Splitting water to release hydrogen for a future zero-carbon energy economy makes perfect sense unless you take into account just how precious a resource is pure water. Fortunately, a team from Stanford University are working on a way to use the much more abundant resource of seawater instead. [Kuang, Y. et al. Proc Natl Acad Sci (2019) 201900556 DOI: 10.1073/pnas.1900556116]
Theoretically, to power cities and cars, "you need so much hydrogen it is not conceivable to use purified water," explains team member Hongjie Dai, "We barely have enough water for our current needs in California." Nevertheless, hydrogen is an appealing alternative to fossil fuels as it avoids the notorious carbon emissions. Using hydrogen in a fuel cell or even burning it produces only water as a byproduct. Dai's team has now demonstrated a proof-of-concept that avoids the problem of electrode and system corrosion when salt water is used in electrolysis. Fundamentally, the team has found that they by coating the anode with layers that have a net negative charge, they could repel the corrosive chloride ions and slow the decay of the underlying electrode metal.
The team layered nickel-iron hydroxide on top of nickel sulfide, which covers a nickel foam core. The nickel foam acts as a conductor while the nickel-iron hydroxide sparks the electrolysis, separating water into oxygen and hydrogen. During electrolysis, the nickel sulfide evolves into a negatively charged layer that protects the anode.
The team showed that the same electrolytic cell failed after twelve hours using seawater, the anode simply crumbles and falls apart. With the coating in place, it can work for more than a thousand hours. The approach avoids the need to run at a lower current, which was previously the only way to slow anode decay in seawater electrolysis. The team's coated electrode will operate at 10 times the usual "electrode-safe" current and so generates hydrogen from seawater much faster than any other approach.
The lab tests were conclusive, but the team has also successfully demonstrated a system closer to a real-world application that uses a current from a solar cell and actual sweater from the San Francisco Bay. The system runs at industry standard currents for water electrolysis. In terms of transferring the technology, "One could just use these elements in existing electrolyzer systems and that could be pretty quick," Dai explains. "It's not like starting from zero - it's more like starting from 80 or 90 percent."