Lingling
Zhang
,
Isabel
Álvarez-Martos
,
Alexander
Vakurov
and
Elena E.
Ferapontova
*
Interdisciplinary Nanoscience Center (iNANO), Science and Technology, Aarhus University, Gustav Wieds Vej 1590-14, DK-8000 Aarhus C, Denmark. E-mail: elena.ferapontova@inano.au.dk; Tel: +45-87156703
First published on 9th March 2017
Access to fresh water and energy is ranked as one of the most severe challenges to humankind. The restricted availability of fossil fuels and clean water does not match the increasing energy demands and growing population needs, which, desirably, should be satisfied in the most sustainable, clean and inexpensive way. Here, we report clean and sustainable conversion of solar energy into electricity by photo- and bio-electrocatalytic recycling of the H2O/O2 redox couple in a hybrid bio-photovoltaic (BPV) membraneless cell comprising a sunlight-illuminated water-oxidizing semiconductor anode (either Zn-doped hematite or TiO2) and an oxygen-reducing enzymatic biocathode, in such environmental media as seawater. Upon simulated solar light illumination (AM 1.5G, 100 mW cm−2), the maximum power density (Pmax) generated by the cell was 236 and 21.4 μW cm−2 in 1 M Tris–HCl and seawater, both at pH 8, respectively. In seawater its ionic content inhibited mostly the activity of the photoanode, but not that of the biocathode. The obtained Pmax values were orders of magnitude higher than those of a photo-electrochemical cell with a Pt mesh cathode (0.32 μW cm−2 in seawater). The demonstrated thermodynamically feasible coupling of cost-effective photoactive materials such as TiO2 or hematite semiconductors and enzymatic counterparts in seawater media opens a prospective clean and sustainable way of transformation of the most abundant, clean and renewable source of energy – solar light – and the Earth's most massive water resource – seawater – into electricity, which can also be used for fresh water production.
Dye-sensitized solar cells,15 polymer solar cells16,17 and perovskite solar cells18,19 that rely on more abundant semiconductor nanomaterials seem to provide some cost-effective solutions for utilization of solar energy. For example, in dye-sensitized solar cells, inexpensive titania nanoparticles offer certain advantages over more expensive silicon. Along with that, the incident light conversion efficiency restricted by 12.3–14.1%, discussed stability issues of dye components,20 and relatively high market prices21 interfere with their wider applications.
Among sustainable solutions, biological photosystems wired to electrodes for bioelectrocatalytic water oxidation have been explored.22–25 Such systems are of huge fundamental interest, but practically they are quite expensive, complex in preparation, often insufficiently stable and of low-efficiency, and thus have fewer prospects for commercialization.
In this context, one of the attractive solar energy transformation solutions is photoelectrochemical (PEC) cells that harvest solar energy for electrochemical splitting of water into molecular oxygen and hydrogen that can be further used as a fuel; with that, solar energy is transformed and stored in the form of chemical bonds.26 Such artificial photosynthesis devices pioneered in the early 70s (ref. 27) can be routinely used for the production of H2 fuel (e.g. at a Pt cathode) by oxidation of water to O2 and H+ at a sunlight-illuminated semiconductor anode at potentials far less positive than the standard potential of electrochemical water decomposition (1.23 V) or H2 evolution.
Photo-driven electrooxidation of water can be catalysed by a variety of n-type metal oxide semiconductor materials,28–32 and TiO2, Fe2O3 nanomaterials and/or their nanocomposites may be considered as cost-effective alternatives to biological photocatalysts.28–31 Due to it having one of the most negative potentials for water splitting and low cost, TiO2 represents one of the most promising photoanode materials for PEC applications,8,33 while hematite (α-Fe2O3), not so competitive in the sense of water oxidation potentials, has its own advantages, such as a favourable light absorption ability with a band gap of 1.9–2.2 eV, excellent mechanical stability and chemical inertness in neutral and alkaline environments.34,35 A significant improvement of the photo-electrocatalytic activity of hematite photoanodes has been achieved during the last few years, e.g. hematite's inherent high electron–hole recombination rate had been overcome by material nanostructuring, doping and surface passivation.36–41
Hitherto, artificial photosynthesis devices have been considered mostly for the production of H2 fuel.7,8 Here, we explore an alternative, low-cost and sustainable way for photoelectrochemical transformation of solar energy into electricity (that can also be used for fresh water production) in the Earth's most available electrolyte, seawater. For this, we coupled photoelectrocatalytic oxidation of H2O to O2 at a semiconductor anode42 and O2 reduction to water at a biocathode comprising an O2 reducing enzyme immobilized on an inexpensive carbon-cloth cathode43 (Fig. 1). Potentials for photoelectrocatalytic oxidation of water at both titania electrodes33 and Zn-doped hematite38 are thermodynamically compatible with the (bio)electrocatalytic reduction of O2 to water by Pt and a number of multi-copper enzymes, such as bilirubin oxidase (BOD), that can be directly wired to electrodes and are currently widely used for construction of enzymatic biofuel cells, as a green, cost-effective and sustainable alternative to precious metal catalysts.23,44–48
The performance of such a solar energy transforming device in such renewable media as seawater is a challenging task, and if found feasible, opens new energetic and clean water production prospects. Due to the high salt content, seawater represents a natural electrolyte solution for an electrochemical cell, and H2O/O2 PEC cells operating in seawater will be free from any extra fuels and electrolyte, by directly converting solar energy into electrical power within the H2O–O2–H2O redox cycle. Along with that, large concentrations of chloride and some other anions present in seawater and its basic pH may have detrimental effects on both the photoanode and the biocathode performance, to the extent that redox cycling would be thermodynamically impossible. Hitherto, most of the photoelectrocatalytic studies of semiconductor photoanodes have been performed in a strong (1 M) alkaline medium,39,42,49 and most of the O2-biocathode studies have been performed in more acidic solutions,23,44,45,50,51 with just a few examples of low-potential bacterial enzymes, operating directly in seawater.52
Here, we demonstrate that a hybrid bio-photovoltaic (BPV) cell is capable of recycling the H2O/O2 couple in environmental condition-mimicking basic media and directly in seawater and produces electrical energy under sunlight illumination indeed (Fig. 1). To the best of our knowledge, that is the first example of a hybrid semiconductor-enzymatic BPV cell operating in seawater, verifying the thermodynamic and kinetic feasibility of extracting energy from those environmental resources. It provides a sustainable and cost-effective solution to mitigating the energy and water crisis in future, with such BPV cells applied for direct fresh water production at the biocathode.
As can be seen in Fig. 2, in 1 M NaOH Zn-doped hematite shows the highest photoelectrocatalytic currents and the lowest onset potential for water oxidation, consistent with the previous reports.38,53 In 1 M Tris–HCl, pH 8, Zn-doped hematite still exhibits a sufficiently high photoelectrocatalytic activity and low potential for water oxidation, with a photocurrent density of 0.44 mA cm−2 at 0.56 V (vs. Ag/AgCl, which is equivalent to 1.23 V vs. the RHE) (Fig. 2A). In the seawater medium, the photoelectrooxidation currents decrease 64%, to 0.16 mA cm−2, and the catalysis onset shifts from −100 mV (in Tris–HCl) to 70 mV (in seawater, pH 8), suggesting that photoelectrocatalysis in seawater is significantly inhibited by seawater components.
Such complex media as seawater may be approximated by the following ion content: [Cl−]: 559.40 mM, [Na+]: 480.57 mM, [K+]: 10.46 mM, [Mg+]: 54.14 mM, [Ca2+]: 10.53 mM, [SO42−]: 28.93 mM, [HCO3−]: 2.11 mM, [B(OH)3]: 0.43 mM, and [PO43−]: 3.2 μM.21,54 Based on this composition, we evaluated the effect of Na+, Cl−, CO32−/HCO3−, B4O72−, and HPO42−/H2PO4− on the photo-electrocatalytic activity of Zn-doped hematite (Fig. 2A). No distinct changes in the current or onset potential have been observed in buffer solutions with a high NaCl content. However, the nature of the anion of the buffer solution, such as carbonate, borate and phosphate anions, dramatically affected the photoelectrocatalytic performance of the photoanode (Fig. 2A, inset).
The most pronounced (though reversible) depression of photoelectrocatalysis, reducing the photo-oxidation current densities to 0.05 mA cm−2 at 0.56 V, was observed in the phosphate buffer solutions, which very likely is connected with the formation of insoluble Zn phosphate deposits55 on the surface of Zn-doped hematite, thus eliminating Zn from the reaction zone and fouling the photoanode surface. A strong inhibition of photoelectrocatalysis by phosphate anions excludes Zn-hematite-based BPV cell operation in phosphate-rich media, but not in seawater, where the phosphate content is quite low (3.2 μM). The inhibition of photoelectrocatalysis in carbonate solutions (2.11 mM in seawater) was quite similar to that observed in seawater, and for borate, which actual concentration in seawater is in a sub-mM range, it approached that characteristic of phosphate (Fig. 2A, inset). In agreement with previous reports,56 the Cl− anion itself did not contribute too much to the inhibition (see data for Tris–HCl), and thus most of the inhibition of photoelectrocatalysis in seawater (compared to Tris–HCl of the same pH) may be associated with its borate and carbonate contents.
Photoelectrocatalytic oxidation of seawater was also studied at TiO2 electrodes routinely used for photo-electrocatalytic water splitting.57 In contrast to hematite, at the TiO2 photoanodes photoelectrocatalysis of water oxidation started at much lower potentials, ca. −1.0 V, and the highest efficiency of bioelectrocatalyses was observed in Tris–HCl buffer solutions, with limiting currents approaching 0.37 mA cm−2 (Fig. 2B). These currents are comparable with those at Zn-doped hematite observed at 0.6 V. Along with that, the onset of photoelectrocatalysis and the limiting current plateau occurred, as earlier mentioned, at much more negative potentials. The photoelectrocatalytic activity of the TiO2 electrodes in seawater (and alkaline media) drops down to a larger extent than that of Zn-doped hematite, and the photocurrent at 0.56 V decreases by 75% (and 88%, respectively). Thus, despite the less favorable potential for photoelectrocatalytic water oxidation, Zn-doped hematite residual activity and stability in environmental media are higher, and its performance in such an environment as seawater is somehow superior to TiO2.
Water oxidation on the TiO2 surface58 involves mechanistic routes different from those at hematite,42,59 including formation of several intermediates whose redox transformation is strongly pH dependent, and, in particular, in basic solutions (pH 13) the formed surface Ti–O− species can be quickly oxidized by photogenerated holes, thus slowing down the water oxidation reaction.60 The involvement of O2 and such products of its reduction (by the photoinduced e− in the conductance band) as superoxide anion radical O2˙−, and further routes of its transformation to H2O2, H2O and O2, cannot be excluded; all these steps depend both on pH61 and solution composition62 and thus can contribute to the energy losses in such complex matrices as seawater. Therewith, the Cl− anion itself does not inhibit photoelectrocatalysis (Fig. 2B, data for 1 M Tris–HCl) and was even discussed to enhance the photoelectrocatalytic efficiency.63
To design a simple, cost-effective, and still efficient BOD biocathode for O2 reduction, we have immobilized and cross-linked BOD on the surface of an activated graphitized carbon-cloth (GCC) electrode.69 GCC is an inexpensive and widely industrially used micro-fibrous textile material (Fig. 3A) with outstanding flexibility and mechanical strength, which holds great promise as a high surface area, micro-structured substrate for enzyme immobilization and bioelectrocatalysis. However, GCC is inherently hydrophobic and to make it appropriate for enzyme immobilization it was activated (hydrophilized) by oxidation in concentrated H2SO4, to generate more surface oxide functionalities.70
Cross-linked on the GCC surface, BOD directly, with no mediators, bioelectrocatalytically reduced oxygen in air-saturated 1 M Tris–HCl, pH 8, staring from 0.5 V (Fig. 3B), with O2 diffusion-limited current densities approaching 0.4 mA cm−2. In seawater, the onset potential for bioelectrocatalysis slightly shifted to less positive potentials (0.49 V), and the efficiency of the reduction process decreased to 79% of the original bioelectrocatalytic activity in 1 M Tris–HCl, with current densities reaching now 0.30 mA cm−2. Therewith, the Cl−-tolerance of BOD was much higher than that of fungal laccases, dropping down 30% in 0.15 M NaCl-containing pH 5 solutions69 or 75% in seawater shown for low potential bacterial laccases such as SLAC.52
More basic pH significantly affected the electrocatalysis of the 4e−/2H+-coupled reduction of O2 to H2O, mostly in its onset, shifting it to less positive values, though the efficiency of electrocatalysis in terms of current densities remained almost the same (0.15 mA cm−2, Fig. 4B). However, in seawater a further almost 0.4 V shift of the half-wave potential to less positive values occurred, very likely connected with the electrode fouling. Such performance apparently limits applications of Pt-based electrocatalysts in this medium.
The BPV cell comprising the Zn-doped hematite photoanode and the BOD/GCC biocathode gave the open-circuit voltage (Voc) of 0.66 V and 0.64 V in 1 M Tris–HCl and seawater, respectively (Fig. 5, Table 1). The maximum power density (Pmax) produced by the cell in seawater was 4.2 ± 0.3 μW cm−2, representing ca. 25% of the Pmax in Tris–HCl (18.6 ± 2.1 μW cm−2) (Fig. 5A). These results are consistent with the data in Fig. 2A, demonstrating the essential drop of the photoanode activity in seawater (NB: but not that of the biocathode, Fig. 3B), whose performance becomes the limiting factor in the BPV operation. Even so, the power extracted from the Zn-doped hematite BPV cell is more than an order of magnitude higher than that generated by the photovoltaic cell comprising the Zn-doped hematite photoanode and Pt mesh cathode (0.32 ± 0.01 μW cm−2), in which the performance of the Pt cathode is greatly inhibited by the electrode fouling in seawater (Fig. 4B and 6). It also essentially exceeds 0.87 μW cm−2 (8.7 mW m−2) reported for a low temperature polymer electrolyte fuel cell driven by the water/proton concentration gradient between two electrodes electrochemically recycling the H2O/O2 redox couple at 70 °C and pH 11.73
Design of the BPV cell | 1 M Tris–HCl (pH 8.0) | Seawater (pH 8.0) | ||||
---|---|---|---|---|---|---|
Photoanode-cathode type | V oc (V) | P max (μW cm−2) | FF | V oc (V) | P max (μW cm−2) | FF |
a Without the sunlight illumination (dark cell conditions) the Pmax did not exceed 44 nW cm−2 for Zn-hematite systems (ESI, Fig. S2–S5); TiO2 systems did not properly work as a galvanic element in the dark. | ||||||
Zn-Doped hematite – BOD/GCC | 0.66 ± 0.01 | 18.6 ± 2.1 (at 0.20 V) | 0.16 | 0.64 ± 0.01 | 4.2 ± 0.3 (at 0.16 V) | 0.10 |
TiO2 – BOD/GCC | 1.47 ± 0.02 | 236 ± 38 (at 1.06 V) | 0.57 | 1.01 ± 0.01 | 21.4 ± 4.1 (at 0.50 V) | 0.28 |
Zn-Doped hematite – Pt mesh | 0.39 ± 0.01 | 1.56 ± 0.11 (at 0.19 V) | 0.12 | 0.48 ± 0.02 | 0.32 ± 0.01 (at 0.15 V) | 0.13 |
As may be expected, the BPV cell comprising the TiO2 photoanode and the BOD/GCC biocathode showed the Voc and the Pmax of 1.47 V and 236 μW ± 38 cm−2 in 1 M Tris–HCl (Fig. 5B) that actually approach the best results shown for direct ET-based enzymatic biofuel cells operating under the conditions of O2-limited mass-transfer reactions43,74,75 characteristic of environmental media. The cell fill factor FF of 0.57 was also quite high and approached that of dye-sensitized solar cells,76 while for the hematite cell it was only 0.16 (Table 1) consistent with a poorer performance of hematite, for which FF typically did not exceed 0.3.77 In seawater (Fig. 2B) the essential inhibition of photoelectrocatalytic currents of water oxidation at the TiO2 photoanodes resulted in a Voc decrease to 1.01 V, and the Pmax dropped sharply to less than 10% of its value in 1 M Tris–HCl (to 21.4 ± 4.1 μW cm−2) at 0.5 V. As in the case of the Zn-doped hematite BPV cell, such TiO2 cell characteristics are consistent with the limiting performance of the photoanode, exhibiting the photoelectrocatalytic activity in seawater lower than the bioelectrocatalytic activity of the biocathode. The higher Pmax generated from the TiO2 BPV cell (Table 1) is mainly due to the higher cell voltage provided by the galvanic element formed by (H2O)TiO2/(O2)BOD rather than from the higher photocurrent values. Along with that, the higher tolerance of the Zn-doped hematite electrode towards operation in seawater implies the superior adaptability of Zn-doped hematite for operation in this environmental medium.
Finally, the operational stability of the BPV cells in basic media was evaluated by the on–off cyclic illumination of the photoanodes, and the variation of the Pmax with time, in response to the repeated light stimulation, was recorded (Fig. 5C and D). Both Zn-doped hematite and TiO2 BPV cells exhibited fast photo-responses to constantly switching dark and light operations. With time, though, the power generated by both BPV cells slightly decreased, in seawater quite quickly, in 40 min for Zn-doped hematite and in 60 min for TiO2 reaching a steady-state level, suggesting that it can be continuously used as a sustainable electrical energy supply. Among the two studied systems, the Zn-doped hematite BPV cell was found to be most stable, with a minimal activity loss upon repeated use, currently demonstrating preferential stability features important for continuous energy transformations in the sea medium.
Cell design | Operational conditions | V oc, V | J sc, μA cm−2 | P, μW cm−2 | FF | Ref. |
---|---|---|---|---|---|---|
TiO2 NPs/FTO vs. BOD/carbon cloth cathode | Photoelectrocatalytic oxidation of H2O and bioelectrocatalytic reduction of O2, 1 M Tris, pH 8, 100 mW cm−2 illumination | 1.47 V | 280 μA cm−2 | 236 μW cm−2 at 1.06 V | 0.57 | This work |
Chlorine-e6/TiO2vs. bilirubin oxidase/ABTS cathode | Photooxidation of NADH for enzymatic oxidation of glucose, 10 mM Tris, pH 7, 100 mW cm−2 | 0.53 V | 9 μA cm−2 | 1.7 μW cm−2 at 0.4 V | 0.36 | 79 |
Cyanobacteria/carbon nanotubes (CNTs) vs. laccase/carbon paper cathode | Bioelectrocatalytic oxidation of H2O and reduction of O2, 0.1 M phosphate, pH 5.8, 76 mW cm−2 illumination | 0.57 | 24 μA cm−2 | 3.5 μW cm−2 at 0.33 V | 0.26 | 80 |
Thylacoid membranes/multiwall carbon nanotubes vs. laccase/MWCNT cathode | Bioelectrocatalytic oxidation of water and reduction of O2, 0.1 M phosphate, pH 6.8, 80 mW cm−2 illumination | 0.35 V | 68 μA cm−2 | 5.3 μW cm−2 at 0.2 V | 0.22 | 25 |
Thylacoid membranes/Toray paper vs. Nafion/laccase/anthracene-modified MWCNTs | Bioelectrocatalytic oxidation of H2O and reduction of O2, citrate buffer pH 5.5, light: 250 W halogen lamp at 5200 lumens | 0.72 V | 14 μA cm−2 | ∼1.8 μW cm−2 at 0.3 V | 0.17 | 24 |
TiO2 nanotubes vs. BOD/carbon nanotube cathode | Photoelectrocatalytic oxidation of glucose – reduction of O2, 0.1 M phosphate, pH 7, 50 mW cm−2 UV light illumination | 1 V | Not reported | 47 μW cm−2 at 0.79 V | — | 30 |
Poly(mercapto-p-benzoquinone)/photosystem II/Au vs. BOD/CNT cathode | Bioelectrocatalytic oxidation of water and reduction of O2, 0.1 M phosphate, pH 7.4, 0.10 W at wavelength > 400 nm | 0.43 V | 114 μA cm−2 | 17 μW cm−2 at 0.28 V | 0.35 | 23 |
ITO/SnO2 NPs/tetraaryl-porphyrin sensitizer vs. Hg/HgSO4 cathode | Photo-oxidation of NADH for enzymatic photo-oxidation of glucose, 0.25 M Tris, pH 8, 1 mW cm−2 at 520 nm | 0.75 V | 60 μA | 18 μW cm−2 at 0.42 V | 0.42 | 81 |
In these hybrid bio-photovoltaic devices, the solar-light-excited electrons flow up in the conduction band of the semiconductor photoanode and then in the external circuit, and the holes in the valence band of the semiconductor allow oxidation of water to molecular oxygen and protons. The electrons transferred through the external circuit to the biocathode allow re-reduction of O2 diffusing from the photoanode to the biocathode into water (or reduction of environmentally supplied O2). The net reaction is the solar energy conversion to electricity in the ceaseless cycle of water consumption and regeneration with no need of a membrane between two electrodes. That lowers the internal resistance of the setup and improves the energy utilization efficiency (Fig. 1).
The current state-of-the-art data show that the cell performance can be improved to 3 mW cm−2 once open circuit cell voltages >1 V and current densities >10 mA cm−2 in neutral solutions are achieved.63,82 Then, it may become comparable with the existing technologies (in average ∼10 mW cm−2 for silicon solar cells), along with that, offering a ca. 5 fold decrease in W cm−2 costs at the expense of cheaper and more sustainable materials used (estimations based on all material costs except of FTO). Attributed to the low expenditure and simple preparation, such types of BPV cells can be expected to find their practical industrial applications and potentially contribute to lessening of the energy crisis.
Another possible application of such BPV cells is self-powered fresh water production at biocathodes that can be directly correlated with electricity costs. The lack of sufficient safe water supplies to satisfy human needs in many regions of the Earth, also in well-developed regions such as California, positions clean water production among the main global challenges for humanity.83 In future, these BPV systems may contribute not only to electricity production, but also to direct fresh water production at the biocathodes or be used indirectly, to power seawater desalination systems.84
However, the current performance of the BPV cells in seawater is strongly limited by the photoanode operation. While existing biotechnologies allow the development of up to 1 year stabilized biocathodes,85 exhibiting 5–10 mA cm−2 steady-state current densities and >8 mW cm−2 power output in advanced enzymatic biofuel cell designs (air-breathing gas-diffusion biocathodes),86,87 whose performance in seawater is not expected to be essentially different (Fig. 3B), the performance of photoanodes in seawater requires focusing of research efforts.
Two problems should be solved, both contributing to the development of “ideal” photoanodes. These are stabilization of the existing semiconductor systems against inhibition in seawater and further lowering of the potential for water oxidation. While extensive research is conducted to enhance the photoelectrocatalytic performance of photoanodes in media such as NaOH, with the reported impressive photocurrent densities between 2 and 5 mA cm−2, at 1.23 V versus the RHE, for example at silicon-doped α-Fe2O3 electrodes34 or Pt-doped single-crystalline hematite electrodes,88 their operation in less conventional but more abundant media is not straightforward. It is also a matter of the efficiency achieved versus the cost and sustainability of the materials used. The cheapest materials so far are iron, titanium and zinc oxides, plentifully available from naturally existing minerals, and their modification by similarly inexpensive and sustainable metal dopants or surface protective layers/islets of relevant catalytic activity89 may allow clean production of energy on a massive scale indeed. For example, the efficient electrocatalytic and photoelectrocatalytic oxidation of water can be achieved at graphite and hematite electrodes modified with trace amounts of quite expensive Ir oxide nanoparticles;38,90 however, even in the case of trace amounts, the relationship between the cost and efficiency in such systems significantly limits their wider applications.
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Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00051k |
This journal is © The Royal Society of Chemistry 2017 |