How Light-Harvesting Semiconductors Can Alter the Bias of Reversible Electrocatalysts in Favor of H₂ Production and CO₂ Reduction

Abstract

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO2/CO and H+/H2 interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO2 or H+ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO2 nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction.

Solar-driven proton and carbon dioxide reduction to fuels — lessons from metalloenzymes

“…On TiO 2 , CdS, and NiO, the naturally intense electrocatalytic activity is controlled instead by the properties of the electrode material [60 ,67 ,68 ], specifically the availability of the majority carriers. The reduction potentials of the H + /H 2 and CO 2 /CO couples lie close (within k B T) to the flatband potentials E fb of the n-type materials TiO 2 and CdS: as a result, the normally reversible electrocatalytic activity of CODH and [NiFeSe]-H2ase is rectified because the n-type materials become increasingly insulating as the potential is raised through and above E fb [67 ]. In contrast, when attached to p-type NiO, CODH behaves as a unidirectional CO oxidiser in the dark: holes are the majority carriers and the depletion layer prevents electrocatalytic CO 2 reduction [60 ].…”

Recent progress on advanced design for photoelectrochemical reduction of CO2 to fuels

“…Recently, Armstrong's group have exploited highly active carbon monoxide dehydrogenase (CODH) to serve as the co-catalyst for PEC CO 2 reduction [32,164]. For instance, a CODH-form carboxydothermus hydrogenoformans was employed to functionalize a dye-sensitized p-NiO photocathode, which could selectively reduce CO 2 to CO under visible-light illumination with good stability (Fig.…”

Recent Progress and Novel Applications in Enzymatic Conversion of Carbon Dioxide

“…Of the semiconductor materials that they investigated, photocatalysts prepared with Evonik Degussa P25 TiO2 (a commercially available 3:1 mixture of anatase/rutile) and ordinary anatase TiO2 nanoparticles were found to be the most effective, in terms of overall turnover rate. More recently, Bachmeier et al [23] demonstrated that semiconducting electrodes can be used to impose directionality on reversible catalysts operating in the region of the flatband potential. Although these enzymes are unsuited for long-term, large-scale systems, their results provide valuable insight for developing integrated artificial systems (based ultimately on abundant chemical catalysts) [23].…”

“…The reaction rate can be increased More recently, Bachmeier et al [23] demonstrated that semiconducting electrodes can be used to impose directionality on reversible catalysts operating in the region of the flatband potential. Although these enzymes are unsuited for long-term, large-scale systems, their results provide valuable insight for developing integrated artificial systems (based ultimately on abundant chemical catalysts) [23].…”