École polytechnique fédérale de Lausanne in is a hotbed of solar research, and this week’s technology tour of Switzerland hooked CleanTechnica up with two of the experts — namely, Professor Michael Graetzel, head of the school’s Laboratory of Photonics and Interfaces, and power-to-gas specialist Professor Kevin Sivula, who heads up the Molecular Engineering of Optoelectronic Nanomaterials Lab. And yes, one or both of those names should ring a bell.
CleanTechnica is on the tech tour thanks to Presence Switzerland and swissnex. The tour included a stopover at École polytechnique fédérale de Lausanne (EPFL), and the opportunity to speak with Graetzel and Sivula.
The Inside Scoop On Dye-Sensitized & Perovskite Solar Cells
Michael Graetzel (or Grätzel) is the world’s leading expert on dye-sensitized solar cells (after all, he invented the stuff). His lab’s work on that topic crossed CleanTechnica’s radar back in 2011, when it seemed that the work would lead to “solar skyscrapers” and other building-integrated solar applications, as well as consumer products.
Sure enough, you can find dye-sensitized solar cells in many commercial applications today, including a showcase installation in Switzerland, where transparent solar panels form a facade for the new SwissTech Convention Center on the EPFL campus (shown in the image at the top of this article).
At our EPFL meeting, Professor Graetzel caught us up on a number of key points about dye-sensitized solar cells. For those of you new to the topic, these solar cells create an electrical charge without using a semiconductor. The secret sauce consists of nanoparticles of dye perched on particles of titanium oxide nanocrystals. The process mimics chlorophyll, the key ingredient in natural photosynthesis.
As Graetzel describes it, plants use the charge they generate very quickly, which is why you don’t get an electric shock every time you touch a plant.
You can actually DIY a Graetzel cell if you have some hibiscus tea or blueberries at home, along with some other equipment.
Although dye-sensitized solar cells don’t boast the solar conversion efficiency of silicon, their flexibility of use and their colorful aesthetic appeal makes them ideal for architectural applications. Unlike silicon solar cells, they can operate efficiently in a vertical position.
Dye sensitized solar cells can also harvest indoor light more efficiently than silicon, as Graetzel demonstrated by holding his phone flashlight over a small thin film panel, which instantly revved up a miniature plastic windmill (you can see the blades spinning over another solar cell sample).
As for perovskite solar cells, Graetzel was brutally honest about the state of current technology.
Perovskites refers to a class of crystalline materials that are emerging as low-cost, easily fabricated alternatives to silicon. Unfortunately, the most promising platform — metal halide perovskite solar cells — uses lead, which is of course a toxic substance, so the race is on for substitutes.
We took note when researchers discovered a hint that tin-based perovskite solar cells could become just one such “coal-killing” alternative, but Graetzel explains that it has an irritating tendency to self-oxidize.
For now, Graetzel foresees that lead-based perovskite solar cells could find a market in centralized solar systems, where containment, site security, and regular maintenance could practically eliminate the risk of contamination. Those risks, however, would probably preclude the use of lead-based perovskite solar cells in distributed applications.
For much more detail, see Graetzel’s article, “The light and shade of perovskite solar cells” in Nature Materials.
A Power-To-Gas Primer
As Graetzel emphasized, all this is leading to solutions for the twin problems of an over-abundance of renewable energy, particularly solar, into the grid, along with the need to integrate enough storage capacity to make these intermittent forms more useful.
His colleague at EPFL, the aforementioned Professor Kevin Sivula, described the emerging solar-dependent grid model as an “energy gap” caused by the simple fact that solar peaks during the day, and in general electricity peaks at night.
That’s not a particularly difficult situation to address with today’s energy storage technology, because only a matter of hours are involved. However, when you throw seasonal and latitudinal variations into the mix, you need to project months-long energy storage requirements.
That dynamic is among the reasons why EPFL is one of a number of A-list research institutions turning attention to solar-powered hydrogen production, aka a “solar refinery” that can churn out hydrogen — essentially, a means of storing solar energy in a simple chemical bond.
The idea, as Sivula describes it, is that solar hydrogen would provide a stable solar energy storage platform for plastics manufacturing and the many other economic sectors that currently rely on fossil sources for foundational ingredients, in addition to enabling solar-based fuel for aircraft as well as ground vehicles.
In a chat after the presentation, Sivula emphasized to CleanTechnica that basic “solar hydrogen” technology is readily available. The problem, of course, is to make it readily available and economical at scale.
That is quite a challenge. Sivula provided a rundown of the state of photovoltaic science today, and arrived at a cost of about $10 per kilogram of hydrogen using conventional silicon photovoltaics to power electrolysis, the reaction that splits water into oxygen and hydrogen.
If that looks a little pricey, it is. According to Sivula, the price of hydrogen produced from natural gas using steam reformation is only in the $1.00 to $2.00 range. (Editor’s Note: Hence this article, and this one.)
That’s not the only force standing in the way of hydrogen from silicon-based photovoltaics. According to Sivula — and seconded by Graetzel — while silicon is dutifully pulling its fair share now, manufacturing silicon solar cells is a very energy-intensive process compared to other emerging solar technologies. Sivula further emphasized that the rate of growth for the silicon market would have to hit a much steeper upwards incline in order for solar hydrogen to fill the place of fossil fuels.
Speaking of those other technologies, Sivula foresees potential in less expensive forms of solar that can be used to generate hydrogen from water using a photoelectrochemical (PEC) reaction, rather than electricity from a solar cell.
Somewhat colorfully describing PEC as a more direct, “brute force” application of solar energy, Sivula also gave us a brief rundown on earlier PEC attempts, which typically involved the use of a single semiconductor.
One emerging solution is to use two semiconductors, one for the anode and one for the cathode, which not coincidentally is analogous to the two-system process of natural photosynthesis. However, under the best of conditions, the operation costs out to about $10.40 per kilogram of hydrogen.
That’s the bad news. The good news is that Sivula described another path to success. Similar to the Graetzel cell, this approach involves the use of nanoengineered iron oxide “paint” formed of suspended particles. Once applied, the organic materials in the suspension can be burned off, leaving the nanoparticles to do their work.
For (many) more details, check out Sulvia’s paper, “Emerging Semiconductor Materials for Direct Photoelectochemical Water Splitting,” which will be presented at the 228th meeting of the Electrochemical Society in the US this October.