The factors that control the rate and selectivity of 4e − /4H + O 2 reduction are important for efficient energy transformation as well as for understanding the terminal step of respiration in aerobic organisms. Inspired by the design of naturally occurring enzymes which are efficient catalysts for O 2 and H 2 O 2 reduction, several artificial systems have been generated where different second-sphere residues have been installed to enhance the rate and efficiency of the 4e − /4H + O 2 reduction. These include hydrogen-bonding residues like amines, carboxylates, ethers, amides, phenols, etc. In some cases, improvements in the catalysis were recorded, whereas in some cases improvements were marginal or nonexistent. In this work, we use an iron porphyrin complex with pendant 1,10-phenanthroline residues which show a pHdependent variation of the rate of the electrochemical O 2 reduction reaction (ORR) over 2 orders of magnitude. In-situ surface-enhanced resonance Raman spectroscopy reveals the presence of different intermediates at different pH's reflecting different rate-determining steps at different pH's. These data in conjunction with density functional theory calculations reveal that when the distal 1,10-phenanthroline is neutral it acts as a hydrogen-bond acceptor which stabilizes H 2 O (product) binding to the active Fe II state and retards the reaction. However, when the 1,10-phenanthroline is protonated, it acts as a hydrogen-bond donor which enhances O 2 reduction by stabilizing Fe III −O 2 .− and Fe III −OOH intermediates and activating the O−O bond for cleavage. On the basis of these data, general guidelines for controlling the different possible ratedetermining steps in the complex multistep 4e − /4H + ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.

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One of the major goals of bioinorganic chemistry has been to mimic the function of elegant metalloenzymes. Such functional modeling has been difficult to attain in solution, in particular, for reactions that require multiple protons and multiple electrons (nH + /ne − ). Using a combination of heterogeneous electrochemistry, electrode and molecule design one may control both electron transfer (ET) and proton transfer (PT) of these nH + /ne − reactions. Such control can allow functional modeling of hydrogenases (H + + e − → 1/2 H 2 ), cytochrome c oxidase (O 2 + 4 e − + 4 H + → 2 H 2 O), monooxygenases (RR′CH 2 + O 2 + 2 e − + 2 H + → RR′CHOH + H 2 O) and dioxygenases (S + O 2 → SO 2 ; S = organic substrate) in aqueous medium and at room temperatures. In addition, these heterogeneous constructs allow probing unnatural bioinspired reactions and estimation of the inner-and outersphere reorganization energy of small molecules and proteins.

In-operando spectroscopic observation of the intermediates formed during various electrocatalytic oxidation and reduction reactions is crucial to propose the mechanism of the corresponding reaction. Surface enhanced resonance Raman spectroscopy coupled to rotating disc electrochemistry (SERRS-RDE), developed about a decade ago, proved to be an excellent spectroscopic tool to investigate the mechanism of heterogeneous oxygen reduction reaction (ORR) catalysed by synthetic iron porphyrin complexes under steady state conditions in water. The mechanistic information helped develop better ORR catalysts with 2nd sphere residues in the porphyrin rings. To date, the application of this SERRS-RDE setup is limited to ORR only because the thiol self-assembled monolayer (SAM) modified Ag electrode, used as the working electrode in these experiments, suffers from stability issues at more cathodic and anodic potential where H2O oxidation, CO2 reduction, H+ reduction reaction occurs. The current investigation shows the development of 2nd generation SERRS-RDE setup consisting of Ag-nanostructure (AgNS) modified graphite electrode as the working electrode. These electrodes show higher stability (compare to the conventional thiol SAM modified Ag electrode) upon exposure to very high cathodic and anodic potential with a good signal to noise (S/N) ratio in the Raman spectra. The behaviour of this modified electrode towards ORR is found to be the same as SAM modified Ag electrode and same ORR intermediates are observed during electrochemical ORR. At higher cathodic potential the signatures of Fe(0) porphyrin, an important intermediate in H+ and CO2 reduction reactions, was observed at the electrode-water interface in-operando.

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