A photodiode can trigger bias-free redox reactions but is often hindered by thermodynamic barriers. Now, a bacteria-conjugated silicon biophotochemical diode allows simultaneous conversion of various carbon molecules with high efficacy.
Decarbonization aims to promote environmental sustainability and future energy security by mitigating the emission of greenhouse gases associated with the extraction and combustion of fossil fuels, as well as emissions from farming, thereby facilitating industrial transformation and agricultural advancement. The transition from finite fossil fuels to renewable alternatives, especially energy from sunlight, will be a critical component of global decarbonization efforts over the next decade.
Sunlight-powered reduction of carbon dioxide through photochemical approaches draws inspiration from the botanical principles of harnessing solar energy to convert water and carbon dioxide into oxygen and energy-storing organic molecules — a process known as artificial photosynthesis. Just as green plants use chlorophyll, artificial photosynthetic systems use materials such as semiconductors that are excited by light, creating photoinduced charge separation to carry out various chemical reactions. Direct overall water splitting in a photoelectrochemical cell using a single-crystalline semiconducting titanium dioxide electrode wired to a platinum electrode was demonstrated in 19721. A subsequent design of a sustainable practical device, based on a photodiode, gave rise to an artificial leaf2,3. An artificial leaf photodiode, comprising two complementary light-absorbing electrodes connected wirelessly via an ohmic contact, can, when immersed in an aqueous electrolyte and illuminated with light, drive both anodic (oxidation) and cathodic (reduction) reactions — thus functioning as an analogue of natural photosynthesis2. A photochemical diode can be configured to conduct conversion of solar energy to fuels and the valorization of industrial waste to value-added chemicals without the need for an applied bias voltage4.
Next, going beyond conventional approaches, the integration of semiconductor materials with biological machineries enables sophisticated photocatalytic systems that inherit intrinsic biological specificity, environmental sustainability, a low activation-energy barrier, enhanced efficiencies and synthetic versatility under ambient conditions. The biological entities used in these bionic hybrids might include photoresponsive molecules, such as tetrapyrrole complexes5, rhodopsin-rich microbial membranes6, as well as non-photosensitive enzymes, either isolated7 or contained within the entire microbial organism8. Enzymes naturally function using organic coenzymes as electrons and energy carriers.
Now, writing in Nature Catalysis, Kim and colleagues report a silicon nanowire artificial leaf that incorporates the biocatalytic machinery of the whole chemotrophic bacteria Sporomusa ovata (Fig. 1), thus merging semiconductor nanotechnology with microbial biotechnology9. Their approach details straightforward steps for fabricating a large artificial leaf, beginning with the synthesis of silicon nanowire arrays by etching two semiconductor wafers. The authors endowed two different silicon nanowire arrays with catalytic capacity by sputtering a platinum–gold (Pt–Au) nanocatalyst to produce an n-type photoanode, and by integrating the living bacteria to produce a p-type biophotocathode. The two photoelectrode arrays were assembled into a single photodiode device through a transparent ohmic contact, leading to a 15-cm-long artificial biophotonic leaf (Fig. 1a).
When the photodiode device was illuminated with low-intensity (20 milliwatts per square centimetre) red light with a wavelength of 740 nanometres in an aqueous environment, two parallel reactions occurred, one on each side of the leaf lamina. On the biophotocathode side of the silicon nanowire artificial leaf, the whole-cell bacterial Wood–Ljungdahl pathway was fuelled by photoinduced electrons as the reducing equivalents instead of NAD(P)H or ferredoxin (which are typical for bacteria). This enables the photocatalytic transformation of carbon dioxide into acetic acid (Fig. 1b). The researchers show that, with the aid of the photovoltage generated by a silicon nanowire, the whole-cell metabolic Wood–Ljungdahl pathway operates at more positive potentials than the thermodynamic potential for the reduction of two carbon dioxide molecules to one acetate molecule (0.123 volts compared to the reversible hydrogen electrode).
An interesting strategy presented here is to apply an adaptive laboratory evolution on S. ovata to dispel their metabolic fatigue, attaining a fivefold increase in catalytic efficiency of acetate production over 60 hours compared to the wild-type S. ovata on the silicon nanowire biophotocathode. In parallel, the photocathode side of the Pt–Au-loaded silicon nanowire leaf carried out a glycerol oxidation reaction, converting glycerol to glyceric acid, a value-added chemical (Fig. 1b). This silicon leaf achieved a bias-free photocurrent density of around 1.2 milliamperes per square centimetre under low-intensity red light with a Faradaic efficiency of about 80% for both cathodic and anodic products (Fig. 1c)9.
Overall, in their work, Kim and colleagues have demonstrated the implementation of the next generation of artificial photosynthesis, with living non-photosynthetic and adaptive bacteria integrated into a biohybrid electronic device a few inches long, powered by photon energy. Not only does the biophotonic silicon leaf supply photoexcited charges as a sustainable energy source for the cost-efficient microbial catalytic reduction of carbon dioxide, but it also simultaneously oxidizes glycerol, a typical waste from plant and animal feedstocks, thereby valorizing both carbon dioxide and biodiesel byproducts into useful platform molecules that could be used for the synthesis of fine organics.
Such biophotonic redox transformation could bolster the current multi-billion-dollar biomass valorization market10. Scaling this method to the industrial scale could be another way to promote global decarbonization, by reducing reliance on fossil fuels and lowering the carbon footprint linked with traditional petrochemical processes, thus catalysing the transition to a circular economy and ultimately aiming for net-zero carbon emissions. Moreover, the conversion of C1 to C2 (carbon dioxide to acetate) compounds and the valorization of C3 compounds (glycerol to glyceric acid) in the work of Kim and colleagues offers the possibility of conducting simultaneous cross-linking and modification of multi-carbon (Cn>3) compounds by utilizing free solar power in combination with microbial metabolism. One potential challenge for the bionic leaf could be the selective valorization of glycerol or Cn>3 compounds into more profitable and enantiomeric products (such as d/l-glyceric acid, antibiotics), thus mimicking living things while achieving decarbonization.
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Original title: Carbon Conversion on Biophotonic Leaf
Author: Jinhyeong Jang & Elena A. Rozhkova