This is an audio version of the Wikipedia Article: 00:01:27 1 Background/Introduction
00:05:05 2 Principle
00:05:14 2.1 Why is COsub2/sub reduction difficult?
00:05:24 2.1.1 Thermodynamic challenges
00:05:42 2.1.2 Kinetic challenges
00:06:02 2.2 Is it feasible to photoelectrochemically reduce COsub2/sub on semiconductor surface?
00:06:37 3 Solvent effect
00:06:46 3.1 Aqueous media
00:07:26 3.2 Non-aqueous media
00:07:41 4 See also
00:09:34 5 References
00:09:44 The photo-reduction of CO2 on p-type semiconductor photo-electrodes has been achieved in both aqueous and non-aqueous media. Main difference between aqueous and non-aqueous media is the solubility of CO2. The solubility of CO2 in aqueous media at 1 atm. of CO2 is around ≈ 35 mM; whereas solubility of CO2 in methanol is around 210 mM and in acetonitrile is around 210 mM.
00:10:20 Aqueous media
00:10:29 Halmann had first shown CO2 photoreduction to formic acid on p-GaP as photocathode in aqueous media in 1978. Apart from several other reports of CO2 photoreduction on p-GaP, there are other p-type semiconductors like p-GaAs, p-InP, p-CdTe, and p+/p-Si have been successfully used for photoreduction of CO2. The lowest potential for CO2 photoreduction was observed on p-GaP. This may be due to high photovoltage excepted from higher band gap p-GaP (2.2 eV) photocathode. Apart from formic acid, other products observed for CO2 photoreduction are formaldehyde, methanol and carbon monoxide. On p-GaP, p-GaAs and p+/p-Si photocathode, the main product is formic acid with small amount of formaldehyde and methanol. However, for p-InP and p-CdTe photocathode, both carbon monoxide and formic acid are observed in similar quantities. Mechanism proposed by Hori based on CO2 reduction on metal electrodes predicts formation of both formic acid (in case of no adsorption of singly reduced CO2●− radical anion to the surface) and carbon monoxide (in case of adsorption of singly reduced CO2●− radical anion to the surface) in aqueous media. This same mechanism can be evoked to explain the formation of mainly formic acid on p-GaP, p-GaAs and p+/p-Si photocathode owing to no adsorption of singly reduced CO2●− radical anion to the surface. In case of p-InP and p-CdTe photocathode, partial adsorption of CO2●− radical anion leads to formation of both carbon monoxide and formic acid. Low catalytic current density for CO2 photoreduction and competitive hydrogen generation are two major drawbacks of this system.
00:12:44 Non-aqueous media
00:12:54 Maximum catalytic current density for CO2 reduction that can be achieved in aqueous media is only 10 mA cm−2 based solubility of CO2 and diffusion limitations. The integrated maximum photocurrent under Air Mass 1.5 illumination, in the conventional Shockley-Quiesser limit for solar energy conversion for p-Si (1.12 eV), p-InP (1.3 eV), p-GaAs (1.4 eV), and p-GaP (2.3 eV) are 44.0 mA cm−2, 37.0 mA cm−2, 32.5 mA cm−2 and 9.0 mA cm−2, respectively. Therefore, non-aqueous media such as DMF, acetonitrile, methanol are explored as solvent for CO2 electrochemical reduction. In addition, Methanol has been industrially used as a physical absorber of CO2 in the Rectisol method. Similarly to aqueous media system, p-Si, p-InP, p-GaAs, p-GaP and p-CdTe are explored for CO2 photoelectrochemical reduction. Among these, p-GaP has lowest overpotential, whereas, p-CdTe has moderate overpotential but high catalytic current density in DMF with 5% water mixture system. Main product of CO2 reduction in non-aqueous media is carbon monoxide. Competitive hydrogen generation is minimized in non-aqueous media. Proposed mechanism for CO2 reduction to CO in non-aqueous media involves single electron reduction of CO2 to CO2●− radical anion and adsorption of radical anion to surface followed by disproportionate reaction between unreduced CO2 and CO2●− radical anion to form CO32− and CO.
00:15:05 See also
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