Effect of W on PtSn/C catalysts for ethanol electrooxidation

Binary and ternary Pt-based catalysts were prepared by the Pechini–Adams modified method on carbon Vulcan XC-72, and different nominal compositions were characterized by TEM and XRD. XRD showed that the electrocatalysts consisted of the Pt displaced phase, suggesting the formation of a solid solution between the metals Pt/W and Pt/Sn. Electrochemical investigations on these different electrode materials were carried out as a function of the electrocatalyst composition, in acid medium (0.5 mol dm⁻³ H₂SO₄) and in the presence of ethanol. The results obtained at room temperature showed that the PtSnW/C catalyst display better catalytic activity for ethanol oxidation compared to PtW/C catalyst. The reaction products (acetaldehyde, acetic acid and carbon dioxide) were analyzed by HPLC and identified by in situ infrared reflectance spectroscopy. The latter technique also allowed identification of the intermediate and adsorbed species. The presence of linearly adsorbed CO and CO₂ indicated that the cleavage of the C–C bond in the ethanol substrate occurred during the oxidation process. At 90 °C, the Pt₈₅Sn₈W₇/C catalyst gave higher current and power performances as anode material in a direct ethanol fuel cell (DEFC).     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

Ethanol crossover and direct ethanol PEM fuel cell performance modeling and experimental validation

In the present work, a one-dimension, steady-state and single phase model is developed with the purpose of describing the mass transport within a PtRu/Nafion^®-115/Pt membrane-electrode assembly and the performance of a direct ethanol proton exchange membrane fuel cell (DE-PEMFC). The effect of the most important cell operating parameters on the ethanol crossover rate and the fuel cell performance is investigated. According to the results, in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover could pose serious problems to the DEFC operation. Moreover, it was pointed out that the ethanol crossover rate dependence on the ethanol feed concentration is an almost linear function presenting a maximum at about . A further increase of the ethanol feed concentration leads to a steep decrease of ethanol crossover rate. This behavior could be attributed to the membrane swelling which is responsible for the membrane volume fraction decrement. It was also found that by the aid of the same model the performance of a direct ethanol PEM fuel cell over three different anode catalysts can be predicted. A relatively good agreement between theory and experimental results related to both ethanol crossover rates and direct ethanol fuel cell performance was found.     

Development of anode catalysts for a direct ethanol fuel cell

Ethanol electrooxidation was investigated at platinum based electrodes: Pt, Pt—Sn, Pt—Re dispersed on a high surface area carbon powder. The atomic composition of the bimetallic catalyst was varied and the best results were obtained with an atomic ratio Pt:X close to 100:20. The electrocatalytic activity of Pt, PtSn and PtRe was compared using cyclic voltammetry and long-term electrolyses at constant potential. Under voltammetric conditions and in a single direct ethanol fuel cell, PtSn was the most active catalyst. During electrolysis ethanol was oxidized to acetaldehyde (AAL), acetic acid (AA) and carbon dioxide. On PtSn/C and PtRe/C, the ratio AA/AAL was found to be always lower than unity. Otherwise, PtSn electrocatalysts were the most selective towards the production of CO₂ compared to Pt and PtRe electrodes.     

Studies of some operating parameters and cyclic voltammetry for a direct ethanol fuel cell

A direct ethanol fuel cell (DEFC) of 5 cm² membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm⁻² of Pt–Ru/C (40%:20%) as the anode and 1 mg cm⁻² of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm⁻² and a power density of 10.3 mW cm⁻². The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm⁻² catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.     

Enhanced activity of PtSn/C anodic electrocatalyst prepared by formic acid reduction for direct ethanol fuel cells

The Pt₃Sn/C catalyst with high electrochemical activity was synthesized under optimizing preparation conditions. The surface of carbon support pretreated by strong acid contains many O-H and CO groups, which will increase the active sites of PtSn/C catalysts. The catalyst structure was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and temperature programmed reduction (TPR). The co-reduction of Pt⁴⁺ and Sn²⁺ ions causes Sn to enter Pt crystal lattice to form PtSn alloy whose surface, however, contains tin oxides with Sn⁴⁺ and Sn²⁺ valences, which can promote the ethanol oxidation. The crystallinity of PtSn decreases with the reduction of the atomic ratio of Pt:Sn. By prolonging the reaction time of formic acid, the forward anodic peak current of ethanol oxidation reaches 16.2 mA on the Pt₃Sn/C catalyst with 0.025 mg Pt loading.     

Thermodynamic analysis for a solid oxide fuel cell with direct internal reforming fueled by ethanol

In the present study, a detailed thermodynamic analysis is carried out to provide useful information for the operation of solid oxide fuel cells (SOFC) with direct internal reforming (DIR) fueled by ethanol. Equilibrium calculations are performed to find the ranges of inlet steam/ethanol (H₂O/EtOH) ratio where carbon formation is thermodynamically unfavorable in the temperature range of 500-1500 K. Two types of fuel cell electrolytes, i.e., oxygen-conducting, and hydrogen-conducting electrolytes, are considered. The key parameters determining the boundary of carbon formation are temperature, type of solid electrolyte and extent of the electrochemical reaction of hydrogen. The minimum H₂O/EtOH ratio for which the carbon formation is thermodynamically unfavored decreases with increasing temperature. The hydrogen-conducting electrolyte is found to be impractical for use, due to the tendency for carbon formation. With a higher extent of the electrochemical reaction of hydrogen, a higher value of the H₂O/EtOH ratio is required for the hydrogen-conducting electrolyte, whereas a smaller value is required for the oxygen-conducting electrolyte. This difference is due mainly to the water formed by the electrochemical reaction at the electrodes.     

NiCoFe/C cathode electrocatalysts for direct ethanol fuel cells

A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion^® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (V_oc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion^®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations.     

直接硼氢化物燃料电池阳极电催化剂研究进展

以硼氢化物作为燃料电池的燃料因其高的理论电动势和比能量而引起研究者的广泛关注。理论上,BH-4的电氧化反应为八电子反应,但实际上由于所用阳极电催化剂的不同,BH-4电氧化释放出的电子数也不同。如何抑制BH-4在阳极的水解反应,促进其八电子氧化反应一直是直接硼氢化物燃料电池研究中的核心问题。综述了近几年来国内外在直接硼氢化物燃料电池阳极电催化剂方面所取得的研究进展,并对这一领域中需要深入研究的主要问题进行了论述。     

Preparation of a PVA/HAP composite polymer membrane for a direct ethanol fuel cell (DEFC)

A novel PVA/Hydroxyapatite (HAP) composite polymer membrane was prepared by the direct blend process and solution casting method. The characteristic properties of the PVA/HAP composite polymer membranes were investigated using thermal gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-Raman spectroscopy and the AC impedance method. An alkaline direct ethanol fuel cell, consisting of an air cathode with MnO₂ carbon inks based on Ni-foam, an anode with PtRu black on Ni-foam, and the PVA/HAP composite polymer membrane, was assembled and investigated. It was found that the alkaline direct ethanol fuel cell comprising of a novel cheap PVA/HAP composite polymer membrane showed an improved electrochemical performance in ambient temperature and air. As a result, the maximum power density of the alkaline DEFC, using a PtRu anode based on Ni-foam (10.74 mW cm⁻²), is higher than that of DEFC using an E-TEK PtRu anode based on carbon (7.56 mW cm⁻²) in an 8M KOH + 2M C₂H₅OH solution at ambient temperature and air. These PVA/HAP composite polymer membranes are a potential candidate for alkaline DEFC applications.     

Structure and chemical composition of supported Pt-Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnO_x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO_x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO_x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO₂ nanoparticles, characterized with the nominal 0.264 nm spacing of SnO₂ (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO_x catalyst. PtSnO_x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO_x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Nano-composite of PtRu alloy electrocatalyst and electronically conducting polymer for use as the anode in a direct methanol fuel cell

Nano-composites comprised of PtRu alloy nanoparticles and an electronically conducting polymer for the anode electrode in direct methanol fuel cell (DMFC) were prepared. Two conducting polymers of poly(N-vinyl carbazole) and poly(9-(4-vinyl-phenyl)carbazole) were used for the nano-composite electrodes. Structural analyses were carried out using Fourier transform nuclear magnetic resonance spectroscopy, AC impedance spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Electrocatalytic activities were investigated by voltammetry and chronoamperometry in a 2 M CH₃OH/0.5 M H₂SO₄ solution and the data compared with a carbon-supported PtRu electrode. XRD patterns indicated good alloy formation and nano-composite formation was confirmed by TEM. Electrochemical measurements and DMFC unit-cell tests indicate that the nano-composites could be useful in a DMFC, but its performance would be slightly lower than that of a carbon-supported electrode. The interfacial property between the PtRu-polymer nano-composite anode and the polymer electrolyte was good, as evidenced by scanning electron microscopy. For better performance in a DMFC, a higher electric conductivity of the polymer and a lower catalyst loss are needed in nano-composite electrodes.     

High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell

This research aims to enhance the activity of Pt catalysts, thus to lower the loading of Pt metal in fuel cell. Highly dispersed platinum supported on single-walled carbon nanotubes (SWNTs) as catalyst was prepared by ion exchange method. The homemade Pt/SWNTs underwent a repetition of ion exchange and reduction process in order to achieve an increase of the metal loading. For comparison, the similar loading of Pt catalyst supported on carbon nanotubes was prepared by borohydride reduction method. The catalysts were characterized by using energy dispersive analysis of X-ray (EDAX), transmission electron micrograph (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectrum (XPS). Compared with the Pt/SWNTs catalyst prepared by borohydride method, higher Pt utilization was achieved on the SWNTs by ion exchange method. Furthermore, in comparison to the E-TEK 20 wt.% Pt/C catalyst with the support of carbon black, the results from electrochemical measurement indicated that the Pt/SWNTs prepared by ion exchange method displayed a higher catalytic activity for methanol oxidation and higher Pt utilization, while no significant increasing in the catalytic activity of the Pt/SWNTs catalyst obtained by borohydride method.     

Electrodeposited PtCo and PtMn electrocatalysts for methanol and ethanol electrooxidation of direct alcohol fuel cells

PtCo and PtMn electrocatalyst particles were successfully synthesized on Ti substrate by the electrodepostion method. PtCo particles deposited are star-shaped particles with size of 100–200 nm and very porous with many slices of 10 nm. On the other hand, PtMn particles are spherical and have no obvious conglomeration, and the particle is in the range of 100–200 nm. The results reveal that the effect of the incorporation of Co and Mn on the electrochemical active surface area of Pt nanoaprticles is very small. However, incorporation of trace Co and Mn in Pt (e.g., Pt₁₀₀₀Co and Pt₁₀₀₀Mn) has dramatic effect on the electrochemical oxidation reaction of alcohol. The mass specific peak current for the methanol oxidation in alkaline media is 49 mA cm⁻² and 39 mA cm⁻² on Pt₁₀₀₀₀Mn and Pt₁₀₀₀Co, which is three and two times higher, respectively, than that on pure Pt electrocatalyst nanoparticles. PtMn and PtCo electrocatalysts also show significant enhanced stability for methanol oxidation. However, the electrocatalytic enhancement of Co or Mn to Pt is relatively small for the electrooxidation reactions of ethanol in alkaline media.     

Carbon supported Pd-Co-Mo alloy as an alternative to Pt for oxygen reduction in direct ethanol fuel cells

Carbon black (CDX975) supported Pd and Pd-Co-Mo alloy nanoparticles are prepared by the reduction of metal precursors with hydrazine in reverse microemulsion of water/Triton-X-100/propanol-2/cyclohexane. The as-synthesized Pd-Co-Mo/CDX975 is heat treated at 973, 1073 and 1173 K to promote alloy formation. The prepared materials are characterized by powder XRD and EDX. Face-centred cubic structure of Pd is evident from XRD. The chemical composition of the respective elements in the catalysts is evaluated from the EDX analysis and observed that it is in good agreement with initial metal precursor concentrations. Oxygen reduction measurements performed by linear sweep voltammetry indicate the good catalytic activity of Pd-Co-Mo alloys compared to Pd. This is due to the suppression of (hydr)oxy species on Pd surface by the presence of alloying elements, Co and Mo. Among the investigated catalysts, heat-treated Pd-Co-Mo/CDX975 at 973 K exhibited good ORR activity compared to the catalysts heat treated at 1073 and 1173 K. This is due to the small crystallite size and high surface area. Rotating disk electrode (RDE) measurements indicated the comparable ORR activity of heat-treated Pd-Co-Mo/CDX975 at 973 K with that of commercial Pt/C. Kinetic analysis reveals that the ORR on Pd-Co-Mo/CDX975 follows the four-electron pathway leading to water. Moreover, Pd-Co-Mo/CDX975 exhibited substantially higher ethanol tolerance during the ORR than Pt/C. Good dispersion of metallic nanoparticles on the carbon support is observed from HRTEM images. Single-cell direct ethanol fuel cell tests indicated the comparable performance of Pd-Co-Mo/CDX975 with that of commercial Pt/C. Stability under DEFC operating conditions for 50 h indicated the good stability of Pd-Co-Mo/CDX975 compared with that of Pt/C.     

Synthesis of sulfated ZrO2/MWCNT composites as new supports of Pt catalysts for direct methanol fuel cell application

Sulfated zirconia supported on multi-walled carbon nanotubes as new supports of Pt catalyst (Pt–S-ZrO₂/MWCNT) was synthesized with aims to enhance electron and proton conductivity and also catalytic activity of Pt electrocatalyst in terms of larger concentrations of ionizable OH groups on surfaces. Fourier transform infrared spectroscopy analysis shows that the sample surfaces were modified with sulfate. Transmission electron microscopy results show that the Pt and sulfated ZrO₂ particles dispersed relatively uniformly on the surface of the multi-walled carbon nanotube. X-ray diffraction shows that S-ZrO₂ and Pt coexist in the Pt–S-ZrO₂/MWCNT composites and S-ZrO₂ has no effect on the crystalline lattice of Pt. Pt–S-ZrO₂/MWCNT catalyst was evaluated in terms of the electrochemical activity for methanol electro-oxidation using cyclic voltammetry, steady-state polarization experiments and electrochemical impedance spectroscopy technique at room temperature. Pt–S-ZrO₂/MWCNT catalyst show higher catalytic activity for methanol electro-oxidation compared with Pt catalyst on non-sulfated ZrO₂/MWCNT support and commercial Pt/C (E-TEK).     

Carbon supported PtBi catalysts for direct formic acid fuel cells

Carbon supported PtBi bimetallic catalysts (PtBi/C) prepared by depositing Bi on a commercial Pt/C catalyst and by codeposition of Pt and Bi have been compared for formic acid oxidation in a multi-anode direct formic acid fuel cell. Both types of catalyst gave much higher cell performances than the Pt/C, with only low amounts of Bi (Pt to Bi mole ratios of 11:1 and 14:1, respectively) required for optimum performance. The high Pt to Bi ratio for the best codeposited catalyst indicates that the Bi was concentrated at the surface, and this is consistent with X-ray diffraction and X-ray photoelectron spectroscopy results. However, cyclic voltammetry revealed a strong electronic effect that is inconsistent with surface decoration. The effects of the Bi have been attributed to selective blocking of sites at which CO is formed.     

Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs)

Direct ethanol fuel cells (DEFCs) belong to the family of proton exchange membrane fuel cells (PEMFCs), in which ethanol is directly used as the fuel. In the present work, the main aspects related to DEFCs such as electrocatalysts, membrane electrode assembly (MEA) preparation and their corresponding effects on the total cell performance are summarized and discussed. Furthermore, the issues about the disadvantages such as ethanol crossover and the electrolyte membrane's thermal and mechanical stability, as well as the challenges for DEFC's rapid development and commercialization are addressed.     

Electrocatalysis for the direct alcohol fuel cell

The basic principles of a direct alcohol fuel cell are first presented. Low temperature fuel cells (working between ambient temperature and 80–120 °C) need improved catalysts to reach performance levels sufficient for practical applications, particularly for the electric vehicle and for portable electronic devices. This is the case of proton exchange membrane fuel cells (PEMFC) and of direct alcohol fuel cells (DAFC) for which the kinetics of the electrochemical reactions involved (oxidation of reformate hydrogen containing some traces of carbon monoxide, oxidation of alcohols, reduction of oxygen) is rather slow. Basic understanding of electrocatalysis is then examined, showing how to increase the reaction rate both by the nature and the structure of the catalytic electrode and by the electrode potential. Finally the most used Pt-based electrocatalysts to activate the electrode reactions occurring in a direct ethanol fuel cell (DEFC) are discussed on the basis of electrochemical, spectro-electrochemical and fuel cell experiments.