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.     

An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane

An alkaline direct ethylene glycol fuel cell (DEGFC) with an alkali-doped polybenzimidazole membrane (APM) is developed and tested. It is demonstrated that the use of APMs enables the present fuel cell to operate at high temperatures. The fuel cell results in the peak power densities of 80 mW cm⁻² at 60 °C and 112 mW cm⁻² at 90 °C, respectively. The power output at 60 °C is found to be 67% higher than that by DEGFCs with proton exchange membranes, which is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline media.     

Electro-oxidation of ethylene glycol on PtCo metal synergy for direct ethylene glycol fuel cells: Reduced graphene oxide imparting a notable surface of action

Slow electro-oxidation reaction and low power output are two major limiting factors in successful commercialization of fuel cell technology. An efficient and stable electro-catalyst with effectual metal combination supported on a durable matrix may provide a viable solution to overcome these issues. The direct ethylene glycol fuel cell consisting of bimetallic anode catalysts are expected to lead out the high-power output issues. In the present paper, we emphasized on the synthesis of a high performing CO poisoning resistant Pt based binary anode catalysts for the electro-oxidation of ethylene glycol (EG) using a chemical reduction route. The electrocatalysts consists of PtCo alloy nanoparticles with different composition of Pt and Co, supported on reduced graphene oxide (rGO). Physical characterizations revealed the formation of bi-metallic catalysts within the size ranges from 2 nm to 3 nm. Electrochemical analysis revealed that Pt_xCo_y/rGO electrocatalyst with x: y molar ratio of 1:9 imparts the highest peak current and power density as compared to commercially available Pt/C and PtCo/C anode catalysts for ethylene glycol electro-oxidation. The power density (81.1 mW/cm²) obtained using Pt_xCo_y/rGO with x:y molar ratio of 1:9 metal catalyst in DEGFC is more than other synthesized catalysts at an operating temperature of 100 °C and the operating pressure of 1 bar with 2 M ethylene glycol as anode fuel and anode and cathode platinum metal loading of 2 mg/cm².     

Facile construction of pompon-like PtAg alloy catalysts for enhanced ethylene glycol electrooxidation

In the drive toward energy crisis, direct ethylene glycol cells have received a great deal of attention, but its commercial development has been greatly restricted due to the lack of cost-effective anodic catalysts. Therefore, it is essential to design a suitable catalyst with excellent performance. To this end, we herein report a facile one-pot tactic for successfully synthesizing the pompon-like PtAg nanocrystals (NCs) with marvelous electrocatalytic activity and amazing long-term stability. More specifically, the results have proved that the pompon-like PtAg possessed the superior mass activity of 5042.9 mA mg^?1 towards ethylene glycol oxidation reaction (EGOR) under the alkaline condition, 5.4-fold enhancements than that of commercial Pt/C. Upon the basis of our investigation, we attributed the better catalytic activity to abundant available surface active sites and synergistic effect, as well as the brushy hairs on the surface of PtAg NCs. There is no doubt that the as-obtained nanocatalysts show a great prospect for serving as excellent anode catalyst in fuel cells and beyond.     

Performance of a direct ethylene glycol fuel cell with an anion-exchange membrane

This paper reports on the development and performance test of an alkaline direct ethylene glycol fuel cell. The fuel cell consists of an anion-exchange membrane with non-platinum electrocatalysts at both the anode and cathode. It is demonstrated that this type of fuel cell with relatively cheap membranes and catalysts can result in a maximum power density of 67 mW cm⁻² at 60 °C, which represents the highest performance that has so far been reported in the open literature. The high performance is mainly attributed to the increased kinetics of both the ethylene glycol oxidation reaction and oxygen reduction reaction rendered by the alkaline medium with the anion-exchange membrane.     

Highly efficient carbon hybrid supported catalysts using nano-architecture as anode catalysts for direct methanol fuel cells

PtCo based nanoparticles in alloy structure were synthesized using the microwave-assisted reduction method. These nanoparticles were deposited on different carbon supporting materials. Here, these supporting materials such as rGO (reduced graphene oxide), rGO-VC (vulcan carbon) and AC-VC (activated carbon-vulcan carbon) were used and the methanol oxidation reaction (MOR) activity of single carbon support and hybrid carbon support material in the presence of PtCo nanoparticles were investigated at the same molar concentration. The average particle size of the PtCo nanoparticles detected in the TEM analysis was found to be 3.55 ± 0.64 nm. The MOR activity of the PtCo@rGO, PtCo@rGO-VC and PtCo@AC-VC catalysts was determined, where the anodic peak current of PtCo@AC-VC was determined as 73 mA/cm². It has been observed that PtCo nanoparticles with carbon hybrid support structures are more advantageous than single support structures due to the synergistic effect between carbon support structures and providing a larger surface area. Compared to previous studies, the MOR activity of PtCo@AC-VC is quite high. It can be stated that PtCo@AC-VC has comparable catalytic activity compared to the commercial available anode catalyst.     

Agni-catalyzed anode for direct borohydride fuel cells

Ag and AgNi powders were comparatively tested as anodic catalysts for direct electrochemical oxidation of borohydride. Discharge experiments demonstrated for the first time that both Ag and AgNi electrode can catalyze the electrooxidation of borohydride, delivering a high capacity of >7e$>7\mathrm{e}$ oxidation for a borohydride ion. In comparison, AgNi-catalyzed borohydride fuel cells exhibited a higher discharge voltage and capacity, possibly due to a combined action of the electrocatalytic activity of Ni component for borohydride electrooxidation and the depression of borohydride hydrolysis by Ag atoms.     

High-performance anode for solid acid fuel cells prepared by mixing carbon substances with anode catalysts

Optimization of Pt-based electrode structure is a key to enhance power generation performance of fuel cells and to reduce the Pt loading. This paper presents a new methodology for anode fabrication for solid acid fuel cells (SAFCs) operating at ca. 200 °C. Our membrane electrode assembly for SAFCs consisted of a CsH₂PO₄/SiP₂O₇ composite electrolyte and Pt-based electrodes. To obtain the anode, a commercial Pt/C catalyst and carbon substance, such as carbon black and carbon nanofiber, were mixed. The composite anode with Pt loading = 0.5 mg cm⁻² demonstrated superior current-voltage characteristics to a benchmark Pt/C anode with Pt loading = 1 mg cm⁻². We consider that the mixing of Pt/C catalyst and carbon substrate facilitated H₂ mass transfer and increased the number of active sites.     

Perovskite-type oxides La1−xSrxMnO3 for cathode catalysts in direct ethylene glycol alkaline fuel cells

Carbon-supported La_1−xSr_xMnO₃ (LSM/C) was prepared by reversible homogeneous precipitation method, and its catalytic activities for oxygen reduction under the existence of ethylene glycol (EG) were investigated by using rotating disk electrode. LSM/C exhibited the high activity for oxygen reduction irrespective with the presence of EG, indicating that EG is not oxidized by LSM/C at the cathode side in the present system. Consequently, LSM/C can serve as a cathode catalyst in alkaline direct alcohol fuel cells with no crossover problem. Performance test for fuel cells operation also supported these results and showed cathodic polarization curves were not affected by the concentration of EG supplied to anode even at 5 mol dm⁻³.     

Performance of a hybrid direct ethylene glycol fuel cell

In this work, a hybrid fuel cell is developed and tested, which is composed of an alkaline anode, an acid cathode, and a cation exchange membrane. In this fuel cell, ethylene glycol and hydrogen peroxide serve as fuel and oxidant, respectively. Theoretically, this fuel cell exhibits a theoretical voltage reaching 2.47 V, whereas it is experimentally demonstrated that the hybrid fuel cell delivers an open‐circuit voltage of 1.41 V at 60°C. More impressively, this fuel cell yields a peak power density of 80.9 mW cm^?2 (115.3 mW cm^?2 at 80°C). Comparing to an open‐circuit voltage of 0.86 V and a peak power density of 67 mW cm^?2 previously achieved by a direct ethylene glycol fuel cell operating with oxygen, this hybrid direct ethylene glycol fuel cell boosts the open‐circuit voltage by 62.1% and the peak power density by 20.8%. This significant improvement is mainly attributed not only to the high‐voltage output of this hybrid system design but also to the faster kinetics rendered by the reduction reaction of hydrogen peroxide.     

Effect of methanol, ethylene glycol and their oxidation by-products on the activity of Pt-based oxygen-reduction catalysts

Direct-oxidation fuel cells (DOFC) are promising electrochemical devices for various applications. In addition to methanol (MeOH), alternative fuels are being tested in a search for lower toxicity, safer handling, and higher energy density. Ethylene glycol (EG) was employed as one of such fuels. However, DOFCs face several problems, such as fuel crossover through the membrane during its operation. This not only lowers the cell potential but also poisons the catalyst for the oxygen-reduction reaction (ORR). Experiments were performed on the poisoning of Pt and Pt-alloy ORR catalysts (both commercial and homemade, by electroless deposition), by fuels and their oxidation by-products. At 25 °C, methanol poisoning was found to be reversible and the catalytic activity measured afterwards in a fuel-free solution and the electrochemical surface area (ECSA) were enhanced. The effect of poisoning by methanol and ethylene glycol and their oxidation intermediates is reported here for the first time. The severity of poisoning was found to be MeOH ? formaldehyde < formic acid. In solutions of EG and its oxidation by-products, the poisoning order was EG ≤ glycolic acid < oxalic acid, the poisoning of all three being more severe than that of methanol. The catalysts most resistant both to MeOH and EG poisoning were commercial acid-treated PtCo and homemade PtCoSn. The reasons for the enhanced tolerance were investigated and PtCoSn was found to be the less active both in the methanol and ethylene glycol oxidation processes.     

Eco-friendly and facile synthesis of novel bayberry-like PtRu alloy as efficient catalysts for ethylene glycol electrooxidation

Direct fuel cells have attracted an increasing notice over last decade, while its large-scale commercial development is also seriously impeded by the lack of cost-efficient electrocatalysts. The shape-controlled syntheses of binary Pt-based nanocrystals bounded with abundant surface active areas and tunable atomic ratio have been of vital importance in the fabrication and modification of outstandingly excellent electrocatalysts. Therefore, embodying the morphology advantages and composition effects are significant for synthesizing the cost-efficient electrocatalysts. In view of this, we herein report our efforts for demonstrating an eco-friendly approach to successfully synthesize a novel type of bayberry-like PtRu nanocatalyst with different compositions. Different from some other reported PtRu binary nanostructures, such as-prepared bayberry-like PtRu nanocrystals with rough surface can meet the requirement of both high mass activity and long-term stability, which shed light for the commercial development of direct fuel cells.     

NaBH4-assisted ethylene glycol reduction for preparation of carbon-supported Pt catalyst for methanol electro-oxidation

A carbon-supported Pt catalyst (40 wt.% loading) is prepared by a modified ethylene glycol reduction method (Pt–EG-complex). In this procedure, a complex produced by reacting ethylene glycol with sodium borohydride (NaBH₄), serves as a reducing agent for the Pt precursor and as a stabilizer for preventing the growth of Pt particles. For purposes of comparison, two types of carbon-supported Pt catalyst (40 wt.% loading) are also prepared by a NaBH₄ reduction method, in which the Pt precursor is reduced in a ethylene glycol solution (Pt–EG–NaBH₄) and in de-ionized water (Pt–H₂O–NaBH₄). Analysis by X-ray diffraction and transmission electron microscopy reveal that the Pt–EG-complex catalyst is comprised of highly-dispersed Pt nanoparticles with a uniform size (2.9–3.1 nm) on the carbon support, while large Pt particles are observed in the Pt–EG–NaBH₄ (3.3–3.6 nm) and Pt–H₂O–NaBH₄ (5.7–6.2 nm) catalysts. The Pt–EG-complex catalyst has the highest electrochemical surface area and shows the highest catalytic performance for methanol electro-oxidation.     

CeO2 nanoparticles improved Pt-based catalysts for direct alcohol fuel cells

We developed an ultrasonic co-deposition technique to enhance the activity of Pt/C catalyst (and Pt/CNT, PtRu/C catalysts) for direct alcohol fuel cells (DAFCs) by CeO₂ nanoparticles. The composite catalyst architecture is obtained by an ultrasonically mixing commercial Pt/C catalyst and CeO₂ nanoparticles. Both Pt and CeO₂ are dispersed uniformly in the electrodes resulting in a great deal of CeO₂–Pt–C triple junction interfaces. Unlike traditional preparation of metal oxide supported Pt catalysts, CeO₂ will not cut the connection between Pt and C in this composite catalyst structure. Electrochemical measurements confirm that CeO₂ can improve almost all Pt based catalysts (Pt/C, Pt/CNT, and PtRu/C) for almost all small molecular alcohols (methanol, ethanol, ethylene glycol, and glycerol) electro-oxidation. EIS measurement shows that reaction resistance between Pt and alcohols is decreased much by adding small CeO₂ nanoparticles. Besides, these composite catalysts have high stability. It proves CeO₂ a very promising co-catalyst of Pt based catalysts for DAFCs.     

Nanostructured Pt and Pt-Sn catalysts supported on oxidized carbon nanotubes for ethanol and ethylene glycol electro-oxidation

Pt and Pt-Sn catalysts supported on oxidized carbon nanotubes were prepared by multiple potentiostatic pulses and tested for ethanol and ethylene glycol electro-oxidation in sulfuric acid. The composed nanostructured materials were characterized via SEM, TEM, EDX and XRD analysis. Small metal nanoparticles (4-6 nm) forming 3-D nanostructured agglomerates (25-100 nm) distributed over the carbon substrate were formed. XRD results showed that the bimetallic electrocatalysts consisted of a Pt single-phase material, suggesting the formation of solid solutions over the entire composition range. The tin content in the alloys was between 10 and 40 at. %.Cyclic voltammetry and chronoamperometry measurements at room temperature showed that at potentials below 0.5 V, the bimetallic catalyst with 40 at. % Sn exhibited the highest activity for ethanol and ethylene glycol oxidation, whereas at potentials above 0.5 V, the alloy with 25 at. % Sn displayed better performance. This behavior can be explained by the synergistic effect between the facilitation of alcohol oxidation via oxygen-containing species adsorbed on Sn atoms, the alteration of the electronic structure of Pt atoms that weakens CO and intermediates adsorption, and the adequate Pt ensembles size. Besides, the increment of the lattice parameter and the presence of grain boundaries can enhance the adsorption of the alcohols and favor the splitting of the C-C bond.     

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO₂ gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high F_Ar. Polarization performance of the chars under high F_Ar was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO₂. Thermal pretreated AC displays the best durability performance for its stable and moderate CO₂ gasification rate. Additionally, the coal ash does not affect the cell performance significantly.     

Fabrication and characterization of Ni modified TiO2 electrode as anode material for direct methanol fuel cell

Highly ordered porous titanium dioxide nanotube (TiO₂-NT) surfaces were prepared with anodization method to obtain a larger specific surface area that plays a very important role in methanol oxidation. In this regard, optimum conditions such as various anodization voltages and times were determined. The largest surface area of TiO₂ occurred at anodization voltage and time of 60 V and 2 h, respectively. After obtaining the high specific surface area, very small amounts of Nickel (Ni) nanoparticles were deposited on TiO₂-NT surface and their behaviors of methanol electro-oxidation were investigated by Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) methods. Characterizations of the TiO₂-NT and Ni modified electrodes are exerted by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The average tube length and diameter are 36.32 μm and 93.6 nm according to SEM images. XRD results indicated the tetragonal structured anatase of TiO₂ and Ni (111) and (200). While methanol oxidation peak does not observe on TiO₂-NT surface, behaviors of methanol oxidation depend on the Ni content on TiO₂-NT surface. Oxidation response increases by the increasing amount of Ni nano-particles in the deposits. High surface coverage (Γ) with 3.87 × 10⁻⁹ mol cm⁻² and very low activation energy (E_a) with 11.0 kJ/mol are measured on Ni modified TiO₂-NT with the highest Ni content. Charge transfer resistance either reduced or provided long stability and durability with the deposition of Ni on TiO₂-NT. This may associate that TiO₂-NTs with the large surface areas may play a significant role in the methanol oxidation efficiency. Modification of TiO₂-NT surface with Ni particles is an effective plan for high-performance electrocatalysis. Besides, the strong electronic interaction between Ni and TiO₂ may facilitate the adsorption of methanol through the bi-functional mechanism on the electrode surface.     

Preparation of nano-sized nickel as anode catalyst for direct urea and urine fuel cells

Nano-sized nickel with primary particle size of 2-3 nm has been successfully prepared for use as efficient anode catalysts in urea and urine fuel cells. XRD, SEM and TEM were used for characterisation of nano-sized nickel. Based on the previous communication, the performance of urea and urine fuel cells has been further improved when the relative humidity at the cathode was 100%. A maximum power density of 14.2 mW cm⁻² was achieved when 1 M urea was used as fuel, humidified air as oxidant. The performance of urine fuel cells operating above room temperature was also reported for the first time and a power density of 4.23 mW cm⁻² was achieved at 60 °C indicating potential application in urea-rich waste water treatment.     

Sustainable hydrogen production for fuel cells by steam reforming of ethylene glycol: A consideration of reaction thermodynamics

The use of renewable biomass, such as ethylene glycol (EG), for hydrogen production offers a more sustainable system compared to natural gas and petroleum reforming. For the first time, the reaction thermodynamics of steam reforming and sorption enhanced steam reforming of EG have been investigated. Gibbs free energy minimization method was used to study the effect of pressure (1-5 atm), temperature (500-1100 K) and water to EG ratio (WER 0-8) on the production of hydrogen and the formation of associated by-products (CH₄, CO₂, CO, C). The results suggest that hydrogen production is optimum when steam reforming occurs at atmospheric pressure, 925 K and with a WER of 8. Moreover, working at high temperature (>900 K) and with a WER above 6 inhibits almost entirely the production of methane and carbon. The main source of hydrogen in the system is found to be steam reforming of methane and water gas shift reaction by the analysis of the response reactions (RERs). Hydrogen production is governed by the former reaction at low temperatures while the latter one comes into prominence as temperature increases. By coupling with in situ CO₂ capture using CaO, the formation of CO₂ and CO can be avoided and high purity of hydrogen (>99%) can be achieved.     

Optimizing operating conditions and electrochemical characterization of glucose-gluconate alkaline fuel cells

The direct oxidation of glucose to produce electrical energy has been widely investigated because of renewability, abundance, high energy density and easy handling of the carbohydrate. Most of the previous studies have been conducted in extreme conditions in order to achieve complete glucose oxidation to CO₂, neglecting the carbohydrate chemical instability that generally leads to useless by-products mixtures. The partial oxidation to gluconate, originally studied for implantable fuel cells, has the advantage of generating a commercially valuable chemical.In the present paper we optimized fuel composition and operating conditions in order to selectively oxidize glucose to gluconate, maximizing the power density output of a standard commercial platinum based anode material. A deep electrochemical characterization concerning reversible potential, cyclic voltammetry and overpotential measurements have been carried out at 25 °C in the d-(+)-glucose concentration range 1.0 × 10⁻² to 1.0 M. NMR and EIS investigation clarify the role of the buffer in enhancing the electrochemical performance.