ABSTRACT
Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors.
Subject(s)
Photosynthesis , Solar Energy , Catalysis , Light , SemiconductorsABSTRACT
We report on the interplay between light absorption, charge transfer, and catalytic activity at molecular-catalyst-modified semiconductor liquid junctions. Factors limiting the overall photoelectrosynthetic transformations are presented in terms of distinct regions of experimental polarization curves, where each region is related to the fraction of surface-immobilized catalysts present in their activated form under varying intensities of simulated solar illumination. The kinetics associated with these regions are described using steady-state or pre-equilibrium approximations yielding rate laws similar in form to those applied in studies involving classic enzymatic reactions and Michaelis-Menten-type kinetic analysis. However, in the case of photoelectrosynthetic constructs, both photons and electrons serve as reagents for producing activated catalysts. This work forges a link between kinetic models describing biological assemblies and emerging molecular-based technologies for solar energy conversion, providing a conceptual framework for extracting kinetic benchmarking parameters currently not possible to establish.
ABSTRACT
Rationally designed material interfaces offer opportunities to control matter and energy across multiple length scales, yet remain challenging to synthetically prepare. Inspired by nature, where amino acid residues and soft-material coordination environments regulate the midpoint potentials of metals in proteins, thin-film polymeric coatings have been developed to assemble molecular components, including catalysts, onto solid-state (semi)conducting surfaces. In this report, we describe the immobilization of metalloporphyrins onto transparent conductive oxide supports using either direct grafting to the oxide surface or coordination to an initially applied thin-film polypyridyl coating. The composite materials enable direct measurements of electrochemical and optical properties associated with the surface-immobilized components. Despite the similarity of the core cobalt porphyrin units used in assembling these hybrid architectures, the redox potentials assigned to the CoIII/II relays span a 350 mV range across the distinct constructs. This range in redox potential is extended to 960 mV when including comparisons to constructs utilizing polymer-immobilized cobaloxime catalysts in place of cobalt porphyrins, where reduction of the cobaloximes requires significantly more-negative bias potentials. This work illustrates the use of soft-material interfaces for assembling molecular-modified electrodes where the nanoscale connectivity of the surface coatings determines the electrochemical properties of the macroscopic assemblies.
ABSTRACT
We report the immobilization of hydrogen-producing cobaloxime catalysts onto p-type gallium phosphide (111)A and (111)B substrates via coordination to a surface-grafted polyvinylimidazole brush. Successful grafting of the polymeric interface and subsequent assembly of cobalt-containing catalysts are confirmed using grazing angle attenuated total reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Photoelectrochemical testing in aqueous conditions at neutral pH shows that cobaloxime modification of either crystal face yields a similar enhancement of photoperformance, achieving a greater than 4-fold increase in current density and associated rates of hydrogen production as compared to results obtained using unfunctionalized electrodes tested under otherwise identical conditions. Under simulated solar illumination (100 mW cm(-2)), the catalyst-modified photocathodes achieve a current density ≈ 1 mA cm(-2) when polarized at 0 V vs the reversible hydrogen electrode reference and show near-unity Faradaic efficiency for hydrogen production as determined by gas chromatography analysis of the headspace. This work illustrates the modularity and versatility of the catalyst-polymer-semiconductor approach for directly coupling light harvesting to fuel production and the ability to export this chemistry across distinct crystal face orientations.