Ruthenium selenide catalysts for cathodic oxygen reduction in direct methanol fuel cells

The oxygen reduction reaction in sulphuric acid on commercial carbon supported platinum and ruthenium catalysts as well as on a home-made carbon supported ruthenium selenide catalysts (RuSe _x /C) was investigated. The RuSe _x /C catalysts were synthesised using similar procedures to those found in the literature. A dependency of H₂O₂ formation on the selenium content was found using the thin-film rotating ring disc electrode technique, namely that the H₂O₂ formation in the typical operation range of a Direct Methanol Fuel Cell (0.7–0.4 V) on Pt/C is below 1% and 1–4% on Ru/C and RuSe _x /C catalysts. Finally for comparing the intrinsic activities of the catalysts the electrochemically active surface areas were determined in-situ by means of copper underpotential deposition. Our results indicate a comparable activity of the present RuSe _x /C catalyst to commercial Pt/C if the activities are related to the electrochemical active areas.     

Carbon supported Pt–Cr alloys as oxygen-reduction catalysts for direct methanol fuel cells

Electrocatalytic activities of various carbon-supported platinum–chromium alloy electrocatalysts towards oxygen reduction in 1 mol l⁻¹ H₂SO₄ and in 1 mol l⁻¹ H₂SO₄/1–3 mol l⁻¹ CH₃OH, were investigated by means of rotating disc electrode experiments and in solid polymer electrolyte direct methanol fuel cells. The activity of these electrocatalysts for methanol oxidation was evaluated using cyclic voltammetry. It was found that Pt₉Cr/C prepared by reduction with NaBH₄ exhibits the lowest activity for methanol oxidation and the highest activity for oxygen reduction in the presence of methanol, in comparison to commercial Pt/C, Pt₃Cr/C and PtCr/C electrocatalysts.     

Electronic-level interactions of tungsten oxide with unsupported Se/Ru electrocatalytic nanoparticles

Se/Ru nanoparticles - a potent non-platinum catalyst towards oxygen reduction reaction - were modified by hydrated WO₃ and investigated using the rotating disk/ring electrode methods and by synchrotron X-ray photoelectron spectroscopy. The modification resulted in an enhanced catalytic activity towards oxygen reduction reaction (ORR). Our data indicate that the oxygen reduction current starts ca. 70 mV more positive and formation of undesirable hydrogen peroxide has significantly decreased following the modification of Se/Ru with WO₃. X-ray photoelectron spectroscopy reveals that WO₃ interacts electronically with Se/Ru as the W 4f and Se 3d line-shapes change. We therefore conclude that the electronic interactions between Se/Ru and WO₃ are primarily responsible for the increase in activity and selectivity of the WO₃-modified Se/Ru towards ORR.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part II: preparation and characterization of MEAs

Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe _x catalysts. Preparation parameters varied were: catalyst loading (0.5–2 mg RuSe _x cm⁻²), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe _x in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe _x catalysts investigated, i.e. ∼40 mW cm⁻² at 80 °C under ambient pressure and λ_MeOH = λ_air = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe _x catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe _x particle size (∼5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe _x catalyst layers.     

Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition

The kinetics of oxygen reduction in methanol-containing acid electrolyte was investigated at platinum-based electrodes using the porous rotating disk electrodes (RDE) technique. Utilization of commercial-grade (E-TEK) carbon-supported Pt particles with narrow size distribution provided evidences for a particle size effect on the tolerance of oxygen reduction electrocatalysts to methanol competition. In methanol-containing perchloric acid electrolyte, the mass activity (MA, A g⁻¹ Pt) for oxygen reduction increases continuously with a decrease in particle size from d=4.6 to 2.3 nm, whereas in methanol-free electrolyte MA is roughly independent of the size, when d≤3.5 nm. Effects of addition of a second metal to Pt were also investigated. Based on particle size considerations Pt:Cr-C appears to be a more active catalyst than Pt-C for oxygen reduction in methanol-containing electrolyte.     

The role of the WO x ad-component to Pt and PtRu catalysts in the electrochemical CH3OH oxidation reaction

High surface area carbon-supported platinum-based catalysts, Pt/C, PtWO _x /C, PtRu/C and PtRuWO _x /C, were prepared via a chemical reduction route using single metal precursor salts. The catalyst particles were found to be in the nanoscale range, and the addition of Ru clearly decreased the particle size. The Ru was found to be partially incorporated into the face centered cubic lattice of Pt and to form a single Ru catalyst component. X-ray diffraction and X-ray photon spectroscopy did not provide evidence for electronic interactions between WO _x and Pt as well as WO _x and Ru. However, the addition of tungsten to the PtRuWO _x /C catalyst resulted in a high degree of catalyst particle agglomeration. Both Ru containing catalysts showed significantly higher activities for the CH₃OH oxidation reaction in terms of Pt + Ru mass as well as electroactive Pt + Ru surface area than the Pt/C and PtWO _x /C catalysts. The addition of tungsten appeared to mainly result in some ‘physical’ modification of the catalytically active Pt and Ru surface components such as differences in electroactive surface area rather than promotion of the CH₃OH oxidation reaction via a true catalytic mechanism.     

A highly active Pd coated Ag electrocatalyst for oxygen reduction reactions in alkaline media

We synthesized and characterized a highly active electrocatalyst for oxygen reduction reactions (ORRs) in alkaline media by coating carbon-supported silver nanoparticles with Pd (Pd@Ag/C) via a galvanic displacement method. The electrochemical measurements were carried out using an ultrathin film rotating disk electrode. Compared to the Pt/C electrocatalyst, the specific and mass activities of the Pd@Ag/C were enhanced by a factor of 3 and 2.5, respectively. The potentiostatic measurements showed that the Pd@Ag/C is less stable than the Pt/C at the potential of −0.1 V vs. Hg/HgO/OH⁻ in alkaline media. The Pd@Ag/C is insensitive to alcohol, and, as a cathode electrocatalyst of a direct alcohol fuel cell, can resist poisoning by the possible alcohol crossover from the anode.     

Kinetics and mechanism of the electrochemical reduction of molecular oxygen on platinum in KOH: influence of preferred crystallographic orientation

The oxygen electroreduction reaction has been studied at both preferred oriented and conventional polycrystalline platinum rotating disc electrodes in x m KOH (0.05 ⩽ x ⩽ 3.0) aqueous solutions under oxygen saturation at 25°C. At low current densities, Tafel lines with slope -0.060 V decade⁻¹ have been obtained at all platinum electrodes. At high current densities, higher Tafel slopes ranging from -0.18 to -0.40 V decade⁻¹ have been observed, depending on the type of preferred oriented Pt and KOH concentration. Rotating ring-disc electrode data have shown that a higher amount of H₂O₂ is produced on one type of preferred oriented surface at all KOH concentrations. A complex reaction scheme has been used to evaluate the electrochemical rate constants of the reaction steps at three platinum electrodes.     

Carbon-supported hafnium oxynitride as cathode catalyst for polymer electrolyte membrane fuel cells

Highly stable carbon-supported hafnium oxynitride (HfO_xN_y-C) was synthesized by heating carbon-supported hafnium oxide, prepared using an impregnation method, under NH₃ gas in various conditions. X-ray diffraction patterns, X-ray photoelectron spectra, and field-emission transmission electron microscope images confirmed that HfO_xN_y nanoparticles were dispersed onto commercial carbon black, Vulcan XC-72. The stability of HfO_xN_y-C in 0.1 mol dm⁻³ H₂SO₄ at 303 K was evaluated by measuring the mass ratio of dissolved hafnium to immersed HfO_xN_y-C using inductively coupled plasma atomic emission spectroscopy. It saturated at a low level of 0.8–4.0 mg g⁻¹ with increasing immersion time up to ∼24 h. The oxygen reduction reaction (ORR) activity and rate were evaluated by obtaining cyclic voltammograms and rotating disk electrode voltammograms, respectively. The HfO_xN_y-C exhibited higher ORR activity and a lower Tafel slope than NH₃-treated C under identical conditions, demonstrating that HfO_xN_y is active toward ORR. The ORR activity most depended on the heating temperature. The ORR rate increased with increasing the heating time at 1223 K which could be due to the increased y in HfO_xN_y-C. The maximum onset potential for ORR was 0.78 V vs. standard hydrogen electrode, which is 0.18 V lower than that of carbon-supported platinum.     

Novel bismuth/multi-walled carbon nanotubes-based electrochemical sensor for the determination of neuroprotective drug cilostazol

A very sensitive electrochemical sensor has been developed by modification of glassy carbon electrode (GCE) with nanoparticles of bismuth (III) oxide (Bi₂O₃) and multi-walled carbon nanotubes (MWCNTs). The sensor was applied for the determination of cilostazol, cyclic nucleotide phosphodiesterase inhibitors in pharmaceutical formulation and human plasma. The voltammetric responses were compared with those obtained at bare GCE under optimum conditions. The cyclic and square-wave voltammograms of cilostazol showed 3.3 and 4.9 times enhancement in the oxidation peak current at MWCNTs–Bi₂O₃/GCE as compared to a bare GCE. Bi₂O₃–MWCNTs/GCE showed a linear response for cilostazol in standard solution over the concentration range of 0.8–13 μg mL⁻¹ with the detection limit 0.76 μg mL⁻¹, whereas human plasma over the concentration range 0.8–12.5 μg mL⁻¹ with the detection limit 0.66 μg mL⁻¹.     

Investigation of the Pt/YSZ interface at low oxygen partial pressure by solid electrochemical mass spectroscopy under high vacuum conditions

The Pt/YSZ interface was investigated at low oxygen partial pressure under high vacuum (HV) conditions at 400 °C. Two different electrochemical techniques were coupled to mass spectrometric gas analysis using a new solid electrochemical mass spectrometric monitoring device. Under cathodic polarization, the lack of oxygen in the gas phase induces the reduction of the YSZ solid electrolyte which acts as oxygen source for the formation of O²⁻ ions migrating to the anode. Under anodic polarization, both platinum oxidation and oxygen evolution reaction are identified. PtO _x is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms. At the Pt/YSZ interface, PtO _x formation is an electrochemical process following a parabolic growth law, while the presence of PtO _x at the Pt/gas interface is related to the diffusion of PtO _x formed at the triple phase boundary towards the Pt/gas interface. It is proposed that the side oxygen evolution reaction stabilizes thermodynamically the PtO _x diffusion toward the gas exposed interface during the anodic polarization.     

Preparation and oxygen reduction activity of stable RuSex/C catalyst with pyrite structure

Carbon supported RuSe_x (x = 0.35-2) catalysts of controlled stoichiometry and phase are synthesized via precipitating ruthenium nanoparticles on Vulcan XC-72R, then selenizing ruthenium with hydrogen annealing. The competition for Se between solid-state Ru selenization and volatile selenium hydride formation results in RuSe_x nanoparticles with the pyrite structure, the Ru hcp structure, or a mixture of the two. These catalysts are methanol tolerant, and catalytically active toward oxygen reduction reaction (ORR). RuSe_x/C (x ≅ 2) with a pyrite structure, produced at 400 °C, exhibits catalytic activity and stability superior to that of RuSe_x/C with a ruthenium hcp structure or a mixed phase. And this pyrite RuSe_x/C (x ≅ 2) catalyst yields H₂O₂ less than 1.5% in the technically pertinent potential range of 0.4-0.6 V (vs. Ag/AgCl). Its stability is verified in 100 CV cycles, which shows that the catalyst annealed at 400 °C is more enduring in potential cycling over 0.65 V (vs. Ag/AgCl), compared with the RuSe_x/C catalyst annealed at 300 °C and the RuSe_cluster/C reference catalyst prepared by thermolysis of Ru₃(CO)₁₂. After CV cycles, the 400 °C-annealed catalyst still exhibits a higher ORR activity than the other two catalysts.     

Rational determination of exchange current density for hydrogen electrode reactions at carbon-supported Pt catalysts

The rotating disk electrode (RDE) is a useful technique for precise determination of exchange current density (j₀) in electrochemistry. For the study of powder catalysts, a common practice is to apply the powder onto an inert disk substrate (such as glassy carbon). However, this approach in its usual version will lead to wrong results for the exchange current density of hydrogen electrode reactions at carbon-supported Pt nanoparticles (Pt/C) because of the poor utilization of the loaded Pt nanoparticles. Our new approach is to dilute the Pt/C powder with a large amount of pristine carbon support to make the catalyst layer. In this way, all the catalyst particles in the catalyst layer have nearly the same and much enhanced mass transport so that rational exchange current density can be obtained. Using the new approach, the current density for hydrogen electrode reactions at Pt/C in 0.1 M perchloric acid at 25 °C is found to be 27.2 ± 3.5 mA/cm² with an apparent activation energy 43 kJ/mol. These results are in agreement with the j₀ estimation based on real fuel cell experiments.     

Enhancement of oxygen reduction by incorporation of heteropolytungstate into the electrocatalytic ink of carbon supported platinum nanoparticles

Nafion stabilized inks of Vulcan XC-72 supported platinum (20 wt.%) nanoparticles (Pt/XC-72) were utilized to produce electrocatalytic films on glassy carbon. The catalysts were modified (activated) with phosphododecatungstic acid H₃PW₁₂O₄₀ (PW₁₂). Comparison was made to bare (PW₁₂-free) electrocatalytic films. Electroreduction of dioxygen was studied at 25 °C in 0.5 mol dm⁻³ H₂SO₄ electrolyte using rotating disk voltammetry. For the same loading of platinum (≈95 μg cm⁻²) and for the approximately identical distribution of the catalyst, the reduction of oxygen at a glassy carbon electrode modified with the ink containing PW₁₂ proceeded at ca. 30-60 mV more positive potential (depending on the PW₁₂ content), and the system was characterized by a higher kinetic parameter (rate of heterogeneous electron transfer), when compared to the PW₁₂-free electrocatalyst. Gas diffusion electrodes with Pt/XC-72 supported on carbon paper (Pt loading 1 mg cm⁻²) were also tested. Under the same experimental conditions, while the exchange current density and the total resistance contribution to polarization components, computed from the galvanostatic polarization curves were found to be clearly higher and lower, respectively, for the ink modified with PW₁₂ relative to the unmodified system. The results demonstrate that addition of heteropolytungstatic acid (together with Nafion) enhances the electrocatalytic activity of platinum towards reduction of oxygen.     

Oxygen reduction reaction on carbon-supported CoSe2 nanoparticles in an acidic medium

We investigated the effect of CoSe₂/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H₂O₂ production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe₂ nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H₂O₂ production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe₂ loading on the disk electrode. H₂O₂ production also decreased with increasing catalytic site density, a phenomenon related to the CoSe₂ loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe₂/C nanoparticles at 22 μg cm⁻², we determined that the transfer process involves about 3.5 electrons.     

Methanol electrooxidation on carbon-supported Pt-WO3−x electrodes in sulphuric acid electrolyte

Electrooxidation of methanol has been studied in sulphuric acid electrolyte at 60°C on carbon-supported Pt-WO_3–x electrodes employing varying amounts of WO_3–x . It is found that the electrodes containing (3:1) Pt-WO_3–x composite catalyst exhibit a higher catalytic activity towards methanol electrooxidation than platinized carbon electrodes without WO_3–x . In the light of the XPS and XRD data on the carbon-supported (3:1) Pt-WO _x sample, it is speculated that the WO _x is present in the form of an oxyhydroxide, which can promote surface oxy-species on platinum by proton transfer.     

Characterization of methanol-tolerant Pd–WO3 and Pd–SnO2 electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

Palladium (Pd) catalysts containing nanosized metal oxides, tungsten oxide (WO₃) and tin oxide (SnO₂), supported on carbon black (Pd–MO_x/C) were synthesized, and the effect of the metal oxide on the oxygen reduction reaction (ORR) in a direct methanol fuel cell (DMFC) was investigated. The SEM images showed that the Pd nanoparticles were highly dispersed on the carbon black, and the metal oxide particles were also distributed well. Pd/C and Pd–WO₃/C catalysts as cathode materials for the ORR in DMFCs showed activity similar to or better than that of Pt/C, whereas Pd–SnO₂/C showed no improvement in catalytic activity.     

O2 reduction at the Pt/Nafion interface in 85% concentrated H3PO4

The kinetics of oxygen electroreduction have been studied on a smooth platinum electrode coated with Nafion® in concentrated 85% H₃PO₄. The effects of Nafion® coatings of different thickness on O₂ electroreduction at a smooth Pt rotating disk electrode with 85% phosphoric acid as the bulk electrolyte were examined. The kinetic current increases with increasing Nafion® film thickness while the diffusion limiting current decreases with increasing Nafion® film thickness. A O₂ concentration profile model for the Pt/Nafion®/bulk electrolyte has been established, and this model can be used to explain the O₂ reduction polarization results. The performance of Nafion®-modified, high surface area Pt/carbon air cathodes for use in the H₂–air concentrated phosphoric acid fuel cell was also studied.     

Novel Graphitised Carbonaceous Materials for Use as a Highly Corrosion‐Tolerant Catalyst Support in Polymer Electrolyte Fuel Cells

Cathode catalysts for polymer electrolyte fuel cells (PEFCs) are prepared by depositing Pt nanoparticles on carbon nanospheres (CNSs) and graphitised carbon nanospheres (GCNSs), and their corrosion‐tolerance and electrocatalytic activities for the oxygen reduction reaction are evaluated. Transmission electron micrographs show that the deposited Pt nanoparticles are well dispersed on CNSs. In Pt/GCNS, Pt nanoparticles accumulate selectively along the edges of GCNSs' polygonal surfaces. Electrochemical measurements with a rotating‐ring disk electrode in an O₂‐saturated H₂SO₄ solution show that Pt/GCNS and Pt/CNS produce less H₂O₂ during oxygen reduction, compared to that obtained with a Pt catalyst on carbon black (CB). Thermogravimetric analysis reveals that GCNSs show greater combustion‐tolerance than CNSs and CB. Furthermore, GCNSs show excellent electrochemical corrosion‐tolerance in a H₂SO₄ solution. These results indicate that GCNSs are superior for use as carbon supports, and can serve as cathode catalysts in PEFCs even under oxidative conditions.     

Single-step polyol synthesis of alloy Pt<Subscript>7</Subscript>Sn<Subscript>3</Subscript> versus bi-phase Pt/SnO<Subscript>x</Subscript> nano-catalysts of controlled size for ethanol electro-oxidation

Disordered alloy and bi-phase PtSn nanoparticles of nominal Pt:Sn ratio of 70:30 atomic % with controlled size and narrow size distribution were synthesized using a single-step polyol method. By adjusting the solution pH it was possible to obtain Pt₇Sn₃ nanoparticles of various sizes from 2.8 to 6.5 nm. We found that the presence of NaOH in the synthesis solution not only influenced the nanoparticle size, but as it was revealed by XRD, it apparently also dictated the degree of Pt and Sn alloying. Three catalysts prepared at lower NaOH concentrations (C_NaOH < 0.15 M) showed disordered alloy structure of the nominal composition, while the other three catalysts synthesized at higher NaOH concentrations (C_NaOH > 0.15 M) consisted of bi-phase nanoparticles comprising a crystalline phase close to that of pure Pt together with an amorphous Sn phase. These observations are plausibly due to the phase separation and formation of monometallic Pt and amorphous SnO_x phases. A proposed reaction mechanism of Pt₇Sn₃ nanoparticle formation is presented to explain these observations along with the catalytic activities measured for the six synthesized carbon-supported Pt₇Sn₃ catalysts. The highest catalytic activity towards ethanol electro-oxidation was found for the carbon-supported bi-phase catalyst that formed the largest Pt (6.5 nm) nanoparticles and SnO_x phase. The second best catalyst was a disordered alloy Pt₇Sn₃ catalyst with the second largest nanoparticle size (5 nm), while catalysts of smaller size (4.5–4.6 nm) but different structure (disordered alloy vs. bi-phase) showed similar catalytic performance inferior to that of the 5 nm disordered alloy Pt₇Sn₃ catalyst. This work demonstrated the importance of producing bi-metallic PtSn catalysts with large Pt surfaces in order to efficiently electro-oxidize ethanol.