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| Fig. 1: Schematic showing operation mechanism of a photoanodic PEC device. (Source: K. Lee) |
Although fossil fuel has long stood as the primary energy source of the world even up to nowadays, problems in sustainability and large environmental footprints of fossil fuel have triggered a search towards a new source of energy which is renewable, environmentally friendly, and comparable to fossil fuel in energy density and cost. [1-3] Among many alternatives, solar energy stands out as a strong candidate. Supported by advancements in semiconductor research, solar cells have reached up to 28% efficiency for single-junction cells and 38% efficiency for multi-junction cells, both of which are comparable to fossil fuel efficiency (for example, the efficiency of spark-ignition engines is around 25 ~ 35%). [1,4] However, due to the intermittent nature of sunlight, a separate energy storage mechanism which is effective and environmentally harmless is required for solar energy to fully replace fossil energy; this would imply an increase in cost in the overall sunlight-to-fuel process. This makes photoelectrochemical cell (PEC), a device which can convert sunlight directly to hydrogen fuel, an interesting alternative.
A basic PEC device comprises of a semiconducting photoelectrode, a metallic counter electrode, and electrolyte (Fig. 1). In the case of a semiconducting photoanode, the photoanode absorbs light which has energy greater than or equal to the band gap energy of the semiconductor and generates an electron-hole pair by exciting an electron in the conduction band to the valence band. If the energy bands of the semiconductor straddle the oxygen evolution reaction and hydrogen evolution reaction potentials, then the generated hole can hop into the electrolyte to participate in oxygen evolution reaction. The generate electron will travel through an external circuit to the counter electrode then hop into the electrolyte to participate in hydrogen evolution reaction, thus completing the full water-splitting (i.e. water electrolysis) chemical reaction. The overall outcome of this operation is an absorption of light directly producing hydrogen gas, with only oxygen gas as the side product. [5] The simplicity in design and the direct conversion of sunlight to hydrogen gas, a promising alternative to fossil fuel, make PEC an attractive choice for renewable energy research. [2]
However, there exist some non-trivial challenges in developing and enhancing PEC devices. PEC device operation requires the energy bands of the involved semiconductors to straddle the water-splitting redox potentials. [6] This enforces restrictions on the viable semiconductor materials for a PEC device. Semiconductors with smaller band gaps that are fit for visible- light absorption are chemically unstable. [6] This is problematic in a PEC device which involves a direct contact of semiconductor and electrolyte. Meanwhile, the wide-gap semiconductors with band gap larger than approximately 3 eV are chemically stable but are only sensitive to UV-light, which constitutes only a small portion of the solar spectrum. [6] This greatly limits the efficiency of the PEC device made of these wide-gap semiconductors. According to theoretical calculations, the efficiency limit of a single-junction PEC device with band gap larger than 3 eV does not exceed 5%; this is far less than the maximum efficiency of 30.6% for 1.6 eV band-gap single-junction PEC device. [7]
So how can these limitations be overcome? A promising suggestion has been to modify the stable wide-gap semiconductors so that they can operate at higher-wavelength spectra, especially at the visible light spectrum at which the solar spectrum peaks. This approach involves techniques to shorten the effective band gap of the wide-gap semiconductors, allowing visible-light photons to excite electrons to the conduction band (Fig. 2).
One of the popular approaches of sensitizing the wide-gap semiconductors to visible light is to dope the semiconductors to reduce the band gap. Doping a semiconductor can create accessible states for electrons just above the valance band or just below the conduction band, effectively shortening the band gap. One of the pioneering works using this technique is nitrogen-doping in titanium dioxide (TiO2). [8] The nitrogen-doping introduces states just above the valence band of TiO2, reducing the effective band gap of the semiconductor. This results in an improved absorption in the visible light, raising the absorbance of photons with 400 nm wavelength from nearly 0 to 20%. [8] A critical limitation of doping-induced visible light sensitization is stability, as the liberation of the dopants from the semiconductors over time reduces the photoactivity of the overall device. [9]
Another approach is the appliance of light-absorbing dye molecules on semiconductors. In these dye-sensitized PEC devices, the dye molecules absorb visible light and generate electron-hole pairs. For semiconductor anodes, the holes move to the electrolyte and generate oxygen. The electrons move to the semiconductor, travel to the cathode, and move into the electrolyte to generate hydrogen. This process is essentially identical to a normal PEC device, except that the electron-hole pairs are generated by visible light absorption, which happens at the dye molecules at the semiconductor surface. [10] Applying this technique to TiO2 was shown to maintain the same absorption efficiency under ultraviolet illumination all the way to 600 nm wavelength illumination. [10] However, the weak bonding between the dye molecules and the semiconductor causes loss of the dye molecules during operation, reducing the device efficiency over time. [11] Also, many of the dye sensitizers show limited stability in aqueous solution, which is problematic due to the direct contact with the electrolyte in the PEC device configuration. [11]
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| Fig. 2: Schematic of visible sensitization techniques. (Source: K. Lee) |
A recent development in the visible light sensitization technique utilizes unusual resonance effects in metal nanostructures (i.e. plasmonic resonance). [12] Under visible light illumination, nanostructure arrays of noble metals such as gold and silver are found to show non-propagating resonant excitations of electrons (known as localized surface plasmon resonance, or LSPR). [13] As the LSPR decays, electrons with high kinetic energy are emitted. [13] When the metal nanostructures are implemented on the semiconducting anode, these emitted electrons can be injected to the semiconducting anode of a PEC device and participate in the water-splitting reaction. This technique holds several attractive advantages over doping or dye sensitization. First, in contrast to doping, this technique is not material-sensitive in that the metal nanostructures can be implemented to any semiconductors. In addition, the resonating wavelength spectrum can be tuned over a wide range by adjusting the geometry of the metal nanostructures. Furthermore, the use of chemically stable materials like gold can ensure stability of the device. However, this technique is still at its early stages, and reported photon-to-hydrogen conversion efficiency of PEC devices using this technique are yet to surpass 1%. [14] Significant research and design modifications must be made to increase the efficiency of the plasmonic PEC devices.
The PEC device is an attractive approach to solve the cost issues which arises due to separate implementation of solar-to-electricity and electricity-to-chemical-fuel steps. However, the inability of chemically stable wide-gap semiconductors to operate under visible light significantly brings down the efficiency of PEC devices. To resolve this limitation, various visible light sensitization techniques including doping, dye sensitization, and plasmonic applications are under development.
© Kyuho Lee. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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