Chemistry of personalized solar energy

Abstract

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.

Figure 1

An energy independent home delivers the individual personalized energy. Reproduced with permission from MIT and Technology Review.

Figure 2

Electron configurations for the diradical excited state that is characteristic of most transition metal complexes and the two-electron mixed valence excited state of bimetallic compounds explored in the author’s research program.

Figure 3

The molecular orbital diagram of H₂ (top) and the energy level diagram for H₂ described in the limits of molecular orbital theory (MOT) and valence bond theory (VBT) (bottom). ΔW is the one-electron orbital splitting energy and K is the two-electron exchange energy.

Figure 4

(Top) Qualitative energy level diagram for the δ/δ* orbital manifold arising from the overlap of d_xy orbitals of M–⁴–M complexes in accordance with a valence bond model. The ¹δ*δ* excited state can be accessed by two-photon near-IR absorption and detected via laser-induced fluorescence (LIF) of the ¹δδ* excited state. The energy of the states is given in terms of the one-electron orbital splitting energy ΔW and the two-electron exchange energy K. (Bottom) Emission (red), absorption (purple) spectra and two-photon LIF excitation (brown) spectra of Mo₂Cl₄(PMe₃)₄ in 3-methylpentane at room temperature. The two-photon LIF excitation spectrum is superimposed at twice the laser excitation energy. The spectral region over which the incident dye-laser excitation was scanned is indicated by the gray box.

Figure 5

Metal-to-metal charge transfer (MMCT) produces a zwitterionic excited state in which the formal oxidation states of the metal differ by two. Transient absorption studies show that the excited state reacts with a proton to make a hydride, which back reacts to produce the M–⁴–M grounds state. The hydride is not trapped by a second proton to produce hydrogen.

Figure 6

(Left) The three-atom dfpma ligand shown in Chart 1 possessing an electron donor bridgehead (D) between two π-accepting moieties (A), which can be used to stabilize two-electron mixed-valence cores. The stabilization of a mixed-valence core by the A–D–A stereoelectronic ligand motif results in differing bond orders indicative of an asymmetric electronic distribution. (Right) A crystal structure of Rh₂^0,II(dfpma)₃Br₂ displays the long-short bond alteration of the ligand backbone. Reproduced from ref. .

Figure 7

(Left) Thermal ellipsoid plot (50% probability level) for the solid-state structure of the two-electron mixed valence Mg porphyrinogen compound, [L^ΔMg(NCMe)]•CH₂Cl₂. (Middle) Computed frontier Kohn-Sham orbital diagram of the HOMO and LUMOs of [L^ΔMg(NCMe)] showing the localization of the electron density that gives rise to the ligand-to-ligand intravalence charge transfer transition (IVCT). (Right) UV-visible absorption spectra of [L^ΔMg] (—, red), [L^ΔZn] (– –, orange) and [L^ΔCa] (---, yellow) in CH₂Cl₂. Reproduced with permission from the Royal Society of Chemistry

Figure 8

The porphyrinogen ligand as a two-electron mixed-valence scaffold and its redox parallel to the dirhodium system. The two-electron reduced and oxidized parts of the molecules are color coded blue and red, respectively.

Figure 9

The complete photocycle for H₂ generation by the Rh₂ dfpma photocatalyst from nonaqueous solutions containing HCl or HBr. Identification of the intermediates in the cycle is based on the chemistry of dirhodium and diiridium dfpma, tfepma and tfepm analogs, the crystal structures of which are shown.

Figure 10

A schematic of the apparatus used to perform solid state photoelimination of halogen gas from bimetallic Pt₂ tfepma and Au₂ bisphosphine complexes. The color-coded reactants and photoproducts are that for eq. (8).

Figure 11

Supramolecular assemblies created to investigate PCET mechanisms for electron and proton transport along colinear and orthogonal pathways. In each system, the photoinduced electron transfer must negotiate the transfer of a proton. Reproduced from ref. .

Figure 12

Cyclic voltammogram of a aqueous solution containing 1 mM Co²⁺ and 0.1 M Pi electrolyte. SEM images are shown of catalyst films that form holding the electrode at the indicated pre-catalytic and catalytic potential.

Figure 13

Tafel plots showing higher water oxidation activity of Co-OEC (, gray) at lower overpotentials relative to Pt (, red) in aqueous solutions containing phosphate.

Figure 14

Proposed dicubane structure of the Co-OEC (cobalt in blue, oxygen in red) based on XAS experiments of ref . The Figure is reproduced from ref with permission from the American Chemical Society.

Figure 15

Radioactive isotope tracer studies of the Co-OEC repair process. (a) Percentage of ⁵⁷Co leached from films of Co-OEC on an electrode: with a potential bias of 1.3 V (NHE) turned on and off at the times designated () (the trace begins with the applied potential on); and without an applied potential bias (). (b) ³²P leaching from Co-OEC on an electrode with an applied potential of 1.3 V (NHE) () and without an applied potential bias (). (c) Percentage of ⁵⁷Co leached from a typical Co-oxide catalyst on an electrode under a potential bias of 1.3 V () and 1.5 V () (NHE) and an unbiased electrode (). Pi was added at the time points indicated by the arrows. Figures adapted from ref. and reproduced here with permission from the American Chemical Society.

Figure 16

Tafel plots for Co-OEC in 18 MΩ water (, gray) and water collected from the Charles River in front of MIT (, green).

Scheme 1

Scheme 2

Chart 1

Fluorophosphine ligands that promote two-electron mixed-valency of bimetallic cores.

Chart 2

Comparison of functional properties of the Photosystem II OEC (Oxygen Evolving Complex) and the Co-OEC water-splitting catalyst.