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The electrochemical behavior of a Metal-Organic Framework modified gold electrode for methanol oxidation
Electrochimica Acta 219 (2016) 630–637
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The electrochemical behavior of a Metal-Organic Framework modified gold electrode for methanol oxidation Adriana Vulcu, Liliana Olenic, Gabriela Blanita, Camelia Berghian-Grosan* National Institute for Research and Development of Isotopic and Molecular Technologies, 67-103 Donat Street RO 400293, Cluj-Napoca, Romania
A R T I C L E I N F O
Article history: Received 12 May 2016 Received in revised form 10 October 2016 Accepted 11 October 2016 Available online 13 October 2016 Keywords: Noble-Metal-Free Metal-Organic Frameworks HKUST-1 Anode Methanol Oxidation
A B S T R A C T
Direct Methanol Fuel Cells have received great interest for portable applications and electric vehicles. Even if the development of new catalysts for fuel cell is always under consideration, the platinum-based ones remain the most used for these technologies. Metal-organic-frameworks (MOF) are noble-metalfree promising materials for fuel cell industry. Although HKUST-1 is not a very stable MOF in aqueous solutions, we prove that the growth from its mother solution over different linkers (mercaptoacetic and trimesic acids) lead to durable HKUST modified electrodes: Au_MAA_HKUST and Au_TA_HKUST. Selfassembling or electrochemical procedures are used to prepare the adlayers on gold electrode. Electrochemical experiments show that the current density obtained for the Au_MAA_HKUST is almost 28 times higher than that of bulk gold electrode. The Au_TA_HKUST electrode reveals a similar response regarding the potential range, but the current densities are lower. The possibility to form the trimesic acid (a component of HKUST-1) adlayer on gold electrode offers us the opportunity to investigate the mechanism of methanol oxidation on HKUST-1 regarding both the organic linker and metallic ions. It is worth noting that the role of trimesic acid in methanol oxidation is low comparing the CuII nodes from HKUST-1. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction The possibility to convert, cleanly and efficiently, the chemical energy into electrical energy and, also, to use them in a wide range of applications has led to a growth of fuel cell industry [1,2]. Fuel cells based on methanol oxidation (Direct Methanol Fuel Cell DMFC) require a catalyst layer at anode to transform methanol into carbon dioxide. The performance of DMFC shows a strong dependence of the anode material nature and a lot of carbon and non-carbon based materials have been used over the years as electrocatalysts [3]. To date, platinum is still largely used for fuel cell industry [2] and only some alternatives to avoid Pt and Ru as anode catalysts in DMFC have been noticed [4]. Moreover, for DAFC (Direct Alcohol Fuel Cell) a few noble-metal-free catalyst have previously been reported employing NiTi [5], transition metal oxides and W-based systems [6], hierarchical Ni–Fe layered double hydroxide/MnO2 [7], porous nickel phosphates [8], Ni-Cu alloy [9], Ni hydroxide [10] or MOF materials [11]. Metal organic frameworks (MOFs) are a class of highly porous materials formed by joining together, by coordinative bonds, metal
* corresponding author. E-mail address: [email protected] (C. Berghian-Grosan). http://dx.doi.org/10.1016/j.electacta.2016.10.077 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
ions and organic linkers. The diversity of their extended networks (various pore size, large surface area, and controllable surface properties) makes them promising candidates for gas storage [12], separation [13] and heterogeneous catalysis [14–17], with a real possibility to transfer their synthesis and application from laboratory to the industry [18]. The performance and utility of MOFs as chemical sensors have been tested and several approaches to improve selective recognition of analytes have been proposed [19]. However, due to the fact that a small number of MOF materials exhibit electron conductivity, the electrochemistry of MOFs and their use in electrocatalytic reactions is significantly low [20,21]. To date, a noble-metal-free MOF material based on dimeric copper units and organic ligand (HOC2H4)2dtoa [N,N0 -bis-(hydroxyethyl)dithiooxamide] has been used for ethanol electrooxidation reaction [11,22]. Several approaches to growth MOF thin film on solid surfaces have been developed: the direct growth of MOF thin films on solid surfaces, the assembly of preformed, size and shape selected nanocrystals, the stepwise layer-by-layer growth onto the substrate, the electrochemical deposition of thin film on metal surfaces and the deposition of MOF thin film using a gel-layer method [23,24]. Self-assembled monolayers (SAMs) prove to be a powerful tool to control the lateral structure [25] and the
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crystallographic orientation [26,27] of surface-mounted MOF films [28]. HKUST-1 is a copper-based MOF, namely copper benzene tricarboxylate (Cu3(BTC)2). It was first prepared in 1999 and was intensively studied for different applications, such as: hydrogen storage [29], CO2 adsorbent [30], adsorbtion of NH3 [31], electronic applications [32]. Being largely used as adsorbents, the stability of MOFs in the environmental processes’ conditions is considered as a major challenge. Therefore, a lot of studies about water sensitivity of certain MOFs have been conducted; it was found that water content up to 0.5 mol equiv with respect to copper maintains the HKUST-1 structural characteristics, but it easily decomposes at higher water content [33]. For electrochemical application, the HKUST-1 material must be immobilized on the electrode surface to achieve the contact [28]; one method consists in growing it on different self-assembled monolayers [34]. For an oriented growth of HKUST-1 several monolayers, with different terminations (COOH, OH and CH3), can be used. The monolayer nature has a significant influence on the film orientation. In the case of COOH terminated monolayers, the carboxylic acid functionality imitates the organic linker (1,3,5 benzenetricarboxylic acid (BTC) or trimesic acid) in the open framework structure [26]. In our particular case, we have used a thiocarboxylic acid (mercaptoacetic acid) to control the HKUST-1 film growth. The aim of this paper was to demonstrate the capability of a noble-metal-free MOF, namely HKUST-1, to be used as electrocatalyst in methanol oxidation reaction (MOR). To the best of our knowledge, no other copper-based MOF catalysts have been used for methanol electrooxidation. Due to its low stability in water, the HKUST-1 must be linked to an electrode surface. Self-assembling or
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electrochemical procedures have been employed to prepare the adlayers on gold electrode. Even the organic linker used to prepare HKUST-1, namely trimesic acid, has been involved for electrode surface modification. We have demonstrated that the growth of HKUST-1 film, from the aged mother solution, over the linkers (mercaptoacetic and trimesic acids) led to durable HKUSTmodified electrodes: Au_MAA_HKUST and Au_TA_HKUST. The electrochemical investigations reveal good electrocatalytic properties of HKUST-1 towards the MOR. Due to the fact that trimesic acid is a component of MOF, the mechanism of methanol oxidation on HKUST-1 has been investigated both for the organic linker and metallic ions. 2. Experimental 2.1. Chemicals and apparatus All chemicals were of analytical grade and all the solutions were prepared using double distilled water. Mercaptoacetic acid and chromium (III) oxide were purchased from Sigma Aldrich Germany. 1,3,5 Benzenetricarboxylic acid (trimesic acid) was obtained from Alfa Aesar Germany. Hydrogen peroxide, methanol and ethanol were obtained from Nordic Invest Romania and sulphuric acid from Lachner Czech Republic. HKUST-1 used for electrode preparation was obtained by microwave-assisted synthesis method. This method was developed and optimized at National Institute for Research and Development of Isotopic and Molecular Technologies Cluj-Napoca, Romania [35]. The X-Ray diffraction patterns were performed using a D8 Advance Diffractometer with CuKa1 radiation (l=15.4056 Å) and
Fig. 1. (a) Schematic procedure for Au_MAA_HKUST electrode preparation and (b) CVs of gold electrode in 1 mM TA solution + 0.5 M H2SO4 at 50 mV/s scan rate.
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Ge(111) monochromator. Atomic force microscopy (AFM) images of the HKUST modified gold disk electrode were taken in tapping mode using Keysight 9500 AFM. Raman spectroscopy was carried with 785 nm laser excitation using a JASCO NRS-3300 spectrometer, resolution 6.51 cm1 and laser power 11.8 mW. Electrochemical measurements (cyclic voltammetry CV, electrochemical impedance spectroscopy EIS and chronoamperometry CA) were performed using an Autolab Potentiostat/Galvanostat 302N (Metrohm Autolab). For all the electrochemical experiments, a typical three electrode cell was used: the working electrode (modified gold electrode with 0.28 cm2 or 0.07 cm2 diameter), Ag/AgCl (KCl, 3 M) as reference electrode and a platinum plate as auxiliary electrode. Electrochemical impedance spectroscopy (EIS) measurements were recorded by sweeping frequencies from 0.1 to 105 Hz range by using a small sinusoidal excitation signal (10 mV amplitude). Data fitting to equivalent circuit was performed using Nova 1.10 software (Metrohm Autolab). 2.2. Modified electrode preparation Prior to use, the gold electrode was mechanically polished with chromium (III) oxide powder, followed by chemically treatment with “Piranha” solution (3:1 v/v solution of H2SO4:H2O2). Then the electrode was electrochemically treated by cycling the potential between 0.2 and 1.6 V vs. Ag/AgCl in 0.5 M H2SO4 with a scan rate of 100 mV/s. 2.2.1. Self-assembling procedure The modified electrode was prepared by fixing the gold disk electrode in a closed teflon cell. The first layer was obtained by selfassembling of 0.1 M alcoholic solution of mercaptoacetic acid to the gold disk electrode. The alcoholic solution was maintained for 24 hours on the electrode at 4 C. For the second layer preparation an aged (30 days, 20 C) HKUST-1 mother solution was used. The deposition time was 96 hours at room temperature. The schematic procedure for Au_MAA_HKUST electrode preparation is presented in Fig. 1a. 2.2.2. Electrochemical procedure In order to obtain the trimesic acid (TA) film on gold electrode (Au_TA), 20 cyclic voltammograms (CVs) were realized in the 01.2 V potential range, 1 mM TA + 0.5 M H2SO4 solution and at 50 mV/s sweep rate, Fig. 1b. After the 9th scan, a well-defined oxidation peak appears at about 1.18 V, while the reduction peak
was observed at about 0.75 V. The as-prepared modified gold electrode (Au_TA) was immersed in a HKUST-1 mother liqueur for four days to prepare the Au_TA_ HKUST electrode. 3. Results and discussion 3.1. XRD and AFM characterization 3.1.1. XRD characterization The selective growth of HKUST-1 (further termed HKUST) highly ordered thin films on COOH self-assembled monolayers has been demonstrated by Biemmi et al. [26]. The XRD patterns of HKUST thin film grown on the Au_disk substrate modified with mercaptoacetic acid monolayer (red line), trimesic acid (blue line) and HKUST bulk sample (black line) are presented in Fig. 2a. One can see that the HKUST film grown on the COOH terminated layers (Au_TA_HKUST and Au_MAA_HKUST) is orientated along (222) and (333) planes, demonstrating an alignment along the [111] axis. Peaks belonging to other crystallographic orientations observed in HKUST bulk sample are absent in case of Au_TA_HKUST and Au_MAA_HKUST. 3.1.2. AFM characterization The surface topography of HKUST thin film is investigated using atomic force microscopy (Fig. 2b). AFM is a powerful tool to investigate topography and properties of surfaces. The AFM measurements reveal the formation of insular films about 900 nm thickness. The formation of such large structures is a consequence of the prolonged growth time of HKUST layer as evidenced also by Zacher et al. [24]. 3.2. Cyclic voltammetry of the modified electrode obtained by selfassembling The electrocatalytic activity of HKUST towards the methanol oxidation is first evaluated by cyclic voltammetry (CV) and discussed regarding the response of the bulk gold electrode. For comparison, in Fig. 3a are presented the cyclic voltammograms of Au_disk, Au_MAA and Au_MAA_HKUST electrodes only from the cycle where the response is achieved, Au_disk (fifth cycle), Au_MAA and Au_MAA_HKUST (second cycle) respectively. The CV curves were recorded in 0.5 M CH3OH/0.5 M H2SO4 at a scan rate of 0.5 mV/s. The CV curve (fifth cycle) obtained for Au_disk electrode shows that the methanol electrooxidation starts at 0.98 V, reaching a maximum at 1.18 V. The behavior is similar with
Fig. 2. (a) XRD patterns of HKUST bulk sample (black line), Au_MAA_HKUST (red line) and Au_TA_HKUST (blue line) modified electrodes; (b) 3D Topography image of the Au_MAA_HKUST sample overlaid with the phase image to see better contrast within the individual grains (3 3 mm2).
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Fig. 3. (a) CVs for Au_disk, Au_disk coated with MAA (Au_MAA) and Au_disk coated with MAA (linker) and HKUST (Au_MAA_HKUST) in 0.5 M H2SO4 + 0.5 M CH3OH at a scan rate of 0.5 mV/s and (b) CVs for Au_disk coated with MAA (linker) and HKUST (Au_MAA_HKUST) in 0.5 M H2SO4 (black line) and 0.5 M H2SO4 + 0.5 M CH3OH (red line) at a scan rate of 0.5 mV/s.
that obtained by Avramov-Ivic on Au(111) electrode [36]. The response of Au_MAA modified electrode towards MOR is only slightly enhanced compared to Au_disk one (the Au_MAA current density is about 3.5 times higher than that of gold electrode). The presence of HKUST on the electrode surface improves the response towards the methanol electrooxidation. Thus, the methanol oxidation begins at 0.96 V and reaches a maximum at 1.2 V while
the current density of forward anodic peak jf_Au_MAA_HKUST is 0.2 mA cm2, which is 28 times higher than that of jf_Au_disk (0.007 mA cm2) and 8 times higher than that of jf_Au_MAA (0.025 mA cm2). Furthermore, to ensure a fair analysis, the cyclic voltammetric response of the Au_MAA_HKUST electrode in 0.5 M H2SO4 electrolyte is recorded and compared with the results obtained
Fig. 4. (a) CVs for Au coated with TA (linker) and HKUST (Au_TA_HKUST) in 0.5 M H2SO4 electrolyte (black line) and 0.5 M H2SO4 + 0.5 M CH3OH solution (blue line) at a scan rate of 0.5 mV/s; (b) CVs for Au coated with TA (linker) in 0.5 M H2SO4 (black line) and 0.5 M H2SO4 + 0.5 M CH3OH (blue line) at a scan rate of 0.5 mV/s; (c) CVs of Au_TA in 0.5 M H2SO4 (dash line) and 0.5 M H2SO4 + 0.5 M CH3OH (solid line) and their difference spectrum [Au_TA in (0.5 M H2SO4 + 0.5 M CH3OH) 0.5 M H2SO4].
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in the presence of methanol. Thus, the CV curves of Au_MAA_HKUST electrode recorded in the second cycle are shown in Fig. 3b. Upon adding of methanol to the sulfuric acid (electrolyte), a considerable raise of the oxidative current density both for forward and reverse scan is achieved at 0.5 mV/s scan rate. Moreover, an inert glassy carbon (GC) electrode was modified by drop-casting with a HKUST-based ink prepared according to the protocol from [11] (see also Supplementary data). Then, the modified GC electrode was tested in 0.5 M CH3OH/0.5 M H2SO4 solution using cyclic voltammetry technique at a scan rate of 0.5 mV/s. The results show no response of HKUST towards the methanol oxidation neither in the first nor the second cycle, Fig. S1. We consider that this behavior is due to the degradation of HKUST in aqueous solution because the MOF is not linked by a robust linkage to the electrode surface. In order to obtain some information about the interaction of HKUST with electrode surface, Raman analyses have been achieved. The authors suppose a direct contact between copper from HKUST and COOH group from MAA that is linked to the gold electrode surface. To test this hypothesis, we have analyzed the Raman spectra of HKUST powder and Au_MAA_HKUST respectively (Fig. S2). These results highlight the catalytic activity of copper(II)-based MOF, HKUST, towards MOR and prove that the response to methanol oxidation of HKUST-modified gold electrode is not due to the electrocatalytic properties of Au bulk electrode. However, in order to achieve these results, the HKUST material must be linked to an electrode surface; it seems that the linker-HKUST interaction is sufficiently robust to afford the catalysts stability in water solutions and its use in acidic methanolic solution.
3.3. Cyclic voltammetry of the modified electrode obtained by electrochemical procedure Taking into account that 1,3,5-benzenetricarboxylic acid (trimesic acid TA), used to prepare the HKUST, is a polyfunctional carboxylic acid that can be also employed as linker for metal surfaces. Due to the possibility to create a stable adlayer by a potential-induced adsorption, we have used the electrochemical procedure to prepare the TA modified gold electrode [37,38]. The TA modified gold electrode (Au_TA) is immersed in a HKUST mother solution for four days to prepare the Au_TA_ HKUST electrode. Then, several investigations have been made to clarify the influence of the linker over the electrocatalytic activity of HKUST towards the methanol oxidation, Fig. 4. The results are similar with those obtained for Au_MAA_HKUST electrode (Fig. 3), while the methanol oxidation process seems not to be so intense on Au_TA_HKUST electrode (Fig. 4a). The HKUST is a metal-organic framework which possesses CuIIpaddlewheel-type nodes and trimesic acid struts. In order to elucidate the role of copper and trimesic acid sites in the methanol oxidation reactions (MOR), we are interested in the response of Au_TA electrode in a solution containing 0.5 M H2SO4 and 0.5 M H2SO4 + 0.5 M CH3OH (Fig. 4b). The results reveal only little changes in the CV curves when methanol is added to 0.5 M H2SO4 electrolyte. These changes are in the oxidation peak both in direct and reverse directions, but there are insignificant besides that from Au_TA_HKUST electrode (Fig. 4a and 4c). It seems that the contribution of trimesic acid in the MOR is not so significant like the CuII sites. As has been described previously, there are weakly axially coordinating CuII sites in HKUST to which desired molecules
Fig. 5. Nyquist impedance plots of methanol electrooxidation on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes in 0.5 M H2SO4 containing 0.5 M CH3OH at different potentials (a, c) between 0.4-0.55 V and (b, d) between 0.7-1.2 V. Solid lines represent the fit based on the equivalent circuits (inset).
A. Vulcu et al. / Electrochimica Acta 219 (2016) 630–637 Table 1 Charge transfer resistance (Rct) values of Au_MAA, Au_MAA_HKUST and Au_TA_HKUST modified electrodes at various potentials. Potential (V)
0.40 0.45 0.55 0.70 1.00 1.20
Rct (kV) Au_MAA
Au_MAA_HKUST
Au_TA_HKUST
255 622 898 922 155 83.8
205 550 765 110 45.4 27.4
113 273 467 563 198 42.3
635
of the HKUST-modified electrodes and it could be associated to the COOH species from trimesic acid ligand of HKUST (Figs. 3 and 4). The reduction peak from 0.83 V could be due to the reduction of some sulfur-oxygen or oxygen species adsorbed to the Au_MAA electrode surface. The absence of a similar peak for Au_TA electrode suggests a different organization of the two linkers at the gold electrode surface. At the same time, the presence of the reduction peak in the CV of the Au_MAA_HKUST electrode (Fig. 3) indicates the existence of some uncovered areas in the HKUST thin film; this is also confirmed by AFM image (Fig. 2). 3.4. Electrochemical behavior of the modified electrodes
can be coordinated [39]. These open coordination sites on CuII centers could be responsible for electrochemical reactivity of HKUST towards the methanol oxidation. Similar studies have been made with a more stable copper-based MOF for ethanolic oxidation. In this latter case, no covalent linkage between electrode and MOF was implied: the alcoholic solution of catalysts was transferred to a GC electrode [11]. The mechanism proposed by Ishimoto et al. for the ethanol electrooxidation by the stable copper-based MOF involves two states: one attributed to CuI/CuII species (at about 0.35 V vs Ag/AgCl) and the second to the oxidation couple CuII/CuIII (at about 0.95 V vs Ag/AgCl) [22]. In our case, the onset potential for MOR on HKUST-modified electrode is 0.96 V and the forward anodic peak is located at 1.2 V (second cycle at a scan rate of 0.5 mV/s). The broad oxidation peak from the forward scan could be ascribed to methanol oxidation and CuII/CuIII process of the active Cu nodes. For the backward scan, based on the data presented in Fig. S3, we suppose that the sharp peak at about 1.2-1.25 V is related to the COOH species (from MAA and TA linkers) response in methanolic and acidic conditions. This peak is also observed in the reverse scan
3.4.1. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is a powerful tool to study the interface properties of modified electrodes and the electron transfer in electrochemical reactions. EIS is used to investigate the methanol electrooxidation behavior on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes. EIS spectra are recorded at different potentials from 0.40 to 1.20 V in 0.5 M H2SO4 containing 0.5 M CH3OH. The EIS results indicate that the methanol electrooxidation on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes show different impedance behaviors (Fig. 5). The equivalent circuits used to fit EIS experimental data are showed as inset. For all modified electrodes, in the range of 0.4 V to 0.55 V the equivalent circuit include the ohmic resistance of the electrolyte solution (Rs), the charge transfer resistance (Rct), two constant phase elements (CPE-1 and CPE-2) and the resistance resulted from the oxidation of the adsorbed species (R1) (Fig. 5a and 5c inset). In the higher potentials range the corresponding equivalent circuit contains the ohmic resistance of the electrolyte solution (Rs), the charge
Fig. 6. Chronoamperometry measurements in 0.5 M H2SO4 containing 0.5 M CH3OH (a) for Au_disk and Au_MAA_HKUST electrode at 1.2 V vs Ag/AgCl for 3600 s; (b) for Au_disk and Au_MAA_HKUST electrodes at 1.2 V vs Ag/AgCl for 60 s; (c) potential-dependent current density (recorded at 60 s) for Au_disk and Au_MAA_HKUST electrodes.
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transfer resistance (Rct), the constant phase element (CPE) and the Warburg impedance (W) which results from ions diffusion from the electrolyte to electrode surface (Fig. 5b and 5d inset). The electrolyte resistance (Rs) depends on the properties of the electrolyte, while Rct depends on the dielectric properties of the electrode/electrolyte interface. Due to the roughness and the nonhomogenity of electrode surface, the electronic properties of the interface cannot be modeled with a purely capacitive element, such as Cdl, but with a constant phase element (CPE). The electrical impedance of CPE is presented in equation (1): Z CPE ¼
1 1 ðpÞnj ¼ e 2 Q 0 ðvjÞn Q 0 vn
ð1Þ
Where Q0 = 1/|Z| at v = 1 rad/s. The constant phase is always (90 n) degrees, with n varying from 0 to 1 (n = 1 describes an ideal capacitor; n = 0 describes a pure resistor) [40]. One can see that at potentials between 0.4 and 0.55 V (Fig. 5a and 5c) the charge transfer resistance (Rct) increases with potential increasing, while at higher potentials, between 0.7 to 1.2 V (Fig. 5b and 5d), the charge transfer resistance decreases with potential increasing. The charge transfer resistance values obtained by fitting EIS data are presented in Table 1. In case of Au_MAA modified electrode the Rct values are higher suggesting that the monolayer slightly decrease the electron transfer, while the presence of HKUST on the electrode surface conduce to a decrease of Rct, due to the fact that HKUST have a better conductivity and facilitates the electron transfer. The authors suppose that the different MOR behavior of Au_MAA_HKUST and Au_TA_HKUST electrodes is mainly influenced by the organization of the organic linkers (adlayer) at the gold surface. The impedance results are in good agreement with those obtained by cyclic voltammetry; the minimum Rct is observed at +1.20 V and correspond to the potential of methanol oxidation peak obtained by cyclic voltammetry, for Au_MAA, Au_MAA_HKUST and Au_TA_HKUST modified electrodes. 3.4.2. Chronoamperometry Further, the stability of the modified electrodes, Au_disk and Au_MAA_HKUST, is evaluated by chronoamperometric measurements performed in 0.5 M H2SO4 containing 0.5 M CH3OH at 1.2 V vs Ag/AgCl for 3600 s. As shown in Fig. 6a, the current density of Au_MAA_HKUST decays rapidly within first seconds, then the current density stabilizes over time and the steady state is achieved, while Au_disk electrode exhibits a surface degradation during the process after1300 s. For comparison, the chronoamperometric response of Au_disk and Au_MAA_HKUST electrodes in 0.5 M H2SO4 containing 0.5 M CH3OH at steady potential 1.2 V vs Ag/AgCl is investigated for 60 s (Fig. 6b) while their potential-dependent current densities recorded at 60 s are presented in Fig. 6c. The experiments reveal that the transient current density on the HKUST modified electrode is almost ten times higher than that on bare gold one and that the best current density is obtained at 1.2 V vs Ag/AgCl using the HKUST modified electrode. These results are in good agreement with those obtained by cyclic voltammetry and electrochemical impedance spectroscopy. 4. Conclusions This work demonstrates the possibility to use a copper-based MOF material as electrocatalyst for methanol oxidation reaction. We have tested the HKUST, a copper-based MOF not very stable in water, as catalyst for methanol electro-oxidation reaction in acidic medium. In order to use it as anode catalyst, the HKUST is “immobilized” on a gold electrode by a two-step strategy. Firstly,
on the gold electrode a linker film is formed by self-assembling (for mercaptoacetic acid) or electrochemical procedure (for trimesic acid). Secondly, the as-prepared modified electrode is immersed in the HKUST mother solution for four days at room temperature. The response of the HKUST-modified electrode towards the methanol oxidation is investigated by different electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy, chronoamperometry). Thus, the modified electrodes have shown an excellent electrocatalytic effect towards the methanol oxidation in acidic conditions at low scan rate. The highest anodic current density is obtained using the Au_MAA_HKUST electrode prepared by self-assembling procedure. However, further investigations are needed to elucidate the differences between the response of the two electrodes, Au_MAA_HKUST and Au_TA_HKUST. The possibility to prepare the trimesic acid adlayer on gold electrode offers us the opportunity to investigate the mechanism of methanol oxidation on HKUST. Due to the fact that trimesic acid is a component of HKUST, we have demonstrated that the role of trimesic acid in methanol oxidation is low comparing that of CuII nodes from HKUST. Acknowledgement This work was financially supported by Romanian National Authority for Scientific Research and Innovation (ANCSI), NUCLEU Program PN16-30 01 04. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.077. References [1] U.S Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy Fuel cell technologies market report, (2014) http://energy.gov/sites/ prod/files/2015/10/f27/fcto_2014_market_report.pdf (accessed 04.05.16). [2] 4th Energy Wave, The fuel cell and hydrogen annual review, (2015) http:// www.h2fcsupergen.com/wp-content/uploads/2016/01/2_Website_FuelCelland-Hydrogen-Annual-Review-2015.pdf (accessed 04.05.16). [3] S. Sharma, B.G. Pollet, Support materials for PEMFC and DMFC electrocatalysts A review, J. Power Sources 208 (2012) 96. [4] N. Kakati, J. Maiti, S.H. Lee, S.H. Jee, B. Viswanathan, Y.S. Yoon, Anode catalysts for direct methanol fuel cells in acidic media: Do we have any alternative for Pt or Pt-Ru? Chem. Rev. 114 (2014) 12397. [5] R. Manohara, J.B. Goodenough, Methanol oxidation in acid on ordered NiTi, Mater. Chem. 2 (1992) 875. [6] F.G. Welsh, K. Stowe, W.F. Maier, Fluorescence-based high throughput screening for noble metal-free and platinum-poor anode catalysts for direct methanol fuel cell, ACS Comb. Sci. 13 (2011) 518. [7] Z. Jia, Y. Wang, T. Qi, Hierarchical Ni–Fe layered double hydroxide/MnO2 sphere architecture as an efficient noble metal-free electrocatalyst for ethanol electro-oxidation in alkaline solution, RSC Adv. 5 (2015) 83314. [8] J. Yang, J. Tan, F. Yang, X. Li, X. Liu, D. Ma, Electro-oxidation of methanol on mesoporous nickel phosphate modified GCE, Electrochem. Commun. 23 (2012) 13. [9] M. Jafarian, R.B. Moghaddam, M.G. Mahjani, F. Gobal, Electro-catalytic oxidation of methanol on Ni-Cu alloy in alkaline medium, J. Appl. Electrochem. 36 (2006) 913. [10] W. Huang, Z. Li, Y. Peng, Z. Niu, Transition of oscillatory mechanism for methanol electro-oxidation on nano-structured nickel hydroxide film (NNHF) electrode, Chem. Commun. 12 (2004) 1380. [11] L. Yang, S. Kinoshita, T. Yamada, S. Kanda, H. Kitagawa, M. Tokunaga, T. Ishimoto, T. Ogura, R. Nagumo, A. Miyamoto, M. Koyama, A metal-organic framework as an electrocatalyst for ethanol oxidation, Angew. Chem. Int. Ed. 49 (2010) 5348. [12] M.P. Suh, H.J. Park, T.K. Prasad, D.-W. Lim, Hydrogen Storage in Metal–Organic Frameworks, Chem. Rev 112 (2012) 782. [13] J.-R. Li, J. Sculley, H.-C. Zhou, Metal–Organic Frameworks for Separations, Chem. Rev 112 (2012) 869. [14] M. Yoon, R. Srirambalaji, K. Kim, Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis, Chem. Rev 112 (2012) 1196.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
The electrochemical behavior of a Metal-Organic Framework modified gold electrode for methanol oxidation Adriana Vulcu, Liliana Olenic, Gabriela Blanita, Camelia Berghian-Grosan* National Institute for Research and Development of Isotopic and Molecular Technologies, 67-103 Donat Street RO 400293, Cluj-Napoca, Romania
A R T I C L E I N F O
Article history: Received 12 May 2016 Received in revised form 10 October 2016 Accepted 11 October 2016 Available online 13 October 2016 Keywords: Noble-Metal-Free Metal-Organic Frameworks HKUST-1 Anode Methanol Oxidation
A B S T R A C T
Direct Methanol Fuel Cells have received great interest for portable applications and electric vehicles. Even if the development of new catalysts for fuel cell is always under consideration, the platinum-based ones remain the most used for these technologies. Metal-organic-frameworks (MOF) are noble-metalfree promising materials for fuel cell industry. Although HKUST-1 is not a very stable MOF in aqueous solutions, we prove that the growth from its mother solution over different linkers (mercaptoacetic and trimesic acids) lead to durable HKUST modified electrodes: Au_MAA_HKUST and Au_TA_HKUST. Selfassembling or electrochemical procedures are used to prepare the adlayers on gold electrode. Electrochemical experiments show that the current density obtained for the Au_MAA_HKUST is almost 28 times higher than that of bulk gold electrode. The Au_TA_HKUST electrode reveals a similar response regarding the potential range, but the current densities are lower. The possibility to form the trimesic acid (a component of HKUST-1) adlayer on gold electrode offers us the opportunity to investigate the mechanism of methanol oxidation on HKUST-1 regarding both the organic linker and metallic ions. It is worth noting that the role of trimesic acid in methanol oxidation is low comparing the CuII nodes from HKUST-1. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction The possibility to convert, cleanly and efficiently, the chemical energy into electrical energy and, also, to use them in a wide range of applications has led to a growth of fuel cell industry [1,2]. Fuel cells based on methanol oxidation (Direct Methanol Fuel Cell DMFC) require a catalyst layer at anode to transform methanol into carbon dioxide. The performance of DMFC shows a strong dependence of the anode material nature and a lot of carbon and non-carbon based materials have been used over the years as electrocatalysts [3]. To date, platinum is still largely used for fuel cell industry [2] and only some alternatives to avoid Pt and Ru as anode catalysts in DMFC have been noticed [4]. Moreover, for DAFC (Direct Alcohol Fuel Cell) a few noble-metal-free catalyst have previously been reported employing NiTi [5], transition metal oxides and W-based systems [6], hierarchical Ni–Fe layered double hydroxide/MnO2 [7], porous nickel phosphates [8], Ni-Cu alloy [9], Ni hydroxide [10] or MOF materials [11]. Metal organic frameworks (MOFs) are a class of highly porous materials formed by joining together, by coordinative bonds, metal
* corresponding author. E-mail address: [email protected] (C. Berghian-Grosan). http://dx.doi.org/10.1016/j.electacta.2016.10.077 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
ions and organic linkers. The diversity of their extended networks (various pore size, large surface area, and controllable surface properties) makes them promising candidates for gas storage [12], separation [13] and heterogeneous catalysis [14–17], with a real possibility to transfer their synthesis and application from laboratory to the industry [18]. The performance and utility of MOFs as chemical sensors have been tested and several approaches to improve selective recognition of analytes have been proposed [19]. However, due to the fact that a small number of MOF materials exhibit electron conductivity, the electrochemistry of MOFs and their use in electrocatalytic reactions is significantly low [20,21]. To date, a noble-metal-free MOF material based on dimeric copper units and organic ligand (HOC2H4)2dtoa [N,N0 -bis-(hydroxyethyl)dithiooxamide] has been used for ethanol electrooxidation reaction [11,22]. Several approaches to growth MOF thin film on solid surfaces have been developed: the direct growth of MOF thin films on solid surfaces, the assembly of preformed, size and shape selected nanocrystals, the stepwise layer-by-layer growth onto the substrate, the electrochemical deposition of thin film on metal surfaces and the deposition of MOF thin film using a gel-layer method [23,24]. Self-assembled monolayers (SAMs) prove to be a powerful tool to control the lateral structure [25] and the
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crystallographic orientation [26,27] of surface-mounted MOF films [28]. HKUST-1 is a copper-based MOF, namely copper benzene tricarboxylate (Cu3(BTC)2). It was first prepared in 1999 and was intensively studied for different applications, such as: hydrogen storage [29], CO2 adsorbent [30], adsorbtion of NH3 [31], electronic applications [32]. Being largely used as adsorbents, the stability of MOFs in the environmental processes’ conditions is considered as a major challenge. Therefore, a lot of studies about water sensitivity of certain MOFs have been conducted; it was found that water content up to 0.5 mol equiv with respect to copper maintains the HKUST-1 structural characteristics, but it easily decomposes at higher water content [33]. For electrochemical application, the HKUST-1 material must be immobilized on the electrode surface to achieve the contact [28]; one method consists in growing it on different self-assembled monolayers [34]. For an oriented growth of HKUST-1 several monolayers, with different terminations (COOH, OH and CH3), can be used. The monolayer nature has a significant influence on the film orientation. In the case of COOH terminated monolayers, the carboxylic acid functionality imitates the organic linker (1,3,5 benzenetricarboxylic acid (BTC) or trimesic acid) in the open framework structure [26]. In our particular case, we have used a thiocarboxylic acid (mercaptoacetic acid) to control the HKUST-1 film growth. The aim of this paper was to demonstrate the capability of a noble-metal-free MOF, namely HKUST-1, to be used as electrocatalyst in methanol oxidation reaction (MOR). To the best of our knowledge, no other copper-based MOF catalysts have been used for methanol electrooxidation. Due to its low stability in water, the HKUST-1 must be linked to an electrode surface. Self-assembling or
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electrochemical procedures have been employed to prepare the adlayers on gold electrode. Even the organic linker used to prepare HKUST-1, namely trimesic acid, has been involved for electrode surface modification. We have demonstrated that the growth of HKUST-1 film, from the aged mother solution, over the linkers (mercaptoacetic and trimesic acids) led to durable HKUSTmodified electrodes: Au_MAA_HKUST and Au_TA_HKUST. The electrochemical investigations reveal good electrocatalytic properties of HKUST-1 towards the MOR. Due to the fact that trimesic acid is a component of MOF, the mechanism of methanol oxidation on HKUST-1 has been investigated both for the organic linker and metallic ions. 2. Experimental 2.1. Chemicals and apparatus All chemicals were of analytical grade and all the solutions were prepared using double distilled water. Mercaptoacetic acid and chromium (III) oxide were purchased from Sigma Aldrich Germany. 1,3,5 Benzenetricarboxylic acid (trimesic acid) was obtained from Alfa Aesar Germany. Hydrogen peroxide, methanol and ethanol were obtained from Nordic Invest Romania and sulphuric acid from Lachner Czech Republic. HKUST-1 used for electrode preparation was obtained by microwave-assisted synthesis method. This method was developed and optimized at National Institute for Research and Development of Isotopic and Molecular Technologies Cluj-Napoca, Romania [35]. The X-Ray diffraction patterns were performed using a D8 Advance Diffractometer with CuKa1 radiation (l=15.4056 Å) and
Fig. 1. (a) Schematic procedure for Au_MAA_HKUST electrode preparation and (b) CVs of gold electrode in 1 mM TA solution + 0.5 M H2SO4 at 50 mV/s scan rate.
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Ge(111) monochromator. Atomic force microscopy (AFM) images of the HKUST modified gold disk electrode were taken in tapping mode using Keysight 9500 AFM. Raman spectroscopy was carried with 785 nm laser excitation using a JASCO NRS-3300 spectrometer, resolution 6.51 cm1 and laser power 11.8 mW. Electrochemical measurements (cyclic voltammetry CV, electrochemical impedance spectroscopy EIS and chronoamperometry CA) were performed using an Autolab Potentiostat/Galvanostat 302N (Metrohm Autolab). For all the electrochemical experiments, a typical three electrode cell was used: the working electrode (modified gold electrode with 0.28 cm2 or 0.07 cm2 diameter), Ag/AgCl (KCl, 3 M) as reference electrode and a platinum plate as auxiliary electrode. Electrochemical impedance spectroscopy (EIS) measurements were recorded by sweeping frequencies from 0.1 to 105 Hz range by using a small sinusoidal excitation signal (10 mV amplitude). Data fitting to equivalent circuit was performed using Nova 1.10 software (Metrohm Autolab). 2.2. Modified electrode preparation Prior to use, the gold electrode was mechanically polished with chromium (III) oxide powder, followed by chemically treatment with “Piranha” solution (3:1 v/v solution of H2SO4:H2O2). Then the electrode was electrochemically treated by cycling the potential between 0.2 and 1.6 V vs. Ag/AgCl in 0.5 M H2SO4 with a scan rate of 100 mV/s. 2.2.1. Self-assembling procedure The modified electrode was prepared by fixing the gold disk electrode in a closed teflon cell. The first layer was obtained by selfassembling of 0.1 M alcoholic solution of mercaptoacetic acid to the gold disk electrode. The alcoholic solution was maintained for 24 hours on the electrode at 4 C. For the second layer preparation an aged (30 days, 20 C) HKUST-1 mother solution was used. The deposition time was 96 hours at room temperature. The schematic procedure for Au_MAA_HKUST electrode preparation is presented in Fig. 1a. 2.2.2. Electrochemical procedure In order to obtain the trimesic acid (TA) film on gold electrode (Au_TA), 20 cyclic voltammograms (CVs) were realized in the 01.2 V potential range, 1 mM TA + 0.5 M H2SO4 solution and at 50 mV/s sweep rate, Fig. 1b. After the 9th scan, a well-defined oxidation peak appears at about 1.18 V, while the reduction peak
was observed at about 0.75 V. The as-prepared modified gold electrode (Au_TA) was immersed in a HKUST-1 mother liqueur for four days to prepare the Au_TA_ HKUST electrode. 3. Results and discussion 3.1. XRD and AFM characterization 3.1.1. XRD characterization The selective growth of HKUST-1 (further termed HKUST) highly ordered thin films on COOH self-assembled monolayers has been demonstrated by Biemmi et al. [26]. The XRD patterns of HKUST thin film grown on the Au_disk substrate modified with mercaptoacetic acid monolayer (red line), trimesic acid (blue line) and HKUST bulk sample (black line) are presented in Fig. 2a. One can see that the HKUST film grown on the COOH terminated layers (Au_TA_HKUST and Au_MAA_HKUST) is orientated along (222) and (333) planes, demonstrating an alignment along the [111] axis. Peaks belonging to other crystallographic orientations observed in HKUST bulk sample are absent in case of Au_TA_HKUST and Au_MAA_HKUST. 3.1.2. AFM characterization The surface topography of HKUST thin film is investigated using atomic force microscopy (Fig. 2b). AFM is a powerful tool to investigate topography and properties of surfaces. The AFM measurements reveal the formation of insular films about 900 nm thickness. The formation of such large structures is a consequence of the prolonged growth time of HKUST layer as evidenced also by Zacher et al. [24]. 3.2. Cyclic voltammetry of the modified electrode obtained by selfassembling The electrocatalytic activity of HKUST towards the methanol oxidation is first evaluated by cyclic voltammetry (CV) and discussed regarding the response of the bulk gold electrode. For comparison, in Fig. 3a are presented the cyclic voltammograms of Au_disk, Au_MAA and Au_MAA_HKUST electrodes only from the cycle where the response is achieved, Au_disk (fifth cycle), Au_MAA and Au_MAA_HKUST (second cycle) respectively. The CV curves were recorded in 0.5 M CH3OH/0.5 M H2SO4 at a scan rate of 0.5 mV/s. The CV curve (fifth cycle) obtained for Au_disk electrode shows that the methanol electrooxidation starts at 0.98 V, reaching a maximum at 1.18 V. The behavior is similar with
Fig. 2. (a) XRD patterns of HKUST bulk sample (black line), Au_MAA_HKUST (red line) and Au_TA_HKUST (blue line) modified electrodes; (b) 3D Topography image of the Au_MAA_HKUST sample overlaid with the phase image to see better contrast within the individual grains (3 3 mm2).
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Fig. 3. (a) CVs for Au_disk, Au_disk coated with MAA (Au_MAA) and Au_disk coated with MAA (linker) and HKUST (Au_MAA_HKUST) in 0.5 M H2SO4 + 0.5 M CH3OH at a scan rate of 0.5 mV/s and (b) CVs for Au_disk coated with MAA (linker) and HKUST (Au_MAA_HKUST) in 0.5 M H2SO4 (black line) and 0.5 M H2SO4 + 0.5 M CH3OH (red line) at a scan rate of 0.5 mV/s.
that obtained by Avramov-Ivic on Au(111) electrode [36]. The response of Au_MAA modified electrode towards MOR is only slightly enhanced compared to Au_disk one (the Au_MAA current density is about 3.5 times higher than that of gold electrode). The presence of HKUST on the electrode surface improves the response towards the methanol electrooxidation. Thus, the methanol oxidation begins at 0.96 V and reaches a maximum at 1.2 V while
the current density of forward anodic peak jf_Au_MAA_HKUST is 0.2 mA cm2, which is 28 times higher than that of jf_Au_disk (0.007 mA cm2) and 8 times higher than that of jf_Au_MAA (0.025 mA cm2). Furthermore, to ensure a fair analysis, the cyclic voltammetric response of the Au_MAA_HKUST electrode in 0.5 M H2SO4 electrolyte is recorded and compared with the results obtained
Fig. 4. (a) CVs for Au coated with TA (linker) and HKUST (Au_TA_HKUST) in 0.5 M H2SO4 electrolyte (black line) and 0.5 M H2SO4 + 0.5 M CH3OH solution (blue line) at a scan rate of 0.5 mV/s; (b) CVs for Au coated with TA (linker) in 0.5 M H2SO4 (black line) and 0.5 M H2SO4 + 0.5 M CH3OH (blue line) at a scan rate of 0.5 mV/s; (c) CVs of Au_TA in 0.5 M H2SO4 (dash line) and 0.5 M H2SO4 + 0.5 M CH3OH (solid line) and their difference spectrum [Au_TA in (0.5 M H2SO4 + 0.5 M CH3OH) 0.5 M H2SO4].
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in the presence of methanol. Thus, the CV curves of Au_MAA_HKUST electrode recorded in the second cycle are shown in Fig. 3b. Upon adding of methanol to the sulfuric acid (electrolyte), a considerable raise of the oxidative current density both for forward and reverse scan is achieved at 0.5 mV/s scan rate. Moreover, an inert glassy carbon (GC) electrode was modified by drop-casting with a HKUST-based ink prepared according to the protocol from [11] (see also Supplementary data). Then, the modified GC electrode was tested in 0.5 M CH3OH/0.5 M H2SO4 solution using cyclic voltammetry technique at a scan rate of 0.5 mV/s. The results show no response of HKUST towards the methanol oxidation neither in the first nor the second cycle, Fig. S1. We consider that this behavior is due to the degradation of HKUST in aqueous solution because the MOF is not linked by a robust linkage to the electrode surface. In order to obtain some information about the interaction of HKUST with electrode surface, Raman analyses have been achieved. The authors suppose a direct contact between copper from HKUST and COOH group from MAA that is linked to the gold electrode surface. To test this hypothesis, we have analyzed the Raman spectra of HKUST powder and Au_MAA_HKUST respectively (Fig. S2). These results highlight the catalytic activity of copper(II)-based MOF, HKUST, towards MOR and prove that the response to methanol oxidation of HKUST-modified gold electrode is not due to the electrocatalytic properties of Au bulk electrode. However, in order to achieve these results, the HKUST material must be linked to an electrode surface; it seems that the linker-HKUST interaction is sufficiently robust to afford the catalysts stability in water solutions and its use in acidic methanolic solution.
3.3. Cyclic voltammetry of the modified electrode obtained by electrochemical procedure Taking into account that 1,3,5-benzenetricarboxylic acid (trimesic acid TA), used to prepare the HKUST, is a polyfunctional carboxylic acid that can be also employed as linker for metal surfaces. Due to the possibility to create a stable adlayer by a potential-induced adsorption, we have used the electrochemical procedure to prepare the TA modified gold electrode [37,38]. The TA modified gold electrode (Au_TA) is immersed in a HKUST mother solution for four days to prepare the Au_TA_ HKUST electrode. Then, several investigations have been made to clarify the influence of the linker over the electrocatalytic activity of HKUST towards the methanol oxidation, Fig. 4. The results are similar with those obtained for Au_MAA_HKUST electrode (Fig. 3), while the methanol oxidation process seems not to be so intense on Au_TA_HKUST electrode (Fig. 4a). The HKUST is a metal-organic framework which possesses CuIIpaddlewheel-type nodes and trimesic acid struts. In order to elucidate the role of copper and trimesic acid sites in the methanol oxidation reactions (MOR), we are interested in the response of Au_TA electrode in a solution containing 0.5 M H2SO4 and 0.5 M H2SO4 + 0.5 M CH3OH (Fig. 4b). The results reveal only little changes in the CV curves when methanol is added to 0.5 M H2SO4 electrolyte. These changes are in the oxidation peak both in direct and reverse directions, but there are insignificant besides that from Au_TA_HKUST electrode (Fig. 4a and 4c). It seems that the contribution of trimesic acid in the MOR is not so significant like the CuII sites. As has been described previously, there are weakly axially coordinating CuII sites in HKUST to which desired molecules
Fig. 5. Nyquist impedance plots of methanol electrooxidation on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes in 0.5 M H2SO4 containing 0.5 M CH3OH at different potentials (a, c) between 0.4-0.55 V and (b, d) between 0.7-1.2 V. Solid lines represent the fit based on the equivalent circuits (inset).
A. Vulcu et al. / Electrochimica Acta 219 (2016) 630–637 Table 1 Charge transfer resistance (Rct) values of Au_MAA, Au_MAA_HKUST and Au_TA_HKUST modified electrodes at various potentials. Potential (V)
0.40 0.45 0.55 0.70 1.00 1.20
Rct (kV) Au_MAA
Au_MAA_HKUST
Au_TA_HKUST
255 622 898 922 155 83.8
205 550 765 110 45.4 27.4
113 273 467 563 198 42.3
635
of the HKUST-modified electrodes and it could be associated to the COOH species from trimesic acid ligand of HKUST (Figs. 3 and 4). The reduction peak from 0.83 V could be due to the reduction of some sulfur-oxygen or oxygen species adsorbed to the Au_MAA electrode surface. The absence of a similar peak for Au_TA electrode suggests a different organization of the two linkers at the gold electrode surface. At the same time, the presence of the reduction peak in the CV of the Au_MAA_HKUST electrode (Fig. 3) indicates the existence of some uncovered areas in the HKUST thin film; this is also confirmed by AFM image (Fig. 2). 3.4. Electrochemical behavior of the modified electrodes
can be coordinated [39]. These open coordination sites on CuII centers could be responsible for electrochemical reactivity of HKUST towards the methanol oxidation. Similar studies have been made with a more stable copper-based MOF for ethanolic oxidation. In this latter case, no covalent linkage between electrode and MOF was implied: the alcoholic solution of catalysts was transferred to a GC electrode [11]. The mechanism proposed by Ishimoto et al. for the ethanol electrooxidation by the stable copper-based MOF involves two states: one attributed to CuI/CuII species (at about 0.35 V vs Ag/AgCl) and the second to the oxidation couple CuII/CuIII (at about 0.95 V vs Ag/AgCl) [22]. In our case, the onset potential for MOR on HKUST-modified electrode is 0.96 V and the forward anodic peak is located at 1.2 V (second cycle at a scan rate of 0.5 mV/s). The broad oxidation peak from the forward scan could be ascribed to methanol oxidation and CuII/CuIII process of the active Cu nodes. For the backward scan, based on the data presented in Fig. S3, we suppose that the sharp peak at about 1.2-1.25 V is related to the COOH species (from MAA and TA linkers) response in methanolic and acidic conditions. This peak is also observed in the reverse scan
3.4.1. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) is a powerful tool to study the interface properties of modified electrodes and the electron transfer in electrochemical reactions. EIS is used to investigate the methanol electrooxidation behavior on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes. EIS spectra are recorded at different potentials from 0.40 to 1.20 V in 0.5 M H2SO4 containing 0.5 M CH3OH. The EIS results indicate that the methanol electrooxidation on Au_MAA, Au_MAA_HKUST and Au_TA_HKUST electrodes show different impedance behaviors (Fig. 5). The equivalent circuits used to fit EIS experimental data are showed as inset. For all modified electrodes, in the range of 0.4 V to 0.55 V the equivalent circuit include the ohmic resistance of the electrolyte solution (Rs), the charge transfer resistance (Rct), two constant phase elements (CPE-1 and CPE-2) and the resistance resulted from the oxidation of the adsorbed species (R1) (Fig. 5a and 5c inset). In the higher potentials range the corresponding equivalent circuit contains the ohmic resistance of the electrolyte solution (Rs), the charge
Fig. 6. Chronoamperometry measurements in 0.5 M H2SO4 containing 0.5 M CH3OH (a) for Au_disk and Au_MAA_HKUST electrode at 1.2 V vs Ag/AgCl for 3600 s; (b) for Au_disk and Au_MAA_HKUST electrodes at 1.2 V vs Ag/AgCl for 60 s; (c) potential-dependent current density (recorded at 60 s) for Au_disk and Au_MAA_HKUST electrodes.
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transfer resistance (Rct), the constant phase element (CPE) and the Warburg impedance (W) which results from ions diffusion from the electrolyte to electrode surface (Fig. 5b and 5d inset). The electrolyte resistance (Rs) depends on the properties of the electrolyte, while Rct depends on the dielectric properties of the electrode/electrolyte interface. Due to the roughness and the nonhomogenity of electrode surface, the electronic properties of the interface cannot be modeled with a purely capacitive element, such as Cdl, but with a constant phase element (CPE). The electrical impedance of CPE is presented in equation (1): Z CPE ¼
1 1 ðpÞnj ¼ e 2 Q 0 ðvjÞn Q 0 vn
ð1Þ
Where Q0 = 1/|Z| at v = 1 rad/s. The constant phase is always (90 n) degrees, with n varying from 0 to 1 (n = 1 describes an ideal capacitor; n = 0 describes a pure resistor) [40]. One can see that at potentials between 0.4 and 0.55 V (Fig. 5a and 5c) the charge transfer resistance (Rct) increases with potential increasing, while at higher potentials, between 0.7 to 1.2 V (Fig. 5b and 5d), the charge transfer resistance decreases with potential increasing. The charge transfer resistance values obtained by fitting EIS data are presented in Table 1. In case of Au_MAA modified electrode the Rct values are higher suggesting that the monolayer slightly decrease the electron transfer, while the presence of HKUST on the electrode surface conduce to a decrease of Rct, due to the fact that HKUST have a better conductivity and facilitates the electron transfer. The authors suppose that the different MOR behavior of Au_MAA_HKUST and Au_TA_HKUST electrodes is mainly influenced by the organization of the organic linkers (adlayer) at the gold surface. The impedance results are in good agreement with those obtained by cyclic voltammetry; the minimum Rct is observed at +1.20 V and correspond to the potential of methanol oxidation peak obtained by cyclic voltammetry, for Au_MAA, Au_MAA_HKUST and Au_TA_HKUST modified electrodes. 3.4.2. Chronoamperometry Further, the stability of the modified electrodes, Au_disk and Au_MAA_HKUST, is evaluated by chronoamperometric measurements performed in 0.5 M H2SO4 containing 0.5 M CH3OH at 1.2 V vs Ag/AgCl for 3600 s. As shown in Fig. 6a, the current density of Au_MAA_HKUST decays rapidly within first seconds, then the current density stabilizes over time and the steady state is achieved, while Au_disk electrode exhibits a surface degradation during the process after1300 s. For comparison, the chronoamperometric response of Au_disk and Au_MAA_HKUST electrodes in 0.5 M H2SO4 containing 0.5 M CH3OH at steady potential 1.2 V vs Ag/AgCl is investigated for 60 s (Fig. 6b) while their potential-dependent current densities recorded at 60 s are presented in Fig. 6c. The experiments reveal that the transient current density on the HKUST modified electrode is almost ten times higher than that on bare gold one and that the best current density is obtained at 1.2 V vs Ag/AgCl using the HKUST modified electrode. These results are in good agreement with those obtained by cyclic voltammetry and electrochemical impedance spectroscopy. 4. Conclusions This work demonstrates the possibility to use a copper-based MOF material as electrocatalyst for methanol oxidation reaction. We have tested the HKUST, a copper-based MOF not very stable in water, as catalyst for methanol electro-oxidation reaction in acidic medium. In order to use it as anode catalyst, the HKUST is “immobilized” on a gold electrode by a two-step strategy. Firstly,
on the gold electrode a linker film is formed by self-assembling (for mercaptoacetic acid) or electrochemical procedure (for trimesic acid). Secondly, the as-prepared modified electrode is immersed in the HKUST mother solution for four days at room temperature. The response of the HKUST-modified electrode towards the methanol oxidation is investigated by different electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy, chronoamperometry). Thus, the modified electrodes have shown an excellent electrocatalytic effect towards the methanol oxidation in acidic conditions at low scan rate. The highest anodic current density is obtained using the Au_MAA_HKUST electrode prepared by self-assembling procedure. However, further investigations are needed to elucidate the differences between the response of the two electrodes, Au_MAA_HKUST and Au_TA_HKUST. The possibility to prepare the trimesic acid adlayer on gold electrode offers us the opportunity to investigate the mechanism of methanol oxidation on HKUST. Due to the fact that trimesic acid is a component of HKUST, we have demonstrated that the role of trimesic acid in methanol oxidation is low comparing that of CuII nodes from HKUST. Acknowledgement This work was financially supported by Romanian National Authority for Scientific Research and Innovation (ANCSI), NUCLEU Program PN16-30 01 04. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.077. References [1] U.S Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy Fuel cell technologies market report, (2014) http://energy.gov/sites/ prod/files/2015/10/f27/fcto_2014_market_report.pdf (accessed 04.05.16). [2] 4th Energy Wave, The fuel cell and hydrogen annual review, (2015) http:// www.h2fcsupergen.com/wp-content/uploads/2016/01/2_Website_FuelCelland-Hydrogen-Annual-Review-2015.pdf (accessed 04.05.16). [3] S. Sharma, B.G. Pollet, Support materials for PEMFC and DMFC electrocatalysts A review, J. Power Sources 208 (2012) 96. [4] N. Kakati, J. Maiti, S.H. Lee, S.H. Jee, B. Viswanathan, Y.S. Yoon, Anode catalysts for direct methanol fuel cells in acidic media: Do we have any alternative for Pt or Pt-Ru? Chem. Rev. 114 (2014) 12397. [5] R. Manohara, J.B. Goodenough, Methanol oxidation in acid on ordered NiTi, Mater. Chem. 2 (1992) 875. [6] F.G. Welsh, K. Stowe, W.F. Maier, Fluorescence-based high throughput screening for noble metal-free and platinum-poor anode catalysts for direct methanol fuel cell, ACS Comb. Sci. 13 (2011) 518. [7] Z. Jia, Y. Wang, T. Qi, Hierarchical Ni–Fe layered double hydroxide/MnO2 sphere architecture as an efficient noble metal-free electrocatalyst for ethanol electro-oxidation in alkaline solution, RSC Adv. 5 (2015) 83314. [8] J. Yang, J. Tan, F. Yang, X. Li, X. Liu, D. Ma, Electro-oxidation of methanol on mesoporous nickel phosphate modified GCE, Electrochem. Commun. 23 (2012) 13. [9] M. Jafarian, R.B. Moghaddam, M.G. Mahjani, F. Gobal, Electro-catalytic oxidation of methanol on Ni-Cu alloy in alkaline medium, J. Appl. Electrochem. 36 (2006) 913. [10] W. Huang, Z. Li, Y. Peng, Z. Niu, Transition of oscillatory mechanism for methanol electro-oxidation on nano-structured nickel hydroxide film (NNHF) electrode, Chem. Commun. 12 (2004) 1380. [11] L. Yang, S. Kinoshita, T. Yamada, S. Kanda, H. Kitagawa, M. Tokunaga, T. Ishimoto, T. Ogura, R. Nagumo, A. Miyamoto, M. Koyama, A metal-organic framework as an electrocatalyst for ethanol oxidation, Angew. Chem. Int. Ed. 49 (2010) 5348. [12] M.P. Suh, H.J. Park, T.K. Prasad, D.-W. Lim, Hydrogen Storage in Metal–Organic Frameworks, Chem. Rev 112 (2012) 782. [13] J.-R. Li, J. Sculley, H.-C. Zhou, Metal–Organic Frameworks for Separations, Chem. Rev 112 (2012) 869. [14] M. Yoon, R. Srirambalaji, K. Kim, Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis, Chem. Rev 112 (2012) 1196.
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