Nanowire−Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals

Nanowire−Bacteria Hybrids for Unassisted Solar Carbon Dioxide

Fixation to Value-Added Chemicals

Chong Liu †⊥, Joseph J. Gallagher ‡, Kelsey K. Sakimoto †, Eva M. Nichols †, Christopher J. Chang *†‡§#, Michelle C. Y. Chang *†‡#, and Peidong Yang *†⊥∥%

†Department of Chemistry, ‡Department of Molecular and Cell Biology, §Howard Hughes Medical Institute, and ∥Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States

⊥Materials Sciences Division and #Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

% Kavli Energy NanoSciences Institute, Berkeley, California 94720, United States

Nano Lett., 2015, 15 (5), pp 3634–3639

DOI: 10.1021/acs.nanolett.5b01254

Publication Date (Web): April 7, 2015

Copyright © 2015 American Chemical Society

ABSTRACT: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b01254

In this paper, the authors describe a two-step method for artificial photosynthesis: harvesting the energy of light and its reducing power to reduce carbon dioxide into useful chemicals. The artificial photosynthesis uses a combination of a nanowire array for absorbing light interfaced with bacteria and their biocatalysts in their native cellular environments. The authors were motivated by the power of natural photosynthesis to create biomass from carbon dioxide and water and the lack of viable methods for photosynthetically harnessing the energy of the sun as a way to synthesize carbon-based compounds.  Natural photosynthetic capacity is estimated by the authors to be 130 TW annually producing 115 billion metric tons of biomass.

The silicon nanowire array is capable of efficiently absorbing and converting solar energy and provides the high surface area for bacterial integration.  The bacteria used is S. ovata, a strictly anaerobic “homoacetogen” that can metabolize carbon dioxide. It has been observed to reduce carbon dioxide to acetic acid in the presence of graphite electrodes.  To integrate the bacteria onto the nanowire, S. ovata was cultured directly within the Si nanowire array. See SEM pictures given in the article.

The proposed half-reaction of CO2 reduction is

The authors report a relatively high rate of reaction equivalent to about 1 x 10⁶ acetate molecules per second or 2 mol/m³/s under simple electrochemical conditions and no solar illumination (continuous  20% carbon dioxide/80% nitrogen sparging and less than 200 mV vs 0 V reversible hydrogen electrode).  This rate is comparable to gas phase catalysts at much higher temperatures.

These promising rates allowed to authors to construct a solar-driven artificial photosynthetic device for reducing carbon dioxide to the important biochemical feedstock acetic acid.  The photo-absorbing capacity and efficiency of the silicon titanium dioxide nanowires provide the thermodynamic driving force to reduce the CO2.

Without any other input, the set-up generated  a stable 0.3 mA / cm2 photocurrent under simulated sunlight and acetate formation was observed with a product selectivity close to 90%. The maximum photocurrent measured was 0.35 mA/cm2, corresponding to a 0.38% energy efficiency (conversion to acetic acid requires 1.08 V thermodynamically). Separate control experiments showed no acetic acid production in the absence of S. ovata bacteria.  Isotopic labeling indicates that the acetic acid came from the carbon dioxide.

Under aerobic conditions (21% oxygen), the researchers observed that only the nanowires coated with platinum resulted in acetic acid production but at a reduced efficiency of 70%.

Promising uses include:

1)      Scrubbing of carbon dioxide from exhaust gas or even open-air operations

2)      The acetic acid produced was used as a biochemical feedstock by genetically engineered E.coli to produce the important intermediate for biochemical synthesis, acetyl-CoA.  This makes viable the potential to build aerobic reactors using carbon-dioxide reducing “nano-biohybrids” using solar power and acetate consuming E. coli bacteria. In a test-of-concept experiment, the researchers were able to optimize the production of organic compounds:  “the yield of target molecules was as high as 26% for n-butanol, 25% for one of the isoprenoid compounds (amorphadiene), and up to 52% for PHB biopolymer”.  PHB biopolymer is a renewable and biodegradable plastic.