2. Formic Acid as An Efficient Liquid Carrier for H2 Storage
FA’s high density (1.22 g cm
3) translates into a volumetric H
2 concentration of 53 g H
2/L, which is equivalent to 1.77 kWh L
−1. This surpasses the industry standards of the Toyota Mirai (700 bar H
2 storage tank with 1.4 kWh L
−1), which consumes 0.76 kg H
2/100 km [
26,
27]. Despite the advent of electricity-driven technologies and the manufacturing of FA from green feedstock, the FA market is still restricted until new breakthroughs impose its broader commercialization [
28]. FA, a typical industrial reagent, is non-flammable, non-toxic, simple to handle at room temperature, and does not require high-pressure storage or a catalytic reforming unit. It is one of the most widely used organic chemical raw ingredients in the pesticide, leather, dyes, medicine, and rubber sectors [
29]. FA supplied at a concentration of 85% is the worldwide industry standard, while 99% is required for particular applications [
30]. The global output of FA ranged from 750,000 to 800,000 tons between 2014 and 2019, with the price being $400/ton. The Global FA Market was worth 756.5 million $ in 2018 and is projected to reach 828.1 million $ by the end of 2025, expanding at a CAGR (compound annual growth rate) of 1.3% between 2018 and 2025 [
29]. The reduced labor and capital costs, along with the constantly expanding market, will enable China to boost its FA production capacity, making it the leading producer and exporter of FA [
28].
Regarding the involvement of FA in energy issues, FA decomposition occurs via the two routes, as depicted in
Figure 3a. The first equation describes HCOOH dehydrogenation into H
2 and CO
2, whereas in the second one, HCOOH dehydration provides H
2O and CO, which is responsible for the inactivation of fuel cell electrodes. Both reactions are dependent on the employed catalysts, the temperature, and the concentration of FA [
31]. Using an appropriate catalyst at low temperatures, both the dehydrogenation of FA (FADH) to CO
2 and the hydrogenation of CO
2 to FA (FAH) are possible, resulting in a “carbon-neutral cycle” (See
Figure 3b) [
32].
Figure 3. (a) FA properties and decomposition reactions (b) Cycle of FA dehydrogenation and regeneration by CO2 hydrogenation.
CO
2 hydrogenation to FA is a crucial challenge targeting a double benefit for the planet since it prevents environmental pollution caused by CO
2 emissions from industrial effluents and, at the same time, it regenerates a substantial organic feedstock. A carbon-neutral H
2 storage/release cycle is feasible and it consists of the quantitative hydrogenation of CO
2 to form FA, followed by its selective dehydrogenation [
23]. Up-to-day, dehydrogenation of FA, using a molecular catalyst, has gained a lot of interest in the research community, due to its higher performance in mild conditions vs. the hydrogenation path [
20]. Based on this potential, it is possible to envision automotive applications in which gasoline is substituted by FA and automobiles are equipped with technology based on fuel cells; either direct formic acid fuel cells (DFAFC) [
33] or hydrogen fuel cells (HFC) such as PEMFCs (Proton-exchange membrane fuel cells) [
26].
DFAFCs are advantageous for compact portable HFC applications and promising for automotive batteries of electric vehicles. They offer the potential for a carbon-neutral cycle in which CO
2 is collected and subsequently electrolyzed to produce FA. Although DFAFCs conversion of FA to electricity seems to be simple, their performance is restricted by fuel crossover and anode catalyst poisoning [
34]. PEMFCs, on the other hand, are well-established for use in electric vehicles, with maximum power density higher than DFAFCS, i.e., 1400–2000 mW/cm
−2 [
35] vs. 550 mW/cm
−2 [
36] (
Figure 4).
Figure 4. The reactions that occur at the anode and cathode of (a) Direct Formic Acid Fuel Cells (DFAFC), (b) Proton-exchange membrane fuel cells (PEMFC), (c) Schematic representation of a PEMFC.
An efficient catalyst for H
2 generation to be used in PEMFC must meet the following requirements: selectivity, high stability (TON), high activity (TOF), and low cost [
37]. In order to obtain customer acceptance of a new technology, economic factors are of paramount significance. Huang recently evaluated eight homogenous systems which rely on the use of two dimension-free equations, CON (Catalyst cost normalized to the TON value) and COF (Catalyst cost normalized to the TOF value) [
37] (
Figure 5). In this methodology, it is proposed that for a catalyst to be considered valuable for use in automobile applications, it should have a maximum CON and COF value of 0.35 [
37], a range of TOF between 5000 to 10,000 h
−1, and TON in the order of multi-millions [
38]. The same research group, in another perspective, enrich their methodology with a life cycle assessment (LCA) data analysis of FA with CO
2 capture and storage (CCS) and CO
2 capture and utilize prospects (CCU) [
23]. As depicted in
Figure 5, the set thresholds for a product (H
2 in this example) and the overall reactor system (tank, converter, controller) allow for a comparison of the economic viability of new technologies to those now in use. For each process, there exists a region in the CON/COF diagram where catalyst costs satisfy the corresponding cost threshold requirements. Catalysts with greater CON or COF values result in a higher-than-desired product cost (CON > CON
0) or system cost (COF > COF
0). Consequently, the green region in the diagram denotes the aim for catalysts that economically meet all the required criteria.
Figure 5. Μargins of catalyst price normalized vs. TON (CON) or TOF (COF). According to cost-analysis methodology, “product” refers to the total cost of all raw materials, while Cs is the system expenses without a catalyst (e.g., fuel tank, pumps, FA converter, and controller). Adapted from [
37] with permission.
Moreover, to determine how close a technology is to operational deployment and to evaluate the development status of emerging technologies, the U.S. Department of Defense and Horizon 2020 proposed the TRL definition (see
Figure 6) [
39,
40]. Based on this technological assessment, the scale-up implementation of some homogeneous catalysts for the majority of FA-to-power technologies ranges between TRL 7 and TRL 8 [
30,
41,
42,
43]. However, for these systems to be applied in a real operational environment [TRL 9], a lot of further improvement is needed as the requirements for this level take into consideration the catalytic activity in tandem with the decrease of the cost.
Figure 6. Technology Readiness Levels (TRL) for FA dehydrogenation catalytic systems.