The development of low-carbon technologies that will facilitate the efficient use of hydrogen (H₂) as an energy carrier is a critical requirement of contemporary society. To this end, it is anticipated that the cost of H₂ production will become a key factor in tandem with production efficiency, process safety, and transport. Much effort has been made to create and develop new, reversible, and sustainable H₂ storage systems. Among current techniques, formic acid (FA) has been identified as an efficient energy carrier for H₂ storage. Numerous homogeneous catalysts based on transition metals with high activity and selectivity have been reported for selective FA dehydrogenation. In this review, we outline the recent advances in transition-metal molecular catalysts for FA dehydrogenation. Selected catalytic systems that could be implemented on an industrial scale and considered potential materials in fuel cell (FC) technology have been cost-evaluated. We highlight some critical engineering challenges faced during the technology’s scale-up process and explain other factors that are frequently ignored by academic researchers. Finally, we offer a critical assessment and identify several system limitations on an industrial scale that are currently impeding future implementation.

Commercial use of H₂ production catalysts requires a repeated use/stop/store and reuse of the catalyst. Ideally, this cycle should be possible under ambient O₂. Herein we exemplify the concept of Use-Store-Reuse (USR) of a (Ru-phosphine) catalyst in a biphasic catalytic system, for H₂ production via dehydrogenation of HCOOH. The catalytic system can operate uninterrupted for at least four weeks, including storage and reuse cycles, with negligible loss of its catalytic efficiency. The catalytic system consisted of a RuP(CH₂CH₂PPh₂)₃ (i.e. RuPP3) in (tri-glyme/water) system, using KOH as a cocatalyst, to promote HCOOH deprotonation. In a USR cycle of 1 week, followed by storage for three weeks under ambient air and reuse, the system achieved in total TONs > 90,000 and TOFs > 4000 h⁻¹. Thus, for the first time, a USR concept with a readily available stable ruthenium catalyst is presented, operating without any protection from O₂ or light, and able to retain its catalytic performance.

Two types of iron-based catalysts, [Fe/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/PP3], for the dehydrogenation of formic acid (FADH), were synthesized. These catalysts were developed using a double-ligand approach combining a PNP ligand and a PP3 ligand, demonstrating functionality without the need for additional cocatalysts or additives. Furthermore, hybrid catalysts [Fe/SiO₂@^iProPNP/PP3] and [Fe/SiO₂@^tBuPNP/PP3] were created by covalently grafting PNP ligands onto SiO₂ particles. The hybrid [Fe/SiO₂@^iProPNP/PP3] exhibited enhanced recyclability, with turnover numbers (TONs) exceeding 74,000. In situ ATR-FTIR and UV-Vis spectroscopies were used to monitor the structure and dynamics of the catalysts under catalytic conditions, revealing the formation of active catalysts through the involvement of all components: [Fe (metal)/PNP (first ligand)/PP3 (second ligand)/FA (substrate)], which are crucial to FADH catalysis. An Arrhenius study revealed that the hybrid [Fe/SiO₂@^iProPNP/PP3] had a lower activation energy (E_a = 42.5 kJ/mol) compared to its homogeneous counterpart (E_a = 48.2 kJ/mol), indicating superior catalytic performance. Conversely, [Fe/SiO₂@^tBuPNP/PP3] showed an increased activation energy (E_a = 48.3 kJ/mol) compared to its homogeneous form (E_a = 46.4 kJ/mol). This study discusses the differing roles of ^tBuPNP and ^iProPNP in catalyst configuration, highlighting the potential of double-ligand catalysts to enhance the performance and recyclability of PNP ligands in FADH, offering significant implications for the development of efficient and reusable catalytic systems.

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