Novel bioinspired and biohybrid electrode materials for hydrogen production

Abstract: This thesis was accomplished under the scope of eSCALED project administrated by EU MSCA horizon 2020 program, which aimed to develop a device called “artificial leaf” responsible for generating fuels or liquid chemicals (H2 production or CO2 reduction) using solar electrolysis. My objective was to develop a noble metal free, efficient cathode materials for H2 production for the device which yield this thesis. For developing catalysts, we took inspiration from [FeFe] hydrogenase enzymes on merit of their impressive H+/H2 conversion activity (TOF ~10,000 s-1) with negligible overpotential requirements at neutral pH using earth abundant metals. Subsequently, we choose {Fe2(μ-S2)(CO)6} based active site core and designed it as per requirements. Firstly, our aim was to develop a robust anchoring strategy to immobilize the {Fe2(μ-S2)(CO)6}based catalyst on electrode. We designed the diiron site with an anchoring group i.e. pyrene to graft it on multiwalled carbon nanotubes (MWNT) using π-π interaction. The resulting catalyst showed moderate electrochemical H2 production activity at neutral pH while immobilized on electrode. Post operando assessment revealed the degradation of active sites while anchoring group remained intact throughout the catalysis.Secondly, to improve the catalysis further, the active site was encapsulated inside a designed water-soluble polymeric scaffold comprising pyrene as an anchoring group. The resulting metallopolymers functionalized MWNT showed about two-fold increase in electrochemical H2 production activity with relatively low overpotential requirements than isolated complex discussed earlier. However, the catalysis was limited by degradation of the active site. In addition, life cycle assessments (LCA) were performed to evaluate the environmental footprint for H2 production by metallopolymers.Thirdly, we aimed to replace the active site inside metallopolymers with a relatively robust diiron site which resulted in a marginal improvement of durability with an expense of about three times lower activity than previous metallopolymers. Finally, we aimed to study semi-artificial hydrogenases by replacing the native cofactor of the [FeFe] hydrogenase with a synthetic cofactor. Combination of spectroscopy, electrochemistry and site-directed mutagenesis revealed some key insights on structural orientation of active site, activity, sensitivity towards inhibitors like CO, O2 etc., due to changes in structural and electronic properties of the active site.

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