Solar-powered hydrogen generation using two of the most abundant elements on Earth
One potential clean energy future requires an economical, efficient, and relativelysimple way to generate copious amounts of hydrogen for use in fuel-cells and hydrogen-poweredvehicles. Often achieved by using electricity to split water molecules into hydrogen and oxygen, the ideal method would be to mine hydrogen from water using electricity generated directly from sunlight without the addition of any external power source.Hematite – the mineral form of iron – used in conjunction with silicon has shown some promise in this area,but low conversion efficiencies have slowed research. Now scientists havediscovered a way to make great improvements, giving hope to using twoof the most abundant elements on earth to efficiently produce hydrogen.
Hematite holds potential for use in low-powerphotoelectrochemical water splitting (where energy, inthe form of light, is the input and chemical energy is the output) to release hydrogen due to its low turn-on voltage of less than 0.3 voltswhen exposed to sunlight. Unfortunately, that voltage is too low to initiatewater-splitting so anumber of improvements to the surface of hematite have been sought to improve currentflow.
In this vein, researchersfrom Boston College, UC Berkeley, and China's University of Science andTechnology have hit upon the technique of "re-growing" the hematite, so that asmoother surface is obtained along with a higher energy yield. In fact, thisnew version has doubled the electrical output, and moved one step closer toenabling practical, large-scale energy-harvesting and hydrogen generation.
"Bysimply smoothing the surface characteristics of hematite, this close cousin ofrust can be improved to couple with silicon, which is derived from sand, toachieve complete water splitting for solar hydrogen generation," said BostonCollege associate professor of chemistry Dunwei Wang. "This unassisted watersplitting, which is very rare, does not require expensive or scarce resources."
Workingon previous work that realized gains in the photoelectrochemical turn-onvoltage from the use of smooth surface coatings, the team re-assessed thehematite surface structure by employing a synchrotron particle accelerator atthe Lawrence Berkeley National Laboratory. Concentrating on massaging the hematite'ssurface deficiencies to see if this would result in improvements, theresearchers used physical vapor deposition to layer hematite onto a borosilicateglass substrate and create a photoanode. They then baked the devices to produce athin, even film of iron oxide across their surfaces.
Subsequenttests of this new amalgam resulted in an immediate improvement in turn-onvoltage, and a substantial increase in photovoltage from 0.24 volts to 0.80volts. Whilst this new hydrogen harvesting process only realized an efficiency of0.91 percent, it is the very first time that the combination of hematite andamorphous silicon has been shown to produce any meaningful efficiencies ofconversion at all.
As aresult, this research has shown that progress has been made towards thepossibility of producing photoelectrochemicalenergy harvesting that is totally self-sufficient, uses abundantly availablematerials, and is easy to produce.
"Thisoffers new hope that efficient and inexpensive solar fuel production by readilyavailable natural resources is within reach," said Wang. "Getting there willcontribute to a sustainable future powered by renewable energy."
The results of this research were recently published in the journal Angewandte Chemie International Edition