Sequential electrodeposition of Pt and Ru on boron-doped diamond (BDD) films, in 0.5 M H2SO4 by cyclic voltammetry, has been prepared. The potential cycling, in the aqueous solutions of the respective metals, was between 0.00 and 1.00 V versus Ag/AgCl. The catalyst composites, Pt and PtRu, deposited on BDD film substrates, were tested for methanol oxidation. The modified diamond surfaces were also characterized by scanning electron microscopy-X-ray fluorescence-energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The scanning Auger electron spectroscopy mapping showed the ruthenium signal only in areas where platinum was electrodeposited. Ruthenium does not deposit on the oxidized diamond surface of the boron-doped diamond. Particles with 5–10% of ruthenium with respect to platinum exhibited better performance for methanol oxidation in terms of methanol oxidation peak current and chronoamperometric current stability. The electrogenerated •OH radicals on BDD may interact with Pt surface, participating in the methanol oxidation as shown in oxidation current and the shift in the peak position. The conductive diamond surface is a good candidate as the support for the platinum electrocatalyst, because it ensures catalytic activity, which compares with the used carbon, and higher stability under severe anodic and cathodic conditions.
The direct methanol fuel cells (DMFC) are electrochemical cells that convert chemical energy in electrical energy that can be use to power all kind of appliances. Similar to polymer electrolyte membrane fuel cells (PEMFCs), in the DMFC the anode catalyst draws the hydrogen from the methanol, and both systems use a solid electrolyte, reducing the corrosion of the device and improving the power density. Various catalytic composite systems have been studied, including PtRuOsIr [
One of the subjects of research on high-efficiency fuel cells is how to minimize the electrocatalytic noble metal loading without losing the high catalytic activity. This is achieved by dispersing nanoparticles of the catalytic materials (mainly Pt-based alloys) on high surface area materials used as supports. The supports need to, in addition to having high surface, be stable and conductive. The most commonly used particle support is carbon blacks, these undergo irreversible oxidation at positive potentials, and this is a challenge particularly on the oxygen reduction electrode where they undergo high positive potentials (0.7–1.0 versus NHE), but it can occur in the anode during fuel starvation [
Boron-doped diamond (BDD) films exhibit very high chemical and electrochemical stability. The potential range in which high stability exists is greater than 2.5 V, much greater than conventional carbon-based electrode materials such as glassy carbon and carbon blacks. BBD films exhibit very high stability to vigorous chemical and electrochemical treatments [
In the present work, we have used the polished boron-doped microcrystalline diamond film as a model system, in order to study the deposition of noble metal electrocatalysts and to characterize their morphology and electrocatalytic behavior on the diamond surface. As a first step, we have characterized typical BDD films and then have sequentially electrodeposited platinum and ruthenium particles on the surface. The PtRu modified electrodes were used for methanol oxidation.
BDD films were bought from Element Six, the specifications given by the manufacturer were 10 mm × 10 mm × 1 mm thickness, polished surface finish grown by chemical vapor deposition with a resistivity 0.038–0.105 Ω-cm, and a doping level of [B] > 1020 cm−3. Each diamond film was cleaned by using a solution of aqua-regia, followed by cycling from −0.8 V to +1.6 versus Ag/AgCl in sulfuric acid 0.5 M, and tested as clean sample of boron-doped diamond.
All the experiments were performed in an in-house designed electrochemical cell. The boron-doped diamond (BDD) films were used as working electrodes, Ag/AgCl as reference electrode, and a Pt wire as counter electrode, respectively. Sulfuric acid optima (95–98%, Fisher) was used to prepare a 0.5 M aqueous solution as electrolyte. All the electrochemical measurements were performed using a PARSTAT 2273 potentiostat.
Potassium hexachloroplatinate (IV; 98%, Sigma-Aldrich) and ruthenium (III) chloride (45–55%, Sigma-Aldrich), dissolved in 0.5 M H2SO4, were used as metal precursors for the sequential electrodeposition of Pt : Ru. Pt deposition was carried out using K2PtCl6 1 mM cycling between −0.2 V and 1.0 V versus Ag/AgCl at a scan rate of 500 mVs−1. Then the sample was cleaned using cyclic voltammetry between −0.2 V and 1.2 V versus Ag/AgCl at a scan rate of 500 mVs−1 in H2SO4 0.5 M until the voltammogram of polycrystalline platinum was perfectly reproduced. For the Ru sequential deposition, four different concentrations were used 1 mM, 0.5 mM, 0.1 mM, and 0.05 mM of RuCl3. All the electrodepositions were carried out by cycling the potential between −0.2 V and 0.8 V versus Ag/AgCl at a scan rate of 500 mVs−1. Finally, methanol (99%, Sigma-Aldrich) 1 M in H2SO4 0.5 M was oxidized between −0.2 V and 0.8 V versus Ag/AgCl at a scan rate 50 mVs−1. Chronoamperometry measurements were performed at 0.35 V versus Ag/AgCl. All electrochemical measurements were done at room temperature.
The various electrodeposited catalysts were characterized by scanning electron microscopy-X-ray energy-dispersive analysis (SEM/EDS), X-ray diffraction, and cyclic voltammetry. For the SEM micrographs a JEOL 5800LV microscope was used at 15 kV. Platinum particle size distribution measurements were performed using the UTHSCSA Image Tool program [
X-ray diffraction analyses were performed using a SIEMENS D5000 X-ray diffractometer using a Cu K
The XPS analysis data was obtained using a PHI 5600ci spectrometer with an Al Ka X-ray source at 15 kV and 350 W. Binding energies were corrected to the C1s sp3 signal at 284.5 eV [
The BDD films used were of high quality, including predominantly (111-) type and (100-) type grains and facets, as confirmed by X-ray diffraction analysis, which has a peak pattern characteristic for diamond crystals. The first peak, appeared at 44.1°, due to (111) plane, and a second peak, at 75.3°, was characteristic of the (220) plane. The (220) peak were more intense than the (111) peak; this is because the growth process occurred preferentially in the (220) plane. The third peak, at 91.6°, was assigned to the (311) plane.
As a standard characterization of the BDD films bought, Raman spectral analysis was done. From the Raman spectra, the peak at 1332 cm−1 confirmed the presence of sp3 carbons, as expected for diamond materials. A small broad peak in the 1598 cm−1 region indicated the presence of sp2 carbon impurities. The films were of good quality, although sp2 carbon contamination was present, the width of the 1332 cm−1 full width at half-maximum diamond line was 4.7–7.1 cm−1, compared with 2.6 cm−1 and 10.5 cm−1 for crystals with best and poorest crystallinity, respectively [
The cyclic voltammetry measured in 0.5 M H2SO4 showed a wide potential range in which there was relatively small background current, close to 3 V. This contrasts with gold, platinum, glassy carbon, and other commonly used electrodes and is due to the lack of catalytic activity for the hydrogen and oxygen electrolytic gas generation reactions, together with the slow tendency to undergo electrochemically induced surface oxidation [
Platinum electrodeposition was carried out by sweeping the potential back and forth (cyclic voltammetry) between −0.2 and 1.0 V in a K2PtCl6/0.5 M H2SO4 solution at 500 mV/s, where the platinum concentration was varied from 0.5 mM to 2.0 mM. The particles were observed by scanning electron microscopy (SEM), and their elemental composition was verified by X-ray fluorescence-energy dispersive spectroscopy (EDS; not shown). The average size of the Pt particles was approximately 574 nm ± 200 nm, the size of the platinum particles did not change with the platinum solution concentration used during the deposition voltammetry. The particle density was approximately 1.01 × 107, 1.25 × 107, and 4.27 × 107 particles-cm−2 for 0.5 mM, 1.0 mM, and 2.0 mM, respectively. This value compares with the values obtained by Enea et al. in the 200–800 nm range [
Figure
Cyclic voltammogram of Pt-BDD films at 50 V/s in a N2 spurged 0.5 M H2SO4 solution. Pt particles deposited by cyclic voltammetry −0.2 V and 1.0 V versus Ag/AgCl at Pt 500 mVs−1 for (a) 0.5, (b) 1.0, and (c) 2.0,
SEM images of Pt-Ru sequential deposition on BDD films obtained at 20 kV and magnification of 10,000x, by cyclic voltammetry, between −0.2 V and 1.0 V versus Ag/AgCl for Pt and −0.2 V and 0.8 V versus Ag/AgCl for Ru at sweep rate 500 mVs−1 for (a) 1 : 1, (b) 1 : 0.5, (c) 1 : 0.1, and (d) 1 : 0.05, 1 mM K2PtCl6 and
Platinum and ruthenium were electrochemically deposited sequentially by means of cyclic voltammetry, potential sweep between 0.2 and 1.0 V at 50 mV s in 1 mM K2PtCl6/0.5 M H2SO4 and 0.05, 0.1, 0.5, and 1 mM RuCl3/0.5 M H2SO4 solutions. The particles were observed by SEM, EDS, and Auger electron spectroscopy were used to verify their composition.
The range particle size was approximately 100–800 nm for Pt-Ru with particle density of (a) 2.32 × 108 particles/cm2, (b) 2.02 × 108 particles/cm2, (c) 1.86 × 108 particles/cm2 and (d) 5.12 × 107 particles/cm2. The particles were distributed in a nonhomogenous way throughout the surface of the diamond film, as can be seen in Figures
Scanning electron microscopy (SEM) images of Pt-Ru sequential deposition on BDD films obtained at 20 kV and magnification of 3,000x, by cyclic voltammetry, between −0.2 V and 1.0 V versus Ag/AgCl for Pt and −0.2 V and 0.8 V versus Ag/AgCl for Ru at sweep rate 500 mVs−1 for (a) 1 : 1, (b) 1 : 0.5, (c) 1 : 0.1, and (d) 1 : 0.05, 1 mM K2PtCl6 and
Histogram for particle size analysis of Pt-Ru sequential deposition on boron-doped diamond electrodes obtained at 20 kV and magnification of 3,000x, by cyclic voltammetry, between −0.2 V and 1.0 V versus Ag/AgCl for Pt and −0.2 V and 0.8 V versus Ag/AgCl for Ru at sweep rate 500 mVs−1 for (a) 1 : 1, (b) 1 : 0.5, (c) 1 : 0.1, and (d) 1 : 0.05, 1 mM K2PtCl6 and
The chemical composition of the particles was verified with several techniques. Auger spectroscopy and EDS verified the elemental composition of the PtRu particles decorated boron-doped diamond films.
Figure
Scanning electron microscopy and scanning Auger electron spectroscopy mapping images for the C KLL, O KLL, Pt MNN, and Ru LMM on Pt-Ru particle deposited sequentially on BDD films using 1.0 mM K2PtCl6 and 1.0 mM RuCl3 solution.
Further chemical characterization was done by high-resolution XPS spectra for the binding energy regions of C 1s, O 1s, Pt 4f, and Ru 3p (Figures
High-resolutions X-ray photoelectron spectra of the Pt particle decorated boron-doped diamond film. C 1s binding energy region.
High-resolutions X ray photoelectron spectra of the Pt particle decorated boron-doped diamond film. (b) Pt 4f region (c) Ru 3p1/2 energy region (a) O 1s region.
The high-resolution X-ray photoelectron spectrum shows the binding energy peak of Pt0 4f5/2 and Pt0 4f7/2, and with an increase of +1.2 eV in the binding energy, the signal for platinum dioxide which consist of 20% relative to the Pt0 signal. Ruthenium 3p1/2 region shows two main peaks, ruthenium in its reduced form and ruthenium oxide.
The catalytic activity of the electrodeposited metal particles was tested by cyclic voltammetry (see Figure
Cyclic voltammetry on 0.5 M CH3OH/0.5 M H2SO4 Pt particles deposited on boron-doped diamond films by cyclic voltammetry. (a) Chronoamperometry at 350 mV versus Ag/AgCl at 24°C of Pt particles deposited on boron-doped diamond films by cyclic voltammetry on 0.5 M CH3OH/0.5 M H2SO4 (b).
Of greater importance, however, for the fuel cell catalysis is the potential range in which significant anodic current begins to be observed, that is, the oxidation onset potential. Although an arbitrary criterion must be established for this, for example, a given minimum current density, if we take a value of 10 mA/cm2, the onset potentials were in the range of 332–369 mV versus Ag/AgCl, for Pt-Ru particles deposited on diamond film surface [
As observed in Figure
Also the oxidized surface of diamond that possesses hydroxy (–OH) groups may be involved on the water activation on the platinum surface. Desai and Neurock showed that the presence of surface hydroxyl groups adsorbed to Ru can induce water to adsorb and activate at neighboring Pt sites [
Chronoamperometry was performed at 0.350 V versus Ag/AgCl for all the electrodeposited particles for 1800 s, Figures
Current at the final point of the chronoamperometry at 350 mV versus Ag/AgCl at 24°C of Pt particles deposited on boron-doped diamond films.
It is accepted that PtRu is the most promising bimetallic catalyst for methanol oxidation. Its superior activity has been observed for different PtRu materials, such as alloys, codeposits, sequential electrodeposits, and Ru adsorbed on Pt-single-crystal and on Pt carbon-supported electrodes. We have seen that the potential cycling technique yields results that are similar to those of the pulsed galvanostatic technique for platinum itself. For platinum-ruthenium, it is expected that further improvements can be made in reaching higher dispersion, although it has been reported that the dispersion of particles on diamond films is low [
In this work we have examined the possibility of the use of boron-doped diamond as the support for electrocatalyst particles in a fuel cell. The electrochemical behavior of oxidized boron-doped diamond (BDD) films in 0.5 M H2SO4 using cyclic voltammetry showed a very wide electrochemical window. Platinum particles deposited a different platinum concentrations did not showed a different particle size; although it did show a difference in the particle distribution. Pt and Pt-Ru particles were deposited on BDD film substrate; the modified diamond surfaces were also characterized by SEM/EDS, XPS, and Auger. High-resolution XPS peak fitting showed that the BDD films were oxidized with C–O, C–OH, and C–O–C. Auger electron spectroscopy mapping showed that a complete oxidized surface of diamond and that ruthenium does not deposit on the oxidized diamond surface of the boron-doped diamond. Particles with 5–10% of ruthenium with respect to platinum exhibited better performance for methanol oxidation in terms of methanol oxidation peak current and current stability. Particles with greater amounts of 20% of ruthenium sequentially deposited the ruthenium covers the platinum, and the catalytic property of the particle is lost. In the case of cyclic voltammogram for methanol oxidation, a current for the reverse peak is higher than what it is usually observed on carbon supports, a possible explanation is that the electrogenerated hydroxyl radical on BDD may interact with Pt surface, participating in the methanol oxidation as showed in the oxidation current and the shift in the peak position.
This paper was supported in part by NASA Grant no. NASA- NNX08BA48A. I. González-González and C. Lorenzo-Medrano gratefully acknowledge support from Center for Advanced Nanoscale Materials and Research Initiative Scientific Enhancement. The authors also thank the UPR Materials Characterization Center for all their help with the surface characterization measurements.