Researchers at Pacific Northwest National Laboratory (PNNL), with colleagues from Oregon State University, have developed PNNL a durable, inexpensive molybdenum-phosphide catalyst that efficiently converts wastewater and seawater into hydrogen. Details of the team’s study appear in the journal ACS Catalysis. The findings stem from a three-year project funded by the Department of Energy’s Fuel Cell Technologies Office.

The researchers tested a molybdenum-phosphide (MoP) catalyst with wastewater in a microbial electrolysis cell (MEC). Test results showed that MoP worked better than platinum, a precious and expensive metal typically used for its high catalytic performance. The MoP catalyst also produced hydrogen five times as fast as other non-platinum catalysts reported in related studies. The catalyst also worked well with seawater.

If you can produce hydrogen from seawater, the resource pool is pretty much unlimited.

—Yuyan Shao, corresponding author

Like seawater, the MoP catalyst material is widely available, and therefore, cheap. The catalyst also worked with wastewater, another ubiquitous resource.

In MECs, an electrical current is coupled with bacteria to decompose organics and make hydrogen. Unfortunately, the cells also use costly platinum for the reaction surface, and if non-platinum catalysts are used, hydrogen yields remain low.

At OSU, researchers developed a hybrid MEC design in which fermentation and electrolysis take place in a single pot as opposed to separate steps, and the byproducts are directly consumed in the process. This integrated design increases productivity and lowers equipment costs. But with the high cost of platinum, the team needed a catalyst that could lower production costs to around two dollars per kilogram of hydrogen.

Building on prior discoveries with the MoP catalyst, PNNL researchers investigated the catalyst for use in MECs. The research team started with the MoP combination because of its affinity for activating, or separating, water molecules. The catalyst is also tunable—the amount of each mineral can be adjusted. According to the team’s hypothesis, this tuning would optimize the amount of hydrogen produced in a single reaction.

They found that the catalyst assembled into a mixture of two distinct crystal phases: MoP and MoP₂. The atomic structure for each phase was different, leading to different reactions. While MoP₂ released hydrogen atoms from the water molecules, MoP converted the hydrogen atoms to hydrogen gas molecules. The two active sites boosted the overall reaction.

Colorized high-resolution TEM microscope image shows the catalyst’s two phases, MoP in purple, MoP₂ in yellow. Source: PNNL

We did not expect simultaneous formation of the two crystal phases. The two phases work way better than the single phase.

—Yuyan Shao

The researchers ran their experiments under neutral pH conditions in both the hybrid cell at OSU using wastewater and in another reactor at PNNL using seawater, with consistent results. Shao said these findings give the researchers confidence that the method is sound, eliminates platinum and other byproducts, and holds great promise for advancing hydrogen and fuel cell technologies.

In addition to DOE’s Fuel Cell Technologies Office, the materials characterization in this research was supported by DOE’s Office of Science, Office of Basic Energy Sciences, and PNNL’s Laboratory-Directed Research and Development Program.

Electron microscopy and X-ray photoelectron spectrometer analyses were performed at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Science and located at PNNL.

The OSU and PNNL team will soon be joined by researchers from Texas A&M University on a new project which builds on their research for hydrogen production with MEC. The authors are filing a patent.

Resources

• Xiaohong Xie, Miao Song, Luguang Wang, Mark H. Engelhard, Langli Luo, Andrew Miller, Yayun Zhang, Lei Du, Huilin Pan, Zimin Nie, Yuanyuan Chu, Luis Estevez, Zidong Wei, Hong Liu, Chongmin Wang, Dongsheng Li, and Yuyan Shao (2019) “Electrocatalytic Hydrogen Evolution in Neutral pH Solutions: Dual-Phase Synergy” ACS Catalysis 9 (9), 8712-8718 doi: 10.1021/acscatal.9b02609