Currently, CO₂ reduction based on catalysis is divided into two main categories: photocatalysis and electrocatalysis. This article will discuss photocatalytic CO₂ reduction and electrocatalytic CO₂ reduction from several aspects, including their basic principles, product types and yields, and key performance evaluation parameters.
▷ Photocatalytic CO₂ Reduction
The photocatalytic CO₂ reduction reaction is a complex multi-step process. Generally, this process mainly involves the following three steps[1]:
① A semiconductor photocatalyst is excited by light with energy greater than its bandgap (Eg);
② Separation of photogenerated electrons and holes;
③ Photogenerated electrons migrate to the surface of the photocatalyst, where they react with CO₂ and H⁺ to form reduction products, while photogenerated holes react with H₂O to produce O₂.
Illustration of Photocatalytic CO₂ Reduction[2]
Photocatalytic CO₂ reduction primarily relies on light as its energy source, making it possible to utilize sunlight outdoors. It is a purely green CO₂ reduction technology, aligning with national policies on sustainable development and carbon neutrality.
Electrocatalytic CO₂ Reduction involves converting electrical energy into chemical energy by producing carbon-based fuels through the oxidation of water and the reduction of CO₂. The process includes three main steps: electron transfer, electrochemical reaction, and catalytic reaction.
In electrocatalytic CO₂ reduction, hydrogen ions produced by water electrolysis at the anode combine with CO₂ and electrons at the cathode to form high-value organic compounds.
At the Anode:
Under the action of an oxygen evolution reaction (OER) catalyst: 2H₂O - 4e⁻ → 4H⁺ + O₂, hydrogen ions migrate to the cathode.
At the Cathode:
Under the action of an electrochemical catalyst, CO₂ is adsorbed on the electrode and undergoes two reaction pathways: forming CO or formic acid (salt), which is further converted into C1 or C2 organics.
As mentioned above, electrocatalysis derives its energy from electricity, which can come from a variety of sources, including hydropower, thermal power, wind power, nuclear power, and photovoltaics. This diversity provides unique advantages and flexibility in industrial applications. Electrocatalysis can efficiently utilize surplus electricity, such as wind and solar energy, avoiding waste. Through this technology, surplus electricity can be converted into high-value chemicals and fuels, supporting energy storage and stable utilization. Additionally, electrocatalysis can repurpose industrial surplus electricity, providing an economical solution for enterprises with excess power.
After introducing the basic principles, the following sections will elaborate on the product types, yield levels, and key performance evaluation parameters to further differentiate between photocatalytic and electrocatalytic CO₂ reduction.
Product Types and Yield Levels
Key Performance Evaluation Parameters
In the introductory table of this article, we listed the key performance evaluation parameters for photocatalytic and electrocatalytic CO₂ reduction. Notably, the reaction environment and catalytic quality parameters apply to both photocatalysis and electrocatalysis. Next, we will discuss in detail the specific aspects (as listed in the table) that most significantly impact photocatalysis and electrocatalysis.
1. Reaction Environment
In general, photocatalytic CO₂ reduction reactions primarily occur in either gas-phase or liquid-phase systems[3].
In liquid-phase systems, the reaction takes place in CO₂-saturated solutions with photocatalysts uniformly dispersed. Due to the agitated state of the solid catalysts, charge transfer and heat transfer are more efficient[4,5]. However, the limited solubility and diffusion coefficient of CO₂ in water restrict its mass transfer efficiency[3].
At 25°C and 101.325 kPa, the solubility of CO₂ in water is less than 0.033 mol·L⁻¹, weakening the diffusion of CO₂ molecules from the gas phase to the photocatalyst surface[6]. Increasing the solution's pH or adding organic solvents (e.g., acetonitrile and ethyl acetate) can improve CO₂ solubility[7,8].
In gas-phase systems, the photocatalyst is fixed on a substrate, and CO₂ and water vapor react directly with it (see below)[9]. Compared to liquid-phase reactions, gas-phase reactions are unaffected by sacrificial agents, photosensitizers, and solvents, resulting in a simpler system. The diffusion coefficient of CO₂ in the gas phase (~0.1 cm²·s⁻¹) is four orders of magnitude higher than in the liquid phase[10,11], leading to higher mass transfer efficiency. Gas-phase reactions also effectively suppress hydrogen evolution reactions, improving CO₂ conversion rates[9,12].
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Comparison of Gas-phase and Liquid-phase Photocatalytic CO₂ Reduction Models[9]
Currently, gas-phase photocatalytic CO₂ reduction reactions mainly involve two methods: one is coating the photocatalyst onto a substrate, over which CO₂ flows (as shown in Figure a); the other is a fixed-bed gas-phase reaction, where CO₂ directly passes through the catalyst bed (as shown in Figure b). The latter ensures more adequate mass transfer, which helps improve conversion rates.
Thus, different systems for photocatalytic CO₂ reduction have their specific application advantages and limitations.
(a) Thin-film Gas-phase Reaction Mode and (b) Fixed-bed Gas-phase Reaction Mode
2. Light Wavelengths
In photocatalytic CO₂ reduction, light of different wavelengths (UV, visible, and infrared) significantly affects reaction efficiency. This is mainly related to the light absorption characteristics of the photocatalyst and the generation of photogenerated charge carriers.
UV light, due to its high energy (wavelength < 400 nm), can efficiently excite wide-bandgap semiconductors such as TiO₂ to form highly efficient photogenerated electron-hole pairs. However, its limited presence in the solar spectrum affects practical applications. Visible light (400–700 nm), with lower photon energy, covers most of the solar spectrum and therefore has higher practical utilization. Through appropriate catalyst design (e.g., doping or sensitization), reaction efficiency can be improved. Infrared light (wavelength > 700 nm), due to its low energy, typically cannot directly excite semiconductors. However, it can indirectly enhance reaction activity through thermal effects or utilize low-energy photons via nonlinear optical processes.
3. Catalyst Quality
The quality of the catalyst primarily impacts photocatalytic CO₂ reduction in terms of its composition, structure, surface properties, and stability. A high-quality catalyst should possess excellent light absorption capabilities, efficient photogenerated charge separation and transfer, suitable surface active sites, and outstanding stability. Through scientific design and optimization of the catalyst, the efficiency and selectivity of photocatalytic CO₂ reduction can be significantly enhanced.
1. Electrolyte Concentration and Type
The electrolyte serves as a conductive medium in electrocatalytic CO₂ reduction. Its concentration and type directly influence the pH, conductivity, and stability of intermediate products in the reaction environment. Generally, higher electrolyte concentration increases conductivity, thereby reducing ohmic resistance on the electrode and enhancing electrochemical performance. However, high electrolyte concentrations may also lead to side reactions, such as hydrogen evolution, which lowers selectivity and Faraday efficiency.
Researchers have found (DOI:10.1038/s41467-018-02906-4) that in KCl electrolyte, increasing the concentration from 0.1 M to 1.0 M improved CO selectivity from 65% to 82%. This result indicates that appropriately increasing electrolyte concentration can significantly enhance the selectivity and efficiency of CO₂ reduction.
Furthermore, the type of electrolyte has a significant impact on CO₂ reduction performance. Common types include alkaline electrolytes (e.g., KOH), neutral electrolytes (e.g., NaHCO₃), and acidic electrolytes (e.g., H₂SO₄). Different electrolyte types result in variations in pH at the electrode interface, affecting catalyst activity and selectivity.
2. Electrolyzer Types
The electrolyzer is the core equipment for electrocatalytic CO₂ reduction. Different designs directly impact current distribution, gas transfer, and product collection efficiency. Common types include H-cell electrolyzers, membrane electrode assembly (MEA) electrolyzers, and flow electrolyzers.
H-cell electrolyzers are simple in structure, suitable for small-scale laboratory research, and widely used in fundamental studies. However, their drawbacks include low conductivity and unstable gas-liquid interfaces, resulting in low efficiency in practical applications. According to a study published in JACS (DOI:10.1021/jacs.8b06288), using a flow electrolyzer for CO₂ reduction improved product selectivity and stability significantly compared to H-cell electrolyzers. At a current density of 200 mA/cm², the Faraday efficiency of CO exceeded 90%.
Flow electrolyzers stabilize the interface state on electrodes through the continuous flow of electrolyte and gas supply, thereby improving electrochemical reaction efficiency and product selectivity. As a result, flow electrolyzers are becoming the mainstream choice in industrial applications.
3. Electrolysis Current Density/Electrolysis Voltage
Electrolysis current density and voltage directly influence the reaction rate and product selectivity of electrocatalytic CO₂ reduction. High current density can increase reaction rates but may reduce product selectivity and cause catalyst deactivation. For example, high current density reduces ethylene selectivity while increasing methane selectivity (DOI:10.1002/adma.201707568).
Electrolysis voltage must be appropriately chosen; excessively high voltage can increase by-product generation. For instance, -0.8 V is optimal for CO selectivity, while at -1.2 V, H₂ generation significantly increases (DOI:10.1039/C8CC02905A). Therefore, selecting appropriate current density and voltage is key to achieving efficient and selective CO₂ reduction.
Photocatalytic and electrocatalytic CO₂ reduction reactions each have unique mechanisms, advantages, and challenges. Photocatalytic CO₂ reduction utilizes outdoor sunlight for an environmentally friendly solution, producing small molecules such as CO, CH₄, and alcohols. While reaction rates are slower, it offers a sustainable pathway for long-term carbon emission solutions. Electrocatalytic CO₂ reduction, on the other hand, features higher reaction rates and flexibility, seamlessly integrating with power facilities and systems. It primarily produces hydrocarbons, alcohols, and lipids (C₁₋₄). Although the green nature of the electrocatalysis process depends on the energy source, it has significant application prospects within the current power grid and effectively mitigates CO₂ emissions, aligning with national energy-saving and emission-reduction policies and the macro trends of green industry development.
To achieve photocatalytic/electrocatalytic CO₂ reduction, comprehensive consideration of reaction environments, catalyst quality, electrolyte concentration, and electrolyzer types is required for specific implementation.
