A Better Solar Powered Hydrogen Generator

National Institute of Standards and Technology (NIST) researchers have new information on what may become a cost-effective way to generate hydrogen gas directly from water and sunlight.

The NIST team used a powerful combination of microanalytic techniques that simultaneously image photoelectric current and chemical reaction rates across a surface on a micrometer scale.  The target is a potentially efficient, cost-effective, photoelectrochemical (PEC) cell that’s essentially a solar cell that produces hydrogen gas instead of electric current.

Daniel Esposito, a NIST chemical engineer sets up the background with, “A major challenge with solar energy is dealing with solar intermittency. We demand energy constantly, but the sun’s not always going to be shining, so there’s an important need to convert solar energy into a form we can use when the sun’s not out. For large-scale energy storage or transportation, hydrogen has a lot of benefits.”

The paper exploring the team’s work has been published in Nature Materials.

A simple PEC cell contains a semiconducting photoelectrode that absorbs photons and converts them into energetic electrons that are used to facilitate chemical reactions that split water molecules into hydrogen and oxygen gases.

Water splitting is not easy. Esposito explained he best PEC cell was been demonstrated with efficiency around 12.5% in the 1990s by the U.S. Department of Energy’s National Renewable Energy Laboratory.  But, “it’s been estimated that such a cell would be extremely expensive – thousands of dollars per square meter – and they also had issues with stability,” he said.

One big problem is that the semiconductors used to achieve the best conversion efficiency also tend to be highly susceptible to corrosion by the cell’s water-based electrolyte. A PEC electrode that is efficient, stable and economical to be productive has been elusive.

The NIST team’s proposed solution is a silicon-based device using a metal-insulator-semiconductor (MIS) design that can overcome the efficiency/stability trade-off. The key is to deposit a very thin, but very uniform, layer of silicon dioxide – an insulator – on top of the semiconductor – silicon – that is well suited for doing the photon-gathering work.

On top of that is a polka-dot array of tiny electrodes consisting of platinum-covered titanium. The stable oxide layer protects the semiconductor from the electrolyte, but it’s thin enough and transparent enough that the photons will travel through it to the semiconductor, and the photo-generated electrons will “tunnel” in the opposite direction to reach the electrodes, where the platinum catalyzes the reaction that produces hydrogen.

The MIS device requires good production controls because the oxide layer in particular has to be deposited precisely.  Esposito notes that they used fabrication techniques that are standard in the electronics industry, which has decades of experience in building low-cost, silicon-based devices.

To study the system in detail, the NIST team scanned the surface of the device with a laser beam, illuminating only a small portion at a time to record photocurrent with micrometer resolution.  In tandem with the beam, they also tracked an “ultramicroelectrode” across the surface with scanning photocurrent microscopy (SPCM) and scanning electrochemical microscopy (SECM) to measure the rate of molecular hydrogen generation, the chemical half of the reaction.  The combination allowed them to observe two bonus effects of the MIS photoelectrode design: a secondary mechanism for hydrogen generation caused by the channeling of electrons through the oxide layer, and a more efficient transport of electrons to the reaction site than predicted.

The NIST team calculates an efficiency of 2.9 percent for their device, which also exhibits excellent stability during operation. While this efficiency is far lower than more costly designs, they note that it is 15 times better than previously reported results for similar silicon-based MIS devices, and the new data from their microanalysis of the system points towards several potential routes to improving performance.

While today’s 2.9% isn’t gong to revolutionize the hydrogen production business, the possible improvements should get to numbers that may make economic sense.

We’re a long way from consumers choosing their own solar powered energy or fuel system from a long list of choices.  Saving hydrogen for later use could make more sense than trying to buy electrical storage for storing photovoltaic energy.