The research of p-type dye-sensitized solar cells (p-DSSCs) has attracted growing attention because of the potential for integration with conventional n-type DSSCs (n-DSSCs) into the more efficient tandem-DSSCs. However, to date the performance of p-DSSCs is lagging behind that of n-DSSCs. One main reason is the lack of optimal photocathode materials. This article reviews the most recent progress in utilizing Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr), as photocathodes in p-DSSCs. As alternative materials to the commonly used NiO, the CuMO2 compounds have their intrinsic advantages such as lower valence band edge, larger optical bandgap and higher conductivity. By providing an insight into these materials and their applications in p-DSSCs, this perspective aims to stimulate more exciting research in the development of p-DSSCs as well as of tandem-DSSCs.

The delafossite CuGaO2 is an important p-type transparent conducting oxide for both fundamental science and industrial applications. An emerging application is for p-type dye-sensitized solar cells. Obtaining delafossite CuGaO2 nanoparticles is challenging but desirable for efficient dye loading. In this work, the phase formation and crystal growth mechanism of delafossite CuGaO2 under low-temperature (<250 °C) hydrothermal conditions are systematically studied. The stabilization of Cu(I) cations in aqueous solution and the controlling of the hydrolysis of Ga(III) species are two crucial factors that determine the phase formation. The oriented attachment (OA) growth is proposed as the crystal growth mechanism to explain the formation of large CuGaO2 nanoplates. Importantly, by suppressing this OA process, delafossite CuGaO2 nanoparticles that are 20 nm in size were successfully synthesized for the first time. Moreover, considering the structural and chemical similarities between the Cu-based delafossite series compounds, the understanding of the hydrothermal chemistry and crystallization mechanism of CuGaO2 should also benefit syntheses of other similar delafossites such as CuAlO2 and CuScO2.

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Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion−Jacobson phase HLa 2 Ti 2 TaO 10 in the temperature range of interest (250−475°C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa 2 Ti 2 TaO 10 , which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa 2 Ti 2 TaO 10 ·nH 2 O (n = 1−2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa 2 Ti 2 TaO 10 ·nH 2 O lost intercalated water at temperatures between 100 and 200°C, but X-ray diffraction patterns up to 500°C did not show evidence of collapse of the interlayer galleries that has been observed with the structurally similar compound HCa 2 Nb 3 O 10 . Under humid hydrogen atmosphere, the conductivity of HLa 2 Ti 2 TaO 10 followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10 −9 to 10 −5 S cm −1 depending on the preparation conditions and temperature. Modification of the stoichiometry to produce A-site or B-site (vacancy or substitution) defects decreased the conductivity slightly. The conductivity was approximately 1 order of magnitude higher in humid hydrogen than in humid air atmospheres, suggesting that the dominant mechanism in the intermediate temperature range is electronic. A-site substitution (Sr 2+ for La 3+ ) beyond the Ruddlesden−Popper phase limit converted the layered pervoskite to a cubic perovskite Sr 2.5 □ 0.5 Ti 2 TaO 9 with 2 orders of magnitude higher conductivity than HLa 2 Ti 2 TaO 10 at 475°C. ■ INTRODUCTIONElectrochemical energy conversion technologies are becoming increasingly important as the global energy economy evolves toward higher efficiency and sustainability. The development of these technologies is to a large extent driven by the discovery of higher performing and lower cost materials. In the case of fuel cells, which are devices that electrochemically oxidize fuels to produce electrical energy, the current technology is dominated by low temperature (<150°C) and high temperature (>600°C ) devices that contain polymer and solid oxide electrolytes, respectively. 1 A gap exists in the intermediate temperature regime (250−450°C) where there are reasons to expect that fuel cells could be made more efficient and less expensive. The loading of precious metal anode and cathode catalysts could be substantially decreased because of the higher activity of catalysts at higher temperature. Also, carbon-containing species such as CO, which poison noble metal catalysts in polymer fuel cells, are easily oxidized at the anode at intermediate temperatures.However, the development of intermediate temperature fuel cell technology is hampered by the lack of electrolyte materials that have sufficiently high proton conductivity in the temperature range of interest. 2,3 Experiments on intermediate temperature fuel cells have employed ceramic proton conductors th...

The delafossite structured CuScO2 is a p-type, wide band gap oxide that has been shown to support significant oxygen intercalation, leading to darkened color and increased conductivity. Control of this oxidation proves difficult by the conventional high-temperature solid-state syntheses. In addition, a pure hexagonal (2H) or rhombohedral (3R) polytype of CuScO2 requires careful control of synthetic parameters or intentional doping. Lower-temperature hydrothermal syntheses have thus far led to only a mixed 2H/3R product. Herein, control of hydrothermal conditions with the consideration of copper and scandium hydrolysis led to the synthesis of light beige, hierarchically structured particles of 2H-CuScO2. Absorption of the particles in the visible range was found to increase upon annealing of the sample in air, most likely due to the Cu(II) formation from oxygen interstitials. X-ray photoelectron spectroscopy confirmed purely Cu(I) in the as-synthesized 2H-CuScO2 and increased Cu(II) amounts upon annealing. Oxidation of the samples also led to shifts of the Fermi level toward the valence band as observed by increases in the measured flat band potentials versus normal hydrogen electrode, confirming increased hole carrier densities.