In a recent issue of Nature, Zhou et al. report an artificial photosynthesis scheme that splits water into hydrogen and oxygen with an overall energy efficiency of nearly 10%, close to the value needed for commercial viability. This preview provides a technological context for this advancement and discusses needed next steps.

Electrochemical reduction of CO2 in aqueous media is a strategy for sustainable production of fuels and commodity chemicals. Cu is the only catalyst which converts CO2 to significant quantities of hydrocarbons and oxygenates. Here we demonstrate that oxygenate products can be favored over hydrocarbons by positioning a local source of CO generated by a CO producing catalyst (Au or Ag) in close proximity to a Cu catalyst. Use of a bimetallic device comprising interdigitated and independently controllable lines of Au and Cu allows the local CO concentration to be modulated. Notably, diffusional simulations show that the saturation concentration of CO can be exceeded locally. Actuating both the Au and Cu lines increases the oxygenate to ethylene ratio compared to actuating Cu only. Increasing the relative area of CO-producing Au relative to Cu also increases this ratio. These insights are translated into a second bimetallic system comprising Cu dots/lines patterned directly onto a Ag substrate, allowing for the distance between Cu and the CO generating metal to be precisely controlled. Controlling the relative areas of Ag and Cu allows for tuning of the oxygenate to ethylene ratio from 0.59 to 2.39 and an increase in oxygenate faradaic efficiency from 21.4% to 41.4%, while maintaining the selectivity to C2/C3 products in the 50-65% range. We attribute this change in selectivity to be due to an increased ∗CO coverage on Cu. By utilizing diffusional transport of CO to the Cu, a sequential catalysis pathway is created which allows for the control of oxygenate selectivity in aqueous CO2 reduction.

Oxide-derived (OD) Cu catalysts have high selectivity towards the formation of multi-carbon products (C₂ /C₃ ) for aqueous electrochemical CO₂ reduction (CO₂ R). It has been proposed that a large fraction of the initial oxide can be surprisingly resistant to reduction, and these residual oxides play a crucial catalytic role. The stability of residual oxides was investigated by synthesizing ¹⁸ O-enriched OD Cu catalysts and testing them for CO₂ R. These catalysts maintain a high selectivity towards C₂ /C₃ products (ca. 60 %) for up to 5 h in 0.1 m KHCO₃ at -1.0 V vs. RHE. However, secondary-ion mass spectrometry measurements show that only a small fraction (<1 %) of the original ¹⁸ O content remains, showing that residual oxides are not present in significant amounts during CO₂ R. Furthermore, we show that OD Cu can reoxidize rapidly, which could compromise the accuracy of ex situ methods for determining the true oxygen content.

Carbon dioxide electroreduction in aqueous media using Cu catalysts can generate many different C2 and C3 products, which leads to the question whether all products are generated from the same types of active sites or if product-specific active sites are responsible for certain products. Here, by reducing mixtures of 13CO and 12CO2, we show that oxide-derived Cu catalysts have three different types of active sites for C–C coupled products, one that produces ethanol and acetate, another that produces ethylene and yet another that produces 1-propanol. In contrast, we do not find evidence of product-specific sites on polycrystalline Cu and oriented (100) and (111) Cu surfaces. Analysis of the isotopic composition of the products leads to the prediction that the adsorption energy of *COOH (the product of the first step of CO2 reduction) may be a descriptor for the product selectivity of a given active site. These new insights should enable highly selective catalysts to be developed.

Because single catalysts cannot selectively convert CO2 into a specific reduced product more complex than CO either electrochemically or photoelectrochemically, an alternative is to use multiple catalysts organized into a cascade, which offers a means of generating complex chemicals with improved specificity by providing a channel for the reduction of CO and similar intermediates. The rules for how to select catalysts for this purpose and design systems that have a high degree of chemical control using them are not known however. The efficiency and selectivity for individual catalysts may be optimized, but the resulting efficiency of an entire system will depend on how well the timings of all the chemical steps in the cascade as well as transport between the catalysts are managed. In this Perspective, we discuss these challenges and examine the ways in which control is exerted in natural photosynthetic systems which use cascaded reactions to convert CO2 from the air into sugars using sunlight as the sole source of energy. These cascades take place within complex architectural elements that ensure that the chemistry is as efficient as possible. Adaptations of the functions of these natural design elements to artificial systems may offer ways to attain the promise of artificial photosynthesis.

Numerous studies have examined the electrochemical reduction of CO (COR) to oxygenates (e.g., ethanol). None have considered the possibility that oxygen in the product might arise from water rather than from CO. To test this assumption, C¹⁶O reduction was performed in H₂¹⁸O electrolyte. Surprisingly, we found that 60-70% of the ethanol contained ¹⁸O, which must have originated from the solvent. We extended our previous all-solvent density functional theory metadynamics calculations to consider the possibility of incorporating water, and indeed, we found a new mechanism involving a Grotthuss chain of six water molecules in a concerted reaction with the *C-CH intermediate to form *CH-CH(¹⁸OH), subsequently leading to (¹⁸O)ethanol. This competes with the formation of ethylene that also arises from *C-CH. These unforeseen results suggest that all previous studies of COR under aqueous conditions must be reexamined.

Recycling of waste CO₂ to bulk chemicals has a tremendous potential for the decarbonization of the chemical industry. Quantitative analysis of the prospects of this technology is hindered by the lack of flexible techno-economic assessment (TEA) models that enable evaluation of the processing costs under different deployment scenarios. In this protocol, we explain how to convert literature data into metrics useful for evaluation of the emerging electrolysis technologies, derive TEA models, and illustrate their use with a CO₂-to-ethylene example. For complete details on the use and execution of this protocol, please refer to Barecka et al. (2021a).