As a society, we have seen a tremendous increase in sustainable technology over the last decade. From recycling, to LEDs, to LEED Certified buildings, and to battery-powered cars, clear progress has been made. Today, scientists continue to push boundaries on sustainable technology, shaping public policy and the future in the process.

One area of active research is sustainable solar-produced fuels. Researchers are developing artificial photosynthetic systems that are designed to replicate the natural process of photosynthesis, which harnesses solar energy to convert water and carbon dioxide into oxygen and sugars. These systems, both natural and synthetic, involve chemically converting water into oxygen gas and hydrogen gas.

Usually, our water-splitting processes rely on electrolysis—running electricity through water to trigger a reaction that splits it. In order to carry out this process using solar energy, systems require stable, light-absorbing electrodes. Unfortunately, the solution conditions required to carryout water electrolysis often cause electrodes to degrade, which has hampered progress toward developing efficient, stable artificial photosynthetic systems.

Though scientists have investigated materials to overcome these limitations, many of them have exhibited low efficiencies and/or limited stability. Recently, a team at Caltech has developed sputtered nickel-oxide films that can be used to coat electrodes, enabling stable, solar-driven oxidation of water. To fabricate these films, nickel was deposited onto substrates via reactive sputtering under high vacuum.

These films are optically transparent and antireflective, meaning the underlying electrodes would still be able to obtain sunlight. They're also conductive, stable, and highly catalytically active, which makes them a promising coating for light-activated electrodes.

Because metallic electrocatalysts typically absorb and/or reflect sunlight, there's usually a tradeoff between maximum catalytic activity and retaining optical transparency and low reflectance. The Caltech researchers found the level of oxygen and the temperature at which the sputtering process occurred influenced the coating conductivity and catalytic activity. After further characterization of the nickel oxide films, they chose to use 75nm thick layers of nickel oxide to optimize the antireflective properties in air.

After characterizing, and ultimately optimizing, the coating thickness, they tested the effectiveness of these coatings on various photoelectrodes that typically exhibited rapid surface oxidation in use, leading to electrode degradation. The team found that passing a current through the bare electrodes did not yield significant amounts of oxygen and only resulted in oxidation, which destroys the films in a fashion similar to the rusting of iron. In contrast, when these photoelectrodes were coated in nickel oxide film, they not only produced oxygen, but they also displayed stability under water-oxidation conditions with artificial sunlight for over 100 hours.

The researchers also found that the electric potential of these photoelectrode systems remained unchanged over the 100 hour period when exposed to 1.0 M aqueous potassium hydroxide and simulated sunlight; they exhibited decreased electric potential after 1,000 hours of continuous operation. Other, silicon-based photoelectrodes with a thin (2 nm) coating of nickel metal exhibited strong signs of oxidation within five rounds of experimentation that simulates a charge and discharge cycle; the oxidation of silicon completely destroyed these photoelectrodes.

These results suggest that these nickel oxide coatings may be the solution to the stability issues of photoelectrodes used for artificial photosynthetic systems. The increased oxygen production and enhanced photoelectrode stability attributed to these coatings suggest that nickel oxide coatings may enable such systems to become a real contender in the race for sustainable energy solutions.

PNAS, 2015. DOI: 10.1073/pnas.1423034112 (About DOIs).