Electrochemical activity and stability of dealloyed Pt–Cu and Pt–Cu–Co electrocatalysts for the oxygen reduction reaction (ORR)

A comparative study of the electrochemical stability of Pt₂₅Cu₇₅ and Pt₂₀Cu₂₀Co₆₀ alloy nanoparticle electrocatalysts in liquid electrolyte half-cell environment was conducted. The aforementioned catalysts were shown to possess improved resistance to electrochemical surface area (ECSA) loss during voltage cycling relative to commercially available pure Pt electrocatalysts. The difference in ECSA loss was attributed to their initial mean particle size, which varied depending on the temperature at which the alloy catalysts were prepared (e.g. 600, 800 and 950 °C). Higher preparation temperatures resulted in larger particles and lead to lower ECSA loss. Liquid electrolyte environment short-term durability testing (5000 voltages cycles) revealed the addition of cobalt to be beneficial as ternary compositions exhibited stability advantages over binary catalysts.     

Carbon-supported Pd-Pt cathode electrocatalysts for proton exchange membrane fuel cells

A series of carbon-supported Pd-Pt alloy (Pd-Pt/C) catalysts for oxygen reduction reaction (ORR) with low-platinum content are synthesized via a modified sodium borohydride reduction method. The structure of as-prepared catalysts is characterized by powder X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The prepared Pd-Pt/C catalysts with alloy form show face-centered-cubic (FCC) structure. The metal particles of Pd-Pt/C catalysts with mean size of around 4-5 nm are uniformly dispersed on the carbon support. The electrocatalytic activities for ORR of these catalysts are investigated by rotating disk electrode (RDE), cyclic voltammetry (CV), single cell measurements and electrochemical impedance spectra (EIS) measurements. The results suggest that the electrocatalytic activities of Pd-Pt/C catalysts with low platinum are comparable to that of the commercial Pt/C with the same metal loading. The maximum power density of MEA with a Pd-Pt/C catalyst, the Pd/Pt mass ratio of which is 7:3, is about 1040 mW cm⁻².     

Pt-based electrocatalysts for polymer electrolyte membrane fuel cells prepared by supercritical deposition technique

Pt-based electrocatalysts were prepared on different carbon supports which are multiwall carbon nanotubes (MWCNTs), Vulcan XC 72R (VXR) and black pearl 2000 (BP2000) using a supercritical carbon dioxide (scCO₂) deposition technique. These catalysts were characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and cyclic voltammetry (CV). XRD and HRTEM results demonstrated that the scCO₂ deposition technique enables a high surface area metal phase to be deposited, with the size of the Pt particles ranging from 1 to 2 nm. The electrochemical surface areas (ESAs) of the prepared electrocatalysts were compared to the surface areas of commercial ETEK Pt/C (10 wt% Pt) and Tanaka Pt/C (46.5 wt% Pt) catalysts. The CV data indicate that the ESAs of the prepared Pt/VXR and Pt/MWCNT catalysts are about three times larger than that of the commercial ETEK catalyst for similar (10 wt% Pt) loadings. Oxygen reduction activity was investigated by hydrodynamic voltammetry. From the slope of Koutecky–Levich plots, the average number of electrons transferred in the oxygen reduction reaction (ORR) was 3.5, 3.6 and 3.7 for Pt/BP2000, Pt/VXR and Pt/MWCNT, correspondingly, which indicated almost complete reduction of oxygen to water.     

Investigation of a platinum catalyst supported on a hydrogen peroxide-treated carbon black

A hydrogen peroxide-treated carbon support was used to prepare electrocatalysts for low temperature fuel cells. The electrochemical stability of the Pt/C catalysts was evaluated by potential scan tests. The results show that the oxidative treatment introduces oxygen-bearing functional groups onto the support surface. These functional groups connect with metal nanoparticles via O atoms so as to alleviate the agglomeration of supported nanocatalysts. During the test, the mean particle size of catalysts increased from 2.5 nm and 2.6 nm to 5.3 nm and 4.8 nm for untreated and treated carbon support, respectively. Electrochemical surface area measurements also show that the oxidative treatment of carbon support enhanced the electrochemical stability of Pt/C catalysts. It is suggested that the chemical interaction between the metal particle and the oxygen-bearing functional group plays an important role in immobilizing catalyst nanoparticles.     

Tungsten carbide modified high surface area carbon as fuel cell catalyst support

Phase pure WC nanoparticles were synthesized on high surface area carbon black (800 m² g⁻¹) by a temperature programmed reaction (TPR) method. The particle size of WC can be controlled under 30 nm with a relatively high coverage on the carbon surface. The electrochemical testing results demonstrated that the corrosion resistance of carbon black was improved by 2-fold with a surface modification by phase pure WC particles. However, the WC itself showed some dissolution under potential cycling. Based on the X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis, most of the WC on the surface was lost or transformed to oxides after 5000 potential cycles in the potential range of 0.65-1.2 V. The Pt catalyst supported on WC/C showed a slightly better ORR activity than that of Pt/C, with the Pt activity loss rate for Pt/WC/C being slightly slower compared to that of Pt/C. The performance and decay rate of Pt/WC/C were also evaluated in a fuel cell.     

Carbon supported nano-sized Pt–Pd and Pt–Co electrocatalysts for proton exchange membrane fuel cells

Nano-sized Pt–Pd/C and Pt–Co/C electrocatalysts have been synthesized and characterized by an alcohol-reduction process using ethylene glycol as the solvent and Vulcan XC-72R as the supporting material. While the Pt–Pd/C electrodes were compared with Pt/C (20 wt.% E-TEK) in terms of electrocatalytic activity towards oxidation of H₂, CO and H₂–CO mixtures, the Pt–Co/C electrodes were evaluated towards oxygen reduction reaction (ORR) and compared with Pt/C (20 wt.% E-TEK) and Pt–Co/C (20 wt.% E-TEK) and Pt/C (46 wt.% TKK) in a single cell. In addition, the Pt–Pd/C and Pt–Co/C electrocatalyst samples were characterized by XRD, XPS, TEM and electroanalytical methods. The TEM images of the carbon supported platinum alloy electrocatalysts show homogenous catalyst distribution with a particle size of about 3–4 nm. It was found that while the Pt–Pd/C electrocatalyst has superior CO tolerance compared to commercial catalyst, Pt–Co/C synthesized by polyol method has shown better activity and stability up to 60 °C compared to commercial catalysts. Single cell tests using the alloy catalysts coated on Nafion-212 membranes with H₂ and O₂ gases showed that the fuel cell performance in the activation and the ohmic regions are almost similar comparing conventional electrodes to Pt–Pd anode electrodes. However, conventional electrodes give a better performance in the ohmic region comparing to Pt–Co cathode. It is worth mentioning that these catalysts are less expensive compared to the commercial catalysts if only the platinum contents were considered.     

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

Performance of carbon nanofiber supported Pd-Ni catalysts for electro-oxidation of ethanol in alkaline medium

Carbon nanofibers (CNF) supported Pd-Ni nanoparticles have been prepared by chemical reduction with NaBH₄ as a reducing agent. The Pd-Ni/CNF catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical voltammetry analysis. TEM showed that the Pd-Ni particles were quite uniformly distributed on the surface of the carbon nanofiber with an average particle size of 4.0 nm. The electro-catalytic activity of the Pd-Ni/CNF for oxidation of ethanol was examined by cyclic voltammetry (CV). The onset potential was 200 mV lower and the peak current density four times higher for ethanol oxidation for Pd-Ni/CNF compared to that for Pd/C. The effect of an increase in temperature from 20 to 60 °C had a great effect on increasing the ethanol oxidation activity.     

A facile method to synthesize well-dispersed PtRuMoOx and PtRuWOx nanoparticles and their electrocatalytic activities for methanol oxidation

PtRuMoO_x and PtRuWO_x catalysts supported on multi-wall carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction method. XRD measurements indicate that Pt exists as face-centered cubic structure, Ru is alloyed with platinum, and the metal oxides exist as an amorphous structure. TEM pictures show that PtRuMoO_x and PtRuWO_x catalysts are well dispersed on the surface of MWCNTs with the particle size of about 3 nm and a narrow particle size distribution. The electrochemical properties of the catalysts for methanol electrooxidation are studied by cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP). The onset potentials for methanol oxidation on PtRuMoO_x and PtRuWO_x are more negative than that of pure Pt catalyst, shifting negatively by about 0.20 V and have better electrocatalytic activities than PtRu/MWCNTs.     

A solution-phase synthesis method to highly active Pt-Co/C electrocatalysts for proton exchange membrane fuel cell

A solution-phase synthesis method was studied to prepare carbon supported Pt-Co alloy catalysts. The organic precursors of Pt acetylacetonate and Co acetylacetonate were reduced in a high boiling point solvent of octyl ether in the presence of oleic acid (OAc) and oleylamine (OAm) to produce fine Pt-Co nanoparticles, which were subsequently deposited on carbon support to obtain Pt-Co/C catalysts. Thermogravimetric analysis suggests that the stabilizers (OAc and OAm) can be removed by copious ethanol washing and subsequent moderate temperature heat-treatment (250 °C, under Argon atmosphere). X-ray diffraction patterns indicate that the average particle size is around 2.3 nm, and the lattice parameter is 3.868 Å for the heat-treated Pt-Co/C (40 wt%). Transmission electron microscopy images show very small Pt-Co alloy nanoparticles homogeneously dispersed on the carbon support with a particle size distribution of 2-4 nm for all Pt-Co/C samples. The elements composition of Pt and Co in the final Pt-Co/C catalyst can be well controlled, as evidenced by inductively coupled plasma atomic emission spectroscopy and energy dispersive spectroscopy analyses. Proton exchange membrane fuel cell tests show the heat-treated Pt-Co/C cathode catalyst has higher mass activity of oxygen reduction reaction than Pt/C at an operation voltage of 0.9 V, this can be attributed to its smaller particle size and reduced lattice parameter.     

Activity and durability of electrocatalytic layers with low platinum loading prepared by magnetron sputtering onto gas diffusion electrodes

This article presents development of magnetron sputtering technology for catalytic Pt coating production. Low Pt-loaded gas diffusion electrodes were prepared by a single-step magnetron sputtering in different regimes. Pt and Pt-carbon films (95–97 wt% Pt) were deposited directly onto the gas diffusion layers (GDLs): carbon paper (Sigracet® 39 BC) and carbon cloths (GDL-CT, ELAT® LT 1400 W). A bipolar pulsed direct current (DC) magnetron sputtering with pulsed frequency 100 kHz was used as the method for catalysts deposition. Total Pt loading in the thin films varied in the range 0.04–0.13 mg*cm⁻². Obtained catalysts onto GDLs were investigated by scanning/transmission electron microscope (S/TEM), X-ray Absorption Near Edge Structure (XANES) and potentiodynamic methods. The deposited catalysts had a high electrochemical surface area (ECSA) and stability determined by the durability stress test (DST) method. The highest ECSA was obtained with Pt target in the pulsed (100 kHz) sputtering mode. This ECSA values were rather stable during 3000 cycles of stress-tests versus 500 cycles for Pt deposited by polyol method on the same gas diffusion layers.     

Pulse-microwave assisted polyol synthesis of highly dispersed high loading Pt/C electrocatalyst for oxygen reduction reaction

In the present investigation, a high loading Pt/C up to 50% (weight ratio) was prepared by a pulse-microwave assisted polyol synthesis method, in which the metal reduction can be accomplished in 2 min. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that the Pt particles were highly dispersed on the support and had a narrow particle distribution with a mean particle size of about 2.7 nm. Based on the electrochemical properties characterized by cyclic voltammetry and rotating disk electrode techniques, it was found that the as-prepared 50% Pt/C exhibited a comparable activity for oxygen reduction reaction with respect to the commercial one.     

Electro-deposition on carbon black and carbon nanotubes of Pt nanostructured catalysts for methanol oxidation

An electrochemical method for the Pt nanoparticles deposition on porous and high surface carbon substrates (carbon black and carbon nanotubes), as an alternative way to prepare gas diffusion electrodes for polymer electrolyte fuel cells (PEFCs), is herein described. Pt nanoparticles well distributed and localized on the electrode surface were obtained by using an electric field. The electro-catalysts were prepared by single and multiple pulse galvanostatic polarizations in 1 M sulphuric acid + 5 mM exachloroplatinic acid solution. Chemical analysis, cyclic voltammetry and field emission gun scanning electron microscopy were used to determine the electrochemical features of Pt deposits and the influence of electro-deposition method on their nano-morphology. Electro-catalytic performances were studied by investigating the methanol oxidation reaction and the results are presented in form of surface specific activity and mass specific activity to take into account the electrochemical real surface and Pt loading. A comparison with commercial E-TEK Pt/C catalysts, prepared by traditional chemical reduction and heat treatment in hydrogen, shows that the electrodeposited catalyst presents higher activity at lower Pt loading.     

Synthesis and activation of Pt nanoparticles with controlled size for fuel cell electrocatalysts

Well-dispersed Pt nanoparticles with controlled size and narrow size distribution were prepared by polyalcohol reduction of platinum acetylacetonate, using oleylamine as a capping agent. The particle size was varied from 3.5 nm to 11.5 nm by decreasing the amount of oleylamine added in the synthesis. Size selection of the as-prepared particles by solvent fractionation yielded nearly monodispersed Pt particles. The as-prepared particles were loaded on a carbon support by physical deposition, but showed no electrocatalytic activity due to the oleylamine bound to the particle surface. The particles were activated for electrocatalysis after heating the particles in air at 185 °C for 5 h, conditions that gave no particle-sintering and no oxidation. Cyclic voltammetry showed that the particles after the heat treatment in air were electrocatalytically active for methanol oxidation. The smaller 3.5 nm and 4.0 nm Pt particles had a higher intrinsic activity for methanol oxidation, but a lower tolerance to CO poisoning, compared with 6.0 nm, 9.5 nm and 11.5 nm particles. CO-stripping results suggest that CO is more easily oxidized on larger Pt particles.     

Pt and PtRu nanoparticles deposited on single-wall carbon nanotubes for methanol electro-oxidation

Platinum (Pt) and platinum–ruthenium (PtRu) nanoparticles supported on Vulcan XC-72 carbon and single-wall carbon nanotubes (SWCNT) are prepared by a microwave-assisted polyol process. The catalysts are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which are uniformly dispersed on carbon, have diameters of 2–6 nm. All the PtRu/C catalysts display the characteristic diffraction peaks of a face centred cubic Pt structure, excepting that the 2θ values are shifted to slightly higher values. The results from XPS analysis reveal that the catalysts contain mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV) and Ru(IV). The electrooxidation of methanol is studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. Both PtRu/C catalysts have high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a single direct methanol fuel cell using the SWCNT supported PtRu alloy as the anode catalyst delivers high power density.     

Preparation of Pt supported on WO3-C with enhanced catalytic activity by microwave-pyrolysis method

The WO₃-C hybrid materials are prepared by intermittently microwave-pyrolysis using ammonium tungstate as the precursor, and then Pt nano-particles are deposited by microwave-assited polyol process on WO₃-C. The TEM images show the dispersion of ∼10 nm WO₃ particles size supported on carbon and ∼3 nm Pt metal crystallites supported on WO₃-C. XRD results illustrate that WO₃ presented as monoclinic phase and the content of WO₃ in WO₃/C and Pt/WO₃-C catalysts is further characterized by EDAX. Furthermore, XPS characterizations indicate that the interaction between Pt and WO₃ is dramatically enhanced after heat treatment at 200 °C. The activities of Pt/WO₃-C for the electrochemical oxidation of methanol are compared with Pt/C in acid solution by cyclic voltammetry, CO-stripping and chronoaperometry. Pt/WO₃-C catalyst calcined at 200 °C exhibits the highest activity per electrochemical active surface area for methanol oxidation and is 60 mV more negative for CO electro-oxidation than that of Pt/C and Pt/WO₃-C without heat treatment. The great enhancement of electrochemical performance may be due to the improvement of the synergistic effect between Pt and WO₃ in Pt/WO₃-C catalyst after heat treatment.     

Effects of anatase TiO2 with different particle sizes and contents on the stability of supported Pt catalysts

Pt/TiO₂-C catalyst with TiO₂ and carbon black as the mixed support has been synthesized by the microwave-assisted polyol process (MAPP). Effects of anatase TiO₂ with different particle sizes and contents on the stability of supported Pt catalysts have been systematically studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammograms (CV), and accelerated potential cycling tests (APCT) have been carried out to present the influence degree. The experimental results indicate that the original electrochemically active specific surface areas (ESA) of the catalysts decrease with the increase of mean particle sizes of TiO₂ and TiO₂ contents. However, the activity of Pt/TiO₂-C-20 is very close to that of Pt/TiO₂-C-5 and the stability of Pt/TiO₂-C-20 is the best after 1000 cycles APCT, illustrating that the optimized particle size of TiO₂ in Pt/TiO₂-C catalyst is 20 nm. Furthermore, the stability of the catalysts increase with the increase of TiO₂ contents in the mixed support. Taking into account both the activity and stability of various Pt/TiO₂-C catalysts, the optimized particle size of TiO₂ is 20 nm and the optimal TiO₂ content existed in the mixed support is 40%.     

Durability studies on performance degradation of Pt/C catalysts of proton exchange membrane fuel cell

In the present paper, a proton exchange membrane fuel cell (PEMFC) using 20 wt.% Pt/C as anode and cathode catalysts, and ambient air at cathode was operated at a current density of 160 mA cm⁻² for 2250 h. The measurement results showed that electrochemically active specific areas (S_EAS) of both electrode catalysts calculated from CV curves after test evidently decreased. The decay rate of S_EAS of anode catalyst was much lower than that of cathode one. X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), and X-ray photoelectron spectrometry (XPS) were employed to characterize the anode and cathode catalysts before and after the life test. The XRD results showed that their crystal structures were perfect, the particle size of new Pt/C catalyst was about 2.5 nm, however, the particle sizes of anode and cathode ones markedly increased, and were about 4.9 nm and 6.8 nm, respectively, after the life test. Furthermore, the size of cathode catalyst was much bigger than that of anode one after test. The Pt element was also found in Nafion^® film as shown in EDAX result. The XPS results presented that the content of Pt oxidation states in cathode was much more than that in anode, and the corrosion of carbon support in cathode was also more severe than that in anode after the life test. The experimental results indicated that the increase of particle size of Pt/C catalyst was illustrated with the dissolution/redeposition mechanism. The degradation of cathode catalyst for oxygen electroreduction was one of the main factors affecting on the performance decay of PEMFC.     

Preparation of platinum nanoparticles on carbon black with mixed binary surfactants: Characterization and evaluation as anode catalyst for low-temperature fuel cell

Platinum nanoparticles supported on Vulcan XC-72R prepared by a surfactant-stabilized colloidal method exhibit excellent properties as an anode catalyst for a low-temperature fuel cell. A Pt/C catalyst prepared with a 10-fold critical micelle concentration of mixed non-ionic surfactants [polyoxyethylene (23) lauryl ether + polyoxyethylene (20) sorbitan monolaurate (Brij 35 + Tween 20)] shows the highest catalytic activity and the greatest electrochemical surface-active area among those prepared. The maximum current density of this catalyst is much higher than that of a commercial Pt/C catalyst (E-TEK). Moreover, X-ray diffraction and transmission electron microscopy analyses reveal that Pt/C prepared with Brij 35 + Tween 20 has an average particle size of 2.4 nm with quite a narrow distribution between 2 and 3 nm, which is the smallest among all the catalysts prepared. This is attributed to the formation of more compact micelles. Mixtures of non-ionic and anionic surfactants result in less compact micelles.     

Carbon xerogels as Pt catalyst supports for polymer electrolyte membrane fuel-cell applications

Carbon xerogels prepared by the resorcinol-formaldehyde (RF) sol-gel method with ambient-pressure drying were explored as Pt catalyst supports for polymer electrolyte membrane (PEM) fuel cells. Carbon xerogel samples without Pt catalyst (CX) were characterized by the N₂ sorption method (BET, BJH, others), and carbon xerogel samples with supported Pt catalyst (Pt/CX) were characterized by thermogravimetry (TGA), powder X-ray diffraction (XRD), electron microscopy (SEM, TEM) and ex situ cyclic voltammetry for thin-film electrode samples supported on glassy carbon and studied in a sulfuric acid electrolyte. Experiments on Pt/CX were made in comparison with commercially obtained samples of Pt catalyst supported on a Vulcan XC-72R carbon black support (Pt/XC-72R). CX samples had high BET surface area with a relatively narrow pore size distribution with a peak pore size near 14 nm. Pt contents for both Pt/CX and Pt/XC-72R were near 20 wt % as determined by TGA. Pt catalyst particles on Pt/CX had a mean diameter near 3.3 nm, slightly larger than for Pt/XC-72R which was near 2.8 nm. Electrochemically active surface areas (ESA) for Pt as determined by ex situ CV measurements of H adsorption/desorption were similar for Pt/XC-72R and Pt/CX but those from CO stripping were slightly higher for Pt/XC-72R than for Pt/CX. Membrane-electrode assemblies (MEAs) were fabricated from both Pt/CX and Pt/XC-72R on Nafion 117 membranes using the decal transfer method, and MEA characteristics and single-cell performance were evaluated via in situ cyclic voltammetry, polarization curve, and current-interrupt and high-frequency impedance methods. In situ CV yielded ESA values for Pt/XC-72R MEAs that were similar to those obtained by ex situ CV in sulfuric acid, but those for Pt/CX MEAs were smaller (by 13-17%), suggesting that access of Nafion electrolyte to Pt particles in Pt/CX electrodes is diminished relative to that for Pt/XC-72R electrodes. Polarization curve analysis at low current density (0.9 V cell voltage) reveals slightly higher intrinsic catalyst activity for the Pt/CX catalyst which may reflect the fact that Pt particle size in these catalysts is slightly higher. Cell performance at higher current densities is slightly lower for Pt/CX than the Pt/XC-72R sample, however after normalization for Pt loading, performance is slightly higher for Pt/CX, particularly in H₂/O₂ and at lower cell temperatures (50 °C). This latter finding may reflect a possible lower mass-transfer resistance in the Pt/CX sample.