H2: Carbon Dioxide Reduction Catalysts (Carbon Fuels)

Reduce CO₂ to carbon fuels — CO, formate, methanol, or even multi-carbon products like ethylene.

Idea

Catalyze the reduction of CO₂ (plus protons and electrons) into reduced carbon fuels – possibilities include carbon monoxide (syngas), formate, methanol, methane, or even multicarbon fuels like ethylene or ethanol. Various catalysts have been explored: transition-metal complexes (Re, Ru, Mn bipyridyls for CO or formate), metal-organic frameworks and metal oxides, as well as heterogeneous catalysts (copper is famous for reducing CO₂ to a mix of hydrocarbons in electrolysis).

Rationale

A carbon-based liquid fuel is more energy-dense and easier to store than hydrogen, and directly closes the carbon loop by consuming CO₂. However, CO₂ is a very stable molecule, so it’s much harder to reduce than protons.

Prerequisite Themes

CO₂ electrochemistry; multi-electron redox catalysis; adsorption and activation of CO₂ on surfaces.

Dependencies

Needs a robust source of electrons/protons. Often CO₂ reduction is paired with water oxidation on the other side. This path can piggyback on advances in H₂ catalysts by first making H₂ and then reacting H₂ with CO₂ via a separate catalyst.

Signs of Progress

Selectivity of catalysts – e.g. a molecular catalyst that produces mostly one fuel under light-driven conditions; operation at atmospheric CO₂; reaching beyond one-carbon products to C2+ fuels; coupling of CO₂ reduction with light.

Base Camp H2.1: CO₂ Electrochemistry and Products

Scope: Understand the challenge of CO₂ reduction: its possible products, required electrons/protons for each, and thermodynamics. Learn about competition with hydrogen evolution – a major issue in aqueous CO₂ electroreduction.

Stepping-stones: Write half-reactions for key reductions (e.g., CO₂ + 2H⁺ + 2e⁻ → CO + H₂O). Calculate standard potentials at pH 7. Understand that CO₂ needs to be activated (bent CO₂⁻ radical), often the rate-determining step.

Resources:

• Hori, Yasuaki – “Electrochemical CO₂ reduction on metal electrodes,” In: Modern Aspects of Electrochemistry, No. 42, Springer, 2008, pp. 89–189. Why: Definitive chapter by Hori, who mapped out which metals produce which CO₂ reduction products.
• Whipple, Devin & Kenis, Paul – “Prospects of CO₂ utilization via direct heterogeneous electrochemical reduction,” J Phys Chem Lett 1.24 (2010): 3451–3458. Why: Perspective summarizing state of CO₂ reduction to fuels, including efficiency and selectivity challenges.
• Costentin, Cyrille et al. – “Concerted proton-electron transfer mechanisms in the context of electrochemical CO₂ reduction,” Accounts of Chemical Research 45.5 (2012): 769–777. Why: Digs into how PCET applies to CO₂ reduction mechanisms.

Base Camp H2.2: Molecular CO₂ Reduction Catalysts

Scope: Survey known molecular catalysts: ruthenium and rhenium bipyridine carbonyls (CO₂ to CO), iron porphyrins, cobalt phthalocyanines, nickel cyclam. Understand their mechanisms.

Stepping-stones: Look at the example of Re(bpy)(CO)₃Cl (Lehn’s catalyst): how light or potential triggers CO loss to open a site for CO₂, then yields CO. Note its selectivity for CO vs formate. Compare to an iron porphyrin that can produce formate under certain conditions.

Resources:

• Saverick, John et al. – “A re-bipyridine catalyst for CO₂ reduction: mechanistic insights,” J Am Chem Soc 133.23 (2011): 8912–8920. Why: Focused on Re(bpy)(CO)₃ catalysts, providing mechanistic detail.
• Costentin, Cyrille et al. – “Electrocatalytic reduction of CO₂ by iron(0) porphyrins,” PNAS 115.50 (2018): 13138–13143. Why: Describes an iron porphyrin catalyst working at low overpotential for CO production.
• Toda, Takashi et al. – “Metallic copper as an electrocatalyst for CO₂ reduction to C₂H₄,” J Am Chem Soc 138.2 (2016): 434–437. Why: Shows Cu metal is unique in making appreciable C₂H₄, informing what properties a catalyst needs to couple carbon atoms.

Base Camp H2.3: Heterogeneous Catalysts for CO₂

Scope: Investigate catalysts like copper, silver and gold (which make CO efficiently), zinc, tin, and indium (favor formate), and bimetallics or oxide-derived surfaces. Understand how surface morphology affects product distribution.

Stepping-stones: Study the hypothesis that *CO dimerization on Cu is the route to C₂ products. Examine how oxide-derived Cu performs better – possibly due to remaining subsurface oxygen or grain boundaries.

Resources:

• Hori, Yasuaki (referenced above). Why: Contains data on various metals’ product distributions – invaluable to ground understanding.
• Raciti, David et al. – “Low-overpotential electroreduction of carbon monoxide using copper nanowires,” ACS Catalysis 7.7 (2017): 4467–4472. Why: Looks at CO being further reduced on Cu nanowires, relevant for tandem systems.
• Li, Fan et al. – “Molecular tuning of CO₂-to-ethylene conversion,” Nature 577.7791 (2020): 509–513. Why: Cutting-edge showing how combining molecular insight and surfaces can yield better outcomes.

Base Camp H2.4: Gas Diffusion Electrodes and CO₂ Supply

Scope: A practical aspect: CO₂ is a gas and sparingly soluble in water. Efficient systems often use gas diffusion electrodes (GDEs) or flow cells to supply CO₂ to the catalyst. Learn how these work.

Stepping-stones: Evaluate why many lab demos use CO₂-saturated solutions (limited current density). Understand the design of a flow cell where CO₂ gas flows on one side of a porous electrode and electrolyte on the other.

Resources:

• Dinh, Cao-Thang et al. – “CO₂ electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface,” Science 360.6390 (2018): 783–787. Why: Demonstrates high-rate CO₂-to-ethylene using a GDE, with insight into cell configuration.
• Sargent, Edward – “Carbon dioxide electroreduction: the possibility of scaling up,” Nature Energy 4.1 (2019): 8–9. Why: Commentary on scale-up considerations, including gas-fed systems.
• Weekes, David et al. – “Electrochemical CO₂ reduction: Classified activity and scaling relations,” ACS Energy Letters 3.7 (2018): 1531–1538. Why: Review touching on modes of operation (batch vs flow cell) and compiling data across catalysts.