOF is shown in SEM images Figure 6. 6 It is interesting to see the marked change in the morphology of the MOF crystal with increasing concentration of GO intercalants in the MOF The crystal phase of the composite was investigated by Powder X-ray diffraction (Figure 6. 6(g) & Table 6. 1) Cu-MOF is . 6: Graphene Oxide and Copper-centered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER and ORR Abstract: A composite of graphene oxide and copper-centered metal organic. (101) 2 Intensity 8.28 o (004) 18.04 o (213) 20.91 o (231) 26. 17 o (323) 29.20 o (307) 38 .68 o (110) 8.29 o (002) 11.80 o (320) 21. 36 o (220) 16. 71 o (312) 18.70 o (132) 26. 46 o (340) 29.79 o (114) 38.58 o ( 460 ) 43.52 o (211) 16. 53 o (530) 42.27 o (i) Figure 6. 6 Evolution in the morphology and the crystal structure. 2 1 1 16. 53 2 0 0 11.8 0 0 4 18.04 2 2 0 16. 71 2 1 3 20.91 3 1 2 18.7 2 3 1 26. 17 3 2 0 21. 36 3 2 3 29.2 1 3 2 26. 46 3 0 7 38 .68 3 4