Formic Acid as a Hydrogen Energy Carrier - ACS Energy Letters

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Perspective

Formic Acid as a Hydrogen Energy Carrier Jörg Eppinger, and Kuo-Wei Huang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00574 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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Formic Acid as a Hydrogen Energy Carrier Jörg Eppinger* and Kuo-Wei Huang* KAUST Catalysis Center and Division of Physical Sciences & Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia

Abstract: The high volumetric capacity (53 g H2/L) and its low toxicity and flammability under ambient conditions make formic acid a promising hydrogen energy carrier. Particularly in the last decade significant advancements have been achieved in the catalyst development for selective hydrogen generation from formic acid. This Perspective highlights the advantages of this approach with discussions focused on potential applications in the transportation sector together with an analysis of technical requirements, limitations, and costs.

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In 2013, the estimated world population of 7.14 billion people consumed 13.6 Gtoe of energy (at an average rate of 18.0 TW), of which 19% were required for transportation and road traffic being responsible for the majority (approx. 80%) of the demand.1,2 Recent estimates project that population, average energy consumption and numbers of vehicles per person will steadily increase during the coming century.3 Globally, burning of carbon-based fossil fuels supplies over 81% of the energy demand,2 and hence the prospering industrial societies are responsible for the observed increase in carbon dioxide levels form preindustrial 280 ppm to the record high of 409.5 ppm measured this year.4 The constantly increasing atmospheric CO2 concentration is very likely to result in global warming, sea level rise and ocean acidification. To reduce the environmental footprint of modern societies and address the limitations of fossil recourses, the projected increase in global energy demand must go along with the implementation of carbon neutral energy production and carrier systems.5 The most prominent candidates for such technologies are renewable biofuels, or electricity from nuclear power plants, solar and wind energy, as well as hydroelectricity. While competition with food production and their uncertain CO2 balance render the global substitution of fossil fuels by biofuels unlikely,6 legislation and industry are strongly pushing the build-up of renewable electric energy production.7 The existing power grid makes this technology attractive, because initial infrastructure investments are moderate and therefore the barrier to market entrance is low. However, the electrification of the transport sector faces serious challenges, since the on-board storage of electric energy requires large batteries, which for the foreseeable future suffer from low gravimetric and volumetric energy densities (Fig. 1a). Hydrogen (H2) is considered a promising alternative for intermediate energy storage. It is expected to play a crucial role as a secondary fuel and energy carrier in the new energy system.8-13 H2 has a high gravimetric energy density of 33.3kW·h/kg and can be converted into energy in an internal combustion engine or fuel cells with the production of water as the only byproduct. It is, however, expected that the hydrogen economy will not materialize until significant technological advances in H2 production, storage and delivery systems are made.13 Particularly, development of a safe and efficient 2 ACS Paragon Plus Environment

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system for hydrogen storage represents a great challenge. Conventional H2 storage in high-pressure compressed gas cylinders or cryogenic liquid tanks is straight forward, but suffers from excessive energy losses (H2 compression, liquefaction and boil-off) and low volumetric energy capacity.14,15 State-of-the-art on-board hydrogen storage (700 bar) reaches 5.7 wt-% of H2, which translates into energy densities of 1.9 kW·h/kg and 1.4 kW·h/L.16 Alternative approaches through physical adsorption of H2 in high-surface-area materials, such as metal-organic frameworks, zeolites, nanostructured carbon materials, etc., experience the limitation of temperature and pressure ranges and generally achieve lower gravimetric and volumetric energy densities.17-19 While chemical hydrides (CH) could deliver high gravimetric H2 capacities of up to 20 wt-%, the poor reversibility prohibits their widespread applications (Fig. 1a).20-23 In this regard, the liquid hydrogen carrier, formic acid (FA), becomes an attractive choice. Although FA contains only 4.4 wt-% of H2, because of its high density of 1.22 g/cm3, its volumetric capacity reaches 53 g H2/L. This is equivalent to an energy density of 1.77 kW·h/L, which exceeds those of commercial 70 MPa hydrogen pressure tanks (e.g. 1.4 kW·h/L for the Toyota Mirai) and hence may be suitable for automotive and mobile applications (Fig. 1a). As illustrated in figure 1c, a carbon neutral H2 storage system is at hand, if efficient CO2 hydrogenation and selective FA dehydrogenation can be developed.24-26 Research on catalytic FA decomposition has intensified rapidly during the past decade, and the developments on the catalytic systems related to this particular reaction were summarized recently in several excellent reviews,27-34 therefore herein we aim to emphasize on evaluating technical implementations of FA as a hydrogen carrier and its potential in the transportation sector with technical requirements, limitation, and cost analysis. Applications are assessed with reference to current H2 fuel cells utilizations, followed by a discussion on the technological challenges and enabling requirements. The perspective concludes with an outlook on the future opportunities with this energy storage option.


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Figure 1. The case for formic acid as a hydrogen carrier. A): FA competes well with other reversible hydrogen storage options35 (figure adapted from ref. 35; data for Toyota Mirai15 and BMW 5 GT36 taken from company publications). Dashed lines indicate DOE 2010 targets. B): FA compares well with hydrogen or a current average midsized ICV (engine power: 100 kW, range: 600 km). Recent progress in commercial fuel cell technology has led to a significant weight-reduction electric power train,15 rendering H2-powered FCVs competitive to ICVs in terms of overall weight. While H2-consumption only results in a negligible loss of mass, FA utilization releases CO2 and correspondingly the power train’s energy density exceeds that of an H2-FCV and even an ICV for middle to low fuel tank fillings. C): FA as an H2 carrier for the transport sector. Depending on the hydrogen production method, for an current FCV15 CO2 emissions per km range from 235 g / km (water electrolysis with present grid electricity production,37 to 85 g / km (thermal processes like steam reforming of methane, coal gasification or the Cu-Cl cycle,38 to less than 10 kg/km (water electrolysis with electricity production from renewable sources and CO2 from flue-gas or atmospheric carbon capture (CC).38 CDH: carbon dioxide hydrogenation; FADH: formic acid dehydrogenation.


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General Assessment on formic acid The high gravimetric capacity of FA has been appreciated and its potential application as a secondary fuel has been proposed and explored in direct formic acid fuel cells (DFAFCs).39 While early models suffered from a low performance of the platinum catalyst, better performance could be achieved using palladium.40 The commercial feasibility of the DFAFC technology was examined with investment from the industrial sector (Tekion and Motorola, partnering with BASF) to design and manufacture power packs.41 Presumably because the catalyst poisoning issue in long-term application could not be overcome, there were no further updates about these developments and Tekion’s assets were acquired by Neah Power in 2013.42 While DFAFCs face major challenges, hydrogen fuel cells are a mature technology, which is commercialized in fuel cell vehicles (FCV) with over 140 kW and a range exceeding 600 km (e.g. Toyota Mirai, Hyundai Tucson, Honda Clarity, etc.). Hence, the selective production of H2 from FA to power hydrogen fuel cells is a promising approach with a short way to market. Like in a conventional fuel, energy discharge implies a consumption of FA, which results in a significant release of mass in form of CO2. In combination with a light-weight electrical motor and fuel cell, a FAbased power train can achieve better energy-to-mass ratios than current fossil fuels driven combustion engines (Fig. 1b). Moreover, the cost associated with building and maintaining the distribution infrastructure represents the major hurdle for large-scale consumer applications of gaseous H2. Since FA is a non-toxic and environmentally benign liquid with low flammability under ambient conditions, the existing gasoline infrastructure may be easily adapted for FA distribution.

Consideration on Catalytic Systems Desirable Catalyst Properties. A catalytic FA converter system, which generates on-board the hydrogen powering a fuel cells in an automotive application introduces specific requirements that must be addressed during catalyst development. Key factors are 1) the selectivity for H2 production, 2) catalyst activity, characterized by the catalysts


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turn over frequency (TOF) at a specific reaction temperature, 3) catalyst durability or life time typically determined as the catalysts turnover numbers (TONs), and 4) catalyst costs. Economic considerations are of major importance to achieve consumer’s acceptance of a new technology. Selectivity for H2 production. As FA is an intermediate of the water gas shift (WGS) reaction, in the absence of a suitable catalyst decomposition of FA can occur along two competing low enthalpy pathways:43 i)

dehydrogenation (decarboxylation) yielding H2 and CO2 (∆G° = -32.9 kJ•mol-1, ∆H° = 31.2 kJ·mol-1, ∆S° = 216 J·mol-1·K-1) and

ii)

dehydration (decarbonylation) giving H2O and CO (∆G° = -12.4 kJ·mol-1, ∆H° = 29.2 kJ·mol-1, ∆S° = 139 J·mol-1·K-1).

When FA serves as a chemical hydrogen carrier, any CO generating process must be suppressed since CO formation not only reduces the overall H2 yield, but also leads to poisoning of the fuel cell’s catalyst. CO-poisoning of the Pt-catalysts in proton-exchange membrane fuel cell (PEMFC) is one of the most serious obstacles in developing commercial fuel cells with typical critical CO-concentrations >10 ppm.7,44-48 Hence, any suitable FA-decomposition catalyst should provide a dehydrogenation/dehydration selectivity of. 105, which translates under the assumption of a Curtin-Hammett regime into ∆∆G‡ of 34.7 kJ/mol at a typical reaction temperature of 90 °C. Catalysts that involve reduced metal species49 to encourage decarbonylation and/or strong acidic components for dehydration50 are thus not favorable. While in general a heterogeneous system facilitates easy separation of catalysts and products, for FA decomposition, heterogeneous reactions typically conducted in aqueous FA solutions do not offer any obvious advantages compared to the homogeneous counterparts. Heterogeneous transition metal nanoparticle catalysts provide far too low selectivities and commonly produce hydrogen with a considerable CO content of over 1000 ppm.49 Alloy nanoparticles51,52 can show improved hydrogen selectivities, yet they do not meet the selectivity target53-57 that the homogenous counterparts offer until very recent developments.58,59 Hence, the high degree of control for the dehydrogenation pathway renders homogenous catalysts presently more suitable for an application. From an 6 ACS Paragon Plus Environment

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engineering perspective, a homogeneous converter system will require nothing else than a small reactor, where FA is pumped at the same rate at which it is consumed, while the homogeneously dissolved catalyst remains in the reactor. Such a system can quickly adjust the hydrogen production rate on demand and therefore fulfills the requirements of transportation applications. Since the evolved gas consists of hydrogen and carbon dioxide in a 1 : 1 molar ratio, membrane separation of the two gases may be required to prevent CO2 accumulation on the anode and achieve high energy efficiencies. Catalyst Activity. The rated maximum power output of the fuel cell directly determines the amount of H2-evolution catalyst required in the FA-converter. Assuming a (high) fuel cell energy efficiency of 58% at maximum power, the fuel cell will consume hydrogen at a rate of rFC(H2) = 0.713 mmol·(s·kW)-1. Since there is no on-board reservoir of gaseous H2 in a FA powered FCV, the converter must be able to sustain this hydrogen flow. Considering a typical design reserve factor RF = 200%, the required catalyst amount can be calculated by ncat = [rFC(H2) · RF] / TOFcat = 1.43 (mmol/kW) / TOFcat,[s-1]

(1)

Correspondingly, the minimal amount of catalyst in the converter is reciprocal to the catalyst’s activity at a given operating temperature. Catalyst lifetime. The stability of the catalyst applied will directly determine the driving distance per catalyst loading and thus the service intervals. The available driving distance is directly proportional to the catalyst’s TON and catalyst amount, which is accepted to degrade until a performance loss is observed. Thus, the length of a service interval of the FA-converter catalyst follows from equation 2: dservice = TONcat · ncat / ṅH2,[mmol/(kW·km)]

(2)

The minimal catalyst amount in the converter is reciprocal to the catalyst’s TOF (eqn. 1), and therefore the service intervals will depend on the TON/TOF ratio: dservice = TONcat / TOFcat · [rFC(H2) · (RF-100%)] / ṅH2


(3)

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For an average molar hydrogen consumption of ṅH2 = 36 mmol/(kW·km)60 and the assumptions described above, the service intervals are directly proportional the TON/TOF ratio (eqn. 4) dservice = 0.2 km · s-1 · TON / TOF,[s-1]

(4)

Stability towards water and acid. As FA from commercial sources contains water, a marketable homogenous catalyst should perform well in the presence of water. To minimize or eliminate problems associated with the loss of a volatile organic solvent and additives such as amines during the hydrogen generation process, reactions in an aqueous mixture are considered a favorable option. Hence, catalysts that can perform well in aqueous solutions are expected to play a major role especially in the early development stage.19-26 Catalysts that show reasonable stability and activity in high concentration or neat formic acid in the absence of base promoters represent an important opportunity for the field. These properties do not only allow the exploitation of FA’s full volumetric hydrogen density of 53g H2/L, they may also enable the direct production of high pressure H2 from formic acid under acidic conditions. Since the conversion of FA to H2 and CO2 is an equilibrium, the theoretical highest pressures of H2 and CO2 can be achieved are estimated to be 225 MPa (2250 bar) over neat FA. Indeed, a remarkable system for continuous high-pressure (>120 MPa) H2 production from FA was recently realized using an iridium catalyst at 80 ºC.61 This is a great advance as it opens the opportunities for more applications of high pressure H2 such as that in an H2 gas filling station. The existing technology to feed the high-pressure H2 to FCVs is costly mainly due to the requirement of extensive use of mechanical hydrogen compressors and the energy associated with the H2 compression, liquefaction and boil-off processes.9-10

Economic factors. While FA as hydrogen carrier combines several benefits, market acceptance of a new consumer-targeted technology is usually very price sensitive. Infrastructural costs for adding a FA tank and pump at normal filling stations should be relatively low and thus facilitate the set-up of a FA distribution network. Yet, an


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important factor will be the initial investment and the operational costs of a proposed FApowered vehicle relative to the alternatives. As illustrated by Figure 2a, the FA market prices of around 400 – 650 USD/kg compares favorable with both, H2 sales prices at US filling stations as well as gasoline prices in Europe, if normalized to the “energy-atwheels”. At a sales price below 300 USD/t, FA will be priced comparably to US gasoline and 190 USD/t would equal the DOE price target of 4 USD/kg H2. It can be expected, that up-scaling of the current world production capacity (8·105 t/a)62 to the quantities required for fuel supply by the factor 104 would lead to strongly reduced production costs.

Figure 2. A): Price-comparison of hydrogen,15 formic acid63 and gasoline normalized to the energy available the “energy-at-wheels”, which takes the different tank-to-wheel efficiencies into account.64 B): According to equations 5 and 6, the CON and COF numbers represent the fractions that the catalyst price contributes to the target price of the product (CON) or the capital costs of the system (COF). Peducts: price of all starting materials; Cs, empty: costs of the system without catalyst (here: fuel tank, pumps, FA-converter and controller).

While assessment of the fuel contribution to a vehicle’s operating costs is straight forward, a comparison of catalyst economics requires a more detailed analysis. Since catalysts are commonly characterized by their TON and TOF, we suggest the use of these two dimension-free key-parameters for normalizing catalyst costs. This approach should 9 ACS Paragon Plus Environment

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be applicable to any catalyst, however CON or COF values of a catalyst are always process specific. The CON (catalyst price normalized to TON, eqn. 5) describes the catalyst’s contribution to the operating costs. It is the fraction of the effective fuel costs, which can be attributed to the catalyst. The COF (catalyst cost normalized to TOF, eqn. 6) represents the share of the targeted capital costs of the (fuel) system, which arises from the catalyst. CON = Pcat / TON · KProd

with KProd = (PProdo)-1 · Mwcat / MwProd

(5)

COF = Ccat / TOF · KS

with KS = (rFC(H2) / CSo) · Mwcat / MwProd

(6)

Hence if the targets for the price of the product H2 (PProdo) and the target for the cost of the process related system CSo are known, CON and COF values will allow an economic assessment of different catalyst systems. As illustrated by figure 2b, the thresholds defined for a product (hydrogen in this case) entire reactor system (tank, converter, controller) allow an economic comparison with existing technologies. For each process an area in the CON/COF diagram exists, where catalyst costs meet the respective cost threshold criteria. Catalysts with higher CON or COF values lead to either a higher than targeted product price (CON > CON0) or system cost (COF > COF0). Hence, the green area in the diagram defines the target for catalysts, which economically fulfill all the defined criteria, e.g. to make a FA-powered FCV price-competitive to hydrogenfueled ones. We evaluated eight homogeneous catalyst systems, for which reasonably high TOFs and TONs were reported (Table 1). In System 1, the addition of 6 equiv. of 1,2bis(diphenylphosphino)ethane

(DPPE)

in

N,N-dimethylhexylamine

(HexNMe2)

effectively enhanced the TOF and TON of the Ru catalyst compared to those using 5HCO2H·4HexNMe2 at room temperature.65,66 The cationic Ru system 2 lives for several million cycles in the aqueous solution of HCOOH/HCOONa,67,68 however the reported TOF of 230 h-1 at 100 °C is more than two orders of magnitude short of the performance established for the Ir systems at 80 °C. With a well-defined PNP-Ru catalyst (3), when the less volatile trihexylamine (NHex3) base was employed, the TON was improved from


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326,500 (NEt3) to 706,500 with a similar high TOF.69 While being significantly slower, the PN3P-Ru catalyst (4) showed a long catalyst life time of 150 hours in DMSO/NEt3.70 Cationic pentamethylcyclopentadienyl(Cp*)Ir complexes 5 and 6 demonstrated excellent reactivities in the aqueous solution of HCOOH/HCOONa, but one notes that the initial TOFs of 158,000 h-1 for 571 and 487,500 h-1 for 672 are approx. 3-6 times faster than the average TOFs, suggesting the apparent degradation of the catalysts in 12-14 hours. It is always exciting to see base metals, such as Fe,73,74 showing similar or superior activities to those of their precious metal counterparts, but the cost of the ligand appears to the dominating factor for practical consideration. System 8 further showed that the presence of a Lewis acid, such as LiBF4, dramatically increased the TOF, unfortunately, it can only be achieved under a very low catalyst loading of 0.0001 mol%,74 limiting the overall hydrogen production rate. Since the exact activity of these catalysts under the same reaction condition (solvent, base additives, temperature, etc.) is not available, our analysis is based on the reported results assuming that these values represent the best performance achievable under individual optimized conditions. It is evident from figure 3 that the cheapest iron-pincer system 8 already fulfills both assumed cost criteria, while the Ru-system/complexes 1 and 3 and the Ir-chelate 6 meet the COF threshold (0.5). The low calculated price and high TON place Ru-catalyst 4 below the CON limit (0.35). Under the minimal catalyst requirement calculated form equation 1 the service distance is proportional to the TON / TOF ration of the respective catalyst. Hence, in figure 3 the catalysts fall into three groups: those with a low TON / TOF ratios (complexes 3 and 8) would typically need to be replaced in short intervals, possibly at every stop at the filling station, yet the initial costs are low. Catalysts with a higher estimated service distance could be replaced at every service (1, 5, 6, and 7). Catalysts 2 and 4, which show a high TON / TOF ratio may last for an entire car life cycle, yet the initial costs for the catalyst filling are significant enough to mandate a sales price increase of the FCV.


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Table 1. Selected recent catalysts of high TONs.

Catalyst

Operation conditions

Cost($/mole) a

TON

TOF (h-1)

165,66

in Me2NHex at 25 ºC

60,000

800,000

47,970

267,68

HCOOH/HCOONa in H2O at 100 ºC

250,000

3,000,000b

230

369

In DMF/NHex3 at 90 ºC

150,000

706,500

256,000

470

In DMSO/NEt3 at 90 ºC

35,000

1,100,000

7,333c

571


98,000

308,000

25,700c

672


125,000

2,400,000

171,000c

773

in propylene carbonate at 80 ºC

62,000

92,417

9,425

874

in dioxane at 80 ºC

11,000

983,642

196,728

a

The price estimation was based on commercial prices of available starting materials at 100g or smaller

quantities assuming the same reaction yields reported in the literature without counting solvent and manpower costs. These values are only intended for an initial economic evaluation. bAccording to authors’ presentation in ICEF 2016. cAn average over the catalyst life time.


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Figure 3. Graphical representation of the CON and COF values calculated for the FAdehydrogenation catalysts listed in table 1.

Challenges and Opportunities – an Outlook Formic acid has been recognized as a readily available material to implement a hydrogen storage technology, which circumvents installation of the expensive infrastructure required for refilling hydrogen driven FCVs. Normal filling stations could in principle set up another pump delivering formic acid and might also use the HCOOH storage tank to produce hydrogen on-site, making the HCOOH technology a catalyst for the global implementation of FCVs through lowering the infrastructural barrier. In fact, a working group meeting following the G7 Energy Ministerial Meeting (G7WG) was held in the 2016 Innovation for Cool Earth Forum (ICEF),75 for scientists, industry representatives and policy makers to exchange views and to discuss potential joint R&D efforts in the research field on hydrogen carrier systems based on CO2. Catalysts and converter technologies have made tremendous progress and should currently allow building a technology demonstrator. For example, direct CO2–H2 reaction 13 ACS Paragon Plus Environment

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systems in acidic media have been realized.62,76 One encouraging early example of a Rucatalyzed hydrogen generator using formic acid was developed, successfully meeting 1 kW power output (30 L/min of H2/CO2).30 A similar system at a larger scale should be achievable and a group of students in Eindhoven are currently building a bus fuelled by this formic acid concept.77 Earlier this year, a 400W model car that can carry 45 kg at approx. 8 km/hr was also demonstrated.78 However, as summarized above, in order to establish an economically feasible system for initial commercialization, a significant reduction of the catalyst costs under mild conditions without scarifying the catalyst’s activity and selectivity for hydrogen generation is required, in addition to the advances in formic acid synthesis and CO2 capture. While limitation of recourses and legislative actions will strongly influence the future of transportation, the strategy of utilizing formic acid as an energy carrier to selectively release hydrogen for electricity generation by fuel cells looks promising. Moreover, many mobile applications as well as the foundation of stationary power plants can also be envisioned. With the worldwide research efforts on hydrogen production from renewables, this storage and distribution concept will certainly play a role in the future hydrogen economy.

■ AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected] (J.E.) and [email protected] (K.-W.H.) Notes The authors declare no competing financial interest.

Biographies Jörg Eppinger obtained his Ph.D. from the Technische Universität München (1999) and joined The Scripps Research Institute as a postdoctoral DFG-fellow (2002). He is a


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recipient of the Hans-Fischer Award, held a ForschungsDozentur in molecular catalysis and founded the biotech startup company Alceis Sarl.. Currently, he is a faculty member at the KAUST Catalysis Center. His research interests include hydrogen economy and the interface of biology and inorganic chemistry with a focus on catalytic applications (boc.kaust.edu.sa).

Kuo-Wei Huang obtained his B.S. from National Taiwan University as a Dr. Yuan T. Lee Fellow, and Ph.D. from Stanford University as a Regina Casper Fellow. He is currently Associate Professor of Chemical Sciences at KAUST. Prior to joining KAUST, he was Assistant Professor at National University of Singapore and a Goldhaber Distinguished Fellow at Brookhaven National Laboratory. His research interests include renewable energy and synthetic and mechanistic studies of small molecules activation.

Acknowledgement We are grateful for financial support from the King Abdullah University of Science and Technology (KAUST).

References (1) The International Energy Outlook 2016 (IEO2016)2016. (2) World Energy Outlook; International Energy Agency, 2015. (3) The number of cars in the world is predicted to steadily increase from 1 billion in 2010 to two billions in 2030: a) Sperling, D.; Gordon, D. Two Billion Cars: Driving Toward Sustainability. Oxford Academic Press 2010; b) Gross, M. A planet with two billion cars. Current Biology 2016, 26, R307-R318. (4) Measured on April 10th 2016 at the Mauna Loa Observatory, Hawaii. Published by the Earth Systems Research Laboratory, Global Monitoring Division, National Oceanic and Atmospheric Administration. For more information see: http://www.esrl.noaa.gov/gmd/ccgg/trends/graph.html (5) Climate Change 2014-Mitigation of Climate Change. Fifth assessment report of the IPCC, WGIII; Cambridge University Press: New York, 2014.


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(6) Michel, H. The nonsense of biofuels. Angew. Chem. Int. Ed. 2012, 51, 2516-2518. (7) All major economies have currently governmental incentives and other policies in place to promote market penetration of electric vehicles. Typical instruments are tax reductions or exemptions as well as purchase incentives, e.g. reduced VAT or subsidies. A noteworthy development is the latest initiative of the Ministry of industry and information technology of the PRC. According to the announcement the government of the country with the world’s largest number of registered BEVs will require car makers to sell to meet specific quotas for sales of “new energy vehicles”, otherwise they must reduce the production of gasoline or diesel powered cars in China. Announced quotas are 8% in 2018, 10% in 2019 and 12 % in 2020. (8) Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972974. (9) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. (10) Whitesides, G. M.; Crabtree, G. W. Don't forget long-term fundamental research in energy. Science 2007, 315, 796-798. (11) Moriarty, P.; Honnery, D. Hydrogen's role in an uncertain energy future. Int. J. Hydrogen Energy 2009, 34, 31-39. (12) Moriarty, P.; Honnery, D. A hydrogen standard for future energy accounting? Int. J. Hydrogen Energy 2010, 35, 12374-12380. (13) Armaroli, N.; Balzani, V. The hydrogen issue. ChemSusChem 2011, 4, 2136. (14) Felderhoff, M.; Weidenthaler, C.; von Helmolt, R.; Eberle, U. Hydrogen storage: the remaining scientific and technological challenges. Phys. Chem. Chem. Phys. 2007, 9, 2643-2653. (15) Alazemi, J.; Andrews, J. Automotive hydrogen fuelling stations: An international review. Renewable Sustainable Energy Rev. 2015, 48, 483-499. (16) See: https://pressroom.toyota.com/releases/2016+toyota+mirai+fuel+cell+product.htm. (17) Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Metal hydride materials for solid hydrogen storage: A review. Int. J. Hydrogen Energy 2007, 32, 1121-1140. (18) Yueruem, Y.; Taralp, A.; Veziroglu, T. N. Storage of hydrogen in nanostructured carbon materials. Int. J. Hydrogen Energy 2009, 34, 3784-3798. (19) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen storage in metal-organic frameworks. Chem. Rev. 2012, 112, 782-835. (20) Peng, B.; Chen, J. Ammonia borane as an efficient and lightweight hydrogen storage medium. Energy Environ. Sci. 2008, 1, 479-483. (21) Demirci, U. B.; Miele, P. Chemical hydrogen storage: "material" gravimetric capacity versus "system" gravimetric capacity. Energy Environ. Sci. 2011, 4, 3334-3341. (22) Sanyal, U.; Demirci, U. B.; Jagirdar, B. R.; Miele, P. Hydrolysis of ammonia borane as a hydrogen source: fundamental issues and potential solutions towards implementation. ChemSusChem 2011, 4, 1731-1739. (23) Ley, B. M.; Jepsen, L. H.; Lee, Y.-S.; Cho, Y. W.; von Colbe, J. M. B.; Dornheim, M.; Rokin, M.; O., e. J.; Sloth, M.; Filinchuk, Y.; Jørgensen, E. E.;


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Besenbacher, F.; Jensen, T. R. Complex hydrides for hydrogen storage - new perspectives. Materials Today 2014, 17, 122-128. (24) Fukuzumi, S. Bioinspired energy conversion systems for hydrogen production and storage. Eur. J. Inorg. Chem. 2008, 1351-1362. (25) Enthaler, S. Carbon dioxide - the hydrogen-storage material of the future? ChemSusChem 2008, 1, 801-804. (26) Joo, F. Breakthroughs in hydrogen storage - formic acid as a sustainable storage material for hydrogen. ChemSusChem 2008, 1, 805-808. (27) Enthaler, S.; Langermann, J. v.; Schmidt, T. Carbon dioxide and formic acid—the couple for environmental-friendly hydrogen storage? Energy Environ. Sci. 2010, 3, 1207-1217. (28) Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen generation from formic acid and alcohols using homogeneous catalysts. Chem. Soc. Rev. 2010, 39, 81-88. (29) Czaun, M.; Goeppert, A.; May, R.; Haiges, R.; Prakash, G. K. S.; Olah, G. A. Hydrogen Generation from Formic Acid Decomposition by Ruthenium Carbonyl Complexes. Tetraruthenium Dodecacarbonyl Tetrahydride as an Active Intermediate. ChemSusChem 2011, 4, 1241-1248. (30) Grasemann, M.; Laurenczy, G. Formic acid as a hydrogen source - recent developments and future trends. Energy Environ. Sci. 2012, 5, 8171-8181. (31) Laurenczy, G.; Dyson, P. J. Homogeneous catalytic dehydrogenation of formic acid: progress towards a hydrogen-based economy. J. Braz. Chem. Soc. 2014, 25, 2157-2163. (32) Wang, W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. CO2 hydrogenation to formate and methanol as an alternative to photo- and electrochemical CO2 reduction. Chem. Rev. 2015, 115, 12936-12973. (33) Singh, A. K.; Singh, S.; Kumar, A. Hydrogen energy future with formic acid: a renewable chemical hydrogen storage system. Catal. Sci. Technol. 2016, 6, 12-40. (34) Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M. Formic acid as a hydrogen storage material –development of homogeneous catalysts for selective hydrogen release. Chem. Soc. Rev. 2016, 45, 3954-3988. (35) Review of the Reseach Program of the Freedom CAR and Fuel Partnership, 3rd report, National Academy of Sciences, the national academies press Washington, D.C., 2010. (36) Kunze, K.; Kirchner, O. Cryo-compressed hydrogen storage, Cryogenic Cluster Days, Oxford, Sept. 28th 2012. (37) Bhandari, R.; Trudewind, C. A.; Zapp, P. Life cycle assessment of hydrogen production via electrolysis e a review. J. Clean. Prod. 2014, 85, 151-163. (38) Cetinkaya, E.; Dincer, I.; Natere, G. F. Life cycle assessment of various hydrogen production methods. Int. J. Hydrogen Energy 2012, 37, 2071-2080. (39) Yu, X.; Pickup, P. G. Recent advances in direct formic acid fuel cells (DFAFC). J. Power Sources 2008, 182, 124-132. (40) Ha, S.; Larsen, R.; Masel, R. I. Performance characterization of Pd/C nanocatalyst for direct formic acid fuel cells. J. Power Sources 2005, 144, 28-34. (41) Formic acid fuel cell gets boost. http://www.chemicalprocessing.com/industrynews/2006/035/. (42) See www.neahpower.com for more details. 17 ACS Paragon Plus Environment

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(43) Watanabe, M.; Sato, T.; Inomata, H.; Smith, R. L.; Arai Jr., K.; Kruse, A.; Dinjus, E. Chemical reactions of C(1) compounds in near-critical and supercritical water. Chem. Rev. 2004, 104, 5803-5821. (44) Baschuk, J. J.; Li, X. Carbon monoxide poisoning of proton exchange membrane fuel cells. Int. J. Energy Res. 2001, 25, 695-713. (45) Cheng, X.; Shi, Z.; Glass, N.; Zhang, L.; Zhang, J.; Song, D.; Liu, Z.-S.; Wang, H.; Shen, J. A review of PEM hydrogen fuel cell contamination: impacts, mechanisms, and mitigation. J. Power Sources 2007, 165, 739-756. (46) Das, S. K.; Reis, A.; Berry, K. J. Experimental evaluation of CO poisoning on the performance of a high temperature proton exchange membrane fuel cell. . J. Power Sources 2009, 193, 691-698. (47) Kim, G.; Jhi, S.-H. Carbon monoxide-tolerant platinum nanoparticle catalysts on defect engineered graphene. ACS Nano 2011, 5, 805-810. (48) Springer, T. E.; Rockward, T.; Zawodzinski, T. A.; Gottesfeld, S. Model for polymer electrolyte fuel cell operation on reformate feed–effects of CO, dilution, and high fuel utilization. J. Electrochem. Soc. 2011, 148, A11-A23. (49) Solymosi, F.; Koos, A.; Liliom, N.; Ugrai, I. Production of CO-free H2 from formic acid. A comparative study of the catalytic behavior of Pt metals on a carbon support. J. Catal. 2011, 279, 213-219. (50) Yasaka, Y.; Yoshida, K.; Wakai, C.; Matubayasi, N.; Nakahara, M. Kinetic and equilibrium study on formic acid decomposition in relation to the water–gasshift reaction. J. Phys. Chem. A 2006, 110, 11082-11090. (51) Gilroy, K. D. R., A.; Peng, H.-C.; Qin, D.; Xia, Y. Bimetallic nanocrystals: syntheses, properties, and applications. Chem. Rev. 2016, 116, 1041410472. (52) Gawande, M. B.; Goswami, A.; Asefa, T.; Guo, H.; Biradar, A. V.; Peng, D.-L.; Zboril, R.; Varma, R. S. Core-shell nanoparticles: synthesis and applications in catalysis and electrocatalysis. Chem. Soc. Rev. 2015, 44, 7540-7590. (53) Gu, X.; Lu, Z.-H.; Jiang, H.-L.; Akita, T.; Xu, Q. Synergistic catalysis of metal–organic framework-immobilized Au–Pd Nanoparticles in dehydrogenation of formic acid for chemical hydrogen storage. J. Am. Chem. Soc. 2011, 133, 11822-11825. (54) Tedsree, K.; Li, T.; Jones, S.; Chan, W. A. C.; Yu, K. M. K.; Bagot, P. J. A.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst. Nature Nanotech. 2011, 6, 302-307. (55) Huang, Y.; Zhou, X.; Yin, M.; Liu, C.; Xing, W. Novel PdAu@Au/C core−shell catalyst: superior activity and selectivity in formic acid decomposition for hydrogen generation. Chem. Mater. 2010, 22, 5122-5128. (56) Zhou, X.; Huang, Y.; Xing, W.; Liu, C.; Liao, J.; Lu, T. High-quality hydrogen from the catalyzed decomposition of formic acid by Pd–Au/C and Pd–Ag/C. Chem. Commun. 2008, 3540-3543. (57) Zhang, S.; Metin, Ö.; Su, D.; Sun, S. Monodisperse AgPd alloy nanoparticles and their superior catalysis for the dehydrogenation of formic acid Angew. Chem. Int. Ed. 2013, 52, 3681-3684.


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(58) Chen, Y.; Zhu, Q.-L.; Tsumori, N.; Xu, Q. Immobilizing highly catalytically active noble metal nanoparticles on reduced graphene oxide: A non-noble metal sacrificial approach. J. Am. Chem. Soc. 2015, 137, 106-109. (59) Zhu, Q.-L.; Tsumori, N.; Xu, Q. Immobilizing extremely catalytically active palladium nanoparticles to carbon nanospheres: A weakly-capping growth approach. J. Am. Chem. Soc. 2015, 137, 11743-11748. (60) Calculated from H2-consumption data published for the 2017 Honda Clarity Fuel Cell. Range calculation based on Japanese JC08 drive cycle. For more information see: http://world.honda.com/news/2016/4160310eng.html. (61) Iguchi, M.; Himeda, Y.; Manaka, Y.; Matsuoka, K.; Kawanami, H. Simple continuous high-pressure hydrogen production and separation system from formic acid under mild temperatures. ChemCatChem 2016, 8, 886-890. (62) Moret, S.; Dyson, P. J.; Laurenczy, G. Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media. . Nat. Comm. 2014, article 4017. (63) Perez-Fortes, M.; Schoneberger, J. C.; Boulamanti, A.; Harrison, G.; Tzimas, E. Formic acid synthesis using CO2 as raw material: Techno-economic and environmental evaluation and market potential. Int. J. Hydrogen Energy 2016, 41, 1644416462. (64) Huang, W.-D.; Zhang, Y.-H. P. Energy efficiency analysis: biomass-towheel efficiency related with biofuels production, fuel distribution, and powertrain systems. PLoS ONE 2011, 6, e22113. (65) Boddien, A.; Loges, B.; Junge, H.; Gaertner, F.; Noyes, J. R.; Beller, M. Continuous hydrogen generation from formic acid: highly active and stable ruthenium catalysts. Adv. Synth. Catal. 2009, 351, 2517-2520. (66) Boddien, A.; Federsel, C.; Sponholz, P.; Mellmann, D.; Jackstell, R.; Junge, H.; Laurenczy, G.; Beller, M. Towards the development of a hydrogen battery. Energy Environ. Sci. 2012, 5, 8907-8911. (67) Fellay, C.; Dyson, P. J.; Laurenczy, G. A viable hydrogen-storage system based on selective formic acid decomposition with a ruthenium catalyst. Angew. Chem. Int. Ed. 2008, 47, 3966-3968. (68) Fellay, C.; Yan, N.; Dyson, P. J.; Laurenczy, G. Selective formic acid decomposition for high-pressure hydrogen generation: a mechanistic study. Chem. Eur. J. 2009, 15 3752-3760. (69) Filonenko, G. A.; van Putten, R.; Schulpen, E. N.; Hensen, E. J. M.; Pidko, E. A. Highly efficient reversible hydrogenation of carbon dioxide to formates using a ruthenium PNP-pincer catalyst. ChemCatChem 2014, 6, 1526-1530. (70) Pan, Y.; Pan, C. L.; Zhang, Y.; Li, H.; Min, S.; Guo, X.; Zheng, B.; Chen, H.; Anders, A.; Lai, Z.; Zheng, J.; Huang, K.-W. Selective hydrogen generation from formic acid with well‐defined complexes of ruthenium and phosphorus-nitrogen PN3‐pincer ligand. Chem. Asian J. 2016, 11, 1357-1360. (71) Hull, J. F.; Himeda, Y.; Wang, W.-H.; Hashiguchi, B.; Periana, R.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Reversible hydrogen storage using CO2 and a protonswitchable iridium catalyst in aqueous media under mild temperatures and pressures. Nat. Chem. 2012, 4, 383-388.


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(72) Wang, Z.; Lu, S.-M.; Li, J.; Wang, J.; Li, C. Unprecedentedly high formic acid dehydrogenation activity on an iridium complex with an N,N’-diimine ligand in water. Chem. Eur. J. 2015, 21, 12592-12595. (73) Boddien, A.; Mellmann, D.; Gaertner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient dehydrogenation of formic acid using an iron catalyst. Science 2011, 333, 1733-1736. (74) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.; Würtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. Lewis acid-assisted formic acid dehydrogenation using a pincer-supported iron catalyst. J. Am. Chem. Soc. 2014, 136, 10234-10237. (75) http://www.icef-forum.org/. (76) Sordakis, K.; Tsurusaki, A.; Iguchi, M.; Kawanami, H.; Himeda, Y.; Laurenczy, G. Carbon dioxide to methanol: the aqueous catalytic way at room temperature. Chem. Eur. J. 2016, 22, 15605-15608. (77) http://www.teamfast.nl/. (78) Also seen in a presentation by X. Guo, J. Zheng, and K.-W. Huang in ICEF2016.


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Formic Acid as a Hydrogen Energy Carrier - ACS Energy Letters

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