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.     

Ternary PtRuC co-sputtered electrocatalysts having reaction selectivity for anodes and cathodes in direct methanol fuel cells

In the present study, the reaction selectivity of methanol oxidation and O₂ reduction at PtRuC co-sputtering samples has been described. The samples were prepared using the polygonal barrel-sputtering method and tested by linear sweep voltammetry in 0.5 mol/L H₂SO₄ containing methanol and O₂. When sputtering was performed at a constant Ar gas pressure of 3 Pa, methanol oxidation became highly selective for the samples prepared at AC powers of 30 and 50 W. This characteristic was likely induced by the existence of O₂ reduction intermediates, which can enhance the oxidation of CO, a methanol oxidation intermediate. The samples displaying O₂ reduction selectivity were obtained at Ar gas pressure ranging between 0.4 and 3 Pa (AC power: 100 W). The O₂ reduction selectivity may be attributed to the deposition of the electron-rich metallic Ru, which exhibits poor methanol oxidation performance. Based on these results, PtRuC samples that show reaction selectivity in methanol oxidation and O₂ reduction are independently prepared by the polygonal barrel-sputtering method.     

Synthesis and characterization of quaternary PtRuIrSn/C electrocatalysts for direct ethanol fuel cells

To find a more durable anode with high performance for direct ethanol fuel cells (DEFCs), the present study investigates a series of quaternary electrocatalysts, Pt₃₀Ru₃₀Ir_40−xSn_x/C (wt.%), for the ethanol electro-oxidation reaction (EOR). The carbon-supported Pt₃₀Ru₃₀Ir_40−xSn_x/C electrocatalysts were prepared by a known impregnation-reduction (borohydride) method. The microstructure and chemical composition were determined by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The activity of the electrocatalysts for EOR was compared to commercial Pt₆₇Ru₃₃/C (HISPEC5000) using linear sweep voltammetry (LSV) based on similar Pt loading. The results of this study show that electrocatalyst composition with 10 and 20% Ir (wt.%) exhibit higher electrocatalytic activity than the commercial PtRu electrocatalyst. The single fuel cell testing at 90 °C comparing Pt₃₀Ru₃₀Ir_40−xSn_x/C to commercial Pt₆₇Ru₃₃/C and Pt₈₃Sn₁₇/C anodes showed an enhancement of Pt activity (normalized to Pt loading) in the following order: Pt₃₀Ru₃₀Ir₁₀Sn₃₀ > Pt₃₀Ru₃₀Sn₄₀ ≥ Pt₃₀Ru₃₀Ir₄₀ ≥ Pt₈₃Sn₁₇ > Pt₆₇Ru₃₃. After a long-term performance test, the activity changed to the following order: Pt₃₀Ru₃₀Ir₁₀Sn₃₀ > Pt₃₀Ru₃₀Ir₄₀ > Pt₃₀Ru₃₀Sn₄₀ > Pt₈₃Sn₁₇ > Pt₆₇Ru₃₃. Pt₃₀Ru₃₀Ir₁₀Sn₃₀/C exhibited both a higher performance with a specific power density of 29 mW mg_Pt⁻¹ without O₂ backpressure at the cathode and an excellent long-term stability in a DEFC operating at 90 °C.     

Direct ammonia fuel cell performance using PtIr/C as anode electrocatalysts

Direct ammonia fuel cell (DAFC) performance was investigated using as anode PtIr/C electrocatalysts (Pt:Ir atomic ratios of 50:50, 70:30, 80:20 and 90:10) prepared by the borohydride reduction process and NH₄OH 1.0, 3.0 and 5.0 mol L⁻¹ in KOH 1.0 mol L⁻¹ as fuel. X-ray analyses of PtIr/C electrocatalysts suggested the formation of PtIr alloy and the transmission electron micrographs showed the average particle diameters between 4.5 and 6.0 nm. Using PtIr/C 50:50 electrocatalyst and NH₄OH 5.0 mol L⁻¹ in KOH 1.0 mol L⁻¹ at 40 °C a maximum power density was 48% and 70% higher than that obtained using Pt/C and Ir/C electrocatalysts, respectively. The increase of electroactivity using PtIr/C electrocatalysts might be related to a decrease of poisoning on catalyst surface by N_ads species and to an improved kinetic for ammonia oxidation reaction.     

Microwave assisted synthesis of nitrogen doped and oxygen functionalized carbon nano onions supported palladium nanoparticles as hybrid anodic electrocatalysts for direct alkaline ethanol fuel cells

In this study, we present the synthesis of pristine carbon (p-CNO), nitrogen doped (N–CNO) and oxygen functionalized (ox-CNO) nano onions, using flame pyrolysis, chemical vapour deposition, and reflux methods, respectively. Pd/p-CNO, Pd/N–CNO and Pd/ox-CNO electrocatalysts are prepared using a simple and quick microwave-assisted synthesis method. The various CNO and Pd/CNO electrocatalysts are fully characterized and the FTIR and XPS results reveal that the synthesized CNOs contain oxygen and nitrogen functional groups that facilitates the attachment and dispersion of the Pd nanoparticles. Electrochemical tests show that the N–CNO and Pd/N–CNO electrocatalysts exhibit high current density (4.2 mA cm ^?2 and 17.4 mA cm ^?2), long-term stability (1.2 mA cm ^?2 and 6.9 mA cm ^?2), and fast electron transfer when compared to the equivalent pristine and oxidized catalysts (and their Pd counterparts), and a commercial Pd/C electrocatalyst, towards ethanol oxidation reactions in alkaline medium.     

Synthesis and characterization of anion exchange membranes for alkaline direct methanol fuel cells

Alkaline fuel cells suggest solution for the problems of low methanol oxidation kinetics and methanol crossover, which are limiting the development of direct methanol fuel cells. In this work, a novel anion exchange membrane, quaternized poly(aryl ether oxadiazole), was prepared through polycondensation, grafting and quaternization. The ionic conductivity of as-synthesized anion exchange membrane can reach up to 2.79 × 10⁻² S/cm at 70 °C. The physical and chemical stability of the anion exchange membranes could also meet the requirement for alkaline direct methanol fuel cells.     

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.     

Carbon supported Pt–Pd alloy as an ethanol tolerant oxygen reduction electrocatalyst for direct ethanol fuel cells

A carbon supported Pt–Pd catalyst with a Pt:Pd atomic ratio 77:23 was prepared by reduction of metal precursors with formic acid and characterized by EDX, XRD and XPS techniques. A decrease of the lattice parameter compared with that of pure Pt was observed, indicating the formation of a Pt–Pd alloy. Tests in H₂SO₄ solution in the absence of ethanol showed that the Pd-containing is slightly more active than pure Pt for the oxygen reduction reaction (ORR). In the presence of ethanol a larger increase in overpotential of the ORR on pure Pt than that on Pt–Pd was found, indicating a higher ethanol tolerance of the binary catalyst. The enhanced performance at 90 °C of the direct ethanol fuel cell with Pt–Pd/C as cathode material confirmed the results of half cell tests, and was essentially ascribed to a reduced ethanol adsorption on Pt–Pd.     

Pt supported on mesoporous material for methanol and ethanol oxidation in alkaline medium

Pt nanoparticles supported on a mesoporous material of zeolite Faujasite-C composite is a highly active catalyst for methanol and ethanol oxidation in alkaline media. Pt was synthesized by a simple methodology of chemical reduction using ultrasound method. Faujasite-C composite was prepared by sol-gel method using fly ash as economic precursor. Pt/Faujasite-C was characterized by X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy to investigate its structure, morphology, composition and size. The electrochemical activity of catalyst towards methanol and ethanol oxidation reaction in alkaline media was evaluated by cyclic voltammetry and chronoamperometry techniques. The results obtained were compared with Pt/C synthesized and tested at the same conditions. According to TEM results Pt/Faujasite-C electrocatalyst exhibits a higher Pt agglomeration compared to Pt/C. Pt/Faujasite-C is more active for alcohol oxidation reactions compared to Pt/C. Pt electrocatalysts are more active for ethanol oxidation than methanol oxidation. Chronoamperometric results indicated that Pt deactivation by intermediate poisoning is more severe for ethanol than methanol. Pt/Faujasite-C can be used as anodic electrocatalyst in direct liquid fuel cells.     

Alkaline direct alcohol fuel cells

The faster kinetics of the alcohol oxidation and oxygen reduction reactions in alkaline direct alcohol fuel cells (ADAFCs), opening up the possibility of using less expensive metal catalysts, as silver, nickel and palladium, makes the alkaline direct alcohol fuel cell a potentially low cost technology compared to acid direct alcohol fuel cell technology, which employs platinum catalysts. A boost in the research regarding alkaline fuel cells, fuelled with hydrogen or alcohols, was due to the development of alkaline anion-exchange membranes, which allows the overcoming of the problem of the progressive carbonation of the alkaline electrolyte. This paper presents an overview of catalysts and membranes for ADAFCs, and of testing of ADAFCs, fuelled with methanol, ethanol and ethylene glycol, formed by these materials.     

Performance of direct ammonia fuel cell with PtIr/C,PtRu/C,and Pt/C as anode electrocatalysts under mild conditions

While ammonia (NH₃) is an attractive alternative to pure hydrogen, its direct use in fuel cells is fraught with difficulties. A direct ammonia fuel cell (DAFC) with PtIr/C (Pt:Ir = 1:1), PtRu/C (Pt:Ru = 1:1), and Pt/C anode electrocatalyst was investigated at 25 °C and 100 kPa inlet gas pressure. Due to the synergistic and electronic effects of the PtIr alloy, their open-circuit voltages (OCVs) were rated as PtIr/C > PtRu/C > Pt/C, with the DAFC with PtIr/C anode achieving the highest OCV of 0.50 V and peak power density (PPD) of maximum 1.68 mW cm^?2. Meanwhile, an online Fourier transform infrared (FTIR) spectrometer detected an increase in ammonia permeation in the cathode exhaust gas, indicating a possibility of fuel permeation and cathode electrocatalyst degradation. The degradation of DAFC efficiency with rising cycle numbers may be due to ammonia cross-over and poisoning over the surface of the electrocatalyst.     

Pulsed plasma-polymerized alkaline anion-exchange membranes for potential application in direct alcohol fuel cells

Pulsed plasma polymerization is adopted to synthesize alkaline anion-exchange membranes (AAEMs) with high contents of functional groups. The attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermo gravimetric analysis demonstrate that the benzyltrimethylammonium cationic groups can be successfully introduced into the polymer matrix. The content of the quaternary nitrogen in pulsed plasma-polymerized membrane is up to 1.93 atom%. The ultra-flat and undamaged morphology structure of the AAEMs indicates a low plasma ablation effect in the pulsed plasma polymerization. The excellent properties of the pulsed plasma-polymerized AAEMs, including good adhesion to the substrate, acceptable chemical stability and thermal stability, high ion-exchange capacity (1.42 mmol g⁻¹) and water uptake (59.73 wt%), interesting ionic conductivity (0.0205 S cm⁻¹ in deionized water at 20 °C) and ethanol permeability (3.37 × 10⁻¹¹ m² s⁻¹), suggest a great potential for application in direct alcohol fuel cells.     

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.     

MEA for alkaline direct ethanol fuel cell with alkali doped PBI membrane and non-platinum electrodes

This paper reports on the fabrication of MEA for alkaline direct ethanol fuel cell (ADEFC). The MEA was fabricated using non-platinum electrocatalysts and a membrane of alkali doped polybenzimidazole (PBI). The employed oxygen reduction catalyst was prepared by pyrolysis of 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine cobalt(II) supported on XC72 carbon. This catalyst is tolerant to ethanol. Electrocatalyst at the anode was RuV alloy supported on XC72 carbon. It was synthesized by reduction of respective salts at elevated temperature. Single cell power density of 100 mW cm⁻² at U = 0.4 V was achieved at 80 °C using air at ambient pressure and 3 M KOH + 2 M EtOH anode feed. The developed MEA is considered viable for use in emergency power supply units and in power sources for portable electronic equipment.     

Investigation of PtNi/C anode electrocatalysts for direct borohydride fuel cell

In this study, carbon-supported PtNi alloys with different molar ratios synthesized by borohydride reduction were evaluated as anode catalysts for sodium borohydride fuel cells. The higher angle shifts of the Pt peaks from X-ray diffraction (XRD) account for the alloy formation between Pt and Ni. The negative shift of Pt 4f XPS spectrum for PtNi(7:3)/C also indicates an electronic structural change of Pt in the alloyed PtNi/C catalyst. The cyclic voltammetry (CV) results show that the PtNi(x:10 − x)/C catalysts are electrochemically active toward borohydride oxidation at the potential range between −0.6 V and +0.1 V vs. Hg/HgO electrode, and PtNi(7:3)/C presents the strongest peak current density among three catalysts with different molar ratios. The results of amperometric i–t curves (i–t) tests also show that the steady-state current density is the highest on PtNi(7:3)/C among alloy catalysts. The higher electrocatalytic activity of the PtNi(7:3)/C can be attributed to the alloy effect and the Pt electronic structure change due to the addition of Ni.     

Pt-modified Au/C catalysts for direct glycerol electro-oxidation in an alkaline medium

Au-based catalysts promoted with Pt were prepared by using polyvinyl alcohol protection method. Different amounts of Pt (5, 10 and 15% of total metal) were added in the Au sol formation step to improve the activity of Au/C toward glycerol electro-oxidation in an alkaline medium. The physical and electrochemical properties of the as-prepared catalysts were explored. The average particle sizes of the Au/C and Pt-modified Au/C catalysts measured by transmission electron microscopy (TEM) were the same at around 4 nm. The PtAu/C alloy formation in the PtAu/C catalysts was confirmed by the increase of lattice parameter calculated from the X-ray diffraction (XRD) patterns and by the absence of Pt ring in the electron diffraction pattern. The change of binding energy in X-ray photoelectron spectroscopy (XPS) results indicated the interaction between Pt and Au. For glycerol electro-oxidation in an alkaline medium, the PtAu/C catalysts were more active than the Au/C catalyst as observed from an early onset potential and a shift of potential at maximum current density to a lower potential. Among the Pt-modified Au/C catalysts, the most active catalyst was Pt₁Au₉/C. The synergistic effects between Pt–Au was proven by a better performance of the PtAu/C compared to the physical mixed catalyst of Au/C and Pt/C at the same Pt:Au ratio. The Pt-modified Au/C catalysts were more stable than the Au/C, especially in a high potential region. This enhancement may be caused by the promotion effect of highly active PtO on the surface of the bimetallic catalyst.     

Electrochemical and fuel cell evaluation of PtAu/C electrocatalysts for ethanol electro-oxidation in alkaline media

PtAu/C electrocatalysts in different atomic ratios and supported on Vulcan XC 72 carbon were tested for ethanol electro-oxidation in alkaline media. The electrocatalysts were prepared using borohydride as reducing agent. PtAu/C X-ray diffraction (XRD) patterns showed peaks characteristic of Pt face-centered cubic (fcc) structure and carbon. PtAu/C (70:30) and (50:50) XRD patterns showed a shift to lower values of 2θ when compared to Pt/C, this way suggesting the formation of PtAu alloy. Transmission electron micrographs showed the nanoparticles with particle size between 4 and 6.5 nm for all PtAu/C electrocatalysts. Electrochemical characterization of the PtAu/C materials suggested the PtAu/C (50:50) as the most promising material for ethanol electro-oxidation while experiments in single fuel cell suggested PtAu/C (70:30). The discrepancy in the results obtained can be explained by the electrode construction since PtAu/C (50:50) yields a much thicker electrode than PtAu/C (70:30), due to the Pt load is the same. The best results obtained with PtAu/C electrocatalysts could be explained by the presence of Pt and Au in close contact (alloy) associated to the extend in the platinum lattice parameters since these properties could contribute to the C–C breaking bond.     

Analysis of Pt/C electrode performance in a flowing-electrolyte alkaline fuel cell

We characterize the performance of Pt/C-based electrodes under alkaline conditions using a microfluidic H₂/O₂ fuel cell as an analytical platform. Both anodes and cathodes were investigated as a function of electrode preparation procedures (i.e., hot pressing, acclimatization) and fuel cell operating parameters (i.e., electrolyte composition) via chronoamperometric and electrochemical impedance analyses. X-ray micro-computed tomography was employed to link electrode structure to performance. In addition, the flowing electrolyte stream is used to study the effects of carbonates on individual electrode and overall fuel cell performance. Our studies provide direct evidence that the performance of hydrogen-fueled room-temperature alkaline fuel cells (AFCs) is limited by transport processes to and from the anode primarily due to water formation. Furthermore, the presence of carbonate species in the electrolyte appears to impact only anode performance whereas cathode performance remains unchanged.     

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.     

Theoretical means for searching bimetallic alloys as anode electrocatalysts for direct liquid-feed fuel cells

The present paper reviews the best anode electrocatalysts, mainly the alloys, which have been tested in direct liquid-feed fuel cells fed with methanol, ethanol or formic acid. It attempts to interpret the alloys catalytic behaviours by using the Nørskov and co-workers’ theoretical work [A. Ruban, B. Hammer, P. Stoltze, H.L. Skriver, J.K. Nørskov, J. Mol. Catal. A 115 (1997) 421; B. Hammer, J.K. Nørskov, Adv. Catal. 45 (2000) 71; J. Greeley, J.K. Nørskov, M. Maurikakis, Annu. Rev. Phys. Chem. 53 (2002) 319], who proposed surface theories and databases about the metals d-band centre shift and the segregation. It also attempts to suggest new alloys combinations. For example, for the methanol oxidation, the best catalyst is Pt-Ru and the following features make this catalyst stand out: the d-band centre of Pt shifts down what supposes weaker molecules adsorption and Pt strongly segregates. From this analysis, it is suggested that the Pd-Ni alloy may be a potentially good catalyst. Similar interpretations are given for the three fuel cell systems regarded in the present paper.