Hybrid photocatalysts transform CO2 into environmentally friendly solar fuels
Mimicking nature, artificial photosynthesis uses sunlight to convert CO2 and water into energy-giving and -releasing compounds. But instead of producing sugars, as with green plants, artificial photosynthesis can produce carbon monoxide (CO), methane (CH4), methanol (CH3OH) and hydrogen (H2) – of interest as green fuels. The HyMAP (Hybrid Materials for Artificial Photosynthesis) project, funded by the European Research Council (ERC) was set up to develop a new generation of hybrid organic-inorganic materials and devices to perform the chemical transformations necessary for artificial photosynthesis. This would open the door for the development of green alternatives to electrochemical storage electrodes for batteries. The team investigated photo(electro)catalysis, at different scales, from nanoscaled catalysts to pilot-plant reactors, creating novel photoactive hybrid materials. “Our results, especially those increasing yield, are at the forefront of knowledge in the field of CO2 conversion, marking a milestone for this research area,” says principal researcher Victor A de la Peña O’Shea from the IMDEA Energy Institute. Accordingly, scientific findings are being widely disseminated in high-profile journals. The project’s new family of organic semiconductors, made from conjugated porous polymers, have already been patented for solar fuel production.
Hybrid materials
HyMAP’s main objective was to develop multifunctional systems with improved capabilities to harvest light from the whole solar spectrum. To achieve this, the team explored hybrid photocatalysts, investigating several materials and approaches. The different strategies adopted were (i) band gap engineering of inorganic and (ii) organic semiconductors; (iii) as well as their related heterojunctions; (iv) metal-organic frameworks (MOFs) and (v) up-conversion (UC). The first four options can harvest ultraviolet along with the visible regions of the light spectrum, while UCs improve the harvesting of infrared wavelengths. Crucially, inorganic and organic semiconductors enhance charge generation and transfer, increasing the photocatalytic yield. “Combining different materials, with each specialising in the separate functions of photocatalytic reactions – principally of light absorption, charge separation and catalysis – improved overall efficiency,” explains de la Peña O’Shea. The reaction mechanisms of these materials were characterised in the laboratory using a variety of advanced in situ techniques, including near-ambient pressure X-ray photoelectron spectroscopy, X-ray diffraction and synchrotron radiation.
Solar reactor
As the team’s studies revealed the hybrid organic/inorganic semiconductor heterojunctions made from a conjugated porous polymer to be particularly highly performing, they designed a gas phase solar reactor. It comprised a solar reflector – a compound parabolic collector – that redirects all solar radiation received towards the reactor, and a tubular annular reactor made out of borosilicate glass, which is more resistant to high temperatures. This prototype reactor was demonstrated to successfully produce solar hydrogen from both water and biomass, as well as from other fuels or chemicals, such as CO, CH4 and CH3OH, using CO2 as a reagent. “These excellent results for solar fuel production have already led to a pilot plant, increasing our knowledge and allowing us to fine-tune processes before considering market opportunities,” says de la Peña O’Shea. “We need to broaden the use of these hybrid materials for other reactions, beyond photo- to include photo(electro)catalysis, for more sophisticated fuels and chemicals, such as ammonia, ethylene and dimethyl ether.”
Scaling up to meet new challenges
The HyMAP team has already started an associated ERC-funded proof of concept project NanoCPPs, to develop a proof of concept which scales up their nanostructured conjugated porous polymers. “This polymer’s nanostructure offers enhanced properties, opening the door to better performance,” he adds. A remaining challenge is to truly maximise the electronic properties of these systems so that the proposed environmentally friendly alternatives to current electrochemical storage electrodes for batteries can really be brought forward and made a reality.
Keywords
HyMAP, artificial photosynthesis, hydrogen, solar fuels, nano, photocatalytic, organic, inorganic, semiconductors, hybrid materials, green