Overview of the development of CO-tolerant anode electrocatalysts for proton-exchange membrane fuel cells

Poisoning of Pt anode electrocatalysts by carbon monoxide (CO) is deemed to be one of the most significant barriers to be overcome in the development of proton-exchange membrane fuel cell systems (PEMFCs). The use of CO-tolerant electrocatalysts serves as the most hopeful way to solve this problem. It is well established that Pt-based alloy systems of CO-tolerant electrocatalysts can substantially withstand the presence of CO in the fuel stream. Based on literature starting in 2000, a few efforts have still been conducted at developing a more CO-tolerant anode electrocatalyst than the traditional Pt/C or PtRu/C systems. This review introduces and discusses these efforts.Pt-based electrocatalysts, including PtSn/C, PtMo/C (atomic ratio = 5:1), PtRuMo/C (Mo = 10 wt.%), PtRu–H_xMoO₃/C and PtRu/(C nanotubes), appear to be poisoned by CO at the same, or a lower, level than traditional Pt/C or PtRu/C electrocatalysts. Platinum-free electrocatalysts, such as PdAu/C, have proven to be less strongly poisoned by CO than PtRu/C counterparts at temperatures of 60 °C.A greater tolerance to CO can be achieved by modifying the structure of the electrocatalyst. This involves the use of a composite or double-layer that is designed to make the CO react with one of the electrocatalyst in advance while the main hydrogen reacts at another layer with a traditional Pt/C electrocatalyst.     

High performance rare earth oxides LnOx (Ln = Sc,Y, La,Ce, Pr and Nd) modified Pt/C electrocatalysts for methanol electrooxidation

In this paper, the LnO_x (Ln = Sc, Y, La, Ce, Pr and Nd) modified Pt/C catalysts were prepared by wet precipitation and reduction method. The catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). TEM showed that the Pt-PrO_x nanoparticles were uniformly dispersed on carbon with an average particle size of 5.0 nm in the Pt₃-(PrO_x)₁/C catalyst. EDX showed that Pt and Pr were successfully loaded on the carbon support without obvious loss. XRD showed that all the Pt/C and LnO_x modified Pt/C electrocatalysts (except for the Pt₃-(ScO_x)₁/C electrocatalyst) displayed the typical character of Pt face centered cubic (fcc) phase, whereas the Pt₃-(ScO_x)₁/C electrocatalyst contained the diffraction pattern of Pt face centered cubic and Sc₂O₃ phase. LnO_x modified Pt/C electrocatalysts were compared with Pt/C in terms of the electrochemical activity and stability for methanol electrooxidation using cyclic voltammetry (CV) and chronoamperometry (CA) in 0.5 M H₂SO₄ + 0.5 M CH₃OH solutions. The results showed that all the LnO_x (except for NdO_x) modified the Pt/C electrocatalysts gave higher catalytic activity and stability than Pt/C. In particular, the Pt₃-(PrO_x)₁/C eloctrocatalyst was found to be superior than others. Under this respect, several Pt-PrO_x/C catalysts with different atomic ratio of Pt/Pr were also identically prepared and characterized. It was found by CV and CA that the Pt₃-(PrO_x)₁/C and Pt₁-(PrO_x)₁/C catalysts showed better catalytic activity and stability than the Pt₅-(PrO_x)₁/C, Pt₁-(PrO_x)₃/C and Pt/C catalysts. The Pt₃-(PrO_x)₁/C and Pt₁-(PrO_x)₁/C catalysts had high catalytic activity and good stability, which could be used as novel electrocatalysts for direct methanol fuel cell.     

Carbon supported Pt70Co30 electrocatalyst prepared by the formic acid method for the oxygen reduction reaction in polymer electrolyte fuel cells

Carbon supported Pt and Pt₇₀Co₃₀ electrocatalysts for the oxygen reduction reaction (ORR) were prepared by reduction with formic acid and tested in polymer electrolyte fuel cells. In the presence of Co an increase of the Pt particle size was observed in the as-prepared electrocatalyst and no evidence of Pt–Co alloy formation was detected from XRD measurements. Following thermal treatment (TT) at 900 °C of the Pt₇₀Co₃₀/C electrocatalyst, the presence in the XRD pattern of secondary Pt reflexions shifted to higher angles indicated partial alloy formation. The fuel cell performance with the as-prepared Pt₇₀Co₃₀/C electrocatalyst was inferior than that with Pt/C. The electrocatalytic activity increased with a TT of the binary electrocatalyst, and the value of the mass activity of the Pt₇₀Co₃₀/C electrocatalyst thermally treated at 900 °C was only slightly lower than that of Pt/C, notwithstanding the larger metal particle size, about five times that of pure Pt. On the other hand, there was a remarkable increase of the specific activity for the ORR of the Co-containing catalyst after TT at 900 °C with respect to Pt alone, which was ascribed to both the increased metal particle size and alloy formation. At high current densities the performance of PEMFC electrodes decreased with increasing Pt particle size.     

Char oxidation study of sugar cane bagasse,cotton stalk and Pakistani coal under 1% and 3% oxygen concentrations

Chars of Sugar cane bagasse (1 & 2), Cotton stalk and low rank Pakistani coal have been studied by TGA under low oxidative environments with O₂ concentrations of 1% and 3%. The maximum reactivity of the chars was found to be greater by a factor of 2 under 3% oxygen compared to 1% O₂ conditions. Overall conversion levels at 3% O₂ for Sugar cane bagasse-2 increased from 63% to 100%, Sugar cane bagasse-1; 54% to 97%, Cotton stalk; 45% to 100% and Pakistani coal; 63% to 90% in comparison to 1% O₂. The maximum average rate of weight loss was found in Region III compared to Region I and II supported by CO/CO₂ FTIR Chemigram analysis. On the other hand, % conversion was maximum in Region II under 1% and 3% O₂ concentration. Overall average rates of weight losses were dependant on O₂ concentration and temperature ranges, however for all the regions % conversion and average weight loss were twice in 3% compared to 1% O₂ concentration. Biomass chars were found to be more reactive than the coal studied here during each region of the oxidation process. Evaluated apparent energy of activations for biomass chars was found within range of 41.2–105.8 kJ mole^?1 under 1%, 46.9–125.6 kJ mole^?1 under 3% compared to coal; 70.3–183.9 kJ mole^?1 under 1% and 83.1–167.4 kJ mole^?1 in 3% O₂ concentration for order of reaction (n) varying between 0.5 ≤ n ≤ 2. From the tests carried under O₂ levels of 1% and 3%, it is possible to give the following sequence to the apparent activation energies under any of the fixed value of n, obtained for the biomasses and coal; Pakistani coal > Cotton stalk > Sugar cane bagasse-2 > Sugar cane bagasse-1.     

Performance of direct methanol fuel cell using carbon nanotube-supported Pt–Ru anode catalyst with controlled composition

The performance of a single-cell direct methanol fuel cell (DMFC) using carbon nanotube-supported Pt–Ru (Pt–Ru/CNT) as an anode catalyst has been investigated. In this study, the Pt–Ru/CNT electrocatalyst was successfully synthesized using a modified polyol approach with a controlled composition very close to 20 wt.%Pt–10 wt.%Ru, and the anode was prepared by coating Pt–Ru/CNT electrocatalyst on a wet-proof carbon cloth substrate with a metal loading of about 4 mg cm⁻². A commercial gas diffusion electrode (GDE) with a platinum black loading of 4 mg cm⁻² obtained from E-TEK was employed as the cathode. The membrane electrode assembly (MEA) was fabricated using Nafion^® 117 membrane and the single-cell DMFC was assembled with graphite endplates as current collectors. Experiments were carried out at moderate low temperatures using 1 M CH₃OH aqueous solution and pure oxygen as reactants. Excellent cell performance was observed. The tested cell significantly outperformed a comparison cell using a commercial anode coated with carbon-supported Pt–Ru (Pt–Ru/C) electrocatalyst of similar composition and loading. High conductivity of carbon nanotube, good catalyst morphology and suitable catalyst composition of the prepared Pt–Ru/CNT electrocatalyst are considered to be some of the key factors leading to enhanced cell performance.     

The preparation of Pt/C catalysts using various carbon materials for the cathode of PEMFC

Various 60 wt% Pt/C catalysts were prepared using LG precipitation method with Vulcan XC-72, Ketjen black EC 300J, and Ketjen black EC 600JD. The average Pt particle size decreases with increasing the surface area of carbon black supports. The 60 wt% Pt/C catalyst with 1.6 nm of Pt particle size and good dispersion could be prepared using Ultra high Surface Area Carbon (USAC, Ketjen black EC 600JD). In single cell test, the activity of electrode catalysts was enhanced with Pt surface area increase above 0.6 V but the correlation between activity of catalysts and Pt surface area was not clear below 0.6 V. Pt catalyst supported on USAC showed good oxygen reduction activity in all voltage regions and also showed stable voltage of 0.6 V at 900 mA cm⁻² without degradation over 180 h of durability test.     

Electrocatalysis of oxygen reduction on carbon supported Ru-based catalysts in a polymer electrolyte fuel cell

Powder of nanosized particles of Ru-based (Ru_x, Ru_xSe_y and Ru_xFe_ySe_z) clusters were prepared as catalysts for oxygen reduction in 0.5 M H₂SO₄ and for fuel cells prepared by pyrolysis in organic solvent. These electrocatalysts show a high uniformity of agglomerated nanometric particles. The reaction kinetics were studied using rotating disk electrodes and an enhanced catalytic activity for the powders containing selenium and iron was observed. The Ru-based electrocatalysts were used as the cathode in a single prototype PEM fuel cell, which was prepared by spray deposition of the catalyst on the surface of Nafion^® 117 membranes. The electrochemical performance of each single fuel cell was compared to that of a platinum/platinum conventional membrane electrode assembly (MEA), using hydrogen and oxygen feed streams. A maximum power density of 140 mW cm⁻², at 80 °C with 460 mA cm⁻² was obtained for the Ru_xFe_ySe_z catalysts; approximately 55% lower power density than that obtained with platinum.     

Development of ruthenium-based bimetallic electrocatalysts for oxygen reduction reaction

Ruthenium-based bimetallic electrocatalysts with non-noble metals such as Ti, Cr, Fe, Co and Pb were synthesized on a porous carbon support using a chelation process. Rotating ring disk electrode measurements indicated that RuFeN_x/C showed the catalytic activity and selectivity toward the four-electron reduction of oxygen to water comparable to those of the conventional Pt/C catalysts. The performance of the membrane-electrode assembly prepared with the RuFeN_x/C cathode catalyst was evaluated for 150 h of continuous operation.     

The influence of carbon support porosity on the activity of PtRu/Sibunit anode catalysts for methanol oxidation

In this paper we analyse the promises of homemade carbon materials of Sibunit family prepared through pyrolysis of natural gases on carbon black surfaces as supports for the anode catalysts of direct methanol fuel cells. Specific surface area (S_BET) of the support is varied in the wide range from 6 to 415 m² g⁻¹ and the implications on the electrocatalytic activity are scrutinized. Sibunit supported PtRu (1:1) catalysts are prepared via chemical route and the preparation conditions are adjusted in such a way that the particle size is constant within ±1 nm in order to separate the influence of support on the (i) catalyst preparation and (ii) fuel cell performance. Comparison of the metal surface area measured by gas phase CO chemisorption and electrochemical CO stripping indicates close to 100% utilisation of nanoparticle surfaces for catalysts supported on low (22–72 m² g⁻¹) surface area Sibunit carbons. Mass activity and specific activity of PtRu anode catalysts change dramatically with S_BET of the support, increasing with the decrease of the latter. 10%PtRu catalyst supported on Sibunit with specific surface area of 72 m² g⁻¹ shows mass specific activity exceeding that of commercial 20%PtRu/Vulcan XC-72 by nearly a factor of 3.     

Investigation of ethanol electrooxidation on a Pt–Ru–Ni/C catalyst for a direct ethanol fuel cell

This research is aimed to improve the utilization and activity of anodic alloy catalysts and thus to lower the contents of noble metals and the catalyst loading on anodes for ethanol electrooxidation. The DEFC anodic catalysts, Pt–Ru–Ni/C and Pt–Ru/C, were prepared by a chemical reduction method. Their performances were tested by using a glassy carbon working electrode and cyclic voltammetric curves, chronoamperometric curves and half cell measurement in a solution of 0.5 mol L⁻¹ CH₃CH₂OH and 0.5 mol L⁻¹ H₂SO₄. The composition of the Pt–Ru–Ni and Pt–Ru surface particles were determined by EDAX analysis. The particle size and lattice parameter of the catalysts were determined by means of X-ray diffraction (XRD). XRD analysis showed that both of the catalysts exhibited face centered cubic structures and had smaller lattice parameters than a Pt-alone catalyst. Their particle sizes were small, about 4.5 nm. No significant differences in the ethanol electrooxidation on both electrodes were found using cyclic voltammetry, especially regarding the onset potential for ethanol electrooxidation. The electrochemically active specific areas of the Pt–Ru–Ni/C and Pt–Ru/C catalysts were almost the same. But, the catalytic activity of the Pt–Ru–Ni/C catalyst was higher for ethanol electrooxidation than that of the Pt–Ru/C catalyst. Their tolerance to CO formed as one of the intermediates of ethanol electrooxidation, was better than that of the Pt–Ru/C catalyst.     

Methoxy methane (dimethyl ether) as an alternative fuel for direct fuel cells

The electrooxidation of methoxy methane (dimethyl ether) was studied at different Pt-based electrocatalysts in a standard three-electrode electrochemical cell. It was shown that alloying platinum with ruthenium or tin leads to shift the onset of the oxidation wave towards lower potentials. On the other hand, the maximum current density achieved was lower with a bimetallic catalyst compared to that obtained with a Pt catalyst. The direct oxidation of dimethoxy methane in a fuel cell was carried out with Pt/C, PtRu/C and PtSn/C catalysts. When Pt/C catalyst is used in the anode, it was shown that the pressure of the fuel and the temperature of the cell played important roles to enhance the fuel cell electrical performance. An increase of the pressure from 1 to 3 bar leads to multiply by two times the maximum achieved power density. An increase of the temperature from 90 to 110 °C has the same effect. When PtRu/C catalyst is used in the anode, it was shown that the electrical performance of the cell was only a little bit enhanced. The maximum power density only increased from 50 to 60 mW cm⁻² at 110 °C using a Pt/C anode and a Pt_0.8Ru_0.2/C anode, respectively. But, the maximum power density is achieved at lower current densities, i.e. higher cell voltages. The addition of ruthenium to platinum has other effect: it introduces a large potential drop at relatively low current densities. With the Pt_0.5Ru_0.5/C anode, it has not been possible to applied current densities higher than 20 mA cm⁻² under fuel cell operating conditions, whereas 250 and almost 400 mA cm⁻² were achieved with Pt_0.8Ru_0.2/C and Pt/C anodes. The Pt_0.9Sn_0.1/C anode leads to higher power densities at low current densities and to the same maximum power density as the Pt/C anode.     

Anodic Pt dissolution in concentrated trifluoromethanesulfonic acid

We investigated the anodic Pt dissolution in concentrated trifluoromethanesulfonic acid (TFMSA). The dependence of the Pt dissolution rate on the TFMSA concentration was first measured from the weight difference of a Pt-flag electrode before and after successive potential cycles. From this measurement, the Pt dissolution rate in 10 mol dm^?3 TFMSA is found to be over 40 times greater than those in 1 and 4 mol dm^?3 TFMSA. Next, the anodic Pt dissolution was assessed in 10 mol dm^?3 TFMSA by a potential step technique using a Pt dual microelectrode having generator and collector electrodes. The obtained result shows that the anodic Pt dissolution in 10 mol dm^?3 TFMSA occurs when the Pt generator electrode potential is stepped from 0.25 to 1.0–2.0 V vs. Ag/Ag₂SO₄. Furthermore, the absolute steady-state current-based coulomb charges obtained at the generator (|Q_G|) and collector (|Q_C|) reflect the anodic Pt dissolution and the reduction of the dissolved Pt, respectively. The magnitude of |Q_G| and |Q_C| linearly increase when the generator potential shifts from 1.0 to 2.0 V vs. Ag/Ag₂SO₄. The absolute ratio, |Q_C/Q_G|, also gradually increases according to the shift in the generator electrode potential. These results demonstrate that the anodic Pt dissolution in 10 mol dm^?3 TFMSA occurs at ≥1.0 V vs. Ag/Ag₂SO₄ and that the ratio of the anodic Pt dissolution per total reaction charges increases according to the positive shift of the Pt electrode potential.     

Formulating and optimizing a combustion pathways for oil shale and its semi-coke

Enhanced technologies from oil recovery to unconventional fuels - oil shale, oil sands and extra-heavy oil – have in common complex chemical reactions processes. This paper is about the formulation and optimization of the chemical mechanism especially in oil shale and semi-coke combustion. The Levenberg–Marquardt algorithm was used to minimize the error between estimated values and the thermogravimetric data for combustion mechanisms of 4-steps and 3-steps proposed for the oil shale and its semi-coke respectively. The kinetic parameters such as reaction order, pre-exponential factor, activation energy and stoichiometric coefficients that affect drying, pyrolysis, oxidation and decarbonation reactions were estimated with success. The values of activation energies were 54–67 kJ mol^?1 for oil shale drying, 62–65 kJ mol^?1 for pyrolysis reaction, up to 100 kJ mol^?1 for Fixed Carbon (FC) oxidation reaction, and 162–418 kJ mol^?1 for decarbonation reaction. Regarding to the semi-coke combustion, the activation energies were 33 kJ mol^?1 for drying reaction, 211 kJ mol^?1 for oxidation reaction and 291 kJ mol^?1 for decarbonation reaction. The chemical reactions suggest reaction order superior to one, except to the decarbonation reaction at 3 K min^?1. Considering the estimated parameters, as well as a heating rate at 3 K min^?1, an oil shale containing about 20 wt.% of organic matter and 34.6 wt.% of CaCO₃, the species mass fractions formed during combustion process were 3.4 wt.% of FC, 10.6 wt.% of Oil, 3.3 wt.% of HC and 1.8 wt.% of CO. The fraction of CO₂ formed accounts a total of 21.6 wt.%. For a semi-coke containing 3.4 wt.% of FC and 40.6 wt.% of CaCO₃, its combustion formed 2.1 wt.% of CO. The CO₂ fraction from oxidation and decarbonation reactions accounts 10.2 wt.%, considering that the stoichiometric mass coefficient γ = 0.75 in decarbonation reaction.     

Preparation of Ni/Pt catalysts supported on spinel (MgAl2O4) for methane reforming

MgAl₂O₄ was synthesized through hydrolysis of metallic alkoxides of Mg²⁺ and Al³⁺. The formed spinel precursor phase was calcined at temperatures between 600 and 1100 °C, for 4 h. The spinel was utilized as a Ni/Pt catalyst support. The Ni/MgAl₂O₄ catalysts (15% Ni, w/w) containing small amounts of Pt were tested for methane steam reforming. The solids were analyzed by X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂ and catalytic tests. The spinel phase was formed at temperatures above 700 °C. The addition of small amounts of Pt to Ni/MgAl₂O₄ promoted an increase in surface area. This probably caused the considerable increase in methane conversion.     

Preparation and evaluation of electrodeposited platinum nanoparticles on in situ carbon nanotubes grown carbon paper for proton exchange membrane fuel cells

Multi-walled carbon nanotubes (MWCNTs) based microporous layer on the non-woven carbon paper substrates was prepared by in situ growth in a chemical vapor deposition method. Pt with a loading of ～0.13 mg cm^?2 was electrodeposited at ?0.3, ?0.6, ?1.2, ?2.4, and ?3.6 V vs SCE in a chloroplatinic acid (60 g/L) and hydrochloric acid (10 g/L) bath using a potentiostat. Scanning electron micrographs showed that the Pt nanoparticles decorated on the MWCNTs/carbon paper are highly uniform, especially at an electrodeposition voltage of ?0.6 V vs SCE. Pt particles' size at various deposition potentials, as estimated by X-ray diffraction analysis is in nanosize range with an average diameter of 6 nm. Fuel cell performance of the Pt deposited in situ grown MWCNTs carbon paper was evaluated using Nafion-212 membrane at various operating conditions. The cathode with Pt deposition at ?0.6 V showed a power density of ～640 mW cm^?2 at 80 °C using H₂ and O₂ at 90% RH and 101 kPa.     

Oxygen reduction at carbon supported ruthenium–selenium catalysts: Selenium as promoter and stabilizer of catalytic activity

Carbon supported ruthenium-based catalysts (Ru/C) for the oxygen reduction in acid electrolytes were investigated. A treatment of Ru/C catalysts with selenious acid had a beneficial effect on catalytic activity but no influence on intrinsic kinetic properties, like Tafel slope and hydrogen peroxide generation. Reasons for the increased activity of RuSe_x/C catalysts are discussed. Potential step measurements suggest that at potentials around 0.8 V (NHE) a selenium or selenium-oxygen species protects the catalyst from formation of inactive RuO₂-films. This protective effect leads to an enhanced activity of RuSe_x/C compared to Ru/C. No evidence was found for a catalytically active stoichiometric selenium compound. The active phase may be described as a ruthenium suboxide RuO_x (x < 2) layer integrated in a RuSe_y phase or RuSe_yO_v (y < 2, v < 2) layer at the particle surface.     

Studies of performance degradation of a high temperature PEMFC based on H3PO4-doped PBI

In this paper, a 600 h life test of a high temperature PEMFC based on phosphoric acid (H₃PO₄)-doped polybenzimidazole (PBI) (H₃PO₄/PBI HT-PEMFC) at a current density of 714 mA cm⁻² (the beginning 510 h continuous test) and 300 mA cm⁻² (the last 90 h intermittent test) was carried out. After the life test, degradation of the MEA occurred. The H₂ crossover rate through the PBI membrane and the open circuit voltage (OCV) of the cell were tested with time. The results showed that, at the beginning of 510 h continuous test, the PBI membrane did not show much physical degradation, but during the last 90 h test there was a remarkable physical degradation which resulted in a higher H₂ crossover. The catalysts, PBI membranes and the membrane electrode assemblies (MEAs) before and after the life test were comprehensively examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). TEM results showed that the particle size of the Pt/C catalysts in the anode and cathode increased from 3.72 to 7.40 and 8.39 nm, respectively. SEM images of MEA in cross-section revealed that the PBI membrane became thin after the life test. EDS analysis implied the leaching of H₃PO₄ from the PBI membrane had occurred. Therefore, we conclude that physical degradation of PBI membrane, agglomeration of the electrocatalysts (both anode and cathode) and the leaching of H₃PO₄ from the PBI membrane were responsible for the performance degradation of the H₃PO₄/PBI HT-PEMFC.     

Alcohol reduction-mediated preparation of a nano-scale Pt/C electrocatalyst for the oxygen reduction reaction in PEM fuel cells

A nano-scale Pt/C electrocatalyst for oxygen reduction in PEM fuel cells was prepared by alcohol-mediated reduction of the PtCl₆^2? ion complex on the carbon Vulcan XC-72. The effects of various parameters, including the types of precursor and reducing agent and their concentrations, the initial solution pH and the reaction time, were explored. The preliminary results indicated that the electrocatalyst prepared using ammonium hexachloroplatinate ((NH₄)₂PtCl₆) as the Pt²⁺ source provided a similar catalytic efficiency as that prepared from hexachloroplatinic acid (H₂PtCl₆). The nano-scale Pt/C electrocatalyst prepared using methanol (CH₃OH) as a reducing agent provided the smallest sized platinum particles with a uniform distribution in the nanometer range, a good particle dispersion and a high Pt content compared with that prepared using ethanol (C₂H₅OH) or 2-propanol (C₃H₇OH). The electrocatalyst prepared in an acidic solution yielded smaller sized platinum particles and a higher Pt content than that prepared in a basic solution. In addition, the concentration of the reducing agent and reaction time slightly affected both the Pt particle size and the Pt yield of the obtained electrocatalyst. Under apparent optimal conditions, the nano-scale Pt/C electrocatalyst had an electrochemical surface area of ～39.7 m²/g, which was ～1.9-fold higher than that of the commercial one. The performance of the electrocatalyst was also tested in a single PEM fuel cell in a H₂/O₂ atmosphere where compared to a commercial electrode a lower activation loss but higher ohmic loss was observed.     

PtM/C (M = Sn,Ru, Pd,W) based anode direct ethanol–PEMFCs: Structural characteristics and cell performance

In the present work, the role of the structural characteristics of Pt-based catalysts on the single direct ethanol proton exchange membrane fuel cell (PEMFC) performance is examined. Several PtM/C (M = Sn, Ru, Pd, W) catalysts were characterized by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) and then evaluated as anode catalysts in single direct ethanol fuel cells. XRD spectra showed that Pt lattice parameter decreases with the addition of Ru or Pd and increases with the addition of Sn or W. According to the obtained experimental results, PtSn catalysts presented better electrocatalytic activity towards ethanol electro-oxidation. Based on these results, PtSn/C catalysts with different Pt/Sn atomic ratio were tested and compared. The maximum power density values obtained were correlated with the structural characteristics of the catalysts. A volcano type behaviour between the fuel cell maximum power density and the corresponding atomic percentage of Sn (Sn%) was observed. It was also observed that Sn% affects almost linearly the Pt_xSn_y catalysts’ lattice parameter.     

Catalytic effect of ZrCrNi alloy on hydriding properties of MgH2

MgH₂ nanocomposites with ZrCrNi alloy obtained by high energy ball-milling were studied as-milled and after several hydriding-deydriding cycles. The microstructure and morphology of the samples was characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD patterns show that no phase formation between MgH₂ and elements of the alloys takes place during milling and after cycling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure-composition isotherms of these composites were obtained in the pressure and temperature range of 0.1–15 bar and 200–300 °C respectively. The maximum reversible storage capacity was found to be 6.2 wt% at 300 °C. Absorption/desorption kinetics data at pressures of 0.1–5.0 bar and temperatures of 275 °C and 300 °C show that an activation process of about 20 cycles at 300 °C is necessary for stabilization of the kinetics and for achievement of the full hydrogen capacity. The thermodynamic parameters, i.e. enthalpy of formation and dissociation calculated using Van't Hoff plots, were found to be 73.53 kJ mol^?1 and 87.63 kJ mol^?1 respectively, in agreement with MgH₂ data reported in literature.