Two decades of solar energy research, since the “Holy Grails” Account on Artificial Photosynthesis, has delivered astounding discovery that sets the stage for a paradigm shift from a fuels and ...chemicals industry powered by fossil fuels to one powered by the sun.
Proton-coupled electron transfer (PCET) underpins energy conversion in chemistry and biology. Four energy systems are described whose discoveries are based on PCET: the water splitting chemistry of ...the Artificial Leaf, the carbon fixation chemistry of the Bionic Leaf-C, the nitrogen fixation chemistry of the Bionic Leaf-N and the Coordination Chemistry Flow Battery (CCFB). Whereas the Artificial Leaf, Bionic Leaf-C, and Bionic Leaf-N require strong coupling between electron and proton to reduce energetic barriers to enable high energy efficiencies, the CCFB requires complete decoupling of the electron and proton so as to avoid parasitic energy-wasting reactions. The proper design of PCET in these systems facilitates their implementation in the areas of (i) centralized large scale grid storage of electricity and (ii) decentralized energy storage/conversion using only sunlight, air and any water source to produce fuel and food within a sustainable cycle for the biogenic elements of C, N and P.
The Artificial Leaf Nocera, Daniel G
Accounts of chemical research,
05/2012, Letnik:
45, Številka:
5
Journal Article
Recenzirano
To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and ...electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs. The holes of this wireless current are captured by the oxygen evolving complex (OEC) of photosystem II (PSII) to oxidize water to oxygen. The electrons and protons produced as a byproduct of the OEC reaction are captured by ferrodoxin of photosystem I. With the aid of ferrodoxin–NADP+ reductase, they are used to produce hydrogen in the form of NADPH. For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 sun (100 mW/cm2) illumination. This Account describes the construction of an artificial leaf comprising earth-abundant elements by interfacing a triple junction, amorphous silicon photovoltaic with hydrogen- and oxygen-evolving catalysts made from a ternary alloy (NiMoZn) and a cobalt–phosphate cluster (Co-OEC), respectively. The latter captures the structural and functional attributes of the PSII-OEC. Similar to the PSII-OEC, the Co-OEC self-assembles upon oxidation of an earth-abundant metal ion from 2+ to 3+, may operate in natural water at room temperature, and is self-healing. The Co-OEC also activates H2O by a proton-coupled electron transfer mechanism in which the Co-OEC is increased by four hole equivalents akin to the S-state pumping of the Kok cycle of PSII. X-ray absorption spectroscopy studies have established that the Co-OEC is a structural relative of Mn3CaO4–Mn cubane of the PSII-OEC, where Co replaces Mn and the cubane is extended in a corner-sharing, head-to-tail dimer. The ability to perform the oxygen-evolving reaction in water at neutral or near-neutral conditions has several consequences for the construction of the artificial leaf. The NiMoZn alloy may be used in place of Pt to generate hydrogen. To stabilize silicon in water, its surface is coated with a conducting metal oxide onto which the Co-OEC may be deposited. The net result is that immersing a triple-junction Si wafer coated with NiMoZn and Co-OEC in water and holding it up to sunlight can effect direct solar energy conversion via water splitting. By constructing a simple, stand-alone device composed of earth-abundant materials, the artificial leaf provides a means for an inexpensive and highly distributed solar-to-fuels system that employs low-cost systems engineering and manufacturing. Through this type of system, solar energy can become a viable energy supply to those in the non-legacy world.
Self‐sustained NiI/III cycles are established as a potentially general paradigm in photoredox Ni‐catalyzed carbon–heteroatom cross‐coupling reactions through a strategy that allows us to recapitulate ...photoredox‐like reactivity in the absence of light across a wide range of substrates in the amination, etherification, and esterification of aryl bromides, the latter of which has remained, hitherto, elusive under thermal Ni catalysis. Moreover, the accessibility of esterification in the absence of light is especially notable because previous mechanistic studies on this transformation under photoredox conditions have unanimously invoked energy‐transfer‐mediated pathways.
Turning the light off: Self‐sustained NiI/III cycles are established as a potentially general paradigm in photoredox cross‐coupling reactions. It is shown that photoredox‐like reactivity can be recapitulated with high fidelity in the complete absence of light across multiple substrates and transformations, thus obviating the need for high‐energy photons and precious‐metal photocatalysts.
Self-healing catalysis in water Costentin, Cyrille; Nocera, Daniel G.
Proceedings of the National Academy of Sciences,
12/2017, Letnik:
114, Številka:
51
Journal Article
Recenzirano
Odprti dostop
Principles for designing self-healing water-splitting catalysts are presented together with a formal kinetics model to account for the key chemical steps needed for self-healing. Self-healing may be ...realized if the catalysts are able to self-assemble at applied potentials less than that needed for catalyst turnover. Solution pH provides a convenient handle for controlling the potential of these two processes, as demonstrated for the cobalt phosphate (CoPi) water-splitting catalyst. For Co2+ ion that appears in solution due to leaching from the catalyst during turnover, a quantitative description for the kinetics of the redeposition of the ion during the self-healing process has been derived. The model reveals that OER activity of CoPi occurs with negligible film dissolution in neutral pH for typical cell geometries and buffer concentrations.
Personalized energy (PE) is a transformative idea that provides a new modality for the planet’s energy future. By providing solar energy to the individual, an energy supply becomes secure and ...available to people of both legacy and nonlegacy worlds and minimally contributes to an increase in the anthropogenic level of carbon dioxide. Because PE will be possible only if solar energy is available 24 h a day, 7 days a week, the key enabler for solar PE is an inexpensive storage mechanism. HY (Y = halide or OH−) splitting is a fuel-forming reaction of sufficient energy density for large-scale solar storage, but the reaction relies on chemical transformations that are not understood at the most basic science level. Critical among these are multielectron transfers that are proton-coupled and involve the activation of bonds in energy-poor substrates. The chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new hydrohalic acid- and water-splitting catalysts are delineated. The latter water-splitting catalyst captures many of the functional elements of photosynthesis. In doing so, a highly manufacturable and inexpensive method for solar PE storage has been discovered.
Conspectus Sunlight is an abundant energy source for a sustainable society. Indeed, photosynthetic organisms harness solar radiation to build the world around us by synthesizing energy-rich compounds ...from water and CO2. However, numerous energy conversion bottlenecks in the natural system limits the overall efficiency of photosynthesis; the most efficient plants do not exceed solar storage efficiencies of 1%. Artificial photosynthetic solar-to-fuels cycles may occur at higher intrinsic efficiencies, but they typically terminate at hydrogen, with no process installed to complete the cycle for carbon fixation. This limitation may be overcome by interfacing solar-driven water splitting to H2-oxidizing microorganisms. To this end, hybrid biological–inorganic constructs have been created to use sunlight, air, and water as the only starting materials to accomplish carbon fixation in the form of biomass and liquid fuels. This artificial photosynthetic cycle begins with the Artificial Leaf, which accomplishes the solar process of natural photosynthesisthe splitting of water to hydrogen and oxygen using sunlightunder ambient conditions. To create the Artificial Leaf, an oxygen evolving complex of Photosystem II was mimicked, the most important property of which was the self-healing nature of the catalyst. Self-healing catalysts permit water splitting to be accomplished using any water source, which is the critical development for (1) the Artificial Leaf, as it allows for the facile interfacing of water splitting catalysis to materials such as silicon, and (2) the hybrid biological–inorganic construct, called the Bionic Leaf, as it allows for the facile interfacing of water splitting catalysis to bioorganisms. Hydrogenases in the bioorganism allow the hydrogen to be coupled to NADPH and ATP production, thus allowing the solar energy from water splitting to be converted into cellular energy to drive cellular biosynthesis. In the design of the hybrid system, water splitting catalysts must be designed that support hydrogen generation at low applied potential to ensure a high energy efficiency while avoiding reactive oxygen species. Using the tools of synthetic biology, a bioengineered bacterium, Ralstonia eutropha, converts carbon dioxide from air, along with the hydrogen produced from such catalysts of the Artificial Leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. The Bionic Leaf operates at solar-to-biomass and solar-to-liquid fuels efficiencies that greatly exceed the highest solar-to-biomass efficiencies of natural photosynthesis.
First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical ...characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.
The utilization of solar energy on a large scale requires its storage. In natural photosynthesis, energy from sunlight is used to rearrange the bonds of water to oxygen and hydrogen equivalents. The ...realization of artificial systems that perform "water splitting" requires catalysts that produce oxygen from water without the need for excessive driving potentials. Here we report such a catalyst that forms upon the oxidative polarization of an inert indium tin oxide electrode in phosphate-buffered water containing cobalt (II) ions. A variety of analytical techniques indicates the presence of phosphate in an approximate 1:2 ratio with cobalt in this material. The pH dependence of the catalytic activity also implicates the hydrogen phosphate ion as the proton acceptor in the oxygen-producing reaction. This catalyst not only forms in situ from earth-abundant materials but also operates in neutral water under ambient conditions.
A versatile method for the hydromethylation and hydroalkylation of alkenes at room temperature is achieved by using the photooxidative redox capacity of the valence band of anatase titanium dioxide ...(TiO2). Mechanistic studies support a radical-based mechanism involving the photoexcitation of TiO2 with 390 nm light in the presence of acetic acid and other carboxylic acids to generate methyl and alkyl radicals, respectively, without the need for stoichiometric base. This protocol is accepting of a broad scope of alkene and carboxylic acids, including challenging ones that produce highly reactive primary alkyl radicals and those containing functional groups that are susceptible to nucleophilic substitution such as alkyl halides. This methodology highlights the utility of using heterogeneous semiconductor photocatalysts such as TiO2 for promoting challenging organic syntheses that rely on highly reactive intermediates.
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