We describe the formation of stable, adherent, mesoporous films of 2 nm diameter IrIVO x nanoparticles on glassy carbon electrodes, by a previously unreported method of controlled potential electro-flocculation from pH 13 nanoparticle solutions. These films initiate O2 evolution from water oxidation and then achieve 100% current efficiency, at overpotentials only ∼0.15 and ∼0.25 V higher, respectively, than the reversible H2O/O2 potential. The overpotentials, measured at ∼0.5 mA/cm2, are independent of pH and are the smallest yet reported for electrochemical water oxidation, a property important in possible uses in electrochemical solar cells. The films appear to be mesoporous and microscopically accessible, since O2 evolution currents increase proportionately to multilayer nanoparticle film coverage but without a concurrent increase in overpotential.

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We describe the first example of redox catalysis using a dissolved electroactive nanoparticle, based on the oxidation of water by electrogenerated IrO(x) nanoparticles containing Ir(VI) states, in pH 13 solutions of 1.6 +/- 0.6 nm (dia.) Ir(IV)O(x) nanoparticles capped solely by hydroxide. At potentials (ca. +0.45 V) higher than the mass transport-controlled plateau of the nanoparticle Ir(V/IV) wave, rising large redox catalytic currents reflect electrochemical generation of Ir(VI) states, which by +0.55 V and onward to +1.0 V are shown by rotated ring disk electrode experiments to lead with 100% current efficiency to the oxidation of water to O(2). O(2) production at +0.55 V corresponds to an overpotential eta of only 0.29 V, relative to thermodynamic expectations of the four electron H(2)O-->O(2) reaction. The Ir site turnover frequency (TO, mol O(2)/Ir sites/s) is 8-11 s(-1). Controlled potential coulometry shows that all Ir sites in these nanoparticles (average 66 Ir each) are electroactive, meaning that the nanoparticles are small enough to allow the required electron and proton transport throughout. Both the overpotential and TO values are nearly the same as those observed previously for films electroflocculated from similar IrO(x) nanoparticles, providing the first comparison of electrocatalysis by nanoparticle films with redox catalysis by dissolved, diffusing nanoparticles.

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We have developed the passive-type biofuel cell, in which two-electron oxidation of glucose and four-electron reduction of O2 occur at pH 7 in mediated bioelectrochemical processes, having the maximum power density of 5.0 mW/cm2 at 500 mV. This performance has been achieved by using a new electrolyte solution (Imidazole/HCl buffer, 2 M, pH 7), instead of the general solution (Sodium phosphate buffer, 1 M, pH 7); the maximum power density was a 3.0 mW/cm2 at the same conditions. By introducing the new electrolyte solution into our biofuel cell, we have succeeded in enhancing the catalytic current because the proton transfer into electrodes would increase compared with the phosphate buffer system.

An enzyme-modified electrode was prepared producing a diffusion-limited bioelectrocatalytic current for the reduction of O2 to water at neutral pH and at ambient temperature. The electrode used bilirubin oxidase as an enzyme and [Fe(CN)6]3−/4− as a mediator, both of which were immobilized on the surface of a glassy carbon electrode by electrostatic entrapment with poly-l-lysine.