01-09-2025
Introduction – global CO₂ problem, why electrochemical reduction is useful.
Fundamentals of CO₂ electroreduction (CO₂RR)
Thermodynamics (standard potentials, overpotentials).
General equations.
Competition with hydrogen evolution reaction (HER).
Mechanism of CO₂RR
Key intermediates (*COOH, *OCHO, *CHO, *CO).
Reaction steps to CO, formate, methane, ethylene, alcohols.
Potential-dependent pathways.
Noble metal catalysts (Au, Ag, Pd, etc.) – activity, selectivity, mechanisms.
Non-noble metal catalysts (Zn, Bi, Fe, Co, Ni, Sn, etc.) – performance, examples.
Carbon-based catalysts (graphene, CNTs, N-doped, quantum dots).
Reaction pathways & useful products (CO, HCOOH, CH₃OH, CH₄, C₂H₄, C₂H₅OH).
Challenges in CO₂RR – stability, selectivity, mass transfer, energy cost.
Future outlook & conclusion.
Why electrochemical CO₂ reduction (CO₂RR)?
Uses electricity + catalysts to convert CO₂ into valuable products.
Can be powered by renewables (solar, wind) → sustainable.
Operates at room temperature and pressure, unlike high-energy thermochemical
methods.
Possible useful products:
2e⁻ products: Carbon monoxide (CO), formate (HCOO⁻).
4e⁻ products: Methanol (CH₃OH), formaldehyde (HCHO).
6e⁻ products: Methane (CH₄).
12e⁻ products: Ethylene (C₂H₄), ethanol (C₂H₅OH).
These are fuels, chemicals, and feedstocks for industry. For example:
CO → syngas (for Fischer–Tropsch fuels, plastics).
Formate → fuel cells, chemical industry.
Methanol/Ethanol → liquid fuels.
Ethylene → plas cs, polymers.
So, CO₂RR helps fight climate change and produce useful products.
1
, 01-09-2025
Fundamentals of Electrochemical CO₂ Reduction
Electrochemical CO₂ reduction happens at the cathode of an electrochemical cell, while water oxidation
occurs at the anode.
2.1 General reaction for CO₂RR
Potentials are close to zero or slightly negative,
CO₂ + nH⁺ + ne⁻ → Products
which means in theory CO₂ reduction is feasible.
Where n depends on the product (2, 4, 6, 12 electrons, etc.).
2.2 Standard reactions and potentials (in aqueous solution, vs RHE)
Product Reaction Electrons (n) E° (V vs RHE)
CO CO₂ + 2H⁺ + 2e⁻ → CO + H₂O 2 –0.11
Formate (HCOO⁻) CO₂ + 2H⁺ + 2e⁻ → HCOO⁻ 2 –0.20
Methanol (CH₃OH) CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O 6 +0.03
Methane (CH₄) CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O 8 +0.17
2CO₂ + 12H⁺ + 12e⁻ → C₂H₄ +
Ethylene (C₂H₄) 12 +0.08
4H₂O
2CO₂ + 12H⁺ + 12e⁻ → C₂H₅OH +
Ethanol (C₂H₅OH) 12 +0.09
3H₂O
Hydrogen Evolution Reaction (HER)
Competing side reaction:
2H+ + 2e− → H2 HER is often faster than CO₂RR, wasting electrons.
A good catalyst must suppress HER while favoring CO₂ reduction.
Mechanism: General Pathways
CO₂ reduction follows stepwise proton–electron transfers with adsorbed intermediates:
Activation of CO₂
Linear CO₂ molecule bends and accepts an electron:
CO₂ + e⁻ → CO₂•⁻ (adsorbed) Very unstable; catalyst surface stabilizes it.
Formation of key intermediates
For CO pathway: *COOH intermediate forms.
CO₂•⁻ + H⁺ → *COOH
For formate pathway: *OCHO intermediate forms.
CO₂•⁻ + H⁺ → *OCHO
Product formation
*COOH → *CO → CO (desorbs). *OCHO → HCOOH (formic acid).
Further hydrogenations lead to CH₃OH, CH₄, C₂H₄, etc. depending on catalyst and potential.
2
Introduction – global CO₂ problem, why electrochemical reduction is useful.
Fundamentals of CO₂ electroreduction (CO₂RR)
Thermodynamics (standard potentials, overpotentials).
General equations.
Competition with hydrogen evolution reaction (HER).
Mechanism of CO₂RR
Key intermediates (*COOH, *OCHO, *CHO, *CO).
Reaction steps to CO, formate, methane, ethylene, alcohols.
Potential-dependent pathways.
Noble metal catalysts (Au, Ag, Pd, etc.) – activity, selectivity, mechanisms.
Non-noble metal catalysts (Zn, Bi, Fe, Co, Ni, Sn, etc.) – performance, examples.
Carbon-based catalysts (graphene, CNTs, N-doped, quantum dots).
Reaction pathways & useful products (CO, HCOOH, CH₃OH, CH₄, C₂H₄, C₂H₅OH).
Challenges in CO₂RR – stability, selectivity, mass transfer, energy cost.
Future outlook & conclusion.
Why electrochemical CO₂ reduction (CO₂RR)?
Uses electricity + catalysts to convert CO₂ into valuable products.
Can be powered by renewables (solar, wind) → sustainable.
Operates at room temperature and pressure, unlike high-energy thermochemical
methods.
Possible useful products:
2e⁻ products: Carbon monoxide (CO), formate (HCOO⁻).
4e⁻ products: Methanol (CH₃OH), formaldehyde (HCHO).
6e⁻ products: Methane (CH₄).
12e⁻ products: Ethylene (C₂H₄), ethanol (C₂H₅OH).
These are fuels, chemicals, and feedstocks for industry. For example:
CO → syngas (for Fischer–Tropsch fuels, plastics).
Formate → fuel cells, chemical industry.
Methanol/Ethanol → liquid fuels.
Ethylene → plas cs, polymers.
So, CO₂RR helps fight climate change and produce useful products.
1
, 01-09-2025
Fundamentals of Electrochemical CO₂ Reduction
Electrochemical CO₂ reduction happens at the cathode of an electrochemical cell, while water oxidation
occurs at the anode.
2.1 General reaction for CO₂RR
Potentials are close to zero or slightly negative,
CO₂ + nH⁺ + ne⁻ → Products
which means in theory CO₂ reduction is feasible.
Where n depends on the product (2, 4, 6, 12 electrons, etc.).
2.2 Standard reactions and potentials (in aqueous solution, vs RHE)
Product Reaction Electrons (n) E° (V vs RHE)
CO CO₂ + 2H⁺ + 2e⁻ → CO + H₂O 2 –0.11
Formate (HCOO⁻) CO₂ + 2H⁺ + 2e⁻ → HCOO⁻ 2 –0.20
Methanol (CH₃OH) CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O 6 +0.03
Methane (CH₄) CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O 8 +0.17
2CO₂ + 12H⁺ + 12e⁻ → C₂H₄ +
Ethylene (C₂H₄) 12 +0.08
4H₂O
2CO₂ + 12H⁺ + 12e⁻ → C₂H₅OH +
Ethanol (C₂H₅OH) 12 +0.09
3H₂O
Hydrogen Evolution Reaction (HER)
Competing side reaction:
2H+ + 2e− → H2 HER is often faster than CO₂RR, wasting electrons.
A good catalyst must suppress HER while favoring CO₂ reduction.
Mechanism: General Pathways
CO₂ reduction follows stepwise proton–electron transfers with adsorbed intermediates:
Activation of CO₂
Linear CO₂ molecule bends and accepts an electron:
CO₂ + e⁻ → CO₂•⁻ (adsorbed) Very unstable; catalyst surface stabilizes it.
Formation of key intermediates
For CO pathway: *COOH intermediate forms.
CO₂•⁻ + H⁺ → *COOH
For formate pathway: *OCHO intermediate forms.
CO₂•⁻ + H⁺ → *OCHO
Product formation
*COOH → *CO → CO (desorbs). *OCHO → HCOOH (formic acid).
Further hydrogenations lead to CH₃OH, CH₄, C₂H₄, etc. depending on catalyst and potential.
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