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Researchers use efficient microbial electrosynthesis cells to convert CO2 to butyric acid; upgrade to butanol

Researchers from University of Girona (Spain) successfully used electrically efficient microbial electrosynthesis cells (MES) to convert CO2 to butyric acid. In an open-access paper published in the journal Environmental Science and Ecotechnology, they reported operating the low ohmic resistance (15.7 mΩ m2) cells in a batch-fed mode, alternating high CO2 and hydrogen (H2) availability to promote the production of acetic acid and ethanol.

Chain elongation resulted in the selective (78% on a carbon basis) production of butyric acid, a valuable chemical used in pharmaceuticals, farming, perfumes, and the chemical industry.

At an applied current of 1.0 or 1.5 mA cm−2, the study achieved an average production rate of 14.5 g m−2 d−1 of butyric acid.

The key player in the chain elongation process was identified as Megasphaera sp. Inoculating a second cell with the enriched community replicated the butyric acid production rate, but with an 82% reduction in the lag phase.

Butyric acid was successfully upgraded to butanol, a valuable biofuel compatible with existing gasoline infrastructure and used as a precursor in pharmaceutical and chemical industries for acrylate and methacrylate production.


Romans-Casas et al.

Solventogenic butanol production was triggered at a pH below 4.8 by interrupting CO2 supply and maintaining specific pH and hydrogen partial pressure conditions.

The MES cell design proved highly efficient, with average cell voltages of 2.6–2.8 V and an electric energy requirement of 34.6 kWhel kg−1 of butyric acid produced. Despite some limitations due to O2 and H2 crossover through the membrane, the study identified optimal operating conditions for energy-efficient butyric acid production from CO2.

This study showcases the potential of bioelectrochemical conversion of CO2 to butyric acid and its subsequent upgrade to butanol in microbial electrolysis cells.

Further research and development are needed to optimize the process for large-scale applications.


  • Meritxell Romans-Casas, Laura Feliu-Paradeda, Michele Tedesco, Hubertus V.M. Hamelers, Lluis Bañeras, M. Dolors Balaguer, Sebastià Puig, Paolo Dessì (2023) “Selective butyric acid production from CO2 and its upgrade to butanol in microbial electrosynthesis cells,” Environmental Science and Ecotechnology, Volume 17, doi: 10.1016/j.ese.2023.100303



' The MES cell design proved highly efficient, with average cell voltages of 2.6–2.8 V and an electric energy requirement of 34.6 kWhel kg−1 of butyric acid produced'

I can't get back to their figures to show something 'highly efficient'

Googling ' butyric acid energy content kwh per kg'

Turns up:

' Life-Cycle Assessment of Corn-Based Butanol as a Potential ...
Department of Energy (.gov)
https://afdc.energy.gov › files › publication › A...
Butanol has unique properties as a fuel. The energy content of butanol — 99,840 Btu per gallon (low heating value [LHV]) — is 86% of the energy content of ...
59 pages'

Which comes to 7.7KWh/Liter, which is more or less 1:1 in KWh/Kg

7.7KWh/Kg from an input of 34Kwh is hardly 'high energy efficiency' unless I have dropped a clanger, which is probable!

Thomas Pedersen

Wikipedia says heat of combustion: 2670 kJ/mol, and with a molar mass of 74 g/mol this amounts to 36 MJ/kg = 10 kWh/kg butanol.

But where does the hydrogen come from?

The hydrogen contained in the butanol contains 16 MJ/kg -butanol (LHV; 120 MJ/kg), which also needs to be counted as energy input. It has probably required 25 MJ/kg-butanol to generate said amount of H2 by electrolysis, so that adds another 7 kWh/kg to the 36 supplied to the cell.

I struggle to see how this is supposed to be great news in any way..?


Efficiency rarely matters in the chemicals industry. Economics is what matters. If you have a source of cheap electrons such as off peak or better yet curtailment wind energy at one cent per kWh or at times negative in Texas. Or off peak or better yet on-site dedicated nuclear power.ask the S.Koreans to build you a candu reactor for $2800 kw fuel it with the cheapest source of BTU heat energy on the planet aka natural uranium. Candu can make 2 cents per kWh to the busbars on-site.

Either way it doesn't matter if it takes 36 or 36+7 kWh to make a kg of butanol that's less than 45 cents per kg which is near 1:1 per liter just in energy alone it's only $1.70 per gallon.

Given that butanol can be used directly in existing ICE and hybrid engines with an order of magnitude higher energy density than batteries. Butanol is liquid at room temperature, has low vapor pressure and is hydrophobic. The ideal liquid energy storage medium.

In a previous post I showed mathematically that even at $9 a US gallon it makes better economical sense to burn synthetic fuels in a 5 passenger mid sized VW luxury vehicle vs the cheapest used Model 3 Tesla over a total driving lifetime till both vehicles reach 150,000 miles.

The math is here.


The math gets even better for a hybrid using a 2022 Prius at 58mpg vs a 2022 model 3 both used and both with similar miles on them synthetic fuels could be in the $11 US gallon range and over a 100,000 mile life you come out ahead with the ICE why? The BEV is 20,000 dollars more expensive you can buy an ENORMOUS amount of synthetic fuels for 20 grand.

This says nothing of the advantages of having a 500+ mile range in any weather conditions brutally hot or Arctic cold. At the US fuel pump limit of 10 gal per min a Prius takes less than a min to put the 9 gallons it needs to cover 500 miles. That fuel will sit for months or years and unlike a BEV wont lose 2% per day or more in self discharge. There is no substitute for liquid fuel.energy density that is physics and it's the immutable laws of physics that liquids will always be denser than lithium or any other battery. Only metal air fuel cells can come close and no one has ever make one rechargeable nor will they likely ever will again it's physics.

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