Title: Extended Operational Lifetime of a Photosystem-Based Bioelectrode
Authors: Fangyuan Zhao, Adrian Ruff, Matthias Rogner,Wolfgang Schuhmann and Felipe Conzuelo F
Journal: Journal of American Chemical Society
Nature is the greatest inspiration for human beings in developing efficient chemical and biochemical processes artificially. There has been an ever-growing interest in mimicking photosynthesis artificially to harvest solar energy in recent times. In that context, the development of an artificial leaf by a team of researchers at MIT marked an important milestone in 2011 (Science, 2011, 332, 6025, 25). This leaf, which concsists of bacteria and inorganic molecule convrts carbon dioxide to liquid fuels with 10% efficiency (compared to 1% efficiency achived by the plants). While such an artificial system can offer solutions to several contemporary energy and transportation problems, e.g., an artificial solar energy harvesting device can charge electrical equipment, generate fuel for airplanes traveling long distance without stopping for fuels in between and cutting the total flight time.
Apart from these artificial photovoltaic devices, there has also been a recent interest in developing semi-artificial devices that incorporate biological enzymes and proteins along with synthetic molecules. This offers the advantage of higher efficiency and selectivity of naturally optimized biological molecules. In particular, photosystem I (PS I), the protein complex responsible for photosynthesis is a suitable protein to be incorporated in a biophotovoltaic device owing to its high stability after extraction from the leaves, efficient light harvesting (measured using a parameter known as quantum yield. A group of scientists at Ruhr University in Germany integrates PSI as a building block in a biophotovoltaic device without compromising its efficiency and operational lifetime by judicial choice of electron carrier molecules and more importantly .
One of the major challenges in making an efficient device is to prevent the neutralization of the negative (electron), and positive (hole) charges that are generated within a molecule after it absorbs sun light. This event of charge neutralization is also known as charge recombination. A photovoltaic device is great if it prevents the recombination as much as possible so that negative charge or the electron, known as photo electron, can be extracted and used to generate high photocurrent. Most efforts have been employed in improving the photocurrent efficiency in these devices, but not so much to increase the operational time. Most of the biopholtaic devices involving PSI reported till date are great when they are used for a small amount of time and then stored under proper condition to be used later. They lack continuous operational stability, and that limits their use commercially. They are only stable for 30 min to a few hours when in use continuously.
PSI gets excited by absorbing light to PSI* and transfers electrons in this excited state to a suitable electron acceptor. Conventionally methyl viologen (MV) is the molecules that are employed in accepting the electrons (primary electron scavenger) before transferring it to O2 molecules (terminal electron scavenger) in these devices (Scheme 1). However, this process generates a partially reduced oxygen species that are detrimental to the stability of PSI. Unlike Nature that has its own damage control mechanism to regenerate PSI within a very short time, these devices do not have such an ability. Hence O2 presents a serious problem to the stability of these kinds of bioelectrodes.
To prevent this issue, the researchers have embedded PSI within material support that consists of a polymeric osmium molecule (P-Os). P-Os is also responsible to replenish electron back to PSI+ from the electrode surface.
Scheme 1. Electron Transfer Pathway in PSI-Photoelectrode. (Figure made by Moumita Bhattacharya)
Now, MV cannot work efficiently in the absence of O2. Hence another set of molecules known as ubiquinones (Q) are employed as the substitute for MV. In this way this work suggests an alternative method of generating high photocurrent from PSI without using O2.
After choosing suitable components of the photovoltaic device, it is crucial to choose a potential at which the device will operate efficiently. We must remember more negative the operational potential, higher the operational cost will be. So, the more positive operational potential will be desirable.
The process of electron transfer is also known as redox process, and electrochemistry is the most useful analytical method to understand redox processes accurately.
Figure 1.Cycllic Voltammograms of molecular components in the biophotovoltaic cell. (Figure from the main article used with permission)
Figure 1. shows cyclic voltammograms of MV, P-Os, and Q. Assuming that the potential is scanned from negative to a positive direction, peaks facing upwards signify oxidation (giving up electrons) while those facing downwards indicate reduction (accepting electrons).
It can be qualitatively understood that at any potential more positive than – 0.4V MV will remain oxidized. Below -0.4V it will remain reduced. Since P-Os needs to accept an electron from the electrode to replenish the electron deficiency in PSI+ (Scheme 1), we need to apply a potential more negative than 0.4V. However, MV will be oxidized in such potential preventing the generation of photocurrent.
Figure 2. Schematic diagram of the reduction potential of important molecular components in the biophotovoltaic cell. (Figure made by Moumita Bhattacharya)
The good news is that Q has a higher reduction potential (0.6V vs. SCE) suggesting that at a potential lower than 0.4V, it will still remain reduced and hence able to retain the photo generated electron. Scheme 2 shows the respective reduction potentials of MV, P-Os, and Q. It also shows the redox states attained by each of them in various ranges of potential. To use MV effectively along with P-Os, we need to apply a very negative potential (theoretically below – 0.4V). However, by switching to Q, we can apply a potential just below 0.4 V (0.3V vs. SCE has been chosen in this work) to operate the device under study.
All these manipulations finally brought the operational time of the device up to 4 hrs with a loss of only 20% efficiency in the absence of O2. Hence this work shows a possible solution to overcome the long-term instability of biophotovoltaic devices by modifying the electron acceptors attached to the electrode material and can indeed be an inspiration for the future design of photovoltaic devices as well as organic solar cells.