ELECTROCATALYTICALLY ACTIVE NANOCOMPOSITE MATERIAL AND A PRODUCTION METHOD THEREFOR

Abstract

A first aspect of the invention relates to an electrocatalytically active nanocomposite material, comprising electrically conductive carbon material decorated with platinum nanoparticles or nanoclusters anchored thereon. The decorated electrically conductive carbon material is overcoated with catecholamine-based polymer. Another aspect of the invention relates to a method for producing electrocatalytically active nanocomposite material.

Claims

1. An electrocatalytically active nanocomposite material, comprising electrically conductive carbon material decorated with platinum nanoparticles or nanoclusters anchored thereon, wherein the electrically conductive carbon material and the platinum nanoparticles or nanoclusters anchored thereon are overcoated with catecholamine-based polymer.

2. The electrocatalytically active nanocomposite material as claimed in claim 1, wherein the electrically conductive carbon material comprises graphitized carbon material.

3. The electrocatalytically active nanocomposite material as claimed in claim 1, wherein the electrically conductive carbon material and the platinum nanoparticles or nanoclusters anchored thereon are overcoated with polydopamine.

4. The electrocatalytically active nanocomposite material as claimed in claim 1, wherein the electrically conductive carbon material comprises a catecholamine surface functionalization and wherein the platinum nanoparticles or nanoclusters are anchored on the surface functionalization of the electrically conductive carbon material.

5. The electro catalytically active nanocomposite material as claimed in claim 1, forming a porous three-dimensional network.

6. The electrocatalytically active nanocomposite material as claimed in claim 1, provided in the form of a suspension.

7. An electrocatalyst, comprising the electrocatalytically active nanocomposite material as claimed in claim 1.

8. A fuel cell comprising the electro catalyst of claim 7.

9. An electrolyser comprising the electrocatalyst of claim 7.

10. A method for producing electrocatalytically active nanocomposite material, comprising: depositing platinum nanoparticles or nanoclusters on electrically conductive carbon material using a suspension of the electrically conductive carbon material in a solution of chloroplatinic acid so as to form platinum-decorated carbon material; and overcoating the platinum-decorated carbon material with catecholamine-polymer.

11. The method as claimed in claim 10, wherein the electrically conductive carbon material comprises graphitized carbon material.

12. The method as claimed in claim 10, wherein the platinum-decorated carbon material is overcoated with polydopamine using a dispersion of the platinum-decorated carbon material in a dopamine salt solution.

13. The method as claimed in claim 10, wherein the electrically conductive carbon material is provided with a catecholamine surface functionalization before deposition of the platinum nanoparticles or nanoclusters.

14. A method for producing an electrocatalyst, comprising: dispersing electrocatalytically active nanocomposite material comprising electrically conductive carbon material decorated with platinum nanoparticles or nanoclusters anchored thereon, the electrically conductive carbon material and the platinum nanoparticles or nanoclusters anchored thereon being overcoated with catecholamine-based polymer, and depositing the dispersed electrocatalytically active nanocomposite material on a substrate so as to create a porous three-dimensional network of the electrocatalytically active nanocomposite material.

15. The method as claimed in claim 14, wherein the substrate comprises at least one of a polymer electrolyte membrane and a gas diffusion layer.

16. The electrocatalytically active nanocomposite material as claimed in claim 2, wherein the graphitized carbon material comprises carbon nanotubes, graphene, or graphite flakes.

17. The electrocatalytically active nanocomposite material as claimed in claim 6, wherein the suspension is a colloidal suspension.

18. The fuel cell of claim 8, wherein the fuel cell is a direct methanol fuel cell or a polymer electrolyte membrane fuel cell.

19. The method as claimed in claim 11, wherein the graphitized carbon material comprises carbon nanotubes, graphene, or graphite flakes.

20. The method as claimed in claim 14, wherein depositing the dispersed electrocatalytically active nanocomposite material on the substrate is carried out using an LBL spraying or dipping technique.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings illustrate several aspects of the present invention and, together with the detailed description, serve to explain the principles thereof. In the drawings:

[0029] FIG. 1: is a schematic illustration of a Pt-decorated CNT overcoated with catecholamine-based polymer;

[0030] FIG. 2: is a schematic illustration of a Pt-decorated graphene particle overcoated with catecholamine-based polymer;

[0031] FIG. 3: is a schematic illustration of a Pt-decorated carbon black particle overcoated with catecholamine-based polymer;

[0032] FIG. 4: is a schematic exploded view of a multi-layered electrode assembly comprising an electrocatalyst according to a preferred aspect of the invention

[0033] FIG. 5: shows a transmission electron microscopy (TEM) of Pt-decorated multiwalled carbon nanotubes (MWNT/Pt) on the left and the corresponding Pt particle size distribution histogram on the right;

[0034] FIG. 6: shows a TEM of PDA-coated Pt-decorated multiwalled carbon nanotubes (PDA-MWNT/Pt) on the left and the corresponding Pt particle size distribution histogram on the right;

[0035] FIG. 7: shows the XPS spectrum of the MWNT/Pt of FIG. 5;

[0036] FIG. 8: shows the XPS spectrum of the PDA-MWNT/Pt of FIG. 6;

[0037] FIG. 9: is a cyclic voltammogram of the MWNT/Pt of FIG. 5 recorded in 0.5 M H.sub.2SO.sub.4 at 25 C. and a sweep rate of 50 mV/s;

[0038] FIG. 10: is a cyclic voltammogram of the PDA-MWNT/Pt of FIG. 6 recorded in 0.5 M H.sub.2SO.sub.4 at 25 C. and a sweep rate of 50 mV/s;

[0039] FIG. 11: shows the normalized peak current plots for the MWNT/Pt of FIG. 5 and the PDA-MWNT/Pt of FIG. 6;

[0040] FIG. 12: shows the polarization curves for an electrocatalyst obtained by LBL spray deposition of PDC-coated Pt-decorated MWNTs ([PDA-MWNT/Pt].sub.50) and an electrocatalyst obtained by LBL spray deposition of Pt-decorated MWNTs ([MWNT/Pt].sub.50);

[0041] FIG. 13: shows the power density curves of [PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50;

[0042] FIG. 14: is a scanning electron microscopy (SEM) cross-section of [MWNT/Pt].sub.50 on a substrate (PEM);

[0043] FIG. 15: is a SEM cross-section of [PDA-MWNT/Pt].sub.50 on a substrate (PEM);

[0044] FIG. 16: is a cyclic voltammogram corresponding to the electrooxidation of LBL-spray deposited PDA-coated CNTs not bearing Pt nanoparticles ([PDA-MWNT].sub.50) recorded in an Ar-saturated 0.5 M H.sub.2SO.sub.4 solution at 25 C. and a sweep rate of 50 mV/s;

[0045] FIG. 17: is a cyclic voltammogram corresponding to the electrooxidation of LBL-spray deposited CNTs not bearing Pt nanoparticles ([MWNT].sub.50) recorded in an Ar-saturated 0.5 M H.sub.2SO.sub.4 solution at 25 C. and a sweep rate of 50 mV/s;

[0046] FIG. 18: is a comparison of the normalized coulomb charge plots for [MWNT].sub.50 and [PDA-MWNT].sub.50.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0047] FIGS. 1 to 3 schematically show different examples of electrocatalytically active nanocomposite material 10, 10, 10 according to preferred embodiments of the invention.

[0048] FIG. 1 illustrates a CNT 12 decorated with Pt nanoparticles 14 anchored thereon. The decorated CNT is sheathed with a coating 16 of catecholamine-based polymer.

[0049] FIG. 2 illustrates a graphene particle 18 decorated with Pt nanoparticles 14 anchored thereon. The decorated graphene is overcoated with catecholamine-based polymer 16.

[0050] FIG. 3 illustrates carbon black 20 decorated with Pt nanoparticles 14 anchored thereon. The decorated carbon black is overcoated with catecholamine-based polymer 16.

[0051] Two different catalyst supports were prepared: [0052] i) multiwalled carbon nanotubes (MWNTs) decorated with Pt nanoparticles (abbreviated MWNT/Pt; as a comparative example); and [0053] ii) MWNTs decorated with Pt nanoparticles and overcoated with PDA (abbreviated PDA-MWNT/Pt; as an example according to a preferred embodiment of the invention).

[0054] For the obtainment of the MWNT/Pt and the PDA-MWNT/Pt, MWNTs were first oxidized, creating nucleation sites for Pt nanoparticles formation during the reduction of H.sub.2PtCl.sub.4. For each example preparation, 100 mg multi-walled carbon nanotubes (MWNT) in 20 ml ethylene glycol (EG) were stirred under sonication for 10 min. 100 mg of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.6H.sub.2O) in 30 ml EG were added to the suspension under agitation, and then the solution was heated to 140 C. for 1.5 h under reflux. The solution was cooled down to room temperature and kept under agitation for 24 h. MWNT/Pt was collected by filtration and washed with deionized water.

[0055] The so-obtained MWNT/Pt exhibited a fairly uniform distribution of Pt nanoparticles over the entire length of the MWNTs, as can be seen in the transmission electron microscopy (TEM) investigations (FIG. 1). The Pt particles ranged in size from 7.25 to 8.2 nm.

[0056] The polydopamine-modified MWNT/Pt were prepared in the following way. 100 mg of the MWNT/Pt were dispersed in 200 ml of deionized water (10 mM Tris-HCl (pH 8.5)) containing 30 mM of cupric sulphate and 0.1 mg/ml of dopamine hydrochloride, before being stirred for 24 h at room temperature. The obtained PDA-MWNT/Pt were rinsed with deionized water.

[0057] One difficulty in the modification of MWNT/Pt by PDA is precipitation, which may introduce impurities to the MWNT/Pt. Such impurities can affect the structure of the PEMFCs as well as their performances. As it has been shown that the amount of precipitates depends on the dopamine concentration, .sup.[23] the latter was chosen equal to 0.1 mg/mL. FIG. 2 shows a TEM of the PDA-MWNT/Pt obtained this way.

[0058] FIGS. 7 and 8 show the XPS survey spectra of MWNT/Pt and PDA-MWNT/Pt respectively. Apart from the similar C, O of MWNT/Pt, additional N moieties are found in the spectrum of PDA-MWNT/Pt. These are due to the nitrogen-containing functional groups of polydopamine.

[0059] To investigate the electrochemical performance of PDA-MWNT/Pt and MWNT/Pt, characterizations were performed by cyclic voltammetry (CV). First, the prepared PDA-MWNT/Pt and MWNT/Pt catalysts were respectively mixed with 1%-wt of Nafion ionomer which plays the role of dispersing agent. The CVs were cycled between 0.2 and 1 V vs. SCE (saturated calomel electrode) reference electrode. For each measurement, 1.2 mg of PDA-MWNT/Pt and MWNT/Pt, respectively, were deposited on a 1 cm diameter glassy carbon electrode to perform a CV measurement. The corresponding cyclic voltammograms can be seen in FIGS. 9 and 10. The corresponding normalized peak current plots are shown in FIG. 11. The PDA-MWNT/Pt and MWNT/Pt catalysts showed similar increase in the hydrogen adsorption/desorption (H.sub.ads/des) and oxygen adsorption/desorption (O.sub.ad/des). This proves that the coverage of the MWNT/Pt system with PDA does not affect the catalytic reactions. Moreover, these results are also due to high electron and proton conductivities of the porous structure (nanotube network) allowing the reactants to reach the catalyst. The voltammograms can be compared by the calculation of the electrochemical active surface area (ECSA) of Pt of each catalyst. The double-layer capacitance is subtracted from the H.sub.ads peaks and the resulting area under the peaks is integrated. The average peak area corresponds to the total charge consumed by the Pt surface. The ECSA (in m.sup.2g.sup.1) is then obtained by:

ECSA=H.sub.ads/(210 C.Math.cm.sup.2.Math.Pt.sub.loading),

where H.sub.ads stands for hydrogen adsorption in C.Math.cm.sup.2, Pt loading is the electrode Pt loading in g.Math.m.sup.2 and the standard charge of 210 C correlates with the coverage of one cm.sup.2 surface area of three basal Pt planes.

[0060] For the performed CV experiments, the ECSA of PDA-MWNT/Pt and MWNT/Pt was found to be 18.81 m.sup.2g.sup.1 and 11.68 m.sup.2g.sup.1, respectively. The Pt loadings of PDA-MWNT/Pt and MWNT/Pt were measured with thermogravimetric analysis (TGA; Netzsch STA 409) under N.sub.2 from ambient temperature to 120 C. at a rate of 10 C./min; the temperature was then kept constant at 120 C. for 30 min in order to remove all solvent. When that was done, the composite was heated to 1000 C. with a heating rate of 10 C./min.

[0061] The numerical results show that overcoating of the MWNT/Pt catalyst with PDA improves the ECSA value. This result seems to be due to the presence of PDA which enhances the proton conductivity, the hydrophilic property of the MWNT/Pt as well as the catalytic reaction. This last feature can be observed on FIG. 11 showing the relative currents versus the cycle number.

[0062] Membrane electrode assemblies (MEAs) were prepared by alternated spraying of PDA-MWNT/Pt and MWNT/Pt catalyst, respectively, onto a Nafion 117 membrane. To this end, the membranes were first pre-treated by boiling in 3%-wt hydrogen peroxide and deionized water for 1 h, then rinsed in boiling deionized water for 1 h, put into boiling 0.5 M sulfuric acid for 30 min, and finally rinsed again in boiling deionized water for 1 h. PDA-MWNT/Pt were dispersed in isopropanol under sonication for 15 min and Nafion perfluorinated resin solution was added to the suspension to obtain a stable dispersion for LBL assembly. The suspension was sprayed onto one of the membranes in the following conditions. Each deposition layer was obtained by 1 s of spraying and left to dry for 1 s before the next layer was applied. The process was repeated until the desired thickness was obtained. In total, 50 layers of PDA-MWNT/Pt were applied. The multilayered nanocomposite stack is noted [PDA-MWNT/Pt)].sub.n, n representing the number of layers sprayed with the same suspension (here n=50). FIG. 4 shows a MEA 22 comprising a PEM 24 serving as the substrate of two electrocatalyst layers 26 (one on each side of the substrate) obtained by LBL spray deposition of PDA-MWNT/Pt. The porous network 28 formed on the PEM 24 is illustrated as a magnified detail. The porous network 28 is composed of Pt-decorated MWNTs 12 sheathed with PDA coating 16.

[0063] A multilayered nanocomposite stack of MWNT/Pt was obtained by LBL spray deposition using similar deposition parameters, in particular, the same number of spraying cycles, but slightly longer deposition times for each layer (2-3 s). The multilayered nanocomposite stack of MWNT/Pt was thus [MWNT/Pt)].sub.50.

[0064] The schematic view of the multilayered electrode assembly is shown in FIG. 4. The anodes and the cathodes of the two systems (PDA-MWNT/Pt and MWNT/Pt) were assembled in a way allowing a fine tuning of the contact area of the functional species: the Pt nanoparticles, electric conductive support (MWNT), and the proton conductive media (PDA). The intersection of these components and the gas phase constitute the so-called TPB (triple-phase boundary). In order to enhance the area of the TPB, the components were sprayed successively to form the multilayered cathodes and anodes ([PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50). This assembly technique resulted in a 3D porous network with good reactant/product transport.

[0065] In situ fuel cell measurements were performed. The polarization curves for [PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50 and the power density curves are displayed in FIGS. 12 and 13. The fuel cell measurements of the MEAs were carried out by sandwiching the MEAs between two pieces of H2315-13 carbon paper serving as gas diffusion layers (GDL), then fixing them between two bipolar plates with flow field. The electrodes were fed with H.sub.2 at a rate of 150 mL/min and high-purity O.sub.2 at a rate of 75 mL/min. A humidifier was used in order to hydrate the fuel cell. 100% humidified gases were been injected in the cell during the fuel cell test. Electrode temperature was set to 80 C. The polarization curves were collected in galvanostatic mode by using a FuelCon Evaluator C50 test bench (FuelCon AG, Germany). Maximum power densities of 780 mW cm.sup.2 and 530 mW cm.sup.2 were obtained for [PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50, respectively. The measured [PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50 had a Pt loading of 0.014 mg cm.sup.2 and 0.020 mg cm.sup.2, respectively. The loading of the anode was kept the same for both electrodes. Starting with the maximum power density and the Pt loading of the anode and cathode, Pt utilization was calculated. Pt utilizations for [PDA-MWNT/Pt].sub.50 and [MWNT/Pt].sub.50 were 6051 W g.sub.(Pt).sup.1 and 2912 W g.sub.(Pt).sup.1. Note that the two values of Pt utilization reflect the results from the CV experiments, where both systems had different ECSA. It is remarkable that both systems show differences in terms of performance and that the presence of PDA seems to strongly boost fuel cell performance.

[0066] FIGS. 14 and 15 scanning electron microscopy (SEM) micrographs of [MWNT/Pt].sub.50 and [PDA-MWNT/Pt].sub.50 taken using a Quanta FEG 200 (FEI Nova, Netherland) field-emission gun scanning electron microscope. Each sample was cut to a piece of 1 cm0.2 cm on a SEM holder along the middle axis to get an insight into the electrode's morphology. The SEM micrographs of the [MWNT/Pt].sub.50 and [PDA-MWNT/Pt].sub.50 electrodes indicate very porous structures. The average thicknesses were measured to be 4.5 m and 5.6 m, respectively.

[0067] In order to compare the stability of the catalyst supports, LBL-spray deposited PDA-coated CNTs (not bearing Pt nanoparticles, [PDA-MWNT].sub.50) and LBL-spray deposited naked CNTs (not bearing Pt nanoparticles, [MWNT].sub.50) were subjected to continued cyclic voltammogram tests (1800 cycles). The results are shown in FIGS. 16-18. FIG. 16 clearly evidences that the amount of oxygen moieties on the surface of the MWNTs increases with the cycle number. This could be due to quinone (Q) and hydroquinone (HQ) redox reactions. FIG. 17 shows no inclination of the curve during cycling, which suggests that there is no change in the surface resistance and that the conductivity of PDA-MWNT/Pt remains stable during cycling. During cycling, the area under the voltammogram of PDA-MWNT does not change substantially, meaning that there are not redox peaks. This proves that PDA-MWNT films exhibit very good electrochemical stability (resistance to corrosion). FIG. 18 depicts the normalized coulomb charge HQ/Q plots for [PDA-MWNT].sub.50 and [MWNT].sub.50. The charge rate of the catalytic support can be determined by comparing the charge integral. The hydraquinone-quinone (HQ-Q) peak, indicates the peak surface oxidation currents after a certain duration. The surface charge density due to the surface reaction of the electrodes can be calculated by subtracting the pseudo-capacitance charge from total charge in the HQ-Q region and integrating the area under the peak. This can give a representation of the amount of surface oxide (C/O) groups, formed due to voltage cycling. FIG. 18 clearly suggests that [MWNT].sub.50 tends to corrode, while [PDA-MWNT].sub.50 remains stable and is thus strongly corrosion-resistant.

[0068] The cyclic voltammograms were acquired with a Gamry Reference 600 potentiostat (USA) in a standard glass three-compartment electrochemical cell with a glassy carbon working electrode (=3 mm), a Pt wire serving as counter electrode and an SCE as the reference electrode. The potential between the working electrode (WE) and reference electrode was cycled between 0.2 to 1 V with a sweep rate of 50 mV s.sup.1. The measurement curves were recorded after a stable response had been obtained. All electrochemical measurements were carried out in 0.5 M (molar) H.sub.2SO.sub.4 (ACS reagent 70%) at 25 C. The electrolyte was purged for 5 min with Ar to eliminate oxygen before testing. After each experiment, the WE was polished with 1-m diamond paste and cleaned using 2000 CVs restructuration cycles between 0-1.6 V with a sweep rate of 10 V s.sup.1 in 0.5 M H.sub.2SO.sub.4 solution to remove remaining surface oxides.

[0069] All materials in these examples (multi-walled carbon nanotubes, Nafion 117 solution, etc.) were used as received from Sigma-Aldrich.

[0070] In the example according to a preferred embodiment of the invention, a novel electrocatalytically active [PDA-MWNT/Pt].sub.50 multilayered nanocomposite film was obtained via spray deposition. The [PDA-MWNT/Pt].sub.n multilayered films showed very high performance in terms of power densities as well as very high Pt utilizations. Furthermore, we proved that PDA-MWNT supports exhibit a better oxidation resistance than MWNT. Hence PDA-MWNT supports are very interesting candidate for replacing carbon black without a loss in performance. Furthermore, a simple preparation technique for high performance and long lasting advanced electrode structures was demonstrated. A porous network of Pt-decorated MWNTs overcoated with PDA was produced using LBL spray-deposition. The porous architecture seems to ease the gas permeability leading to a better accessibility of the Pt nanoparticles.

[0071] While specific examples and embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

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