In June I wrote a blog post titled “Artificial Leaf or Solar Powered Electrosynthesis?” about a photoelectrochemical cell (PEC) described in the Proceedings of the National Academy of Sciences (PNAS). The goal of this research was to create a PEC that can use sunlight to split water (H2O) into oxygen (O2) and hydrogen (H2). The stored energy in hydrogen can then be used to generate electricity via a hydrogen fuel cell.
In that post I described the device and how it operates. It essentially has a water oxidation catalyst on top of a p-n junction silicon solar cell. It was a step toward a solar driven device, but unfortunately the solar cell alone did not provide the force (>1.23 V) necessary to drive water oxidation and proton reduction. The cell had to be supplemented with an external power supply. The device marked a great step forward but not quite a standalone earth-abundant PEC. I concluded the post with the sentence “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.” While I was technically right in my timeline (< a few years) my estimate was clearly too pessimistic.
A follow up paper to the PNAS publication was published last week in Science. The article “Wireless Solar Water Splitting Using Silicon-based Semiconductors and Earth-Abundant Catalysts” by Daniel Nocera and the team at Sun Catalytix introduces a fully functional hydrogen and oxygen generating PEC. In fact two different architectures for the PEC were investigated and both can be seen below.
The article describes a triple junction amorphous silicon solar cell (3jn a-Si) as the light absorbing and charge separating component which creates the electricity necessary to run the electrolysis of H2O. The 3jn a-Si is composed of alternating layers of amorphous silicon and amorphous silicon-germanium alloy on a stainless steel back plate. Unlike the devices described in the previous post, the 3jn a-Si can produce > 2 V of driving force and thus it does not require a power supply to run catalysis.
Fluorine doped tin oxide (FTO) was deposited on the solar cell to prevent oxidation of the silicon. The cobalt phosphate oxygen evolving catalyst, discovered by Matthew Kanan and Daniel Nocera, was then electrodeposited on the FTO. Two different devices, based on the way the cell was completed, were created. In the wired configuration (a) the NiMoZn proton reduction catalyst, electrodeposited on nickel mesh, is connected to the steal back plate via a wire. In the wireless configuration (2) the NiMoZn catalyst is deposited directly on the steel back plate.
When the devices are submerged in water and hit with light, H2O is photocatalytically split into O2 and H2 without the assistance of an external power supply. The device’s efficiency – the stored energy created during H2 and O2 bond formation per the solar energy that is absorbed – was 2.5% and 4.7% for the wireless and wired cell, respectively. The efficiency of catalysis is primarily limited by the efficiency of the solar cell which were 6.2% and 7.7% in the wireless and wired devices, respectively. This means that even if catalysis is 100% efficient these are the maximum device efficiencies that can be reached. With a more efficient solar cell, the device performance will no doubt improve.
The use of a tandem solar cell to run water oxidation is by no means a new strategy. Almost the exact same wired device architecture shown above was published more than a decade ago by Kaselev and Turner (below).
The key differences are that the previous system 1) used platinum as the catalyst and 2) it was in a strongly basic solution (pH~14) which can lead to corrosion and thus a short device lifetime. The platinum containing device had a significantly higher efficiency (7.8 % from a 9% solar cell). While the efficiency of the recently published system is lower, it is important because it is made with relatively abundant and inexpensive materials (Zn, Ni, Co…) and can operate at a lower base concentration (pH~10). The use of earth-abundant materials hints of a future where such devices are an economically feasible solution to our energy needs.
The pursuit and publication of these wireless and wired devices is less interesting from a chemical perspective, yet quite interesting in an economic and engineering sense. I am not versed in the cost-benefit analysis of these strategies but I know that each has their advantages and disadvantages. For example, the wireless architecture is potentially easier to manufacture and could possibly be adapted as photocatalytic nanoparticles in solution. On the other hand, both H2 and O2 are generated in the same compartment and must be separated for use in a fuel cell. This is not an issue in the wired configuration where each electrode can be operated in its own compartment. The efficiency of the wireless configuration is also lower due to slow transport of protons from one side of the electrode to the other. These are all issues that may be solvable with the right engineering and manufacturing strategies.
What is it going to take for me to have one of these devices at my home? Ignoring the hydrogen storage and fuel cell aspect of the equation, there are a few notable issues about these PECs that need to first be addressed before they can populate our daily lives. First, the efficiency and lifetime of the device needs to be improved. Increases in efficiency and lifetime are likely with better manufacturing techniques. Regardless of how well you put the current components together, manufacturing techniques will only increase the efficiency by a moderate amount because it is limited by the efficiency of the solar cell. Which leads us to the crux of the problem: the price. The complex structure of a multijunction tandem solar cell greatly increases the cost of these solar cells compared to single junction cells. Unless you plan on sending the cells to Mars, it is unlikely that your return on investment will be worth the purchase. The current high cost of this architecture is likely inhibitory for mass production and release to the general public, especially to supply power to the “non-legacy world”. Increases in amorphous silicon tandem solar cells in conjunction with more efficient and inexpensive manufacturing techniques are likely required.
A side note: I really appreciated that this paper explicitly noted that the reported results were for their highest performing cells. It is commonly accepted that many of the devices we make in lab do not perform very well or at all. This makes sense being that every device we make is a prototype often put together by hand. The inconstancies in these manual manipulations often result in a wide range of measured results. In basic research settings were new devices are developed and tested the highest performing cells are known as “Hero devices.” The distinction of “hero devices” from the rest often goes unnoted since most scientists in the field recognize this practice and understand that, with further optimization of manufacturing techniques, we should be able to get at least that much efficiency.