Novel solar-microbial device harnesses sun and sewage to produce hydrogen

Collaboration between researchers at the University of California Santa Cruz (UCSC) and researchers at Lawrence Livermore National Laboratory (LLNL) resulted in development of a solar-microbial device that produces hydrogen gas and electricity using sunlight and wastewater as the only energy sources. By combining solar and microbial technology, it provides a sustainable energy source while improving the efficiency of wastewater treatment.

The device is composed of microbial fuel cell (MFC) and a type of solar cell called photoelectrochemical cell (PEC). The MFC is a bioelectrochemical system filled with electrogenic bacteria that are capable of moving electrons to solid phase materials including electrodes. Bacteria oxidize organic matter in the wastewater and then transfer the electrons to an electrode and generate electrical energy.

The MFC is a self-sustained bio-battery that provides extra voltage and energy to the PEC. The generated electricity is delivered to the PEC component to support the solar-powered splitting of water (electrolysis) that generates hydrogen and oxygen.

Both the PEC and MFC can be used alone in order to produce hydrogen gas. Unlike the MFC-PEC device that is self-sustained and self-driven, both PEC and MFC require a small additional voltage to overcome the thermodynamic energy barrier for proton reduction into hydrogen gas when used separately.

In the initial testing of the MFC-PEC device, the research team used a well-studied strain of electrogenic bacteria grown in the lab on artificial growth medium. They also used municipal wastewater which contains both rich organic nutrients and a diverse mix of microbes that feed on those nutrients, including naturally occurring strains of electrogenic bacteria.

According to the researchers, the MFC-PEC device showed continuous production of hydrogen gas at an average rate of 0.05 cubic meters per day. Simultaneously, the black wastewater became clearer. The chemical oxygen demand (COD) test, which is commonly used to indirectly measure the amount of organic compounds in water, showed decline in the amount of organic compounds by 67 percent over 48 hours.

Generation of hydrogen gas declined over time as the bacteria deplete the organic matter in wastewater, so researchers had to replenish wastewater in each feeding cycle.

“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”, said Yat Li, associate professor of chemistry at UCSC.

According to LLNL researcher Fang Qian, they are working to commercialize the MFC-PEC device. Their next goal is to scale up the device by making a 40-liter prototype which would be continuously fed with municipal wastewater. If results from the 40-liter prototype are promising, the research team will test the device on site at the wastewater treatment plant.

“The MFC will be integrated with the existing pipelines of the plant for continuous wastewater feeding, and the PEC will be set up outdoors to receive natural solar illumination”, said Qian.

For more information, read the paper published in the journal ASC Nano: “Self-biased solar-microbial device for sustainable hydrogen generation”.