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The sluggish kinetics of oxygen reduction to water remains a significant limitation in the viability of proton‐exchange‐membrane fuel cells, yet details of the four‐electron oxygen reduction reaction remain elusive. Herein, we apply in situ infrared spectroscopy to probe the surface chemistry of a commercial carbon‐supported Pt nanoparticle catalyst during oxygen reduction. The IR spectra show potential‐dependent appearance of adsorbed superoxide and hydroperoxide intermediates on Pt. This strongly supports an associative pathway for oxygen reduction. Analysis of the adsorbates alongside the catalytic current suggests that another pathway must also be in operation, consistent with a parallel dissociative pathway. [ABSTRACT FROM AUTHOR]

Highlights: [•] Superoxide and peroxide with upright orientation are ORR intermediates at alkaline pH. [•] Vibrational frequency of superoxide intermediate changes with electrolyte. [•] Relation between absorbance and current is discussed in a simple kinetic scheme. [•] A way is shown to obtain rate constants of individual steps in multistep reactions. [•] Reaction from superoxide to peroxide is faster in chloride than in perchlorate. [Copyright &y& Elsevier]

We describe a method for addressing redox enzymes adsorbed on a carbon electrode using synchrotron infrared microspectroscopy combined with protein film electrochemistry. Redox enzymes have high turnover frequencies, typically 10-1000 s-1, and therefore, fast experimental triggers are needed in order to study subturnover kinetics and identify the involvement of transient species important to their catalytic mechanism. In an electrochemical experiment, this equates to the use of microelectrodes to lower the electrochemical cell constant and enable changes in potential to be applied very rapidly. We use a biological cofactor, flavin mononucleotide, to demonstrate the power of synchrotron infrared microspectroscopy relative to conventional infrared methods and show that vibrational spectra with good signal-to-noise ratios can be collected for adsorbed species with low surface coverages on microelectrodes with a geometric area of 25 ? 25 μm². We then demonstrate the applicability of synchrotron infrared microspectroscopy to adsorbed proteins by reporting potential-induced changes in the flavin mononucleotide active site of a flavoenzyme. The method we describe will allow time-resolved spectroscopic studies of chemical and structural changes at redox sites within a variety of proteins under precise electrochemical control. [ABSTRACT FROM AUTHOR]