A research team led by Yat Li, associate professor of chemistry at the University of California, Santa Cruz, developed the solar-microbial a hybrid device that combines a microbial fuel cell (MFC) and a type of solar cell called a photoelectrochemical cell (PEC).
In the MFC component, bacteria degrade organic matter in the wastewater, generating electricity in the process. The biologically generated electricity is delivered to the PEC component to assist the solar-powered splitting of water (electrolysis) that generates hydrogen and oxygen.
Either a PEC or MFC device can be used alone to produce hydrogen gas. Both, however, require a small additional voltage (an external bias) to overcome the thermodynamic energy barrier for proton reduction into hydrogen gas.
The hybrid solar-microbial device is self-driven and self-sustained, because the combined energy from the organic matter (harvested by the MFC) and sunlight (captured by the PEC) is sufficient to drive electrolysis of water.
In effect, the MFC component can be regarded as a self-sustained bio-battery that provides extra voltage and energy to the PEC for hydrogen gas generation. The only energy sources are wastewater and sunlight, Li said.
The successful demonstration of such a self-biased, sustainable microbial device for hydrogen generation could provide a new solution that can simultaneously address the need for wastewater treatment and the increasing demand for clean energy, Li said.
Microbial fuel cells rely on unusual bacteria, known as electrogenic bacteria, that are able to generate electricity by transferring metabolically-generated electrons across their cell membranes to an external electrode.
Li's group collaborated with researchers at Lawrence Livermore National Laboratory (LLNL) who have been studying electrogenic bacteria and working to enhance MFC performance.
Initial proof-of-concept tests of the solar-microbial (PEC-MFC) device used a well-studied strain of electrogenic bacteria grown in the lab on artificial growth medium.
Subsequent tests used untreated municipal wastewater from the Livermore Water Reclamation Plant. The wastewater contained both rich organic nutrients and a diverse mix of microbes that feed on those nutrients, including naturally occurring strains of electrogenic bacteria.