Photoelectrochemical reduction of carbon dioxide

Last updated February 26, 2024

Photoelectrochemical reduction of carbon dioxide, also known as photoelectrolysis of carbon dioxide, is a chemical process whereby carbon dioxide is reduced to carbon monoxide or hydrocarbons by the energy of incident light. This process requires catalysts, most of which are semiconducting materials. The feasibility of this chemical reaction was first theorised by Giacomo Luigi Ciamician, an Italian photochemist. Already in 1912 he stated that "[b]y using suitable catalyzers, it should be possible to transform the mixture of water and carbon dioxide into oxygen and methane, or to cause other endo-energetic processes."

Thermodynamics

Thermodynamic potentials for the reduction of CO₂ to various products is given in the following table versus NHE at pH = 7. Single electron reduction of CO₂ to CO₂^●− radical occurs at E° = −1.90 V versus NHE at pH = 7 in an aqueous solution at 25 °C under 1 atm gas pressure. The reason behind the high negative thermodynamically unfavorable single electron reduction potential of CO₂ is the large reorganization energy between the linear molecule and bent radical anion. Proton-coupled multi-electron steps for CO₂ reductions are generally more favorable than single electron reductions, as thermodynamically more stable molecules are produced.^[1]

Summary of thermodynamic potentials of CO₂ reduction to various products
CO^₂ + 2 H⁺ + 2 e⁻ → CO + H^₂O	E⁰ = −0.53 V
CO^₂ + 2 H⁺ + 2 e⁻ → HCOOH	E⁰ = −0.61 V
CO^₂ + 4 H⁺ + 4 e⁻ → HCHO + H^₂O	E⁰ = −0.48 V
CO^₂ + 6 H⁺ + 6 e⁻ → CH^₃OH + H^₂O	E⁰ = −0.38 V
CO^₂ + 8 H⁺ + 8 e⁻ → CH^₄ + 2 H^₂O	E⁰ = −0.24 V
CO^₂ + e⁻ → CO⁻ ₂	E⁰ = −1.90 V

Kinetics

Thermodynamically, proton coupled multiple-electron reduction of CO₂ is easier than single electron reduction. But to manage multiple proton coupled multiple-electron processes is a huge challenge kinetically. This leads to a high overpotential for electrochemical heterogeneous reduction of CO₂ to hydrocarbons and alcohols. Even further heterogeneous reduction of singly reduced CO₂^●− radical anion is difficult because of repulsive interaction between negatively biased electrode and negatively charged anion.

Figure 2 shows that in case of a p-type semiconductor/liquid junction photo generated electrons are available at the semiconductor/liquid interface under illumination. The reduction of redox species happens at less negative potential on illuminated p-type semiconductor compared to metal electrode due to the band bending at semiconductor/liquid interface. Figure 3 shows that thermodynamically, some of the proton-coupled multi-electron CO₂ reductions are within semiconductors band gap. This makes it feasible to photo-reduce CO₂ on p-type semiconductors. Various p-type semiconductors have been successfully employed for CO₂ photo reduction including p-GaP, p-CdTe, p-Si, p-GaAs, p-InP, and p-SiC. Kinetically, however, these reactions are extremely slow on given semiconductor surfaces; this leads to significant overpotential for CO₂ reduction on these semiconductor surfaces. Apart from high overpotential; these systems have a few advantages including sustainability (nothing is consumed in this system apart from light energy), direct conversion of solar energy to chemical energy, utilization of renewable energy resource for energy intensive process, stability of the process (semiconductors are really stable under illumination) etc. A different approach for photo-reduction of CO₂ involves molecular catalysts, photosensitizers and sacrificial electron donors. In this process sacrificial electron donors are consumed during the process and photosensitizers degrade under long exposure to illumination.

Solvent effect

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.

Aqueous media

Photoreduction of CO₂ to formic acid was demonstrated on an p-GaP photocathode in aqueous media.^[2] Apart from several other reports of CO₂ photoreduction on p-GaP, there are other p-type semiconductors like p-GaAs,^[3] p-InP, p-CdTe,^[4] and p⁺/p-Si^[5] 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^[6] 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.

Non-aqueous media

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.^[7] 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.^[8] 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.^[9] 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.^[10] 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.

Related Research Articles

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity".

Redox is a type of chemical reaction in which the oxidation states of a reactant change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state.

Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells in which methanol is used as the fuel. Their main advantage is the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions.

<span class="mw-page-title-main">Alkaline fuel cell</span> Type of fuel cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, Francis Thomas Bacon, is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70%.

A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.

Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis to convert sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term artificial photosynthesis is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel. Photocatalytic water splitting converts water into hydrogen and oxygen and is a major research topic of artificial photosynthesis. Light-driven carbon dioxide reduction is another process studied that replicates natural carbon fixation.

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.

In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically-determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential implies that the cell requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density is achieved.

A polymer-based battery uses organic materials instead of bulk metals to form a battery. Currently accepted metal-based batteries pose many challenges due to limited resources, negative environmental impact, and the approaching limit of progress. Redox active polymers are attractive options for electrodes in batteries due to their synthetic availability, high-capacity, flexibility, light weight, low cost, and low toxicity. Recent studies have explored how to increase efficiency and reduce challenges to push polymeric active materials further towards practicality in batteries. Many types of polymers are being explored, including conductive, non-conductive, and radical polymers. Batteries with a combination of electrodes are easier to test and compare to current metal-based batteries, however batteries with both a polymer cathode and anode are also a current research focus. Polymer-based batteries, including metal/polymer electrode combinations, should be distinguished from metal-polymer batteries, such as a lithium polymer battery, which most often involve a polymeric electrolyte, as opposed to polymeric active materials.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.

The electrochemical reduction of carbon dioxide, also known as CO2RR, is the conversion of carbon dioxide to more reduced chemical species using electrical energy. It represents one potential step in the broad scheme of carbon capture and utilization.

Photochemical reduction of carbon dioxide harnesses solar energy to convert CO₂ into higher-energy products. Environmental interest in producing artificial systems is motivated by recognition that CO₂ is a greenhouse gas. The process has not been commercialized.

Andrew Bruce Bocarsly is currently a professor at Princeton University, New Jersey. His primary research interests lie in physical inorganic chemistry. He conducts research in electrochemistry, photochemistry, solids state chemistry, and fuel cells, and is known for his work on alternate energy solutions involving processes and materials for photo-reduction and electro-reduction.

Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems. It is an active domain of investigation. One of the pioneers of this field of electrochemistry was the German electrochemist Heinz Gerischer. The interest in this domain is high in the context of development of renewable energy conversion and storage technology.

Transition metal oxides are compounds composed of oxygen atoms bound to transition metals. They are commonly utilized for their catalytic activity and semiconducting properties. Transition metal oxides are also frequently used as pigments in paints and plastics, most notably titanium dioxide. Transition metal oxides have a wide variety of surface structures which affect the surface energy of these compounds and influence their chemical properties. The relative acidity and basicity of the atoms present on the surface of metal oxides are also affected by the coordination of the metal cation and oxygen anion, which alter the catalytic properties of these compounds. For this reason, structural defects in transition metal oxides greatly influence their catalytic properties. The acidic and basic sites on the surface of metal oxides are commonly characterized via infrared spectroscopy, calorimetry among other techniques. Transition metal oxides can also undergo photo-assisted adsorption and desorption that alter their electrical conductivity. One of the more researched properties of these compounds is their response to electromagnetic radiation, which makes them useful catalysts for redox reactions, isotope exchange and specialized surfaces.

<span class="mw-page-title-main">Alkaline anion exchange membrane fuel cell</span>

An alkaline anion exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion exchange membrane to separate the anode and cathode compartments.

A solar fuel is a synthetic chemical fuel produced from solar energy. Solar fuels can be produced through photochemical, photobiological, and electrochemical reactions.

Dioxide Materials was founded in 2009 in Champaign, Illinois, and is now headquartered in Boca Raton, Florida. Its main business is to develop technology to lower the world's carbon footprint. Dioxide Materials is developing technology to convert carbon dioxide, water and renewable energy into carbon-neutral gasoline (petrol) or jet fuel. Applications include CO₂ recycling, sustainable fuels production and reducing curtailment of renewable energy(i.e. renewable energy that could not be used by the grid).

Krishnan Rajeshwar is a chemist, researcher and academic. He is a Distinguished University Professor and Founding Director of the Center for Renewable Energy Science & Technology at The University of Texas at Arlington.

References

↑ Benson, Eric E.; Kubiak, Clifford P.; Sathrum, Aaron J.; Smieja, Jonathan M. (2009). "Electrocatalytic and homogeneous approaches to conversion of CO₂ to liquid fuels". Chem. Soc. Rev. 38 (1): 89–99. doi:10.1039/b804323j. PMID 19088968.
↑ Halmann, M. (1978). "Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells". Nature . 275 (5676): 115–116. Bibcode:1978Natur.275..115H. doi:10.1038/275115a0. S2CID 4257841.
↑ Aurian-Blajeni, B.; Halmann, M.; Manassen, J. (1983). "Electrochemical measurement on the photoelectrochemical reduction of aqueous carbon dioxide on p-Gallium phosphide and p-Gallium arsenide semiconductor electrodes". Solar Energy Materials . 8 (4): 425–440. doi:10.1016/0165-1633(83)90007-2.
↑ Yoneyama, Hiroshi; Sugimura, Kenji; Kuwabata, Susumu (1988). "Effects of electrolytes on the photoelectrochemical reduction of carbon dioxide at illuminated p-type cadmium telluride and p-type indium phosphide electrodes in aqueous solutions". J. Electroanal. Chem. Interfacial Electrochem. 249 (1–2): 143–153. doi:10.1016/0022-0728(88)80355-3.
↑ Junfu, Liu; Baozhu, ChunYu (1992). "Photoelectrochemical reduction of carbon dioxide on a p⁺/p-Si photocathode in aqueous electrolyte". J. Electroanal. Chem. 324 (1–2): 191–200. doi:10.1016/0022-0728(92)80045-6.
↑ Hori, Yoshio; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, Osamu (1994). "Electrocatalytic process of CO selectivity in electrochemical reduction of CO₂ at metal electrodes in aqueous media". Electrochim. Acta . 39 (11–12): 1833–1839. doi:10.1016/0013-4686(94)85172-7.
↑ Kumar, Bhupendra; Smieja, Jonathan M.; Kubiak, Clifford P. (2010). "Photoreduction of CO₂ on p-type Silicon Using Re(bipy-Bu^t)(CO)₃Cl: Photovoltages Exceeding 600 mV for the Selective Reduction of CO₂ to CO". J. Phys. Chem. C . 114 (33): 14220-14223. doi:10.1021/jp105171b.
↑ Walter, Michael G.; Warren, Emily L.; McKone, James R.; Boettcher, Shannon W.; Mi, Qixi; Santori, Elizabeth A.; Lewis, Nathan S. (2010). "Solar Water Splitting Cells". Chem. Rev. 110 (11): 6446–6473. doi: 10.1021/cr1002326 . PMID 21062097.
↑ Kaneco, Satoshi; Katsumata, Hideyuki; Suzuki, Tohru; Ohta, Kiyohisa (2006). "Photoelectrochemical reduction of carbon dioxide at p-type gallium arsenide and p-type indium phosphide electrodes in methanol". Chemical Engineering Journal . 116 (3): 227–231. Bibcode:2006ChEnJ.116..227K. doi:10.1016/j.cej.2005.12.014.
↑ Taniguchi, I.; Aurian-Blajeni, B.; Bockris, J. O'M. (1984). "The reduction of carbon dioxide at illuminated p-type semiconductor electrodes in nonaqueous media". Electrochim. Acta . 29 (7): 923–932. doi:10.1016/0013-4686(84)87137-6.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] Benson, Eric E.; Kubiak, Clifford P.; Sathrum, Aaron J.; Smieja, Jonathan M. (2009). "Electrocatalytic and homogeneous approaches to conversion of CO₂ to liquid fuels". Chem. Soc. Rev. 38 (1): 89–99. doi:10.1039/b804323j. PMID 19088968.

[2] Halmann, M. (1978). "Photoelectrochemical reduction of aqueous carbon dioxide on p-type gallium phosphide in liquid junction solar cells". Nature . 275 (5676): 115–116. Bibcode:1978Natur.275..115H. doi:10.1038/275115a0. S2CID 4257841.

[3] Aurian-Blajeni, B.; Halmann, M.; Manassen, J. (1983). "Electrochemical measurement on the photoelectrochemical reduction of aqueous carbon dioxide on p-Gallium phosphide and p-Gallium arsenide semiconductor electrodes". Solar Energy Materials . 8 (4): 425–440. doi:10.1016/0165-1633(83)90007-2.

[4] Yoneyama, Hiroshi; Sugimura, Kenji; Kuwabata, Susumu (1988). "Effects of electrolytes on the photoelectrochemical reduction of carbon dioxide at illuminated p-type cadmium telluride and p-type indium phosphide electrodes in aqueous solutions". J. Electroanal. Chem. Interfacial Electrochem. 249 (1–2): 143–153. doi:10.1016/0022-0728(88)80355-3.

[5] Junfu, Liu; Baozhu, ChunYu (1992). "Photoelectrochemical reduction of carbon dioxide on a p⁺/p-Si photocathode in aqueous electrolyte". J. Electroanal. Chem. 324 (1–2): 191–200. doi:10.1016/0022-0728(92)80045-6.

[6] Hori, Yoshio; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, Osamu (1994). "Electrocatalytic process of CO selectivity in electrochemical reduction of CO₂ at metal electrodes in aqueous media". Electrochim. Acta . 39 (11–12): 1833–1839. doi:10.1016/0013-4686(94)85172-7.

[7] Kumar, Bhupendra; Smieja, Jonathan M.; Kubiak, Clifford P. (2010). "Photoreduction of CO₂ on p-type Silicon Using Re(bipy-Bu^t)(CO)₃Cl: Photovoltages Exceeding 600 mV for the Selective Reduction of CO₂ to CO". J. Phys. Chem. C . 114 (33): 14220-14223. doi:10.1021/jp105171b.

[8] Walter, Michael G.; Warren, Emily L.; McKone, James R.; Boettcher, Shannon W.; Mi, Qixi; Santori, Elizabeth A.; Lewis, Nathan S. (2010). "Solar Water Splitting Cells". Chem. Rev. 110 (11): 6446–6473. doi: 10.1021/cr1002326 . PMID 21062097.

[9] Kaneco, Satoshi; Katsumata, Hideyuki; Suzuki, Tohru; Ohta, Kiyohisa (2006). "Photoelectrochemical reduction of carbon dioxide at p-type gallium arsenide and p-type indium phosphide electrodes in methanol". Chemical Engineering Journal . 116 (3): 227–231. Bibcode:2006ChEnJ.116..227K. doi:10.1016/j.cej.2005.12.014.

[10] Taniguchi, I.; Aurian-Blajeni, B.; Bockris, J. O'M. (1984). "The reduction of carbon dioxide at illuminated p-type semiconductor electrodes in nonaqueous media". Electrochim. Acta . 29 (7): 923–932. doi:10.1016/0013-4686(84)87137-6.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]