The tendency for sensationalism in science reporting is a problem. Phrases in a peer-reviewed article that say “this discovery could lead to applications such as x, y, and z” undergo a sensationalist spin when it’s reported that scientists have “discovered a cure for cancer,” “found THE cause of schizophrenia,” or “increased solar cell efficiency by 50%!” Sometimes the reporter facilitates the translation. Other times it is the researcher. The unfortunate result of this type of reporting is desensitization and, even worse, an increased skepticism of scientific claims. When a really important discovery comes along it is appropriately met with “AGAIN? Really?” and “well, then where is my flying car?” For the sake of maintaining the public’s trust and support, scientists should do what they can to avoid sensationalism. To avoid sensationalism in the area of solar fuels research we should be more thoughtful and critical about the use of the term “artificial leaf.”
A leaf in nature uses the energy in sunlight to split water and convert carbon dioxide into energy-rich sugars, adenosine triphosphate, and other organic molecules in a process we know as photosynthesis. This complex process contains a number of stepwise events involving geometrically organized proteins and small molecules located in the chloroplast. I am not going to discuss the individual steps but I encourage everyone to read up on this incredible machinery. The question I now pose is this: how close to natural photosynthesis does a solar fuel cell have to be for us to reasonably consider it an artificial leaf? Is it enough that a device absorbs sunlight and makes chemical bonds? If that is the case then a bond forming reaction driven by a solar-powered hotplate could be considered an artificial leaf. Is it defined by the chemical bonds that are formed or is a well-defined molecular geometry for the photon absorption and electron transfer events sufficient? An official line between artificial photosynthesis and solar powered chemistry has not yet been drawn. I do not set out to define that line here but I do want to call attention to the differences in recent “artificial leaf” devices and describe how they fall short of their aspiring names while simultaneously indicating that a true artificial leaf may be imminent.
One “artificial leaf” receiving buzz at the moment is being publicized by MIT professor Daniel Nocera. The catalytic portion of this artificial leaf has a cobalt phosphate thin-film anode and a yet to be published nickel cathode. When these electrodes are submerged in a pH 7 phosphate-buffered aqueous solution and a potential of 1.3 V vs NHE is applied, water is catalytically oxidized at the anode to give O2. The remaining protons are reduced at the cathode to give hydrogen. The hydrogen and oxygen can then be used for hydrogen fuel cells.
The applied potential to run the catalysts can come from any source: a water-wheel, a wind turbine, a person riding a bicycle equipped with a generator or any other device that generates electricity. In the case of Nocera’s artificial leaf the potential is created by hooking the electrocatalytic device described above to a silicon solar cell. By describing the device in this way I try to emphasize why I dislike its designation as an “artificial leaf.” It is only an artificial leaf in the most superficial sense in that it converts sunlight into molecular bonds. However, in a more logistical sense it is simply a solar cell (a more than 125 year old technology) attached by wires to a electrocatalytic cell (a more than 200 year old technology).
I am in no way trying to belittle the research of Professor Nocera but I am questioning the use of the descriptor “artificial leaf” rather than focusing on the device’s interesting and important materials. The water oxidation catalyst is not only composed of relatively inexpensive cobalt ions but it can also be electrodeposited on an electrode from a solution of Co2+ and phosphate. The importance of electrodeposition is twofold: 1) it makes production of the catalyst relatively easy and 2) it offers a mechanism for self-repair of the catalytic material.
When the line is drawn a true artificial leaf should, at the very least, integrate the two devices into one operational component that generates charges (ideally on a molecular level) and delivers oxidative/reductive equivalents to the catalysts. A simplified, one component system may also reduce the production and operational costs of such a device. Professor Nocera, his collaborator Professor Buonassisi and their colleagues have taken one step closer to a true artificial leaf in their recent publication in the Proceedings of the National Academy of Sciences. In this paper they describe a device where they combined the two component system into one by depositing the electrocatalytic cobalt material directly onto a silicon solar cell. The device can be seen below (not pictured are the Ag/AgCl reference and the platinum counter electrodes):
In this device’s architecture a standard p-n junction silicon solar cell is coated on one side with metal contact and semi-transparent photoresist. The other side of the solar cell is coated with an indium tin oxide (ITO) layer and then the catalytic cobalt phosphate thin film. Under illumination the silicon absorbs photons to generate an exciton (a bound electron hole pair) which is separated at the p-n interface to give a free hole and an electron. The electron travels to the metal contact and then to the external circuit while the holes travel through the ITO layer to the cobalt film. The cobalt film then catalytically oxidizes H2O to O2.
When a silicon wafer is put under illumination in an aqueous solution an insulating layer of SiO2 will form that kills the photocurrent. The key to using a silicon solar cell in this architecture is to passivate the surface of silicon with photoresist and ITO so the silicon will not get oxidized. Using this strategy they created a device that can generate oxygen from water consistently for at least 6 days.
Although the device is predominantly driven by photon energy, a single silicon solar cell unfortunately does not have the driving force to oxidize water. An external applied potential was still necessary to generate O2. Despite this shortcoming, this is a great proof-of-concept device that, as the paper states, is “analogous to the wireless current in natural photosynthesis.” (It can be argued that the ITO in this case is acting as the wire but that is just more semantics). With further optimization, possibly involving a tandem solar cell architecture, I have no doubt we will see a fully functioning device within the next few years.
Although I do not think we have yet created a true “artificial leaf” and that we should be vigilant to avoid sensationalism, results like those described above as well as progress in other solar fuel strategies signal that man-made photosynthesis is on the horizon and a future powered by solar fuels is within our grasp.