A membrane-free solid-state reactor turns captured carbon dioxideinto clean formic acid that feeds microbes, growing protein-rich yeast and suggesting how electricity and waste carbon could supply future protein.
(Nanowerk Spotlight) A future low carbon menu might not start in fields or barns at all. Instead, it could begin in boxy devices bolted to the side of a factory, quietly drinking in carbon dioxide and electric power. Inside, gases bubble over metal catalysts, producing simple liquid molecules. Next door, stainless steel tanks full of microbes feed on those molecules and grow into dense protein. Pipes, wires and pumps would play the role that sunlight, soil and crops play today.
This idea joins two pressures that already shape policy and industry. One is the push to capture carbon dioxide from power stations, cement plants and, eventually, from air. The other is the growing search for protein sources that do not demand vast tracts of land, heavy fertilizer use and large greenhouse gas emissions. If engineers can turn streams of captured carbon dioxide and renewable electricity into protein rich biomass, carbon capture stops being only a disposal problem and becomes part of a food and feed supply chain.
Bridging electricity and biology is not straightforward. Electrochemical reactors run on ions, high pH or low pH, and strong electric fields. Microbes prefer gentle, buffered conditions and clean nutrients. Products that leave an electrochemical cell often carry salts or impurities that inhibit growth, or they are so dilute that concentrating them cancels any efficiency gains.
Meanwhile, devices that can make clean, concentrated products often rely on specialized membranes that are fragile in harsh conditions and can dominate the cost of the reactor. The central technical question is whether it is possible to build a simpler, more robust reactor that turns carbon dioxide and electricity into a clean liquid that microbes can use and then show that this liquid can support real protein production.
A paper in Advanced Energy Materials (“Power‐to‐Protein: Anion‐Exchange‐Membrane‐Free Solid‐State Electrolysis for Efficient Formic Acid Production and Microbial Protein Synthesis”) addresses this question directly. It reports an anion exchange membrane free porous solid state electrolyte reactor that produces concentrated, pure formic acid from CO₂ using commercial bismuth nanoparticles. It then links this reactor to a two-stage microbial process that converts formic acid into acetate and finally into yeast biomass rich in protein. The study presents this as a Power to Protein pathway that ties together renewable electricity, CO₂ reduction and microbial protein production in a continuous chain.
Performance of the sandwich-structured gas diffusion electrode (GDE). (a) Schematic illustration of the sandwich-structured GDE. (b) Faradaic Efficiency (FE) of different GDE configurations. (c) Full-cell voltages of various GDE configurations. (d) Effect of PDDA–GO mass loading on FE. (e) Influence of PDDA molecular weight on FE. (f) Effect of PDDA-to-GO ratio on FE. The PDDA–GO mass loading was 1.5 mg cm−2 for (b, c) and 1.0 mg cm−2 for (e, f). Optimization was conducted through single-factor experiments. Unless specified, current density was 60 mA cm−2, PDDA molecular weight was 400 000–500 000, and PDDA-to-GO ratio was 1:1. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The electrochemical part centers on a redesigned gas diffusion electrode, the component that brings together gas, liquid and catalyst. In the new design, a layer of bismuth nanoparticles on carbon provides the catalytic surface. On top of this sits a thin interlayer made from a positively charged polymer, poly(diallyldimethylammonium chloride) or PDDA, mixed with graphene oxide, a conductive carbon material with negative charges. A final coating of Sustainion ionomer, another positively charged polymer that conducts anions, covers this structure and shields it from the strongly acidic electrolyte.
This three-part sandwich tunes the microscopic environment at the catalyst. Graphene oxide helps hold PDDA close to the bismuth surface. That local enrichment of positive charge raises the pH near the catalyst and makes hydrogen formation less favorable, so more current goes into turning CO₂ into formate, the ion form of formic acid. By adjusting the amount and composition of the PDDA graphene oxide interlayer, the study identifies operating conditions that balance selectivity and electrical resistance.
With this optimized gas diffusion electrode in a solid-state electrolyte reactor, the system reaches a Faradaic efficiency for formic acid of 63.6 % at 100 mA cm⁻². Faradaic efficiency is the fraction of the electric current that ends up in the desired product. At 200 mA cm⁻², the reactor produces formic acid at a concentration of 1.5 m in a single pass, with hydrogen as the only gaseous byproduct.
The reactor layout also departs from standard designs. Instead of a prefabricated anion exchange membrane, the solid-state electrolyte chamber is packed with a commercial cation exchange resin such as Amberlite IRC120, which stores and moves protons. A cation exchange membrane separates the resin from an acidic anolyte on the anode side. On the cathode side, the new gas diffusion electrode faces the resin, with a sheet of cellulose filter paper between them. During operation, CO₂ flows behind the gas diffusion electrode. At the catalyst surface it is reduced to formate ions, which move into the resin and combine with protons to form formic acid. Deionized water rinses the resin bed and carries the acid out of the reactor.
The filter paper plays an important role. It lengthens the path for proton transport, which helps maintain a more alkaline microenvironment at the electrode even though the bulk electrolyte is acidic. It also stabilizes the electrode mechanically and blocks carbon-based particles such as graphene oxide from entering the product stream. Nuclear magnetic resonance measurements show that the collected liquid contains pure formic acid with no detectable PDDA, indicating that the cationic polymer remains confined to the electrode.
Compared with a more conventional solid-state reactor that uses a commercial bismuth gas diffusion electrode and an anion exchange membrane, the membrane free device performs markedly better. The conventional cell reaches Faradaic efficiencies below 21 % for formic acid and current densities below 50 mA cm⁻² even at 15 V. The new reactor delivers an average outlet concentration of 950 mM and an average Faradaic efficiency of 42 % over 150 h at 100 mA cm⁻². Periodic sparging of the resin bed with inert gas restores performance without disassembly, and analysis finds no significant leaching of metal or carbon components into the product. A larger prototype with a 100 cm² active area supports currents above 10 A at 7 V and operates stably at 60 mA cm⁻² for 3 h.
To probe why the microenvironment design matters, the study uses in situ vibrational spectroscopies that monitor chemical bonds at the electrode surface during operation. Raman spectra at controlled electrode potentials show that the sandwich electrode maintains a more alkaline region near the catalyst, indicated by signals from carbonate (CO₃²⁻) rather than only bicarbonate (HCO₃⁻).
Surface enhanced infrared and Raman measurements reveal fingerprints of key intermediates such as adsorbed *OCHO groups, which are precursors to formate, and activated *CO₂⁻ species. These signals are strong for the sandwich electrode but weak for a simpler bismuth on carbon electrode under the same conditions, consistent with more effective CO₂ activation in the engineered structure.
The second part of the paper tests whether the formic acid stream from this reactor can support microbial protein production. The team builds a two-chamber bioreactor. One chamber contains Acetobacterium woodii, a bacterium that converts formate to acetate under anaerobic conditions. The other holds Saccharomyces cerevisiae, a wild type yeast used in food and brewing. An anion exchange membrane between the chambers allows acetate to diffuse from the first culture to the second.
In a typical run, formate in the Acetobacterium chamber starts at 118.3 mM and drops to 12.1 mM as the bacteria convert it into acetate. The acetate moves into the yeast chamber, where the optical density at 600 nm, a standard proxy for cell growth, rises to 0.65. Under these conditions the yeast uses acetate as its only carbon source, and the resulting biomass contains protein at up to 50 % (g g⁻¹). Isotope tracing with ¹³C labeled formate and ¹³C nuclear magnetic resonance confirms that carbon from CO₂ passes through formate and acetate into amino acids in the yeast.
Taken together, the electrochemical and biological data outline a full chain from CO₂ and electricity to microbial protein. The integrated process yields 0.02 g of yeast protein per g of formic acid consumed and reaches an overall carbon conversion efficiency of 6.3 %, compared with a theoretical single pass yield of 13 % estimated from literature values for each step. That gap points to scope for improving the biological stages, including handling of acetate and nutrient balance.
The Advanced Energy Materials study also examines how flexible the reactor concept is. When potassium ions replace the PDDA graphene oxide interlayer, the system still reduces CO₂ to formate, though with somewhat lower Faradaic efficiency. Substituting tin oxide for bismuth as the catalyst also preserves substantial activity. These tests indicate that the cation tuned sandwich gas diffusion electrode can work with different ions and catalyst materials.
One practical advantage highlighted in the cost analysis is that removing the prefabricated anion exchange membrane cuts materials costs sharply. In a typical solid-state device of this type, the membrane contributes more than 80 % of the bill of materials. The reported reactor instead relies on commercial ion exchange resins, a simple cation exchange membrane on the anode side and low-cost components such as filter paper. Together with the use of off the shelf bismuth nanoparticles, this shifts the design toward something that could be manufactured without specialized parts.
This work does not present an off the shelf technology. It does assemble, in one experimentally validated chain, the steps from CO₂ gas to a purified liquid intermediate and on to protein rich microbial biomass, using a reactor architecture that avoids some persistent cost and stability barriers. As solid-state electrolytes and microbial processes are refined, this kind of Power to Protein framework offers a concrete reference for turning captured carbon and renewable electricity into usable protein rather than waste.
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