Ellis Davies reports on an artificial photosynthesis process that could produce clean energy.
Photosynthesis is a natural process with an attractive ability – turning something harmful into something useful. Researchers at the University of Central Florida, USA, have found a way to harness this artificially, turning greenhouse gases into energy. The team found a way to trigger the photosynthesis reaction in metal-organic frameworks (MOFs), but instead of producing food or oxygen like a plant, it has the potential to create solar fuel from CO2.
MOFs are a class of crystals that combine metal oxides with organic molecules (see Materials World, October 2016, page 61). They have been used in previous studies exploring their gas storage properties with hydrogen. The MOFs resemble honeycomb structures, containing walls and pores that trap the CO2 molecules. Dr Fernando Uribe-Romo, Assistant Professor of Chemistry at the University of Central Florida, told Materials World, ‘The MOF is able to absorb the energy of light and transfer it to CO2, facilitating its transformation to a higher energy form of carbon – in this case formate.’
The researchers added the MOFs to titanium, where they act as light-harvesting antennas. Titanium itself can reduce CO2, but requires high-energy ultraviolet (UV) light to do so. Sunlight is made up of just 4% UV light, which makes this process inefficient on its own. ‘In our case, we used a titanium MOF that can absorb specific colours of light, thus increasing the efficiency. Moreover, titanium is a very cheap and abundant element on Earth, so cheap that it is utilised as the most common white pigment, costing something between US$3–4 per kilogram. This is important to mention, because there are other materials that produce solar fuel from CO2, but they require the use of very expensive and rare metals, such as platinum. Currently one kilogram of platinum can cost around US$30,000,’ Uribe-Romo explained.
The team aims to use the material to absorb all types of light from the sun, from high-energy UV to low-energy red and infrared. The material can currently only absorb blue light, which sits between violet and green in terms of wavelength. Being able to absorb green, yellow, orange, red and infrared will allow for the production of solar fuel with high efficiency and fast rates, the team says.
In testing, CO2 was bubbled into the titanium MOF, with the addition of a solvent the team called a ‘sacrificial electron donor’. Blue LEDS were then shone onto the material. ‘During illumination we sampled the reaction mixture to study the chemicals that were formed and observed the presence of formate and derivatives of formate, evidencing the transformation of the CO2 into the solar fuel. We measured this using a technique called nuclear magnetic resonance, an instrument similar to a hospital MRI, and we were able to determine how fast and efficient this CO2 transformation was,’ said Uribe-Romo. The process currently has an efficiency of 1.8%, and the team is aiming for an overall efficiency of 2–3%.
Uribe-Romo mentioned the potential use of this technology in conjunction with power stations as a way of negating some of their environmental effects. ‘The idea of incorporating it into a power plant comes from the fact that power plants – after burning the fuel – purify the CO2 using scrubbing technology, only to be released into the atmosphere. If our materials are economically competitive, they could be implemented so that after scrubbing the emitted gas from the plant, instead of being released, it could be treated with our materials to produce solar fuel. This fuel can be used at the power plant, ultimately reducing the carbon footprint and increasing energy efficiency,’ he said. The material also has potential application in other chemical transformations such as synthesis of pharmaceuticals and agrochemicals.
Working on greater efficiency will be the researchers’ goal moving forward, as well as expanding the material’s capability to absorb all colours of sunlight. A more effective material could lead towards a new way of producing clean energy.