Highly effective anode structure in a direct formic acid fuel cell

We report the mass transport characteristics of formic acid and performance enhancement in a direct formic acid fuel cell in terms of the property of anode components. The effect of hydrophobicity of anode diffusion media as well as catalyst layer was investigated applying different cell temperature and fuel concentration. The operation over 80 °C and concentrated formic acid is of great advantage to the enhancement of catalytic activity and better water management. On the other hand, the conductivity of formic acid decreases by means of the formation of more complex chains of formic acid and the fuel cell resistance increases by membrane dehydration effect due to the hygroscopic property of formic acid, resulting in overall decrease of cell performance and long-term stability. Optimizing operating conditions, the use of 60% PtRu/C with only 1 mg/cm² on plain carbon paper can be one of the good choice to achieve both sustainable power performance and higher utilization of anode catalysts keeping cell resistance.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

钯基催化剂应用于甲酸电氧化反应的研究进展

甲酸是一种很有前途的化学储氢材料,可作为低温液体燃料电池的直接燃料。钯基催化剂作为直接甲酸燃料电池（DFAFC）阳极材料,对甲酸氧化具有良好的催化活性,能克服一氧化碳的毒化,在甲酸电化学氧化反应中主要按直接途径进行。降低贵金属含量、提高催化活性、提升稳定性是当前钯基催化材料研究领域的主要方向。主要介绍了当前研究中钯催化剂对甲酸电氧化的催化机理,综述了近5 a的钯合金催化剂制备、特殊形貌控制、碳负载对甲酸氧化活性增强的研究,对钯基催化剂的持续开发具有实际应用意义。     

Performance characterization of direct formic acid fuel cell using porous carbon-supported palladium anode catalysts

Palladium particles supported on porous carbon of 20 and 50 nm pore diameters were prepared and applied to the direct formic acid fuel cell (DFAFC). Four different anode catalysts with Pd loading of 30 and 50 wt% were synthesized by using impregnation method and the cell performance was investigated with changing experimental variables such as anode catalyst loading, formic acid concentration, operating temperature and oxidation gas. The BET surface areas of 20 nm, 30 wt% and 20 nm, 50 wt% Pd/porous carbon anode catalysts were 135 and 90 m²/g, respectively. The electro-oxidation of formic acid was examined in terms of cell power density. Based on the same amount of palladium loading with 1.2 or 2 mg/cm², the porous carbon-supported palladium catalysts showed higher cell performance than unsupported palladium catalysts. The 20 nm, 50 wt% Pd/porous carbon anode catalyst generated the highest maximum power density of 75.8 mW/cm² at 25 °C. Also, the Pd/porous carbon anode catalyst showed less deactivation at the high formic acid concentrations. When the formic acid concentration was increased from 3 to 9 M, the maximum power density was decreased from 75.8 to 40.7 mW/cm² at 25 °C. Due to the high activity of Pd/porous carbon catalyst, the cell operating temperature has less effect on DFAFC performance.     

Formic acid oxidation by carbon-supported palladium catalysts in direct formic acid fuel cell

The oxidation of formic acid by the palladium catalysts supported on carbon with high surface area was investigated. Pd/C catalysts were prepared by using the impregnation method. 30 wt% and 50 wt% Pd/C catalysts had a high BET surface area of 123.7 m²/g and 89.9 m²/g, respectively. The fuel cell performance was investigated by changing various parameters such as anode catalyst types, oxidation gases and operating temperature. Pd/C anode catalysts had a significant effect on the direct formic acid fuel cell (DFAFC) performance. DFAFC with Pd/C anode catalyst showed high open circuit potential (OCP) of about 0.84 V and high power density at room temperature. The fuel cell with 50 wt% Pd/C anode catalyst using air as an oxidant showed the maximum power density of 99 mW/cm². On the other hand, a fuel cell with 50 wt% Pd/C anode catalyst using oxygen as an oxidant showed a maximum power density of 163 mW/cm² and the maximum current density of 590 mA/cm² at 60 °C.     

Influence of underpotentially deposited Sb onto Pt anode surface on the performance of direct formic acid fuel cells

We first reported on electrocatalytic activity and stability of antimony modified platinum (PtSb_upd) as anode catalyst in direct formic acid fuel cells. Sb modified Pt (PtSb_upd) was prepared by underpotential deposition technique applying constant potential of 0.2 V (vs. Ag/AgCl, 3M KCl) and its modified surface was characterized by XRD and XPS. The electrocatalytic oxidation activity by cyclic voltammograms and the single cell power performance of Sb modified Pt were measured and their results were compared with the data of unmodified Pt electrode. PtSb_upd induced lower onset potential of formic acid oxidation and twice higher power density of 250 mW cm⁻² was observed.     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

Improved electrochemical CO removal via potential oscillations in serially connected PEM fuel cells with PtRu anodes

The polarization performance of two PEM fuel cells (with anode PtRu/C catalyst) connected either in parallel or serial, was compared to the performance of a single PEM fuel cell in galvanostatic operation using CO-free H₂ or 200 ppm CO-containing H₂ stream as anode feed at ambient temperature. Spontaneous potential oscillations were observed experimentally for the coupled configuration with two cells connected in serial or parallel using CO-containing H₂ feed at various current densities applied. The potential oscillations are ascribed by the dynamic CO adsorption and subsequent electrochemical CO oxidation on the anode. The measured anode outlet CO concentration was found to decrease with the order: single cell > parallel cells > serial cells at various current densities and anodic flow rates. The low anode outlet CO concentration (<10 ppm) at high current densities applied showed that CO in the anode feed was removed efficiently by the electrochemical CO oxidation occurring on the PtRu anode. The anode outlet CO concentration decreased as follows: a single cell > the parallel cells > the serial cells at broad range of current densities and anodic flow rates. The highest CO conversion and the highest average power output at equal hydrogen recovery degree were obtained with serially coupled fuel cells.     

Evaluation of direct formic acid fuel cells with catalyst layers coated by electrospray

We investigated cell performance and performed phenomenological analyses of direct formic acid fuel cells (DFAFCs) incorporating anode (palladium) and cathode (platinum) catalysts prepared using a new electrospray coating technique. To optimize the design of the DFAFC, we examined the cell performance by the Pd catalyst loading and formic acid feed rate. Of Pd catalyst loaded samples, 3 mg/cm² sample showed the highest electrical performance with formic acid feed rate of 5 ml/min. This behavior was caused by discrepancies in the mass transfer limitation. When the feed rate was greater than 10 mL/min, however, the 7 mg/cm² sample provided the highest electrical performance, which was attributed to enhanced electrooxidation reactions. For comparison of the effect of the catalyst coating method on the cell performance of DFAFC, polarization curves of the DFAFC incorporating catalysts prepared using a conventional airspray coating method were also measured. As a result of the comparison, the electrospray coatingused DFAFC showed better cell performance. Based on these results, the cell performance of the DFAFCs was optimized when the catalysts using the electrospray catalyst coating were employed, the amount of Pd loaded on the anode electrode was 3 mg/cm² (Pd thickness: ∼6 μm), and the formic acid feed rate was 10 mL/min.     

Passive micro tubular direct formic acid fuel cells (DFAFCs) with chemically assembled Pd anode nano-catalysts on polymer electrolytes

Wet-chemical assembling process has been developed for the formation of the anode electrocatalyst layers of a micro tubular direct formic acid fuel cell. By using this method, a porous layer of Pd nano-catalyst was bonded onto the inner surface of a tubular polymer electrolyte membrane by chemical reduction of Pd complex impregnated in the membrane. The performance characteristics as a function of parameters such as catalyst loading amount of Pd or the cell temperature were evaluated by using a half-cell testing method. The micro tubular DFAFC with a 2.5 mg-Pd cm⁻² anode and 6 mg-Pt cm⁻² cathode fabricated by wholly wet-chemical assembling process exhibited a peak power density over 4 mW cm⁻² under passive and air breathing conditions at ambient temperature and pressure.     

One-step growth of 3-5 nm diameter palladium electrocatalyst in a carbon nanoparticle-chitosan host and characterization for formic acid oxidation

The electrochemical formation of a palladium nanoparticle catalyst composite material has been investigated. A carbon nanoparticle-chitosan host film deposited onto a carbon substrate electrode has been employed to immobilize PdCl₂ as catalyst precursor. A one-step electrochemical reduction process gave Pd nanoparticles within the chitosan matrix with different levels of loading, on different carbon substrates, and with a reproducible catalyst particle diameter of ca. 3-5 nm. High activity for formic acid oxidation has been observed in aqueous phosphate buffer medium. The oxidation of formic acid has been investigated as a function of pH and maximum catalyst activity was observed at pH 6. When varying the formic acid concentration, limiting behaviour consistent with a “resistance effect” has been observed. A flow cell system based on a screen-printed carbon electrode has been employed to establish the effect of hydrodynamic conditions on the formic acid oxidation. Both increasing the convective-diffusion mass transport rate and increasing the concentration of formic acid caused the oxidation peak current to converge towards the same “resistance limit”. A mechanistic model to explain the resistance effect based on CO₂ flux and localized CO₂ gas bubble formation at the Pd nanoparticle modified carbon nanoparticle-chitosan host film has been proposed.     

Pb and Sb modified Pt/C catalysts for direct formic acid fuel cells

PtPb/C and PtSb/C bi-metallic catalysts were synthesized by chemical deposition of Pb or Sb on a commercial 40% Pt/C catalyst. The performances of catalysts with a range of compositions were compared in a multi-anode direct formic acid fuel cell in order to optimize compositions and evaluate the statistical significance of differences between catalysts. The catalytic activity for formic acid oxidation increased approximately linearly with adatom coverage for both PtPb/C and PtSb/C, to maxima at fractional coverages of ca. 0.7. At a cell voltage of 0.5 V, the currents at the optimum Pb or Sb coverages were ca. 8 times higher than at unmodified Pt/C. CO-stripping results indicate that the presence of Pb or Sb facilitates the oxidation of adsorbed CO. In addition, both metals appear to produce electronic effects that inhibit poison formation on the modified Pt surface.     

Probing anodic reaction kinetics and interfacial mass transport of a direct formic acid fuel cell using a nanostructured palladium-gold alloy microelectrode

The anodic reaction kinetics and interfacial mass transport of a direct polymer electrolyte membrane formic acid fuel cell have been investigated in an all solid-state electrochemical cell using a highly active nanostructured palladium-gold alloy microelectrode as an in situ probe. Well-defined “S-shaped” steady-state cyclic voltammograms exhibiting current-rising region at lower overpotentials and limiting current region at higher overpotentials have been first obtained for the electrochemical oxidation of formic acid at varying temperature. The “S-shaped” steady state polarization curves and chronoamperometric curves enable convenient measurements of the anodic reaction kinetics and interfacial mass transport of formic acid under real polymer electrolyte membrane conditions. It is encouragingly found that formic acid can be directly oxidized to CO₂ with the first electron transfer being the likely rate-determining step and the formation of surface poison can be neglected. The exchange current density for the electrooxidation of formic acid is on the order of magnitude of 10⁻⁷ A cm⁻² in the temperature range of 20-60 °C. The permeability and diffusion coefficient of formic acid through a Nafion^® 117 membrane are of the order of magnitude of 10⁻⁹ mol cm⁻¹ s⁻¹ and 10⁻⁶ cm² s⁻¹, respectively. The combination of a nanostructured microelectrode and an all solid-state electrochemical cell offers a versatile approach to evaluate potential electrocatalysts for fuel cells and electrochemical sensors employing polymer electrolyte membranes.     

Investigation of fuel and media flexible laminar flow-based fuel cells

We investigate the performance of air-breathing laminar flow-based fuel cells (LFFCs) operated with five different fuels (formic acid, methanol, ethanol, hydrazine, and sodium borohydride) in either acidic or alkaline media. The membraneless LFFC architecture enables interchangeable operation with different fuel and media combinations that are only limited by the actual anode catalyst used. Furthermore, operating under alkaline conditions significantly improves methanol and ethanol oxidation kinetics and stabilizes sodium borohydride. LFFCs operated with hydrazine and sodium borohydride as fuels exhibit power densities of 80 and 101 mW/cm², respectively. To optimize anode performance, particularly for ethanol electro-oxidation, we introduced a hydrogen cathode to the membraneless LFFC design which renders the cell an ideal platform for anode investigation. Here, we highlight two simple diagnostic methods, in situ single electrode studies and electrochemical impedance spectroscopy (EIS), for characterizing and optimizing the performance of a direct ethanol LFFC anode.     

Improved electrocatalytic performance of Pd nanoparticles with size-controlled Nafion aggregates for formic acid oxidation

This work focuses on the effect of Nafion ionomer aggregation within the Pd catalytic electrode on electrocatalytic oxidation of formic acid. By a simple heat-treatment, the particle sizes of both Nafion ionomers in Nafion solution and congeries formed between Pd nanoparticles and Nafion ionomers in the catalyst ink decrease and their size distribution becomes narrow. Heat treatment of the catalyst ink leads to a significantly enhanced catalytic activity for formic acid oxidation on the Pd catalytic electrode. Such an enhancement is ascribed to the improvement in catalyst utilization verified by CO stripping voltammograms and to the decrease in charge-transfer resistance of oxidation reaction confirmed by impedance analysis. Typical XPS analysis shows that there are at least two kinds of Pd and S surface states within the catalytic electrode with the ink pre-heated at 25 °C and only one kind of Pd and S surface state at 80 °C, indicative of a better dispersion between Pd nanoparticles and smaller Nafion ionomers at a higher heat treatment temperature. Furthermore, the decrease in congeries size within the anode catalyst ink leads to a significant decrease in Nafion loading within the catalytic layer and a remarkable improvement in direct formic acid fuel cell's performance.     

Poisoning and regeneration of Pd catalyst in direct formic acid fuel cell

Palladium catalyst poisoned in the anode of direct formic acid fuel cell (DFAFC) during constant current discharging can be fully regenerated by a non-electrochemical method, i.e. just switching pure water to DFAFC for 1 h. Electrochemical impedance spectrum of DFAFC during the discharging and regeneration were recorded and analyzed. No much difference could be found for the high-frequency resistance of DFAFC after discharging while the charge transfer resistance in the mediate-frequency region increased significantly. The voltage variation during the regeneration showed that one platform of 0.35 V was formed by the intermediate species of formic acid oxidation, which is proven to be critical for cell performance regeneration. The results indicated that the absorption of poisoning species on Pd was the main reason for the decaying of cell performance.     

CO tolerance of PdPt/C and PdPtRu/C anodes for PEMFC

The performance of H₂/O₂ proton exchange membrane fuel cells (PEMFCs) fed with CO-contaminated hydrogen was investigated for anodes with PdPt/C and PdPtRu/C electrocatalysts. The physicochemical properties of the catalysts were characterized by energy dispersive X-ray (EDX) analyses, X-ray diffraction (XRD) and “in situ” X-ray absorption near edge structure (XANES). Experiments were conducted in electrochemical half and single cells by cyclic voltammetry (CV) and I-V polarization measurements, while DEMS was employed to verify the formation of CO₂ at the PEMFC anode outlet. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with the PdPt/C and PdPtRu/C anodes containing 0.4 mg metal cm⁻², with the cell presenting potential losses below 200 mV at 1 A cm⁻², with respect to the system fed with pure H₂. For the PdPt/C catalysts no CO₂ formation was seen at the PEMFC anode outlet, indicating that the CO tolerance is improved due to the existence of more free surface sites for H₂ electrooxidation, probably due to a lower Pd-CO interaction compared to pure Pd or Pt. For PdPtRu/C the CO tolerance may also have a contribution from the bifunctional mechanism, as shown by the presence of CO₂ in the PEMFC anode outlet.     

Microwave heated polyol synthesis of Pt3Te/C catalysts

Pt₃Te/C nanoparticles supported on Vulcan XC-72 carbon were prepared within a few minutes under different reaction conditions by using a microwave-polyol method. Their physical and electrochemical characterization were carried out by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectrometer (XPS), selected-area electron diffraction (SAED), H₂ adsorption-desorption, ethanol oxidation and CO stripping. TEM shows that the Pt₃Te/C (pH 3) catalyst has uniform nanoparticles and is well dispersed with average particle size of about 2.8 nm. Electrochemical results show that the electrochemical activity of Pt₃Te/C catalysts synthesized at different pH values are in the order of pH 3 > pH 7 > pH 9 > pH 13. The Pt₃Te/C catalyst (pH 3) is also better than the Pt₃Te/C catalyst synthesized by formic acid as reductant. From a practical point of view, the microwave-polyol method at the pH value of 3 could be an appropriate method for synthesizing nanocatalysts.     

Sonochemical synthesis of Pt-doped Pd nanoparticles with enhanced electrocatalytic activity for formic acid oxidation reaction

Pt-doped Pd nanoparticle catalysts (Pd _n Pt, n is 12, 15 and 19) supported on carbon were synthesized by an ultrasound assisted polyol method. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The electrochemical activity of the electrocatalysts was investigated in terms of formic acid oxidation reaction (FAOR) at low concentration of formic acid in 0.1 M perchloric acid at room temperature. Formic acid oxidation on the Pd _n Pt/C commences at lower potential than a commercial Pt/C. Pd₁₉Pt/C catalyst showed the highest catalytic activity in FAOR compared to that of other catalysts. The obtained electrochemical results from voltammograms indicate that Pt-doped Pd catalysts can be a promising candidate for the anode material in direct formic acid fuel cells. The synthesis procedure is not only a very facile route but also a mass producible method for preparing carbon supported alloy nanoparticles.     

Recent progress in selective CO removal in a H2-rich stream

In this review, recent works related to the selective CO removal in a H₂-rich stream for the application of the low-temperature fuel cell are discussed. The membrane separation, the selective CO hydrogenation, and the preferential CO oxidation (PROX) have been generally studied to meet the requirement for the polymer electrolyte membrane fuel cell (PEMFC) where the CO concentration should be controlled to be less than 10 ppm not to degrade the electrochemical performance of Pt anode. For the membrane separation, the thin layer of Pd-based alloy metal on the porous ceramic material coupled with the catalytic purification is the most advanced method at present. For PROX catalysts, supported Ru catalysts and Pt-based alloy catalysts have been successfully developed so far. The combination of highly selective PROX catalysts and the CO methanation catalyst can provide the extended temperature range to achieve the acceptable CO removal. Because each method has presently its own weak points, the further advance is still in need. The non-noble metal-based membrane requiring smaller pressure differentials is highly plausible in the membrane separation. The highly selective catalyst for CO methanation in the presence of excess CO₂ and H₂O can simplify the CO removal step. The PROX catalyst should be operative over a wide reaction temperature as well as at low temperatures not to cause the reverse water–gas shift reaction. During the development of these catalysts, the progress on the high-temperature PEM fuel cell or the CO-tolerant anode should be carefully evaluated.