Triiodide Reduction Reaction Research Articles

Boron-doped carbon nanotubes as metal-free electrocatalyst for dye-sensitized solar cells: Heteroatom doping level effect on tri-iodide reduction reaction

Heteroatom-doped carbons have been substantially applied on electrochemical applications for their exceptional electrocatalytic ability and electric conductivity. Among the doping elements, electron-deficient boron (B) is considered to be a promising heteroatom for the chemical doping of carbon materials to modify the chemically inert sp2 carbon structure and thus activate the abundant free‐flowing π electrons. In this study, B-doped carbon nanotubes (BCNTs) with various boron doping atomic percentages (0.40–3.92 at%) are synthesized and used as a electrocatalyst on the counter electrode (CE) of dye-sensitized solar cells (DSSCs) for investigating the effect of boron doping on carbon materials. A solar-to-electricity conversion efficiency (η) of 7.17 ± 0.11% is achieved for a DSSC with a CE containing BCNTs with optimized B doping concentration (BCNT-0.40 at%), which is higher than that of the cells with CEs consisting of pristine carbon nanotubes (CNT, η = 5.98 ± 0.20%) and is comparable to that of the cell with a Pt CE (η = 7.98 ± 0.05%). It is also noteworthy from a practical viewpoint that the developed atmospheric-pressure synthesis method for synthesizing BCNT is amenable to industrial-scale production since a requirement for vacuum system can be avoided.

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

A new nanoarchitecture of cobalt sulfide (CoSx) is designed by exploiting a Prussian blue analogue. Depending on the sulfidation temperatures, CoSx materials with different compositions and morphologies are obtained. This investigation of the composition-dependent electrocatalytic activity of CoSx for triiodide reduction reaction (IRR) reveals that sulfur-deficient CoSx is more active than sulfur-rich CoSx. When utilized in dye-sensitized solar cells (DSSCs), sulfur-deficient CoSx 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 CoSx 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.

Electrochemical deposition of Pt-Ni on reduced graphene oxide as counter electrode material for dye-sensitized solar cell

Abstract In dye-sensitized solar cell (DSSC) alloys of Pt with other metals or replacement of it with Pt-free alloys can be used to reduce the fabrication cost. Herein, we present the formation of Pt-Ni alloy/reduced graphene oxide (Pt-Ni/RGO) counter electrode (CE) by a simple and environmental-friendly electrochemical deposition method for fabrication of DSSC. First, RGO/FTO CE was prepared through electrophoretic deposition (EPD) method and then reduced electrochemically. Then, Pt-Ni alloy was loaded on CE by cyclic voltammetry (CV) strategy. The morphology of the Pt-Ni/RGO film was characterized by field emission scanning electron microscopy (FESEM) analysis. Pt-Ni nanoparticles were uniformly dispersed on the surface of RGO after electrodeposition. The Pt-Ni/RGO CE showed remarkably enhanced electrocatalytic activities toward triiodide (I3−) reduction reaction in comparable to RGO CE. CV and electrochemical impedance spectroscopy (EIS) confirmed that the Pt-Ni/RGO CE has superior electrocatalytic activity than that of RGO and Pt-Ni CEs. The enhanced electrocatalytic activities can be in association with the synergistic effect of transition metals. As a result, an efficiency of 6.14% with Jsc = 14.08 mA cm−2, Voc = 749 mV, and FF = 0.58 was achieved for DSSC assembled with the Pt-Ni/RGO CE under AM 1.5 illumination of 100 mW cm−2. Thus, the fabricated Pt-Ni/RGO CE could be used as a promising material for a low-cost CE for DSSC.

Rational Design of LaNiO3/Carbon Composites as Outstanding Platinum‐Free Photocathodes in Dye‐Sensitized Solar Cells With Enhanced Catalysis for the Triiodide Reduction Reaction

In many photovoltaics (PVs), including dye‐sensitized solar cells (DSSCs), triiodide/iodide (I3−/I−) redox couple plays an important role, and an active and stable electrocatalyst is required for promoting I3− reduction reaction (IRR) to minimize efficiency losses. Platinum (Pt) is the state‐of‐the‐art electrocatalyst for IRR, which unfortunately suffers from high cost and poor stability. Herein, strongly coupled LaNiO3 perovskite/carbon composites are developed for the first time as highly efficient, stable and low‐cost electrocatalysts for IRR to enable better DSSCs. High‐energy ball milling of crystallized LaNiO3 with carbon material is applied for the facile synthesis of LaNiO3/carbon composites. This ball‐milling process simultaneously leads to partial reduction of LaNiO3 by carbon with the creation of oxygen vacancies inside the perovskite oxide lattice, a decrease in LaNiO3 particle size and the creation of strong coupling between LaNiO3 and carbon. Particularly, DSSC with LaNiO3/multi‐walled carbon nanotubes photocathode delivers a high power conversion efficiency of 9.81% with an attractive enhancement of 21% compared with Pt electrocatalyst, superior to most state‐of‐the‐art highly efficient Pt‐free photocathodes in DSSCs. LaNiO3 or carbon alone demonstrates a much poorer performance. Moreover, LaNiO3/carbon composites demonstrate excellent operational stability. This study highlights the extended applications of perovskites in other I3−/I−‐mediated PVs, electrochromic devices or batteries.

Facile fabrication of activated charcoal decorated functionalized multi-walled carbon nanotube electro-catalyst for high performance quasi-solid state dye-sensitized solar cells

The proposed research presents significant progress in the photovoltaic performance of quasi-solid state dye-sensitized solar cells (DSSCs) by synthesizing a highly electro-catalytic active activated charcoal decorated functionalized multi-walled carbon nanotube (MWCNT) composite electro-catalyst as a counter electrode (CE). The proposed carbon composite structure was synthesize by facile acid functionalization of MWCNTs followed by the addition of mesoporous activated charcoal, decorating the tubular graphitic structure of the CNTs. The carbon composite paste deposited on FTO glass by a sequential process of doctor blade coating under an air-drying technique. The porous functionalized mesoporous carbon (f-MC) with a dominant oxygen rich surface displays greatly enhanced electro-catalytic activity, low charge transfer resistance (RCT), and exceptional cyclic stability as compared with pristine CNTs. The DSSC fabricated with f-MC CE demonstrated efficient electrochemical characteristics and photovoltaic performance when fabricated with a high-viscosity quasi-solid electrolyte. The highly conductive and porous carbon structure locates manifold sites for tri-iodide reduction reaction. High mobility of the quasi-solid electrolyte within defect rich (f-MC) surface confirmed a low RCT of (0.60Ω.cm2), and exhibited superior electrocatalytic activity compared to a conventional platinum (Pt) reference CE. The f-MC CE based DSSCs showed high power conversion efficiency (PCE) of 8.42%, exceeding the Pt reference CE of 8.11%. Based on the facile synthesis of f-MC composites and fabrication of CE, the proposed DSSCs stand out as efficient next generation solar cells.

Phosphorus-doped porous graphene nanosheet as metal-free electrocatalyst for triiodide reduction reaction in dye-sensitized solar cell

Phosphorus-doped porous graphene nanosheet with a large specific surface area (1627.8m2g−1) was prepared via chemical vapour deposition (CVD) with porous Mg3(PO4)2 as phosphorus (P) source and template, then it was applied as the counter electrode (CE) for dye-sensitized solar cell (DSSC). Due to the enhanced intrinsic catalytic activity induced by P doping, along with abundant open edge sites originated from the unique porous nanosheet, DSSCs with P doped porous graphene nanosheets CEs exhibited a high power conversion efficiency of 7.08%, which was comparable to that of DSSCs with Pt CEs.

Ozonization, Amination and Photoreduction of Graphene Oxide for Triiodide Reduction Reaction: An Experimental and Theoretical Study

This work proposes a mild and environmentally-friendly approach to prepare a highly efficient functional graphene (termed as AGO-hv) using methods of ozone oxidation, solvothermal synthesis, and photoreduction. The use of ozone oxidation in the first step can effectively increase the interlaminar distance between graphite oxide sheets, and create active sites for nucleophilic attack on the epoxy carbon from ammonia. The amino groups were successfully grafted on the surface of graphene as evidenced by the amidation reaction, with a maximum nitrogen content of 10.46wt% and a C/N molar ratio of 8.46. After further photoreduction of the aminated graphite oxide (AGO), the residual oxygen functionalities, such as C-OH, were effectively removed and the conductivity of the graphene sheet was further recovered. The dye-sensitized solar cell (DSC) exhibited a power conversion efficiency (PCE) of 7.51% based on AGO-hv counter electrode (CE), close to that of Pt counterpart (7.79%). The experimental results indicated that the amidation and photoreduction processes were significantly facilitated by the initial ozonization of graphene oxide, and this process significantly improved the electrochemical activity and the conductivity of graphene oxide. Density functional theory (DFT) calculations revealed that AGO-hv had the lowest ionization energy (a better electron-donating ability) and also suitable binding energy with I atoms as well. The combination of ozonization, amination and photoreduction is an efficient route to obtain electrocatalysts with desired compositional distributions and performance for triiodide reduction reaction in DSCs.

Counter electrodes from polymorphic platinum-nickel hollow alloys for high-efficiency dye-sensitized solar cells

Precious platinum counter electrode (CE) has been an economic burden for future commercialization of dye-sensitized solar cells (DSSCs). Low-platinum alloy CE catalysts are promising in bringing down the solar cell cost without reducing photovoltaic performances. We present here a facile strategy of fabricating ZnO nanorods assisted platinum-nickel (PtNi) alloy microtube CEs for liquid-junction DSSCs. By adjusting the concentration of zinc precursors, the ZnO nanostructures and therefore PtNi alloys are optimized to maximize the electrocatalytic behaviors toward triiodide reduction reaction. The maximal power conversion efficiency is determined as high as 8.43% for liquid-junction DSSC device with alloyed PtNi microtube CE synthesized at 75 mM Zn(NO3)2 aqueous solution, yielding a 32.8% enhancement in cell efficiency in comparison with the solar cell from pristine platinum electrode. Moreover, the dissolution resistance and charge-transfer ability toward redox couples have also been markedly enhanced due to competitive dissolution reactions and alloyed effects.

Theoretical study of triiodide reduction reaction on nitrogen-doped graphene for dye-sensitized solar cells

Graphene and its derivatives are attractive for electrocatalytical application in dye-sensitized solar cells because of their unique structures and electronic properties. By means of density functional theory calculations, the mechanism of triiodide reduction reaction on nitrogen-doped graphene (NDG) was studied in acetonitrile environment. The computations demonstrated that the rate-determining step was the ability of NDG to release electrons to active iodine atoms. According to the calculation, the optimal NDG was designed with nitrogen contents of 4.0 % graphite N and 3.0 % pyridinic N approximately. In order to precisely distinguish these two nitrogen species in the optimal NDG, we proposed the chemical shift of 15N NMR of nitrogen doped in graphene provided guidance for the experiments.

Revealing the Volcano-Shaped Activity Trend of Triiodide Reduction Reaction: A DFT Study Coupled with Microkinetic Analysis

Triiodide/iodide (I3–/I–) represents a widely used redox couple and plays an important role in some photovoltaic devices. However, the understanding of the triiodide reduction kinetics occurring at the liquid/electrode interface is very limiting, which largely hinders the identification of highly efficient electrode material. In this work, by virtue of DFT calculations, we systematically investigated the I3– electroreduction at some acetonitrile/electrode interfaces and uncovered two new BEP relations for the key elementary steps, I2 dissociation and I* desorption through one-electron reduction. Furthermore, by utilizing a steady-state microkinetic model, we successfully identified a general volcano-shaped activity trend of triiodide electroreduction as a function of a single descriptor, the adsorption energy of I atom (EadI) at the interface. Our results show that a good catalyst should possess an EadI within the range of 0.3–0.6 eV, while the optimal EadI is 0.43 eV, where the surface coverages of free ...

Alloying of Pt with Ni microtubes and Co nanosheets for counter electrode of dye-sensitized solar cell

Counter electrode (CE) is crucial in catalyzing triiodide reduction reaction and therefore in enhancing power conversion efficiency of a dye-sensitized solar cell (DSSC). We present here the alloying of Pt with Ni microtubes and Co nanosheets for cost-effective CE electrocatalyst using ZnO microrod templates. In comparison with pristine Pt electrode, the resultant PtNiCo alloy CE displays significantly elevated electrocatalytic activity and charge transfer ability, yielding an impressive efficiency of 8.85% in its liquid-junction DSSC platform.

Multi-interfacial polyaniline-graphene/platinum counter electrodes for dye-sensitized solar cells

Pursuit of multi-interfacial counter electrodes (CEs) for enhanced triiodide (I3−) reduction reaction has been a persistent objective for dye-sensitized solar cells (DSSCs). Here we report the synthesis of multilayer CEs consisting of positively charged polyaniline-graphene (PANi-G) complex and negatively charged platinum (Pt) nanoparticles. The (PANi-G/Pt)n (n represents the bilayer number) multilayer displays multi-interfaces for I3− reduction and charge transfer. Moreover, the complexation between PANi and G can markedly accelerate the electron migration from G to PANi. The DSSC with (PANi-10wt‰G/Pt)9 electrode yields an impressive power conversion efficiency of 7.45% under simulated air mass 1.5 global sunlight. The promising efficiency along with cost-effectiveness and scalable materials demonstrates the multi-interfacial CEs to be good candidates for robust DSSCs.

Highly Electrocatalytic Activity of RuO2 Nanocrystals for Triiodide Reduction in Dye‐Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are promising alternatives to conventional silicon devices because of their simple fabrication procedure, low cost, and high efficiency. Platinum is generally used as a superior counter electrode (CE) material, but the disadvantages such as high cost and low abundance greatly restrict the large-scale application of DSCs. An efficient and sustainable way to overcome the limited supply of Pt is the development of high-efficiency Pt-free CE materials, which should possess both high electrical conductivity and superior electrocatalytic activity simultaneously. Herein, for the first time, a two-step strategy to synthesize ruthenium dioxide (RuO₂) nanocrystals is reported, and it is shown that RuO₂ catalysts exhibit promising electrocatalytic activity towards triiodide reduction, which results in comparable energy conversion efficiency to that of conventional Pt CEs. More importantly, by virtue of first-principles calculations, the catalytic mechanism of electrocatalysis for triiodide reduction on various CEs is investigated systematically and it is found that the electrochemical triiodide reduction reaction on RuO₂ catalyst surfaces can be enhanced significantly, owing to the ideal combination of good electrocatalytic activity and high electrical conductivity.

Nitrogen-doped graphene for dye-sensitized solar cells and the role of nitrogen states in triiodide reduction

Nitrogen-doped graphene was demonstrated as an efficient and alternative metal-free electrocatalyst for dye-sensitized solar cells. Electrochemical measurements showed that the nitrogen-doping process can remarkably improve the catalytic activity of graphene toward triiodide reduction, lower the charge transfer resistance, and thus enhance the corresponding photovoltaic performance. Furthermore, the nitrogen doping levels ranging from 3.5 at% to 18 at%, as well as the nitrogen states (including pyrrolic, pyridinic and quaternary configurations) in graphene, were controlled to interpret the roles of graphene structure in catalytic activity and device performance. The results suggested that the nitrogen states, rather than the total N content, have a significant effect on the catalytic activity. Both pyridinic and quaternary nitrogen states can provide active sites for promoting triiodide reduction reaction, probably due to the shift in redox potential and the lowered adsorption energy.

High performance low temperature carbon composite catalysts for flexible dye sensitized solar cells

Roll-to-roll manufacturing of dye sensitized solar cells (DSSCs) requires efficient and low cost materials that adhere well on the flexible substrates used. In this regard, different low temperature carbon composite counter electrode (CE) catalyst ink formulations for flexible DSSCs were developed that can be simply and quickly coated on plastic substrates and dried below 150 °C. The CEs were investigated in terms of photovoltaic performance in DSSCs by current-voltage measurements, mechanical adhesion properties by bending and tape tests, electro-catalytic performance by electrochemical impedance spectroscopy and microstructure by electron microscopy. In the bending and tape tests, PEDOT-carbon composite catalyst layers exhibited higher elasticity and better adhesion on all the studied substrates (ITO-PET and ITO-PEN plastic, and FTO-glass), compared to a binder free carbon composite and a TiO2 binder enriched carbon composite, and showed lower charge transfer resistance (1.5-3 Ω cm(2)) than the traditional thermally platinized CE (5 Ω cm(2)), demonstrating better catalytic performance for the tri-iodide reduction reaction. Also the TiO2 binder enriched carbon composite showed good catalytic characteristics and relatively good adhesion on ITO-PET, but on ITO-PEN its adhesion was poor. A DSSC with the TiO2 binder enriched catalyst layer reached 85% of the solar energy conversion efficiency of the reference DSSC based on the traditional thermally platinized CE. Based on the aforementioned characteristics, these carbon composites are promising candidates for replacing the platinum catalyst in a high volume roll-to-roll manufacturing process of DSSCs.

Carbon nanotube/graphene nanocomposite as efficient counter electrodes in dye-sensitized solar cells

We demonstrated the replacement of the Pt catalyst normally used in the counter electrode of a dye-sensitized solar cell (DSSC) by a nanocomposite of dry spun carbon multi-walled nanotube (MWNT) sheets with graphene flakes (Gr-F). The effectiveness of this counter electrode on the reduction of the triiodide in the iodide/triiodide redox (I−/I3−) redox reaction was studied in parallel with the use of the dry spun carbon MWNT sheets alone and graphene flakes used independent of each other. This nanocomposite deposited onto fluorinated tin-oxide-coated glass showed improved catalytic behavior and power conversion efficiency (7.55%) beyond the use of the MWNTs alone (6.62%) or graphene alone (4.65%) for the triiodide reduction reaction in DSSC. We also compare the use of the carbon MWNT/Gr-F composite counter electrode with a DSSC using the standard Pt counter electrode (8.8%). The details of increased performance of graphene/MWNT composite electrodes as studied are discussed in terms of increased catalytic activity permitted by sharp atomic edges that arise from the structure of graphene flakes or the defect sites in the carbon MWNT and increased electrical conductivity between the carbon MWNT bundles by the graphene flakes.

Improved Tri-iodide Reduction Reaction of Co-TMPP/C as a Non-Pt Counter Electrode in Dye-Sensitized Solar Cells

ABSTRACT: We report Co-tetramethoxyphenylporphyrin on carbon particles (Co-TMPP/C) as a non-Pt catalystfor tri-iodide reduction in dye-sensitized solar cells (DSSCs). The presence of well-dispersed carbonand cobalt source in the catalyst surface is confirmed by transmission electron microscopy, scanningelectron microscopy, and energy dispersive X-ray analysis. In the C 1s, Co 2p, and N 1s peaks mea-sured by X-ray photoelectron spectroscopy, the C-N, Co-N 4 , and N-C are assigned to the componentat 285.7, 781.8, and 401 eV, respectively. Especially, the Co-TMPP/C shows improved current density,diffusion coefficient, and charge-transfer resistance in the I 3− /I − redox reaction compared to conventionalcatalysts. Furthermore, in the DSSCs performance, the Co-TMPP/C shows increased short circuitcurrent density, higher open circuit voltage, and improved cell efficieny in comparison with Pt/C.Keywords : Tri-iodide reduction, Co-TMPP, Non-Pt, Counter electrode, Dye-sensitized solar cells Recieved November 24, 2010 : Accepted December 28, 2010