The Liquid Sunlight Alliance (LiSA) has demonstrated two photochemical architectures that combine a light absorber and a multi-catalyst cascade to achieve conversion of CO₂ and water into liquid fuels using sunlight. Both are complete systems including a light absorber and a multi-catalyst cascade to achieve CO₂ reduction to carbon-based fuels. Realization required careful management of light absorption and conversion and supply of electrons to multiple catalyst sites, while at the same time controlling reactant, intermediate, and product fluxes. I will describe the work of two task forces within LiSA that designed the systems, synthesized, and integrated the components, and evaluated their performance.

The first architecture is a three-terminal tandem (3TT) system which uses a monolithic semiconductor photoelectrode using sunlight to drive chemical reactions in a cascade that produces a liquid fuel. The semiconductor architecture, based on a 3TT solar cell, absorbs light and generates electrons that are used for two separate reduction reactions at two of the terminals. The product of the CO₂ reduction reaction in the first location, CO, moves from where it is formed to the second location where it is reduced further to form methanol. The protons needed for the reduction reaction are generated from water at the third terminal. The photoelectrochemical cascade has two steps: (1) two-electron reduction of CO₂ to CO (driven by the GaInP subcel) and subsequent four-electron reduction of CO to methanol (driven by the GaInP/GaAs tandem subcell).

The second architecture combines photoelectrochemical and solar-driven thermocatalytic environments. A photoelectrochemical (PEC) reactor is used to reduce CO₂ to ethylene (with minimal coproduction of H₂, CO, and CH₄). Its design minimizes losses due to crossover between the electrodes and can achieve efficient conversion of CO₂ as well as high selectivity. The PEC cell can achieve an ethylene Faradaic efficiency (FE) of ~30% and a single pass concentration of > 0.5 vol.% to ethylene. Ethylene is then oligomerized in a second reactor using a supported Ni catalyst. This PEC – solar thermal tandem system has successfully produced butene and hexene. Notably also, the thermocatalytic reactor operating by itself in batch mode can produce C₇ – C₂₄ products from a pure ethylene feed using 1 Sun illumination.