Engineered Electron‐Transfer Chain in Photosystem 1 Based Photocathodes Outperforms Electron‐Transfer Rates in Natural Photosynthesis

Abstract

Photosystem 1 (PS1) triggers the most energetic light-induced charge-separation step in nature and the in vivo electron-transfer rates approach 50 e(-) s(-1) PS1(-1). Photoelectrochemical devices based on this building block have to date underperformed with respect to their semiconductor counterparts or to natural photosynthesis in terms of electron-transfer rates. We present a rational design of a redox hydrogel film to contact PS1 to an electrode for photocurrent generation. We exploit the pH-dependent properties of a poly(vinyl)imidazole Os(bispyridine)2Cl polymer to tune the redox hydrogel film for maximum electron-transfer rates under optimal conditions for PS1 activity. The PS1-containing redox hydrogel film displays electron-transfer rates of up to 335±14 e(-) s(-1) PS1(-1), which considerably exceeds the rates observed in natural photosynthesis or in other semiartificial systems. Under O2 supersaturation, photocurrents of 322±19 μA cm(-2) were achieved. The photocurrents are only limited by mass transport of the terminal electron acceptor (O2). This implies that even higher electron-transfer rates may be achieved with PS1-based systems in general.

The Role of Hydrophobicity of Os-Complex-Modified Polymers for Photosystem 1 Based Photocathodes

“…In comparison, the addition of the freely diffusing mediator does not change the photocurrent for hydrogel films, such as PS1-G43, that display non-limiting ET rates. 15 This clearly demonstrates that the low photocurrent obtained for the DOs modified polymers is not due to inefficient PS1 entrapment in the film, but to the limiting ET processes. The latter limitation may be due to the increased hydrophobicity of the DOs electron relay which reduces their solvation in aqueous media and thus their mobility, leading to an inefficient ET inside the hydrogel network.…”

“…Hence, the recombination of the methyl viologen radical cation with the oxidized Os-complex or the electrode is impeded. 15,24 This strategy ensures that the resulting photocurrents are primarily limited by the electron transfer chain from the electrode to PS1, which is the subject of the current study. To verify that the photocurrents are generated from PS1 rather than from the polymer matrix, control experiments with electrodes lacking PS1 were conducted.…”

“…To avoid compromising on the hydrophobic/hydrophilic requirements, we recently reported a strategy based on a polymer backbone with switchable properties. 15 We start from a fully solvated hydrophilic polymer for homogeneous mixing with the protein and deposition on electrode surfaces. The polymer properties are then switched to hydrophobic via a pH shift causing polymer collapse and efficient entrapment of PS1.…”

“…The highest electron transfer rate between PS1 and an electrode (335 e − s −1 PS1 −1 ) was achieved by tuning the properties of a pHresponsive Os-complex modified hydrogel. 15 Photocurrents and the associated electron transfer rate were only limited by the mass transport of the electron acceptor. In theory, a PS1-based device may allow even higher electron-transfer (ET) rates.…”

Has the Sun Set on Quantum Dot-Sensitized Solar Cells?

“…The similarities of the two systems are readily apparent with a light-harvesting unit (chlorophyll or quantum dot) becoming excited by light to generate an electron-hole pair which is separated and each carrier is transferred through different media (either across a thylakoid membrane or within nanoparticle network) along the energetic gradient of the chargecarriers. However, the electron-transfer rate in photosystem I is 47 e -/s [12] which is minimal compared to the electron-transfer rate between quantum dots and TiO 2 , which is reported to be between 10 7 and 10 10 e -/s depending on the binding environment of the quantum dots [13][14][15][16].…”