*5.2. Biophotoelectrocatalysis (PEC)*

Biophotoelectrodes fabricated on planar carbon or SAM-modified metal electrode usually show low photocurrent density which limits their applications in bioelectrocatalysis. However, the development of redox polymer electrodes, layer-by-layer assembly and 3D architectures has enhanced the performance of biophotoelectrodes [176,177]. PSII is the only natural protein able to catalyse the photooxidation of water. The electron flow can be blocked by herbicide compounds since they bind to the terminal plastoquinone Q<sup>B</sup> of PSII. This inhibition effect can be exploited for designing PSII light-driven biosensor to detect herbicides [178]. A similar approach to detect herbicides was taken with purple bacteria RCs [179].

As water oxidation biophotocatalyst, PSII attracts more attention for solar energy conversion to generate electricity or fuels. As mentioned above, PSII photoanodes have been connected to PSI photocathodes (both in Os redox polymers) to mimic natural photosynthesis Z-scheme for solar-to-electricity generation [62,63]. In a similar approach using a benzoquinone redox polymer, a PSII photoanode (O<sup>2</sup> evolution) has been connected to a bilirubin oxidase cathode for O<sup>2</sup> reduction [180]. Recently, PSII was integrated together with PbS quantum dots within a TiO<sup>2</sup> inverse opal electrode to perform H2O photooxidation. This was combined with an inverse opal antimony-doped tin oxide (ATO) cathode modified with bilirubin oxidase to catalyse oxygen reduction. This system achieved a high open-circuit voltage of about 1V under illumination (Figure 4a,b) [181].

A full water splitting process can be realised by combining a PSII photoanode with a cathode modified with hydrogenases. In contrast to EBCs (Section 5.1), water-soluble hydrogenases (typically [FeFe] hydrogenases, but also [NiFe] hydrogenases; H2ases) are most commonly used for water splitting systems. When combining PSII and H2ases, there is an energy gap between the terminal electron acceptor Q<sup>B</sup> within PSII and one of the FeS cluster within H2ase. A biophotoelectrochemical cell with a PSII photoanode and a H2ase cathode thus requires an applied bias voltage of 0.8 V to drive H2O splitting [93]. This was improved by wiring H2ase and p-Si on an inverse opal TiO<sup>2</sup> photocathode, lowering the required applied bias voltage to 0.4 V for H2O splitting [182]. Finally, by integrating PSII on a diketopyrrolopyrrole dye-sensitised TiO<sup>2</sup> photoanode and connecting it with a H2ase cathode, a bias-free photoelectrochemical cell for H2O splitting was developed by the group of Reisner [183]. Coupling a dye-sensitised PSII photoanode with a W-dependent formate dehydrogenase (FDH) cathode, a biophotoelectrochemical cell was constructed for CO<sup>2</sup> reduction at a small bias voltage of 0.3 V (Figure 4c,d) [184]. The latter study showed the possibility for rational design of biophotoelectrochemical cells for value-added chemicals generation beyond H2.

Unlike PSII, PSI does not directly catalyse technologically valuable reactions such as water oxidation. However, photoexcitation of PSI provides the reductive potential to drive reactions with other catalysts, such as Pt and H2ase [185]. Recent studies show that it is possible to drive H<sup>2</sup> production from light with PSI and H2ase by electrode design. Photoelectrodes were manufactured in a 'layered' fashion using an Os redox polymer, PSI and, finally, a polymer/H2ase mix [186]. Photoelectrochemical H<sup>2</sup> production is achieved at an onset potential of +0.38 V vs. SHE. In this study, the PSI was randomly orientated and did not form a compact layer, likely limiting efficiency by charge recombination between the carrier and mediator or electrode. In a more recent study, an anisotropically oriented PSI monolayer was formed using Langmuir-Blodgett deposition [187]. A compact and oriented PSI layer minimises charge recombination and enables unidirectional electron transfer to H2ase for H<sup>2</sup> evolution. Combining this PSI/H2ase photocathode with a PSII photoanode created a system able of bias-free light-driven water splitting [187]. Langmuir-Blodgett deposition transfers only a monolayer of PSI and this might limit the performance of the biophotoelectrode as this limits the loading or coverage of PSI.

One of the limitations with biophotoelectrodes is the limited lifetime of the isolated proteins, especially PSII [154]. Light-induced formation of reactive oxygen species can further limit the lifespan of proteins [115]. Another limitation of biophotoelectrodes is that photosynthetic proteins only use a limited range of the solar spectrum which reduces solar conversion efficiency. The absorption spectral range can be enhanced by attaching complementary chromophores to light-harvesting complex

proteins [188]. It also can be improved by integrating biological light-harvesting antenna complexes or organic dyes/synthetic compounds to the RCs [189,190].

**Figure 4.** Photoelectrochemical (PEC) cell for electricity or biofuels generation. (**a**) Schematic of the electron transfer steps and energetic level of the components of the light-driven signal chain composed of TiO<sup>2</sup> , PbS QDs, redox polymer (POs), and PSII. (**b**) A scheme of a PEC cell consisting of an IO-TiO<sup>2</sup> |PbS|POs|PSII anode and an IO-ATO|PC|BOD cathode. Reprinted with the permission from ref [181]. Copyright (2019), Wiley. (**c**) A schematic representation of a semi-artificial photosynthetic tandem PEC cell coupling CO<sup>2</sup> reduction to water oxidation. A blend of POs and PSII adsorbed on a dpp-sensitized photoanode (IO-TiO<sup>2</sup> |dpp|POs-PSII) is wired to an IO-TiO<sup>2</sup> |FDH cathode. (**d**) Energy level diagram showing the electron-transfer pathway between PSII, the redox polymer (POs), the dye (dpp), the conduction band (CB) of IO-TiO<sup>2</sup> electrodes, four [Fe4S4] clusters, and the [WSe]-active site in FDH. All potentials are reported vs. SHE at pH 6.5. Reprinted with the permission from ref [184]. Copyright (2018), American Chemical Society.
