The project started in 2009, when Li's wife, Fang Qian, staff scientist at Lawrence Livermore National Laboratory, saw potential for a groundbreaking collaboration. Li and Qian both specialize in devices that create hydrogen gas—Li's using the sun, and Qian's using bacteria.
Led by Li, the UCSC research team has coupled the two devices—a photoelectrochemical cell (PEC) and a microbial fuel cell (MFC)—to make one self-sustaining, water-treating, hydrogen-fuel-producing mega-device. We'll call the hybrid device the PEC-MFC.
The PEC captures sunlight, and uses its energy to kick-start a process known as electrolysis. During electrolysis, water is split into hydrogen and oxygen, and then released as gas. The MFC not only creates hydrogen gas, but also uses electrogenic bacteria to produce electrical energy. While splitting water to make hydrogen seems reasonable, Li recognized that California is in a drought and water regulation is tight, so they tweaked their initial approach.
"The idea is to replace the MFC 'solution' [pure water and cultured bacteria] with wastewater that contains these kinds of bacteria naturally," says Li.
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WHAT SMELLS? A sustainable-energy future, that’s what.
To ensure authenticity, researchers swapped the pure water solution with wastewater directly from the water treatment facility in Livermore, Calif. If you know someone who lives in Livermore, thank them—they may have unknowingly contributed to this research.
Though it may be counterintuitive, Livermore's dirty water is what makes the clean energy process sustainable. Human wastewater naturally contains organic material and electricity-generating bacteria.
Separately, the PEC and MFC both require an additional energy boost to drive full-fledged hydrogen production, and the added voltage is pricey. But putting them together creates a symbiotic solution: the bacteria in the MFC feed off of the organic matter in the wastewater, and during digestion, produce energy in the form of electrons. These electrons are channeled to the PEC, supplying the final electrical nudge needed to create the hydrogen fuell source. In that sense, the MFC acts as a battery, providing energy and enabling the PEC to continuously make hydrogen gas. Essentially, as long as wastewater is available to the bacteria in the MFC, the PEC bubbles out hydrogen at a fairly constant rate.
On the flip side, as the PEC splits water, the hydrogen is stored in a cylinder and doled out as useful energy for the MFC. Between the chemical fuel-kick from the PEC and the bacteria-generated electrical jolt from the MFC, the hybridized device can take care of itself. It's a self-perpetuating cycle that works to balance and propagate its counterpart.
But that's not to say Li and his team didn't experience their share of unique issues. Occasionally, and for no apparent reason, the PEC-MFC simply stopped working. Wang suspects that the different bacteria in each batch of wastewater may have caused the PEC-MFC to briefly misbehave—some pools may have more electrogenic bacteria than other, especially because the wastewater is allocated for a small-scale setup.
"Each batch of wastewater is different. Maybe the bacteria aren't happy that day, or are feeling lazy. They are live cells—they have their temper too," Wang jokes. "There wasn't a clear reason why some batches were better than others. So I'd try again, and it'd work."
With the overall striking success of the PEC-MFC in the lab, Wang and Li have big plans for future projects. Literally. The ultimate goal is to create a large-scale PEC-MFC that harnesses the same stinky raw materials and functions in the same self-perpetuating cycle—the only difference being the sheer quantity of hydrogen gas produced.
A PEC-MFC that's large enough to convert and pump hydrogen directly from a wastewater treatment facility is still just a schematic, but Li and Wang are actively taking steps toward achieving such a mega-device.