**4. The use of components/systems of the photosynthetic apparatus to generate electricity**

Nowadays, solar cells containing mono- and polycrystalline silicon as inorganic semiconduc‐ tors are used for commercial applications in small devices, such as solar panels on roofs, pocket calculators, water pumps, and also in space technologies. Common traditional solar batteries can use less than 20% of the incident solar light [4]. Production of silicon solar cells requires energy-intensive processes, high temperatures (400-1400 оС) and pure vacuum conditions, which results in high cost of such cells [24]. In contrast, production of solar cells based on biological photoactive components does not require these conditions. It suggests that biolog‐ ical-based solar cells are less expensive. The main disadvantage is the fact that they do not reach the efficiency of the inorganic solar cells [21, 24].

In recent researches, the thylakoid membrane and isolated PSI and PSII have been used in solar cells [26-28] and in optoelectronic devices by immobilizing these photoactive components directly onto the electrode surface [29-31] or via linker molecule [32-39].

#### **4.1. Thylakoids as photobiocatalysts**

On the I–V curve, it is possible to determine four parameters of the cell: short-circuit current, open-circuit voltage, values of current and voltage defining maximum power generated by

Short-circuit current *I*sc (the current at an external voltage equal to zero) is the point of I–V intersection with the vertical axis. Open-circuit voltage *V*oc is the voltage equal in absolute value to photovoltage and opposite to it in sign: if it is applied to the cell, no current flows. Opencircuit voltage is determined by the I–V curve intersection with the axis of abscissa. Current

There is a point on the I–V curve, where the value *P* reaches its maximum, *P*max. The product of *I*sc and *V*oc presents the value proportionate to an area of rectangle AB'C'D' (Fig. 4). The ratio of maximum power *P*max, corresponded to the area of rectangle ABCD, to the product of *I*sc by

> *ABCD* max *AB C D sc oc*

Thereby, the maximum coefficient of efficiency could be expressed by the following equation:

light light *P I V FF sc oc J J*

**4. The use of components/systems of the photosynthetic apparatus to**

Nowadays, solar cells containing mono- and polycrystalline silicon as inorganic semiconduc‐ tors are used for commercial applications in small devices, such as solar panels on roofs, pocket calculators, water pumps, and also in space technologies. Common traditional solar batteries can use less than 20% of the incident solar light [4]. Production of silicon solar cells requires energy-intensive processes, high temperatures (400-1400 оС) and pure vacuum conditions, which results in high cost of such cells [24]. In contrast, production of solar cells based on biological photoactive components does not require these conditions. It suggests that biolog‐ ical-based solar cells are less expensive. The main disadvantage is the fact that they do not

In recent researches, the thylakoid membrane and isolated PSI and PSII have been used in solar cells [26-28] and in optoelectronic devices by immobilizing these photoactive components

*S P FF S IV* ¢¢¢

max

max

h

reach the efficiency of the inorganic solar cells [21, 24].

directly onto the electrode surface [29-31] or via linker molecule [32-39].

*P IV ell* = × (2)

= = <sup>×</sup> (3)

× × = = (4)

power generated on the cell is determined by the voltage *V* and current *I*.

the cell [25].

168 Applied Photosynthesis - New Progress

*V*oc is called fill factor.

**generate electricity**

Thylakoid membranes can be isolated from plant leaves and immobilized on the electrode surface to generate a photocurrent. A team of researchers led by Robert Carpentier [40] has been the first to begin using thylakoid membranes isolated from spinach leaves as a photo‐ sensitizer. In their work, a platinum electrode was used as a final acceptor. Studies were carried out in the light and in the dark, in the presence and in the absence of potassium ferrocyanide as a mediator. Native thylakoids generated a photocurrent up to 6-9 μA without a mediator, and four times more current in the presence of potassium ferrocyanide. This study has shown that the photocurrent generation without any mediators is associated with direct electron transfer from the membrane proteins to the electrode surface or through the molecules in the electrolyte that can function as mediators. Oxygen capable of producing the superoxide radical may be viewed as a mediator. In 2011, Bedford et al. [41] immobilized thylakoids on conductive nanofibers, using the electrospinning technique. The maximum electric power generated by the cell surface was 24 mW/cm2 upon illumination by red light with a wavelength of 625 nm.

It is possible to create a stable solar cell by combining the photosynthetic anode and biocatalytic cathode. There is an idea to use photosynthetic organisms/organelle/photosystems for the water oxidation at the anode and the conversion of oxygen into water at the cathode.

Calkins et al. [32] created solar cells using thylakoids isolated from spinach. Thylakoids were immobilized on the anode modified with multi-walled carbon nanotubes (MWCNT). Glass electrode modified by laccase/MWCNT system was used as the cathode (Fig. 5a). The study has demonstrated a maximum current density of 68 mA/cm2 and a maximum power density of 5.3 mW/cm2 (Fig. 5b). Composite electrode based on thylakoid/MWCNT produced a current density of 38 mA/cm2 that is by two orders higher than predicted. The fact that the transmem‐ brane chlorophyll-protein complexes remain in their native state during the isolation process is the main advantage of the usage of membrane thylakoids for photocurrent. This may lead to greater stability and greater power output as compared to the results that can be achieved by using isolated chlorophyll-protein complexes or RCs.

#### **4.2. Photosystem I as photobiocatalyst**

Besides the thylakoid membrane preparations, some researchers have conducted studies of photocurrent generating using cells based on isolated photosystems. There are two major benefits of using photosystems as a photosensitizers compared to thylakoids [20]:


Fourmond et al. [42] developed a photobioelectrochemical system with PSI as the main photocatalytic subunit, cytochrome C6 and ferredoxin as electron carriers and FNR as an electron acceptor (Fig. 1). They used a gold electrode in the experiment. In an earlier investi‐ gation, Frolov et al. [43] created a photobioelectrochemical cell that could generate a voltage of 0.498±0.02 V. They used the PSI preparations isolated from the cyanobacteria *Synechocystis*

**Figure 5** Schematic representation of the functioning photobioelectrochemical cells based on, a) the thylakoid/ MWCNT; b) cyanobacteria *Nostoc*/MWCNT; c) and d) the dependences of the voltage and the flux density of the re‐ ceived energy on the current density for each of the cells shown (adapted from [20, 32]).

*sp*. PCC 6803. These systems are more stable than plant systems due to the antenna pigment molecule's integration into the core subunits. More specifically, unlike plant systems, the antenna pigments are associated only with chlorophyll-protein complex attached to core subunits. Surfactant peptides necessary for the stabilization of other plant and bacterial RCs were not required to stabilize such PSI. In their work, another important factor was the mutation-based replacement of specific amino acids of the PSI by cysteines. Properly oriented stable monolayer of PSI was formed through the formation of Au-S bonds between the thiol group of cysteine and purified hydrophilic gold surface. The procedure for creating the corresponding gold electrode included thermal treatment at 350 оС. In studies carried out by Das et al [24], mutation-modified PSI complexes were attached to the gold electrode by Ni2+ nitrilotriacetic acid (Ni2+-NTA). In these complexes, native subunit PsaD was replaced by PsaD-His6 one (Fig. 6a).

**Figure 6** Models used for immobilizing photosystem I on the electrode. a) Native PsaD subunit of PSI replaced by PsaD-His, which clings to the histidine tag, and the entire structure is associated with the gold electrode through a Ni2+-NTA (adapted from [20, 24]). b) The scheme of the cysteine mutants of the PSI with Pt ions and the multilayer structure of such PSI on a gold substrate (adapted from [20, 45]).

In another study, Faulkner et al. [44] reported a fast way to create a dense monolayer of PSI isolated from spinach leaves on a gold electrode. This method of the monolayer creation requiring vacuum conditions was 80 times faster than method of photosystem precipitation from a solution. More specifically, PSI was immobilized on the electrode modified with gold nanoparticles (GNP). In the presence of suitable mediators, the cell generated a photocurrent of 100 nA/cm2 [44].

However, photobioelectrochemical elements based on the PSI monolayer were not sufficiently effective in cases when a large cross-sectional area of light absorption was required. A photobioelectrochemical cell based on multilayer structures of PSI was created in the same year [45]. The PSI complexes were platinised on the stromal side to form the multilayer structures. The platinum ions facilitated the binding of the lumenal side of PSI and the stromal side of another PSI complex that resulted in the electrically connected multilayer. The first PSI monolayer was attached to the gold surface through the bonds between the cysteine's thiol groups of the mutant PSI and the gold atoms. Then, the next layer was formed through the connection between the photosystem donor side of the next layer and the platinum atoms (Fig. 6b). The devices developed on the basis of the two and three layers generated photovolt‐ age outputs of 0.330 and 0.386 V, respectively [45]. Hereafter, the investigations of solar cells based on multilayer structures of PSI were continued. Method suggested by Ciesielski et al. [46] did not require the use of photosystems isolated from mutated cyanobacteria, nor the use

*sp*. PCC 6803. These systems are more stable than plant systems due to the antenna pigment molecule's integration into the core subunits. More specifically, unlike plant systems, the antenna pigments are associated only with chlorophyll-protein complex attached to core subunits. Surfactant peptides necessary for the stabilization of other plant and bacterial RCs were not required to stabilize such PSI. In their work, another important factor was the mutation-based replacement of specific amino acids of the PSI by cysteines. Properly oriented stable monolayer of PSI was formed through the formation of Au-S bonds between the thiol group of cysteine and purified hydrophilic gold surface. The procedure for creating the

**Figure 5** Schematic representation of the functioning photobioelectrochemical cells based on, a) the thylakoid/ MWCNT; b) cyanobacteria *Nostoc*/MWCNT; c) and d) the dependences of the voltage and the flux density of the re‐

CURRENT DENSITY **(***μ***A**⋅**cm–2)** CURRENT DENSITY **(mA**⋅**m–2)**

POTENTIONAL

(V vs. Ag/.AgCl)

H2O H2O

H++O2 H++O2

**ANODE**

**CATHODE**

Light Light

LOAD LOAD

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6

**cell**

Laccase Laccase

**MWNT MWNT MWNT**

(b)

POWER DENSITY

**(***μ***w**⋅**cm–2)**

Thylakoid **cyanobacterial**

e– e–

POTENTIAL POWER DENSITY

0 0

50 100 150 200 250

POWER DENSITY

**(mw**⋅**m–2)**

**CATHODE**

0.00

0.05

0.10 0.15 0.20 0.25 0.30 0.35 0.40

**CELL VOLTAGE (V)**

(a)

**ANODE**

**MWNT**

170 Applied Photosynthesis - New Progress

0 10 20 30

40 50

60 70 80

ceived energy on the current density for each of the cells shown (adapted from [20, 32]).

VOLTAGE

POWER

(c) (d)

of a high vacuum, so it was more economical and less time-consuming. In their study, a plate of gold (thickness of about 125 nm) immobilized on a silicon substrate served as cathode and a working surface of transparent plastic plate coated with lead oxide doped with indium served as anode of the photoelement, respectively. A cavity between them was half-filled with an electrolyte composed of 5 mM 2,6-dichlorphenolindophenol, 100 mM ascorbic acid (Asc), and 100 mM NaCl in 5 mM phosphate buffer at pH 7.0. In the other half, there was a buffer solution containing PSI complexes (about 9 μM), Triton-X100 (0.05% w/v), 0.14 M in 0.2 M NaH2PO4 at pH 7.0. The PSI complexes were precipitated on a gold electrode for seven days. As a result, a multilayer of the PSI complexes with a thickness of 1-2 μm was obtained. The obtained solar cell generated a photocurrent at a density of about 2 mA/cm2 under illumination by a standard light intensity (clear sky at noon). The device demonstrated a considerable stability and retained activity under ambient conditions for at least 280 days [46].

According to the review of the recent advances in photosynthetic energy conversion made by Sekar and Ramasamy [2], to the present day the highest current density of 362 mA/cm2 and the energy flux density of 81 mkW/cm2 using PSI were obtained by Mershin et al. [47]. In their work, they compared the efficiency of solar cells with two different semiconductor substrates: nanocrystalline titanium dioxide TiO2 and nanowires of zinc oxide (ZnO) (Fig. 7). The measurements were carried out under normal sunlight. The PSI complexes were adsorbed on each of these two substrates. Stability of the isolated PSI complexes was increased by the treatment with surfactant Ac-AAAAAAK-NH2 – a sequence of six alanines and one lysine (A6K). This also promoted the selective adsorption of the PSI on the substrates and increased the absorption of light. Such approach improved the photovoltaic performance. In this artificial system, cobalt electrolyte performed the role of plastocyanine, and ferredoxin was replaced by nanocrystalline TiO2, or nanofiber ZnO [47]. Overall, PSI is a good photobiocatalyst, but it has several disadvantages as a photosensitizer. First, the process of the complex isolation is more laborious compared with the isolation of thylakoid membranes. Second, the isolated PSI complex is less stable. Third, for getting a continuous electron transfer to P700, RC requires an external electron donor with a redox potential approximately equal to the redox potential of plastocyanin. Thus, the photosystem depends on other electron sources.

#### **4.3. Photosystem II as photobiocatalyst**

The main advantage of PSII against PSI is the fact that water is the electron source required to activate the electron transfer, and it is abundant in the environment [2]. Unlike the PSI, which requires an electron donor, PSII has an internal oxygen-evolving complex, also known as water-splitting complex. Thus, PSII depends on the availability of water and light. Here, electrons from P680 are transferred to pheophytin, then to plastoquinone and further to the other ETC components [8]. There are two major rate-limiting steps in this process: reduction of plastoquinone in QB-site by two electrons from plastoquinone in QA-site and diffusion of the double reduced quinone (PQH2) inside membrane [8]. Therefore, it is assumed that the water oxidation in PSII should be accelerated if electrons from QA-site could be efficiently transferred to an external electron acceptor [48]. Thus, in order for the electrons from PSII to be transferred onto the electrode, the complex should come in contact with the surface of the

of a high vacuum, so it was more economical and less time-consuming. In their study, a plate of gold (thickness of about 125 nm) immobilized on a silicon substrate served as cathode and a working surface of transparent plastic plate coated with lead oxide doped with indium served as anode of the photoelement, respectively. A cavity between them was half-filled with an electrolyte composed of 5 mM 2,6-dichlorphenolindophenol, 100 mM ascorbic acid (Asc), and 100 mM NaCl in 5 mM phosphate buffer at pH 7.0. In the other half, there was a buffer solution containing PSI complexes (about 9 μM), Triton-X100 (0.05% w/v), 0.14 M in 0.2 M NaH2PO4 at pH 7.0. The PSI complexes were precipitated on a gold electrode for seven days. As a result, a multilayer of the PSI complexes with a thickness of 1-2 μm was obtained. The

by a standard light intensity (clear sky at noon). The device demonstrated a considerable

According to the review of the recent advances in photosynthetic energy conversion made by Sekar and Ramasamy [2], to the present day the highest current density of 362 mA/cm2 and

work, they compared the efficiency of solar cells with two different semiconductor substrates: nanocrystalline titanium dioxide TiO2 and nanowires of zinc oxide (ZnO) (Fig. 7). The measurements were carried out under normal sunlight. The PSI complexes were adsorbed on each of these two substrates. Stability of the isolated PSI complexes was increased by the treatment with surfactant Ac-AAAAAAK-NH2 – a sequence of six alanines and one lysine (A6K). This also promoted the selective adsorption of the PSI on the substrates and increased the absorption of light. Such approach improved the photovoltaic performance. In this artificial system, cobalt electrolyte performed the role of plastocyanine, and ferredoxin was replaced by nanocrystalline TiO2, or nanofiber ZnO [47]. Overall, PSI is a good photobiocatalyst, but it has several disadvantages as a photosensitizer. First, the process of the complex isolation is more laborious compared with the isolation of thylakoid membranes. Second, the isolated PSI complex is less stable. Third, for getting a continuous electron transfer to P700, RC requires an external electron donor with a redox potential approximately equal to the redox potential of

The main advantage of PSII against PSI is the fact that water is the electron source required to activate the electron transfer, and it is abundant in the environment [2]. Unlike the PSI, which requires an electron donor, PSII has an internal oxygen-evolving complex, also known as water-splitting complex. Thus, PSII depends on the availability of water and light. Here, electrons from P680 are transferred to pheophytin, then to plastoquinone and further to the other ETC components [8]. There are two major rate-limiting steps in this process: reduction of plastoquinone in QB-site by two electrons from plastoquinone in QA-site and diffusion of the double reduced quinone (PQH2) inside membrane [8]. Therefore, it is assumed that the water oxidation in PSII should be accelerated if electrons from QA-site could be efficiently transferred to an external electron acceptor [48]. Thus, in order for the electrons from PSII to be transferred onto the electrode, the complex should come in contact with the surface of the

using PSI were obtained by Mershin et al. [47]. In their

under illumination

obtained solar cell generated a photocurrent at a density of about 2 mA/cm2

plastocyanin. Thus, the photosystem depends on other electron sources.

the energy flux density of 81 mkW/cm2

172 Applied Photosynthesis - New Progress

**4.3. Photosystem II as photobiocatalyst**

stability and retained activity under ambient conditions for at least 280 days [46].

**Figure 7** Schematic presentation of two Mershin's cells with zinc oxide and titanium dioxide. FTO – a layer of fluorine doped with tin oxide, ITO – a layer of indium doped with tin oxide, and PsaE-ZnO – mutant subunit (adapted from [20, 47]).

electrode, or the electron transfer should be carried out by a mediator. In fact, it is difficult to achieve direct electron transfer from the PSII to the electrode due to the deep localization of the Pheo-PQ site inside the PSII [38].

For the creation of efficient solar cell based on PSII, it is important to improve its stability and increase electron transport efficiency. To achieve that, Vittadello et al. [49] reported the application of histidine-tagged protein complex of PSII from *Synechococcus elongatus* covalently bound to a gold electrode treated with Ni2+-nitrilotriacetic acid (Ni2+-NTA). The current density of the resulting photobioelectrochemical cell has reached 43 mkA/cm2 [49]. On the other hand, while the photochemical energy conversion efficiency of the freshly isolated PSII was 0.7, the same parameter for the PSII immobilized on gold was 0.53. This clearly indicated that the PSII complexes were photochemically stable even after immobilization [49].

Utilization of osmium-containing redox polymer based on poly-1-vinylimidazole is also an effective immobilization method, which could help maintain the stability as well as enhance the coating degree of the electrode by the PSII complexes (Fig. 8a) [38]. The polymer works both as an immobilization matrix and a mediator. This kind of system could facilitate the electron transfer from the PSII complex to the electrode. The correct orientation of the immo‐ bilized complex could also support the electron transfer. Recently, Noji et al. [50] developed a nanodevice for the artificial water decomposition controlled by light, using a conjugate of PSII-GNP. The core of the PSII complex comprising a histidine tag on the C-terminus of CP47 protein was immobilized on a GNP by Ni2+-NTA (Fig. 8b). In this work, the diameter of GNPs was about 20 nm, and GNPs could bind four or five PSII complexes. The efficiency of oxygen evolution by the developed PSII-GNP was comparable to that of the unbound PSII [50].

Israeli scientists developed the photocell on the basis of bacterial PSII complexes isolated from the thermophilic cyanobacterium *Mastigocladus laminosus*. The photoanode consisting of a matrix of 2-mercapto-1,4-benzoquinone was electro-polymerized on the gold surface. Then, PSII complexes were immobilized on this surface. The anode was electrically connected to the cathode by bilirubin oxidase/carbon nanotubes (BOD/CNT). It is claimed that photo-induced quinone-mediated electron transfer led to the generation of photocurrent with an output power of 0.1 W [37].

**Figure 8** Models used for immobilizing photosystem II on the electrode. a) PSII associated with an osmium redox poly‐ mer containing a mediator network (adapted from [20, 38]). Yellow arrows depict the electron transfer pathway by a hopping mechanism. b) Connection of PSII with gold nanoparticle through histidine tag with Ni2+-nitrilotriacetic acid (Ni2+-NTA) attached to the C-terminus of the CP47 protein (adapted from [20, 50]).

Special protective compounds located inside the chloroplast protect highly sensitive photo‐ systems (PSI and PSII) from photoinhibition [2]. It is evident that the stability of isolated photosystems will be impaired after their isolation from native environments. It should be noted that isolated PSII is less stable compared to PSI. Thus, photocurrent of higher density could be achieved in cells using PSI complexes [20].

#### **4.4. The bacterial cell as photobiocatalyst**

Photocells with isolated photosynthetic structures such as thylakoids, PSI and PSII suffer from significant disadvantages. The components of these cells are relatively unstable; they have a short running time and require labour-consuming laboratory procedures such as isolation/ purification. These limitations could be overcome if whole cells of photosynthetic microor‐ ganisms were used as a biocatalyst and/or sensitizer. In the past few years, some studies have been conducted to construct a photosynthetic microbial fuel cell (PMFC) based on whole cells of photosynthetic organisms such as cyanobacteria [2]. In the anode chamber of PMFCs, there are photosynthetic organisms that are able to oxidize water using light. PMFC requires only sunlight and water for the functioning, whereas traditional MFCs based on bacteria, for example, *Gejbacter* and *Shewanella*, require organic carbon sources such as glucose/lactate, and they produce CO2 as final product. Figure 9 shows the general scheme of the combined cell.

Israeli scientists developed the photocell on the basis of bacterial PSII complexes isolated from the thermophilic cyanobacterium *Mastigocladus laminosus*. The photoanode consisting of a matrix of 2-mercapto-1,4-benzoquinone was electro-polymerized on the gold surface. Then, PSII complexes were immobilized on this surface. The anode was electrically connected to the cathode by bilirubin oxidase/carbon nanotubes (BOD/CNT). It is claimed that photo-induced quinone-mediated electron transfer led to the generation of photocurrent with an output

**Figure 8** Models used for immobilizing photosystem II on the electrode. a) PSII associated with an osmium redox poly‐ mer containing a mediator network (adapted from [20, 38]). Yellow arrows depict the electron transfer pathway by a hopping mechanism. b) Connection of PSII with gold nanoparticle through histidine tag with Ni2+-nitrilotriacetic acid

Special protective compounds located inside the chloroplast protect highly sensitive photo‐ systems (PSI and PSII) from photoinhibition [2]. It is evident that the stability of isolated photosystems will be impaired after their isolation from native environments. It should be noted that isolated PSII is less stable compared to PSI. Thus, photocurrent of higher density

Photocells with isolated photosynthetic structures such as thylakoids, PSI and PSII suffer from significant disadvantages. The components of these cells are relatively unstable; they have a short running time and require labour-consuming laboratory procedures such as isolation/ purification. These limitations could be overcome if whole cells of photosynthetic microor‐ ganisms were used as a biocatalyst and/or sensitizer. In the past few years, some studies have been conducted to construct a photosynthetic microbial fuel cell (PMFC) based on whole cells of photosynthetic organisms such as cyanobacteria [2]. In the anode chamber of PMFCs, there are photosynthetic organisms that are able to oxidize water using light. PMFC requires only sunlight and water for the functioning, whereas traditional MFCs based on bacteria, for

(Ni2+-NTA) attached to the C-terminus of the CP47 protein (adapted from [20, 50]).

could be achieved in cells using PSI complexes [20].

**4.4. The bacterial cell as photobiocatalyst**

power of 0.1 W [37].

174 Applied Photosynthesis - New Progress

Various cyanobacteria were used in the most effective PMFCs [51-53]. In particular, the ability of the cyanobacteria *Nostoc sp*. in generating a photocurrent was investigated using various electrochemical methods. In a recent investigation, the mechanism of direct electron transfer from ETC of *Nostoc* to electrode was studied using the site-specific photosynthetic inhibitors (Fig. 1) [54]. It was shown that the solar cell with *Nostoc* immobilized on the MWCNT-modified carbon electrode as an anode and laccase/MWCNT-modified cathode (Fig. 5c) generated a current density of 25 mA/cm2 , while the maximum energy flux density achieved without mediators was only at 3.5 mW/cm2 (Fig. 5d). In comparison, the cell based on thylakoids generated a maximum current density of 10 mA/cm2 (Fig. 5a), and the maximum energy flux density achieved without mediators was of 5 mW/cm2 (Fig. 5b). Overall, the maximum current density from the solar element based on the native photosynthetic cells was higher than that from the photoelement based on thylakoids.

One of the main advantages of cyanobacteria compared with individual components of the photosynthetic apparatus is that they are considerably less susceptible to dehydration. Currently, the power that could be generated by PMFCs is less than that achieved by the biofuel cell [2]. However, many of their advantages such as simplicity of operation, the utilization of available substrates, for example, water, as well as stress-resistance of PMFCs in comparison with the other biofuel cells mark them as promising solar cell structure for the future.

**Figure 9** Schematic representation of the different forms of fuel cells: a) hydrogen fuel cell with a platinum catalyst on the anode and the cathode and b) photobioelectrochemical cell based on cyanobacterial cell (CB) on the anode and lac‐ case enzyme on the cathode (adapted from [2, 20]).
