Composition-Dependent Electrocatalytic Activity of Cobalt Sulfides for Triiodide Reduction in Dye-Sensitized Solar Cells

A new nanoarchitecture of cobalt sulfide (CoS_<i>x</i>) is designed by exploiting a Prussian blue analogue. Depending on the sulfidation temperatures, CoS_<i>x</i> materials with different compositions and morphologies are obtained. This investigation of the composition-dependent electrocatalytic activity of CoS_<i>x</i> for triiodide reduction reaction (IRR) reveals that sulfur-deficient CoS_<i>x</i> is more active than sulfur-rich CoS_<i>x</i>. When utilized in dye-sensitized solar cells (DSSCs), sulfur-deficient CoS_<i>x</i> with a hollow nanocube morphology outperforms platinum (Pt), showing great promise as a Pt alternative. This composition dependency on IRR is attributed to different surface characteristics and electrical properties that vary with CoS_<i>x</i> composition. This work highlights the importance of understanding the surface properties of sulfide-based electrocatalysts that are intimately dictated by their compositions as part of a new design principle for a highly active electrocatalyst

Nitrogen-Doped Carbon Nanocoil Array Integrated on Carbon Nanofiber Paper for Supercapacitor Electrodes

Integrating a nanostructured carbon array on a conductive substrate remains a challenging task that presently relies primarily on high-vacuum deposition technology. To overcome the problems associated with current vacuum techniques, we demonstrate the formation of an N-doped carbon array by pyrolysis of a polymer array that was electrochemically grown on carbon fiber paper. The resulting carbon array was investigated for use as a supercapacitor electrode. In-depth surface characterization results revealed that the microtextural properties, surface functionalities, and degree of nitrogen incorporated into the N-doped carbon array can be delicately controlled by manipulating carbonization temperatures. Furthermore, electrochemical measurements showed that subtle changes in these physical properties resulted in significant changes in the capacitive behavior of the N-doped carbon array. Pore structures and nitrogen/oxygen functional groups, which are favorable for charge storage, were formed at low carbonization temperatures. This result showed the importance of having a comprehensive understanding of how the surface characteristics of carbon affect its capacitive performance. When utilized as a substrate in a pseudocapacitive electrode material, the N-doped carbon array maximizes capacitive performance by simultaneously achieving high gravimetric and areal capacitances due to its large surface area and high electrical conductivity

New Insight into Copper Sulfide Electrocatalysts for Quantum Dot-Sensitized Solar Cells: Composition-Dependent Electrocatalytic Activity and Stability

Despite recent significant strides in understanding various processes in quantum dot-sensitized solar cells (QDSSCs), little is known about the intrinsic electrocatalytic properties of copper sulfides that are the most commonly employed electrocatalysts for the counter electrode of QDSSCs. Given that the physical properties of copper sulfides are governed by their stoichiometry, the electrocatalytic activity of copper sulfides toward polysulfide reduction may also be dictated by their compositions. Using a new, simple approach to prepare robust copper sulfide films based on chemical bath deposition (CBD), we were able to delicately control the compositions of copper sulfides, which allowed us to perform a systematic investigation to gain new insight into copper sulfide-based electrocatalysts. The electrocatalytic activity is indeed dependent on the compositions of copper sulfides: Cu-deficient films (CuS and Cu_1.12S) are superior to Cu-rich films (Cu_1.75S and Cu_1.8S) in their electrocatalytic activity. In addition, the stability of the Cu-deficient electrocatalysts is substantially better than that of the Cu-rich counterparts

Metal Selenides as a New Class of Electrocatalysts for Quantum Dot-Sensitized Solar Cells: A Tale of Cu_1.8Se and PbSe

The development of a Pt-free, highly active electrocatalyst for a counter electrode (CE) is vital to the construction of highly efficient quantum dot-sensitized solar cells (QDSSCs). As an alternative to Pt, the use of various metal sulfides, such as Cu₂S, CoS, and PbS, has been successfully demonstrated; however, the studies on the utilization of non-sulfide materials have been scarcely reported. In this regard, we examined eight different types of binary metal selenides as new candidate materials, and found that the electrocatalytic activity of Cu_1.8Se and PbSe toward polysulfide reduction was superior to that of Pt. In depth investigation into these two materials further revealed that, while the electrocatalytic activity of both metal selenides surpasses that of Pt, the long-term utilization of the PbSe CE is hindered by the formation of PbO on the surface of PbSe, which is attributed to the instability of PbSe under air. Unlike PbSe, Cu_1.8Se was found to be chemically stable with a polysulfide electrolyte and was even better than Cu₂S, a commonly used CE material for QDSSCs. Using the Cu_1.8Se CE, we obtained a power conversion efficiency of 5.0% for CdS/CdSe-sensitized solar cells, which was an efficiency almost twice that obtained from Pt CE. This work provides a new application for metal selenides, which have been traditionally utilized as sensitizers for QDSSCs

Carbon Microspheres as Supercapacitors

Carbon microstructures fabricated by ultrasonic spray pyrolysis (USP) of aqueous precursors were tested as supercapacitors. USP carbons (USP-C) possess unique physicochemical characteristics, including substantial microporosity and high surface concentrations of oxygenated functional groups. We find that USP-Cs have higher electrochemical double-layer capacitance compared with other carbon structures. Porous carbon microspheres prepared from USP of lithium dichloroacetate, lithium/potassium propiolate, or sucrose produce electrochemical double layer capacitors (EDLCs) that have gravimetric capacitances of 185, 341, and 360 F/g, respectively. Microstructural and chemical analyses of the carbon materials suggest that the observed capacitance is related to the effects of surface functionality

Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance

Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, we provide deep insight into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, we focused on the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs and reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, this work represents an important stepping stone toward the development of MCSSCs by accomplishing a new PCE record of 3.8%

Highly Electrocatalytic Cu₂ZnSn(S_1–<i>x</i>Se_<i>x</i>)₄ Counter Electrodes for Quantum-Dot-Sensitized Solar Cells

Traditional Pt counter electrode in quantum-dot-sensitized solar cells suffers from a low electrocatalytic activity and instability due to irreversible surface adsorption of sulfur species incurred while regenerating polysulfide (S_<i>n</i>^2–/S^2–) electrolytes. To overcome such constraints, chemically synthesized Cu₂ZnSn­(S_1–<i>x</i>Se_<i>x</i>)₄ nanocrystals were evaluated as an alternative to Pt. The resulting chalcogenides exhibited remarkable electrocatalytic activities for reduction of polysulfide (S_<i>n</i>^2‑) to sulfide (S^2–), which were dictated by the ratios of S/Se. In this study, a quantum dot sensitized solar cell constructed with Cu₂ZnSn­(S_0.5Se_0.5)₄ as a counter electrode showed the highest energy conversion efficiency of 3.01%, which was even higher than that using Pt (1.24%). The compositional variations in between Cu₂ZnSnS₄ (<i>x</i> = 0) and Cu₂ZnSnSe₄ (<i>x</i> = 1) revealed that the solar cell performances were closely related to a difference in electrocatalytic activities for polysulfide reduction governed by the S/Se ratios

Deciphering the Electrocatalytic Activity of Nitrogen-Doped Carbon Embedded with Cobalt Nanoparticles and the Reaction Mechanism of Triiodide Reduction in Dye-Sensitized Solar Cells

The electrocatalytic activity of carbon materials for triiodide (I₃^–) reduction has spurred the development of low-cost electrocatalysts as an alternative to platinum in dye-sensitized solar cells. While many catalytic aspects of nitrogen-doped carbons have been unveiled in recent years, not all underlying factors that dictate their electrocatalytic activity have been fully considered; the current understanding of the electrocatalytic activity of nitrogen-doped carbons is limited. In addition, the synergistic effect of metal nanoparticles embedded in nitrogen-doped carbon, which was recently demonstrated as a facile way to boost the electrocatalytic activity of carbon, remains elusive. This work sheds light on these unknown aspects of carbon’s electrocatalytic activity by carrying out a systematic investigation of nitrogen-doped carbon with incorporated cobalt nanoparticles. Furthermore, the generally accepted mechanism of the I₃^– reduction reaction (IRR) is re-evaluated in this work with the aid of density functional theory calculations and in-depth electrochemical analysis. A new insight into this mechanism, which suggests that there is another possible reaction pathway available for the IRR on carbon, is provided

Deciphering the Electrocatalytic Activity of Nitrogen-Doped Carbon Embedded with Cobalt Nanoparticles and the Reaction Mechanism of Triiodide Reduction in Dye-Sensitized Solar Cells

The electrocatalytic activity of carbon materials for triiodide (I₃^–) reduction has spurred the development of low-cost electrocatalysts as an alternative to platinum in dye-sensitized solar cells. While many catalytic aspects of nitrogen-doped carbons have been unveiled in recent years, not all underlying factors that dictate their electrocatalytic activity have been fully considered; the current understanding of the electrocatalytic activity of nitrogen-doped carbons is limited. In addition, the synergistic effect of metal nanoparticles embedded in nitrogen-doped carbon, which was recently demonstrated as a facile way to boost the electrocatalytic activity of carbon, remains elusive. This work sheds light on these unknown aspects of carbon’s electrocatalytic activity by carrying out a systematic investigation of nitrogen-doped carbon with incorporated cobalt nanoparticles. Furthermore, the generally accepted mechanism of the I₃^– reduction reaction (IRR) is re-evaluated in this work with the aid of density functional theory calculations and in-depth electrochemical analysis. A new insight into this mechanism, which suggests that there is another possible reaction pathway available for the IRR on carbon, is provided

Revival of Solar Paint Concept: Air-Processable Solar Paints for the Fabrication of Quantum Dot-Sensitized Solar Cells

One way to revolutionize solar energy production and expand it to a large scale is to reduce the manufacturing cost and complexity of the fabrication process. The ability to make solar cells on the surface of any shape would further transform this technology. Quantum dot-sensitized solar cells (QDSSCs) are an ideal candidate to push solar cell technology in this direction. In this regard, making a paint that can be applied by a paint brush to any transparent conductive surface to turn it into the photoanode of QDSSCs is the ultimate goal. We herein demonstrate the feasibility of one-coat fabrication of QDSSCs from a lead sulfide (PbS)-based solar paint. This is possible because of its unique ability to regenerate after oxidation occurred during heat treatment in air. Hence, the whole fabrication process can be carried out in air unlike a first-generation solar paint based on cadmium sulfide (CdS) and cadmium selenide (CdSe). Two solar paints using a commercially available titanium dioxide (TiO₂) and a p-type TiO₂ powder were synthesized and evaluated. Also, the performance-limiting parameters are thoroughly investigated using various spectroscopic and electrochemical characterization methods. The implication of new insights into the PbS-based solar paint for further development of paint-on solar cells is discussed