Microbial bioelectrochemical cells for hydrogen generation based on irradiated semiconductor photoelectrodes

Different MFC/MEC architectures are used that comprise single and double-chambered reactors. The utilization of one type instead of another depends on the purpose (Logan et al 2006). The earlier reactors were often composed of two bottles (one per each half cell), which were connected by a tube with a cation-exchange membrane (CEM). Usually, an ion-exchange membrane (IEM) could allow the passage of either cations or anions (anion-exchange membrane). The CEM permits protons or other cations passing to the cathode, thereby avoiding oxygen diffusion to the anode (Min et al 2005). For MFC, the double-chamber configuration could be the preferred solution.

The single-chamber configuration is preferred in MECs, since an IEM may not be absolutely needed (Bond et al 2002, Kim et al 2002, Min and Logan 2004). In this case, the reactor can have different architectures, but the essential feature is that anode and cathode are immersed in the same electrochemical solution (Kim et al 2007, Call and Logan 2008, Hu et al 2008). This configuration decreases the internal resistance by eliminating that across the membrane. For this reason, in the case of a stipulated current density, e.g. at the cathode, the applied voltage is expected to be reduced, resulting to energy savings. Alternatively, in the case of a stipulated cell voltage, the current density, and consequently the rate of hydrogen generation, is expected to be enhanced. Nevertheless, some researchers prefer the double-chamber configuration, because the compartments separation avoids the uncontrolled mixing of chemicals produced by the cathode and the anode, as in the case of gaseous product generation at the cathode or of electroactive compounds generated by effluent water cleaning from hazardous chemicals. In the case of gas generation like hydrogen or methane in MECs, a membrane prevents the diffusion and reoxidation of the gaseous product at the anode, therefore avoiding the diminishment of the faradaic yield for the desired product. A disadvantage of the membrane in MECs is that a substantial pH difference occurs. The reason is that the membrane is not only permeable to protons, but also to other cations. The concentration of cations such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ in microbial cultures is usually higher than that of protons, and these cations are transferred faster to the cathode, resulting to a pH gradient (Rozendal et al 2006, 2007, Kim et al 2007).

Bacteria transfer electrons to the anode by an either direct or indirect mechanism, which usually depends on the microorganism properties. Three mechanisms are essentially known: transfer by using a specific mediator (Rabaey et al 2004, 2005); nanowires in bacteria membranes (Reguera et al 2005, Gorby et al 2006); direct membrane-protein associated transfer (Bond and Lovley 2003).

The simplest way is the direct electron transfer from the bacterium membrane to the electrode. In this case, the most representative microorganisms that use this mechanism are: Shewanella (Bretschger et al 2007), Rhodoferax ferrireducens (Chaudhuri et al 2003), and Geobacter (Bond et al 2002). These bacteria transfer electrons by outer membrane cytochromes (Myers and Myers 1992).

A second way to transfer electrons consists in using one or more redox mediators or shuttles (Rabaey et al 2005, Rabaey and Verstraete 2005), which is able to accept electrons from the substrate at close proximity to the bacterium membrane and to the oxidase itself and then transfer them to the anode electrode. The mediators are directly produced by electrogenic microorganisms.

Other microorganisms, like Shewanella, may produce specific pili nanowires, which connect the same species of bacteria. Finally, they transfer electrons to the anode (or to a final electron acceptor) using these conductive structures, in the presence of an electron-accepting mediator (Reguera et al 2005, Gorby et al 2006). Furthermore, pili can equally connect even different species to each other (Gorby et al 2006). This system may enhance electron transfer from bacteria to electron acceptor without direct contact between the cell and the acceptor or the use of an electron shuttle. Sometimes, microorganisms are able to use more than a single electron transfer system: Shewanella can transfer electrons by means of a self-produced mediator, for example flavin mononucleotide (FMN) and riboflavin, to another mediator (Bond and Lovley 2005, Marsili et al 2008, von Canstein et al 2008). Such microorganisms are able to transfer electrons in this way because they can survive in anaerobic conditions, using other final electron acceptors, like metal ions. For instance, Fe³⁺ may be reduced by bacteria to Fe²⁺ (Logan 2009).

An MEC may work using either a pure or a mixed culture. The first solution could be of interest when researchers have to understand the operation of a BEC for a specific bacterium strain: it is usually used for research purposes. Mixed cultures are more widespread in application-oriented systems; in this case, a consortium of microbial strains constitute a biofilm on the anode and the electron transfer is guaranteed by the aforementioned mechanisms. A high number of strains in a mixed culture enhances the bioanode diversity composition (Bond et al 2002, Phung et al 2004). For instance, a specific strain, which is able to produce redox mediators, may provide them to other microorganisms (which do not produce the mediator by themselves) of the consortium, thereby enhancing the system efficiency (Rabaey and Verstraete 2005). Nevertheless, BEC performances may change because of the interplay of different parameters: microorganism metabolism, electron transfer efficiency, membrane ohmic resistance (if any), electrolyte ohmic resistance, and cathode efficiency to reduce an oxidant species (Rabaey et al 2004). The presence of a bacterium strain in a reactor may change due to its architecture and the conditions used (Logan and Regan 2006). BEC research have demonstrated that a bioanode acclimatization may occur using different sources, as marine sediment (Reimers et al 2001, Tender et al 2002) and wastewater or sewage sludge (Kim et al 2004, Pant et al 2010).

In recent years, researchers try to develop more efficient systems to produce more energy in the case of MFC or larger quantity of chemicals with MBECs. The anode is the engine of an MBEC (MFC and MEC): microorganisms able to produce electrons live there. Bioanode performances may be improved overcoming limiting factors in the system. Different parameters, such as pH, medium composition and anode potential influence the microorganism concentration, biofilm conditions and electron transfer rate. Many efforts in research have focused on studies aiming to enhance the performances of the bioanode in order to ensure a higher system efficiency (Pham et al 2009). It is possible to reach this aim by changing several relevant parameters in the two compartments. This concerns acclimatization period and substrate used, and type of electrode materials. For instance, in the case of a system works with an electrode composed of diverse carbon or metal structures and the kind of surface treatment, it was found that carbon nanoparticles on the anode surface enhanced biocompatibility and performance (Hou et al 2014). Electrode design is crucial because it enables to reduce the overvoltage in an MEC, which is a central issue in hydrogen production, where a power supply has usually to be employed. The enhanced performances are possible by using innovative materials, such as nickel alloys (Hu et al 2008) and stainless steel (Call et al 2009), which enhance the catalytic performances (Selembo et al 2009), or semiconductor photoelectrodes. Each solution described has disadvantages; for instance, carbon-based electrodes are expensive and semiconductors are not always stable (Guo et al 2015).

As electrolytic photoelectrochemical cells, or photoelectrolysis cells, will be denoted in this section cells comprising one or two photoexcited semiconductor–electrolyte interfaces in which different reactions take place at the cathode and anode so that a net chemical reaction takes place. These cells are contrasted to electrochemical photovoltaic cells converting light to electricity without any net change in the electrolyte, with the species being oxidized at the anode undergoing reduction at the cathode, such as the dye-sensitized solar cell (DSSC) mentioned in the previous section. In general, photoelectrolysis cells are classified either as photoelectrosynthetic or photocatalytic cells, depending on whether the overall reaction is endoergic (thermodynamically non-spontaneous) or exoergic (thermodynamically spontaneous). In the case of photoelectrosynthetic cells, stored energy is equal to the Gibbs free energy of the overall reaction. However, in the photoelectrolysis hydrogen-evolving MPEC cells considered in the present publication the stored energy corresponds to the endoergic water decomposition reaction

$$\begin{equation}{\text{2}}{{\text{H}}_{\text{2}}}{\text{O}} \to 2{{\text{H}}_{\text{2}}}{{ + }}{{\text{O}}_{\text{2}}}\end{equation} \tag{ 1 }$$

with oxygen originating from the atmosphere.

The reaction at the anode is not of interest to energy storage, with the resulting electrolyte solution after oxidation either being discarded or undergoing, in the case of water effluent remediation, further chemical or biochemical purification.

In the simplest MPEC configuration operating under short circuit with one photoelectrode, the energy storage efficiency ${\eta _{{\text{EFF}}}}$ for hydrogen generation is

$$\begin{equation}{\eta _{{\text{EFF}}}} = \frac{{\left( {\frac{{I{\varphi _{{\text{FAR}}}}}}{{nF}}} \right){\Delta _{\text{R}}}G}}{{{P_{{\text{LIGHT}}}}}} \times 100\end{equation} \tag{ 2 }$$

with:

I current (A),

${\varphi _{{\text{FAR}}}}$ faradaic efficiency,

F Faraday constant (charge per mole of electrons)(C mol⁻¹),

n number of electrons for each of the half reactions constituting the overall chemical reaction,

${\Delta _{\text{R}}}G$ the Gibbs free energy of the overall chemical storage reaction (J mol⁻¹),

${P_{{\text{LIGHT}}}}$ irradiance (W m⁻²).

If the water decomposition reaction is formulated according to equation (1), it is n = 4 with ${\Delta _{\text{R}}}G$ equal to 4F (1.23 V) or 474.3 kJ mol⁻¹.

It is also possible to have simultaneous solar energy storage and generation of electrical energy if an external resistance ${\mathcal{\text R}_{{\text{EXT}}}}$ is inserted across the cell, corresponding to electric power generation equal to ${I^2}{\mathcal{R}_{{\text{EXT}}}}$. In this case the total energy conversion efficiency is

$$\begin{equation}{\eta _{{\text{EFF}}}} = \frac{{\left( {\frac{{I{\varphi _{{\text{FAR}}}}}}{{nF}}} \right){\Delta _R}G + {I^2}{\mathcal{R}_{EXT}}}}{{{P_{{\text{LIGHT}}}}}} \times 100.\end{equation} \tag{ 3 }$$

A series of pioneering water splitting experiments using a semiconductor electrode were conducted by Honda, Fujishima et al in the 1970s (Fujishima and Honda 1971, 1972, Fujishima et al 1975b) based on a n-TiO₂ photoelectrode with a dark Pt counter electrode, with the semiconductor exposition to the sunlight enabling electron excitation from the valence to the conductive band, generating an electron–hole couple on TiO₂. Given its large bandgap energy (E_BG= 3.0 eV), only UV light could be adsorbed. Since UV light represents only <5% of the solar spectrum, the energy conversion efficiency of systems based on TiO₂ is low, so that research efforts have been directed toward the development of materials which are able to adsorb visible light in an efficient and high-performance way. This means not only that the E_G is more suitable for adsorption of a large fraction of solar photons but also that the deleterious recombination of the photogenerated electrons and holes, resulting to the evolution of heat or fluorescent radiation, is not important.

Different semiconductors may either oxidize or reduce water to gaseous oxygen and gaseous hydrogen, respectively; some may carry out both reactions, depending on the E_BG as well as the position of the valence and conduction band edges at the electrode surface. The needed applied thermodynamic voltage for water splitting in photoelectrochemical cells corresponds to 1.23 V under standard conditions: if anodic and cathodic overvoltages are considered, at least 1.7–1.9 V are needed (Chen et al 2012). A material eligible such as photo-electrode semiconductor needs to have the additional feature of being resistant to corrosion in the dark and, in particular, under irradiation (photocorrosion). To avoid the latter problem, during light exposition the electrons or holes generated in the semiconductor does not have to react with the semiconductor instead of participating to useful electrochemical reactions.

A number of semiconductors used in microbial photoelectrochemical cells is presented in table 1. As discussed in the following section, the positions of the band edges are expressed in energy units (eV) rather than, as it is often the case for photoelectrochemistry publications, in equivalent electric potential (V) units.

For the principles of semiconductor photoelectrochemistry, details of various electrode materials, and experimental methods the books by Myamlin and Pleskov (1967), Grätzel (1983), Schiavello (1985), Pleskov and Gurevich (1986), Finklea (1988), Sato (1998), Grimes et al (2008), Rajeshwar (2008), van de Krol and Grätzel (2012), Chen et al (2013a), Giménez and Bisquert (2016), Sharon (2016) can be consulted.

Cu₂O is a p-type semiconductor and one of the most used for hydrogen evolution, with a E_BG of 2 eV bias. Its conductive band, at 0.7 eV/RHE, lies above the apparent redox Fermi level of H⁺/H₂O (Paracchino et al 2011). It can be produced in several ways: electrodeposition (Siripala et al 2003), sputtering and thermal oxidation. A photoelectrode made with this semiconductor, could produce up to −7.6 mA cm⁻² at 0 V vs RHE (Paracchino et al 2011), during an air mass (AM) 1.5G (Global, it refers to the light coming from an angle −48° from the vertical, Bolton et al 1985) sunlight exposition, with the photoelectrode immersed in a 1 M Na₂SO₄ solution. This semiconductor has a solar to hydrogen theoretical efficiency of 18% with one sun light exposition (Paracchino et al 2011). Despite its advantageous features, the bare semiconductor is liable to poor stability and photo-corrosion in aqueous solutions, as shown in Gerischer (1977):

$$\begin{equation}{\text{C}}{{\text{u}}_{\text{2}}}{\text{O + }}{{\text{H}}_{\text{2}}}{\text{O }} + { }2{ }{{\text{e}}^ - } \to 2{\text{ Cu }} + { }2{\text{ O}}{{\text{H}}^ - }.\end{equation} \tag{ 24 }$$

Different studies showed that covering the Cu₂O layer with other superficial protective layers, avoiding electrolyte contact, photo-corrosion was avoided (Wang et al 2013). When the semiconductor is covered by a protective layer, the overvoltage needed for the HER increases, to limit this issue a nickel or ruthenium co-catalyst is usually needed. Effective protection is provided by layers of Al-doped ZnO (AZO) and TiO₂ deposited by atomic layer deposition (ALD), as discussed by Paracchino et al (2011). The configuration of the multilayer system described by these authors is

$$\begin{equation}{\text{TCO/Au/C}}{{\text{u}}_{\text{2}}}{\text{O}}/{\mathbf{AZO}}/{\mathbf{Ti}}{{\mathbf{O}}_{\mathbf{2}}}/{\text{Pt/electrolyte}}\end{equation} \tag{ 25 }$$

with an Au layer deposited on transparent conducting oxide (TCO) glass serving as electrodeposition substrate, and a thin Pt layer serving as electrocatalyst for H₂ evolution. The light impinges from the electrolyte side of the interface. This scheme is termed EE irradiation (Lindquist et al 1983) and for photoelectrolytic cells this is the usual type of irradiation. The alternative option of irradiation from the substrate side, termed SE irradiation. This requires a photoelectrode of sufficient transparency, and for some types of porous electrode it might be advantageous over EE irradiation; it is the preferred method of irradiation for DSSC cells.

In a further development (Pan et al 2018) a more efficient operation was achieved by replacing AZO by ALD-deposited Ga₂O₃, also deposited by ALD. The photoelectrode described by these authors, with the use of either ruthenium oxide or Earth-abundant NiMo as H₂ electrocatalyst was of the configuration

$$\begin{equation}{\text{TCO/Au/C}}{{\text{u}}_{\text{2}}}{\text{O}}/{\mathbf{G}}{{\mathbf{a}}_{\mathbf{2}}}{{\mathbf{O}}_{\mathbf{3}}}/{\text{Ti}}{{\text{O}}_{\text{2}}}/{\text{Ru}}{{\text{O}}_x}{\text{ or NiMo/electrolyte}}{\text{.}}\end{equation} \tag{ 26 }$$

A disadvantage of the Au layer is that it can facilitate electron hole recombination. Therefore, the photoelectrochemical performance can be improved by interposing a hole transport CuSCN layer, well known from solid-state DSSC research, as demonstrated by Pan et al (2020), who proposed the configuration

$$\begin{equation}{\text{TCO/Au/}}{\mathbf{CuSCN}}/{\text{C}}{{\text{u}}_{\text{2}}}{\text{O/G}}{{\text{a}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{/Ti}}{{\text{O}}_{\text{2}}}{\text{/Ru}}{{\text{O}}_x}{\text{/electrolyte}}\end{equation} \tag{ 27 }$$

with the CuSCN layer prepared by cathodic electrochemical deposition. A further disadvantage of the Au layer is its opacity if the photoelectrode is illuminated from the TCO side (back illumination). The use of this configuration is essential in some hybrid photovoltaic–photoelectrochemical cell configurations, as described by Pan et al (2020). In this respect, CuSCN can be directly electrodeposited on TCO, with the resulting configuration

$$\begin{equation}{\text{TCO/}}{\mathbf{CuSCN}}{\text{/C}}{{\text{u}}_{\text{2}}}{\text{O/G}}{{\text{a}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{/Ti}}{{\text{O}}_{\text{2}}}{\text{/Ru}}{{\text{O}}_x}{\text{/electrolyte}}\end{equation} \tag{ 28 }$$

which has the additional advantage of eliminating the costly Au layer. A drawback is the higher ohmic resistance in the absence of the Au layer, resulting to a decrease of the photoelectrochemical performance in comparison to the electrode incorporating both Au and CuSCN.

An alternative method of protecting Cu₂O against photocorrosion is by coating by a NiO_x surface layer, in fact a mixture or NiO and Ni(OH₂), as described by Lin et al (2012).

As regards microbial photoelectrochemical cell applications, Cu₂O PCATs were used by Liang et al (2016b) in H₂-evolving MPECs and by Guo et al (2019) and Jia et al (2019) in MPFCs.

For CuO, E_BG of 1.7 eV is narrower than that of Cu₂O (Hardee and Bard 1977, Chauhan et al 2006). It has the same disadvantages to photo-corrosion as aforementioned for Cu₂O, since CuO can photocorrode by undergoing conversion to Cu₂O according to the reaction (Xing et al 2019)

$$\begin{equation}{\text{2CuO + }}{{\text{H}}_{\text{2}}}{\text{O }} + {\text{ }}2{\text{ }}{{\text{e}}^ - } \to {\text{C}}{{\text{u}}_{\text{2}}}{\text{O }} + {\text{ }}2{\text{ O}}{{\text{H}}^ - }\end{equation} \tag{ 29 }$$

possibly followed by the conversion of Cu₂O to Cu according to equation (23).

Photoelectrochemical hydrogen evolution at Cu₂O have been observed by Hardee and Bard (1977) and by Chiang et al (2012). Sun et al (2015) employed an oxygen Cu₂O PCAT in an MPFC.

TiO₂ is a n-type semiconductor and it was extensively studied in different research groups. In this respect, the pioneering research of Honda and Fujishima in the 1970s (Fujishima and Honda 1971, 1972, Fujishima et al 1975b, 1979), who conducted the first solar-assisted water splitting experiments, should be noted. Usually, TiO₂ is employed as photoanode. It is photo-stable in solution, with a E_BG of 3.0–3.2 eV. The main crystallographic forms of TiO₂ are anatase (E_BG= 3.2 eV) and rutile (E_BG= 3.0). An important advantage of and several other n-type oxide semiconductors is the good stability against photocorrosion. Earlier experiments were conducted with single-crystal rutile electrodes. Later, inexpensive polycrystalline n-TiO₂ forms were preferred. Several methods exist for preparation of polycrystalline TiO₂. A simple method for preparing very thin layers of TiO₂ is the thermal oxidation by oxygen of a Ti foil or a thin layer of Ti on quartz (Lindquist et al 1983, Lindquist and Vidarsson 1986, 1987). Thicker, porous anatase TiO₂ layers, of interest to both electrolysis cells with directly photoexcited electrodes and DSSCs, can be prepared by the thermal decomposition of a metallorganic Ti compound in an alcoholic solution (Stalder and Augustynski 1979, Desilvestro et al 1985, Vlachopoulos et al 1988). The most popular anatase n-TiO₂ electrode preparation method for DSSC applications is by spreading of a colloidal TiO₂ layer on a conducting glass substrate and subsequent sintering, resulting to a mesoporous layer (O'Regan and Grätzel 1991, Barbé et al 1997). In addition of its use as directly excited electrode or sensitized photoelectrode, TiO₂ is also used as protective layer of other semiconductors, against photocorrosion as thin layer of thickness in the nm range. A frequently used method for applying protective TiO₂ layers ALD, as it is the case for Cu₂O electrodes (Paracchino et al 2011). Despite its features, due to the large bandgap, visible light energy is not sufficient to produce a substantial photocurrent. Furthermore, recombination occurs between photogenerated electrons and holes (Ahmad et al 2015), further hindering efficient energy conversion.

A disadvantage if TiO₂ is the limited solar light absorption, mainly to the UV region, due to the large E_BG. The responsivity toward the visible can be enhanced by doping with metal cations, e.g. Be, Ni, Cr, Zn for porous TiO₂ electrodes (Stalder and Augustynski 1979, Monnier and Augustynski 1980).

In recent years, one-dimensional TiO₂ nanostructures like nanotubes, nanorods and nanowires have been extensively used in photoelectrochemistry and other nanostructures. An advantage of such materials is the fast electron transport through the semiconductor layer. A disadvantage is the possibility of the E_BG enlargement due to quantum confinement, resulting by further shift of the light absorption spectrum toward the UV. A way to remedy this disadvantage is doping with metal ions, as explained in the previous paragraph (Pang et al 2014).

In general, n-TiO₂ is used as photoanode, but occasionally also as PCAT. In standard 'textbook' photoelectrochemistry n-semiconductors are photoanodes and p-semiconductors are PCATs, but several exceptions to this rule exist. There are several investigations in which reduction reactions of photogenerated electrons in n-TiO₂ have been evidenced (Lindquist and Vidarsson 1986, Lindström et al 1995, Tsujiko et al 2002). The use of n-TiO₂ as PCAT is related to the increased photoconductivity upon irradiation.

As regards microbial photoelectrochemical cell applications, n-TiO₂ PCATs were used in H₂-evolving MPECs by Chen et al (2013b) and He et al (2014) and in MPFCs by Bhowmick et al (2018), Lu et al (2010), and Sun et al (2015). n-TiO₂ photoanodes were used in a H₂-evolving MPECs by Kim et al (2018a), in an MPFCs by Du et al (2014), in an MPEC effecting the reduction of CO₂ to CH₄ by Du et al (2014) and Kim et al (2018a), and in a photoelectrochemical cell effective both toward electricity generation and conversion of CO₂ to CH₄ by Kim et al (2018b).

By coating TiO₂ by a lower bandgap semiconductor, higher photocurrents can be obtained, analogously to the case of dye sensitization. In this respect, a CdS-coated n-TiO₂ electrode was used (Xiao et al 2020b) in an MPEC converting CO₂ to CH₄.

Hematite (n-type α-Fe₂O₃) has, compared to n-TiO₂ a lower E_G of 2.0 eV. Therefore, it absorbs a significant part of the visible part solar light and has been studies with respect to water decomposition applications (Sivula et al 2009, 2011). However, contrarily to anatase TiO₂, E_CS lies lower than E_F (H⁺/H₂). Therefore, an auxiliary bias voltage would be needed in order to use it in an H₂-evolving MPEC. The Fe₂O₃ photoanode has been studied for microbial oxidation reactions in three-electrode voltammetric cells by Qian et al (2014), Feng et al (2016), Liang et al (2016a), Zhu et al (2017).

Similarly to α-Fe₂O₃, n-WO₃ has the advantage of a favourable E_G of 2.4 eV and the disadvantage of a E_CS below the redox Fermi level of E_F (H⁺/H₂). However, it may be possible by coating to modify the E_CS by coating with a catalyst. Tahir (2019) reported the use of a NiFe₂O₄-coated WO₃ electrode as a PCAT in a H₂-evolving MPEC cell.

p-CaFe₂O₄ with a E_G of 1.9 eV is a promising PCAT material for water oxidation. However, it has been little studied, and according to the available publications, its photoelectrochemical performance is relatively low (Matsumoto et al 1987, Ida et al 2010, Cao et al 2011). Chen et al (2017) applied p-CaFe₂O₄ to an H₂-evolving MPEC.

Silicon, with an E_BG of 1.1 eV close to the optimal value for solar light absorption (E_BG = 1.1 eV), has been used extensively in photoelectrochemistry as both as n- and p-type semiconductor. However, it is known to be subject to photocorrosion (Gerischer 1977). A n-Si PCAT was used by Wan et al (2015) and by Zang et al (2014) in an H₂-evolving MPECs: in the latter case, Si was coated by an electrocatalytic layer of MoS₃.

n-CdS has been extensively studied as photoelectrode and photocatalytic material in microdispersed from, due to the favourable E_BG of 2.4 eV for solar light absorption and the fact that its E_CS and E_VS are located sufficiently below and above the redox Fermi levels of H⁺/H₂ and O₂/H₂O,H⁺ respectively. However, it is prone to photocorrosion. Hou et al (2020a), Hou et al (2020b) used a g-C₃N₄/CdS heterojunction photodiode, where is a graphitic carbon nitride polymer, for the reductive degradation of organic compounds, in conjunction with a microbial abode effecting the oxidation of acetate. As mentioned above, CdS can be also used as sensitizer of TiO₂, as in the case of the photoanode in the MPEC system converting CO₂ to CH₄ according to Xiao et al (2020b).

A large variety of methods has been used for preparing semiconductor electrodes for photoelectrochemical cells. In this section, application examples of a number of methods for materials of interest to microbial photoelectrochemical cells are presented.

ALD, also known as atomic later epitaxy, consists of forming ultrathin layers on a surface by the sequential exposure of a surface to reactants in the gas phase (Puurunnen 2014, Oviroh et al 2019). It differs from chemical vapour deposition (CVD) in the fact that in the latter the surface to be coated can be exposed to several chemical compounds at the same time. For example, for TiO₂ deposition (Ritala et al 1993, Niemelä et al 2017) the surface is at first exposed to a Ti source, usually a metallorganic compound of Ti (TiL₄), e.g. Titanium-isopropoxide, Ti[OCH(CH₃)₂]₄, and then to water as oxygen source. The reaction sequence is (Niemelä et al 2017)

$$\begin{equation}n\,( - {\text{OH}})\,{\text{(s)}} + {\text{Ti}}{{\text{L}}_{\text{4}}}\,{\text{(g)}} \to {{\text{(}} - {\text{O}} - {\text{)}}_n}{\text{Ti}}{{\text{L}}_{{\text{4}} - n}}\,{\text{(s)}}\end{equation} \tag{ 30 }$$

$$\begin{align} {{{\text{(}} - {\text{O}} - {\text{)}}}_n} & {\text{Ti}}{{\text{L}}_{{\text{4}} - n}}{\text{(g)}}\,\, + \,(4 - n)\,{{\text{H}}_{\text{2}}}{\text{O}}({\text{g}})\nonumber \\ & \quad \to {{{\text{(}} - {\text{O}} - {\text{)}}}_n}{\text{Ti(OH}}{{\text{)}}_{{\text{4}} - n}}{\text{(s)}} + n{\text{HL}}({\text{g}}) .\end{align} \tag{ 31 }$$

This method has been applied to the deposition of thin protective layers of AZO, TiO₂, and Ga₂O₃ on Cu₂O (Paracchino et al 2011, Pan et al 2018, 2020).

An example of this method is the generation of an oxide layer on a surface dipped in a solution of a precursor undergoing hydrolysis. For TiO₂, CBD can be applied either at ambient temperature or in an autoclave at a temperature exceeding 100 °C. In all the cases mentioned below the precursor was Titanium-butoxide, Ti[O(CH₂)₃CH₃]₄ in an acidic solution. Chen et al (2013b) used CBD for the preparation of a nanorod array PCAT on TCO glass, by the acidic hydrolysis of Ti-butoxide (in an autoclave) for a hydrogen evolving MEPC. Fu et al (2018) and Xiao et al (2020a, 2020b) used CBD for the preparation of a nanowire array photoanode (at room temperature) and PCAT (in an autoclave), respectively, for an MEPC used for the CO₂ reduction to CH₄.

An α-Fe₂O₃ nanowire array photoanode layer can be produced by dipping a TCO plate in an acidic solution of FeCl₃ (Vayssières et al 2001). This electrode was used by Qian et al (2014), Zhu et al (2017) for the production of a photoanode used in their microbial photobioelectrochemical studies.

A CVD variant is the successive ionic layer adsorption SILAR. An example is the deposition of a CdS overlayer on a TiO₂ nanowire array by dipping the electrode coated by the latter at first in a Cd²⁺-containing and then in a S²⁻-containing aqueous solution (Xiao et al 2020b).

This method has been applied to deposit thin layers of α-Fe₂O₃ on a conducting glass plate (Dotan et al 2011). The particular CVD variety used in this case was atmospheric CVD. The plate was at first heated to 420 °C and then exposed to a mixture of iron carbonyl [Fe(CO)₅] and tetraethylorthosilicate (Si(OCH₂CH₃)₄, TEOS) in a cold AVD chamber. By this method 1% Si-doped α-Fe₂O₃ was obtained.

A frequently used method of preparing TiO₂ nanotubes is the anodic oxidation of a Ti substrate in an acidic solution containing F⁻ ions. Due to the high resistance of the resulting TiO₂ layer, the voltage between the TiO₂ anode and the cathode usually exceeds 10 V. The overall oxidation reaction for TiO₂ generation is

$$\begin{equation}{\text{Ti + 2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{Ti}}{{\text{O}}_{\text{2}}} + 4{{\text{H}}^ + } + 4{{\text{e}}^ - }\end{equation} \tag{ 32 }$$

with details on the individual steps involving Ti⁺, OH⁻ and O²⁻ as intermediate species discussed by Kim et al (2018b) and Vargas et al (2019).

The partial dissolution of the formed TiO₂ by reaction with fluoride ions according to

$$\begin{equation}{\text{Ti}}{{\text{O}}_{\text{2}}}{\text{ + 6}}{{\text{F}}^ - }{\text{ + 4}}{{\text{H}}^{{ + }}} \to {\left[ {{\text{Ti}}{{\text{F}}_{\text{6}}}} \right]^{2 - }} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}\end{equation} \tag{ 33 }$$

is important for the creation of the nanotube layer (Kim et al 2018b). Du et al (2014) and Kim et al (2018b) applied this approach to the TiO₂ photoanode preparation for an MPFC, Kim et al (2018a) for a H₂-evolving MPEC.

The preparation nanowire layer of Cu(OH)₂ was prepared by the electrochemical oxidation of Cu foam is described by Guo et al (2019). Cu(OH)₂ was a precursor layer for the generation of a Cu₂O nanowire layer, used as cathode in an MPFC by thermal decomposition.

Several semiconductors and catalytic layers on semiconductors can be obtained by electrodeposition. p-Cu₂O is generated by reduction of CuSO₄ in an alkaline medium (Golden et al 1996, Paracchino et al 2011).

Fu et al (2014) applied the electrodeposition of FeOOH by anodic oxidation of Fe²⁺ from an aqueous or mixed water-ethylene glycol FeCl₂ solution (pH = 4.1) as a first step to the generation of a nanorod α-Fe₂O₃ film. FeOOH is converted α-Fe₂O₃ to in a subsequent thermal decomposition step. The reaction sequence in the anodic oxidation is

$$\begin{equation}{\text{F}}{{\text{e}}^{{\text{2}} + }} \to {\text{F}}{{\text{e}}^{{\text{3}} + }} + {{\text{e}}^ - }\end{equation} \tag{ 34 }$$

$$\begin{equation}{\text{F}}{{\text{e}}^{{\text{3}} + }} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{FeOOH}} + 3{{\text{H}}^ + }.\end{equation} \tag{ 35 }$$

FeOOH is converted α-Fe₂O₃ to in a subsequent thermal decomposition step. This method was applied by Feng et al (2016) for the production of a photoanode used in their microbial photobioelectrochemical study.

A layer of MoS₃, acting as H₂-evolution electrocatalyst has been deposited on nanowire Si electrode by performing several, cyclic voltammetry scans in a Mo salt solution (Zang et al 2014).

A Si nanowire (Si-NW) layer can be generated by dissolution of a Si crystal layer in a HF solution containing Ag⁺ ions (Peng et al 2012, Zang et al 2014, Wan et al 2015). Si dissolution is coupled to the Ag deposition according to the reactions (Peng et al 2012)

$$\begin{equation}{\text{Si + 6}}{{\text{F}}^ - }\, \to {\text{SiF}}_{\text{6}}^{2 - }{\text{ + 4}}{{\text{e}}^ - }\end{equation} \tag{ 36 }$$

$$\begin{equation}{\text{A}}{{\text{g}}^{{ + }}}{{ + }}{{\text{e}}^ - }\, \to {\text{Ag}}{\text{.}}\end{equation} \tag{ 37 }$$

The Si surface is partially covered by Ag particles with grow fractally, without covering the whole surface. Therefore, the part of Si not covered by the Ag particles dissolves, instead, the part of Si below the Ag particles does not dissolve, so that nanowires capped by Ag are generated. Finally, the Ag particles can be dissolved by immersion in HNO₃. A Si-NW layer generated by this method was used in the MPEC experiments by Zang et al (2014).

Fe₂O₃ has been prepared by a particular variety of spray pyrolysis, ultrasonic spray pyrolysis (Dotan et al 2011). A conductive glass plate is exposed to a mixture of iron acetylacetonate [Fe(C₅H₇O)₃, Fe(acac)₃] and tetramethylorthosilicate [Si(OCH₃)₄, TMOS] at 540 °C. The mixture of the two chemicals is fed in the form of droplets, generated ultrasonically in an air stream.

This is a widely used method among scientist and technologists involved in the preparation colloidal, mesoporous TiO₂ electrodes for DSSCs (O'Regan and Grätzel 1991, Barbé et al 1997, Ito et al 2007). A widespread approach is based on the deposition of a paste, slurry, or other type of mixture containing semiconductor particles, a solvent, and one or more additives, facilitating the uniform and crack-free of the layer to prepare, on a conductive glass or metal substrate. Various deposition methods can be used, including doctor blading (Barbé et al 1997), painting (He et al 2014) or, for large scale electrode production, screen printing (Ito et al 2007). After drying, the layer is often heated at a sufficiently high temperature in order to burn the additives and sinter the particles toward the formation of a continuous layer.

Examples of electrodes prepared by this method applied in microbial photoelectrochemical BECs are presented by Lu et al (2010), for the preparation of a rutile TiO₂ cathode on graphite, used in an MPFC, Chen et al (2017) for the preparation of a p-CaFe₂O₄ PCAT on Pt, used in an H₂-evolving MPEC, and by Hou et al (2020a, 2020b) for the preparation of a g-C₃N₄/CdS PCAT, used in an MPECs for the degradation of an organic pollutant.

The particulate oxide can be obtained from various commercial sources or prepared in the laboratory by a sol–gel method. In the case of TiO₂, a colloidal solution is prepared by the acid or base hydrolysis of a metallorganic compound of Ti, e.g. Ti-isopropoxide [Ti{OCH(CH₃)₂}₄] (O'Regan and Grätzel 1991, Barbé et al 1997). Then the colloidal solution undergoes a hydrothermal treatment in an autoclave so that the smaller particles are agglomerated to larger ones at the desired diameter of ∼20 nm (Ostwald ripening). Subsequently, a large part of the solvent is removed so that the particle concentration is significantly increased so that a viscous dispersion with a high particle concentration is obtained, which can be used for the preparation of the paste by introducing suitable additives, as mentioned above.

Guo et al (2019) generated a nanowire p-Cu₂O layer by thermal decomposition of a Cu(OH)₂ nanowire layer generated by electrochemical anodization of a Cu foam electrode. This layer was used as PCAT in an MPFC.

Fu et al (2014) report the decomposition of FeOOH as a second step for the generation of an α-Fe₂O₃ photoanode as a second step following the electrodeposition of FeOOH:

$$\begin{equation}{\text{F}}{{\text{e}}^{{\text{3}} + }} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{FeOOH}} + 3{{\text{H}}^ + }.\end{equation} \tag{ 38 }$$

This method was applied by Feng et al (2016) for the production of a photoanode used in their microbial photobioelectrochemical study.

A method for preparing porous TiO₂ is based on the thermal decomposition of a thin layer of a metallorganic compound of TiO₂, Ti-alkoxide [Ti(OCH₂CH₃)₄], prepared by the addition of TiCl₄ in a methanol–ethanol solvent (Stalder and Augustynski 1979, Monnier and Augustynski 1980, Vlachopoulos et al 1988). Upon heating, Ti-ethoxide decomposes to TiO₂. This method was used for preparing TiO₂ electrodes for photoelectrolysis based on direct photoexcitation and, until the early 1990s, as substrate for dye-sensitized electrodes. An advantage is the facility of controlled doping, by addition in the Ti precursor solution of controlled amounts of other elements amounts. A disadvantage is that in each thermal decomposition step a rather thin layer is prepared (e.g. ∼1 μm), so that in order to achieve the desired thickness (e.g. 5–10 μm), the preparation step has to repeated several times.

Among the various preparation methods for polycrystalline TiO₂, a simple one is the thermal oxidation of a Ti plate or a Ti thin film on a quartz plate in oxygen or air in a temperature of 400 °C–700 °C. (Lindquist 1983, Lindquist and Vidarsson 1986, 1987). Tachibana et al (2006) prepared p-Cu₂O by thermal oxidation of Cu at 180 °C. p-CuO can be prepared by the thermal decomposition of a Cu(OH)₂ film previously prepared by the wet chemical oxidation of a Cu substrate by (NH₄)₂S₂O₈ (Sun et al 2015).

The reaction sequence for an MPEC in the case of PCAT and dark bioanode, for the case of acetate oxidation at the dark bioanode, can be written in neutral solutions as

$$\begin{equation}\begin{gathered} {\text{2}}{{\text{H}}^ + } + 2{{\text{e}}^ - }\xrightarrow{{h\,\nu }}{{\text{H}}_2} \hfill \\ {\text{MPEC photocathode}} \hfill \\ \end{gathered} \end{equation} \tag{ 42 }$$

and

$$\begin{equation}\begin{array}{*{20}{l}} {{\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - } + 2{{\text{H}}_{\text{2}}}{\text{O}} \to 2{\text{C}}{{\text{O}}_{\text{2}}} + 7{{\text{H}}^ + } + 8{{\text{e}}^ - }} \\ {{\text{MPEC}}\,\,{\text{bioanode}}} \end{array}.\end{equation} \tag{ 43 }$$

There are two categories of possible systems: (a) short-circuited cells; (b) cells with an auxiliary external bias voltage. Two particular cases of (b) are: (b1) cells with the external bias voltage supplied by an MFC with the possibility of simultaneous electricity generation and (b2) cells with the external bias voltage supplied by a photovoltaic cell. No examples of the latter case have been so far mentioned in the literature; the analogous case of water splitting in systems combining directly photoexcited semiconductor electrodes and coupled solar cells is well documented (e.g. Sivula et al 2011, 2013). Figures 4 and 5 show the coupling of a MPEC to an MFC, with the MPEC consisting of either a photoanode and dark photocathode (figure 4) or photocathode and dark photoanode (figure 5).

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Chen et al (2013b) describe an MPEC using a nanorod n-TiO₂ PCAT of 2 cm² area in a deoxygenated 0.2 M Na₂SO₄ electrolyte, with electricity co-generation, operating with an external resistance between PCAT and anode. Under irradiation provided by a 300 W xenon lamp, H₂ was generated at a maximum rate of 2.2 μl h⁻¹ cm⁻².

Chen et al (2017) describe an MPEC using a p-CaFe₂O₄ PCAT of 4 cm² area in a deoxygenated 0.2 M Na₂SO₄ electrolyte, with electricity co-generation, operating with an external resistance between PCAT and anode (figure 3). Under visible light irradiation (λ > 420 nm) of intensity 100 mW cm⁻² provided by a 300 W xenon lamp externally applied bias voltage, H₂ was generated at a maximum rate of 6.7 μl h⁻¹ cm⁻².

He et al (2014) describe an MPEC using a n-TiO₂ porous cathode of 25 cm² area in a deoxygenated 0.1 M Na₂SO₄ electrolyte, operating under short circuit. The UV irradiation provided by a 30 W low-pressure mercury lamp. For an illuminated area of 15 cm², H₂ was generated at an average rate of 3.5 μl h⁻¹; the reactor operated well for a time exceeding 200 h.

Liang et al (2016b) describe an MPEC using a Cu₂O/NiO_x PCAT of 1.8 cm² area, operating with an external bias voltage of 0.2–0.4 V. Under visible light irradiation and 0.2 V external bias, H₂ was generated at a rate of 5.09 μl h⁻¹ cm⁻².

Tahir (2019) describes an MPEC using a n-WO₃ PCAT coated with a NiFe₂O₄ catalyst, operating with an external bias voltage supplied by a coupled MFC. Under visible light irradiation, H₂ was generated at a rate of 10.7 μl h⁻¹ cm⁻² with a 0.3 V bias voltage.

Wan et al (2015) describe an MPEC with an external bias voltage provided by a coupled MFC, where the MPEC PCAT was a p-Si-NW PCAT. The same electrolyte was used in the cathodic and anodic compartment. Under white light irradiation, quantitative generation of H₂ was detected.

Zang et al (2014) describe an MPEC with electricity co-generation using a p-Si-NW PCAT coated with MoS₃ as H₂-evolution catalyst, in a 0.1 M H₂SO₄+ K₂SO₄ (pH = 1) electrolyte, with electricity co-generation, operating with an external resistance between PCAT and anode. Under visible light irradiation, H₂ was generated at an average rate of 7.5 μmol h⁻¹ cm⁻², corresponding to 183 μl h⁻¹ cm⁻² (SATP).

The reaction sequence for an MPEC in the case of PCAT and dark bioanode, for the case of acetate oxidation at the dark bioanode, can be written in neutral solutions as

$$\begin{equation}\begin{gathered} {\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - } + 4{{\text{H}}_{\text{2}}}{\text{O}}\,\,{{ + }}\,\,8{{\text{h}}^ + }\xrightarrow{{h\,\nu }}2{\text{HCO}}_3^ - + 9{{\text{H}}^ + } \hfill \\ {\text{MPEC}}\,\,{\text{photobioanode}} \hfill \\ \end{gathered} \end{equation} \tag{ 44 }$$

and

$$\begin{equation}\begin{array}{*{20}{l}} {{\text{2}}{{\text{H}}^ + } + 2{{\text{e}}^ - }\mathop \to \limits^{h\,\nu } {{\text{H}}_2}} \\ {{\text{dark cathode}}} \end{array}.\end{equation} \tag{ 45 }$$

The possibilities (a) and (b), (b1) and (b2) mentioned in the previous subsection are also possible in the present case. However, no example of this type has been described in the literature.

An alternative system is described by Kim and Lee (2018a) consisting of a PEC using a TiO₂ nanoarray photoanode (not photobioanode), and a Pt cathode, with an external bias voltage provided by a coupled MFC, with electricity co-generation, operating with an external resistance between PCAT and anode. In a strict sense, this cell is not an MPEC but a PEC coupled to an MFC. The particular MFC of these authors is based on the hydrogen evolution in place of the oxygen reduction reaction, so it could be also denoted as MEC. Their configuration uses a common Pt-coated cathode for both the MPEC and the MFC units, covered by an acrylic polymer layer as barrier to oxygen reduction so that the hydrogen evolution is the only cathodic reaction. Figure 6 shows the scheme of Kim et al and figure 7 a current flow diagram explaining the operational concept of this cell. No membrane was used to separate the electrolytes in either the MEC or the MFC subunit. Under simulated AM1.5 irradiation, the increase in the rate of H₂ evolution was 1.35 μmol cm⁻² h⁻¹, based on the cathode area corresponding to 33 μl cm⁻² h⁻¹ (SATP). Under dark the respective rate is 4.39 μmol cm⁻² h⁻¹ (109 μl cm⁻² h⁻¹).

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For the optimization of the dark cathode in this type of cell the publications by and related to cathodes for microbial solar cells, as well as the references on H₂ electrocatalysis mentioned in section 3.2.