Making hydrogen fuel from water and visible light at 100 times higher efficiency

A big step closer to hydrogen as a practical fuel to power vehicles and electrical devices
August 23, 2015

Test unit schematic for temperature-induced photocatalytic hydrogen production from H2O with methanol as a sacrificial agent: (1) thermocouple (temperature sensor), (2) black Pt/TiO2 on SiO2 substrate, (3) quartz wool, (4) quartz tube reactor, (5) electrical tube furnace; (GC) gas chromatograph (analyzes gas components) (credit: Bing Han and Yun Hang Hu/Journal of Physical Chemistry)

Researchers at Michigan Technological University have found a way to convert light to hydrogen fuel more efficiently — a big step closer to mimicking photosynthesis.

Current methods for creating hydrogen fuel are based on using electrodes made from titanium dioxide (TiO2), which acts as a catalyst to stimulate the light–>water–>hydrogen chemical reaction. This works great with ultraviolet (UV) light, but UV comprises only about 4% of the total solar energy, making the overall process highly inefficient.*

The ideal would be to use visible light, since it constitutes about 45 percent of solar energy. Now two Michigan Tech scientists — Yun Hang Hu, the Charles and Carroll McArthur professor of Materials Science and Engineer, and his PhD student, Bing Han — have developed a way to do exactly that.

They report in Journal of Physical Chemistry that by absorbing the entire visible light spectrum, they have increased the yield and energy efficiency of creating hydrogen fuel by up to two magnitudes (100 times) greater than previously reported.**

As described in the paper, they used three new techniques to achieve that:

  • “Black titanium dioxide” (with 1 percent platinum) on a silicon dioxide substrate;
  • A “light-diffuse-reflected surface” to trap light;
  • An elevated reaction temperature (280 degrees Celsius).

In addition, the new setup is “convenient for scaling up commercially,” said Ho.

* TiO2 has a relatively large band gap energy (3.0−3.2 eV) and thus it can absorb only ultraviolet (UV) light (about 4% of the total solar energy), leading to a low photoconversion efficiency (less than 2% under AM 1.5 global sunlight illumination).

** The new method achieves a photo hydrogen yield of 497 mmol/h/g and an apparent quantum efficiency of 65.7% for the entire visible light range at 280 °C.

Abstract of Highly Efficient Temperature-Induced Visible Light Photocatalytic Hydrogen Production from Water

Intensive effort has led to numerous breakthroughs for photoprocesses. So far, however, energy conversion efficiency for the visible-light photocatalytic splitting of water is still very low. In this paper, we demonstrate (1) surface-diffuse-reflected-light can be 2 orders of magnitude more efficient than incident light for photocatalysis, (2) the inefficiency of absorbed visible light for the photocatalytic H2 production from water with a sacrificial agent is due to its kinetic limitation, and (3) the dispersion of black Pt/TiO2 catalyst on the light-diffuse-reflection-surface of a SiO2 substrate provides a possibility for exploiting a temperature higher than H2O boiling point to overcome the kinetic limitation of visible light photocatalytic hydrogen production. Those findings create a novel temperature-induced visible light photocatalytic H2production from water steam with a sacrificial agent, which exhibits a high photohydrogen yield of 497 mmol/h/gcat with a large apparent quantum efficiency (QE) of 65.7% for entire visible light range at 280 °C. The QE and yield are one and 2 orders of magnitude larger than most reported results, respectively.