A nanosized, environmentally friendly hydrogen generator

Could produce hydrogen for cars and generators in the future; we meet reduced graphene oxide (rGO) in yet another radical role
September 23, 2014

Depiction of photocatalytic hydrogen evolution using platinum/titanium oxide (Pt/TiO2) interfaced with reduced graphic oxide (rGO) and photosensitive proton pump bacteriorhodopsin protein (bR) triggered by incoming light to generate electrons that convert H+ from water to H2 (hydrogen gas) (H2 (credit: Peng Wang et al./ACS Nano)

A small-scale “hydrogen generator” that uses light and a two-dimensional graphene platform to boost production of hydrogen has been developed by researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory.

While hydrogen is ubiquitous, it’s typically bonded with other elements, such as oxygen in H2O, where it must be separated to produce free hydrogen. The commercial separation process uses natural gas to react with superheated steam to strip away hydrogen atoms producing hydrogen fuel, which also generates carbon dioxide  — a greenhouse gas byproduct that escapes into the atmosphere.

Argonne’s early-stage generator, composed of many tiny assemblies, is proof that hydrogen can be produced without burning fossil fuels. The experiment was limited to nanoscale dimensions, but scaling this research up in the future may mean that we could one day replace the gasoline used in cars and generators with hydrogen — a greener option, because burning hydrogen fuel emits only water vapor.

“Many researchers are looking to inorganic materials for new sources of energy,” said Elena Rozhkova, chemist at Argonne’s Center for Nanoscale Materials, a DOE Office of Science (Office of Basic Energy Sciences) User Facility. “Our goal is to learn from the natural world and use its materials as building blocks for innovation.”

Photosensitive proton pump accesses some of the lost 96% of solar energy

TEM (transmission electron microscope) image of the Pt/TiO2-rGO. The white arrows point to the transparent, atomically thin rGO (reduced graphene oxide) sheets assembled on the surface of the Pt/TiO2 material. (Credit: Peng Wang et al./ACS Nano)

For Rozhkova, this particular building block is inspired by the function of an ancient protein called bacteriorhodopsin (bR), known to turn light into energy in single-celled organisms by absorbing sunlight and pumping protons (H+) through a membrane, creating a form of chemical energy.

It’s well known that water can be split into oxygen and hydrogen by combining these proteins with titanium dioxide (TiO2) and platinum (Pt) and then exposing the proteins to ultraviolet light.

However,  this process is limited because titanium dioxide only reacts in the presence of ultraviolet light, which makes up a mere four percent of the total solar spectrum.

So the researchers looked for a new material with enough surface area to move electrons across quickly and evenly, and boost the overall electron transfer efficiency. They also needed a platform on which biological components, like bR, could survive and connect with the titanium dioxide catalyst.

Graphene — actually the reduced graphene oxide (rGO) version (“reduced” refers to removing oxygen) — served both purposes. “It’s ultra-thin and biologically inert,” said Rozhkova,  allowing “the other components to self-assemble around it, which totally changes how the electrons move throughout our system.” That’s because rGO allows for interaction with positively charged molecules and can serve as nucleation centers for nanoparticle growth.

How the hydrogen generator works

1. The bR protein and the graphene platform  (see diagram above) sensitize the TiO2 photocatalyst to visible light.

2. Simultaneously, light from the green end of the solar spectrum triggers the bR protein to begin pumping protons (H+) from water along its membrane.

3. These protons make their way to the platinum nanoparticle co-catalyst that sit on top of the titanium dioxide.

4. The photoinduced electrons convert the H+ ions to H2 (hydrogen gas) by the interaction of the protons and electrons as they converge on the platinum co-catalyst.

5. Photoinduced holes (missing electrons) are scavenged by methanol (CH3OH), which converts to CO2 and water.

“The majority of the research out there states that graphene principally conducts and accepts electrons,” said Argonne postdoctoral researcher Peng Wang. “Our exploration using [electron paramagnetic resonance (EPR)] allowed us to prove, experimentally, that graphene also injects electrons into other materials.”

According to Rozhkova, the hydrogen generator proves that nanotechnology, merged with biology, can create new sources of clean energy and that the discovery may provide future consumers a biologically inspired alternative to gasoline.

This research appeared in the July 7 issue of ACS Nano. It was performed at the Center for Nanoscale Materials and supported by the U.S. Department of Energy’s Office of Science.

Abstract of ACS Nano paper

Photocatalytic production of clean hydrogen fuels using water and sunlight has attracted remarkable attention due to the increasing global energy demand. Natural and synthetic dyes can be utilized to sensitize semiconductors for solar energy transformation using visible light. In this study, reduced graphene oxide (rGO) and a membrane protein bacteriorhodopsin (bR) were employed as building modules to harness visible light by a Pt/TiO2 nanocatalyst. Introduction of the rGO boosts the nano-bio catalyst performance that results in hydrogen production rates of approximately 11.24 mmol of H2 (μmol protein)−1 h–1. Photoelectrochemical measurements show a 9-fold increase in photocurrent density when TiO2 electrodes were modified with rGO and bR. Electron paramagnetic resonance and transient absorption spectroscopy demonstrate an interfacial charge transfer from the photoexcited rGO to the semiconductor under visible light.