Electricity generation by biocathode coupled photoelectrochemical cells

Abstract

Biocathode coupled photoelectrochemical cells (Bio-PEC) have the potential for electricity generation and pollutant removal, with the simultaneous utilization of both solar energy and bioenergy. However, their performance is influenced by many factors. In this work, crucial parameters, including the pollutant type, electrolyte concentration and gas atmosphere of the photoanode, were investigated to optimize the operation of the Bio-PEC in terms of electricity generation.

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The energy crisis and environmental pollution are two hurdles that must be overcome to achieve the sustainable development of human society in the 21st century. Solar energy and bioenergy are environmentally friendly, renewable and inexhaustible; the application of solar energy and bioenergy for energy conservation and pollutant remediation has attracted much attention recently. The photoelectrochemical cell (PEC) is attractive due to its twofold environmental benefits, namely, electricity generation and the photocatalytic degradation of organic wastes with the utilization of solar energy.^1–3 The main components of a PEC consist of a photoanode, where electrons in the conduction band and holes in the valence band are produced with excitation by photons, and a cathode, where a reductive reaction, usually the oxygen reduction reaction (ORR), takes place with the assistance of catalysts. The common cathode catalysts, such as noble or transition metal nanoparticles, had the disadvantages of a high cost and short service life due to loss, agglomeration and poisoning during long-term operation.⁴

Biological cathodes (biocathodes) are appealing alternatives to metal catalysts and intensively studied in the field of microbial fuel cells (MFCs).¹⁷ An aerobic biocathode is an electrochemically active biofilm that accumulates at the cathode surface and facilitates the 4-electron ORR.^5,6 Bergel et al. tested a sea-water fuel cell with a stainless steel biocathode, and observed that the maximum power density decreased from 270 to 2.8 mW m⁻² after the biofilm was removed from the cathode, indicating the effective catalysis of the ORR by the marine biofilms.¹⁸ An oxygen-reducing biocathode MFC operated with passive air transfer was reported to yield a maximum power density of 554 ± 0 mW m⁻², which was comparable to that obtained with a platinum cathode (576 ± 16 mW m⁻²).¹⁹ Besides, the strains of biocathode can be selected from the natural environment; the rapid growth and multiplication of the biofilm endows the biocathode with a high efficiency and reproducibility, and the pollutants in the wastewater can be converted or completely eliminated through microbial metabolism.^7–9

The biocathode coupled PEC (Bio-PEC) was originally explored to integrate the advantages of the fast and unspecific photodegradation of organic pollutants and efficient, reproducible, and cost-efficient oxygen reduction of the biocathode.¹⁰ The electricity generation and pollutant degradation by Bio-PECs is influenced by many factors. The optimization of operational parameters is crucial for the application of Bio-PECs. In this work, the effects of influencing factors, such as the substrates, electrolyte concentrations, and air and nitrogen sparging of the anode solution, on electricity generation have been thoroughly investigated. The information may provide valuable insights for the optimization of Bio-PECs.

TiO₂ nanotube arrays fabricated on a titanium sheet using the electrochemical anodization method were used as the photoanode. The biocathode used in this work was firstly enriched in a MFC reactor, as described in detail in the ESI (Fig. S1†). The biocathode was then taken out of the MFC and installed into the PEC reactor to construct the Bio-PEC (Fig. S2†).

The dual chambered Bio-PEC was made of plexiglass, with a quartz window to allow the transmission of UV light. The photoanode chamber and cathode chamber were separated by a cation exchange membrane (Qianqiu Co. Ltd., China). A 150 W xenon lamp (GY-10A, Tuopu Co. Ltd., China) was used as the light source. The cathode solution was aerated with a pump in all of the experiments. The cathode part of the PEC was wrapped in aluminium foil to shield it from UV-visible light. The external resistance was kept at 1000 Ω for all of the experiments, except for the power density and polarization measurements. The anolyte was stirred using a magnetic bar in the dark for 20 min, to ensure an adsorption–desorption equilibrium was reached prior to irradiation, and continuously stirred during the experiment.

The output voltages (V) of the Bio-PECs were recorded with a data acquisition board at one-minute intervals. The polarization curves were measured by recording the current response to a linear potential decrease imposed upon the PECs at a scanning rate of 10 mV s⁻¹ using an electrochemical workstation (Metrohm Autolab 85061). The power density was calculated by normalizing the power (P = IV) by the surface area of the anode. Electrons harvested through the external circuit (Q) were determined by integrating the current over time (Q = ∫Idt).

The working principle of Bio-PECs involves the production of electrons through the photocatalytic process in the photoanode and electron consumption facilitated by exoelectrogens at the biocathode. With xenon illumination, the electrons on the surface of the TiO₂ electrodes are excited from the valence band to the conduction band, yielding positive holes in the valence band and negative electrons in the conduction band (eqn (1)).¹² The electrons travel to the cathode through an external circuit and are harvested as energy; while the holes oxidize the organic substrates, either directly or via ˙OH radicals formed by a reaction with OH⁻ and/or H₂O in the solution (eqn (2)). In the solution, the cation, such as Na⁺, H⁺ or K⁺, will diffuse from the anode chamber to the cathode chamber through the cation exchange membrane to complete the production of electricity. The ability of the photoanode to generate electrons and oxidize the organic substrates is closely related to the separation efficiency of hole/electron pairs, which is mainly determined by the photocatalyst. Other operation parameters, such as the illumination intensity, organic substrates, electrolyte concentration, and gas atmosphere, also influence the performance of the photoanode, which will be discussed in the following sections.

TiO₂ + hv → h_vb⁺ + e⁻ (1)

h_vb⁺ + H₂O → ˙OH + H⁺ or h_vb⁺ + OH⁻ → ˙OH (2)

4H⁺ + O₂ + 4e⁻ → 2H₂O (3)

e⁻ + O₂ → O₂⁻ or 2e⁻ + O₂ + 2H⁺ → H₂O₂ (4)

The biocathode, usually called an aerobic biocathode or nitrifying biocathode from the functionality point of view, was dominated by ammonia oxidizing bacteria and nitrite oxidizing bacteria.¹¹ The electrochemically active biofilm catalyzed the oxygen reduction reaction in the cathode (eqn (3)) and meanwhile converted the ammonia in the catholyte to nitrate through microbial metabolism. The catalytic activity of the oxygen reduction reaction is closely related to the microbial activity: with suitable culture conditions, such as sufficient nutrients and oxygen, the metabolism and multiplication of bacteria is faster, which results in a higher catalytic efficiency.

Various kinds of artificial wastewater were used as anode substrates for electricity generation (Table 1). The open-circuit voltage (V_oc) and short-circuit current density (I_sc) varied when different organic compounds were used. As the cathode potentials in the open-circuit condition (P_cathode) varied over a small range, from 299 to 308 mV, the difference in the voltage and power output was mainly determined by the anode performance. The Bio-PEC produced the highest V_oc (730 mV), I_sc (1.87 A m⁻²) and maximum power density (Power_max) (473 mW m⁻²) with artificial acetate wastewater as the substrate. The substrates in the anode solution functioned not only as the pollutants to be removed by the photocatalytic process, but also as sacrificial agents for holes that improved the separation of hole/electron pairs. Their role is similar to that of methanol that is used as a sacrificial agent for holes during hydrogen production through photocatalytic water splitting.¹³ Acetate has a simple molecular structure, and can be easily destroyed;¹⁴ moreover, acetate radicals formed by the hydrolyzation of acetate are negatively charged, and can easily reach the positively charged holes. These caused the acetate to yield a higher performance in the Bio-PEC.

Table 1 Power generation of the Bio-PEC using various kinds of artificial wastewater as substrates

	Voc (mV)	Isc (A m^−2)	Pcathode (mV)	Panode (mV)	Powermax (mW m^−2)	pH
Methyl orange	690	1.87	302	−388	218	6.9
Acetate	730	1.51	299	−428	473	7.3
Glucose	715	1.74	305	−408	203	6.8
Glutamic acid	642	1.07	301	−352	96	3.5
Urea	668	1.40	308	−359	222	6.2

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The Bio-PEC produced its lowest performance, with a V_oc of 642 mV, I_sc of 1.07 A m⁻² and Power_max of 96 mW m⁻², when glutamic acid was used as the substrate in the artificial wastewater, which was probably due to the acidity of the glutamic acid solution (pH = 3.48). The protons may react with photogenerated electrons and cause a lower current density to be produced.¹⁵

In the typical process in fuel cells, electrons transfer from the anode to the cathode through an external circuit, while the cation moves from the anode to the cathode through the solution. A low electrolyte concentration results in a large cation transition resistance, low power output, and low pollutant degradation; thus, certain actual wastewater with low electrical conductivity is not suitable for treatment by the Bio-PEC.

To study the minimum conductivity requirement of wastewater, glucose (5 g L⁻¹) with different Na₂SO₄ concentrations (0.005, 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5 mol L⁻¹) was used as the substrate of the photoanode (Fig. 1). The correlated electrical conductivities for each anode solution were 1.11, 2.10, 3.94, 8.80, 16.9, 29.8, and 57.8 mS cm⁻¹, respectively. The maximum power density and current density both improved with the increase of the Na₂SO₄ concentration in the anode electrolyte. The maximum power density increased by 2.5 times as the anolyte conductivity increased from 1.11 mS cm⁻¹ (65 mW m⁻²) to 8.8 mS cm⁻¹ (225 mW m⁻²). The increase in the maximum power densities from 270 to 293 mW m⁻² was negligible with the anolyte conductivities between 16.9 and 57.8 mS cm⁻¹. Although wastewater with a higher conductivity is more beneficial for power production, the conductivity is relatively low for most actual wastewater. For example, the electrical conductivities of pharmaceutical wastewater, brewery wastewater, and chicken osteoprotegerin wastewater studied in our lab were 7.78, 3.28, and 44.50 mS cm⁻¹, respectively. Considering the electricity production and practical situation in actual wastewater, the minimum required solution conductivity should be around 10–15 mS cm⁻¹.

Fig. 1 Power generation and polarization curves of Bio-PECs with Na₂SO₄ concentrations in the anode solution of 0.5, 0.2, 0.1, 0.05, 0.02, 0.01 and 0.005 mol L⁻¹.

To evaluate the influence of the gas atmosphere on the electricity generation characteristics, the anode solution (glucose 5 g L⁻¹ and Na₂SO₄ 0.05 mol L⁻¹) was firstly sparged with nitrogen or air for 30 min, before poured into the reactor for measurement. When the anode solution was aerated with air, the anode potential in the open-circuit condition was ca. −408 mV, the maximum current density was 1.76 A m⁻² and the maximum power density was 203 mW m⁻² (Fig. 2). The nitrogen sparging obviously enhanced the performance of the Bio-PEC, with the anode potential under open-circuit conditions decreasing to −457 mV, the maximum current density increasing to 2.22 A m⁻² and the maximum power density increasing to 239 mW m⁻². In the air-sparged solution, the oxygen could react with photogenerated electrons to form O₂⁻ or H₂O₂ (eqn (4)), which is also a strong oxidant. The presence of oxygen in the anode solution may contribute to pollutant degradation. However, it also results in the consumption of electrons and a lower power production. This is in good agreement with a previous study that investigated the effect of dissolved oxygen on the photocatalytic reaction rate.¹⁶

Fig. 2 Power generation and polarization curves of Bio-PEC with an anode solution aerated with nitrogen or air for 30 min.

Conclusions

In this work, various factors, such as the substrates, electrolyte concentration and gas atmosphere, influencing a photoanode were investigated in terms of the electricity generation of a Bio-PEC. As a model compound in artificial wastewater, acetate produces the highest cell performance, with a short circuit current density of 1.87 A m⁻², open-circuit voltage of 730 mV, and maximum power density of 473 mW m⁻². The minimum electrical conductivity of wastewater required for the Bio-PEC system was 10–15 mS cm⁻¹, which would avoid a large ion transfer resistance in the solution. Nitrogen sparging of the anode solution facilitated electricity generation more than air sparging, due to electron capture by oxygen.

Acknowledgements

This work was supported by the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Grant no. 2013DX08) and by the National Natural Science Fund for Distinguished Young Scholars (Grant no. 51125033) and National Natural Science Fund of China (Grant no. 51209061). The authors also acknowledge the support from the Creative Research Groups of China (Grant no. 51121062) and the International Cooperating Project between China and Canada (Grant no. S2012GR01820).

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Footnote

† Electronic supplementary information (ESI) available: The process for the enrichment of the biocathode; (Fig. S1) digital photograph of the biocathode microbial fuel cell (MFC); and (Fig. S2) digital photograph of the biocathode coupled photoelectrochemical cell (Bio-PEC). See DOI: 10.1039/c4ra15965a

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