Solar cell that can directly convert light into fuel

| Hans Wolkers

University of Twente scientists Annemarie Huijser and Kaijian Zhu are developing a new type of solar panel that converts solar energy directly into renewable fuel. This direct conversion has the potential to become more cost-effective than the combination of a conventional solar cell and an electrolyzer.

Associate professor Huijser and PhD candidate Zhu are working on this new design as part of the Advanced Research Center Chemical Building Blocks Consortium. Together with a fellow PhD researcher Lisanne Einhaus, Kaijian Zhu aligns and calibrates their very recently upgraded laser setup that is aimed to shed light on a big unknown: the complicated reactions taking place in their new dye-based solar cells. The table is full of small lenses and mirrors, guiding the laser beam. Connecting electrical wires are everywhere, while sparse light illuminates their working space, creating a futuristic, yellowish atmosphere.

Zhu picks up a microscope glass, containing a reddish, photosensitive dye and places it in the laser beam. Protective laser goggles have to be worn: the powerful laser can easily burn through paper and is dangerous for the eye. ‘With this setup we can follow in real-time all individual photochemical reaction steps that take place after light hits our dye, the basis of our solar cell,’ Zhu explains. ‘We aim to directly convert the light energy into green fuel, while using CO2 from air as a feedstock. But there are still a lot of questions about what exactly happens after the light hits the dye and how we can control and optimize all these reactions.’

‘We aim to directly convert the light energy into green fuel, while using CO2 from air as a feedstock’

Eventually, the scientists aim to increase the efficiency of the reactions taking place, paving the way for a completely new type of solar cell that can directly convert light into fuel. The system could revolutionize the solar technology and effectively deal with the drawbacks of current solar panels.

Dark side

The sun is an almost inexhaustible source of energy. To harvest this energy, silicon-based solar panels are mostly used. These convert the solar energy into electricity with an efficiency of about 20 percent. However, these solar panels have a dark side and multiple problems are associated with their production, use and disposal. Their production requires the use of toxic chemicals, while lots of resources like very pure silicon and a variety of sometimes toxic metals are used. Panels that have reached the end of their lifespan, around 20-30 years, are difficult to recycle and pose a tremendous waste problem.

A second general issue with solar panels is their dependency on the weather and season: they only produce electricity with sufficient daylight, resulting in periods in which too much electricity is produced and periods with little production. To solve this problem, efficient storage during excess electricity production is required. Batteries come with their own environmental problems, while the generation of hydrogen from solar electricity is associated with significant losses. Converting solar electricity into hydrogen for storage, and back to electricity again, may result in as much as two thirds of efficiency loss.

‘A solar panel based on organic molecules like dyes that can convert solar energy directly into fuel does not have the disadvantages of conventional solar panels,’ Huijser says. ‘They are more environmentally friendly, while theoretically the conversion efficiency is much better.’ However, converting solar energy directly into fuel is a complicated process and the scientists lack a thorough understanding of how this works. 

CO2 into alcohol

Major breakthroughs in science often start with fundamental research at a molecular scale. To understand a certain process in detail is key to modify and improve it and solve a technological problem. Annemarie Huijser’s and Kaijian Zhu’s research to convert solar energy directly into fuel is no exception. One of the major problems they need to solve is the low efficiency of the fuel production by their dye-based solar cells, but the whole process still is a kind of black box. Some principles of the process are quite well understood however, and these are the starting points to solve the complicated puzzle piece by piece.

‘The dye absorbs light and this results in the generation of excited electrons’

‘The dye absorbs light and this results in the generation of excited electrons,’ Zhu explains. ‘These have more energy, and are normally the basis of electricity. But in our solar cells, they are not used to generate electricity, but to drive a chemical reaction where CO2 from air dissolved in water can be transformed into a fuel like alcohol.’ So, the more electrons involved in the chemical reaction, the higher the solar conversion efficiency.

Complicated task

Currently, the efficiency of the dye-based solar cell generating fuel is still low, below one percent. Zhu: ‘We think that this lack of efficiency is due to an electron deactivation process, in which the nickel oxide plays a crucial role.’ The scientists suspect that this happens at the surface of the nickel oxide: the electrons that are supposed to drive the chemical reaction might travel back to the nickel oxide. Especially in an aqueous environment, needed for the reaction to take place, this is a problem.

‘We believe that the problem lies at the nickel oxide surface that is in contact with water,’ Zhu explains. ‘At this surface the water dissociates, resulting in hydroxide (OH-) that will bind to the nickel oxide, forming nickel hydroxide. This thin nickel hydroxide layer attracts the electrons from the dye into the nickel oxide, so they can’t drive the fuel-forming reaction.’ With their upgraded laser setup, the scientists can follow these ultrafast electron transfer processes in real-time, at different experimental conditions. A highly complicated task that focusses at the interface of chemistry and physics.

Tremendous potential

So, for the scientists there is a big dilemma. On the one hand, they need water for the reactions to proceed, but at the same time water also seems to be the problem. To solve the impasse, there are several options that are currently investigated. The first solution is to block part of the nickel oxide surface, so less water is dissociated resulting in less OH-, to slow down the undesirable electron transfer from the dye into the nickel oxide. First experiments have indeed shown a 50 percent increase in efficiency.

‘We are also experimenting with replacing the nickel oxide by other materials based on copper. This resulted in a four to five times higher efficiency,’ says Zhu. ‘There are still major challenges to overcome that will take time and a lot of work. At some point, we really need a breakthrough. But the technology is really promising and has tremendous potential!’

The research is part of the general ambition of the PhotoCatalytic Synthesis group chaired by Prof. Guido Mul to develop solar to fuel devices, performed in collaboration with Marie Brands and Prof. Joost Reek of the UvA, and facilitated by the Advanced Research Center Chemical Building Blocks Consortium.  

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