Hydrogen Storage in Formic Acid: A Comparison of Process Options

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Hydrogen storage in formic acid: A comparison of process options Karsten Müller, Kriston P. Brooks, and Tom Autrey Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02997 • Publication Date (Web): 04 Nov 2017 Downloaded from http://pubs.acs.org on November 5, 2017

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Hydrogen storage in formic acid: A comparison of process options Karsten Müller1,2,*, Kriston Brooks1, Tom Autrey1 1

Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, United States

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Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute of Separation Science and Technology, Egerlandstr. 3, 91058 Erlangen, Germany

*To whom correspondence should be addressed: Karsten Müller ([email protected], Tel: +49 9131 8527455) KEYWORDS: formic acid, hydrogen storage, system analysis, process evaluation, energy balance

ABSTRACT Formic acid (53 g H2/liter) is a promising liquid storage and delivery option for hydrogen for fuel cell power applications. In this work we compare and evaluate several process options using formic acid for energy storage. Each process requires different steps, which contribute to the overall energy demand. The first step, i.e. production of formic acid, is thermodynamically unfavorable. However, the energy demand can be reduced if a formate salt is produced via a bicarbonate route instead of forming the free acid from hydrogen and carbon dioxide. This bicarbonate / formate approach also turns out to be comparatively more efficient in terms of hydrogen release than the formic acid route. Even though less energy efficient, catalytic decomposition of formic acid has the advantage of reaching higher volumetric power densities during hydrogen release. Efficiencies of all process options involve aqueous media and are dependent on concentration. Heating water leads to additional energy demand for hydrogen release and thus lowers the overall efficiency. Separation and purification of hydrogen contributes a minor impact to the overall energy demand. However, its effect on efficiency is not negligible. Other process options like thermal decomposition of formic acid or direct formic acid fuel cells thus far do not appear competitive.

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Introduction To overcome the low volumetric storage density of molecular hydrogen, a variety of concepts have been proposed. Potential options are e.g. the physi-sorption on high surface area materials such as metal organic frameworks1 or zeolite templated carbons2. However, this approach requires low temperatures for storage. Storage at ambient temperature can be realized with approaches based on chemi-sorption. Examples for such systems are complex hydrides3 and liquid organic hydrogen carriers4. Chemisorption on the other hand requires high temperature for hydrogen release. Alternative options could be the conversion into fuels like methane5 or methanol6 or mixing hydrogen with methane and feeding it into the natural gas grid7. An overview of different storage materials is given in a number of review articles.8-10 A particularly interesting carrier material for hydrogen that has received recent attention is formic acid (FA).11-14 Hydrogen release from FA is exergonic, but not exothermic. This thermodynamic property leads to a couple of distinct advantages; H2 can be released from the liquid carrier at (i) low temperature, e.g., T < 80 °C15, and (ii) high H2 pressure (P > 600 bar, has been reported16-18). FA (53 g H2/liter), with a formal energy density of 6.41 MJth L-1 (1.76 kWh L-1), may not be capable of meeting the DOE ultimate targets for H2 storage on board vehicles. Nevertheless, interesting projects, like team FAST in the Netherlands19, are currently aiming at the application of FA as a fuel for vehicles. Furthermore, liquid FA may provide an alternative option for long term energy storage, and as a liquid, may provide a means of H2 transport and delivery to H2 refueling stations using existing infrastructure. The first scientific description of properties of FA dates back to the year 1670.20 Today more than 600,000 Mt of FA are produced each year with the main applications in silage and animal feeding, the leather and textile industry, and for pharmaceuticals and food additives.21 FA’s utilization for hydrogen storage has come into the focus of different researchers in recent years. The focus of nearly all these publications is on catalysis. Most of the work describes the catalytic release of hydrogen,16, 22-25 while the challenging hydrogenation of carbon dioxide to FA is the subject of fewer studies26-29. A recent review by Sordakis et al.30 tries to address both aspects. Nonetheless, there are quite a number of potential process routes (Figure 1) and only very limited research so far on the comparative evaluation of the different approaches. Pérez-Fortes et al.31 studied economic and environmental aspects of one of these options in comparison to conventional FA production. Álvarez et al.32 presented a comparison of FA to other potential


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hydrogenation products of CO2 like methanol or dimethyl ether, focusing mainly on catalysis. One of the authors of this work previously published a study on a comparison of process options for the hydrogenation of carbon dioxide to FA.33 However, to the best of our knowledge no comprehensive thermodynamic comparison of the different process options has been published. This is critical information that is necessary to have to provide insight into the possibility of using FA as a low pressure delivery approach. To this end, the aim of this work is to present an evaluation of the different process options for FA and formate based hydrogen storage, i.e., production of FA, recovery of H2 from FA and purification of hydrogen.

Figure 1: Process options for FA based hydrogen storage

Process options overview First, an overview on the different options for FA based hydrogen storage shall be given. Carbon dioxide based options FA can be decomposed catalytically into hydrogen and carbon dioxide: HCOOH ⇋ H2 + CO2

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This reaction is reversible; however, the thermodynamics to make FA from carbon dioxide and H2 are unfavorable (∆Rg = +31 kJ mol-1). Hence, the direct formation of FA by hydrogenation of carbon dioxide is limited by its reaction equilibrium. Fink et al.34 pointed out, that the solvent can have a significant effect on the position of this equilibrium. However, pressures far above 10 MPa are required anyway to even reach conversions in the range of a few percent. To overcome the challenge of direct synthesis the reaction is performed in the presence of an amine.35,

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The amine reacts with FA shifting the equilibrium towards higher conversion. 29

Moret et al.

performed the reaction in the presence of dimethyl sulphoxide at 20 MPa to

enhance conversion (producing a 1.9 M solution) and avoid the formation of a FA-amine complex. Nevertheless, FA remains dilute in all process variants based on hydrogenation of carbon dioxide. As a consequence, energy demand for isolating pure FA from a dilute solution is high. It has been suggested to use a modification of the current state of the art technology for FA production.33 Currently FA is produced in a two-step process; in the first step, methyl formate is produced from methanol and carbon monoxide, followed by a second step where the methyl formate is hydrolyzed into FA and methanol.21 This process could be modified to make FA for hydrogen storage if carbon monoxide is produced by a reversed water-gas shift reaction. On the level of the substance balance for the overall process, it is still a conversion of hydrogen and carbon dioxide to FA, however, the thermodynamically unfavorable direct reaction is avoided. There are a number of process options for FA based energy storage not involving free hydrogen. An option for FA production could be driving the reaction electrochemically (with water and carbon dioxide as reactants). It has been reported that FA can be formed on a cathode with a suitable catalyst (e.g. tin) with good faradic efficiency.37, 38 In this approach high-grade energy (i.e. electricity) is utilized to push the unfavorable reaction. On the other hand, electricity for a separate electrolysis of water is saved. A clear disadvantage is the fact that FA is only produced in highly diluted form. There are several means of recovering the energy stored in the FA. The most straightforward means of recovering the hydrogen from FA can be done by reversing the synthesis of the FA and producing H2 and CO2. The unfavorable thermodynamics of the synthesis become beneficial when it comes to decomposition. Hence, liberating the hydrogen is possible at ambient temperature or slightly above. A number of research groups have studied heterogeneous25, 39-41 as well as homogeneous42-45 catalysts to release H2 from dilute aqueous formic acid solutions. An


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advantage of this technology is the possibility to generate hydrogen at elevated pressures, potentially reducing energy demand for subsequent physical compression. Another means of decomposing FA in aqueous solution into hydrogen and carbon dioxide is a biological route. The respective biological process has been known for over a century46, but has only been the subject of few more recent research47. An alternative option, that completely avoids any gas phase hydrogen, is feeding FA directly into a fuel cell.48 This technology is still at an early stage of development, but is notable, because no energy input for releasing hydrogen is required. The theoretical open current potential is 1.48 V, but actual values observed so far are only about 0.8 to 0.9 V.49, 50 Bicarbonate / formate approach A disadvantage of all the variants described above is the dependence on the thermodynamically unfavorable reduction of carbon dioxide. To synthesize FA pure carbon dioxide must be provided and upon hydrogen release carbon dioxide has to be separated to produce pure hydrogen. For a closed cycle, carbon dioxide has to be stored, which reduces the high storage density achieved using FA. An alternate approach to store H2 that avoids the handling of gaseous CO2 is the bicarbonate / formate system: MHCOO + H2O ⇋ MHCO3 + H2

with M = Na, K

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The FA / carbon dioxide + hydrogen cycle is replaced by an alkali formate / bicarbonate salt cycle, eq 2. An additional advantage is more favorable reaction thermodynamics. The Gibbs free energy of the reaction is close to zero, which allows for easy shifting equilibrium between reactant and product side with moderate changes in pressure and temperature. The reaction is performed in aqueous solution and it is notable that H2O is the source of H2. Concentrated solutions are limited by the lower solubility of the bicarbonate salt, formed upon H2 release, rather than by the solubility of the formate salt. The potassium salts exhibit a higher solubility than the respective sodium salts. A further increase could be achieved by utilizing the cesium salts.51 Cesium formate / bicarbonate have the highest solubility in water among all alkali metal formates / bicarbonates. Nevertheless, the much higher price and limited availability is a clear drawback. Several research groups have studied heterogeneous52-54 as well as homogeneous55, 56 catalysts for this reversible reaction. An interesting feature of this formate/bicarbonate process is


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the fact that conversion of bicarbonate to formate does not necessarily require hydrogen as reducing agent, but carbon monoxide could be used as well. Hence, synthesis gas can be applied directly without previous shift of CO to hydrogen.57 Other options An approach that is applied today for producing FA on an industrial scale is acidolysis of formate salts.21 In this process FA is liberated from sodium or potassium formate, which are byproducts of other chemical processes. Still, the process is limited by the availability of the respective formate salt as a byproduct of other processes. Therefore, this is not really an option for FA as a hydrogen storage and delivery application, since all FA produced this way currently is consumed in other industries. An interesting, renewable source for FA could be biomass. Jin et al.58 demonstrated conversion of carbohydrates to FA with H2O2 as oxidant. The group of Albert and Wasserscheid59-61 was able to produce FA with high yields from different types of biomass with O2 as oxidant applying a homogeneous polyoxometalate catalyst. Recovery of the energy in FA does not necessarily require release of hydrogen. In most concepts proposed so far FA is decomposed catalytically in aqueous solution. If the reaction is performed at elevated temperatures in the gas phase (without water) FA decomposes to carbon monoxide and water: HCOOH(gas) ⇋ H2O(gas) + CO(gas)

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Usually CO is considered an undesired byproduct. However, if repowering options are not restricted to fuel cells, this is not necessarily the case. CO could be fed into an internal combustion engine (ICE). Some modifications on existing ICEs would be needed, but principally it could also run on CO. Selectivity would not be a crucial parameter, because some hydrogen and carbon dioxide in the CO stream will not be detrimental to an ICE. This is important since selectivity for CO in the gas phase reaction is not likely to exceed about 90 %.62 This can be attributed to the fact that water acts as a catalyst for the dehydrogenation of FA.63 Consequently, the thermal decomposition to CO and water leads to some subsequent dehydrogenation of FA. (Partial) removal of water from CO might be advisable, before injection to the ICE. An ICE run with a sufficient excess of oxygen and equipped with a post combustion catalyst could burn the reaction mixture without releasing problematic amounts of carbon monoxide. To our knowledge,


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there are no evaluations available so far on the hazard management of ICEs run with CO. However, if the volume and pressure of the CO-containing gas phase are kept small the risk caused by such a system should be manageable. All processes for recovering energy from FA produce water and CO2 in the end, corresponding to the combustion of FA as the overall reaction. Continuing this thought further, one could also think of directly combusting FA (e.g. in an ICE). However, this is not likely to be a benefit since the advantage of FA as a hydrogen carrier is not related to its production, but its easy decomposition. Hence, this option is not considered in the further evaluation, but has been mentioned only for the sake of completeness. Purification Hydrogen (or carbon monoxide) derived from FA should be purified before repowering. It is well known that hydrogen fed to a fuel cell needs to be free of CO. It has been outlined by many authors that catalytic FA dehydrogenation yields CO-free hydrogen. Nonetheless, it should be taken into account that CO2 is converted to a CO-like species on the surface of the fuel cell catalyst via the reverse water-gas shift reaction, thus poisoning the catalyst. The effect of CO (and thus CO2) on fuel cells is reversible17, 64 and alloying the fuel cell catalyst with ruthenium might suppress the reverse water-gas shift reaction65. However, removal of CO2 (or at least reduction of the CO2 concentration) is still highly advisable and even required to meet the SAE standard. If FA is produced in a process involving amine, even traces of amine must also be removed,66 since amines can do permanent damage to a PEM fuel cell. A disposable activated carbon cartridge should be sufficient to remove residual traces of amines remaining in the hydrogen after CO2 separation. Four common options are available for CO2 separation: adsorption, cryogenic separation, absorption and membranes. The adsorption option can be further distinguished with respect to the means of regeneration: Pressure Swing Adsorption (PSA) and Temperature Swing Adsorption (TSA). In all cases at least two adsorber beds are used in a cyclic procedure. One adsorber bed is brought into contact with the gas stream. Carbon dioxide adsorbs preferably on the surface of the adsorbent, while adsorption of hydrogen is rather weak. After a while the gas stream is switched to another adsorber bed and the first bed is regenerated by a decrease in


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pressure, a strip gas, an increase in temperature, or a combination of these. An adsorber material often applied in CO2 removal is zeolite 13X67. Cryogenic removal of CO2 requires low temperatures. Absorption therefore appears to be the more meaningful alternative to adsorption. Absorption is usually done in form of amine scrubbing (or in a similar process using other solvents such as Ionic Liquids68). Carbon dioxide is dissolved in aqueous solutions of e.g. monoethanolamine. Regeneration can be done by the same means as in case of adsorption. If amine scrubbing is applied for hydrogen purification it should be ensured that no amine contamination of the fuel cell occurs. Membrane separation is a further option for CO2 removal. Membranes are often used in biogas treatment. In this case, CO2 permeates through the membrane leaving enriched methane. When purifying hydrogen the situation is different due to its high diffusivity. Hence, hydrogen must be the permeating compound. Membranes suited for this task could be palladium or palladium alloy membranes.69 The permeating compound exhibits a strong pressure drop. If the permeating gas is supposed to be discarded (like CO2 removed from biogas), this pressure drop does not matter. However, hydrogen is the valuable compound and its pressure drop should be avoided. Hence, membrane processes do not appear to be an attractive option for this task. Purification options in case of thermal decomposition to explicitly yield carbon monoxide, instead of hydrogen, are similar. Water could be removed by adsorption as well as condensation. An absorption process is not advisable for drying of carbon monoxide. This study will focus on the purification of hydrogen. Readers interested in the drying of gas streams are referred to the respective literature for more detail.70

Energy balance The energy balances of the options described above are discussed in the following section. The detailed assumptions along with sensitivity analyses can be found in the supporting information. FA production The production of FA from hydrogen and carbon dioxide has been studied previously by one of the authors of this work.33 It has been demonstrated that the modified methyl formate based twostep synthesis currently used for FA production is also the most efficient process for producing FA from hydrogen and carbon dioxide (in this case CO2 is converted in a reversed water-gas


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shift reaction to CO). The energy demand was estimated to about 560 kJ mol-1FA (compared to over 1000 kJ mol-1FA for the direct reaction of CO2 and H2 without additional reactant or in the presence of an amine; it was shown that yields far higher than reported so far might make them competitive). However, only direct catalytic conversion options of CO2 and H2 to FA have been evaluated so far. This work aims to enhance the analysis to include (i) electrochemical FA production and (ii) the bicarbonate / formate approach. Electrochemical synthesis The energy demand of electrochemical FA production is mainly determined by two factors: the applied voltage and the faradic efficiency . Voltage can be viewed as a measure for the energy demand for the transfer of each electron. Faradic efficiency describes which share of the electrons actually leads to the formation of FA. In addition to short-circuit currents the incomplete utilization of electrons for FA production can be attributed to the formation of byproducts such as carbon monoxide, methane or hydrogen.71 The minimum energy demand for FA formation can be calculated as: =

∙ ∙

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with being the Faradic constant and the number of electrons transferred per FA molecule (i.e. 2). Voltages reported in the literature mostly range from about -1.8 to -2.1 V38, 72, but it also has been reported that above -2.75 V current density strongly decreases73. Faradic efficiencies for FA of 80 to 90 % have been reported.37, 73, 74 For the voltages and faradic efficiencies reported in literature an energy demand between about 405 and 590 kJel mol-1FA can be calculated. This corresponds to an energetic efficiency for the production step between 41 and 59 %, which is consistent to the value of 45 % reported by Whipple et al.73. An important aspect to be considered when accessing electrolytic FA production is the low concentration of FA (or formate, depending on the pH). High faradic efficiencies are only achieved at concentration not exceeding a certain threshold (0.0138 and 0.05 74 M, depending on the report). Recovery of pure (or at least enriched) FA will require additional energy. Bicarbonate/formate A large fraction of the energy demand for the conversion of bicarbonate salts to the corresponding formate salts is due to the need for preheating the aqueous solution to reaction temperature and the compression of hydrogen. Energy demand for preheating is not only


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influenced by ambient and reaction temperature, but also strongly by carbonate concentration. If the carbonate concentration is low the amount of water that needs to be preheated per mole carbonate is high. Based on the data reported (reaction temperature of 40 °C) the heat demand was estimated to about 17 ± 5 kJth mol-1FA. This amount of heat might be decreased by internal heat recovery utilizing the (weak) exothermicity of the reaction or heat from intercoolers of hydrogen compression to reaction pressure of about 3 to 6 MPa. The compression contributes a substantial demand of high value electrical/mechanical work. Based on pressure data reported in literature (4 ± 0.5 MPa) this energy demand has been estimated to 19 ± 5 kJel mol-1FA (energy demand for compression represents a namable share of the total energy demand for charging, but its sensitivity on pressure is comparatively low, since energy demand for compression does not depend on pressure difference, but ratio; compare figure S3 in the supporting information). In total an energy demand of 36 ± 7 kJ mol-1FA (or about 26 with internal heat recovery; assuming that about 50 % of the waste heat from the compressor can be utilized) can be estimated for the charging step of the bicarbonate / formate approach. FA decomposition Energy is not only required for FA production, but also for its decomposition. The enthalpy of reaction only plays a minor role in the total energy demand. Energy demand is rather dominated by the heat required for bringing FA to reaction temperature (there is a strong dependence on FA concentration, due to the large heat capacity of the accompanying water). Furthermore, a small heat demand can be attributed to the evaporation of water under the reaction conditions of hydrogen release. Catalytic FA dehydrogenation If neat FA is decomposed energy demand for preheating would be comparatively small (about 6 kJth mol-1FA for a temperature increase from 20 °C to a reaction temperature of 80 °C). In aqueous solution the heating requirements will drastically increase (about 64 kJth mol-1FA for a 4 M solution). Enthalpy of reaction accounts for an energy demand of 16.5 ± 0.3 kJth mol-1FA. Energy demand to compensate the cooling effect of evaporation has been estimated to 28 ± 14 kJth mol-1FA (it is assumed that about 5 % of the mixture is evaporating). Adding these values together results in a total energy demand for catalytic hydrogen release from FA of 108 ± 33 kJth mol-1FA for a 4 M aqueous solution and 24 ± 2 kJth mol-1FA for pure FA. While the


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neat FA case is attractive, it is also challenging, since decomposition without water favors carbon monoxide formation. Thus, there may be an optimum concentration that minimizes energy input and maintains high selectivity for dehydrogenation. Reduced energy demand due to internal heat recovery is estimated to 66 ± 19 kJth mol-1FA for a 4 M aqueous solution and 20 kJth mol-1FA for pure FA (assuming a temperature difference of 20 K for heat transfer; for more details on the assumptions refer to the supporting information). Bicarbonate/formate Energy demand for releasing hydrogen in the bicarbonate / formate approach is dominated by preheating to reaction temperature. An energy demand of 55 ± 16 kJth mol-1FA is calculated (plus about 3 kJth mol-1FA to compensate the cooling effect due to evaporation during reaction). Total energy demand for hydrogen release is estimated to 77 ± 19 kJth mol-1FA, which might be decreased to about 41 kJth mol-1FA by internal heat recovery. FA Dehydration Energy demand for thermal decomposition to release carbon monoxide is slightly different, since evaporation is not an undesired side effect, but a crucial step for the gas phase reaction. Energy demand for preheating is 50 ± 1 kJth mol-1FA for reaction at 500 °C. This amount of heat can be drastically reduced, if the reaction is performed at lower temperatures (only about 31 kJth mol-1FA at 200 °C). Heat of reaction itself again is only a minor contribution to the overall energy demand (30.9 ± 6.3 kJth mol-1FA). Total energy demand (for reaction at 500 °C) is estimated to 80 ± 6 kJth mol-1FA and might be reduced to about 54 kJth mol-1FA if internal heat recovery is applied. An overview over the energy demand for release of H2 or CO from FA or formate salts is shown in Figure 2.


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Figure 2: Energy demand for releasing hydrogen (or carbon monoxide in case of thermal decomposition) from FA or the bicarbonate / formate system

FA Fuel cell Direct formic acid fuel cells are a special case, since they skip the hydrogen release step. Their efficiency is, similar to electrochemical FA production, determined by voltage and faradic efficiency. The highest voltages reported for open circuits are about 0.8 V.49 With increasing current density, voltage drastically decreases. Faradic efficiency is strongly influenced by crossover (i.e. the effect of FA permeating through the membrane instead of undergoing the “regular” fuel cell reaction). Permeation of FA through Nafion® membranes is far weaker than for methanol. Nevertheless, the loss of FA is still not negligible. Rhee et al.75 reported a permeation between about 2.0 · 10-8 (for 1 M FA) and 1.9 · 10-7 mol cm-2 s-1 (for 10 M FA). Based on the data reported by Jeong et al.76 concerning the correlation between electric and crossover flow (which are consistent to the data by Rhee et al.) a loss of about 13 % of the formic acid can be estimated for an electric current density of 90 mA cm-2 and a FA concentration of 6 M. For a voltage of 0.39 V, as reported by Ha et al.77, an overall efficiency for a formic acid fuel cell of about 27.5 % can be calculated. The actual efficiency in real applications will most likely be a little lower, since energy demands for e.g. pumps has to be taken into account. The value can thus be considered to be consistent to the value of 24 % estimated by Reis and Mert for exergetic (not energetic) efficiency.78 For more details on the calculations presented here, refer to the supporting information.


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Purification The main options for removing carbon dioxide from the product gas stream are, as described above: adsorption, with regeneration by pressure decrease (PSA) or temperature increase (TSA), amine scrubbing or cryogenic CO2 removal. The exact energy demand depends on parameters such as initial CO2 content, initial pressure and temperature as well as target purity. Since especially the latter might be a subject of discussion general statements on the energy demand have limited significance. Thus, stating ranges seems more appropriate (Figure 3 a). Comparatively low energy demands for CO2 removal of down to 21 kJ mol-1CO2 have been reported for one-step PSA systems.79 Energy demands reported for all other purification technologies barely reach values below 100 kJ mol-1CO2 (see e.g. ref.80-82). When evaluating these energy demands, one should keep in mind that PSA requires a form of energy with a rather high value (electric/mechanic), while TSA and amine scrubbing only apply heat for desorption. Exergy is a common means of expressing the value of an energy form. Electric or mechanical energy are pure exergy, while heat is weighted by its Carnot factor. This factor accounts for the temperature of the respective heat (150 °C for TSA80 and 120 °C for amine scrubbing83) and the ambient temperature (here assumed as 20 °C). Considering the exergetic value of the energy forms, TSA and amine scrubbing might become attractive options again (Figure 3 b). A final decision on a purification technology should take the conditions of the respective applications into account. If waste heat at a sufficient temperature level is available for regeneration, TSA or amine scrubbing will most likely be the most reasonable purification options. If no such heat is available PSA seems to be the most efficient way of removing CO2 from the hydrogen stream.

Figure 3: Energy (a) and exergy (b) demand for the removal of carbon dioxide from gas streams


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Efficiency of electricity production Producing electricity from FA (or a formate species) requires three steps: decomposition, purification and power production (with the exception of the direct formic acid fuel cell). Most energy is dissipated in the decomposition step (Figure 4; the term “dissipation of energy” is used instead of “energy loss” to be consistent with the laws of thermodynamics). However, one should keep in mind that the amount of energy actually reaching the fuel cell is smaller than the initial amount of energy carried by the hydrogen stored in FA. Electric heating of the decomposition is not reasonable energetically. Hence, it was assumed that a certain share of hydrogen is combusted before entering the fuel cell to provide heat for hydrogen release. Taking this effect into account, it is reasonable to conclude that dissipation of energy during decomposition and in the fuel cell are of similar relevance to overall efficiency.

Figure 4: Loss cascade for recovering electric energy from FA for the bicarbonate / formate approach and for catalytic hydrogen release from a 4 M aqueous solution of FA (assumed fuel cell efficiency: 60 %; purification of H2 after catalytic release by PSA; no energy is required for purification in the bicarbonate / formate approach, because water is assumed to be the only accompanying vapor/gas); note that the starting energy level is the same for both approaches due to the molar reference. For a mass or volume reference the starting points would differ due to the different energy densities.

Comparing all options for obtaining electric energy from formic acid the bicarbonate / formate approach turns out to be the most efficient process option (Figure 5). Catalytic decomposition of pure FA would be competitive to this approach, if carbon monoxide formation is prevented effectively. Catalytic hydrogen release from a 4 M aqueous FA solutions on the other hand


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exhibits rather poor efficiency. Furthermore, the energy demand for producing formate salts from the respective bicarbonates is less energy intensive than formation of FA (see above). Consequently, the bicarbonate / formate approach appears to be the most reasonable option among the FA based variants in terms of its overall energy balance. Thermal decomposition to release CO suffers from the fact that the efficiency of ICEs is lower than of fuel cells. The direct formic acid fuel cell might be an interesting option in the future, if significant progress is made on this technology. However, its current status does not allow considering it an efficient technology for obtaining electric energy from FA. Biological hydrogen release from FA is only in a very early development stage and no reliable conclusions on its energy efficiency are possible so far.

Figure 5: Efficiencies of approaches for electricity generation from FA (considering decomposition, purification and repowering)

Energy and power density The energy density of FA is often referred to as 5.26 kJth g-1 or 4.4 % gravimetric hydrogen capacity. This is formally correct, but not only neglects that conversion is often below 100%, but also ignores two further important aspects for practical application. First, formic acid is not handled in its neat form, but mixed with water. Second, hydrogen release requires energy, which


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lowers the effective energy density. Estimates for the effective storage densities, considering these effects, are summarized in Table 1. If FA or a formate salt is stored in aqueous solution hydrogen capacity and energy density are strongly depending on concentration. Effective energy density is further depending on concentration, since low concentration means that a large share of water has to be heated to reaction temperature. At low concentrations effective energy density might even become negative, because energy demand for release exceeds the energy content of the hydrogen. The bicarbonate / formate based approach reaches far lower gravimetric storage densities due to the additional mass of the cation. Yet, this is only a disadvantage for mobile applications. If energy should be stored at a fixed location (without transportation) volumetric storage density is the only relevant parameter. The volume demand of the formate salt is only slightly higher than for FA. Hence, the higher efficiency of the bicarbonate / formate approach might render it the better option. Table 1: Hydrogen capacity (taking mass/volume of the water into account) and energy density (taking mass/volume of the water and the energy demand for hydrogen release into account); there is no effective energy density for pure, water-free potassium formate, since the reaction requires an aqueous solution

pure

formic acid (catalytic hydrogen release)

potassium formate (bicarbonate / formate approach)

hydrogen capacity energy density hydrogen capacity energy density

g g-1 g L-1

0.04 4 53.4

concentration / mol L-1 1

2

3

4

5

10

0.002

0.004

0.006

0.007

0.009

0.016

2.0

4.0

6.0

8.1

10.1

20.2

-1

4.81

-0.02

0.20

0.41

0.61

0.79

1.61

-1

kJth L

5871

-53

143

339

535

731

1711

g g-1

0.02 4

0.002

0.004

0.005

0.006a

0.008 a

0.012 a

g L-1

45.7

2.0

4.0

6.0

8.1

10.1

20.2

kJth g-1

-

-0.02

0.19

0.37

0.53

0.68

1.24

kJth L-1

-

-19.4

210

439

668

898

2044

kJth g

a

The solubility of the potassium bicarbonate formed upon hydrogen release is lower than solubility of the respective formate. At ambient temperature precipitation will occur in the product mixture for concentrations higher than about 3.3 M.

For a final statement on the volume requirements of a storage system the size of the release unit should be considered as well. The problem is that the volume requirements of this unit depend on the power demand, which can be scaled independently of energy capacity. Hence, a general statement on total system size is not possible today. Nevertheless, to allow a first, rough


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comparison power densities for different approaches have been calculated based on data found in literature16, 25, 39, 42, 47, 54-56, 84-86 (Figure 6). As expected power density for the biological process is comparatively low. The decomposition of FA in aqueous solution by a homogeneous catalyst is likely to reach the highest power densities, but heterogeneous catalysts still might be competitive. The bicarbonate / formate approach clearly reaches lower power densities, but improvements in catalysis to reach a similar level are needed.

Figure 6: Reported hydrogen release rates relative to the reaction volume for different approaches of release from FA

Conclusion Process options for formic acid (FA) based hydrogen storage have been evaluated. Most of these processes can be divided into the following process steps: production of FA, decomposition of FA, purification of product gas and electricity production from the product gas. All these steps contribute to the energy demand of the overall process. The analysis shows that the cycle between formate and the corresponding bicarbonate is an attractive process option. It avoids the handling and storage of carbon dioxide after hydrogen release. The equilibrium constant for the respective reaction is close to unity, allowing its reversibility at rather mild conditions. As a consequence, the bicarbonate / formate system appears to be the most energy efficient approach for FA based hydrogen storage. With effective internal heat recovery overall efficiencies (hydrogen fed to the storage system to electricity


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finally obtained afterwards) of about 50 % seem realistic. It is interesting to note that solid potassium formate provides the same volumetric hydrogen storage density as 700 bar H2. Thus, formate could be further investigated as a means to store energy on a large scale as envisioned by H2@Scale87. To recover the H2, formate can be transported to decentralized sites, dissolved in water, and passed through a catalyst bed, essentially obtaining H2 from the water. Process options based on FA will become more attractive for storage if (i) the low yields of production from hydrogen and carbon dioxide can be overcome and (ii) catalysts are developed to release H2 from concentrated solutions to minimize the heat demand for preheating water in dilute aqueous solutions. More work is needed to understand the overall economics of using formic acid and formate as hydrogen carrier materials in terms of the entire landscape of its use including production, transportation, decomposition, chemical compression, and recycle.

Acknowledgments The authors wish to thank Prof. Wolfgang Arlt for his support of this research. Tom Autrey and Kriston Brooks gratefully acknowledge research support from the Hydrogen Materials Advanced Research Consortium (HyMARC), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830.

Supporting Information Assumptions in the calculations; Calculation results concerning electrochemical FA production and direct FA fuel cell; calculation results concerning electricity production via hydrogen release; sensitivity analysis


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Jeong, K.-J.; Miesse, C. M.; Choi, J.-H.; Lee, J.; Han, J.; Yoon, S. P.; Nam, S. W.; Lim, T.-H.; Lee, T. G., Fuel crossover in direct formic acid fuel cells. Journal of Power Sources 2007, 168, (1), 119-125. Ha, S.; Adams, B.; Masel, R. I., A miniature air breathing direct formic acid fuel cell. Journal of Power Sources 2004, 128, (2), 119-124. Reis, A.; Mert, S. O., Performance assessment of a direct formic acid fuel cell system through exergy analysis. International Journal of Hydrogen Energy 2015, 40, (37), 12776-12783. Liu, Z.; Grande, C. A.; Li, P.; Yu, J.; Rodrigues, A. E., Multi-bed Vacuum Pressure Swing Adsorption for carbon dioxide capture from flue gas. Separation and Purification Technology 2011, 81, (3), 307-317. Merel, J.; Clausse, M.; Meunier, F., Experimental Investigation on CO2 Post−Combustion Capture by Indirect Thermal Swing Adsorption Using 13X and 5A Zeolites. Industrial & Engineering Chemistry Research 2008, 47, (1), 209-215. Mimura, T.; Simayoshi, H.; Suda, T.; Iijima, M.; Mituoka, S., Development of energy saving technology for flue gas carbon dioxide recovery in power plant by chemical absorption method and steam system. Energy Conversion and Management 1997, 38, (Supplement), S57-S62. Leung, D. Y. C.; Caramanna, G.; Maroto-Valer, M. M., An overview of current status of carbon dioxide capture and storage technologies. Renewable and Sustainable Energy Reviews 2014, 39, (Supplement C), 426-443. Li, K.; Cousins, A.; Yu, H.; Feron, P.; Tade, M.; Luo, W.; Chen, J., Systematic study of aqueous monoethanolamine-based CO2 capture process: model development and process improvement. Energy Science & Engineering 2016, 4, (1), 23-39. Enthaler, S.; von Langermann, J.; Schmidt, T., Carbon dioxide and formic acid-the couple for environmental-friendly hydrogen storage? Energy & Environmental Science 2010, 3, (9), 1207-1217. Yoshida, A.; Nishimura, T.; Kawaguchi, H.; Inui, M.; Yukawa, H., Efficient induction of formate hydrogen lyase of aerobically grown Escherichia coli in a three-step biohydrogen production process. Applied Microbiology and Biotechnology 2007, 74, (4), 754-760. Wiener, H.; Sasson, Y.; Blum, J., Palladium-catalyzed decomposition of aqueous alkali metal formate solutions. Journal of Molecular Catalysis 1986, 35, (3), 277-284. H2@ Scale concept of the Fuel Cell Technologies Office of the US Department of Energy. https://energy.gov/eere/fuelcells/h2-scale (accessed on October 3rd 2017)


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Energy & Fuels

Figure 1. Process options for FA based hydrogen storage 79x51mm (300 x 300 DPI)


Energy & Fuels

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Figure 2. Energy demand for releasing hydrogen (or carbon monoxide in case of thermal decomposition) from FA or the bicarbonate / formate system 82x53mm (300 x 300 DPI)


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Energy & Fuels

Figure 3. Energy (a) and exergy (b) demand for the removal of carbon dioxide from gas streams 82x53mm (300 x 300 DPI)


Energy & Fuels

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Figure 3. Energy (a) and exergy (b) demand for the removal of carbon dioxide from gas streams 82x53mm (300 x 300 DPI)


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Energy & Fuels

Figure 4. Loss cascade for recovering electric energy from FA for the bicarbonate / formate approach and for catalytic hydrogen release from a 4 M aqueous solution of FA (assumed fuel cell efficiency: 60 %; purification of H2 after catalytic release by PSA; no energy is required for purification in the bicarbonate / formate approach, because water is assumed to be the only accompanying vapor/gas) 82x53mm (300 x 300 DPI)


Energy & Fuels

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Figure 5. Efficiencies of approaches for electricity generation from FA (considering decomposition, purification and repowering) 82x53mm (300 x 300 DPI)


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Energy & Fuels

Figure 6. Reported hydrogen release rates relative to the reaction volume for different approaches of release from FA 82x53mm (300 x 300 DPI)


Hydrogen Storage in Formic Acid: A Comparison of Process Options

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