Layer-by-Layer Self-Assembled Graphene Multilayers as Pt-Free Alternative Counter Electrodes in Dye-Sensitized Solar Cells

An overview of the preparation and application of counter electrodes for DSSCs

Dye-sensitized solar cells (DSSCs) are potential products for the next generation of photovoltaic technology, which is one of the research hotspots in photovoltaics. The counter electrode in DSSCs collects electron in the external circuit and catalyzes the reduction of the redox electrolyte and hole transport in the solid electrolyte. Thus, it undoubtedly has an important impact on the photovoltaic performance, long-term stability, and cost of DSSCs. In this work, the materials of counter electrodes are classified into metals, carbon materials, conductive polymers, and inorganic compounds. The preparation, mechanism, conversion efficiency, and properties of counter electrodes are reviewed.

Recent progress on nanostructured carbon-based counter/back electrodes for high-performance dye-sensitized and perovskite solar cells

Dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) favor minimal environmental impact and low processing costs, factors that have prompted intensive research and development. In both cases, rare, expensive, and less stable metals (Pt and Au) are used as counter/back electrodes; this design increases the overall fabrication cost of commercial DSSC and PSC devices. Therefore, significant attempts have been made to identify possible substitutes. Carbon-based materials seem to be a favorable candidate for DSSCs and PSCs due to their excellent catalytic ability, easy scalability, low cost, and long-term stability. However, different carbon materials, including carbon black, graphene, and carbon nanotubes, among others, have distinct properties, which have a significant role in device efficiency. Herein, we summarize the recent advancement of carbon-based materials and review their synthetic approaches, structure-function relationship, surface modification, heteroatoms/metal/metal oxide incorporation, fabrication process of counter/back electrodes, and their effects on photovoltaic efficiency, based on previous studies. Finally, we highlight the advantages, disadvantages, and design criteria of carbon materials and fabrication challenges that inspire researchers to find low cost, efficient and stable counter/back electrodes for DSSCs and PSCs.

High-Energy Density Li-O2 Battery with a Polymer Electrolyte-Coated CNT Electrode via the Layer-by-Layer Method

Li-O₂ batteries have attracted considerable attention for several decades due to their high theoretical energy density (>3400 Wh/kg). However, it has not been clearly demonstrated that their actual volumetric and gravimetric energy densities are higher than those of Li-ion batteries. In previous studies, a considerable quantity of electrolyte was usually employed in preparing Li-O₂ cells. In general, the electrolyte was considerably heavier than the carbon materials in the cathode, rendering the practical energy density of the Li-O₂ battery lower than that of the Li-ion battery. Therefore, air cathodes with significantly smaller electrolyte quantities need to be developed to achieve a high specific energy density in Li-O₂ batteries. In this study, we propose a core-shell-structured cathode material with a gel-polymer electrolyte layer covering the carbon nanotubes (CNTs). The CNTs are synthesized using the floating catalyst chemical vapor deposition method. The polymeric layer corresponding to the shell is prepared by the layer-by-layer (LbL) coating method, utilizing Li-Nafion along with PDDA-Cl [poly(diallyldimethylammonium chloride)]. Several bilayers of Li-Nafion and PDDA, on the CNT surface, are successfully prepared and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The porous structure of the CNTs is retained after the LbL process, as confirmed by the nitrogen adsorption-desorption profile and BJH pore-size distribution analysis. This porous structure can function as an oxygen channel for facilitating the transport of oxygen molecules for reacting with the Li ions on the cathode surface. These polymeric bilayers can provide an Li-ion pathway, after absorbing a small quantity of an ionic liquid electrolyte, 0.5 M LiTFSI EMI-TFSI [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide]. Compared to a typical cathode, where only liquid electrolytes are employed, the total quantity of electrolyte in the cathode can be significantly reduced; thereby, the overall cell energy density can be increased. A Li-O₂ battery with this core-shell-structured cathode exhibited a high energy density of approximately 390 Wh/kg, which was assessed by directly weighing all of the cell components together, including the gas diffusion layer, the interlayer [a separator containing a mixture of LiTFSI, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR-14), and PDDA-TFSI], the lithium anode, and the LbL-CNT cathode. The cycle life of the LbL-CNT-based cathode was found to be 31 cycles at a limited capacity of 500 mAh/g_carbon. Although this is not an excellent performance, it is almost 2 times better than that of a CNT cathode without a polymer coating.

Remarkable Enhancement in the Photoelectric Performance of Uniform Flower-like Mesoporous Fe3O4 Wrapped in Nitrogen-Doped Graphene Networks

The porous structure and excellent specific surface area are superior for use as a counter electrode (CE) material. In addition, N-doped graphene possesses a remarkable electron-transfer pathway and many active sites. Therefore, a novel idea is to wrap uniform flower-like mesoporous Fe₃O₄ (Fe₃O₄UFM) in an N-doped graphene (N-RGO) network structure to enhance the power conversion efficiency (PCE). The hybrid materials of Fe₃O₄UFM@N-RGO are first used as a CE in dye-sensitized solar cells (DSSCs), showing a preeminent conductive interconnected 3D porous structure with more catalytic activity sites and a better ability for and a faster reaction rate of charge transfer, resulting in quicker reduction of I₃^- than Pt. A 9.26% photoelectric conversion efficiency has been achieved for the DSSCs with Fe₃O₄UFM@N-RGO as the CE, which is beyond the value of Pt (7.72%). The positive synergetic effect between Fe₃O₄ and N-RGO is mainly responsible for the remarkable photoelectric property enhancement of this uniform flower-like mesoporous Fe₃O₄ wrapped in N-doped graphene networks, as demonstrated by the Tafel polarization, electrochemical impedance spectra, and CV curves. These methods will provide a simple way to effectively reproduce CE materials.

Synergistically Enhanced Electrochemical Performance of Ni3S4-PtX (X = Fe, Ni) Heteronanorods as Heterogeneous Catalysts in Dye-Sensitized Solar Cells

Platinum (Pt)-based alloys are considerably promising electrocatalysts for the reduction of I^-/I₃^- and Co²⁺/Co³⁺ redox couples in dye-sensitized solar cells (DSSCs). However, it is still challenging to minimize the dosage of Pt to achieve comparable or even higher catalytic efficiency. Here, by taking full advantages of the Mott-Schottky (M-S) effect at the metal-semiconductor interface, we successfully strategize a low-Pt-based M-S catalyst with enhanced electrocatalytic performance and stability for the large-scale application of DSSCs. The optimized M-S electrocatalyst of Ni₃S₄-Pt₂X₁ (X = Fe, Ni) heteronanorods is constructed by rationally controlling the ratio of Pt to transition metal in the hybrids. It was found that the electrons transferred from Ni₃S₄ to Pt₂X₁ at their interface under the Mott-Schottky effect result in the concentration of electrons onto Pt₂X₁ domains, which subsequently accelerates the regeneration of both I^-/I₃^- and Co²⁺/Co³⁺ redox shuttles in DSSCs. As a result, the DSSC with Ni₃S₄-Pt₂Fe₁ manifests an impressive power conversion efficiency (PCE) of 8.79% and 5.56% for iodine and cobalt-based electrolyte under AM1.5G illumination, respectively. These PCEs are obviously superior over those with Ni₃S₄-Pt, PtFe, Ni₃S₄, and pristine Pt electrodes. The strategy reported here is able to be further expanded to fabricate other low-Pt-alloyed M-S catalysts for wider applications in the fields of photocatalysis, water splitting, and heterojunction solar cells.

Niobium-Doped (001)-Dominated Anatase TiO2 Nanosheets as Photoelectrode for Efficient Dye-Sensitized Solar Cells

TiO₂ nanocrystals with different reactive facets have attracted extensive interest since they were first synthesized. The anatase TiO₂ nanocrystals with (001) or (100) dominate facets were considered to be excellent electrode materials to enhance the cell performance of dye-sensitized solar cells. However, which reactive facet presents the best surface for benefiting photovoltaic effect is still unknown. We report a systematic study of various anatase TiO₂ surfaces interacting with N719 dye by means of density functional theory calculations in combination with microscopic techniques. The (001) surface interacting with N719 would have the lowest work function, leading to the best photovoltaic performances. To further increase the efficiency, Nb dopant was incorporated into the anatase TiO₂ nanocrystals. Based on the theoretical prediction, we proposed and demonstrated novel Nb-doped (001)-dominated anatase TiO₂ nanosheets as photoelectrode in a dye-sensitized solar cell to further enhance the open-circuit voltage. And a power conversion efficiency of 10% was achieved, which was 22% higher than that of the undoped device (P25 as an electrode).

Extended visible photosensitivity of carboxyethyltin functionalized polyoxometalates with common organic dyes enabling enhanced photoelectric performance

A kind of broad spectral responsive photoelectrode was assembled from carboxyethyltin–POM derivatives and organic dyes.

Non-Volatile ReRAM Devices Based on Self-Assembled Multilayers of Modified Graphene Oxide 2D Nanosheets

2D nanomaterials have been actively utilized in non-volatile resistive switching random access memory (ReRAM) devices due to their high flexibility, 3D-stacking capability, simple structure, transparency, easy fabrication, and low cost. Herein, it demonstrates re-writable, bistable, transparent, and flexible solution-processed crossbar ReRAM devices utilizing graphene oxide (GO) based multilayers as active dielectric layers. The devices employ single- or multi-component-based multilayers composed of positively charged GO (N-GO(+) or NS-GO(+)) with/without negatively charged GO(-) using layer-by-layer assembly method, sandwiched between Al bottom and Au top electrodes. The device based on the multi-component active layer Au/[N-GO(+)/GO(-)]_n /Al/PES shows higher ON/OFF ratio of ≈10⁵ with switching voltage of -1.9 V and higher retention stability (≈10⁴ s), whereas the device based on single component (Au/[N-GO(+)]_n /Al/PES) shows ≈10³ ON/OFF ratio at ±3.5 V switching voltage. The superior ReRAM properties of the multi-component-based device are attributed to a higher coating surface roughness. The Au/[N-GO(+)/GO(-)]_n /Al/PES device prepared from lower GO concentration (0.01%) exhibits higher ON/OFF ratio (≈10⁹ ) at switching voltage of ±2.0 V. However, better stability is achieved by increasing the concentration from 0.01% to 0.05% of all GO-based solutions. It is found that the devices containing MnO₂ in the dielectric layer do not improve the ReRAM performance.