Carbon nanotube-supported Pt-Alloyed metal anode catalysts for methanol and ethanol oxidation

To improve the electrocatalytic activity of alcohol oxidation, functionalized carbon nanotubes (CNTs) decorated with various compositions of metal alloy catalyst nanoparticles (Pt_xM_y, where M = Au and Pd; x and y = 1–3) have been prepared via reduction. The CNTs were treated with an nitric acid solution to promote the oxygen-containing functional groups and further load the metal nanoparticles. X-ray diffraction (XRD) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to probe the formation of catalyst microstructure morphologies. A uniform dispersion of the spherical metal particles with diameters of 2–6 nm was acquired. The catalytic properties of the catalyst for oxidation were thoroughly studied by electrochemical methods that involved in the cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). To maximize the electrocatalytic performance and minimize the metal integration of the loaded CNTs, various compositions of active catalysts with large active surface areas are expected to increase the activity of the enhanced catalysts for alcohol oxidation. Most of the prepared bimetallic catalysts have better alcohol oxidation kinetics than commercial PtRu/C. Among the prepared catalysts, the PtAu/CNTs and PtPd/CNTs catalysts with high electrochemically active surface area (ECSA) show excellent activities for alcohol oxidation resulting in their low onset potentials, small charge transfer resistances and high peak current densities and I_f/I_b ratios, stability, and better tolerance to CO for alcohol oxidation. The integration of Pt and different metal species with different stoichiometric ratios in the CNTs support affects the electrochemical active surface area achieved in the catalytic oxidation reactions.     

Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

A parametric study was conducted on the performance of direct ethanol fuel cells. The membrane electrode assemblies employed were composed of a Nafion^® 117 membrane, a Pt/C cathode and a PtRu/C anode. The effect of cathode backpressure, cell temperature, ethanol solution flow rate, ethanol concentration, and oxygen flow rate were evaluated by measuring the cell voltage as a function of current density for each set of conditions. The effect of the anode diffusion media was also studied. It was found that the cell performance was enhanced by increasing the cell temperature and the cathode backpressure. On the contrary, the cell performance was virtually independent of oxygen and fuel solution flow rates. Performance variations were encountered only at very low flow rates. The effect of the ethanol concentration on the performance was as expected, mass transport loses observed at low concentrations and kinetic loses at high ethanol concentration due to fuel crossover. The open circuit voltage appeared to be independent of most operating parameters and was only significantly affected by the ethanol concentration. It was also established that the anode diffusion media had an important effect on the cell performance.     

Simplified model for the direct methanol fuel cell anode

A kinetic model for the anode of the direct methanol fuel cell (DMFC) is presented. The model is based on the generally accepted dual site mechanism of methanol oxidation, in aqueous solution, on well characterized Pt–Ru catalyst and it can predict the performance of the electrode as a function of cell temperature, anode potential and methanol concentration. In addition the model also generates data regarding the surface coverage of significant adsorbates involved in methanol oxidation on the dual site catalyst.     

Investigation of AuNi/C anode catalyst for direct methanol fuel cells

AuNi nanoparticles supported on the activated carbon (AuNi/C) are synthesized by the impregnation method in the ethyleneglycol system using NH₂NH₂·H₂O as a reducing agent. The alloying of Au and Ni and the removal of unalloyed Ni in the AuNi/C composition are achieved by heat and acid treatments in sequence. Research results reveal that the average size and alloying degree of the AuNi nanoparticles in the AuNi/C catalyst increase with the enhancement of the annealing temperature. However, the Ni content of the AuNi/C catalyst firstly goes up and then down with the rising of heat treatment temperature due to the AuNi system phase-separates. Moreover, the electrocatalytic activity normalized by the electrochemically active surface area of each AuNi/C catalyst is far better than that of the Au/C catalyst, because of the bifunctional mechanism and the electrocatalytic activity of the NiOOH. In particular, the AuNi/C catalyst annealed at 400 °C exhibits the most excellent activity, due to its small AuNi particles and proper alloying degree. Furthermore, its mass-specific electrochemical activity is higher than that of the Au/C catalyst, although the mean diameter of the AuNi nanoparticles in this catalyst is larger than that of the Au nanoparticles.     

Investigation of Ni2+-doped ceria nanorods as the anode catalysts for reduced-temperature solid oxide fuel cells

Tailoring the surface chemistry of oxides has been widely used to adjust their catalytic behavior in the energy conversion and storage devices. Herein, nanorods of Ni²⁺-doped ceria (Ce_1-xNi_xO_2-δ, x = 0, 0.05, 0.1, 0.15) are synthesized via a modified hydrothermal method, and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cells (SOFCs). X-Ray diffraction patterns of as-synthesized powders in air imply successful incorporation of Ni²⁺ into the fluorite lattice of ceria for x = 0.05 and 0.1, with a secondary phase of NiO observed for x = 0.15. Transmission electron microscopy (TEM) examination confirms a rod-like morphology with a diameter of 10–13 nm and a length of 55–105 nm. Exposure of these powders in H₂ at 600°C results in exsolution of some spherical Ni particles of 11 nm in diameter. Electrochemical measurements on both symmetrical anode fuel cells and functioning cathode-supported fuel cells show an order of the catalytic activity toward hydrogen oxidation - CeO_2-δ < Ce_0·95Ni_0·05O_2-δ < Ce_0·9Ni_0·1O_2-δ. The anode polarization resistances in 97% H₂ – 3% H₂O are 0.24, 0.31 and 0.37 Ω?cm² for Ce_0·9Ni_0·1O_2-δ, Ce_0·95Ni_0·05O_2-δ and CeO_2-δ at 600°C, respectively. Thin (La_0·9Sr_0.1) (Ga_0.8Mg_0.2)O_3-δ-electrolyte fuel cells with nanostructured SmBa_0.5Sr_0·5Co₂O_5+δ cathodes and Ce_0·9Ni_0·1O_2-δ anodes yield the highest power densities among the investigated series of anodes, e.g., 820 mW?cm^?2 in 97% H₂ – 3% H₂O and 598 mW?cm^?2 in 68% CH₃OH - 32% N₂. XPS analyses of reduced nanorods indicate that the highest catalytic activities of Ce_0·9Ni_0·1O_2-δ toward fuel oxidation reactions should be correlated to the presence of highly active Ni nanoparticles and increased surface active oxygen, as confirmed by substantially facilitated extraction of the lattice oxygen on the surface by H₂ in temperature-programmed reduction (TPR) measurements.     

Boosting the performance of alkaline direct ethanol fuel cell with low-Pd-loading nickel foam electrode via mixed acid-etching

Nickel foam has been widely used as an electrode supporting material for alkaline direct ethanol fuel cells (ADEFC). However, the smooth skeleton surface of pristine nickel foam results in low specific surface area, such that a high-load catalyst is required to deal with ethanol oxidation, which limits its application as a catalyst support. Therefore, efforts to enhance the roughness of skeleton surface and reduce the catalyst loading have been intensively made. One of the conventional approaches is hydrochloric acid (HCl) etching method, which can remove the inert layer but does not change the surface roughness. In this paper, a mixed acids treated nickel foam anode with low Pd loading (0.35 mg cm^?2) is prepared by simply soaking for three times for ADEFC performance testing. The peak power density reaches 30 mW cm^?2, which is double the performance of the HCl treated anode. The performance improvement is attributed to the micro-holes produced by mixed acids etching, which enhance the roughness of skeleton and improve electrochemical active surface area (ECSA) of the catalyst. This work opens a new platform for in-depth exploration on metal foam electrodes in fuel cells.     

Catalysts for direct ethanol fuel cells

By comparing the performance of fuel cells operating on some low molecular weight alcohols, it resulted that ethanol may replace methanol in a direct alcohol fuel cell. To improve the performance of a direct ethanol fuel cell (DEFC), it is of great importance to develop anode catalysts for ethanol electro-oxidation more active than platinum alone. This paper presents an overview of catalysts tested as anode and cathode materials for DEFCs, with particular attention on the relationship between the chemical and physical characteristics of the catalysts (catalyst composition, degree of alloying, and presence of oxides) and their activity for the ethanol oxidation reaction.     

Fabrication of three-dimensional copper nanodomes as anode materials for direct methanol fuel cells

In this study, we demonstrate a novel approach for fabricating copper nanodomes (Cu-NDs) by combining of soft lithography, nanosphere lithography, physical vapor deposition (PVD) and electrochemical deposition methods. The 3D nano structures were characterized using surface microscopic techniques. The methanol oxidation activity of the Cu-NDs anode was tested by electrochemical methods in 0.1 M KOH +1 M CH₃OH solution and the results were compared with that of bulk Cu as a reference point. The results showed that very well-structured, uniformly and homogeneously distributed Cu-NDs could be fabricated using these combined methods. The peak current density related to methanol oxidation reaction increased and charge transfer resistance reduced almost three times at the Cu-NDs electrode with respect to the bulk Cu. Also, the Cu-NDs electrode has good time stability and high tolerance to CO_ads poisoning. The enhanced activity of the nanostructures was related to good intrinsic activity of Cu for this reaction and their larger available electrochemical active sites.     

Nitrogen-doped carbon xerogel as high active oxygen reduction catalyst for direct methanol alkaline fuel cell

A novel catalyst based on nitrogen-doped carbon xerogel for oxygen reduction reaction (ORR) was prepared via a sol–gel process, following by the subsequent pyrolysis under ammonia atmosphere. The catalytic activity in alkaline media was optimized by tuning the metal (cobalt) ratio to the gel precursor. Sample with the optimum activity was characterized by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis and electrochemical measurements. Results show that the catalyst possesses an amorphous microstructure with nitrogen doped on the surface. The nitrogen-doped carbon xerogel displays comparable ORR activity and superior methanol tolerance than Pt/C in alkaline medium, demonstrating its promising application in direct methanol alkaline fuel cells as non-precious cathode catalyst.     

A liquid electrolyte alkaline direct 2-propanol fuel cell

Prototype alkaline direct 2-propanol fuel cells (AD2PFCs) using commercial Pt/C electrodes and hardware, and a liquid electrolyte, were constructed and compared to the 3-dimensional current-time-potential profiles for the 3-electrode oxidation of 2-propanol. A substantial current maximum occurs at low potentials and is attributed to a change in the mechanism of 2-propanol oxidation. This mechanism change influenced the stability of the AD2PFC; when the cell was polarized to a lower cell voltage limit of 0.5 V, stable and relatively high power densities are achieved. When the cell was polarized to a lower cell voltage limit of 0 V, unstable and only marginally higher power densities were observed. A maximum power density of 22.3 mW mg_Pt⁻¹ was achieved, and most of the cell polarization occurred at the cathode.     

Supported bimetallic nanoparticles as anode catalysts for direct methanol fuel cells: A review

Direct methanol fuel cell (DMFC) is an environment friendly energy source that transforms chemical energy of methanol oxidation into electrical energy. The Pt- and non-Pt based bimetallic nanoparticles (BMNPs) with electrocatalyst support materials are employed as anode electrocatalysts for methanol oxidation. These supported BMNPs have drawn prominent consideration due to their incredible physical and chemical properties. This article reviews the advancements in the field of supported BMNPs of varied structures, compositions and morphologies, using innumerable carbonaceous support materials such as carbon black, carbon nanotubes, carbon nanofibers, graphene, mesoporous carbon as well as non-carbonaceous supports like inorganic oxides, graphitic carbon nitride, metal nitrides, conducting polymers and hybrid support materials. The performance of electrocatalysts on the basis of support material, structure, composition and morphology of BMNPs, and pros and cons of various support materials have been discussed.     

Sn-modified carbon-supported Pt nanoparticles synthesized using spontaneous deposition as electrocatalysts for direct alcohol fuel cells

Sn-modified carbon-supported Pt nanoparticles (Sn(Pt)/C electrocatalysts) were prepared by spontaneous deposition. Sn species were deposited on Pt/C by immersion in 2.0 × 10⁻⁴ M SnCl₂ + 0.1 M HClO₄ for different times, which allowed achieving an adequate control of the coverage (θ). Cyclic voltammetry (CV) in 0.5 M H₂SO₄ was carried out to determine θ and to evaluate the Sn(Pt)/C performance. The activity towards the oxidation reactions of methanol (MOR) and ethanol (EOR) was analyzed using CV in 0.5 M H₂SO₄ + 1.0 M alcohol. A promotional effect for the MOR and the EOR after the partial coverage by the Sn species was shown, as indicated by the significant reduction of the overpotential and the higher oxidation currents in both cases. This activation was explained by the formation of hydroxylated species on the tin deposits, thus facilitating the removal of the adsorbed intermediates. The best performance was achieved for θ ≈ 0.3 in the case of the MOR and for θ ≈ 0.5 in the case of the EOR. The reaction pathway for both alcohols was analyzed according to the obtained kinetic parameters, which significantly depended on the coverage.     

Improved performance of Pd electrocatalyst supported on three-dimensional nickel foam for direct ethanol fuel cells

To improve the performance of direct ethanol fuel cells (DEFCs), a three-dimensional (3D), hierarchically structured Pd electrode has been successfully fabricated by directly electrodepositing Pd nanoparticles on the nickel foam (referred as Pd/Nickel foam electrode hereinafter). The electrochemical properties of the as-prepared electrode for ethanol oxidation have been investigated by cyclic voltammetry (CV). The results show that the oxidation peak current density of the Pd/Nickel foam electrode is 107.7 mA cm⁻², above 8 times than that of Pd film electrode at the same Pd loading (0.11 mg cm⁻²), and a 90 mV negative shift of the onset potential is found on the Pd/Nickel foam electrode compared with the Pd film electrode. Furthermore, the peak current density of the 500th cycle remains 98.1% of the maximum value for the Pd/Nickel foam electrode after a 500-cycle test, whereas it is only 14.2% for the Pd film. The improved electrocatalytic activity and excellent stability of the Pd/Nickel foam electrode make it a favorable platform for direct ethanol fuel cell applications.     

Bubble behaviors in direct methanol fuel cell anode with parallel design

In this paper, the Volume of Fluid (VOF) method for tracking the gas-liquid interface is employed to investigate the carbon dioxide (CO₂) behaviors inside the anode of a direct methanol fuel cell (DMFC). The CO₂ bubble emergence processes from the catalyst layer (CL) to the gas diffusion layer (GDL) and then to the flow channels are studied with two different strategies. In the first strategy, the CL and GDL are modeled as a uniform porous layer; in the second strategy, they are modeled as a well-ordered-path GDL and a uniform CL. The simulation results show that the second modeling strategy can better capture and match the fundamental phenomena of CO₂ bubble formation and evolvement observed from the experiments inside a DMFC anode.     

Systematic approach for modeling methanol mass transport on the anode side of direct methanol fuel cells

A systematic method for modeling direct methanol fuel cells, with a focus on the anode side of the system, is advanced for the purpose of quantifying the methanol crossover phenomenon and predicting the concentration of methanol in the anode catalyst layer of a direct methanol fuel cell. The model accounts for fundamental mass transfer phenomena at steady state, including convective transport in the anode flow channel, as well as diffusion and electro-osmotic drag transport across the polymer electrolyte membrane. Experimental measurements of methanol crossover current density are used to identify five modeling parameters according to a systematic parameter estimation methodology. A validation study shows that the model matches the experimental data well, and the usefulness of the model is illustrated through the analysis of effects such as the choice fuel flow rate in the anode flow channel and the presence of carbon-dioxide bubbles.     

Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization,steam reforming,dry reforming or partial oxidation conditions

There is increasing interest in developing solid oxide fuel cells (SOFC) for portable applications. For these devices it would be convenient to directly use a liquid fuel such as methanol and ethanol rather than hydrogen. The direct utilization of alcohol fuels in SOFC involves several processes, including the deposition of carbon, which can lead to irreversible deactivation of the fuel cell. Several publications have addressed the thermodynamic analysis of the reforming of methanol (MeOH) and ethanol (EtOH) in SOFC, but none have considered the direct utilization of these fuels. The equilibrium compositions, the carbon deposition boundaries, and the electromotive forces for the direct utilization and partial oxidation of methanol and ethanol in SOFC as a function of the fuel utilization are obtained in this study. In addition, the minimum amounts of H₂O, and CO₂ for direct and indirect reforming with MeOH and EtOH to avoid carbon formation are calculated.     

A new application for nickel foam in alkaline fuel cells

The use of nickel foam as an electrode substrate in alkaline fuel cells (AFCs) has been investigated for bi-polar cells incorporating an electrically conducting gas diffusion layer. This contribution focuses on the cathode, and draws comparisons between nickel foam and nickel mesh substrates. One of the principal electrocatalysts for the cathodic reduction of oxygen is silver, so an improvement in electrochemical performance was obtained by electroplating the nickel foam with silver. The electrodeposition process was optimised to maximise electrochemical performance with a minimum of silver deposited. Nickel foam, which is less expensive than the usual nickel mesh, appears to be a good substrate for AFC electrode fabrication, especially when incorporating a conducting gas diffusion layer. Silver deposited by electroplating onto the nickel foam was found to result in significantly enhanced electrochemical performance, with a reduction in both the Ohmic resistance of the electrode, as well as the oxygen reduction reaction charge transfer resistance.     

Pt-Ru electrocatalysts supported on ordered mesoporous carbon for direct methanol fuel cell

Pt-Ru electrocatalysts supported on ordered mesoporous carbon (CMK-3) were prepared by the formic acid method. Catalysts were characterized applying energy dispersive X-ray analyses (EDX) and X-ray diffraction (XRD). Methanol and carbon monoxide oxidation was studied electrochemically by cyclic voltammetry, and current-time curves were recorded in a methanol solution in order to establish the activity towards this reaction under potentiostatic conditions. The physicochemical and electrochemical properties of the Pt-Ru catalysts supported on CMK-3 carbon were compared with those of electrocatalysts supported on Vulcan XC-72 and commercial catalyst from E-TEK. Additionally, in order to complete this study, Pt electrocatalysts supported on CMK-3 and Vulcan XC-72 were prepared by the same method and were used as reference. Results showed that the Pt-Ru/CMK-3 catalyst presented the best electrocatalytic activity towards the CO oxidation and, therefore, good perspectives to its application in DMFC anodes. On the other hand, the activity of the Pt-Ru/CMK-3 catalyst towards methanol oxidation was higher than that of the commercial Pt-Ru/C (E-TEK) catalyst on all examined potentials, confirming the potential of the bimetallic catalysts supported on mesoporous carbons.     

Palladium selenides as active methanol tolerant cathode materials for direct methanol fuel cell

Palladium selenides, PdSe, Pd₃Se and PdSe₂ have been prepared by the hydrothermal method and investigated for their structural and electrocatalytic properties toward the oxygen reduction reaction (ORR) using SEM/TEM, XRD, cyclic and linear sweep voltammetries. The crystallites of PdSe and PdSe₂ are found to follow tetragonal and orthorhombic crystal structures, respectively. The PdSe electrode in 0.5 M H₂SO₄ exhibits significantly higher electrocatalytic activity than the Pd₃Se or PdSe₂ electrode under similar experimental conditions. Further, a change in the palladium/selenium ratio from unity in the catalyst results in low ORR activity.     

Balancing the electron conduction and mass transfer: Effect of nickel foam thickness on the performance of an alkaline direct ethanol fuel cell (ADEFC) with 3D porous anode

Nickel foam has been applying as the electrode support material for alkaline direct ethanol fuel cells, since its unique three-dimensional network structure helps efficiently use the catalyst to improve the cell performance. In this work, the effect of the thickness of nickel foam electrodes on cell performance is investigated. The experimental results show that the nickel foam thickness influences both the electron conduction and mass transfer, and the optimal thickness is a trade-off between them. Through XRD, SEM image, polarization curve test, EIS test and CV test, it is found that the nickel foam electrode with the thickness of 0.6 mm has better performance than that of 0.3 mm and 1.0 mm. The thinner the nickel foam, the better the conductivity. However, the corresponding three-dimensional space becomes narrower, which leads to partial agglomeration of the catalyst and hindrance of mass transfer. In addition, the influence of catalyst loading on the performance of 0.6 mm nickel foam electrode is explored. The maximum power density of 1.0 mg cm⁻² Pd loading reaches 56.3 mW cm⁻² at 60 °C, which is higher than that of 2.0 mg cm⁻² loading, indicating that the three-dimensional network structure of nickel foam can efficiently utilize the catalyst, and fully exert the catalytic function of the catalyst even at a lower catalyst loading. Moreover, the effects of operating temperature and ethanol concentration on cell performance are also studied. The cell performance increases with the increase of temperature, and it reaches the highest with 3 M ethanol.