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Microbial Photoelectrosynthesis for Self-sustaining Hydrogen Generation Lu Lu, Nicholas B Williams, John Turner, Pin-Ching Maness, Jing Gu, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03644 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017
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Environmental Science & Technology
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Microbial Photoelectrosynthesis for Self-sustaining Hydrogen
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Generation
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Lu Lu, † Nicholas B. Williams, ‡ John Turner, § Pin-Ching Maness, § Jing Gu, ‡* Zhiyong
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Jason Ren †*
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†
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Boulder, Boulder, Colorado 80309, USA.
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‡
Department of Civil, Environmental, and Architectural Engineering, University of Colorado
Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile
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Drive, San Diego, California 92182, USA
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§
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Colorado 80401, USA.
National Renewable Energy Laboratory, Chemistry and Nanoscience Center, Golden,
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*Corresponding author e-mail:
[email protected],
[email protected] 15 16 17 18
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ABSTRACT
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Current artificial photosynthesis (APS) systems are promising for the storage of solar energy
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via transportable and storable fuels, but the anodic half-reaction of water oxidation is an
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energy intensive process which in many cases poorly couples with the cathodic half-reaction.
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Here we demonstrate a self-sustaining microbial photoelectrosynthesis (MPES) system that
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pairs microbial electrochemical oxidation with photoelectrochemical water reduction for
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energy efficient H2 generation. MPES reduces the overall energy requirements thereby
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greatly expanding the range of semiconductors that can be utilized in APS. Due to the
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recovery of chemical energy from waste organics by the mild microbial process and
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utilization of cost-effective and stable catalyst/electrode materials, our MPES system
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produced a stable current of 0.4 mA/cm2 for 24 h without any external bias and ~10 mA/cm2
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with a modest bias under one sun illumination. This system also showed other merits, such as
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creating benefits of wastewater treatment and facile preparation and scalability.
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TOC art
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INTRODUCTION
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Sunlight offers an inexhaustible and sustainable source of renewable energy to meet our
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increasing energy demand. Nowadays with the availability of low-cost solar panels, the direct
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harvesting of solar energy is becoming widespread, however powering society’s 24-hour
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demand is still challenging due to the variation of natural sunlight and its intermittent nature.
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Storage of solar energy into transportable and storable fuels, such as H2, through the direct
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photoelectrolysis of water at the interface of semiconductor and electrolyte is promising for
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commercial utilization of solar energy.1, 2 This artificial photosynthesis (APS) system has
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advantages of low cost, high efficiency as well as independence of arable land and a
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flexibility for tailored processes compared to natural photosynthesis.3, 4 However, the lack of
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a stable and efficient light-absorption semiconductor with suitable energetics remains a major
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challenge for water-splitting through the photoelectrochemical (PEC) approach. For example,
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semiconductors, such as Si, InP, and GaAs (with band gaps of 1.1-1.4 eV), do not produce
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sufficient voltage to drive the water-splitting reaction since the practical energy input needed
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for water-splitting ranges from 1.6 to 2.3 eV (1.23 eV being the thermodynamic potential)5
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(Figure 1b).Thus, an external electrical bias is usually applied to provide the extra
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overpotential required to drive the water splitting reaction. Additionally, in order for a
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single-gap semiconductor electrode to do photo-activated water splitting, the conduction band
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(Ecb) and valence band (Evb) edges of the semiconductor must straddle the water reduction
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and water oxidation potentials (Figure 1b).1 This requirement then excludes most of the
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common semiconductor materials, such as GaInP2 and GaP with ideal band-gap, from
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achieving self-sustaining water-splitting. As a result, there are only few semiconductor 3
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materials that could meet requirements with the right energetics and band gap, such as Cu2O
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and TiO2 (Figure 1b). Previous studies with p-Cu2O yielded high H2 evolution Faradaic
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efficiencies, but the material is unstable due to the band edge positions of Cu2O straddling the
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Cu(I) reduction and oxidation potentials.6 TiO2 has a large band-gap and requires activation
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under UV light, making it non-ideal for solar hydrogen production because UV light only
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accounts for
