Enzyme-Inspired Synthetic Proton Relays Generate Fast and Acid-Stable Cobalt-Based H₂ Production Electrocatalysts

Abstract

Construction of a water-soluble, oxygen-tolerant, and acid-stable synthetic H2 production catalyst is vital for the development of an economic and user-friendly H2-based renewable energy infrastructure. Natural enzyme hydrogenases exhibit excellent energy-efficient H2 production activity. However, fragility of the overall protein structure has restricted their sustainable and practical application. Among the synthetic functional models of hydrogenase, cobaloxime-based complexes offer O2-insensitivity. However, they are only active near neutral conditions with moderate rates and poor aqueous solubility properties. Here, in this work, we have specifically stationed a series of enzyme-inspired, multicomponent outer coordination sphere components around the cobaloxime core to simultaneously improve its catalytic rate, aqueous solubility, and activity even under acidic conditions. We have also established that cobaloximes display catalytic H2 production via two independent mechanisms: (i) Co­(II)-centric and (ii) Co­(I)-centric. Initial Co­(II)-centric H2 evolution occurred at a relatively less reducing potential following the substitution of the axial Cl– ligand with solvent water. Dominant Co­(I)-centric H2 production reactivity was observed in further cathodic potential. Incorporation of dynamic peripheral basic functionalities enhanced H+ trafficking around the cobaloxime core to significantly improve (∼2.0–9.5 times) Co­(I)-centric H2 production reactivity. Complementary NMR and electrochemical results suggest that formation of an intricately interactive water-assisted proton relay neighboring the metal core is the prime reason for this improved activity. Additionally, these peripheral basic functionalities, blended with proton relay, provided an alternative protonation site during the catalysis to induce unprecedented H2 production for cobaloximes under acidic aqueous conditions (pH < 5). Thus, this work provides a prime example of catalytic upgradation of an already existing, moderately active synthetic complex core by encompassing it with precisely positioned enzyme-inspired basic functionalities and water molecules.

Hyaluronic Acid Allows Enzyme Immobilization for Applications in Biomedicine

“…In this regard, research by Ben-Zvi et al, utilized algal biocatalysts within the C. reinhardtii for the natural photoproduction of H 2 [ 3 , 29 ]. Authors have focused on hydrogenase, an enzyme capable of catalyzing the production of with incredible activity and efficiency [ 41 , 42 ], and especially [Fe-Fe]-hydrogenase [ 3 ], the well-known enzyme used for catalytic H 2 turnover [ 41 ]. [Fe-Fe]-hydrogenase (HydA) and superoxide dismutase (SOD) were fused through genetic code sequencing to form a complex that was shown to lead to increased ability for catalyzed production, with the Hyd-SOD fusion protein being subsequently cloned.…”

“…Utilizing cobaloxime complexes consisting of six derivatives with varying side chains, an enzyme-inspired, multicomponent outer coordination sphere was studied as a means to decrease the effect of O 2 on hydrogenase activity as well as to increase enzyme’s rate of reaction, solubility in water, and resistance to acidic environments respectively. [ 42 ]. Authors demonstrated that such enzyme-based complexes led to fast, energy efficient, and/or stable catalytic functionality for more profitable integration in enzyme-based renewable energy sector [ 42 ].…”

“…[ 42 ]. Authors demonstrated that such enzyme-based complexes led to fast, energy efficient, and/or stable catalytic functionality for more profitable integration in enzyme-based renewable energy sector [ 42 ].…”

Mechanistic insights into photocatalysis and over two days of stable H2 generation in electrocatalysis by a molecular cobalt catalyst immobilized on TiO2

“…Thus, productive electron transfer from PS − exclusively occurs to the immobilized Co II with the rate k Co . As cyclic cobalt polypyrphyrins such as 6 and 8 are reduced at −0.35 V vs. SHE, much earlier than cobaloxime (Co II/I = −0.6 V vs. SHE 40,41 ) and below the CB potential of TiO 2 , electron injection via cobalt into the TiO 2 can be excluded.…”

Electrocatalytic and Photocatalytic Hydrogen Evolution by Ni(II) and Cu(II) Schiff Base Complexes

“…The redox properties of these phenolate‐rich complexes are reminiscent of the active metal site of GAO, whose catalytic pathway also includes Cu‐phenolate radical intermediates. Meanwhile, we noticed that diglyoxime ligand platform that share the critical C=N moieties with the Schiff‐base type ligand had been proved very effective for homogenous catalytic proton to hydrogen reduction, especially when coordinating with a cobalt center [27–33] . On the contrary, transition metal complexes with Schiff‐base ligands, such as H 2 L , have been barely explored as hydrogen evolution catalysts [21,34,35] .…”