**1. Introduction**

186 Solar Cells – New Aspects and Solutions

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pp. 1263-1275, ISSN 0022-3697

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Oxide Films. *Scripta Materialia*, Vol.38, No.11, (May 1998), pp. 1731-1738, ISSN 1359-

Atmospheric carbon dioxide has been increased and was reached approximately to 390 mg/L at December 2010 (Tans, 2011). Rising trend of carbon dioxide in past and present time may be an indicator capable of estimating the concentration of atmospheric carbon dioxide in the future. Cause for increase of atmospheric carbon dioxide was already investigated and became general knowledge for the civilized peoples who are watching TV, listening to radio, and reading newspapers. Anybody of the civilized peoples can anticipate that the atmospheric carbon dioxide is increased continuously until unknowable time in the future but not in the near future. Carbon dioxide is believed to be a major factor affecting global climate variation because increase of atmospheric carbon dioxide is proportional to variation trend of global average temperature (Cox et al., 2000). Atmospheric carbon dioxide is generated naturally from the eruption of volcano (Gerlach et al., 2002; Williams et al., 1992), decay of organic matters, respiration of animals, and cellular respiration of microorganisms (Raich and Schlesinger, 2002; Van Veen et al., 1991); meanwhile, artificially from combustion of fossil fuels, combustion of organic matters, and cement making-process (Worrell et al., 2001). Theoretically, the natural atmospheric carbon dioxide generated biologically from the decay of organic matter and the respirations of organisms has to be fixed biologically by land plants, aquatic plants, and photosynthetic microorganisms, by which cycle of atmospheric carbon dioxide may be nearly balanced (Grulke et al., 1990). All of the human-emitted carbon dioxide except the naturally balanced one may be incorporated newly into the pool of atmospheric greenhouse gases that are methane, water vapor, fluorocarbons, nitrous oxide, and carbon dioxide (Lashof and Ahuja, 1990). The airborne fraction of carbon dioxide that is the ratio of the increase in atmospheric carbon dioxide to the emitted carbon dioxide variation was typically about 45% over 5 years period (Keeling et al., 1995). Canadell at al (2007) reported that about 57% of human-emitted carbon dioxide was removed by the biosphere and oceans. These reports indicate that the airborne fraction of carbon dioxide is at least 43-45%, which may be the balance emitted by human activity.

The land plants are the largest natural carbon dioxide sinker, which have been decreased globally by deforestation (Cramer et al., 2004). Especially, tropical and rainforests are being

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 189

neutral red (NR) have been used as electron mediator to induce electrochemical redox reaction between electrode and electron carriers that are NAD+, FAD, and cytochrome C (Pollack et al., 1996; Park et al., 1997; Wang and Du, 2002; Kang et al., 2007). In order to *in vivo* drive and maintain bacterial metabolism with electrochemical reducing power as a sole energy source, only NAD+ or NADP+ is required to be reduced by coupling redox reaction between electron mediator and biochemical electron carrier (Park and Zeikus, 1999; 2000). NR can catalyze the electrochemical reduction reaction of NAD+ both *in vivo* and *in vitro* but no electron mediator except the NR can. NR is a water-soluble structure composed of phenazine ring with amine, dimethyl amine, methyl, and hydrogen group as shown in Fig 1. The dimethyl amine group is redox center for electron-accepting and donating in coupling with phenazine ring; meanwhile, the amine, methyl, and hydrogen are structural group. Redox potential of NR is -0.325 volt (vs. NHE), which is 0.05 volt lower than NAD+. The electrochemical redox reaction of NR can be coupled to biochemical redox reaction as

[ NRox + 2e- + 1H+ NRred; NRred + NAD+ NRox + NADH ]

Commonly, NRox and NAD+ are reduced to NRred and NADH, respectively by accepting

Fig. 1. Molecular structure of neutral red, which can be electrochemically oxidized (A) or reduced (B). The reduced neutral red can catalyze reduction reaction of NAD+ (C) to NADH (D) without enzyme catalysis. Ox and Red indicate oxidation and reduction, respectively.

+ 2H+ NADH + H+]

NAD+ can be reduced in coupling with biochemical redox reaction as follows:

[ NAD+ + 2e-

follows:

two electrons and one proton.

cut down for different purpose and by different reason and some of the forest are being burned for slash and burn farming. The atmospheric carbon dioxide and other greenhouse gases are increased in proportion to the deforestation (McKane et al. 1995). Deforestation causes part of the released carbon dioxide to be accumulated in the atmosphere and the global carbon cycle to be changed (Robertson and Tiejei, 1988). The releasing carbon dioxide and changing carbon cycle increase the greenhouse effect and may raise global temperature. The greenhouse effect is generated naturally by the infrared radiation, which is generated from incoming solar radiation, absorbed into atmospheric greenhouse gases and re-radiated in all direction (Held and Soden). The gases contributing to the greenhouse effect on Earth are water vapor (36-70%), carbon dioxide (9-26%), methane (4-9%), and ozone (3-7%) (Kiehl et al., 1977). Especially, water vapor can amplify the warming effect of other greenhouse gases, such that the warming brought about by increased carbon dioxide allows more water vapor to enter the atmosphere (Hansen, 2008). The greenhouse effect can be strengthened by human activity and enhanced by the synergetic effect of water vapor and carbon dioxide because the elevated carbon dioxide levels contribute to additional absorption and emission of thermal infrared in the atmosphere (Shine et al., 1999). The major non-gas contributor to the Earth's greenhouse effect, cloud (water vapor), also absorb and emit infrared radiation and thus have an effect on net warming of the atmosphere (Kiehl et al., 1997). Elevation of carbon dioxide is a cause for greenhouse effect, by which abnormal climate, desertification, and extinction of animals and plants may be induced (Stork, 1997). However, carbon dioxide is difficult to be controlled in the industry-based society that depends completely upon fossil fuel. If the elevation of carbon dioxide was unstoppable or necessary evil, the technique to convert biologically the atmospheric carbon dioxide to stable polymer in the condition without using fossil fuel must be developed. All of the land and aquatic plants convert mainly carbon dioxide to biomolecule in coupling with oxygen generation; however, a total of 16.5% of the forest (230,000 square miles) was affected by deforestation due to the increase of fragmented forests, cleared forests, and boundary areas between the fragmented forests (Skole et al., 1998). Decline of plants may be a cause to activate generation of the radiant heat because the visible radiation of solar energy absorbed for photosynthesis can be converted to additional radiant heat.

Solar cell is the useful equipment capable of physically absorbing solar radiation and converting the solar energy to electric energy (O'Regan and Grätzel, 1991). The radiant heat generated from the solar energy may be decreased in proportion to the electric energy produced by the solar cells. Electrochemical redox reaction can be generated from electric energy by using a specially designed bioreactor equipped with the anode and cathode separated with membrane, which is an electrochemical bioreactor. The electric energy generated from the solar energy can be converted to biochemical reducing power through the electrochemical redox mediator. The biochemical reducing power (NADH or NADPH) is the driving force to generate biochemical energy, ATP. The biochemical reducing power and ATP are essential elements that activate all biochemical reactions for biosynthesis of cell structure and production of metabolites.

### **2. Electrochemical redox mediator**

The electrochemical reduction reaction generated in cathode can't catalyze reduction of NAD+ or NADP+ both *in vitro* and *in vivo* without electron mediator. Various ion radicals that are methyl viologen, benzyl viologen, hydroquinone, tetracyanoquinodimethane, and

cut down for different purpose and by different reason and some of the forest are being burned for slash and burn farming. The atmospheric carbon dioxide and other greenhouse gases are increased in proportion to the deforestation (McKane et al. 1995). Deforestation causes part of the released carbon dioxide to be accumulated in the atmosphere and the global carbon cycle to be changed (Robertson and Tiejei, 1988). The releasing carbon dioxide and changing carbon cycle increase the greenhouse effect and may raise global temperature. The greenhouse effect is generated naturally by the infrared radiation, which is generated from incoming solar radiation, absorbed into atmospheric greenhouse gases and re-radiated in all direction (Held and Soden). The gases contributing to the greenhouse effect on Earth are water vapor (36-70%), carbon dioxide (9-26%), methane (4-9%), and ozone (3-7%) (Kiehl et al., 1977). Especially, water vapor can amplify the warming effect of other greenhouse gases, such that the warming brought about by increased carbon dioxide allows more water vapor to enter the atmosphere (Hansen, 2008). The greenhouse effect can be strengthened by human activity and enhanced by the synergetic effect of water vapor and carbon dioxide because the elevated carbon dioxide levels contribute to additional absorption and emission of thermal infrared in the atmosphere (Shine et al., 1999). The major non-gas contributor to the Earth's greenhouse effect, cloud (water vapor), also absorb and emit infrared radiation and thus have an effect on net warming of the atmosphere (Kiehl et al., 1997). Elevation of carbon dioxide is a cause for greenhouse effect, by which abnormal climate, desertification, and extinction of animals and plants may be induced (Stork, 1997). However, carbon dioxide is difficult to be controlled in the industry-based society that depends completely upon fossil fuel. If the elevation of carbon dioxide was unstoppable or necessary evil, the technique to convert biologically the atmospheric carbon dioxide to stable polymer in the condition without using fossil fuel must be developed. All of the land and aquatic plants convert mainly carbon dioxide to biomolecule in coupling with oxygen generation; however, a total of 16.5% of the forest (230,000 square miles) was affected by deforestation due to the increase of fragmented forests, cleared forests, and boundary areas between the fragmented forests (Skole et al., 1998). Decline of plants may be a cause to activate generation of the radiant heat because the visible radiation of solar energy absorbed for

photosynthesis can be converted to additional radiant heat.

structure and production of metabolites.

**2. Electrochemical redox mediator** 

Solar cell is the useful equipment capable of physically absorbing solar radiation and converting the solar energy to electric energy (O'Regan and Grätzel, 1991). The radiant heat generated from the solar energy may be decreased in proportion to the electric energy produced by the solar cells. Electrochemical redox reaction can be generated from electric energy by using a specially designed bioreactor equipped with the anode and cathode separated with membrane, which is an electrochemical bioreactor. The electric energy generated from the solar energy can be converted to biochemical reducing power through the electrochemical redox mediator. The biochemical reducing power (NADH or NADPH) is the driving force to generate biochemical energy, ATP. The biochemical reducing power and ATP are essential elements that activate all biochemical reactions for biosynthesis of cell

The electrochemical reduction reaction generated in cathode can't catalyze reduction of NAD+ or NADP+ both *in vitro* and *in vivo* without electron mediator. Various ion radicals that are methyl viologen, benzyl viologen, hydroquinone, tetracyanoquinodimethane, and neutral red (NR) have been used as electron mediator to induce electrochemical redox reaction between electrode and electron carriers that are NAD+, FAD, and cytochrome C (Pollack et al., 1996; Park et al., 1997; Wang and Du, 2002; Kang et al., 2007). In order to *in vivo* drive and maintain bacterial metabolism with electrochemical reducing power as a sole energy source, only NAD+ or NADP+ is required to be reduced by coupling redox reaction between electron mediator and biochemical electron carrier (Park and Zeikus, 1999; 2000). NR can catalyze the electrochemical reduction reaction of NAD+ both *in vivo* and *in vitro* but no electron mediator except the NR can. NR is a water-soluble structure composed of phenazine ring with amine, dimethyl amine, methyl, and hydrogen group as shown in Fig 1. The dimethyl amine group is redox center for electron-accepting and donating in coupling with phenazine ring; meanwhile, the amine, methyl, and hydrogen are structural group. Redox potential of NR is -0.325 volt (vs. NHE), which is 0.05 volt lower than NAD+. The electrochemical redox reaction of NR can be coupled to biochemical redox reaction as follows:

$$\left[ \left[ \mathrm{NR}\_{\mathrm{ox}} + 2\mathrm{e}^{\cdot} + 1\mathrm{H}^{+} \right. \right. + \left. \mathrm{NR}\_{\mathrm{red}} \right] \mathrm{NR}\_{\mathrm{red}} + \mathrm{NAD}^{+} \right. \left. \left. \mathrm{NR}\_{\mathrm{ox}} + \mathrm{NADH} \right] \mathrm{J}$$

NAD+ can be reduced in coupling with biochemical redox reaction as follows:

$$\left[\mathrm{NAD^{\*}} + 2\mathrm{e^{\*}} + 2\mathrm{H^{\*}} \rightarrow \mathrm{NADH} + \mathrm{H^{\*}}\right]$$

Commonly, NRox and NAD+ are reduced to NRred and NADH, respectively by accepting two electrons and one proton.

Fig. 1. Molecular structure of neutral red, which can be electrochemically oxidized (A) or reduced (B). The reduced neutral red can catalyze reduction reaction of NAD+ (C) to NADH (D) without enzyme catalysis. Ox and Red indicate oxidation and reduction, respectively.

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 191

Fig. 2. Schematic structure of polyvinyl-NR that is produced by covalent bond between amine of NR and alcohol of polyvinyl alcohol. The polyvinyl-NR can bind physically to

Fig. 3. Schematic structure of a combined anode composed of cellulose acetate film, porous ceramic membrane and porous carbon plate. Water or gas can penetrate across the cellulose

Practically, the hydrogenotrophic methanogens are useful microorganisms for carbon dioxide fixation using the electrochemical bioreactor. However, most of the reducing power that is electrochemically generated in the NR-graphite cathode may be consumed to

graphite cathode surface.

acetate film but solutes can't.

Theoretically, the water-soluble NR may be reduced at the moment when contacted with electrode and catalyze biochemical reduction of NAD+ at the moment when contacted with bacterial cell or enzyme. A part of NR may be contacted with electrode or bacterial cell in water-based reactant but most of that is dissolved or dispersed in the reactant. In order to induce the effective electrochemical and biochemical reaction in the bacterial culture, NR and bacterial cells have to contact continuously and simultaneously with electrode. This can be accomplished by immobilization of NR in graphite felt electrode based on the data that most of bacterial cells tend to build biofilm spontaneously on surface of solid material and the graphite felt is matrix composed of 0.47m2/g of fiber (Park et al., 1999). The amino group of NR can bind covalently to alcohol group of polyvinyl alcohol by dehydration reaction, in which polyvinyl-3-imino-7-dimethylamino-2-methylphenazine (polyvinyl-NR) is produced as shown in Fig 2. The polyvinyl-NR is a water-insoluble solid electron mediator to catalyze electrochemically reduction reaction of NAD+ like the water-soluble NR (Park and Zeikus, 2003). The polyvinyl-NR immobilized in graphite felt (NR-graphite) functions as a cathode for electron-driving circuit, an electron mediator for conversion of electric energy to electrochemical reducing power, and a catalyst for reduction of NAD+ to NADH. The electrochemical bioreactor equipped with the NR-graphite cathode is very useful to cultivate autotrophic bacteria that grow with carbon dioxide as a sole carbon source and electrochemical reducing power as a sole energy source (Lee and Park, 2009).

### **3. Separation of electrochemical redox reaction**

The biochemical reducing power can be regenerated electrochemically by NR-graphite cathode (working electrode) that functions as a catalyst, for which H2O has to be electrolyzed on the surface of anode (counter electrode) that functions as an electron donor. The working electrode is required to be separated electrochemically from the counter electrode by specific septa that are the ion-selective Nafion membrane (Park and Zeikus, 2003; Kang et al., 2007; Tran et al., 2009), the ceramic membrane (Park and Zeikus, 2003; Kang et al., 2007; Tran et al., 2009), the modified ceramic membrane with cellulose acetate film (Jeon et al, 2009B), and the micro-pored glass filter, by which the electrochemical reducing power in the cathode compartment can be maintained effectively. Jeon and Park (2010) developed a combined anode that was composed of cellulose acetate film, porous ceramic membrane and porous carbon plate as shown in Fig 3. The combined anode functions as a septum for electrochemical redox separation between anode and cathode, an anode for electron-driving circuit, and a catalyst for electrolysis of H2O. The major function of anode is to supply electrons required for generation of electrochemical reducing power in the working electrode (NR-graphite cathode), in which H2O functions as an electron donor. The strict anaerobic bacteria that are methanogens, sulfidogens, and anaerobic fermenters grow in the condition with lower oxidation-reduction potential than -300 mV (vs. NHE) (Ferry, 1993; Gottschalk, 1985), which can be induced electrochemically inside of the carbon fibre matrices of NR-cathode under only non-oxygen atmosphere. The NR-cathode can catalyze biochemical regeneration of NADH and generation of hydrogen but can't catalyze scavenging of oxygen and oxygen radicals at around 25oC and 1 atm. The combined anode can protect effectively contamination of catholyte with the atmospheric oxygen by

unidirectional evaporation of water from catholyte to atmosphere through the combined anode as shown in Fig 4. The driving force for the unidirectional evaporation of water may be generated naturally by the difference of water pressure between catholyte and outside atmosphere (Jeon et al., 2009A).

Theoretically, the water-soluble NR may be reduced at the moment when contacted with electrode and catalyze biochemical reduction of NAD+ at the moment when contacted with bacterial cell or enzyme. A part of NR may be contacted with electrode or bacterial cell in water-based reactant but most of that is dissolved or dispersed in the reactant. In order to induce the effective electrochemical and biochemical reaction in the bacterial culture, NR and bacterial cells have to contact continuously and simultaneously with electrode. This can be accomplished by immobilization of NR in graphite felt electrode based on the data that most of bacterial cells tend to build biofilm spontaneously on surface of solid material and the graphite felt is matrix composed of 0.47m2/g of fiber (Park et al., 1999). The amino group of NR can bind covalently to alcohol group of polyvinyl alcohol by dehydration reaction, in which polyvinyl-3-imino-7-dimethylamino-2-methylphenazine (polyvinyl-NR) is produced as shown in Fig 2. The polyvinyl-NR is a water-insoluble solid electron mediator to catalyze electrochemically reduction reaction of NAD+ like the water-soluble NR (Park and Zeikus, 2003). The polyvinyl-NR immobilized in graphite felt (NR-graphite) functions as a cathode for electron-driving circuit, an electron mediator for conversion of electric energy to electrochemical reducing power, and a catalyst for reduction of NAD+ to NADH. The electrochemical bioreactor equipped with the NR-graphite cathode is very useful to cultivate autotrophic bacteria that grow with carbon dioxide as a sole carbon source and

electrochemical reducing power as a sole energy source (Lee and Park, 2009).

The biochemical reducing power can be regenerated electrochemically by NR-graphite cathode (working electrode) that functions as a catalyst, for which H2O has to be electrolyzed on the surface of anode (counter electrode) that functions as an electron donor. The working electrode is required to be separated electrochemically from the counter electrode by specific septa that are the ion-selective Nafion membrane (Park and Zeikus, 2003; Kang et al., 2007; Tran et al., 2009), the ceramic membrane (Park and Zeikus, 2003; Kang et al., 2007; Tran et al., 2009), the modified ceramic membrane with cellulose acetate film (Jeon et al, 2009B), and the micro-pored glass filter, by which the electrochemical reducing power in the cathode compartment can be maintained effectively. Jeon and Park (2010) developed a combined anode that was composed of cellulose acetate film, porous ceramic membrane and porous carbon plate as shown in Fig 3. The combined anode functions as a septum for electrochemical redox separation between anode and cathode, an anode for electron-driving circuit, and a catalyst for electrolysis of H2O. The major function of anode is to supply electrons required for generation of electrochemical reducing power in the working electrode (NR-graphite cathode), in which H2O functions as an electron donor. The strict anaerobic bacteria that are methanogens, sulfidogens, and anaerobic fermenters grow in the condition with lower oxidation-reduction potential than -300 mV (vs. NHE) (Ferry, 1993; Gottschalk, 1985), which can be induced electrochemically inside of the carbon fibre matrices of NR-cathode under only non-oxygen atmosphere. The NR-cathode can catalyze biochemical regeneration of NADH and generation of hydrogen but can't catalyze scavenging of oxygen and oxygen radicals at around 25oC and 1 atm. The combined anode can protect effectively contamination of catholyte with the atmospheric oxygen by unidirectional evaporation of water from catholyte to atmosphere through the combined anode as shown in Fig 4. The driving force for the unidirectional evaporation of water may be generated naturally by the difference of water pressure between catholyte and outside

**3. Separation of electrochemical redox reaction** 

atmosphere (Jeon et al., 2009A).

Fig. 2. Schematic structure of polyvinyl-NR that is produced by covalent bond between amine of NR and alcohol of polyvinyl alcohol. The polyvinyl-NR can bind physically to graphite cathode surface.

Fig. 3. Schematic structure of a combined anode composed of cellulose acetate film, porous ceramic membrane and porous carbon plate. Water or gas can penetrate across the cellulose acetate film but solutes can't.

Practically, the hydrogenotrophic methanogens are useful microorganisms for carbon dioxide fixation using the electrochemical bioreactor. However, most of the reducing power that is electrochemically generated in the NR-graphite cathode may be consumed to

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 193

Fig. 5. Schematic structure of the anode and cathode compartment separated by glass filter. Protons, electrons, and oxygen generated from water by the electrolysis may be transferred

A specially designed electrochemical bioreactor is composed of the combined anode (Fig 4) and NR-graphite cathode for enrichment of the hydrogenotrophic methanogens as shown in Fig 6. Oxygen-free and carbonate-saturated wastewater was supplied continuously from a wastewater reservoir as shown in Fig 7. The electrochemical bioreactor was operated with the electricity generated from the solar panel. The wastewater obtained from sewage treatment plant was used without sterilization, to which 50 mM of sodium bicarbonate was added. The contaminated oxygen was consumed by bacteria growing intrinsically in the wastewater reservoir. Hydrogenotrophic methanogens grow with the free energy and reducing power generated by the coupling redox reaction of carbon dioxide and hydrogen (Ferguson and Mah, 1983; Na et al., 2007; Zeikus and Wolfe, 1972). Hydrogen generated from the electrolysis of water can't function to maintain the proper oxidation reduction

separately to the catholyte, the NR-cathode, and the atmosphere.

**4. Enrichment of hydrogenotrophic methanogens** 

maintain the proper oxidation-reduction potential for growth of the hydrogenotrophic methanogens in the condition without chemical reducing agent. This may be a cause to decrease the regeneration effect of the biochemical reducing power and free energy in the electrochemical bioreactors. In natural ecosystem, hydrogen sulfide produced metabolically by sulfidogens in coupling with oxidation of organic acids functions as the chemical reducing agent to maintain the proper environmental condition for growth of the methanogens (Thauer et al., 1977; Oremland et al., 1989; Zinder et al., 1984).

Fig. 4. Schematic structure of the combined anode composed of cellulose acetate film, porous ceramic membrane, and porous carbon plate, in which protons, electrons, and oxygen generated from water by the electrolysis may be transferred separately to the catholyte, the NR-cathode, and the atmosphere. Water is transferred from catholyte to atmosphere through the combined anode by difference of water pressure between catholyte and atmosphere.

Meanwhile, the growth condition for facultative anaerobic mixotrophs is not required to be controlled electrochemically because the metabolic function of the facultative anaerobic mixotrophs is not influenced critically by the oxidation-reduction potential. Accordingly, the combined anode may be replaced by the glass filter (pore, 1-1.6 m) that permits transfer of water and diffusion of ions and soluble compounds. Water transferred from catholyte to anolyte through the glass filter by difference of pressure and volume is electrolysed into oxygen, protons, and electrons in the anode compartment. The protons, electrons, and oxygen are transferred separately to the catholyte, the NR-cathode, and the atmosphere as shown in Fig 5. The water in the anode compartment equipped at the center of catholyte is consumed continuously by electrolysis and refilled spontaneously from catholyte by difference of volume and pressure between the catholyte and anolyte.

maintain the proper oxidation-reduction potential for growth of the hydrogenotrophic methanogens in the condition without chemical reducing agent. This may be a cause to decrease the regeneration effect of the biochemical reducing power and free energy in the electrochemical bioreactors. In natural ecosystem, hydrogen sulfide produced metabolically by sulfidogens in coupling with oxidation of organic acids functions as the chemical reducing agent to maintain the proper environmental condition for growth of the

methanogens (Thauer et al., 1977; Oremland et al., 1989; Zinder et al., 1984).

Fig. 4. Schematic structure of the combined anode composed of cellulose acetate film, porous ceramic membrane, and porous carbon plate, in which protons, electrons, and oxygen generated from water by the electrolysis may be transferred separately to the catholyte, the NR-cathode, and the atmosphere. Water is transferred from catholyte to atmosphere through the combined anode by difference of water pressure between catholyte

difference of volume and pressure between the catholyte and anolyte.

Meanwhile, the growth condition for facultative anaerobic mixotrophs is not required to be controlled electrochemically because the metabolic function of the facultative anaerobic mixotrophs is not influenced critically by the oxidation-reduction potential. Accordingly, the combined anode may be replaced by the glass filter (pore, 1-1.6 m) that permits transfer of water and diffusion of ions and soluble compounds. Water transferred from catholyte to anolyte through the glass filter by difference of pressure and volume is electrolysed into oxygen, protons, and electrons in the anode compartment. The protons, electrons, and oxygen are transferred separately to the catholyte, the NR-cathode, and the atmosphere as shown in Fig 5. The water in the anode compartment equipped at the center of catholyte is consumed continuously by electrolysis and refilled spontaneously from catholyte by

and atmosphere.

Fig. 5. Schematic structure of the anode and cathode compartment separated by glass filter. Protons, electrons, and oxygen generated from water by the electrolysis may be transferred separately to the catholyte, the NR-cathode, and the atmosphere.
