WO2010045072A2

WO2010045072A2 - Microbial electrolytic cell

Info

Publication number: WO2010045072A2
Application number: PCT/US2009/059697
Authority: WO
Inventors: Bruce Rittmann; Hyung-Sool Lee; Cesar I. Torres
Original assignee: Arizona Board Of Regents For And On Behalf Of Arizona State University
Priority date: 2008-10-17
Filing date: 2009-10-06
Publication date: 2010-04-22
Also published as: WO2010045072A3

Abstract

System and methods for efficiently capturing hydrogen gas from a microbial electrolytic cell. The systems and methods can be configured such that the cathode is located above the anode and proximal to a fluid level and a gas headspace in the single-chamber microbial electrolytic cell.

Description

DESCRIPTION

MICROBIAL ELECTROLYTIC CELL

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Patent Application Serial No. 61/106,225, filed October 17, 2008, entitled "Microbial Electrolytic Cell", the entire disclosure of which is specifically incorporated herein by reference.

BACKGROUND OF THE INVENTION A. Field of the Invention

Embodiments of the present invention relate generally to a system and method for efficiently capturing hydrogen gas from a microbial electrolytic cell. In particular, embodiments of the present invention concern the systems and methods where the cathode is located above the anode and proximal to a fluid level and a gas headspace in a single-chamber microbial electrolytic cell.

B. Description of Related Art

H₂ can become a significant contributor to global energy sustainability if it is produced from renewable, non-fossil fuel resources (e.g., biomass or sunlight). A microbial electrolytic cell (MEC) can be attractive as an alternative to biological H₂ production out of organic compounds. The MEC uses specific bacteria, called anode-respiring bacteria (ARB) that can transfer electrons extracted from organic donors to the anode in the MEC. The electrons transported to the anode pass through a circuit and reach the cathode, where the electrons react with H⁺ ions (or H₂O) to produce H₂.

MECs have two main advantages over other biohydrogen processes. First, a variety of organic donor substrates can be used as fuel, such as glucose, cellulose, acetate, butyrate, lactate, propionate, or valerate (Cheng et al., 2007), by using bacterial consortia involving fermenters and ARB (Ren et al., 2007; Lee et al., 2008). Second, non-fermentable substrates can be completely oxidized to CO2, resulting in high conversion yields of 67-91% (Cheng et al., 2007). An MEC involves two redox steps. The first redox step is oxidation of an electron donor by the ARB, with electrons transferred to the anode. The coulombic efficiency (CE) is the ratio of electron equivalents (e^" equiv) converted to electrical flow (i.e., coulombs) normalized to the number of e^" equiv consumed from the organic donor. The second redox step is a reduction in which the electrons transferred through the circuit react with H+ (or H₂O) at the cathode and produce H₂. The cathodic conversion efficiency (CCE) is the ratio of e^"- equivalents donated to H₂ normalized to the e^~ equivalents transferred in the circuit from the anode to the cathode. A CCE less than 100% means that H2 produced on the cathode gets lost to other reactions (e.g., diffusion to an anode compartment, leak from an MEC, or biological oxidation). The H2 yield is the product of CE and CCE and it can be computed by

Ho V₁₆Id = CE < CCE = ^Col!lombs™ x ^H^^s ₍₁₎

Δ e donor coulombs ^_n

where Δ e^" donor is electron donor removed by ARB in a given time (e^~ equiv), coulombs_cum are cumulative coulombs transferred to an anode in a given time (e^~ equiv), and H2,₀bs is H₂ volume measured in a given time (e^~ equiv).

Equation (1) illustrates that the ability to achieve a high H2 yield is improved when CE is increased. A high CE value can be obtained when the e^" equivalents of the donor substrate do not get "lost" before they are transferred to the anode. Possible other e^" sinks that decrease

CE include biomass synthesis, soluble microbial products (SMP), or CH4 gas in anaerobic condition. H2O can also be a significant electron sink if O₂ leaks into the anode compartment.

Lee et al. (2008) showed that biomass (15-26%) and SMP-like organics (11-18%) were the largest non-electricity sinks in an MFC that had no O₂ leakage.

The ability to achieve a high H2 yield is also improved when the CCE is high. Rozendal et al. (2007) reported a CCE close to 100% in a dual-chamber MEC. However, H₂ loss by diffusion into an anode chamber was large in some cases (Rozendal et al., 2008), decreasing CCE down to 6-33%. Loss of H₂ by diffusion can seriously limit MEC applications, since H₂ yield can be small even though the CE is high. Thus, it is desirable to design an MEC to prevent H2 loss, as well as have a high CE, if the H₂ yield is to be maximized.

Two studies tested single-chamber MECs (Call and Logan, 2008; Hu et al., 2008), but they showed unstable CCEs in the range of 28-96% (Call and Logan, 2008). The most likely sink for H₂ in a single-chamber MEC would seem to be hydrogenotrophic methanogens that consume the H₂ produced at the cathode before it can be recovered (Lee et al., 2008, Hu et al., 2008). Up to now, no studies quantified H₂ consumed by the methanogens of electron donor utilized in single MECs, while CEU gas was observed (Call and Logan, 2008; Hu et al., 2008). Another H₂ sink can be oxidation by ARB, if they are able to utilize H₂ as an electron donor (Torres et al., 2007; Bond and Lovley, 2003). H₂ oxidation by ARB, however, might not be a significant H₂ loss in a single-chamber MEC, since current produced by H₂ oxidation produces H₂ gas on the cathode again; H₂ is simply recycled in the MEC.

To ensure a high CCE when hydrogenotrophic methanogens are a risk, one can try to inhibit H₂-utilizing methanogenesis with a specific inhibitory (e.g., BES), intermittent exposure to air, an acidic pH, or a short solids retention time (SRT). Using inhibitors is generally not practical for field applications, due to their expense, toxicity potential, or difficult handling. Exposure to air also is generally not practical because it adds an alternative electron sink that will reduce the CE significantly. Hu et al. (2008) attempted to use an acidic pH for preventing the methanogens 'growth, but it was not effective. In addition, an acidic pH could lower the current, since substrate-utilization rates are inhibited in acidic pH (Torres et al., 2008). Short SRT less than 0.76 d can be efficient for depressing the methanogens' activity, since absolute minimum SRT of the archaea is 0.76 d (Rittmann and McCarty, 2001) in contrast to infinite SRT of ARB biofilm on the anode.

SUMMARY

Certain embodiments comprise a microbial electrolytic cell comprising: a reservoir containing a fluid; an organic donor material contained within the reservoir; an anode submerged in the fluid; anode-respiring bacteria proximal to the anode; and a cathode, wherein the cathode is located above the anode and proximal to an upper level of the fluid contained within the reservoir. In specific embodiments, the anode and the cathode are not separated by a membrane. Certain embodiments may comprise a pump to circulate the fluid within the reservoir. In specific embodiments, the organic donor material may be selected from the group consisting of: glucose, cellulose, acetate, butyrate, lactate, propionate, or valerate. In particular embodiments, the organic donor material is a waste material. In specific embodiments, the waste material may be selected from the group consisting of: sewage, human waste, animal waste, and industrial waste. In certain embodiments, the anode-respiring bacteria transfer electrons extracted from the organic donor material to the anode.

In particular embodiments, the microbial electrolytic cell can be configured so that the electrons extracted from the organic donor material and transferred to the anode will react with H⁺ to produce H₂ gas at the cathode. In certain embodiments, the microbial electrolytic cell can be configured so that the electrons extracted from the organic donor material and transferred to the anode will react with H₂O to produce H₂ gas at the cathode. Specific embodiments may further comprise a pH measurement device configured to measure the pH of the fluid contained within the reservoir. In certain embodiments, the anode comprises graphite rods. In particular embodiments, the anode and the cathode are less than 3.0 cm apart. In certain embodiments, the anode and the cathode can be approximately 2.0 cm apart. Specific embodiments may comprise a gas flow meter configured to measure an amount of H₂ gas produced by the microbial electrolytic cell. Certain embodiments may comprise a potentiostat configured to apply a voltage between 0.8 volts and 1.2 volts between the anode and the cathode. Particular embodiments may comprise a reference electrode electrically coupled to an electrical circuit comprising the cathode and the anode. Certain embodiments may comprise a method comprising: providing the microbial electrolytic cell as previously described; inducing a transfer of electrons from an organic donor material to an anode; and reacting the electrons with H⁺ or H₂O proximal to a cathode to produce hydrogen gas. In specific embodiments, the transfer of electrons from the organic donor material to the anode involves applying a voltage between the anode and the cathode. It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method or system of the invention, and vice versa. Furthermore, systems of the invention can be used to achieve methods of the invention.

The term "conduit" or any variation thereof, when used in the claims and/or specification, includes any structure through which a fluid may be conveyed. Non-limiting examples of conduit include pipes, tubing, channels, or other enclosed structures.

The term "reservoir" or any variation thereof, when used in the claims and/or specification, includes any body structure capable of retaining fluid. Non-limiting examples of reservoirs include ponds, tanks, lakes, tubs, or other similar structures. The term "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%. The terms "inhibiting" or "reducing" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result. The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or."

As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as

"have" and "has"), "including" (and any form of including, such as "includes" and "include"), or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the examples, while indicating specific embodiments of the invention, are given by way of illustration only. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of an exemplary embodiment of microbial electrolytic cell system according to the present disclosure.

FIG. 2(a) is a graph illustrating the applied voltage and volumetric current density versus time for the embodiment of FIG. 1 under certain operating conditions. FIG. 2(b) is a graph illustrating the coulombic efficiency (CE), cathodic conversion efficiency (CCE), and acetate concentration for the embodiment of FIG. 1 under certain operating conditions.

FIG. 3 is a graph illustrating the cathodic conversion efficiency (CCE) for the embodiment of FIG. 1 under varying operating conditions.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now to FIG. 1, a microbial electrolytic cell (MEC) 100 -comprises a reservoir 110, an anode 120, a cathode 130 and a fluid 140 maintained at a fluid level 145. In this specific embodiment, the anode 120 comprises a plurality of graphite rods coated with anode-respiring bacteria (ARB) 125, and cathode 130 comprises a carbon felt. In the embodiment shown, anode 120 and cathode 130 are electrically coupled to form an electrical circuit 129, which also comprises a potentiostat 128 and a reference electrode 127. Electrical circuit 129 may also comprise a graphite rod 131 inserted into cathode 130 and a graphite rod 121 inserted into anode 120.

In the embodiment shown, MEC 100 also comprises a pump 141 configured to recirculate fluid 140 from a lower portion of anode 120 to an upper portion of anode 120. An inlet 142 allows fluid 140 to enter reservoir 110, and an outlet 143 maintains fluid 140 at fluid level 145. A gas headspace 144 is located above fluid level 145 and below the upper portion 111 of reservoir 110. In this embodiment, fluid 140 also comprises an organic donor material (not visible in FIG. 1) such as acetate.

During operation of MEC 100, potentiostat 128 can be used to apply a voltage between anode 120 and cathode 130 and cause a negative potential at cathode 130. ARB 125 will transfer electrons extracted from the organic donor material to anode 120. The electrons transported to anode 120 pass through an electrical circuit 129 and reach cathode 130, where the electrons react with H⁺ ions (or H₂O) to produce H₂. As shown in FIG. 1, cathode 130 is placed proximal to anode 120. Specifically, cathode 130 is placed above anode 120 and below fluid level 145. In exemplary embodiments, cathode 130 is proximal to fluid level 145 so that the majority of H₂ gas produced by MEC 100 enters gas headspace 144 rather than being consumed by methanogens or other potential H₂ sinks present in MEC 100. Since the solubility of the H₂ molecule is extremely low (K_H = 7.65 x 10^"4 mol/L-atm at one atmosphere and a temperature of 3O⁰C) (CRC, 2008), rapid recovery of H₂ gas should be feasible if the configuration of MEC 100 is optimized for this purpose. Reservoir 110 also comprises a gas outlet 112 that allows H₂ gas produced by MEC 100 to flow through a gas flow meter 113.

It is understood that FIG. 1 represents one exemplary embodiment of the present disclosure, and that other embodiments may comprise a different configuration, including for example, such as a cross-flow reactor, a completely stirred reactor, a sequencing batch reactor, and a reactor using a membrane for gas separation.

EXAMPLE

In a specific exemplary embodiment, an MEC comprised a glass cylinder with a diameter of 4.5 cm and height of 21.6 cm. Graphite rods (OD 0.8 cm, McMaster-Carr, USA) were cut into 2~3-cm long pieces and packed in the single cell up to a height of 10.5 cm to form the anode bed. The total volume of the single MEC was 161 mL, the empty-bed working volume was 140 mL, the effective working volume was 122 mL (excluding electrode volume), and gas headspace was 21 mL. The reported volumetric current density or volumetric H₂ production rate is based on the empty-bed working volume of 140 mL. The average specific surface area of the granular anode was 4.15 m /m of the empty-bedworking volume. Carbon felt (#43199, Alfa Aesar, MA, USA) without a chemical catalyst was used as the cathode, and its geometric surface area was 21.8 cm .

To connect the electrodes, graphite rods (OD 0.4 cm and length 5.4 cm) were inserted into the top areas of the granule anode and the cathode felt was penetrated with the rods. The distance between the top layer of the anode bed and the cathode was 2.0 cm. The MEC was mixed by adjusting the circulation flowrates between the bottom and the middle of the MEC. The circulation flow was generated with a peristaltic pump (Masterflex L/S®, Cole-Parmer, USA). The pH values were monitored by placing a pH probe (Cole-Parmer, USA) inside the MEC. An Ag/AgCl reference electrode (BASI Electrochemistry, MF-2052) was placed 1.5 cm over the top of the anode bed, and H₂ gas produced in the MEC was released at the top of the cell and measured using a Milligas counter (Calibrated Instruments, Inc., NY, USA).

In this example, acetate was the electron donor and organic-carbon source to the MEC.

In order to select a good ARB biofilm on the anodes, the MEC was acclimated in the continuous mode with a recirculation rate of 20 mL/min using a peristaltic pump (Masterflex L/S®, Cole-Parmer, USA) and a feed rate at 0.88 mL/min. The empty-bed contact time was

2.3 h during continuous operation. When the CE reached a steady-state value of 64±2%, the continuous cell was shifted into batch mode for accurately quantifying the H₂ yield. The acetate concentration was 10 mM, and the internal recycle rate 7 mL/min or 0 mL/min.

The cell was operated at 30±2°C, and the medium pH was 7.3-7.4. The medium was operated in the MEC by operating the cell in continuous mode for 5 hydraulic retention times and a parameter was varied between each experiment. The anode potential (Eanode) was fixed at -0.2 V (vs.Ag/ AgCl); Eanode was +0.07 V vs. the standard hydrogen electrode (SHE). Power was provided to the MEC using a potentiostat (VMP3, Applied Princeton Research,

Tennessee, USA), which provided the ability to determine what applied voltage corresponded to the maximum current density. The current, E_anOde, cathode potential, and applied voltage were recorded every 120 seconds using EB lab software (Applied Princeton Research,

Tennessee, USA).

The CCE was computed (coulombs to H₂), according to Equation (2), and determined the percentage H₂ loss with 100% - CCE.

Cafliodic conversion efficiency (%) = CCE = ^-^ x 100 (2) coulombs m „m Cumulative H₂ volume was measured during a given reaction- time using Equation (3) and converted the H₂ volume into electron equivalence (or vice versa) by using the ideal gas law, described at Equation (4).

H₂.ob& = Observed V_H2.t⁼ V_m. _t-i - C_H2._t (V_g , - V_g. _t-1) - V_head (Oc.t - Cm, t-i) (3)

_{Λ r} . _τ . ... , , lmmolH_^ 22AmL 303.15JC . ..

Y_H2 t (niL) = ⁽- ^otιιombs_am x x x (4)

^{K } am} le mmol \mmolH₂ 273.15JC

where Vκ2,t = cumulative H₂ gas volume at time t (mL), V_H2, t-i = cumulative H₂ gas volume at time t-1 (mL), V_g>t = cumulative total gas volume at time t (mL), V_{g; t-}i = cumulative total gas volume at time t-1 (mL), Vhead

H₂ percentage of biogas in headspace at time t, C_{H2, t}-i ⁼ H₂ percentage of biogas in headspace at time t-1, and temperature = 30°C (303.15 K).

Figure 2-a shows the applied voltage and volumetric current density with time in the MEC run with a starting acetate concentration of 10 mM and a recirculation rate of 7 mL/min. The average volumetric current density was 51.4±1.6 A/m³ for 6 h to 22 h. The average CCE was 98±2%, the CE was 60±l% (n = 3), and the H₂ yield was 59±2% (Figure 2-b). For this condition, the upflow single-chamber MEC efficiently prevented H₂ loss to methanogenesis, since CH4 peaks (detected only at the end of the test) were too small to be quantified (< 0.5%). To maintain this high CCE is significant, because previous single-chamber MECs had fluctuating CCE in the range of 28-96% (Call and Logan, 2008), and the CCE dropped to 6% in one dual-chamber MEC (Rozendal etal., 2008).

The applied voltage was 1.06±0.08 V for the average volumetric current density (51.4±1.6 AJm²), which equals a volumetric H₂ production rate of 0.57±0.02 m³ H₂/ m³-d of MEC working volume. This finding is significant, since this example demonstrates the ability to double the H₂-producing rate over the rate obtained (0.3 m³ H₂/m³-d) with a dual-chamber MEC using Pt catalyst at the cathode and a similar applied voltage (Rozendal et al., 2007), even though there was no chemical catalyst on the cathode.

Figure 3 shows that the higher recirculation rate of 40 niL/min (from the baseline of 7mL/min) improved the volumetric current density so that its maximum value rose up to 68.1±1.2A/m³ (a 32% increase over the control) at an applied voltage of 1.15±0.03 V. This corresponds to a 0.75-m³ H₂/m³-d production rate. CE was still stable at 60% with the high recirculation rate. The CCE dropped only slightly to 89±9%, with small CH₄ peak (< 0.5%) observed at the end of the test. The CE and CCE together gave a H₂ yield of 53.4%. These results show that improved mass-transport could increase acetate utilization rate and current generation with minimal negative impact on the CCE and H₂ yield.

REFERENCES

The following references are herein incorporated by reference in their entirety.

Bond, D. R.; Lovley, D. R. Electricity Production by Geobacter sulfurreducens Attached to Electrodes. Applied and Environmental Microbiology, 2003, 69 (3), 1548-1555.

Call, D.; Logan, B.E. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environmental Science & Technology 2008, 42 (9), 3401-3406.

Cheng, S.; Logan, B. E. Sustainable and efficient biohydrogen production via electrohydrogenesis. Proceedings of the National Academy of Sciences of the United States of America 2007, 104 (47), 18871-18873. CRC, Handbook of Chemistry and Physics; 88th eds., 2008. http://www.hbcpnetbase.com/.

Hu, H.; Fan, Y.; Liu, H. Hydrogen production using single-chamber membrane- free microbial electrolysis cells. Water Research 2008 DOI 10.1016/j.watres.2008.06.015. Lee, H. S.; Parameswaran, P.; Kato-Marcus, A.; Torres, C. L; Rittmann, B. E., Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs) utilizing fermentable and non- fermentable substrates. Water Research 2008, 42 (6-7), 1501-1510.

Ren, Z. Y.; Ward, T. E.; Regan, J. M., Electricity production from cellulose in a microbial fuel cell using a defined binary culture. Environmental Science & Technology 2007, 41 (13), 4781-4786.

Rittmann, B. E.; McCarty, P. L. Environmental Biotechnology: Fundamentals and Applications. McGraw-Hill: New York, 2001, Chapter 13.

Rozendal, R. A.; Hamelers, H. V. M.; Molenkmp, R. J.; Buisman, J. N., Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes. Water Research 2007, 41 (9), 1984-1994. Rozendal, R. A.; Jeremiasse, A. W.; Hamelers, H. V. M.; Buisman, C. J. N., Hydrogen production with a microbial biocathode. Environmental Science & Technology 2008, 42 (2), 629-634.

Torres, C. L; Kato Marcus, A.; Rittmann, B. E., Kinetics of consumption of fermentation products by anode-respiring bacteria. Applied Microbiology and Biotechnology 2007, 77 (3), 689-697.

Torres, C. I.; Marcus, A.K.; Rittmann, B.E. Proton transport inside the biofilm limits electrical current generation by anode-respiring bacteria. Biotechnology and Bioengineering 2008, 100 (5), 872-881.

Claims

1. A microbial electrolytic cell comprising: a reservoir containing a fluid; an organic donor material contained within the reservoir; an anode submerged in the fluid; anode-respiring bacteria proximal to the anode; and a cathode, wherein the cathode is located above the anode and proximal to an upper level of the fluid contained within the reservoir.

2. The microbial electrolytic cell of claim 1, wherein the anode and the cathode are not separated by a membrane.

3. The microbial electrolytic cell of any of claims 1-2, further comprising a pump to circulate the fluid within the reservoir.

4. The microbial electrolytic cell of any of claims 1-3, wherein the organic donor material is selected from the group consisting of: glucose, cellulose, acetate, butyrate, lactate, propionate, or valerate.

5. The microbial electrolytic cell of any of claims 1-3, wherein the organic donor material is a waste material.

6. The microbial electrolytic cell of claim 5, wherein the waste material is selected from the group consisting of: sewage, human waste, animal waste, and industrial waste.

7. The microbial electrolytic cell of any of claims 1-6, wherein the anode-respiring bacteria transfer electrons extracted from the organic donor material to the anode.

8. The microbial electrolytic cell of any of claims 1-7 wherein the microbial electrolytic cell is configured so that the electrons extracted from the organic donor material and transferred to the anode will react with H⁺ to produce H₂ gas at the cathode.

9. The microbial electrolytic cell of any of claims 1-7 wherein the microbial electrolytic cell is configured so that the electrons extracted from the organic donor material and transferred to the anode will react with H₂O to produce H₂ gas at the cathode.

10. The microbial electrolytic cell of any of claims 1-9, further comprising a pH measurement device configured to measure the pH of the fluid contained within the reservoir.

11. The microbial electrolytic cell of any of claims 1-10, wherein the anode comprises graphite rods.

12. The microbial electrolytic cell of any of claims 1-11, wherein the anode and the cathode are less than 3.0 cm apart.

13. The microbial electrolytic cell of any of claims 1-11, wherein the anode and the cathode are approximately 2.0 cm apart.

14. The microbial electrolytic cell of any of claims 1-13, further comprising a gas flow meter configured to measure an amount of H₂ gas produced by the microbial electrolytic cell.

15. The microbial electrolytic cell of any of claims 1-14, further comprising a potentiostat configured to apply a voltage between 0.8 volts and 1.2 volts between the anode and the cathode.

16. The microbial electrolytic cell of any of claims 1-15, further comprising a reference electrode electrically coupled to an electrical circuit comprising the cathode and the anode.

17. A method of producing hydrogen gas, the method comprising: providing the microbial electrolytic cell of any of claims 1-16; inducing a transfer of electrons from the organic donor material to the anode; and reacting the electrons with H⁺ or H₂O proximal to the cathode to produce hydrogen gas.

18. The method of claim 17 wherein inducing the transfer of electrons from the organic donor material to the anode involves applying a voltage between the anode and the cathode.