Improved ORR activity of non-noble metal electrocatalysts by increasing ligand and metal ratio in synthetic complex precursors

In an effort to improve oxygen reduction reaction (ORR) activity by increasing the catalytic active site density in carbon-supported non-noble metal catalysts, several nitrogen-containing catalysts were synthesized through a heat treatment process at 900 °C using precursor complexes of Fe(II) and tripyridyl triazine (TPTZ). Fe to TPTZ mole ratios of 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 were used to prepare the precursor complexes. X-ray diffraction and surface electrochemical techniques were used to characterize these catalysts (Fe–N_x/C), and revealed that when the amount of TPTZ in the precursor complex was increased, the decomposition of Fe–N_x sites, which are considered active sites for the ORR, was effectively reduced, resulting in higher Fe–N_x site density and thus improving the catalysts’ ORR activity. This beneficial effect was validated through rotating disk electrode tests and analysis of the ORR kinetics catalyzed by these catalysts. The obtained results showed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–N_x/C increased monotonically in the mole ratio range of 1:2–1:6. Therefore, increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to reduce the decomposition of ORR active site density and thereby enhance the ORR activity of non-noble metal catalysts.     

Fe loading of a carbon-supported Fe–N electrocatalyst and its effect on the oxygen reduction reaction

Carbon-supported non-noble metal catalysts with Fe as the metal and tripyridyl triazine (TPTZ) as the ligand (Fe-TPTZ/C) were synthesized using a simple chemical method. How the Fe loading in this Fe-TPTZ/C catalyst affected the ORR activity was investigated using several Fe loadings: 0.2, 0.4, 0.7, 2.7, 4.7, 5.8 and 7.8 wt%. The as-prepared catalysts were then heat-treated at 800 °C in an N₂ environment to obtain catalysts of Fe–N/C. Energy dispersive X-ray spectroscopy (EDX) was used to identify the Fe–N/C catalysts. These Fe–N/C catalysts showed significant ORR activity improvement over the as-prepared Fe-TPTZ/C catalysts. The kinetics of the ORR catalyzed by the catalysts with different Fe loadings was studied using the rotating disk electrode technique. It was observed that a 4.7 wt% Fe loading yielded the best catalytic ORR activity. Regarding the overall ORR electron transfer number, it was found that as the catalyst's Fe loading increased, the overall ORR electron transfer number changed from 2.9 to 3.9, suggesting that increasing the Fe loading could alter the ORR mechanism from a 2-electron to a 4-electron transfer dominated process. The Tafel method was also used to obtain one important kinetic parameter: the exchange current density. A fuel cell was assembled using a membrane electrode assembly with 4.7 wt% Fe loaded Fe–N/C as the cathode catalyst, and the cell was tested for both performance and durability, yielding a 1000-h lifetime.     

Heat-treated cobalt-tripyridyl triazine (Co-TPTZ) electrocatalysts for oxygen reduction reaction in acidic medium

In this paper, carbon-supported cobalt-tripyridyl triazine (Co-TPTZ) complexes were synthesized by a simple chemical method, then heat-treated at 600, 700, 800, and 900 °C to optimize their activity for the oxygen reduction reaction (ORR). The resulting catalysts (Co-N/C) all showed strong catalytic activity toward the ORR, but the catalyst heat-treated at 700 °C yielded the best ORR activity. Co-N/C catalysts with several Co loadings - 0.64, 2.0, 2.96, 3.33, 5.28, and 7.18 wt% - were also synthesized and tested for ORR activity. X-ray diffraction and energy dispersive X-ray analysis were used to characterize these catalysts in terms of their structure and composition. To achieve further quantitative evaluation of the catalysts in terms of their ORR kinetics and mechanism, rotating disk electrode and rotating ring-disk electrode techniques were used with the Koutecky-Levich theory to obtain several important kinetic parameters: overall ORR electron transfer number, electron transfer coefficiency in the rate-determining step (RDS), chemical reaction rate constant, electron transfer rate constant in the RDS, exchange current density, and mole percentage of H₂O₂ produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.5 with 14% H₂O₂ production, suggesting that the ORR catalyzed by Co-N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process; based on these measurements, an ORR mechanism is proposed based on the literature and our understanding, to facilitate further investigation. The stability of a Co-N/C catalyst was also tested by fixing a current density to record the change in electrode potential with time. For comparison, two other catalysts, Fe-N/C and TPTZ/C, were also tested for stability under the same conditions as the Co-N/C catalyst. Among these three, the 5 wt% Co-N/C was most stable.     

Synthesis of carbon-supported binary FeCo–N non-noble metal electrocatalysts for the oxygen reduction reaction

In this paper, a carbon-supported binary FeCo–N/C catalyst using tripyridyl triazine (TPTZ) as the complex ligand was successfully synthesized. The FeCo–TPTZ complex was then heat-treated at 600 °C, 700 °C, 800 °C, and 900 °C to optimize its oxygen reduction reaction (ORR) activity. It was found that the 700 °C heat-treatment yielded the most active FeCo–N/C catalyst for the ORR. XRD, EDX, TEM, XPS, and cyclic voltammetry techniques were used to characterize the structural changes in these catalysts after heat-treatment, including the total metal loading and the mole ratio of Fe to Co in the catalyst, the possible structures of the surface active sites, and the electrochemical activity. XPS analysis revealed that Co–N_x, Fe–N_x, and C–N were present on the catalyst particle surface. To assess catalyst ORR activity, quantitative evaluations using both RDE and RRDE techniques were carried out, and several kinetic parameters were obtained, including overall ORR electron transfer number, electron transfer coefficient in the rate-determining step (RDS), electron transfer rate constant in the RDS, exchange current density, and mole percentage of H₂O₂ produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was ～3.88, with H₂O₂ production under 10%, suggesting that the ORR catalyzed by FeCo–N/C catalyst is dominated by a 4-electron transfer pathway that produces H₂O. The stability of the binary FeCo–N/C catalyst was also tested using single Fe–N/C and Co–N/C catalysts as baselines. The experimental results clearly indicated that the binary FeCo–N/C catalyst had enhanced activity and stability towards the ORR. Based on the experimental results, a possible mechanism for ORR performance enhancement using a binary FeCo–N/C catalyst is proposed and discussed.     

Novel carbon-supported Fe-N electrocatalysts synthesized through heat treatment of iron tripyridyl triazine complexes for the PEM fuel cell oxygen reduction reaction

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) was used as a ligand to prepare iron-TPTZ (Fe-TPTZ) complexes for the development of a new oxygen reduction reaction (ORR) catalyst. The prepared Fe-TPTZ complexes were then heat-treated at temperatures ranging from 400 °C to 1100 °C to obtain carbon-supported Fe-N catalysts (Fe-N/C). These catalysts were characterized in terms of catalyst composition, structure, and morphology by several instrumental methods such as energy dispersive X-ray, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. With respect to the ORR activity, the Fe-N/C catalysts were also evaluated by cyclic voltammetry, as well as rotating disk and ring-disk electrodes. The results showed that among the heat-treated catalysts, that obtained at a heat-treatment temperature of 800 °C is the most active ORR catalyst. The overall electron transfer number for the catalyzed ORR was determined to be between 3.5 and 3.8, with 10-30% H₂O₂ production. The ORR catalytic activity of this catalyst was also tested in a hydrogen-air proton exchange membrane (PEM) fuel cell. At a cell voltage of 0.30 V, this fuel cell can give a current density of 0.23 A cm⁻² with a maximum MEA power density of 0.070 W cm⁻² indicating that this catalyst has potential to be used as a non-noble catalyst in PEM fuel cells.     

Carbon-supported manganese oxide nanoparticles as electrocatalysts for oxygen reduction reaction (orr) in neutral solution

Manganese oxides (MnO _x ) catalysts were chemically deposited onto various high specific surface area carbons. The MnO _x /C electrocatalysts were characterised using a rotating disk electrode and found to be promising as alternative, non-platinised, catalysts for the oxygen reduction reaction (ORR) in neutral pH solution. As such they were considered suitable as cathode materials for microbial fuel cells (MFCs). Metal [Ni, Mg] ion doped MnO _x /C, exhibited greater activity towards the ORR than the un-doped MnO _x /C. Divalent metals favour oxygen bond splitting and thus orientate the ORR mechanism towards the 4-electron reduction, yielding less peroxide as an intermediate.     

Tantalum (oxy)nitrides prepared using reactive sputtering for new nonplatinum cathodes of polymer electrolyte fuel cell

Tantalum (oxy)nitrides (TaO_xN_y) have been investigated as new cathodes for polymer electrolyte fuel cells without platinum. TaO_xN_y films were prepared using a radio frequency magnetron sputtering under Ar + O₂ + N₂ atmosphere at substrate temperatures from 50 to 800 °C. The effect of the substrate temperature on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the TaO_xN_y films were examined. The catalytic activity of the TaO_xN_y for the ORR increased with the increasing substrate temperature. The ORR current density at 0.4 V vs. RHE on TaO_xN_y prepared at 800 °C was approximately 20 times larger than that on TaO_xN_y prepared at 50 °C. The onset potential of the TaO_xN_y for the ORR was obtained at the ORR current density of −0.2 μA cm⁻². The onset potential of the TaO_xN_y prepared at 800 °C was ca. 0.75 V vs. RHE. The X-ray diffraction patterns revealed that Ta₃N₅ structure grew as the substrate temperature increased. While, the ionization potentials of all specimens were lower than that of Ta₃N₅, and decreased with the increasing substrate temperature. The TaO_xN_y which had Ta₃N₅ structure and lower ionization potential might have a definite catalytic activity for the ORR.     

Phosphate adsorption and its effect on oxygen reduction reaction for PtxCoy alloy and Aucore–Ptshell electrocatalysts

The phosphate adsorption characteristics and its effect on oxygen reduction reaction (ORR) were examined for various carbon-supported catalysts (Pt/C, Pt₃Co/C, PtCo/C, and Au_core–Pt_shell/C). Using cyclic voltammetry (CV) and the addition of phosphoric acid, the degree of phosphate adsorption for each catalyst was evaluated based on the intensity of the phosphate adsorption peaks (0.25–0.3 V and 0.5–0.65 V) and on the decrease in the platinum oxidation current (0.9 V). In the N₂O reduction technique, the surface structures were analyzed using N₂O as an electrochemical probe, which showed that as the Co content increased, (i) steps or defects were introduced by surface reconstruction, (ii) the phosphate adsorbed more strongly compared to Pt/C with a preference for the terrace sites, and (iii) the potential of zero total charge (PZTC) shifted to negative potentials. In the case of the Au_core–Pt_shell/C, the phosphate adsorption was found to be weaker than other catalysts, including Pt/C catalyst. The relative ORR activity with PA addition, normalized by that with no phosphate adsorption, was significantly smaller for Co containing alloy catalysts (PtCo/C: 18.2%) and larger for Au_core–Pt_shell (30.2%) compared with the Pt/C catalyst (27.8%), confirming the phosphate adsorption characteristics of each catalyst, as measured by CV and N₂O reduction analysis.     

Electrochemical behavior of nanostructured nickel phthalocyanine (NiPc/C) for oxygen reduction reaction in alkaline media

Carbon-supported nickel phthalocyanine (NiPc/C) nanoparticle catalysts have been synthesized by a simple solvent-impregnation and milling procedure, then heat-treated at 600, 700, 800 and 900?°C to optimize their activity for the oxygen reduction reaction (ORR). The electrocatalytic activity and electron transfer mechanism of NiPc/C catalysts were demonstrated in oxygen-saturated alkaline electrolyte by cyclic voltammetry, linear sweep voltammetry as well as rotating disk electrode techniques, respectively. The results show that the heat-treatment temperature has a remarkable impact on the ORR activity of NiPc/C. An onset potential of 0.05?V and a half-wave potential of ?0.15?V are achieved in 0.1?M KOH after the catalyst was heat-treated at 800?°C. In addition to an increase in ORR kinetics, the number of electrons transferred for ORR also increased from 2.2 to 2.8 with increasing heat-treatment temperature from 600 to 800?°C. To understand the heat-treatment effect, X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy (XPS) were used to identify the catalyst structure and composition. From XPS analysis, pyridinic-N and graphitic-N were clearly observed after the sample was heat-treated at 800?°C. Both of these species might be assigned to sites catalytically active toward the ORR leading to activity enhancement.     

Ordered mesoporous Fe (or Co)–N–graphitic carbons as excellent non-precious-metal electrocatalysts for oxygen reduction

Novel mesoporous Fe (or Co)–N_x–C non-precious-metal catalysts (NPMCs) have been fabricated by a simple nanocasting-pyrolysis method using 1,10-phenanthroline metal chelates as the precursors. Owing to the ordered hexagonal mesostructures, appropriate surface area, large-pore channels, and well-distributed metal–N_x moieties embedded within the graphitic carbon backbones, the prepared metal–N_x–C materials exhibit excellent catalytic activity for oxygen reduction reaction (ORR) in both alkaline and acidic media. The prepared Fe–N_x–C materials, when prepared with an optimized catalyst loading on the electrode, exhibit more positive ORR onset-potential and half-wave potential (E_1/2) than commercial Pt/C catalysts and the previously reported NPMCs in 0.1 M KOH electrolyte. They also have the comparable ORR onset-potential and current densities to Pt/C electrode in 0.1 M HClO₄ electrolyte. Moreover, ORR over mesoporous Fe–N_x–C was found to proceed by the direct four-electron mechanism with high selectivity in both electrolytes. The mesoporous Fe–N_x–C materials demonstrated higher ORR catalytic activity compared to the NPMCs made by alternative methods. Analysis of the catalytic behavior, structure and nature of surface species of N_x–C materials allows us to ascribe the origin of the excellent ORR catalytic activity of mesoporous Fe (or Co)–N_x–C in both electrolytes to Fe (or Co)–N_x moieties embedded within the graphitic carbon frameworks.     

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.     

Characterisation by TPR, XRD and NOx Storage Capacity Measurements of the Ageing by Thermal Treatment and SO2 Poisoning of a Pt/Ba/Al NOx-Trap Model Catalyst

The deactivation of a Pt/Ba/Al₂O₃ NO _x -trap model catalyst submitted to SO₂ treatment and/or thermal ageing at 800 °C was studied by H₂ temperature programmed reduction (TPR), X-ray diffraction (XRD) and NO _x storage capacity measurements.The X-ray diffractogram of the fresh sample exhibits peaks characteristic for barium carbonate. Thermal ageing leads to the decomposition of barium carbonate and to the formation of BaAl₂O₄. The TPR profile of the sulphated sample shows the presence of (i) surface aluminium sulphates, (ii) surface barium sulphates, (iii) bulk barium sulphates. The exposure to SO₂ after ageing leads to a small decrease of the surface barium-based sulphates, expected mainly as aluminate barium sulphates. This evolution can be attributed to a sintering of the storage material. TPR experiments also show that thermal treatment at 800 °C after the exposure to SO₂ involves the decomposition of aluminium surface sulphates to give mainly bulk barium sulphates, also pointed out by XRD. Thus, the thermal treatment at 800 °C leads to a stabilization of the sulphates.These results are in accordance with the NO _x storage capacity measurements. On non-sulphated catalysts, the treatment at 800 °C induces to a decrease of the NO _x storage capacity, showing that barium aluminate presents a lower NO _x storage capacity than barium carbonate. Sulphation strongly decreases the NO _x storage capacity of catalysts, whatever the initial thermal treatment, showing that barium sulphates inhibit the NO₂ adsorption. Moreover, the platinum activity for the NO to NO₂ oxidation is lowered by thermal treatments.     

PtCo/Polypyrrole‐Multiwalled Carbon Nanotube Complex Cathode Catalyst Containing Two Types of Oxygen Reduction Active Sites Used in Direct Methanol Fuel Cells

Low oxygen reduction reaction (ORR) activity and high cost of noble metal catalysts are two major challenges in direct methanol fuel cells (DMFCs). Pt‐based catalysts are considered as an ideal alternative to deal with these two problems. While the second component metals play only the role of synergy effect with Pt, they themselves are inert towards activity towards ORR. It is necessary to design a new route to ultilize the second component metal by forming CoN_x ORR active site on the base of PtM catalyst. In this paper, PtCo/polypyrrole‐multiwalled carbon nanotubes (PtCo/PPy‐MWCNTs) catalyst containing two types of ORR active site (Pt and CoN_x) was synthesized by one pot synthesis route. The effect and dynamic mechanism of the named CoN_x active site towards ORR was discussed by X‐ray photoelectron sprectroscopy and linear sweep voltammetry. PtCo/PPy‐MWCNTs cathode catalyst showed improved activity towards ORR and great potential in DMFCs.     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

The effect of boron doping into Co-C-N and Fe-C-N electrocatalysts on the oxygen reduction reaction

Co-C-N and Fe-C-N thin film catalysts have been modified by controlled doping with boron. Corresponding novel thin film catalysts Co-C-N-B and Fe-C-N-B were synthesized by combinatorial magnetron sputter deposition in an Ar/N₂ gas mixture followed by subsequent heat-treatment between 700 and 1000 °C in an argon atmosphere. The nitrogen content of the as-prepared thin film catalysts could be increased by the addition of boron. Furthermore, the amount of remaining nitrogen in heat-treated catalyst samples was significantly higher in case of boron containing samples. The thin film catalysts were characterized by means of X-ray diffraction (XRD) analysis, electron microprobe and electrochemical measurements. For electrochemical studies the activity as oxygen reduction reaction (ORR) catalyst was investigated using the rotating ring-disk electrode (RRDE) technique in 0.1 M HClO₄ solution at room temperature. The catalytic activity was found to decrease with the boron content in the thin film catalysts even though the N-content increased.     

Effect of support type and synthesis conditions on the oxygen reduction activity of RuxSey catalyst prepared by the microwave polyol method

Ru_xSe_y nanoparticles supported on different carbon substrates were synthesized by microwave heating of ethylene glycol solutions of Ru(III) chloride and sodium selenite at different pH and Ru/Se mole ratios. The resulting catalysts were used for the electrochemical oxygen reduction reaction (ORR) in acidic solution. The electrochemical activity was highest for the supported catalyst synthesized at pH 8. Increasing the Se concentration of the catalyst up to 15 mol% increased the catalytic activity for the ORR; at this Se concentration, the activity of the catalyst was considerably higher than that observed for pure Ru catalyst synthesized at exactly the same conditions. The influence of the type of carbon support on the activity of the electrocatalyst was also investigated. Among the different supports, including carbon black (Vulcan XC-72R) (C1), and nanoporous carbons synthesized from resorcinol- (C2) and phloroglucinol-formaldehyde (C3) resins, the Ru_xSe_y catalyst supported on C3 exhibited highest activity for ORR.     

Nano-stuctured Pt-Fe/C as cathode catalyst in direct methanol fuel cell

Pt-Fe/C catalysts were prepared by a modified polyol synthesis method in an ethylene glycol (EG) solution, and then were heat-treated under H₂/Ar (10 vol.%) at moderate temperature (300 °C, Pt-Fe/C300) or high temperature (900 °C, Pt-Fe/C900). As comparison, Pt-Fe/C alloy catalyst was prepared by a two-step method (Pt-Fe/C900B). X-ray diffraction (XRD) and transmission electron microscopy (TEM) images show that particles size of the catalyst increases with the increase of treatment temperatures. Pt-Fe/C300 catalyst has a mean particle size of 2.8 nm (XRD), 3.6 nm (TEM) and some Pt-Fe alloy was partly formed in this sample. Pt-Fe/C900B catalyst has the biggest particle size of 6.2 nm (XRD) and the best Pt-Fe alloy form. Cyclicvoltammetry (CV) shows that Pt-Fe/C300 has larger electrochemical surface area than other Pt-Fe/C and the highest utilization ratio of 76% among these Pt-based catalysts. Rotating disk electrode (RDE) cathodic curves show that Pt-Fe/C300 has the highest oxygen reduction reaction (ORR) mass activity (MA) and specific activity (SA), as compared with Pt/C catalyst in 1.0 M HClO₄. However, Pt-Fe/C catalyst does not appears to be a more active catalyst than Pt/C for ORR in 1.0 M HClO₄ + 0.1 M CH₃OH. Pt-Fe/C300 exhibits higher ORR activity and better performance than other Pt-Fe/C or Pt/C catalysts when employed for cathode in direct methanol single cell test, the enhancement of the cell performance is logically attributed to its higher ORR activity, which is probably attributed to more Pt⁰ species existing and Fe ion corrosion from the catalyst.     

Catalytic activity of (CaO)1−x(ZnO)x solids for the N2O decomposition: relation with photoluminescence

(CaO)_1–x (ZnO) _x mixed oxides (x=0–1), heated at 1423 K under atmospheric conditions, were checked for their catalytic activity in the N₂O decomposition in the temperature range of 450–650°C. Although the catalytic activity was measured in the dark, it was found to be linearly related with the photoluminescence intensity of the catalysts.     

Preparation and characterization of PdxAgy/C electrocatalysts for ethanol electrooxidation reaction in alkaline media

Carbon-supported bimetallic PdAg catalysts with Pd/Ag atomic ratios varying from 4/1 to 1/2 were prepared by an impregnation–reduction method. The impregnated black mixture was treated in H₂/N₂ atmosphere at a temperature varying from 180 to 500 °C. The obtained Pd_xAg_y/C catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV) and chronoamperometry (CA). XRD results show that the lattice constant of Pd is dilated, suggesting the formation of PdAg alloy. The lattice constant of Pd for the Pd_xAg_y/C-500 (reduced at 500 °C by H₂) increases linearly and the average metal particle size decreases slightly from 6.8 to 5.1 nm with increasing Ag fractions from 20% to 67% in the PdAg composition. For Pd_xAg_y/C catalysts with a certain specific Pd/Ag atomic ratio, e.g., Pd₂Ag₁/C, the dilated lattice constant of Pd is independent of the reducing temperature, indicating the alloy degree for the Pd₂Ag₁/C-t catalysts is comparable. The average metal particle size for the Pd₂Ag₁/C-t catalysts increases from 3.4 to 5.2 nm with H₂ reduction temperature increasing from 180 to 500 °C. The potentiodynamic measurements on ethanol electrooxidation reaction (EOR) show that the catalytic activities for the Pd_xAg_y/C-t catalysts toward the EOR are improved by alloying Pd with Ag. At typical potential of a working fuel cell, e.g., −0.4 V vs. Hg/HgO, the EOR current density presents a volcano shape as a function of the Ag fractions in PdAg with the maximum occurs at the Pd/Ag atomic ratios between 2/1 and 3/1. The CA tests show that the Pd_xAg_y/C-500 catalysts perform high stability than that of Pd/C-500. The improved EOR activity for the Pd_xAg_y/C-t catalysts, compared with whether Pd/C or Ag/C catalyst, may possibly be attributed to the formation of PdAg alloy and the fitted particle size.     

Activity,selectivity, and methanol tolerance of novel carbon-supported Pt and Pt<Subscript>3</Subscript>Me (Me = Ni,Co) cathode catalysts

The activity, selectivity, and methanol tolerance of novel, carbon supported high-metal loading (40 wt.%) Pt/C and Pt₃Me/C (Me = Ni, Co) catalysts for the O₂ reduction reaction (ORR) were evaluated in model studies under defined mass transport and diffusion conditions, by rotating (ring) disk and by differential electrochemical mass spectrometry. The catalysts were synthesized by the organometallic route, via deposition of pre-formed Pt and Pt₃Me pre-cursors followed by their decomposition into metal nanoparticles. Characteristic properties such as particle sizes, particle composition and phase formation, and active surface area, were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For comparison, commercial Pt/C catalysts (20 and 40 wt.%, E-Tek, Somerset, NJ, USA) were investigated as well, allowing to evaluate Pt loading effects and, by comparison with the pre-cursor-based catalyst with their much smaller particle sizes (1.7 nm diameter), also particle size effects. Kinetic parameters for the ORR were evaluated; the ORR activities of the bimetallic catalysts and of the synthesized Pt/C catalyst were comparable and similar to that of the high-loading commercial Pt/C catalyst; at typical cathode operation potentials H₂O₂ formation is negligible for the synthesized catalysts. Due to their lower methanol oxidation activity the bimetallic catalysts show an improved methanol tolerance compared to the commercial Pt/C catalysts. The results indicate that the use of very small particle sizes is a possible way to achieve reasonably good ORR activities at an improved methanol tolerance at DMFC cathode relevant conditions.