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PHOTOELECTROCHEMICAL CELLS - Coggle Diagram
PHOTOELECTROCHEMICAL CELLS
MATERIALS USED
Titanium dioxide (TiO2)
corrosion resistance and generally acceptable band edge alignments relative to the redox potentials
most prominent catalysts
wide band gap
Gallium nitride (GaN)
narrower band gap than TiO2 but is still large enough to allow water splitting to occur at the surface
GaN nanowires exhibited better performance than GaN thin films
Tungsten(VI) Oxide (WO3)
high conductivity but has a relatively wide, indirect band gap (~2.7 eV)
does not appear to be a viable material for PEC water splitting
Hematite
low cost, ability to be n-type doped, and band gap (2.2eV)
performance is plagued by poor conductivity and crystal anisotropy, enhanced catalytic activity by forming a layer of co-catalysts on the surface
Bismuth vanadate (BiVO4)
narrower, direct band gap (2.4 eV) and proper band alignment with water oxidation potential
APPLICATIONS
Solar hydrogen production: using solar energy to split water, make use of the Earth's abundant, environmental friendly, long-lasting and clean solar energy
1) PEC cells utilize light energy (photons) to perform a chemical reaction, in this case, the splitting of water into hydrogen (H2) and oxygen (O2) gases.
2) They consist of an anode and a cathode immersed in an electrolyte and connected in an external circuit.
3) The anode or the cathode consists of a semiconductor that absorbs sunlight, and the other electrode is typically a metal.
4) Photons with energies greater than the semiconductor band gap can be absorbed by the semiconductor, creating electron-hole pairs which are split by the electric field in the space-charge region between the semiconductor and the electrolyte.
5) The electric field reflects the band bending of the conduction and valence band edges at the semiconductor surface and is necessary to supply the free carriers to the appropriate electrode.
6) Water is oxidized at the anode:
2H+ + H2O (l) -> ½O2 (g) + 2H
7) At the cathode, H+ ions are reduced to form hydrogen gas:
2H+ + 2e- -> 2H2 (g)
8) The overall cell reaction:
2h+ + H2O (l) -> ½O2 (g) + H2 (g)
Conversion of carbon dioxide (CO2) into fuels
convert CO2 into selective gaseous (e.g., methane, ethane, etc.) and liquid products (e.g., formate, methanol, ethanol, etc.) under solar light irradiation, especially for liquid products at ambient temperature and pressure
artificial photosynthesis because it mimics nature’s energy cycle
benefits such as economic feasibility, control of product selectivity, and environmental compatibility along with the use of renewable solar energy
Photocatalysts with wide-band-gap energy of at least 2.88 eV are essential to perform redox reactions such as water oxidation and CO2 reduction
The product yield of CO2 conversion is governed by the choice of electrode materials and electrolytes
It can be defined as electrochemical cells that convert light energy (solar energy) into electric or chemical energy through photoinduced electron transfer reactions. A typical PEC cell consists of a photoactive semiconductor as a working electrode, a counter electrode (usually Pt), and an appropriate supporting electrolyte.
Important requirements for the semiconducting photoelectrode(s)
large enough band gap to split water (1.23V) and appropriate positions relative to redox potentials for H2 and O2
well-tailored redox processes at both electrodes in the PEC
materials must be stable to prevent decomposition and loss of function
high catalytic activity increases efficiency of the water-splitting reaction
determined by band gap and appropriate for solar irradiation spectrum
photoelectrodes must be conductive (or semi-conductive) to minimize resistive losses
low-cost and earth abundant for the widespread adoption of PEC water splitting to be feasible
