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The worldwide energy system developed in the past century is highly dependent on non-renewable fossil feedstocks. Hence, the concentration of carbon dioxide (CO
2) in the atmosphere has continuously increased from 280 ppm (parts per million) to > 420 ppm since the industrial revolution [
1–
5]. This excessive emission of CO
2 causes global warming and ocean acidification. In this context, the Paris Agreement on Climate Change adopted in 2015 recognized the necessity of holding the increase of global average temperature below 2 °C, preferably 1.5 °C, above pre-industrial levels, and reaching a CO
2-neutral process by 2050. As an important component of this, China proposed a two-step “double carbon” target: carbon peak emission by 2030 and carbon neutrality by 2060.
To transform our present energy system to a more sustainable one, green hydrogen (H
2) is widely accepted as a clean high-energy carrier, producing H
2O as the only product in fuel cells, therefore serving as a promising alternative to traditional fossil fuels. However, H
2 gas is explosive in the presence of O
2 and has a very low energy density under normal conditions. Therefore, the key to a competitive H
2 economy is exploring energy-efficient chemical methods of storing and releasing it. As an example of H
2 carrier molecules, formic acid (HCOOH), a biomass-derived source of energy, contains 4.4 wt.% H
2 with 53
/L volumetric storage, making it a good storage medium for H
2 [
6]. Furthermore, formic acid-based reversible H
2 storage and release systems generate CO-free (< 10 ppm) H
2, which is advantageous for fuel cell applications as they can be easily poisoned in the presence of a very small amount of CO.
In this respect, the development of efficient catalysts for both processes of CO
2 hydrogenation to formic acid, and H
2 discharge by dehydrogenation of formic acid is a crucial issue. To date, cost-prohibitive noble metals such as Pd, Ru, Rh, and Ir, are the dominant catalysts for these reactions (Fig.1, left) [
7–
10]. Therefore, the development of efficient non-noble metal-based catalysts is of interest. In this regard, manganese (Mn) complexes received significant research attention in recent years and such catalysts have exhibited their effectiveness for CO
2 hydrogenation to formic acid [
11–
13] and formic acid dehydrogenation [
14–
16]. In addition, we reported that the performance of a ruthenium catalyst for CO
2 fixation to formate is greatly enhanced by adding amino acids, particularly lysine [
8]. We conceived an idea for this system in our recent publication [
17]. In the current study, our group, Leibniz Institute for Catalysis (LIKAT) together with an industrial partner, APEX Energy Teterow GmbH, applied a non-noble metal Mn-pincer complex that can chemically store and release H
2 at will with impressive activity and in a highly pure form (Fig.1, right). On this basis, systems based on the principle of a battery could in the future donate hydrogen anytime and anywhere, for example, to power fuel cells.
Differing from the previously published systems where the produced CO
2 from the process of formic acid dehydrogenation must be released and charged again for the subsequent CO
2 hydrogenation step, the presented reaction system permanently retains the CO
2 (> 99.9%) in the process by employing the common amino acid L-lysine or its potassium salt. The newly developed system follows the principle of an electric battery, but differs because H
2 is used instead of electricity. Such a chemical hydrogen battery is thus filled once at the beginning with CO
2 from air and is able to store energy in the form of H
2 reversibly. Under optimized conditions, Mn-catalyzed CO
2-to-formate transformation reached 92% yield with a turnover number (TON) of 2.3 × 10
5. Applying the same solvent and concentration of L-lysine, the yield of HCOOH-to-H
2 transformation reached 99% with a TON of 2.94 × 10
4. Taking advantage of the biphasic solvent of H
2O/THF, the organic upper layer containing the Mn-pincer complex after CO
2 hydrogenation reactions could be easily separated for a subsequent next run. Consequently, 80% of the initial activity was preserved after 10 consecutive runs of H
2 storage (hydrogenation) and release (dehydrogenation) without reloading of CO
2, which led to an outstanding total TON of 2.0 × 10
6 and 6.0 × 10
5 for CO
2 hydrogenation (93% yield to formic acid) and formic acid dehydrogenation (> 99% yield to H
2), respectively. The reported productivity is even higher than the noble-metal system such as homogeneous base-promoted Ru catalyst [
18]. Furthermore, the system can be easily scaled up to 90 mmol without sacrificing its productivity.
A process on this basis is expected to fulfill its full potential when the H2 to be stored is derived from renewable sources such as wind power, photovoltaics, or even biomass (Fig.1, middle). Although our study proposes an effective strategy for a carbon-neutral vision and will inspire more research on reversible chemical H2 batteries, further investigation is needed for practical applications. For example, the development of cheaper catalysts with facile and scalable approaches should be considered. In addition to high activity and selectivity, durability up to several hundreds or even thousands of hours should be pursued in the future to allow application of the technique. Finally, in-depth kinetic studies and insightful mechanistic analysis should be employed which will be helpful to develop principles of designing more efficient catalysts for this type of reaction.
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