Dye sensitized solar cells (DSSCs) use low-cost materials, feature tunable molecular sensitizers, and exhibit quantum efficiencies near unity. These advantageous features can be exploited in the context of solar water splitting by functionalizing DSSCs with catalysts for water oxidation and reduction. This article will cover the development of photoanodes for water splitting DSSCs from the perspective of water oxidation catalysts, sensitizers, electron transfer mediators, photoanode materials, and system level design. Within each section we will endeavor to highlight critical design elements and how they can affect the efficiency of the overall system.

Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light.artificial photosynthesis | photoelectrochemistry T he design of biomimetic systems for artificial photosynthesis is of fundamental interest in the study of light-driven electron and proton transfer reactions. It also represents a potential route to the efficient conversion of solar energy to energy stored in fuel. System modeling has shown that it should be possible, using complementary dye molecules that absorb in the infrared and the visible, to construct artificial Z-schemes that split water with over 10% energy conversion efficiency (1, 2). It is simpler in many ways to design small molecules with the proper photoredox properties than it is to find a set of semiconductors that can be coupled for visible light water splitting. Nevertheless, the molecular approach has so far lagged behind the semiconductor-based approach where high efficiencies have been realized with expensive materials (3-7).A ubiquitous problem in molecular artificial photosynthesis is back electron transfer, which rapidly thermalizes the energy stored by light-induced charge separation in donor-acceptor pairs. Recently, our group and several others have studied this problem in dye-sensitized solar cells where a molecular dye and a porous TiO 2 electrode act as the donor-acceptor dyad (8-13). The dye is covalently coupled to a colloidal or molecular water oxidation catalyst. Fast back electron transfer, relative to the rate of water oxidation, results in low quantum yields for water splitting in these systems.It is well known that charge-separation lifetimes in molecular donor-acceptor systems can be increased dramatically by adding secondary electron donors or acceptors to form triads, tetrads, and more complex supermolecu...

Iron-doped nickel (oxy)hydroxide catalysts (Fe x Ni 1−x OOH) exhibit high electrocatalytic behavior for the oxygen evolution reaction in base. Recent findings suggest that the incorporation of Fe 3+ into a NiOOH lattice leads to nearly optimal adsorption energies for OER intermediates on active Fe sites. Utilizing electrochemical impedance spectroscopy and activation energy measurements, we find that pure NiOOH and FeOOH catalysts exhibit exceedingly high Faradaic resistances and activation energies 40−50 kJ/mol −1 higher than those of the most active Fe x Ni 1−x OOH catalysts. Furthermore, the most active Fe x Ni 1−x OOH catalysts in this study exhibit activation energies that approach those previously reported for IrO 2 OER catalysts.

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO2 surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO2 film. In this paper we report the effects of electron trapping and protonation in the TiO2 film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis(2,2'-bipyridine)(4,4'-diphosphonato-2,2'-bipyridine)ruthenium(II) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO2 film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen.