Characterization of a high performing passive direct formic acid fuel cell

Small fuel cells are considered likely replacements for batteries in portable power applications. In this paper, the performance of a passive air breathing direct formic acid fuel cell (DFAFC) at room temperature is reported. The passive fuel cell, with a palladium anode catalyst, produces an excellent cell performance at 30 °C. It produced a high open cell potential of 0.9 V with ambient air. It produced current densities of 139 and 336 mA cm⁻² at 0.72 and 0.53 V, respectively. Its maximum power density was 177 mW cm⁻² at 0.53 V. Our passive air breathing fuel cell runs successfully with formic acid concentration up to 10 and 12 M with little degradation in performance. In this paper, its constant voltage test at 0.72 V is also demonstrated using 10 M formic acid. Additionally, a reference electrode was used to determine distinct anode and cathode electrode performances for our passive air breathing DFAFC.     

Methanol conditioning for improved performance of formic acid fuel cells

This paper considers the effect of methanol pretreatment on the performance of a direct formic acid fuel cell (DFAFC). We find that conditioning of the cell in methanol results in a substantial increase in current. The current at 60 °C increases from 95 to 320 mA/cm² at 0.3 V. The maximum power density increases from 33 to 119 mW/cm². The cell resistance decreases from 0.37 to 0.32 Ω cm². CO stripping experiments show that the catalyst is not being greatly affected by these changes. Our interpretation of the data is that the anode layer of membrane electrolyte assembly (MEA) undergoes some change during the methanol conditioning. The change improves the performance.     

Unusually active palladium-based catalysts for the electrooxidation of formic acid

Formic acid fuel cells offer exciting prospects for powering portable electronic and MEMS devices. Pd-based catalysts further improve the performance of direct formic acid fuel cells while reducing catalyst costs over Pt-based catalysts. This study investigates several Pd-based catalysts, both unsupported and carbon-supported, and compares the electrochemical results with results obtained in an operating fuel cell. Power densities of up to 260 mW cm⁻² were achieved in a fuel cell at 750 mA operating at 30 °C. Carbon-supported catalysts and addition of other metals, such as gold, show potential in further improving the performance of Pd-based catalysts.     

A miniature air breathing direct formic acid fuel cell

Small fuel cells are considered likely replacements for batteries in portable power applications. In this paper, the performance of a $\text{2}\text{cm}\text{×2.4}\text{cm}\text{×1.4}$ cm passive miniature air breathing direct formic acid fuel cell (DFAFC) at room temperature is reported. The cell produced current density up to 250 mA/cm² and power density up to 33 mW/cm² at ambient conditions. The fuel cell runs successfully with formic acid concentration ranging from 1.8 and 10 M with little degradation in performance. These results show that passive fuel cells can compete with batteries in portable power applications.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

A study on composite PtRu(1:1)-PtSn(3:1) anode catalyst for PEMFC

Composite PtRu(1:1)/C-PtSn(3:1)/C catalyst layers with various geometries and loadings were designed for a proton exchange membrane fuel cell (PEMFC) anode to improve carbon monoxide (CO) tolerance of the conventional PtRu(1:1)/C catalyst. The idea was based on an experimental finding that the onset potential of the PtSn for CO oxidation was lower than that of the PtRu and the resultant expectation that there seemed to be a possibility of using the PtSn as a CO filter. The CO tolerance of the composite catalyst of each design was judged by the cell performance obtained through a single cell test using H₂/CO gases of various CO concentrations and compared to that of the PtRu/C catalyst. The highest CO tolerance among the composite catalysts tested in this study was obtained for the one with geometry of double layers in the order of PtRu/C and PtSn/C from the electrolyte layer and with respective PtRu and PtSn loadings of 0.25 and 0.12 mg cm⁻². The cell with this composite catalyst showed better performance than the one with the PtRu/C catalyst. When a H₂/100 ppmCO gas was used as the fuel in the single cell test, the cell voltages were measured to be 0.49 and 0.44 V at a current density of 500 mA cm⁻², respectively for the cell with the composite and PtRu/C catalyst.     

Improvement in high temperature proton exchange membrane fuel cells cathode performance with ammonium carbonate

Proton exchange membrane (PEM) fuel cells with optimized cathode structures can provide high performance at higher temperature (120 °C). A “pore-forming” material, ammonium carbonate, applied in the unsupported Pt cathode catalyst layer of a high temperature membrane electrode assembly enhanced the catalyst activity and minimized the mass-transport limitations. The ammonium carbonate amount and Nafion^® loading in the cathode were optimized for performance at two conditions: 80 °C cell temperature with 100% anode/75% cathode R.H. and 120 °C cell temperature with 35% anode/35% cathode R.H., both under ambient pressure. A cell with 20 wt.% ammonium carbonate and 20 wt.% Nafion^® operating at 80 °C and 120 °C presented the maximum cell performance. Hydrogen/air cell voltages at a current density of 400 mA cm⁻² using the Ionomem/UConn membrane as the electrolyte with a cathode platinum loading of 0.5 mg cm⁻² were 0.70 V and 0.57 V at the two conditions, respectively. This was a 19% cell voltage increase over a cathode without the “pore-forming” ammonium carbonate at the 120 °C operating condition.     

Development and demonstration of a higher temperature PEM fuel cell stack

Research and development was conducted on a proton exchange membrane (PEM) fuel cell stack to demonstrate the capabilities of Ionomem Corporation's composite membrane to operate at 120 °C and ambient pressure for on-site electrical power generation with useful waste heat. The membrane was a composite of polytetrafluoroethylene (PTFE), Nafion^®, and phosphotungstic acid. Studies were first performed on the membrane, cathode catalyst layer, and gas diffusion layer to improve performance in 25 cm², subscale cells. This technology was then scaled-up to a commercial 300 cm² size and evaluated in multi-cell stacks. The resulting stack obtained a performance near that of the subscale cells, 0.60 V at 400 mA cm⁻² at near 120 °C and ambient pressure with hydrogen and air reactants containing water at 35% relative humidity. The water used for cooling the stack resulted in available waste heat at 116 °C. The performance of the stack was verified. This was the first successful test of a higher-temperature, PEM, fuel-cell stack that did not use phosphoric acid electrolyte.     

New borohydride fuel cell with multiwalled carbon nanotubes as anode: A step towards increasing the power output

A borohydride fuel cell has been constructed using a platinized multiwalled carbon nanotube (MWCNT) anode and an air cathode having an anionic exchange membrane separating the anode and cathode. The MWCNT was functionalized with carboxylic acid under nitric acid reflux. Platinum metal was subsequently incorporated into it by galvanostatic deposition. The platinized functionalized MWCNT was characterized by thermogravimetric analysis, Fourier transform infrared spectrum, scanning electron microscope and X-ray diffraction. The fuel cell produced a voltage of 0.95 V at low currents and a maximum power density of 44 mW cm⁻² at room temperature in 10% sodium borohydride in a 4 M sodium hydroxide medium. Another borohydride fuel cell under identical conditions using carbon as the anode produced a cell voltage of 0.90 V and power density of about 20 mW cm⁻². The improved performance of the MWCNT is attributed to the higher effective surface area and catalytic activity.     

A 28-W portable direct borohydride–hydrogen peroxide fuel-cell stack

A 28-W direct borohydride–hydrogen peroxide fuel-cell stack operating at 25 °C is reported for contemporary portable applications. The fuel cell operates with the peak power-density of ca. 50 mW cm⁻² at 1 V. This performance is superior to the anticipated power-density of 9 mW cm⁻² for a methanol–hydrogen peroxide fuel cell. Taking the fuel efficiency of the sodium borohydride–hydrogen peroxide fuel cell as 24.5%, its specific energy is ca. 2 kWh kg⁻¹. High power-densities can be achieved in the sodium borohydride system because of its ability to provide a high concentration of reactants to the fuel cell.     

Low Pt loading high performance cathodes for PEM fuel cells

A simple direct mixing of carbon-supported catalysts with Nafion without adding any additional organic solvents was used to make electrodes for oxygen reduction in PEM fuel cells. For E-TEK 20% Pt/C, a Nafion content of 30% in the catalyst layer exhibited the best performance. Electrode dried from 90 to 150 °C showed little difference in performance. Highest power densities increased almost linearly with cell temperature, and values of 0.52, 0.60, 0.63, and 0.72 W/cm² were achieved at 35, 50, 60, and 75 °C, respectively, for a cathode with a Pt loading of 0.12 mg/cm² and operated using air at ambient pressure. A maximum performance was achieved with Pt loadings of 0.20±0.05 and 0.35±0.05 mg/cm² for 20 and 40% Pt/C, respectively, while the maximum performance using 40% Pt/C was only slightly better than that using 20% Pt/C. A Nafion/carbon sublayer with up to 30% Nafion content added between ELAT and the catalyst layer did not show any effect on performance.     

Surface modified Nafion® membrane by ion beam bombardment for fuel cell applications

The interfacial structure between an electrolyte membrane and an electrode catalyst layer plays an important role in determining performance of proton exchange membrane fuel cell (PEMFC) since the electrochemical reactions produce electricity occur on the interfaces that are in contact with hydrogen or oxygen gas, so-called three phase boundaries. To improve performance of the PEMFC by enlarging effective area of the interfaces, surface of Nafion^® 115 membrane was roughened by Ar⁺ ion beam bombardment before being coated with a catalyst ink to form the electrode layer. With increasing ion dose density from 0 to 1 × 10¹⁷ ions cm⁻², roughness and hydrophobicity of the membrane surface increased, which could be favored for a high-performance PEMFC. In fuel cell tests, the single cell using Nafion^® membrane bombarded at an ion dose density of 10¹⁶ ions cm⁻² exhibited maximum power density of 0.62 W cm⁻², which was two times higher than that of the single cell employing untreated Nafion^® 115 membrane, i.e. 0.30 W cm⁻².     

Methoxy methane (dimethyl ether) as an alternative fuel for direct fuel cells

The electrooxidation of methoxy methane (dimethyl ether) was studied at different Pt-based electrocatalysts in a standard three-electrode electrochemical cell. It was shown that alloying platinum with ruthenium or tin leads to shift the onset of the oxidation wave towards lower potentials. On the other hand, the maximum current density achieved was lower with a bimetallic catalyst compared to that obtained with a Pt catalyst. The direct oxidation of dimethoxy methane in a fuel cell was carried out with Pt/C, PtRu/C and PtSn/C catalysts. When Pt/C catalyst is used in the anode, it was shown that the pressure of the fuel and the temperature of the cell played important roles to enhance the fuel cell electrical performance. An increase of the pressure from 1 to 3 bar leads to multiply by two times the maximum achieved power density. An increase of the temperature from 90 to 110 °C has the same effect. When PtRu/C catalyst is used in the anode, it was shown that the electrical performance of the cell was only a little bit enhanced. The maximum power density only increased from 50 to 60 mW cm⁻² at 110 °C using a Pt/C anode and a Pt_0.8Ru_0.2/C anode, respectively. But, the maximum power density is achieved at lower current densities, i.e. higher cell voltages. The addition of ruthenium to platinum has other effect: it introduces a large potential drop at relatively low current densities. With the Pt_0.5Ru_0.5/C anode, it has not been possible to applied current densities higher than 20 mA cm⁻² under fuel cell operating conditions, whereas 250 and almost 400 mA cm⁻² were achieved with Pt_0.8Ru_0.2/C and Pt/C anodes. The Pt_0.9Sn_0.1/C anode leads to higher power densities at low current densities and to the same maximum power density as the Pt/C anode.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

Electrochemical oxidation of 1-methoxy-2-propanol in direct liquid fuel cells

Some interesting features have been observed when 1-methoxy-2-propanol was studied in direct liquid fuel cells. Air flow rate ranging from 180 to 920 ml/min had no effect on performance, but the performance increased largely when the cell temperature was increased from 40, to 60, and then to 80 °C. The open circuit voltage of the cell was around 0.70 V, which was 0.08–0.33 V higher than that when methanol was used. At low air flow rates, 1-methoxy-2-propanol performed much better than methanol in the entire current density region at 60 and 80 °C. At high air flow rates, methanol performed better than 1-methoxy-2-propanol at current densities higher than 100 mA/cm², but the latter performed better than the former at current densities less than ca. 50 mA/cm². The crossover current density of 1.0 M 1-methoxy-2-propanol through a Nafion^® 112 membrane was estimated electrochemically, and it was 25.6, 60.8 and 96.0 mA/cm² at cell temperatures of 40, 60, and 80 °C, respectively, measured at 0.90 V. These numbers were much smaller than those of methanol that, e.g. had a crossover current density of 232 mA/cm² at 0.9 V and 60 °C. One problem with using 1-methoxy-2-propanol as a fuel was that the cell anode seemed to be seriously poisoned by the oxidation intermediates at anode overpotentials lower than ca. 0.2 V.     

Dependence of high-temperature PEM fuel cell performance on Nafion® content

Operating a proton exchange membrane (PEM) fuel cell at elevated temperatures (above 100 °C) has significant advantages, such as reduced CO poisoning, increased reaction rates, faster heat rejection, easier and more efficient water management and more useful waste heat. Catalyst materials and membrane electrode assembly (MEA) structure must be considered to improve PEM fuel cell performance. As one of the most important electrode design parameters, Nafion^® content was optimized in the high-temperature electrodes in order to achieve high performance. The effect of Nafion^® content on the electrode performance in H₂/air or H₂/O₂ operation was studied under three different operation conditions (cell temperature (°C)/anode (%RH)/cathode (%RH)): 80/100/75, 100/70/70 and 120/35/35, all at atmospheric pressure. Different Nafion^® contents in the cathode catalyst layers, 15–40 wt%, were evaluated. For electrodes with 0.5 mg cm⁻² Pt loading, cell voltages of 0.70, 0.68 and 0.60 V at a current density of 400 mA cm⁻² were obtained at 35 wt% Nafion^® ionomer loading, when the cells were operated at the three test conditions, respectively. Cyclic voltammetry was conducted to evaluate the electrochemical surface area. The experimental polarization curves were analyzed by Tafel slope, catalyst activity and diffusion capability to determine the influence of the Nafion^® loading, mainly associated with the cathode.     

Effects of platinum loading on performance of proton-exchange membrane fuel cells using surface-modified Nafion® membranes

The interface between the electrolyte and electrode catalyst plays an important role in determining the performance of proton-exchange membrane fuel cells (PEMFCs) since the electrochemical reactions take place at the interface in contact with the reactant gases. To enhance catalyst activity by enlarging the interfacial area, the surface of a Nafion^® membrane is roughened by Ar⁺ ion beam bombardment that does not change the chemical structure of the membrane, as confirmed by FT-IR spectra. Among the membranes treated with ion dose densities of 0, 10¹⁵, 10¹⁶, 5 × 10¹⁶ and 10¹⁷ ions cm⁻² at ion energy of 1 keV, the membrane treated at ion dose density of 5 × 10¹⁶ ions cm⁻² exhibits the highest performance. Using the untreated and the treated membrane with 5 × 10¹⁶ ions cm⁻², the effects of platinum loading on cell performance are examined with Pt loadings of 01, 0.2, 0.3, 0.4 and 0.55 mg cm⁻². Except for a Pt loading of 0.55 mg cm⁻² where mass transport limits the cell performance, the single cell using a treated membrane gives a higher performance than that using an untreated membrane. This implies that the cell performance can be improved and the Pt loading can be reduced by ion beam bombardment.     

Transient phenomenon of step switching for current or voltage in PEMFC

The transient phenomenon of fuel cell with 5 cm² active area is investigated in this study by current density step increase and switching voltage under different conditions. It is found that there is an undershoot when the current density step increase is at the loading of 60% RH anode cathode, 3 stoic., 70 °C, 15 psi for automobile applications. The voltage is almost zero under 0.2 step increase to 1.0 A/cm² due to the H⁺ transport in membrane or H₂/O₂ in catalyst layer is almost used up. The undershoot phenomenon is more serious under gases stoichiometries of 3.0/3.0 when H₂ is fully humidified due to low gas concentration or flooding on the electrode. This phenomenon would induce the degradation of fuel cell components.     

Direct formic acid fuel cell portable power system for the operation of a laptop computer

A direct formic acid fuel cell (DFAFC) hybrid power system for a laptop computer has been developed at the Korea Institute of Science and Technology, Fuel Cell Research Center. At the heart of the system is a 15 MEA DFAFC stack capable of 30 W at 60 mW cm⁻². Stack characteristics relevant to integration into the power system such as concentration and orientation dependence, dynamic response, and long-term performance are elucidated and the resulting hybrid power system's performance is detailed. The stack's fast dynamic response eliminated the need for significant power buffering in the power conditioning equipment. The MEAs were found to give reduced but stable performance after 3 months of operation. The system is capable of an overall system efficiency of 0.23 (delivered power compared to theoretical power), and can operate under a substantial computing load for 2.5 h using a 280 mL tank of 50 wt.% fuel.     

Electrooxidation of ascorbic acid on polyaniline and its implications to fuel cells

l-Ascorbic acid (AA) has been shown to undergo oxidation on polyaniline (PANI) without a platinum-group catalyst. A direct ascorbic acid fuel cell (DAAFC) has been assembled by employing an anode coated with PANI catalyst. From the experimental studies using cyclic voltammetry, amperometry and IR spectroscopy, it has been concluded that PANI facilitates the oxidation of AA. It has been possible to achieve a maximum power density of 4.3 mW cm⁻² at a load current density of 15 mA cm⁻² at 70 °C. As both AA and PANI are inexpensive and environmental-friendly, the present findings are expected to be useful for the development of cost-effective DAAFCs for several low power applications.