Insights Into Water Splitting Reaction In Electrocatalytic And Biological Environment From First-Principles

The promise of safe, secure, and sustainable energy has been the focus of current society and is driving research across a wide spectrum of scientific disciplines. With the development of human society, on the one hand, the traditional fossil fuel is going to be exhausted gradually, on the other hand, the improper exploitation of fossil fuel causes severe environmental problems, such as green house effect and acid rain. To meet the increasing needs for energy, the research has paved the way to renewable energy, which includes hydrogen, solar energy, bioenergy, geothermal energy, wind energy and tidal energy. Among them, hydrogen emerges as the future energy, as it does no harm to environment and the sole product is water.Several methods could be utilized to produce hydrogen. Some of them have been industrialized, such as steam methane reforming, electrolysis and gasification. Some of them are still on the early period in lab, such as renewable liquid reforming, nuclear high-temperature electrolysis, high-temperature thermochemical water-splitting, photobiological microbes and photoelectrochemical systems. The electrolysis of water seems to be a very promising approach. But it is still limited by a bottleneck, which is caused by the high overpotential of oxygen evolution reaction at anode. Therefore, it drives the development of more efficient water splitting catalyst. To solve the problem, first we should get a comprehensive insight into the mechanism of water splitting reaction, then the design of new water splitting catalyst on the basis of mechanism. For electrolysis of water, the most frequently used catalyst is rutile-type catalyst, such as RuO2and IrO2. Because oxygen evolution reaction at the anode suffers from substantial energy loss owing to the high overpotential (-0.3V) on traditional catalysts (RuO2, IrO2and mixed oxides), huge efforts have been devoted to searching for better water splitting catalysts by means of experimental and theoretical approaches. Recently, a large number of bimetal water splitting catalysts has been synthesized and investigated by doping a second metal element into RuO2catalyst, including Ru-Ir, Ru-Co, Ru-Ni, Ru-Re, Ru-Pt, Ru-Cu, Ru-Ce, Ru-Pb, Ru-Cr, Ru-Fe and Ru-W. Among them, Co, Ni doped RuO2catalysts are shown to increase the activity while Ir doped RuO2catalyst lowers the activity but improves the stability. To improve the performance of water splitting catalyst systematically, it is essential to achieve a better understanding of water splitting reaction on the doped oxide systems, and identify the key parameters that are relevant to the activity.At the same time, the increased interest in direct conversion of solar energy into storable fuels has led to intensified research efforts with respect to artificial photosynthesis. A successful imitation of the natural energy-storing process requires the combination of several processes, including light harvesting, charge separation, electron transferring and water splitting. Whereas a close adaption to the biological process of photosynthesis would involve the transformation of carbon dioxide into a chemical compound of higher energy content, at present the generation of hydrogen by means of proton reduction seems to be a more realistic method for storing solar energy by artificial photosynthesis. The electrons and protons required for fuel generation should be obtained from water splitting. Not only the generation of hydrogen from water requires water splitting, for all substances that are typically considered as fuels, it holds that the utilization of chemically stored energy involves water splitting. For a better utilization, an understanding of the photosynthesis process is a must. The initial steps of photosynthesis take place in Photosystem Ⅱ (PSⅡ). The core part of PSII is a so-called oxygen evolution complex (OEC), which is an oxo-bridged cluster of four manganese and one calcium ions, like a distorted chair. Umena has solved the structure of OEC with1.9A resolution, laying a solid foundation for the research into the water splitting mechanism in PSⅡ.The study of binary rutile-type water splitting electrocatalysts from first principlesWe choose five different rutile-type binary metal oxides, namely, RuNiO2, RuCoO2, RuRhO2, RuIrO2and OsIrO2and their water splitting activities are assessed based on a simplified theoretical model. To allow for a fast screening of potential working catalyst, two key properties relevant to water splitting activity are focused on to assess the performance of different catalysts in this work,(i) the surface O coverage at the concerned potentials and (ii) the H2O activation kinetics on the O-covered surfaces. We found that on all the rutile-type binary metal oxide surfaces, the relevant O coverage (above1.23V) on the surface is1ML, with all the five-fold metal cation sites being terminated by O atoms. On the1ML O covered surface, the terminal O atom bonds most strongly on OsIrO2, and shows the similar bonding strength on RuO2, RuCoO2and RuNiO2. For the first step in water splitting, the activity sequence of the catalysts is RuNiO2, RuCoO2, RuO2, RuRhO2RuIrO2and OsIrO2. By plotting the free energy barrier against the differential adsorption energy of the terminal O atom, we found that the barriers for the group RuNiO2, RuCoO2and RuRhO2are obviously lower than the other group including RuO2, RuIrO2and OsIrO2. Fundamentally, we find that in addition to the terminal O on the surface, the lattice bridging O also plays an important role in affecting the water splitting activity. The presence of active bridge O atoms can help to reduce the water dissociation barrier.The study of mechanism of water splitting reaction in PSⅡ from first principlesIn this work, we make a theoretical attempt to getting knowledge of the water splitting mechanism in PSII with a combination of thermodynamic and kinetic tools based on first principles calculations. We also make an effort to illustrate the Kok cycle theory in PSII. Based on our results, we find that the most probable mechanism for water splitting reaction in PSII follows this pathway:the initial steps are driven by two holes. A water molecule which is coordinated to one of the four manganese atoms (sits at the position of the back of distorted chair) in the first coordination sphere environment deprotons and forms a terminal O atom on manganese atom. Then at the driving force of the third hole, a new water molecule attacks the terminal O atom and breaks it H-OH bond simultaneously. In the next step, the final hole attacks and the OOH species deproton, forming intermediate species with radical characteristics. Finally, the oxygen molecule dissociates and a new water molecule comes to fill the coordination. A cycle is completed. The rate-determining step lies in the dissociation of the second water molecule at terminal O atom. Based on our results, the key S2state in the Kok cycle is the intermediate which has a hydroxyl group linked to the fourth manganese atom. It is matched by experiments. The free energy is downhill from So state to S3state and rate-determining step lies in the process from S3state to S4state. The S4state involves several intermediates and one of them is a radical species, which is testified by the highly reactive S4state in experiment. In the sequential oxidation process of Kok cycle, the ligands for the cluster play a very important role. The substitute of calcium by strontium or manganese by titanium will leads to a poorer catalyst compared with the original OEC. Because the fourth manganese atom is very close to the reacting site, it is very significant in water splitting reaction. It not only offers support to the geometry but also provides support to electronic structure of the cluster. Towards the latter, the fourth manganese atom could stabilize the LUMO from the initial state to the transition state and localizes the LUMO, promoting the reach of transition state. All this verifies the high efficiency of water splitting reaction. Our research represents a trial to uncover the mechanism of water splitting reaction in PSII from an atomic-level and offers support for the rational design of water splitting photocatalysts in the future.