Reversible hydrogenation of carbon dioxide to formic acid using a Mn-pincer complex in the presence of lysine

Efficient hydrogen storage and release are essential for effective use of hydrogen as an energy carrier. In principle, formic acid could be used as a convenient hydrogen storage medium via reversible CO2 hydrogenation. However, noble metal-based catalysts are currently needed to facilitate the (de)hydrogenation, and the CO2 produced during hydrogen release is generally released, resulting in undesirable emissions. Here we report an α-amino acid-promoted system for reversible CO2 hydrogenation to formic acid using a Mn-pincer complex as a homogeneous catalyst. We observe good stability and reusability of the catalyst and lysine as the amino acid at high productivities (CO2 hydrogenation: total turnover number of 2,000,000; formic acid dehydrogenation: total turnover number of 600,000). Employing potassium lysinate, we achieve >80% H2 evolution efficiency and >99.9% CO2 retention in ten charge–discharge cycles, avoiding CO2 re-loading steps between each cycle. This process was scaled up by a factor of 18 without obvious drop of the productivity. Formic acid is a convenient hydrogen storage medium with storage release occurring via reversible hydrogenation of CO2 and facilitated by noble metal-based catalysts. Now, reversible storage release is demonstrated using a non-noble, Mn-based catalyst in the presence of an amino acid.

A s the most significant greenhouse gas, carbon dioxide (CO 2 ) has risen from pre-industrial levels of 280 ppm (parts per million) to 419 ppm in the Earth's atmosphere in February 2022, along with the exponential global energy demand supplied by carbon-rich fossil fuels (Supplementary Fig. 1) 1,2 . The extensive CO 2 valorization and zero-CO 2 emission technologies are crucial to mitigate global warming and related climatic deterioration. To accomplish such purpose, 196 countries/parties have signed the 2015 Paris Agreement committing to reach net-zero CO 2 emissions around the year 2050. Generally, there are two approaches to realize carbon neutrality: reducing carbon emissions by shifting towards alternative energy technologies and balancing carbon emissions with carbon usage.
In this context, the feasibility study of hydrogen (H 2 ) as a clean alternative energy carrier has inspired growing attention because it could be prepared from renewable resources, for example, by electrochemical water splitting, and produces nothing but water and energy in fuel cells 3,4 . However, it is troublesome to transport and store hydrogen gas due to its physical and explosive properties in mixtures containing oxygen. This situation can be solved by converting hydrogen gas to solid or liquid organic hydrogen carriers 5,6 , for example, by catalytic CO 2 hydrogenation. Following this concept, besides methanol and Fischer-Tropsch products (hydrocarbons), formic acid (HCO 2 H, FA) and its formate salts also are readily accessible. Both are stable compounds that can be stored and dehydrogenated on demand to H 2 and CO 2 under milder conditions compared with other liquid organic hydrogen carriers (Fig. 1a) [7][8][9] , separating hydrogen storage and release without the restriction of time and place. It should be noted that hydrogen is wasted partly in the form of water when it is stored in methanol, which is not the case for formic acid, even though the hydrogen content in FA (4.4 wt%) is lower than that in methanol (12.6 wt%). Overall, a FA-based H 2 storage and release system may also benefit from the CO content in the generated hydrogen (usually less than 10 ppm), which is important for its application in fuel cells.
An electric battery is commonly a source of electric power containing electrochemical cells to power electronic products. Therefore, reusable electric batteries can be discharged and recharged multiple times under electric current. According to this concept, a chemical hydrogen battery is a device where energy is stored in the form of hydrogen that is discharged and recharged as needed. Hydrogen is directly converted to electric energy by using fuel cell technologies after its release out of the chemical hydrogen battery. Obviously, such technology offers substantial potential as a clean energy technology working towards carbon neutrality.
Only limited examples of such chemical hydrogen batteries have been demonstrated to date. Clearly, catalysts active in both hydrogenation and dehydrogenation reactions are essential and only a few molecularly defined transition metal-based complexes are known to fulfil this mission. Most systems contain expensive noble metal-based catalysts, for example, Rh (ref. 10 ), Ru (refs. [11][12][13][14][15][16][17][18] and Ir (refs. 19,20 ) (Supplementary Fig. 2). An example of a heterogeneous catalyst was reported in 2016 when a reusable bimetallic catalyst was developed to hydrogenate and dehydrogenate N-heterocycles efficiently 21 . Besides, it's difficult to reach a simple and truly rechargeable hydrogen storage and release device when it comes to non-unified reaction conditions in H 2 recharge and discharge steps, for example, catalysts, solvents 11,22 , bases change 20 , pH control 19 , reloading of storage media (concerning catalysts, H 2 carriers and so on) between each charge-discharge cycle 13 or generally low catalytic efficiency (that is, turnover numbers). Furthermore, in all currently known examples, the release of simultaneously produced CO 2 in the dehydrogenation process results in not only undesirable carbon emissions but also inferior H 2 purity for subsequent fuel cell applications 13,15 . Consequently, there is an actual demand for carbon-neutral hydrogen storage and release technologies that integrate the carbon capture (recycling the released CO 2 , dotted arrows in Fig. 1b) or in an ideal condition avoid CO 2 release in the dehydrogenation reactions (bold arrows in Fig. 1b).
The homogeneous catalyst development for the two individual steps of chemical hydrogen storage and release-CO 2 hydrogenation to FA 23 and its dehydrogenation processes 7 -focused traditionally on noble metals. However, non-noble metal-based complexes have been proven to be valuable, more recently 24,25 . In this context, apart from iron 9,26 , manganese especially is of important interest, owing to its abundant, non-toxic, biocompatible and environmentally friendly features 27 . However, Mn pincer complexes were first recognized for catalytic hydrogenation reactions in 2016 [28][29][30][31][32] . Thus, examples of homogeneous Mn catalysts for CO 2 hydrogenation to FA [33][34][35][36][37] and its dehydrogenation [38][39][40][41] are rather limited and often come with far lower catalytic efficiency compared with their noble-metal counterparts (Fig. 2). Most important of all, Mn catalysts are rarely reported so far to be efficient in both CO 2 hydrogenation to FA and its dehydrogenation under unified reaction conditions (same catalyst, base, solvent and so on). Recently, a ruthenium-catalysed CO 2 fixation to formates in the presence of amino acids (AAs) was reported by our group 42 . We speculated that such a system could also be used to develop state-of-the-art carbon-neutral hydrogen storage and release instead of using conventional CO 2 absorbents, for example, alkanolamines 43 . Among the tested AAs, lysine (Lys) offers several advantages for the fixation of CO 2 as it is an essential AA, industrially produced from microbial fermentation with >2.2 million tonnes scale per year 44 .
Herein we provide a sustainable hydrogen storage and release method integrating the reversible hydrogenation of CO 2 to FA and carbon capture processes. In our catalytic system, we use the α-amino acid Lys and a specific manganese complex as the carbon absorbent and catalyst. We show that such a system can catalyse the reversible hydrogenation of CO 2 to FA in high productivities. With potassium lysinate, we achieve high evolution efficiency of hydrogen and retain CO 2 inside of the cycles.

Catalytic CO 2 hydrogenation
Initially, using several manganese, iron, cobalt, and rhenium complexes with bidentate and tridentate ligands, we performed the Lys-promoted CO 2 hydrogenation (Supplementary Table 1 Tables 2-3). Only when using another basic AA arginine (Arg) bearing a guanidino group in its side chain, formate is produced in 62% yield.

Catalytic dehydrogenation of formic acid
Next, we investigated the H 2 evolution from formic acid (5.0 mmol) in the presence of Lys (1.0 equivalent) at 90 °C using the Mn catalysts previously active in the hydrogenation step. The Mn-PN (H) P-Br complexes Mn-1, 2 and 3 promoted the H 2 formation in yields up to 70% equivalent to 3.5 mmol H 2 and TON 17,500 ( Table 2, entries 1-3). Again, Mn-PN 5 P iPr -Br (Mn-6) catalysed the H 2 release more efficiently with quantitative H 2 yield and a TON of 29,400, while Mn-5 and Mn-7 gave obviously lower productivities ( Table 2, entries 4-6). Lowering the quantity of Mn-6 to 0.10 μmol (20 ppm), the conversion was incomplete with much lower productivity (Table 2, entry 7). Decreasing the temperature to 85 °C, 74% yield and TON >21,000 were still obtained ( Table 2, entry 8). Compared with Lys, Arg was found to be less active, leading to H 2 in 46% yield (Table 2, entry 9). Utilizing the potassium salt of Lys (LysK), a high yield of H 2 also was achieved (94%, Table 2, entry 10). In addition, Table 2 shows the H 2 :CO 2 molar ratios in the gas products that contain generally less than 50% CO 2 , demonstrating again the good CO 2 capture effect induced by Lys in the FA dehydrogenation reactions. Notably, LysK led to >99.9% CO 2 retention, which offers the possibility for further reuse of CO 2 . Additionally, blank reactions reveal that in the absence of Mn-6 or Lys, no product was detected (Supplementary Tables 4  and 9). Applying FA/LysK mixtures in the presence of other organic solvents, for example, 2-methyl-THF, triglyme and ethanol, H 2 was produced in up to 88% yield with >99% CO 2 retention (Supplementary Table 14 and Supplementary Fig. 38). In addition, H 2 evolution from FA/LysK mixtures in an open system (using a manual burette) presented a complete FA conversion within 3 h, which resulted in a higher CO 2 ratio (14%, Supplementary  Figs. [39][40][41]. Time-dependent experiments of CO 2 hydrogenation and FA dehydrogenation applying Lys and Mn-6 were performed (Supplementary Table 16). Slightly decreased formate yield in the CO 2 hydrogenation reaction was observed at 90% in 6 h, which dropped to 12% in 3 h. On the other hand, performing the FA dehydrogenation reaction in a shorter time led to decreased H 2 yields (72% in 6 h and 4% in 3 h).

Reusability of Mn catalyst
To avoid extra addition of catalysts or bases between the H 2 storage-release cycles, we investigated the reusability of catalyst and base beforehand (Fig. 3). Based on the biphasic solvent system ( Supplementary Fig. 26), we achieved a facile recycling of the Mn catalyst and organic solvent by separation of the organic and aqueous layers. Therefore, catalyst Mn-6 could be reused for ten consecutive runs in hydrogenation of CO 2 to formate reactions. About 80% of the initial productivity remains after ten runs, resulting in a remarkable total TON (TTON) of 2,050,000 for formate production ( Fig. 3a and Supplementary Table 8). The decrease of formate yield is probably due to the incomplete catalyst separation between each run rather than catalyst deactivation as no free phosphine ligand is detected by 31 P NMR in the separated organic phase.
Then, we evaluated the reusability of Mn-6 as well as Lys in FA dehydrogenation (Fig. 3b). After each run, a new batch of FA was reloaded into the reaction mixture (first to fifth runs: 5.0 mmol FA and sixth to tenth runs: 20.0 mmol FA). Following this procedure, Mn-6 catalyst and Lys were reused for ten consecutive runs. Fortunately, more than 89% of the theoretical H 2 productivity was achieved, resulting in an excellent TTON of 676,700 for hydrogen. Moreover, the H 2 :CO 2 molar ratios in the gas phase are found to be approaching the theoretical value 50:50 after ten runs (Supplementary Table 10 and Supplementary Figs. [29][30][31]. Besides recycling the Mn-6/Lys catalyst system in up to ten runs, the stability in long-term storage was evaluated. Quantitative H 2 yield in FA dehydrogenation was obtained after storing the corresponding reaction solution for two weeks at room temperature under argon. Overall, these results demonstrate the high stability and reusability of the Mn-6 catalyst in both the CO 2 hydrogenation and FA dehydrogenation processes.

Hydrogen storage-release cycles applying Lys
Next, we combined CO 2 hydrogenation to formate and its dehydrogenation applied Lys and Mn-6 complex. Due to the CO 2 loss in the H 2 release step, CO 2 was reloaded for each hydrogenation step (Fig. 4a). The H 2 storage-release cycles start from the dehydrogenation of FA: Mn-6 (0.17 μmol), FA:Lys (5.0:5.0 mmol), THF:H 2 O (5:5 ml) in a 100 ml autoclave at 90 °C for 12 h. The inside pressure was then carefully released to the manual burettes at room temperature and analysed by GC. Then, CO 2 is reloaded under CO 2 pressure (20 bar for 0.5 h or 2 bar for 6 h). Afterwards, the autoclave was filled with H 2 (80 bar) and heated at 85 °C for 12 h. After the completion of the H 2 storage step, the reactor was subjected to H 2 release. Following this procedure, ten consecutive cycles in total were performed with >90% yield of H 2 evolution (CO 2 reloading at 20 bar). Using lower CO 2 (2 bar) pressure, an average hydrogen yield of >82% is still achieved. Notably, we performed this carbon-neutral hydrogen storage-release methodology even with CO 2 from air (reloading at ambient conditions for 24 h). Following this concept, over 72% yield of H 2 was obtained in ten consecutive cycles (Fig. 4c

Hydrogen storage-release cycles applying lysinate salts
Even though we performed the hydrogen storage-release cycles with Lys in excellent efficiency, the development of a practical rechargeable chemical hydrogen battery is not yet achieved. Ideally, such devices consist of closed autoclaves containing the hydrogen storage media (in this case, CO 2 ), where hydrogen is charged and discharged conveniently. Obviously, the reloading of the storage media in this system between each cycle should be avoided as far as possible because it requires additional feedstock, energy input and reaction steps. To overcome such issues and demonstrate our main goal of a stable and practical chemical hydrogen storage and release with efficient H 2 storage-release cycles (Fig. 4b), we tested different lysinate salts (LysM, M = K, Na, Li; Fig. 4c and Supplementary  [35][36][37]. Hence, applying LysM instead of Lys, the H 2 evolution efficiency reached ≥80% (LysK), ≥60% (LysNa) and ≥46% (LysLi), respectively, in ten charge-discharge cycles. The observed cation-based effect is consistent with the observation of a previous work for the hydrogenation of imines, where potassium tert-butoxide led to a higher hydrogenation rate than the sodium one 47 . Notably, CO 2 reloading was not necessary in H 2 storage-release cycles applying LysM, thus greatly simplifying the process. After one cycle of hydrogen release and storage, 4.6 mmol formate (92%) was obtained applying 5 mmol LysK. As a prototype example (0.3 l autoclave), the H 2 evolution process applying LysK was scaled up to 90.0 mmol without obvious drop of the efficiency in at least ten charge-discharge cycles ( Fig. 4d and Supplementary Table 13), demonstrating its applicability.

Mechanistic investigations
To understand both catalyst and CO 2 capture efficiency, we tried to detect the main reaction intermediates. Thus, we performed stoichiometric reactions in Young-NMR tubes and analysed them by in situ NMR (Fig. 5a) Table 15) revealed the presence of the α-amino acid group and an appropriate basic side chain in the amine molecule that are crucial for both the Mn-catalysed CO 2 hydrogenation reaction and CO 2 capture processes. On the basis of the in situ NMR studies, control experiments above and the previously reported research work 33,47-49 , we propose the following catalytic cycle on the reversible CO 2 hydrogenation catalysed by Mn-PN 5 P iPr complexes (Fig. 5b). The catalyst precursor is first activated by an excess of LysK via N-H deprotonation and dearomatisation of the triazine moiety, leading to active species I-1, which is described as a bimetallic Mn-K species 50,51 . During the H 2 storage step (green pathway), dihydrogen is activated via heterolytic cleavage resulting in a Mn-hydride species I-2 and transforming LysK into Lys. After insertion of C = O bond in CO 2 to Mn-H I-2, the Mn-OOCH intermediate I-3 is formed. Then, HCOOH is liberated by transforming 1 equivalent of Lys to LysK, which regenerates the active species I-1 after dearomatisation of the triazine moiety. The reverse reaction (pink pathway) enables the HCOOH dehydrogenation to H 2 and CO 2 , accordingly.

Conclusions
The present work describes a concept for carbon neutral hydrogen storage and release. Utilizing a molecularly defined manganese Reaction conditions adopted from Table 1, entry 11, and Table 2, entry 5, respectively. The dotted lines serve as guides to the eye.
complex in the presence of naturally occurring Lys, high efficiency for direct CO 2 hydrogenation to formate is achieved (93% yield; 2,000,000 TTON). On the other hand, the same system promotes H 2 generation from FA in the presence of Lys with >99% yield and a TTON of 600,000. This reaction system exhibited high stability and reusability. On the basis of these results, the combination of the individual processes was realised. Notably, we performed such hydrogen storage-release cycles without the addition of extra AA, catalyst and especially CO 2 due to the excellent CO 2 capture effect (99.9%) of LysK. It is noteworthy that the CO content in the produced H 2 gas was below 10 ppm throughout the process. We successfully scaled up this method without obvious drop of the productivity in at least ten charge-discharge cycles. The current methodology represents one of the most productive combinations of CO 2 valorization and FA dehydrogenation applying homogeneous non-noble metal-based catalysts. The results inspire further research towards practical applications and pave the way for building up a carbon-neutral chemical hydrogen storage and release set-up by employing non-hazardous AAs and benign catalysts.

Measurement of formate yield in the hydrogenation of CO 2 .
The respective amount of catalyst (0.10 μmol or 0.02 μmol) was dispensed from the stock solution; Lys (5.0 mmol), THF (5 ml) and H 2 O (5 ml) were added to a 50 ml autoclave equipped with a magnetic stir bar. After pressurizing the reactor with CO 2 and H 2 gas, the reaction mixture was heated and stirred on a pre-heated oil bath for 12 h. Then the reactor was cooled to room temperature and the inside pressure was carefully released. A biphasic reaction mixture was obtained containing a transparent organic upper layer and an aqueous yellow lower layer. Addition of DI water (~3 ml) to the above-mentioned mixture resulted in a homogeneous solution ( Supplementary Figs. 3-4) (ref. 42 ). DMF (250 μl, 3.24 mmol) was added as an internal standard to the reaction mixture. The reaction mixture was then analysed by 1 H NMR with a few drops of D 2 O (~1 ml) to lock the signals 34 . Yield of formate is calculated by (mmol formate)/(mmol Lys) × 100%. TON of formate is calculated by (mmol formate)/(mmol catalyst). The gas mixtures were analysed by GC, and no concomitant of CO/CH 4 was found.
Measurement of H 2 yield in the FA dehydrogenation. Appropriate amount of catalyst (0.10-0.20 μmol) was dispensed from the stock solution, FA (5.0 mmol), Lys (5.0 mmol), THF (5 ml) and H 2 O (5 ml) were added to a 100 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully to the manual burettes. A 5 ml degassed syringe was used to obtain a gas sample analysed by GC. CO is not detectable in all cases (below the CO quantification limit of 10 ppm). Yield of H 2 is calculated by (mmol H 2 )/(mmol HCOOH) × 100%. TON of H 2 is calculated by (mmol H 2 )/(mmol catalyst).
Calculation of the hydrogen volume, mole, yield and TON. The gas evolution was corrected with the blank volume (18 ml), which corresponds to the gas evolution of the reaction without any catalyst. H 2 volume, VH 2 , and CO 2 volume, V CO2 , are calculated with the following equation: Moles of H 2 , nH 2 , and moles of CO 2 , n CO2 , are calculated with the following equation: Hydrogen yield YH 2 is calculated with the following equation: The turnover number (TON) of H 2 is calculated with the following equation: V obs is the gas evolution volume of the catalytic reaction measured in the manual burettes. V blank is the gas evolution volume of the blank reaction measured in the manual burettes.  : where Catalyst recycling studies in CO 2 hydrogenation. Catalyst Mn-6 (0.02 μmol, dispensed from the stock solution), Lys (730.9 mg, 5.0 mmol), THF (5 ml) and H 2 O (5 ml) were added to a 50 ml autoclave equipped with a magnetic stir bar.
After pressurizing the reactor with CO 2 (20 bar) and H 2 (60 bar) gas, the reaction mixture was heated and stirred on a pre-heated oil bath at 115 °C for 12 h. Then the reactor was cooled to room temperature, and the inside pressure was carefully released. A biphasic reaction mixture was obtained containing a transparent organic upper layer and an aqueous yellow lower layer ( Supplementary Fig. 26).
After the separation of the two layers, the aqueous lower layer was washed and extracted with additional THF (1 ml × 2), and the combined organic parts were used directly for the next run by adding a new batch of 5.0 mmol Lys. Meanwhile, DMF (250 μl, 3.24 mmol) was added into the aqueous lower layer as an internal standard, then analysed by 1 H NMR with a few drops of D 2 O (~1 ml) to lock the signals 34 . A total of ten runs of CO 2 hydrogenation to formate were performed by using the above-mentioned procedure. Yield of formate is calculated by (mmol formate)/(mmol Lys) × 100%. TON of formate is calculated by (mmol formate)/ (mmol catalyst). The gas mixtures were analysed by GC, and no concomitant of CO/CH 4 was found.
Catalyst recycling studies in FA dehydrogenation. Catalyst Mn-6 (0.1 mg, 0.17 μmol), FA, Lys (730.9 mg, 5.0 mmol), THF (5 ml) and H 2 O (5 ml) were added to a 100 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath for 12 h. The reactor was cooled to room temperature and the inside pressure was released carefully to the manual burettes, and the content of the gas phase was analysed by GC. CO was not detectable in all cases (below the CO quantification limit of 10 ppm). After each run, a new batch of FA (first to fifth runs: 5.0 mmol FA and sixth to tenth runs: 20.0 mmol FA) was loaded into the reaction mixture, then heated and stirred on a pre-heated oil bath for 12 h. Following this procedure, a total of ten runs of catalytic dehydrogenation of FA in the presence of Lys were performed. Yield of H 2 is calculated by (mmol H 2 )/(mmol HCOOH) × 100%. TON of H 2 is calculated by (mmol H 2 )/(mmol catalyst).

Measurement of hydrogen evolution in the H 2 storage-release cycles applying
Lys combined with CO 2 capture. The H 2 storage-release cycles start from the dehydrogenation of FA (H 2 release): Mn-6 (0.1 mg, 0.17 μmol), FA (188.6 μl, 5.0 mmol), Lys (730.9 mg, 5.0 mmol), THF (5 ml) and H 2 O (5 ml) were added to a 100 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath at 90 °C for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully to the manual burettes. The content of the gas phase was analysed by GC. CO was not detectable in all cases (below the CO quantification limit of 10 ppm). Then CO 2 is replenished under the following conditions-capture with CO 2 (20 bar or 2 bar): after the completion of the above-mentioned FA dehydrogenation, the 100 ml autoclave was charged with CO 2 (20 bar for 0.5 h or 2 bar for 6 h). After releasing the overpressure of CO 2 , the 100 ml autoclave was filled with 80 bar of H 2 . Then the reaction mixture was heated and stirred on a pre-heated oil bath at 85 °C for 12 h. Afterwards, the reactor was cooled to room temperature, and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure. Following this procedure, a total of ten runs were performed via CO 2 recharging under 20 bar or 2 bar of CO 2 . Capture from ambient air: after the completion of FA dehydrogenation, a biphasic reaction mixture containing a transparent organic upper layer and a pale yellow aqueous lower layer was obtained. After the separation of the two layers, the aqueous lower layer was washed with additional THF (1 ml × 2), and the combined organic layer was reserved for the following H 2 storage step. On the other hand, the aqueous layer was subjected to CO 2 capture from ambient air at room temperature for 24 h. After bubbling with argon for 0.5 h, this aqueous layer was combined with the above-mentioned organic layer and loaded to a 100 ml autoclave. The autoclave was then filled with 80 bar of H 2 and heated and stirred on a pre-heated oil bath at 85 °C for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure. Following this procedure, a total of ten runs were performed with CO 2 capture from ambient air. It should be noted that no H 2 evolution was observed at room temperature in the H 2 storagerelease cycles for up to 12 h.

Measurement of hydrogen evolution in the H 2 storage-release cycles applying
LysM (without CO 2 reloading). The lysinate salts (LysM, 5.0 mmol) was prepared via stirring Lys (5.0 mmol) with 1.0 equivalent of corresponding alkali metal hydroxides MOH (KOH, NaOH, LiOH) for 30 min in 1 ml DI water at room temperature before the H 2 storage-release cycles. The H 2 storage-release cycles start from the dehydrogenation of FA (H 2 release): Mn-6 (0.1 mg, 0.17 μmol), FA (188.6 μl, 5.0 mmol), LysM (5.0 mmol), THF (5 ml) and H 2 O (4 ml) were added to a 100 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath at 90 °C for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully to the manual burettes and the content of the gas phase was analysed by GC. CO was not detectable in all cases (below the CO quantification limit of 10 ppm). After the completion of FA dehydrogenation, the 100 ml autoclave was filled with 80 bar of H 2 . Then the reaction mixture was heated and stirred on a pre-heated oil bath at 85 °C for 12 h. Afterwards, the reactor was cooled to room temperature, and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure. Following this procedure, a total of ten runs were performed with 5.0 mmol LysK, LysNa, and LysLi. were added to a 300 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath at 90 °C for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully to the manual burettes and the content of the gas phase was analysed by GC. CO is not detectable in all cases (below the CO quantification limit of 10 ppm). After the completion of FA dehydrogenation, the 300 ml autoclave was filled with 80 bar of H 2 . Then the reaction mixture was heated and stirred on a pre-heated oil bath at 85 °C for 12 h. Afterwards, the reactor was cooled to room temperature, and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure. Following this procedure, a total of ten runs were performed with LysK in 20.0 mmol scale. The 90 mmol lysine potassium salt (LysK, 90.0 mmol) was prepared via stirring Lys (90.0 mmol) with 1.0 equivalent of KOH for 30 min in 20 ml DI water at room temperature before the H 2 storage-release cycles. The H 2 storage-release cycles started from the dehydrogenation of FA (H 2 release); Mn-6 (1.8 mg, 3.05 μmol), FA:LysK (90.0:90.0 mmol), THF (30 ml) and H 2 O (60 ml) were added to a 300 ml autoclave equipped with a magnetic stir bar. The reaction mixture was then heated and stirred on a pre-heated oil bath at 90 °C for 12 h. The reactor was cooled to room temperature, and the inside pressure was released carefully to the manual burettes and the content of the gas phase was analysed by GC. CO was not detectable in all cases (below the CO quantification limit of 10 ppm). After the completion of FA dehydrogenation, the 300 ml autoclave was filled with 80 bar of H 2 . Then the reaction mixture was heated and stirred on a pre-heated oil bath at 85 °C for 12 h. Afterwards, the reactor was cooled to room temperature, and the inside pressure was released carefully. Then the autoclave was subjected to the H 2 release procedure. Following this procedure, a total of ten runs were performed with LysK at 90.0 mmol scale.

Data availability
All the relevant data are included in the published article and its Supplementary Information.