Thermodynamic, Electrochemical and Practical Constraints on Electromicrobial Formate Assimilation

Electromicrobial production (EMP) technologies seek to use renewable electricity, CO2, and engineered microbes to create energy-rich molecules more efficiently than photosynthesis. CO2 is first electrochemically reduced to formate, a simpler molecule to handle in lab settings compared to hydrogen or electrodes. However, formate only supplies two electrons per carbon atom, while biofuels require six, so the remaining electrons must be sourced from oxidizing additional formate, hydrogen oxidation, or extracellular electron uptake (EEU), with no comprehensive comparison of these methods and carbon assimilation pathways currently available.

Researchers calculated upper-limit efficiencies for butanol production using six different carbon assimilation pathways and three electron delivery mechanisms, based on electrochemical data from recent literature. Electrical to butanol energy conversion efficiencies ranged from 35.5 to 51.7%, translating to solar-to-fuel efficiencies of 11.7 to 17%, which still surpass the 8% theoretical limit of algal photosynthesis.

The reductive glycine variant of the serine pathway achieved the highest electrical energy conversion efficiency when using H2 oxidation, only 1.9 points behind the most efficient pathway, and is the only high-efficiency option that can tolerate oxygen. Combining formate delivery with the serine variant of reductive glycine pathway appears highly attractive due to its simplicity, quick iterative engineering potential, and high theoretical efficiency ceiling.

Formate-only systems cost 6.2 points less in electrical energy conversion efficiency compared to hydrogen at a state-of-the-art whole-cell voltage (2.2 V). However, this advantage diminishes dramatically as the CO2-to-formate cell voltage increases, reaching 13.5 points behind H2 at the highest whole-cell voltages for scaled-up CO2-to-formate electrolyzers, where formate-only operation is 11.2 electrical-to-fuel and 3.7% solar-to-fuel efficiency, both below the photosynthesis ceiling, while H2 reaches 24.7 electrical-to-fuel and 8.1% solar-to-fuel efficiency.

The choice between a formate-only system and one using H2 oxidation or EEU depends on predictions about CO2 reduction technology's trajectory. If whole-cell voltages continue to decrease as they have in the past decade, formate alone is the most suitable target, with the added benefit of simplicity at a low cost. Conversely, if improvement plateaus, the electron delivery mechanism should be easily swappable, and the system should be designed from the outset to accommodate this.

At the US Department of Energy SunShot goal of 2 cents per kilowatt hour, producing a US gallon of butanol costs $1.40 for a formate-only system at the state-of-the-art, rising to $5.45 at the highest scaled-up electrolyzer voltage, compared to $1.23 and $2.47 for a formate and H2 system.