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Direct formic acid oxidation for liquid-fed PEM fuel cells Published on February 2012 by Catherine Lepiller, PhD, for Pragma Industries With the ever growing power demand of portable electronic appliances such as cell phones, MP3 players, laptop computers and similar consumer products in the 0.5-100 W range, micro fuel cells, and in particular low temperature PEM micro fuel cells, appear as the best bet for the future due to a higher energy density than batteries. However, the storage and transfer of volatile hydrogen gas in solid systems for mobile use is difficult because of the relatively low volumetric and weight energy density of metal hydride compounds. In such context, hydrogen chemically stored in a primary organic liquid for on-demand in situ release, at room or near-room temperature and without carbon monoxide contamination, from a disposable or recyclable cartridge, seems to be a very valuable option for portable applications. Besides methane and methanol, renewable resources, such as (bio)ethanol and glycerol, have been considered as promising routes for hydrogen production. But all related reforming processes run at temperatures higher than 200°C, which renders their practical use problematic. This explains why the search for improved processes generating hydrogen from liquid fuels at higher reaction rates and under milder, well-controlled conditions, is an active area of interest in the fuel cell field. Among possible liquid fuel candidates, formic acid (HCOOH) definitely shows a great potential as in situ source of hydrogen for fuel cells. Compared with methanol for example, it offers higher energy density and faster electro-oxidation kinetics at room temperature; and not to be overlooked as well, it is non toxic and can be handled in aqueous media and stored safely. Formic acid is even found on the US Food Drug Administration list of food additives! The electro-oxidative decomposition of formic acid can follow two possible pathways, the first one yielding carbon monoxide and water (1), and the second one yielding carbon dioxide and hydrogen (2). Both reactions are dependent on the catalyst surface, formic acid concentration and temperature: HCOOH = CO + H2O (1)
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Reaction (1) leading to CO has to be minimized by a proper choice of selective catalyst and experimental conditions, while the reversible reaction of CO2 hydrogenation (2) is a potential way to produce hydrogen. |
| Figure 1. A CO2- neutral cycle for the storage of hydrogen in formic acid base adducts. | |
| It is only in recent years that the feasibility of generating hydrogen on demand from mixtures of formic acid and amines at low temperatures < 100°C was demonstrated [1,2]. Excellent catalytic activities over homogeneous ruthenium-based compounds were reported: for example, Loges et al. [1] investigated the decomposition of formic acid in excess of amine in the presence of different catalysts at 40°C. A mixture of acid formic HCOOH and triethanolamine NEt3 at a formic acid to nitrogen atom ratio of 5:2 was chosen as base case owing to its prior use as hydrogen source in transfer hydrogenation reactions. The influence of different types of amines was then explored using the best performing Ru(II) catalyst at the same ratio 5:2, and additional experiments showed some improvement with higher amine content in the solution. Activities were only slightly lower at 26.5°C compared to 40°C. Loges et al. were eventually the first to be able to generate hydrogen from formic acid-amine adducts at high rates at ambient temperature with a commercially available Ru(II) complex [formula RuCl2(PPh3)3 with Ph = aromatic group C6H6]. An easy gas cleaning by charcoal is then applied to remove traces of amine. Compared with previous hydrogen generating systems based on organic liquids, their process can be operated at much lower temperatures and hydrogen is fed directly to the fuel cell.
In a close field, a Swiss patent in 2008 by the Ecole Polytechnique Fédérale de Lausanne [2] provided a method for converting formic acid to hydrogen continuously by a homogeneous catalytic reaction in aqueous media at high reaction rates in the low-temperature range 30-180°C. During the process, hydrogen is produced at partial pressures up to 600 bar convenient for direct on-demand feeding of a variety of hydrogen consuming devices such as fuel cells, and free of any CO by the reaction (2). In the invention, the catalyst is a ruthenium-based complex (with general formula Ru(II)Ln) dissolved in acidic aqueous solution at high concentrations > 200 g/L. The solution is buffered at 2.5-4.5 by adding formate salt HCOO-,X+ [with X+ = inorganic cation: Na+, Ca2+, Li+, K+, NH4+], and in contrast to reference [1], there is no other acid or amine added to the reaction medium. The two gaseous products H2 and CO2 are readily separated from the solution, since the pressurized gases just evaporate once generated. Hydrogen is then physically separated from co-product CO2 by exploiting properties such as melting temperature, volatility and/or diffusion coefficient that strongly differ between the two gases (the exact method is not described in detail). The Ru(II) catalyst is easily separated from the reaction products as well, due to the high solubility of the catalyst in the liquid phase and hardly any solubility in the gaseous phase. This way, it is entirely recycled and keeps effective for prolonged time without degradation. The Swiss teamwork evaluated a range of highly water-soluble ligands L having at least one P atom (or a carbene group in certain embodiments), one aromatic group C6H6 and one hydrophilic group, which could satisfy to catalyst activity requirements toward formic acid oxidation under mild reaction conditions, and already at room temperature. Sulfonation of the aromatic group is an efficient way to increase water solubility of the Ru(II) complex. Like in reference [1], a commercial product was identified as sufficiently soluble and stable for producing hydrogen at proper rates and pressures: Ru(TPPTS)2X2 [with TPPTS = tris(3-sulfophenyl)phosphine; X = sulfonated anion: tosylate CF3SO2O− or triflate CH3C6H4SO2O−]. They also found that the rate and conversion efficiency of the oxidation reaction could be controlled by varying the ratio of formic acid to formate ion in the solution, and identified an optimum value of 9:1. Reaction rate directly correlates with temperature, therefore it is slower at room temperature; yet, high conversion efficiencies are already achieved and high-pressure gases >> 1 bar are still produced under these conditions because pressure correlates with conversion yield. In the final system, hydrogen would be directed to the device through a buffer tank, and a valve would allow tuning the entry pressure to match the amount of hydrogen used by the device. These works solved a number of drawbacks related to homogeneous catalytic reactions, like the separation of reactants and products, the use of organic solvents, ligands and additives that may lead to severe problems in scale-up. Nevertheless, the use of solid catalysts is becoming more like a better alternative than catalysts in solution: after a period where, despite the anticipated benefits in catalyst separation and processing, there was a consistent lack of such catalysts that were sufficiently active and/or selective for formic acid oxidation, research in the field finally took off. Initial studies using solid catalysts were all carried out at temperatures higher than 50°C: such conditions raised issues related to volatility of the formic acid/water mixture and to heat management, which both were detrimental to miniaturization. Besides, although some catalysts exhibited a promising initial activity, rapid deactivation and concomitant CO production from parasitic side-reaction (1) at the levels of hundreds of ppm were generally reported. Recently, palladium-based catalysts were identified to possess the highest activity toward formic acid oxidation among the transition metal series – including platinum –due to the optimum adsorption energy of hydrogen atoms on Pd surface. It was also demonstrated that the oxidation reaction mainly occurs through the direct pathway onto Pd instead of the dual pathway on Pt: thus, Pd-based catalysts are highly selective for reaction (2) and can overcome the CO poisoning effect. Following the “Pd line”, better performing catalysts are currently being found, as exemplified below.
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In a first example, Liang et al. [3] improved the conventional precipitation-reduction procedure used to synthesize Pd/C nanoparticles from aqueous solutions by adding a hydrophilic stabilizer BDPA (1,4-butylenediphosphonic acid) and adjusting the pH to 9 (Fig.2). Under these basic conditions, Pd nanoparticles are in oxidized and colloidal state (i.e. hydrated), and BDPA is in its anionic form: one phosphonate group per BDPA molecule adsorbs onto the Pd-O surface while the second group extending outside has a strong electrostatic repulsion effect between nanoparticles.In the consecutive step, the nanoparticles are reduced by NaBH4; finally, the adsorbed phosphonate anion is easily removed by a simple rinsing with NaOH 0.1 M, which is made possible by the weak Pd-O-P bond at high pH. |
| Figure 2. Formation mechanism of PD/C catalyst by the improved precipitation-reduction process. | |
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This procedure allows obtaining a high and stable dispersion of Pd/C nanoparticles with small particle size and very clean surface. Electrochemical performance confirmed that the activity of Pd/C nanoparticles prepared by the BDPA-based method was four times greater than that of Pd/C-NaBH4 prepared by the standard method. However, the initial decay of formic acid oxidation current at 0.15 V at room temperature is indicative of some kind of surface poisoning that must be looked for. In a further effort, researchers at the University of Oxford prepared by wet chemical synthesis various engineered core-shell nanoparticles having an inner core of a metal element and an external shell of palladium [4]. As it has been widely adopted in research for fuel cell oxygen reduction catalysts, the strategy behind the bi-metallic core-shell structure is to retain the desirable Pd surface properties and play with the interacting effects of a particle core of a different metal. Moreover, this configuration, which spares most of the expensive noble metal Pd, is quite attractive from an economical point of view. |
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Among all metals tested, the highest activity toward formic acid oxidation at room temperature was found for Ag@Pd nanoparticles (diameter 8 nm) with the thinnest continuous Pd shell (1-2 atomic layers)Turnover frequencies per surface Pd site were comparable to homogeneous catalysts: 125 h-1 at 20°C and 252 h-1 at 50°C.At 20°C, an equimolar mixture of hydrogen and CO2 was continuously produced without any trace of CO; on the other hand, CO was detected at temperatures higher than 50°C. In contrast to reference [3], no deactivation effects were observed during the time of experiments, and the rate closely followed the first order kinetics of formic acid decomposition by reaction (2)Furthermore, electrochemical experiments were in excellent agreement with theoretical calculations showing a strong correlation between the catalytic activity and the work function of the metal core: the largest net difference with the work function of the Pd shell will lead to the highest adsorption energy by charge transfer from the core to the shell, hence to the best possible activity of the resulting bi-metallic structure for formic acid electro-oxidation (Fig.3). The very short range of this so-called "ligand" electronic effect between the two metals explains why the highest performance is achieved for the thinnest Pd layer. Nanomaterials interface definitely plays a key role in catalysis. |
| Figure 3. Plot of rates of formic acid decomposition monitored by 13C NMR over different metal colloid catalysts. Correlation with the work function of the M core, where M = face-centred cubic (111) Ag, Rh, Au, Ru and Pt or hexagonal close-packed (0001) Ru, Ag, with the largest difference in work function in relation to Pd, gives the strongest electron promotion to the Pd shell. | |
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In the light of previous results, engineered bi-metallic nanoparticles like the ones presented here appear as a very valuable option yielding convenient rates of pure hydrogen gas production from formic acid oxidation at room temperature. Based on the values of reference [4], it was roughly determined that 10-100 mg of catalyst would be enough to supply hydrogen for micro fuel cell operation. Would be current encouraging achievements confirmed in the future, they would no doubt boost efforts in the microfluidic fuel cell field… Coming next! [1] Controlled generation of hydrogen from formic acid amine adducts at room temperature and application in H2/O2 fuel cells. B. Loges, A. Boddien, H. Junge and M. Beller, Angew. Chem. Int. Ed., 47 (2008), 3962-3965. |
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