Breaking a Molecular Scaling Relationship Using an Iron-Iron Fused Porphyrin Electrocatalyst for Oxygen Reduction.

The design of efficient electrocatalysts is limited by scaling relationships governing trade-offs between thermodynamic and kinetic performance metrics. This ″iron law″ of electrocatalysis arises from synthetic design strategies, where structural alterations to a catalyst must balance nucleophilic versus electrophilic character. Efforts to circumvent this fundamental impasse have focused on bioinspired applications of extended coordination spheres and charged sites proximal to a catalytic center. Herein, we report evidence for breaking a molecular scaling relationship involving electrocatalysis of the oxygen reduction reaction (ORR) by leveraging ligand design. We achieve this using a binuclear catalyst (a diiron porphyrin), featuring a macrocyclic ligand with extended electronic conjugation. This ligand motif delocalizes electrons across the molecular scaffold, improving the catalyst's nucleophilic and electrophilic character. As a result, our binuclear catalyst exhibits low overpotential and high catalytic turnover frequency, breaking the traditional trade-off between these two metrics.

Molecular-Modified Photocathodes for Applications in Artificial Photosynthesis and Solar-to-Fuel Technologies.

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.

Photoelectrochemistry of metalloporphyrin-modified GaP semiconductors.

Photoelectrosynthetic materials provide a bioinspired approach for using the power of the sun to produce fuels and other value-added chemical products. However, there remains an incomplete understanding of the operating principles governing their performance and thereby effective methods for their assembly. Herein we report the application of metalloporphyrins, several of which are known to catalyze the hydrogen evolution reaction, in forming surface coatings to assemble hybrid photoelectrosynthetic materials featuring an underlying gallium phosphide (GaP) semiconductor as a light capture and conversion component. The metalloporphyrin reagents used in this work contain a 4-vinylphenyl surface-attachment group at the ß-position of the porphyrin ring and a first-row transition metal ion (Fe, Co, Ni, Cu, or Zn) coordinated at the core of the macrocycle. In addition to describing the synthesis, optical, and electrochemical properties of the homogeneous porphyrin complexes, we also report on the photoelectrochemistry of the heterogeneous metalloporphyrin-modified GaP semiconductor electrodes. These hybrid, heterogeneous-homogeneous electrodes are prepared via UV-induced grafting of the homogeneous metalloporphyrin reagents onto the heterogeneous gallium phosphide surfaces. Three-electrode voltammetry measurements performed under controlled lighting conditions enable determination of the open-circuit photovoltages, fill factors, and overall current-voltage responses associated with these composite materials, setting the stage for better understanding charge-transfer and carrier-recombination kinetics at semiconductor|catalyst|liquid interfaces.

Understanding and Controlling the Performance-Limiting Steps of Catalyst-Modified Semiconductors.

Understanding and controlling factors that restrict the rates of fuel-forming reactions are essential to designing effective catalyst-modified semiconductors for applications in solar-to-fuel technologies. Herein, we describe GaAs semiconductors featuring a polymeric coating that contains cobaloxime-type catalysts for photoelectrochemically powering hydrogen production. The activities of these electrodes (limiting current densities >20 mA cm-2 under 1-sun illumination) enable identification of fundamental performance-limiting bottlenecks encountered at relatively high rates of fuel formation. Experiments conducted under varying bias potential, pH, illumination intensity, and scan rate reveal two distinct mechanisms of photoelectrochemical hydrogen production. At relatively low polarization and pH, the limiting photoactivity is independent of illumination conditions and is attributed to a mechanism involving reduction of substrate protons. At relatively high polarization or pH, the limiting photoactivity shows a linear response to increasing photon flux and is attributed to a mechanism involving reduction of substrate water. This work illustrates the complex interplay between transport of photons, electrons, and chemical substrates in photoelectrosynthetic reactions and highlights diagnostic tools for better understanding these processes.

Photosensing System Using Photosystem I and Gold Nanoparticle on Graphene Field-Effect Transistor.

In this study, a light sensor is fabricated based on photosystem I (PSI) and a graphene field-effect transistor (FET) that detects light at a high quantum yield under ambient conditions. We immobilized PSI on a micrometer-sized graphene FET using Au nanoparticles (AuNPs) and measured the I-V characteristics of the modified graphene FET before and after light irradiation. The source-drain current (Isd) increased upon illumination, exhibiting a photoresponsivity of 4.8 × 102 A W-1, and the charge neutrality point of graphene shifted by -12 mV. This system represents the first successful photosensing system based on proteins and a solution-gated graphene FET. The probable mechanism of this negative shift can be explained by the increase in negative charge carriers in graphene induced by a hole trap in the AuNP resulting from electron transfer from the AuNP to PSI. Photoresponses were only observed in the presence of two surface-active agents, n-hexyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, because they caused the formation of a hydrophobic environment on the surface of the graphene. The lipid layer of these agents caused dissociation of ascorbate ions from the graphene sheet, thereby expanding the Debye screening length of the electrolyte solution. The hydrophobic environment above graphene also enhanced hole storage in the AuNP through electron transfer from the AuNP to PSI.

Photocurrent Generation of Reconstituted Photosystem II on a Self-Assembled Gold Film.

Photosystem II (PSII)-modified gold electrodes were prepared by the deposition of PSII reconstituted with platinum nanoparticles (PtNPs) on Au electrodes. PtNPs modified with 1-[15-(3,5,6-trimethyl-1,4-benzoquinone-2-yl)]pentadecyl disulfide ((TMQ(CH2)15S)2) were incorporated into the QB site of PSII isolated from thermophilic cyanobacterium Thermosynechococcus elongatus. The reconstitution was confirmed by QA-reoxidation measurements. PSII reconstituted with PtNPs was deposited and integrated on a Au(111) surface modified with 4,4'-biphenyldithiol. The cross section of the reconstituted PSII film on the Au electrode was investigated by SEM. Absorption spectra showed that the surface coverage of the electrode was about 18 pmol PSII cm-2. A photocurrent density of 15 nAcm-2 at E = +0.10 V (vs Ag/AgCl) was observed under 680 nm irradiation. The photoresponse showed good reversibility under alternating light and dark conditions. Clear photoresponses were not observed in the absence of PSII and molecular wire. These results supported the photocurrent originated from PSII and moved to a gold electrode by light irradiation, which also confirmed conjugation with orientation through the molecular wire.