Fuel crossover in direct formic acid fuel cells

This work studies formic acid crossover in a direct formic acid fuel cell under different operating conditions. Comparison with methanol crossover in a direct methanol fuel cell is included. The effects of cell conditioning, temperature, fuel concentration, and Nafion^® thickness on the rate of formic acid crossover are examined. The effects of temperature, concentration, and membrane thickness are qualitatively similar to those with methanol, but overall crossover rates are much lower. Under the same operating conditions, it is found that formic acid has a crossover flux rate that is approximately one-sixth that of methanol. Measurement of the CO₂ membrane permeation flux is performed to determine its contribution to a bias that it might cause in the quantification of crossover.     

A parametric study of the direct formic acid fuel cell (DFAFC) performance and fuel crossover

The effect of formic acid concentration (2–20 M), operating temperature (30–70 °C), and relative humidity (RH 40–90%) on the direct formic acid fuel cell (DFAFC) performance and fuel crossover were studied. In addition, air and oxygen were used to investigate the effect of oxidant flow rate on DFAFC performance and fuel crossover by operating the DFAFC under three modes of reactant supply: passive, semi passive (oxidant supplied), and active (both oxidant and fuel supplied). Fuel crossover was determined by measuring the percentage of exhausted carbon dioxide (CO₂) at the cathode using a CO₂ analyzer, from which the equivalent formic acid crossover flux was calculated. The results indicate that the DFAFC performance and fuel crossover were affected by formic acid concentration, temperature, humidity, oxidant flow rate, and the mode of operation. Optimums of these operating parameters were established for obtaining high performance of the DFAFC. The relationships between these parameters and the performance and fuel crossover of the DFAFC are discussed in this paper.     

Catalytic reduction of CO2 by Al–Sn-CNTs composite for synthesizing formic acid

Electrochemical reduction and photocatalytic reduction of CO₂ have attracted more and more attention, but they also face the problem of low utilization efficiency of electricity and solar energy. In this study, a new strategy of applying a novel Al–Sn-CNTs composite in electrochemical corrosion process was proposed to reduce CO₂ without additional electricity and light. In the Al–Sn-CNTs/CO₂ system, micro galvanic cell with Al as anode and Sn or CNTs as cathode was formed, and CO₂ was reduced to formic acid on the cathode surface. The cumulative formic acid concentration of the Al–Sn-CNTs/CO₂ system achieved 21.18 mg/L within 60 min under the conditions of initial pH 9.0, Cl⁻ concentration 10 mmol/L, and Al–Sn-CNTs composite dosage 2 g/L. Based on the morphology, crystal structure and electrochemical test results of the Al–Sn-CNTs composite, a possible mechanism of CO₂ reduction to formic acid in the Al–Sn-CNTs/CO₂ system was proposed.     

Direct formic acid fuel cells

The performance of formic acid fuel oxidation on a solid PEM fuel cell at 60 °C is reported. We find that formic acid is an excellent fuel for a fuel cell. A model cell, using a proprietary anode catalyst produced currents up to 134 mA/cm² and power outputs up to 48.8 mW/cm². Open circuit potentials (OCPs) are about 0.72 V. The fuel cell runs successfully over formic acid concentrations between 5 and 20 M with little crossover or degradation in performance. The anodic polarization potential of formic acid is approximately 0.1 V lower than that for methanol on a standard Pt/Ru catalyst. These results show that formic acid fuel cells are attractive alternatives for small portable fuel cell applications.     

PdBi alloy nanoparticle-enhanced catalytic activity toward formic acid oxidation

Improved performance and reduced costs are crucial to develop catalysts for direct formic acid fuel cells. In this study, PdBi alloy nanoparticles were synthesized using a facile seed-mediated growth method. The as-synthesized Pd₁Bi₁ alloy nanoparticles exhibited a large electrochemical surface area (46.3 m² g⁻¹) and a high mass activity (1.44 A mg⁻¹), which was 1.19- and 4.8-fold higher than that of commercial Pd/C catalysts, respectively. The PdBi alloy nanoparticle is a promising catalyst for direct formic acid fuel cells.     

Extracting high-purity hydrogen via sodium looping-based formic acid dehydrogenation

Formic acid (FA) has been considered as a prospective hydrogen carrier for its potentials to realize hydrogen storage, transportation, and in-situ supply under mild conditions. However, the application of FA dehydrogenation is limited by its unsatisfactory hydrogen concentration and carbon monoxide selectivity. Herein, a sodium looping-based (Na₂CO₃?NaHCO₃) formic acid dehydrogenation (SLFAD) system is proposed for high-purity hydrogen production with ultra-low CO generation via the Na₂CO₃?NaHCO₃ looping. The SLFAD system consists of three parts, which are FA dehydrogenation reactor (FADR), sorption-enhanced carbon oxide removal reactor (CORR), and sodium-based sorbent regeneration reactor (SSRR). Experimental results proved that no sodium formate and sodium oxalate was formed under NaHCO₃ reduction by H₂. A comprehensive assessment of the system was carried out to preliminary verify the feasibility and optimize the operation parameters of the SLFAD system. Results indicated that a maximum hydrogen concentration of 97.905 vol%, a minimum CO concentration of 11.97 ppm, and a high hydrogen production rate of 0.99989 kmol H₂ h^?1 can be obtained under the conditions of atmospheric pressure, FADR temperature at 80 °C, H₂O/HCOOH = 1.2, CORR temperature at 80 °C, and Na₂CO₃/HCOOH = 1.0.     

Ammonia as a hydrogen energy carrier

The gravimetric H₂ densities and the heats of combustion of tanks stored ammonia (ammonia storage tanks) were similar to those of the liquid H₂ tanks at the weight of 20–30ton, although the gravimetric H₂ density of liquid H₂ is 100 wt%. The volumetric H₂ densities and the heats of combustion of ammonia storage tanks were about 2 times higher than those of liquid H₂ tanks at 1–4 × 10⁴ m³. Gray ammonia is synthesized from hydrogen through process known as steam methane reforming, nitrogen separated from air and Haber-Bosch process. Blue ammonia is the same as gray ammonia, but with CO₂ emissions captured and stored. Green ammonia is produced by reacting hydrogen produced by electrolysis of water and nitrogen separated from air with Haber-Bosch process using renewable energies. The energy efficiencies of gray, blue and green ammonia were better than those of liquid hydrogen and methylcyclohexane (MCH) with high H₂ density and similar to the efficiency of H₂ gas. The energy efficiencies of ammonia decreased in the order, gray ammonia > blue ammonia > green ammonia. The production costs of green hydrogen energy carried increased in the order, ammonia < liquid H₂<MCH. The amounts of energy consumption by N₂ production and Haber-Bosch process were below 10% compared with the value of H₂ production from water electrolysis.     

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO₂ and H_2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.     

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.     

Activity of Pd/C for hydrogen generation in aqueous formic acid solution

A commercial Pd/C catalyst was found to exhibit high activity for formic acid (HCOOH) decomposition into CO₂ and H₂ in aqueous solution at near ambient temperatures. The performance of the catalyst toward HCOOH decomposition in aqueous solution was investigated in a batch reactor at temperatures between 21 and 60 °C and HCOOH concentrations between 1.33 and 5.33 M. The apparent activation energy of the overall reaction for the production of H₂ from aqueous HCOOH was determined to be 53.7 kJ/mol on the heterogeneous Pd/C catalyst. This is in good agreement with the previously reported theoretical energy barrier (∼52 kJ/mol) for H₂ evolution on a Pd surface. Under the present experimental conditions, the catalyst lost activity continuously over time and the apparent deactivation energy was estimated to be 39.2 kJ/mol. Furthermore, the deactivated and spent catalyst was studied by temperature-programmed desorption experiments to reveal the possible species that caused the loss of the activity. Combining the results of our previous DFT calculations and the present experimental results, elementary steps of HCOOH decomposition on Pd in aqueous solution were proposed and discussed.     

A miniature direct formic acid fuel cell battery

This work describes miniature formic acid fuel cell batteries, which are built based on a Nafion^® membrane and thin metal foils. The intrinsic advantages of formic acid fuel allow for a very simple design of the fuel cell, and the volume of the complete system, including fuel reservoir, can be as small as 11 mm³. This work examines the effect of membrane thicknesses and fuel concentrations on the cell performance. The optimized cell performance is obtained with N117 membrane and 12 M fuel. Peak power density of the optimized cell is 112 mW cm⁻². Life tests are conducted at various conditions using 6 μL of fuel. An energy density of 70 Wh L⁻¹ with 40% fuel utilization rate is observed when 12 M formic acid is used at 0.5 V.     

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.     

High performance Pd-based catalysts for oxidation of formic acid

Two novel catalysts for anode oxidation of formic acid, Pd₂Co/C and Pd₄Co₂Ir/C, were prepared by an organic colloid method with sodium citrate as a complexing agent. These two catalysts showed better performance towards the anodic oxidation of formic acid than Pd/C catalyst and commercial Pt/C catalyst. Compared with Pd/C catalyst, potentials of the anodic peak of formic acid at the Pd₂Co/C and Pd₄Co₂Ir/C catalyst electrodes shifted towards negative value by 140 and 50 mV, respectively, meanwhile showed higher current densities. At potential of 0.05 V (vs. SCE), the current density for Pd₄Co₂Ir/C catalyst is as high as up to 13.7 mA cm⁻², which is twice of that for Pd/C catalyst, and six times of that for commercial Pt/C catalyst. The alloy catalysts were nanostructured with a diameter of ca. 3–5 nm and well dispersed on carbon according to X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The composition of alloy catalysts was analyzed by energy dispersive X-ray analysis (EDX). Pd₄Co₂Ir/C catalyst showed the highest activity and best stability making it the best potential candidate for application in a direct formic acid fuel cell (DFAFC).     

Performance of direct carbon fuel cell

A direct carbon fuel cell (DCFC) is a variation of the molten carbonate fuel cell (MCFC) which converts the chemical energy of carbon directly into electrical energy. Thus, the energy conversion efficiency is very high and correspondingly CO₂ emission is very low for given power output. DCFC as a high temperature fuel cell performs better at elevated temperatures (>800 °C) but because of the corrosive nature of the molten carbonates at elevated temperatures the degradation of cell components becomes an issue when DCFC is operated for an extended period of time.We explored the DCFC performance at lower temperatures (at 700 °C and less) using different sources of carbon, different compositions of electrolytes and some additives on the cathode surface to increase catalytic activity. Experiments showed that with petroleum coke as a fuel at low temperatures the ternary eutectic (43.4 mol % Li₂CO₃ - 31.2 mol% Na₂CO₃ - 25.4 mol % K₂CO₃) spiked by 20 wt % Cs₂CO₃ performed better than any binary or ternary eutectics described in the published work by other researchers. Maximum power output achieved at 700 °C was 49 mW/cm² at a current density of 78 mA/cm² when modified cathode was fed with O₂/CO₂ gases.     

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.     

Solvent effects on high-pressure hydrogen gas generation by dehydrogenation of formic acid using ruthenium complexes

High-pressure H₂ was produced by the selective dehydrogenation of formic acid (DFA) using ruthenium complexes at mild temperatures in various organic solvents and water. Among the solvents studied, 1,4-dioxane was the best candidate for this reaction to generate high gas pressure of 20 MPa at 80 °C using the Ru complex having a dearomatized pyridine-based pincer PN³P* ligand. This complex shows reusability for the high-pressure DFA in 1,4-dioxiane while maintaining the catalytic performance, however, deactivation occurred in other solvents. In dimethyl sulfoxide, its decomposition products may cause catalytic deactivation. The gas pressure generated in 1,4-dioxane was lower than that in water due to the high dissolution of 1,4-dioxane into CO₂ according the vapor-liquid equilibrium calculations. The role of solvent is crucial since it affected the catalytic performance and also the generated gas pressure (H₂ and CO₂) from FA.     

High activity of Pd-WO3/C catalyst as anodic catalyst for direct formic acid fuel cell

Pd nanoparticles supported on the WO₃/C hybrid are prepared by a two-step procedure and the catalysts are studied for the electrooxidation of formic acid. For the purpose of comparison, phosphotungstic acid (PWA) and sodium tungstate are used as the precursor of WO₃. Both the Pd-WO₃/C catalysts have much higher catalytic activity for the electrooxidation of formic acid than the Pd/C catalyst. The Pd-WO₃/C catalyst prepared from PWA shows the best catalytic activity and stability for formic acid oxidation; it also shows the maximum power density of approximately 7.6 mW cm⁻² when tested with a small single passive fuel cell. The increase of electrocatalytic activity and stability is ascribed to the interaction between the Pd and WO₃, which promotes the oxidation of formic acid in the direct pathway. The precursors used for the preparation of the WO₃/C hybrid support have a great effect on the performance of the Pd-WO₃/C catalyst. The WO₃/C hybrid support prepared from PWA is beneficial to the dispersion of Pd nanoparticles, and the catalyst has potential application for direct formic acid fuel cell.     

Facile synthesis of PtAu alloy nanoparticles with high activity for formic acid oxidation

We report the facile synthesis of carbon supported PtAu alloy nanoparticles with high electrocatalytic activity as anode catalysts for direct formic acid fuel cells (DFAFCs). PtAu alloy nanoparticles are prepared by co-reducing HAuCl₄ and H₂PtCl₆ with NaBH₄ in the presence of sodium citrate and then deposited on Vulcan XC-72R carbon support (PtAu/C). The obtained catalysts are characterized with X-ray diffraction (XRD) and transmission electron microscope (TEM), which reveal the formation of PtAu alloy nanoparticles with an average diameter of 4.6 nm. Electrochemical measurements show that PtAu/C has seven times higher catalytic activity towards formic acid oxidation than Pt/C. This significantly enhanced activity of PtAu/C catalyst can be attributed to noncontinuous Pt sites formed in the presence of the neighbored Au sites, which promotes direct oxidation of formic acid.     

Hydrogen production via electrolysis of aqueous formic acid solutions

Hydrogen was produced via electrolysis of aqueous formic acid solutions, and the effects of the concentrations of formic acid and NaOH on the electrolytic voltage were systematically investigated. The voltage is found to be related to the actual formic acid concentration. When the actual formic acid concentration is higher than 0.8 × 10⁻⁹ M, the initial electrolytic voltage can be as low as 0.30 V, which is much lower than the open circuit voltage in a proton exchange membrane fuel cell. The electrolytic voltage increases with the increase of the current density. Specifically at 1.0 M NaOH and 4.0 M HCOOH, the steady voltage value increases from 0.62 to 0.70 V as the current density increases from 1.0 to 6.0 mA/cm². At 3.0 M HCOOH and 2.5 M NaOH, the hydrogen production rate is 53 μmol/h under 8.0 mA/cm², which is promising for practical industrial-scale hydrogen production.