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.     

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.     

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.     

Effect of temperature, oxidant and catalyst loading on the performance of direct formic acid fuel cell

We investigated the effect of temperature, oxidant and catalyst loading on the performance of direct formic acid fuel cell (DFAFC). When oxidant was changed from air to oxygen, the power density was increased to 17.3 mW/ cm² at 25 ‡C. The power density of DFAFC operated with oxygen showed a maximum value of 40.04 mW/cm² with the temperature rise from room temperature to 70 °C. The highest power density of DFAFC using air was observed for Pt-Ru black catalyst with loading of 8 mgPt/cm² at room temperature. At 70 ‡C; however, the performance of catalyst with the loading of 4 mgPt/cm² was higher than that of 8 mgPt/cm². The DFAFC, operated with oxygen and catalyst of 4 mgPt/cm² loading, showed the best performance at all temperature range. The enhancement of cell performance with an increase of catalyst loading is believed to come from an increase of catalyst active sites. However, operated at higher temperature or with oxygen, the cell with higher catalyst loading showed lower performance than expected. It is speculated that the thick catalyst layer inhibits the proton transport.     

Modification of Pt nanoelectrodes dispersed on carbon support using irreversible adsorption of Bi to enhance formic acid oxidation

In this work, formic acid oxidation on Pt nanoelectrodes modified by irreversible adsorption of Bi is presented. The coverage of Bi, as measured by voltammetry and X-ray photoelectron spectroscopy, was controlled from 0.05 to 0.25. Chronoamperometric measurement of the catalytic activities of the Bi-modified Pt nanoelectrodes revealed that the catalytic enhancement depended on oxidation potential and Bi coverage: elemental Bi in the potential range from −0.1 V to 0.6 V enhanced formic acid oxidation by factor of 4, while partially oxidized Bi in the potential range from 0.3 V to 0.7 V increased by factor of 8. The enhancement in the latter potential range was effective only when the Bi coverage was more than 0.18. Single cell performance of the Pt nanoelectrodes modified by Bi increased by factor of 2-3, depending on operation conditions such as formic acid concentration, temperature, and humidity in the feeding gas into cathode. When the Bi coverage was more than 0.18, the single cell performances were nearly identical. Based on measurement of adsorbed CO and catalytic poison from formic acid, an oxidation path not involving catalytic poison in the potential range from 0.5 V to 0.7 V is discussed in detail.     

Effects of iron-tetrasulfophthalocyanine on the catalytic activities of Pt/C,PtRu/C,and Pd/C catalysts in a multi-anode direct formic acid fuel cell

This paper reports a systematic study of the effects of a promoter, iron-tetrasulfophthalocyanine (FeTSPc), on the catalytic activities of carbon supported Pt, PtRu, and Pd catalysts (Pt/C, PtRu/C, and Pd/C) for formic acid oxidation. A multi-anode direct formic acid fuel cell (DFAFC) was used to compare the effects on each catalyst of adding FeTSPc to the fuel stream. The FeTSPc significantly enhanced the activity of the Pt/C catalyst, but had little effect on the PtRu/C catalyst. The activity of the Pd/C catalyst was inhibited by the FeTSPc. A FeTSPc modified Pt/C was also evaluated in a conventional 5 cm² DFAFC.     

Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide-activated carbon-formic acid system, compared with a hydrogen peroxide-formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.     

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.     

Performance degradation study of a direct methanol fuel cell by electrochemical impedance spectroscopy

A stability test of a direct methanol fuel cell (DMFC) was carried out by keeping at a constant current density of 150 mA cm⁻² for 435 h. After the stability test, maximum power density decreased from 68 mW cm⁻² of the fresh membrane-electrode-assembly (MEA) to 34 mW cm⁻² (50%). Quantitative analysis on the performance decay was carried out by electrochemical impedance spectroscopy (EIS). EIS measurement of the anode electrode showed that the increase in the anode reaction resistance was 0.003 Ω cm². From the EIS measurement results of the single cell, it was found that the increase in the total reaction resistance and IR resistance were 0.02 and 0.05 Ω cm², respectively. Summarizing the EIS measurement results, contribution of each component on the performance degradation was determined as follows: IR resistance (71%) > cathode reaction resistance (24%) > anode reaction resistance (5%). Transmission electron microscopy (TEM) results showed that the average particle size of the Pt catalysts increased by 30% after the stability test, while that of the PtRu catalysts increased by 10%.     

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.     

Upgrading of bitumen with formic acid in supercritical water

Upgrading of bitumen was examined with formic acid in supercritical water (SCW) from 673 to 753 K and at a water/oil ratio from 0 to 3. Decomposition of bitumen in SCW + HCOOH gave higher conversions of asphaltene and lower coke yields than those of pyrolysis or with only SCW. Decomposition of bitumen was also conducted in SCW + H₂, SCW + CO, toluene and tetralin, which revealed that decomposition of asphaltene was promoted and coke formation was suppressed when using SCW + HCOOH. In SCW + HCOOH, an increase in the water/oil ratio promoted both decomposition of asphaltene and suppression of coke formation. Formic acid in SCW seemed to enhance the conversion of bitumen to lower molecular weight compounds because formic acid seems to produce active species in SCW. The low temperature region (ca. 723 K) was suitable for upgrading bitumen with formic acid in SCW since coke formation was strongly promoted at high temperature (>753 K). A reaction model was proposed and the model predicted that hydrogenation of the asphaltene core was important for the suppression of coke formation.     

Polyaniline as cathodic material for electrochemical energy sources: The role of morphology

Polyaniline layers of different morphologies ranging from open and “sponge-like” structures to compact and “pebble-like” surfaces were synthesized from perchlorate solutions and employed as cathode in the galvanic cell with Zn anode and NH₄Cl/ZnCl₂ electrolyte. Cathodic properties of synthesized layers were investigated by the constant current charging/discharging method in 500 cycles. Specific charge capacities and specific energies obtained form the current-time curves strongly depend on the morphology of investigated layers and discharge conditions. The results unambiguously show that charging/discharging reaction of polyaniline layers is limited to relatively thin layer at polymer/solution boundary. Specific charge capacities are inversely related to both the polymer thickness and the discharge current density. In the limit of zero current densities the specific charge capacity as high as 245 A h kg⁻¹ could be achieved for porous structures of polyaniline layers. Specific capacitance higher than 400 F g⁻¹ obtained at 2 mA cm⁻² current density makes polyaniline a promising material for the application in electrochemical supercapacitors. The electrochemical behaviour of the layers was investigated by cyclic voltammetry and electrochemical impedance spectroscopy before and after 500 cycles of charging/discharging experiments. Both, cyclic voltammetry and electrochemical impedance spectroscopy showed that some polyaniline layers develop an increased charged transfer resistance at the carbon support/polymer interface during charging/discharging process. The increased charge transfer resistance does not affect the overall specific charge of the layers. The low-frequency capacities in impedance spectra are attributed to charging/discharging of polymer/electrolyte interface and seem to be related to the specific charge capacities obtained by extrapolation to zero current density discharge reaction.     

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.     

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.     

An alkaline microfluidic fuel cell based on formate and hypochlorite bleach

An alkaline microfluidic fuel cell is demonstrated employing an alkaline version of a formic acid anode and a sodium hypochlorite cathode. Both sodium formate fuel and sodium hypochlorite oxidant are available and stable as highly concentrated solutions, thereby facilitating fuel cell systems with high overall energy density. Sodium hypochlorite is commonly available as hypochlorite bleach. The alkaline anodic half-cell produces carbonate rather than the less-desirable gaseous CO₂, while sustaining the rapid kinetics associated with formic acid oxidation in acidic media. Both half-cells provide high current densities at relatively low overpotentials and are free of gaseous products that may otherwise limit microfluidic fuel cell performance. The microfluidic fuel cell takes advantage of a recently developed membraneless architecture with flow-through porous electrodes. Power densities up to 52 mW cm⁻² and overall energy conversion efficiencies up to 30% per single pass are demonstrated at room temperature using 1.2 M formate fuel and 0.67 M hypochlorite oxidant. The alkaline formate/hypochlorite fuel and oxidant combination demonstrated here, or either one of its individual half-cells, may also be useful in conventional membrane-based fuel cell designs.     

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。     

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.     

CO tolerance effects of tungsten-based PEMFC anodes

The performance of proton exchange membrane fuel cells (PEMFC) fed with CO-contaminated hydrogen was investigated for anodes with PtWO_x/C and phosphotungstic acid (PTA) impregnated Pt/C electrocatalysts. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with anodes containing 0.4 mg PtWO_x cm⁻² and also for those with 0.4 mg Pt cm⁻² impregnated with ca. 1 mg PTA cm⁻². A decay of the single cell performance with time is observed, and this was attributed to an increase of the membrane resistance due to the polymer degradation promoted by the crossover of the tungsten species throughout the membrane.     

Influence of Au contents of AuPt anode catalyst on the performance of direct formic acid fuel cell

We reported that various compositions of AuPt nanoparticles synthesized as an anode material for formic acid fuel cell were investigated. Its surface characteristics were systematically analyzed using XRD and TEM and anodic electrocatalytic activity was studied using a linear sweep voltammetry technique in 0.5 M H₂SO₄ + 1 M HCOOH. In addition, the voltage-current curve and power density of home-made AuPt-based membrane-electrode-assembly (MEA) and commercial Pt_0.5Ru_0.5-based MEA was measured at 60 °C in 9 M formic acid. The maximum power density of Au_0.6Pt_0.4-based MEA was 30% higher than that of PtRu-based MEA which were 200 mW cm⁻² and 155 mW cm⁻², respectively.     

Performance degradation studies on PBI/H3PO4 high temperature PEMFC and one-dimensional numerical analysis

Five hundred hours continuous aging test at constant discharge current (640 mA cm⁻²) was performed on PBI/H₃PO₄ high temperature PEMFC unit cell, electrochemical techniques-linear sweep voltammetry (LSV) and AC impedance measurement were used to investigate the changes of electrochemical surface area (ESA) and high frequency resistance (internal resistance) with time. Initial experimental results showed that during 500 h continuous aging test the main reason for cell performance degradation is the decrease of ESA caused by sintering. In addition, a one-dimensional mathematical model was constructed, the concentration distributions of cathode reactant gases (O₂ and gaseous H₂O) were calculated and polarization curves recorded during aging test were simulated based on the model, the simulated polarization curves compare well with the experimental results.