The original demonstration leaf in 2011 had low efficiencies, converting less than 4.7% of sunlight into fuel. The team’s new analysis shows that efficiencies of 16% or more should now be possible using single-bandgap semiconductors, such as crystalline silicon, or 18% for gallium arsenide cells.

Such a system would use sunlight to produce a storable fuel, such as hydrogen, instead of electricity for immediate use. This fuel could then be used on demand to generate electricity through a fuel cell or other device. This process would liberate solar energy for use when the sun isn’t shining, and open up a host of potential new applications.

This article extends the construction of direct solar-to-fuels devices, such as the artificial leaf based on crystalline silicon. Because a single Si junction has insufficient potential to drive water splitting, it cannot be used for direct solar-to-fuels conversion. This paper performs an equivalent circuit analysis for multiple series-connected devices. The predictive utility of the model is demonstrated in the case of water oxidation at the surface of a Si solar cell, using a cobalt–borate oxygen evolving catalyst. Considering recent cost reductions of Si solar cells, this paper offers a path to the construction of low cost solar-to-fuels devices.

—Winkler et al.

Authors of the paper are MIT associate professor of mechanical engineering Tonio Buonassisi, former MIT professor Daniel Nocera (now at Harvard University), MIT postdoc Mark Winkler (now at IBM T. J. Watson Research Center) and former MIT graduate student Casandra Cox (now at Harvard).

The device combines two technologies: a standard silicon solar cell, which converts sunlight into electricity, and chemical catalysts applied to each side of the cell. Together, these would create an electrochemical device that uses an electric current to split atoms of hydrogen and oxygen from the water molecules surrounding them.

The goal is to produce an inexpensive, self-contained system that could be built from abundant materials. Nocera has long advocated such devices as a means of bringing electricity to billions of people, mostly in the developing world, who now have little or no access to it.

The key to obtaining high solar-to-fuel efficiencies is to combine the right solar cells and catalyst—a matchmaking activity best guided by a roadmap. The approach presented by the team allows for each component of the artificial leaf to be tested individually, then combined.

We have outlined a framework for integrating single-absorber solar cells as power sources for electrochemical processes and understanding the efficiency-limiting elements. The steady-state efficiency of coupled PV-EC systems depends on the individual efficiency of each system, but it also depends critically on the efficiency of coupling the two systems. When coupling the two systems directly, by performing each half-reaction on a terminal of the PV device, the coupling efficiency can be modeled using a steady-state equivalent circuit. Additionally, the efficiency of the coupled PV-EC system can be determined given the behavior of each subsystem. We validate this model by correctly predicting the J–V characteristics of a PV-assisted OER to within <10 mV. A key result of our analysis is that even when using commercially available Si solar cells, SFE over 15% is achievable provided the design yields very low solution resistance. We have proposed strategies for meeting this challenge.

—Winkler et al.

The voltage produced by a standard silicon solar cell, about 0.7 volts, is insufficient to power the water-splitting reaction, which needs more than 1.2 volts. One solution is to pair multiple solar cells in series. While this leads to some losses at the interface between the cells, it is a promising direction for the research, Buonassisi says.

An additional source of inefficiency is the water itself—the pathway that the electrons must traverse to complete the electrical circuit—which has resistance to the electrons, Buonassisi says. So another way to improve efficiency would be to lower that resistance, perhaps by reducing the distance that ions must travel through the liquid.

While the solution resistance is challenging, Cox says, there are “some tricks” that might help to reduce that resistance, such as reducing the distance between the two sides of the reaction by using interleaved plates.

James Barber, the Ernst Chain Professor of Biochemistry at Imperial College London, who was not connected with this work, says, “It is generally agreed that for an effective technology to emerge, the efficiency of the device must be 10 percent or more.” The MIT team’s work suggests such devices “can provide efficiencies as high as 15 percent. This level of energy conversion is considered very good and practical.”

Barber adds that a next step, demonstrating these improvements in a functioning device, is crucial: “It is very important to construct a working system which has a large surface area and operates with solar energy under open field conditions for a long period of time, as is done with the testing of solar cells.” If this can be achieved, he says, “the construction of robust and efficient solar-driven modules which produce hydrogen from water on a large industrial scale would have considerable impact on human society.”

The work was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology, and the Chesonis Family Foundation.

Resources

• Mark T. Winkler, Casandra R. Cox, Daniel G. Nocera, and Tonio Buonassisi (2013) Modeling integrated photovoltaic–electrochemical devices using steady-state equivalent circuits. PNAS doi: 10.1073/pnas.1301532110

• Steven Y. Reece, Jonathan A. Hamel, Kimberly Sung, Thomas D. Jarvi, Arthur J. Esswein, Joep J. H. Pijpers, and Daniel G. Nocera (2011) Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science doi: 10.1126/science.1209816