Homogeneous Carbon Capture and Catalytic Hydrogenation: Toward a Chemical Hydrogen Battery System.

Recent developments of CO2 capture and subsequent catalytic hydrogenation to C1 products are discussed and evaluated in this Perspective. Such processes can become a crucial part of a more sustainable energy economy in the future. The individual steps of this catalytic carbon capture and usage (CCU) approach also provide the basis for chemical hydrogen batteries. Here, specifically the reversible CO2/formic acid (or bicarbonate/formate salts) system is presented, and the utilized catalysts are discussed.

Introduction

Along with the increase of the world’s population, the global economic development, and the will for living in higher standards, the worldwide energy demand supplied by consuming fossil-fuels is still increasing (Figure ). Consequently, the concentration of the most significant greenhouse gas, carbon dioxide (CO2), has increased from a preindustrial level of 280 ppm (parts per million) to currently 416 ppm in the Earth’s atmosphere.[1−3] To mitigate global warming and the resulting climatic changes, renewable energy technologies, including wind, solar, and hydroelectric energy, have to be significantly expanded in the next decade(s). Meanwhile, carbon dioxide emission regulations will gradually take effect since 196 countries/parties have signed the 2015 Paris Agreement aiming to reach carbon neutrality around the year 2050.[4,5] In general, such carbon-neutral status can be achieved on two ways: (1) by replacement of current fossil-based energy technologies toward low-carbon renewable technologies, thus reducing carbon-based energy, and (2) by balancing carbon emissions with carbon offsets via removing carbon dioxide from the atmosphere.

Figure 1

Cumulative fossil fuel (dashed line) with atmospheric CO2 (solid line) increase. Data from Scripps CO2 Program.[10]

In this context, massive CO2 valorization applications are crucial.[6] As a well-known example, carbon capture and storage (CCS) is the process of capturing CO2 before it enters the atmosphere. In general, CCS is based on the separation of CO2 from the off gas of energy conversion or other industrial processes, followed by compression and transport. Although this concept enables the use of fossil fuels with significantly lower CO2 emissions than usual,[7] such storage has to be secured for centuries or even millennia. In addition, constructing CCS units is capital intensive and the overall process is energy intensive. Thus, between 10% and 40% of the energy produced by a power station is required for CCS, whereby 60% originates from the capture process, 30% comes from compression of CO2, and the remaining 10% goes to pumps and fans.[8,9]

As a more concrete example, the energy burden with amine scrubbing constitutes the minimum work (113 kWh/mt CO2 removed) to separate and compress CO2 to 150 bar; as the two demonstration units, Boundary Dam and Thompsons require 210–220 kWh/mt for this purpose.[11] As a consequence, CCS is economically unviable in most countries in the early 21st century.[12]

Alternatively to CCS, carbon capture and utilization (CCU) aims to convert the captured carbon dioxide into more valuable substances or products, such as chemicals, biofuels, and plastics, while retaining the carbon neutrality of the processes.[13] Hence, developing novel methods for CCU processes converting CO2 from air and flue gas not only saves energy from CCS (CO2 desorption and compression steps) but also provides valuable products (Scheme d).[14−21] It is thus an important opportunity for making carbon capture financially viable and achieving a low-carbon economy.[22−24] Apart from fundamental studies, the most prominent example for this approach is the “George Olah Renewable CO2-to-Methanol Plant” in Iceland based on local renewable energy and CO2. The underlying technologies have been steadily developed by Olah, Prakash, and co-workers.[25] The total electrical energy demand is 9.5 MWh/t methanol, and the overall efficiency reaches 60%.

Scheme 1

CO2 Absorbents Based on (a) Conventional Amines, (b) Alkali Hydroxides, and (c) Amino Acids; (d) Concept of CO2 Capture and In Situ Hydrogenation

Carbon Capture

In nature, the most prominent example of carbon capture and utilization is the photosynthesis by which inorganic carbon (in the form of CO2) is converted to organic compounds by living organisms.[26] It is estimated that approximately 258 billion tons of CO2 are converted into biomass by photosynthesis annually.[27] Consequently, a major goal for biological research is the improvement of photosynthesis by the construction of more efficient artificial pathways. For instance, in 2016, Erb’s group reported an improved reaction network of 17 enzymes, which was applied in the synthetic cycle for the continuous fixation and conversion of CO2 into organic molecules in vitro.[28] Recently, another breakthrough of a chemical–biochemical hybrid pathway was reported by Ma and co-workers for starch synthesis from carbon dioxide and hydrogen in a cell-free system.[29] In addition, over the last couple of decades, methods and materials to improve CO2 capture/separation technologies have received a tremendous amount of attention among chemists, including the development of new metal organic frameworks,[30,31] membranes,[32] solid sorbents,[33,34] functionalized ionic liquids,[35−37] reactive separation solvents,[38] and electrochemically driven separations.[39]

As frequently applied in practical postcombustion CO2 capture in large pilot and demonstration plants,[22,40−43] amine-based absorbents are well-known to react with CO2, forming ammonium carbamate and bicarbonate (Scheme a).[7,44−46] To date, alkanolamines are widely studied and applied in this process, although there are also some inherent defects of such amines, for instance, lack of stability, high volatility, and equipment corrosion along with some environmental issues.[47−49] Additionally, alkali hydroxides, e.g., KOH, are also commonly applied for CO2 capture from the atmosphere in industrial scale (Scheme b).[50]

Alternatively, CO2 capture processes with amino acids (AAs) and their alkaline salts have generally attracted much attention due to their ionic nature, low vapor pressure, low toxicity, and high biodegradation potential.[51−54] In the zwitterion mechanism,[55] amino acids react with carbon dioxide to form a zwitterion, followed by the deprotonation step in which the zwitterion reacts with a base, for example, a second molecule of amino acid, water, or hydroxides (Scheme c). Already in 1935, the potassium salt of monomethylaminopropionic acid was first used industrially to remove acid gases (containing H2S and CO2) from gas streams in BASF’s gas treatment known as the Alkazide Process.[56] This gas purification process was originally developed by I.G. Farbenindustrie to treat gas of high sulfur content and has been used since then in many countries all over the world. The carbon capture process was further developed applying various AAs such as glycine, alanine, histidine, lysine, and their alkaline salts.[54,57−61] Those AAs can be produced by microorganisms in a large scale.[62] For example, lysine is industrially produced by microbial fermentation at a >2.2 million tonnes scale per year.[63] Although the amine-based absorbents are generally cheaper than AAs, thermal degradation of amines at high temperature takes place,[64] since cyclic and long chain diamines tend to ring opening and closing at temperatures ≥ 140 °C. Such processes alter the nature and the CO2 sorption capacity of amine-based adsorbents and thus limit their applicability.

Catalytic Hydrogenation of Captured CO2

Catalytic hydrogenation of carbon dioxide dates back to the work of Sabatier and Senderens at the early 20th century utilizing heterogeneous nickel catalysts.[65] Since then, many developments using both heterogeneous and homogeneous catalysts for the hydrogenation of CO2 specifically to methane, methanol, and formic acid (FA) have taken place.[3,16,66−71] We focus in this Perspective on the catalytic hydrogenation of captured CO2. Notably, such reactions have been generally omitted and only during the past decade several homogeneous catalysts based on transition metals, such as Rh and Ru, have been developed in the area of CO2 capture and in situ hydrogenation to C1 products (Scheme d).[72−74] Pioneering works were performed from 2013 by the group of He utilizing RhCl3·3H2O and phosphine containing ligands, for instance, CyPPh2, DPEphos, and PPh3, as catalytic systems, where gaseous CO2 can be chemisorbed by PEI (polyethyleneimine),[75] amidines,[76] and potassium phthalimide[77] then in situ hydrogenated to formates (Scheme ).

Scheme 2

CO2 Capture and Subsequent Hydrogenation Catalyzed by RhCl3·3H2O and Phosphine Ligands

In addition, ruthenium complexes have also been proved as suitable candidates for the in situ hydrogenation of captured CO2 to formates and methanol products. In 2014, Heldebrant and co-workers captured CO2 by DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) in methanol to form methyl carbonate, which then was hydrogenated to formates catalyzed by [RuCl2(PPh3)3] (Scheme ).[78] It is ambiguous at this time whether the alkyl carbonate or CO2 is reduced, as the two species are in equilibrium under such conditions. A mechanism study was demonstrated afterward using a homogeneous [cis-Ru(PNP)2(H)2] (PNP = CH3N[CH2P(CH2CH3)2]) complex that showed the direct insertion of a negatively charged alkyl carbonate into a metal hydride bond.[79] It also showed that the rate of carbonate reduction appears to be faster than the CO2 reduction, potentially due to a different mechanism or because of the lower concentration of CO2 in solution, as compared to the captured CO2 in the form of hexyl carbonate. Last year, the same group reported a 100% atom efficient method where epoxides reacted with CO2 leading to cyclic carbonates with molecular amine as the catalyst. Then, hydrogenation of in situ formed propylene carbonate resulted in good yields (>95% for propylene glycol and 84% for methanol) and selectivity with Ru-MACHO as the catalyst (Scheme ).[80]

Scheme 3

Co2 Capture and In Situ Hydrogenation Catalyzed by Ruthenium-Pincer Complexes

In 2015, Milstein and co-workers reported a novel CCU approach, where CO2 reacted with aminoethanols yielding oxazolidinones which were then hydrogenated to CH3OH in 78–92% yield. The CO2 absorbent aminoethanols and the bipyridine-backbone-based Ru-PNN pincer catalyst were recovered.[81] In 2017, Franciò, Leitner, and co-workers reported a semicontinuous CCU process employing cis-[RuCl2(dppm)2] (dppm: 1,1-bis(diphenylphosphino)methane) as the catalyst and MEA (monoethanolamine) as the base.[82] It is worth mentioning that a TTON (total turnover number) of 150 000 was achieved to produce formate over 11 runs (Scheme ).

In 2011, the Sanford group reported a well-designed cascade reaction for the hydrogenation of CO2 to methanol, where three different homogeneous catalysts, (PMe3)4Ru(Cl)(OAc), Sc(OTf)3, and (PNN)Ru(CO)(H), operate in sequence to promote this transformation (Scheme ).[83] Later on, the same group showed that CO2 capture can be achieved in the presence of NHMe2 to form the corresponding carbamate. Subsequent catalytic hydrogenation yields a mixture of DMF and CH3OH in the presence of the well-known Ru-MACHO-BH complex (Scheme ).[84]

Scheme 4

Cascade Hydrogenation of CO2 to Methanol

Employing the same complex as catalyst and TMG (tetramethylguanidine),[85] metal hydroxides,[86] PEHA (pentaethylenehexamine),[87−89] metal hydroxide/ethylene glycol,[90] TMBDA (tetramethylbutanediamine)/ethylene glycol[91] as CO2 absorbent systems, notable studies were described by Olah, Prakash, and co-workers combining CO2 capture with subsequent hydrogenation to produce formates and methanol. Beside the higher catalyst TON for methanol, Olah, Prakash and co-workers performed the CO2 capture directly with air and flue gas. Recently, our group reported, for the first time, a naturally occurring amino acid (lysine)-based system for CO2 capture and catalytic hydrogenation to produce formates with the same Ru catalyst, giving rise to high TONs up to 55 000.[61] It is noteworthy that, in this method, CO2 can be captured directly from ambient air in the form of carbamates and hydrogenated directly to formates in an one-pot manner (Scheme ). This method offers an environmental benign alternative pathway to access formate in the presence of nontoxic CO2 absorbent amino acid.

To develop a cost-efficient and environmentally benign process of integrated CO2 capture and hydrogenation to produce C1 products,[92−98] inexpensive and nontoxic catalysts are preferred. In this respect, earth-abundant 3d-transition metal catalysts are specifically desired.[99−103] However, up to now, only a few non-noble metal catalysts for such processes, i.e. Fe and Mn, were reported (Scheme ).

Scheme 5

Earth-Abundant Transition Metals in CO2 Capture and In Situ Hydrogenation

For example, Zhou and co-workers described in 2015 an effective, phosphine-free, air- and moisture-tolerant catalyst system based on Knölker’s iron complex for the hydrogenation of in situ captured carbon dioxide to formate.[104] Notably, the catalyst system can hydrogenate bicarbonate at comparably low hydrogen pressures (1–5 bar). In 2016 and 2018, an Fe-PNP pincer-type complex was reported by Olah, Prakash, and co-workers for the CCU process to produce formate using PEHA[85] and NaOH,[86] respectively. Compared with the Ru analogue, Fe shows nearly the same activities (TON) regarding on the formate production; however, the latter was much slower in CO2-to-methanol transformation.[85,87] Last year, the group of Bernskoetter applied morpholine in the presence of a combination of Fe-PNP’ complex/LiOTf to produce methanol via the CCU approach.[105] The CO2 reduction process occurs through initial coupling with H2 and an amine to give N-formyl morpholine. Subsequently, this formamide undergoes catalytic deaminative hydrogenation to afford methanol and regenerates the amine.[106] A net turnover number of 590 is obtained using this protocol (Scheme ).

Apart from iron, recently manganese-based catalysts have gained significant interest for hydrogenations, owing to its abundant, nontoxic, biocompatible, and environmentally friendly nature.[107−110] Thus, a Mn-PNP complex was described by the groups of Prakash and Olah to produce formate[86] and methanol.[87] Like the related iron PNP pincer complex, the manganese one is comparable in activity with the Ru analogue in the formate production, but not in the case of methanol. Apparently, the second hydrogenation step of the formate or formamide toward methanol is more difficult in the presence of non-noble metals. In another work of Olah, Prakash and co-workers, a sequential one-pot CO2 hydrogenation to CH3OH is demonstrated with a slightly higher TON of 36 in the presence of morpholine catalyzed by the same Mn complex.[111] Very recently, beside ruthenium based catalysts,[61] our group realized a high TON of formate (180 000) in the CCU process by selection of the proper combination of triazine based Mn-pincer catalysts and an amino acid (l-lysine, Scheme ).[112]

Development of a Chemical Hydrogen Battery

Hydrogen (H2) attracts increasing attention as a clean energy carrier, which could be produced from renewable resources (namely, green hydrogen in this case), for example, by electrochemical water splitting, and produces only water and heat in the fuel cell processes.[113−116] In fact, transportation and handling of hydrogen gas is quite troublesome due to its physical properties and explosive nature in mixtures with oxygen. However, this situation could be solved by transforming hydrogen gas to solid or liquid compounds,[117−121] e.g., by catalytic CO2 hydrogenation to methanol and formic acid (FA) or its formate salts. As both are stable compounds, they can be stored and decomposed on demand to H2 and CO2 under milder conditions compared to most liquid organic hydrogen carriers (Figure ),[16,66,122−131] thus decoupling hydrogen storage and release regarding place and time. When H2 is stored in FA, no hydrogen is lost in the form of water. The hydrogen content in FA (53 g/L, 4.4 wt %) is comparable to that in H2 storage alloys, e.g., magnesium hydrides (2–6 wt %[132]) and lower than that in methanol (99 g/L, 12.6 wt %).

Figure 2

Concept of chemical hydrogen battery based on the reversible CO2/FA transformation (left) and the corresponding bicarbonate/formate couple (right).

It should be noted that the described CCU processes in the previous section are of interest for the renewable production of fuels or chemicals from carbon dioxide and thereby can contribute to a zero-carbon economy. For example, the produced FA can be applied as a green preservative or methanol provides the basis for green polymers (via methanol-to-olefin processes or as a green additive for ship fuels). Furthermore, catalytic CCU provides possibilities for new energy storage concepts, e.g., so-called hydrogen batteries. Generally, a battery is a source of electric power consisting of electrochemical cells for powering electrical devices. Rechargeable batteries can be discharged and recharged multiple times under applied electric current. Following this concept, a chemical hydrogen battery would be a device through which hydrogen can be charged (stored) and discharged (released) on demand. Obviously, such technology offers significant potential toward clean energy technologies for carbon neutrality.

As first suggested (but never realized) by Williams et al. in the late 1970s,[133] an ideal case of hydrogen storage involves reversible FA and CO2 transformation (Figure , left): here CO2 is combined with green H2 to yield FA with 100% atom efficiency (charging step), and later FA is subjected to on-demand selective hydrogen generation (discharging step). In the presence of base, an equivalent hydrogen storage couple employing formate/bicarbonate salts as chemical carriers in hydrogen storage and transportation is also a viable approach, as initially proposed by Zaidman et al. (Figure , right).[134,135]

Therefore, an eligible chemical hydrogen battery device consists of a closed vessel, for example, an autoclave, containing the hydrogen storage media: CO2/FA (or HCO3–/HCO2–) solvent, catalyst, and any potential additives, where hydrogen will be charged at elevated pressure and discharged at lower pressure through the vessel. The charge/discharge steps could also be controlled by temperature regulation. Clearly, to avoid additional feedstock, energy input, and reaction steps, the reloading of the storage media (including catalysts, bases and so on) between each cycle should be avoided as much as possible. Compared to the CO2/FA couple, it is advantageous to handle the aqueous bicarbonate and formate solutions as they are benign and noncorrosive; however, the H2 storage capacity of the latter system is limited by the solubilities of those carriers in aqueous solution. Furthermore, the solubilities of formate salts vary according to the corresponding cations. For example, at 20 °C, the molar solubilities (mol/100 mL H2O) of formate salts based on alkali metals are observed in the following order: Rb+(4.25) > K+(4.01) > Cs+(2.14) > Na+(1.19) > Li+(0.76).[136] Considering the costs, the capacity of H2 storage might be improved especially applying potassium formate. In the case of a CO2/FA hydrogen storage cycle, it is in general possible to capture the released CO2 and hydrogenate it back to FA in the recharging step, thus achieving carbon-neutrality. However, this additional carbon capturing step makes the process tedious, thus getting neglected in most current applications. Such a chemical H2 battery may offer also advantages regarding the low CO content in the produced hydrogen, which is important for many PEM (Proton Exchange Membrane) fuel cell applications.

Until recently, for chemical H2 batteries, relatively few hydrogen storage-release cycles have been demonstrated and most systems needed noble metal-based catalysts,[135] i.e., Rh, Ru, and Ir (Figure ). One of the first examples of a H2 battery was carried out by Leitner and Gaßner in 1994. They utilized a catalytic system based on [Rh(cod)Cl]2 and dppb (1,4-bis(diphenylphosphino)butane). While the reversible formation of formic acid from CO2 and H2 was achieved at r.t. (room temperature) with a TON up to 2 200 in a polar aprotic solvent (acetone) and tertiary amine (NEt3),[137] the H2 battery cycle was performed only once (Figure ). This concept was further developed by applying ruthenium complexes. Hence, in 2011, both our group[138] and the Joó group[139] reported the reversible hydrogen storage in bicarbonate-formate solutions, through [RuCl2(benzene)]2/dppm and [RuCl2(mTPPMS)2]2/mTPPMS catalytic systems (mTPPMS: sodium diphenylphosphinobenzene-3-sulfonate), respectively. Here, up to three reversible cycles were achieved. As a drawback, using the catalytic system of [RuCl2(benzene)]2/dppm, the hydrogenation and dehydrogenation steps had to be performed in different solvents, DMF/H2O and THF/H2O, respectively, which limits any application. Slightly increased H2 battery cycles (up to 5) were reported by Laurenczy et al. in 2015 via the above-mentioned reactions catalyzed by the [RuCl2(mTPPTS)2]2/mTPPTS system (mTPPTS: trisodium triphenylphosphine-3,3′,3″-trisulfonate).[140]

Figure 3

Reported hydrogen storage-release cycles based on formic acid/formate applied with homogeneous catalysts. DH: dehydrogenation, H: hydrogenation, emim: 1-ethyl-3-methylimidazol-2-ylidene, PNNP: N1,N2-dibenzyl-N1,N2-bis(2-(diphenylphosphino)benzyl) ethane-1,2-diamine, FA: formic acid, PC: propylene carbonate.

In 2012, the concept of reversible hydrogen storage was further verified via employing the [RuH2(dppm)2] complex for the reversible CO2 hydrogenation by us and Laurenczy et al. to increase the H2 storage cycles up to 8.[141] Then, the groups of Pidko,[142] Plietker,[143] and Czaun/Prakash/Olah,[144] found that Ru-PNP-pincer or Ru-PNNP catalysts allow higher stability for the reversible CO2 hydrogenation in the presence of amines or NaOH in cyclic operation with up to 10 H2 storage cycles. In 2018, Laurenczy, Li, and co-workers reported a sodium formate/bicarbonate based rechargeable hydrogen battery in the presence of Ru(acac)3/triphos/Al(OTf)3 (1:1:2.4)[145] whereas the enhancement of the activity and stability by aluminum triflate was proposed by activating the precatalysts and formic acid along with a stabilization effect on the active cationic Ru species (Figure ).

Interestingly, well-defined iridium complexes have shown the highest catalytic activity in CO2-to-formate transformation to date. For example, Nozaki and co-workers reported in 2009 an Ir-PNP pincer-type complex which led to a very high catalyst TON (up to 3 500 000) for the CO2 hydrogenation with KOH as the base.[146] However, the FA dehydrogenation step was studied under different conditions (in the presence of amines like TEA (triethylamine) or TEOA (triethanolamine),[147] which makes it difficult to perform the H2 storage-release cycle in one device. Soon after, Hull, Himeda, Fujita, and co-workers discovered the first water-soluble Ir-based catalyst with N∧N-bidentate ligands capable of reversible H2 storage using CO2 in aqueous media under mild conditions.[148] As the hydrogenation and dehydrogenation steps require specific pH environments, the addition of H2SO4 or KHCO3 is necessary to reach a recyclable system. Similar to the [RuCl2(mTPPMS)2]2/mTPPMS system, Joó and Horváth reported a reversible hydrogen storage battery system in aqueous cesium formate/bicarbonate solutions catalyzed by the Ir(I)-N-heterocyclic carbene complex using flow systems (Figure ).[149,150]

In the past decade, homogeneous catalysis with non-noble metals has become an important topic for many applications. In this respect, Enthaler and Junge et al. reported nickel-PCP pincer-type complexes and applied them in the catalytic decomposition of formic acid/amine mixture and the hydrogenation of sodium bicarbonate to sodium formate.[151] In 2018, Bernskoetter, Hazari, and co-workers reported the PNMeP supported iron precatalysts that are more productive than the PNHP ligand for both CO2 hydrogenation and formic acid dehydrogenation.[152] However, a complete hydrogen storage and release cycle was not demonstrated in both cases. Clearly, to realize any real-world application in this area, it is necessary that the individual hydrogenation and dehydrogenation steps can be performed in the same set up under similar reaction conditions (solvent, no specific additives for the two steps, etc.). In addition, it is important, when using carbon dioxide as the hydrogen acceptor, that the recharging process can be performed without adding carbon dioxide again for each cycle. Notably, all these requirements could be realized most recently in a practical carbon-neutral hydrogen battery based on the reversible hydrogenation of carbon dioxide to formate.[112] By utilizing α-amino acid salts, e.g., potassium lysinate, and a specific Mn-pincer complex, a rechargeable hydrogen battery system was achieved with >80% H2 evolution efficiency and >99.9% CO2 retention in 10 charge–discharge cycles, therefore avoiding additional CO2 reloading steps between each cycle (Figure ).

Conclusions and Take-Home Message

To reach a carbon-neutral society and economy, it will be key to develop new and improved technologies in the next decade (i) for the conversion and storage of renewable energy and (ii) to utilize and recycle nonavoidable carbon dioxide. Both grand challenges can be addressed by utilizing solar, wind, and hydropower to produce “green hydrogen” as a first step, which then will provide the basis for many possibilities such as

Catalysis Enables Viable CCU (Carbon Capture and Utilization) Processes

Compared to carbon capture and storage (CCS), CCU is a more realistic strategy for reducing CO2 amounts because it also creates value-added products and thereby permits economic viability. Key for practical CCU processes is not only the development of suitable catalysts for direct air capture of carbon dioxide and its immediate reductive functionalization (hydrogenation) but also their systems integration. Notably, different CCU methods might be viable in the future depending on the general political and as important regiospecific conditions. One novel concept among the various approaches is the so-called hydrogen battery, which is discussed in this perspective.

Hydrogen Battery as a Concept for Efficient Hydrogen Storage

In general, a hydrogen battery is a device that stores chemical energy in form of hydrogen. After its release, hydrogen can be easily converted to electric energy by using fuel cell technologies. To reach a simple and truly rechargeable hydrogen battery device, both hydrogenation and dehydrogenation steps should be performed under unified reaction conditions (same catalyst, solvent, and base and avoid additional catalyst/base/additive loading between the individual steps or cycles). For example, CO2 is the byproduct in the formic acid (or formate) dehydrogenation step, which can be released together with H2. As CO2 acts as hydrogen carrier, once it is lost from the system, the H2 storage capacity of the system will decrease accordingly. Therefore, retaining CO2 in the system (namely capturing CO2 after every dehydrogenation step) is of benefit in maintaining the H2 storage capacity as well as omitting the reloading of hydrogen carrier materials.

Outlook and Steps to the Next Level

While the hydrogenation of carbon dioxide to formic acid or methanol and the reverse reactions have been extensively studied with both homogeneous and heterogeneous catalysts in the past decade, the combination of these individual steps to create an integrated device (hydrogen battery system) has been largely neglected. In addition, most of the work published in this area was of a more conceptual nature. In order to fully exploit the possibilities this methodology, the following advances are needed:

Process Perfection

By proper design of reaction conditions and the combination of CO2 absorbents and catalyst both hydrogenation and dehydrogenation steps should be performed in the same device without changing solvents, additives, etc. Ideally, the once introduced carbon dioxide stays in solution after every dehydrogenation step, integrating carbon capture and hydrogen storage-release cycles with this approach and, thus, omitting additional carbon dioxide charging steps. Furthermore, it is more important running the process in an optimal energy efficient way. This requires process engineering and intensification apart from molecular understanding. Therefore, a close cooperation with scientists working in chemical engineering and device development must take place.

Next Generation Catalysts

With respect to the catalyst, high reactivity for both reaction steps should be achieved. Notably, the stability of the whole system is a crucial point for any application. To be clear here, for any application, sufficient stability is required for >100 charging and recharging cycles. In the past years, specific transition metal-based pincer complexes dominated for the catalytic hydrogenation of captured CO2. Apart from their reactivity, the low tolerance against air or oxygen makes them inconvenient for practical applications. Therefore, developing water-soluble and air-resistant complexes, especially based on non-noble metals, could be advantageous to avoid the use of any organic solvent and the inert protecting atmosphere.

Development of Heterogeneous Catalysts

Although most research works of this topic utilize homogeneous catalysts, from the viewpoint of application, there is still the practical desire for heterogeneous catalytic systems, with respect to the easier application in many renewable energy systems on larger scale. However, compared with homogeneous catalysts, many of the reported heterogeneous ones provide lower TON and TOF. And relatively few non-noble metal based heterogeneous catalysts have been developed for hydrogen storage applications.

Make Use of Methanol

In addition to FA/formate, methanol, which can also be synthesized easily from CO2 or FA/formate hydrogenation, should be considered as a potential hydrogen carrier in a hydrogen battery device. Obviously, due to its higher hydrogen content, such a system would be potentially more energy efficient. Methanol can be converted to electric energy either by direct methanol fuel cells (DMFCs) or by a combination of methanol reforming to a hydrogen containing gas and PEM fuel cells (PEMFCs). The currently achievable energy conversion efficiencies for operational PEMFCs and DMFCs are 40–80% and 20–40%, respectively.[153] Therefore, intensive research to increase especially the DMFC efficiency is demanded.[154,155] Reversible methanol reforming based on amines has also been reported recently for potential hydrogen storage applications[121,156−158]

General Considerations

To be noted, our present scientific system limits to some extent the implementation of the here presented technology, which is also true for many other high risk/high gain topics. There are many beautiful/innovative basic works in catalysis, organic and inorganic chemistry, which offer significant potential for applications but stay on the small-scale level because most (young) researchers are forced to look for the next fast publication instead of having the time to investigate the “real” difficult problems in detail. Moreover, our traditional boundaries of scientific disciplines have to be overcome because interdisciplinarity is necessary for further advancements.

Finally, we hope that the results summarized in this Perspective and the presented concept will stimulate many colleagues in their activities to develop novel processes for the direct utilization of CO2 to produce liquid fuels or other value-added products.