•The catalytic activities of different counter electrode catalysts are evaluated.•Several methods are applied to enhance the catalytic activities of these catalysts.•The DSCs with carbon sphere counter electrode shows an efficiency of 10.61%.We evaluate the activities of different counter electrode (CE) catalysts on the regeneration of various redox couples in dye-sensitized solar cells (DSCs). Pt-free CE materials such as poly (3,4-ethylenedioxythiophene) (PEDOT), carbon spheres, and tantalum carbide (TaC) exhibit high catalytic activities toward the I3−/I− and Co3+/2+ redox couples with no cost concerns, although Pt remains the ideal CE catalyst. An opposite trend is observed for organic redox couple of di-5-(1-methyltetrazole) disulfide/5-mercapto-1-methyltetrazole sodium salt (T2/T−). Indeed, Pt-free CE catalysts are more suitable for the organic redox couple. Moreover, several methods have been proposed to enhance the catalytic activities of Pt-free CE catalysts. Synthesis of a composite catalyst is an effective path to enhance the catalytic activity of TaC CE. In PEDOT CE, the constructed porous network structure is essential for achieving high catalytic activity. Tuning the diameter, design hollow structure and open-ended surface can strongly affect their catalytic behavior of the carbon spheres CE. After optimization, the cobalt electrolyte based DSCs yield excellent power conversion efficiency (PCE) of 10.61% with open-ended carbon spheres (OCS) CE. Therefore, we can enhance the PCE value and cut the cost of DSCs by selecting a suitable low cost CE catalyst toward a fixed redox couple.

Dye- or quantum dot-sensitized solar cells (DSCs or QDSCs) comprise a sensitizer, a semiconductor, an electrolyte containing redox couple, and a counter electrode (CE), which have inspired a new wave of research. The challenges in realizing the practical application of such photovoltaic devices are the enhancement of photovoltaic performance, stability, and the reduction of fabrication costs. The CE is an important component, and the exploration of low cost CE catalysts to match the redox couples has become a feasible route in the pursuit of high power conversion efficiency and low production cost of the devices. This article reviews the development of CE catalysts for the regeneration of each type of iodide-free redox couple, including inorganic, organic, and transition metal complex-based redox couples, among others.

•A new kind of carbon sphere, with an open-end on the sphere surface (OCS for short) has been synthesized.•This kind of carbon sphere shows higher catalytic behavior than the conventional solid and hollow carbon spheres.•In dye-sensitized solar cells, as the counter electrode catalysts, the prepared OCS performs better than Pt for the sulfide redox shuttle.A new kind of carbon sphere (denoted as OCS) with an open end on the surface has been synthesized successfully. The designed OCS shows higher electrocatalytic activity than the conventional solid and hollow carbon sphere (denoted as SCS and HCS) as counter electrode (CE) catalyst for the regeneration of both iodide (I−/I3−) and sulfide (T−/T2) redox shuttles in dye-sensitized solar cells (DSCs). For the iodide electrolyte, the DSCs using SCS, OCS, and HCS CEs yield high power conversion efficiency (PCE) values of 7.8, 8.7, and 8.1%, respectively, indicating the potential to replace expensive Pt CE. Most importantly, the carbon sphere catalysts exhibit obvious advantages when applied in sulfide redox shuttle. The OCS-CE based DSCs produces a high PCE of 6.4%, much higher than that for Pt-CE based DSCs (4.1%). The high catalytic activity of OCS benefits from the sufficient contact between the redox shuttle with the external and internal surfaces of OCS because an open end exists on the surface of OCS, which provide a diffusion channel for the electrolyte into the inner of OCS. This strategy for designing open-ended structure is highly important for the fields of drug delivery, nanodevice, and adsorption, but not confined to catalysis.Highly effective catalytic activity has been obtained by designing an open end on the surface of carbon sphere. The open-ended structure is beneficial for liquid-solid catalysis, such as the regeneration of the redox shuttle of the electrolyte occurred on the counter electrode in dye-sensitized solar cells.

Molybdenum carbide nanotubes (Mo2C-NTs) were synthesized and showed remarkable catalytic activity for regeneration of an organic sulfide redox shuttle. The dye-sensitized solar cells (DSCs) using Mo2C-NTs as the counter electrode (CE) showed a high power conversion efficiency of 6.22%, which is much higher than the DSCs using a conventional Pt CE (3.91%).

•MoSi2 is proposed as CE catalysts for DSC for the first time.•Highly effective Pt/MoSi2 composite catalysts have been prepared.•The DSC using Pt/MoSi2 composite CE yield a high PCE value of 7.68%.MoSi2 is introduced into dye-sensitized solar cell (DSC) as counter electrode (CE) catalyst for the first time, and the DSC produces power conversion efficiency (PCE) of 4.87%. To improve the catalytic activity, Pt/MoSi2 composite catalyst is synthesized and it is found that 1.13 wt% of Pt loading is enough for achieving high catalytic activity. After optimization, the DSC using the Pt/MoSi2 composite CE shows high PCE of 7.68%, close to the Pt CE based DSC (7.94%).

To realize long-term developments and practical application of the dye-sensitized solar cells (DSCs) requires a robust increase of the power conversion efficiency (PCE) and a significant decrease of the production cost. Fortunately, a new record PCE value of 12.3% was achieved by using cobalt-based redox couples combined with organic dye. Evidently, dye design is the key path to improve the PCE, while developing low cost counter electrode (CE) catalysts is one of the promising paths to reduce the production cost of DSCs by replacing the expensive Pt CE. In this article, we review the recent progress of CE catalysts involving Pt, carbon materials, inorganic materials, multiple compounds, polymers, and composites. We discuss the advantages and disadvantages of each catalyst and put forward ideas for designing new CE catalysts in future research for DSCs and other application fields.

Vanadium nitride (VN) peas and cubes were synthesized by regulating the molar ratio of the starting materials (urea/VOCl3) via the urea–metal chloride route. The as-prepared VN compounds were subsequently introduced into dye-sensitized solar cells (DSCs) as counter electrode (CE) catalysts for the regeneration of novel organic thiolate/disulfide (T–/T2) and traditional iodide/triiodide (I–/I3–) redox couples. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization curve results proved that the catalytic activity of the prepared VN was significantly affected by particle shape and particle size. The VN peas showed the highest catalytic activity, followed by the small VN cubes and large VN cubes. The I–/I3–-electrolyte-based DSCs yielded a high power conversion efficiency (PCE) of 7.29%. The Pt-free VN CE catalysts are more suitable for the new organic redox couples of T–/T2. The DSCs based on VN peas CE showed a PCE of 5.57%, an enhancement of 40.7% relative to the Pt-CE-based DSCs (3.96%).

Titanium carbide/platinum (TiC/Pt) composites were prepared by a simple chemical route and subsequently introduced into dye-sensitized solar cells (DSCs) as counter electrode (CE) catalysts to improve the catalytic activity and reduce the cost of DSCs. The DSC using TiC/Pt (containing 0.12 wt% of Pt) CE showed high power conversion efficiency (PCE) of 7.63%, higher than those of DSCs using TiC and Pt CEs (6.40% and 7.16%, respectively). After a long-term (one year) stability test, the PCE of the DSC using this CE retained 86.9% of its highest value, proving the outstanding durability of the TiC/Pt composite. In large-scale DSCs (55 mm × 75 mm), the DSC using TiC/Pt CE yielded a PCE of 4.98%, comparable to that of a DSC using Pt CE (4.94%). This work points out the feasibility of using TiC/Pt composite CEs in practical applications.

Nano-scaled tungsten oxides and carbides are synthesized using a simple one-step chemical method. They are subsequently introduced into dye-sensitized solar cells (DSCs) as counter electrode (CE) catalysts to replace the expensive Pt. The catalytic mechanism is investigated by measurements of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel-polarization curve for the as-prepared catalysts. The DSCs using WC/W2C, W2C and WO2 as CE catalysts yield high power conversion efficiency (PCE) of 6.23%, 6.68%, and 6.88%, respectively, comparable to that of the DSC using Pt CE. The results demonstrate that tungsten oxides and carbides are the potential alternative to the expensive Pt CE for reducing the cost of DSCs.

Binary composite of Pt decorated transition metal compounds (WO2, TiC, and VN) were introduced into dye-sensitized solar cells and excellent performance were obtained, almost 20% better than unitary materials. Ternary composite of Pt/WO2/TiO2 was also prepared to further improve the photovoltaic performance.Preparing binary and ternary composites as counter electrodes is an effective path to improve performance and reduce cost for dye-sensitized solar cells.

Nano-scaled tungsten oxides and carbides are synthesized using a simple one-step chemical method. They are subsequently introduced into dye-sensitized solar cells (DSCs) as counter electrode (CE) catalysts to replace the expensive Pt. The catalytic mechanism is investigated by measurements of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel-polarization curve for the as-prepared catalysts. The DSCs using WC/W2C, W2C and WO2 as CE catalysts yield high power conversion efficiency (PCE) of 6.23%, 6.68%, and 6.88%, respectively, comparable to that of the DSC using Pt CE. The results demonstrate that tungsten oxides and carbides are the potential alternative to the expensive Pt CE for reducing the cost of DSCs.

Dye- or quantum dot-sensitized solar cells (DSCs or QDSCs) comprise a sensitizer, a semiconductor, an electrolyte containing redox couple, and a counter electrode (CE), which have inspired a new wave of research. The challenges in realizing the practical application of such photovoltaic devices are the enhancement of photovoltaic performance, stability, and the reduction of fabrication costs. The CE is an important component, and the exploration of low cost CE catalysts to match the redox couples has become a feasible route in the pursuit of high power conversion efficiency and low production cost of the devices. This article reviews the development of CE catalysts for the regeneration of each type of iodide-free redox couple, including inorganic, organic, and transition metal complex-based redox couples, among others.

Titanium carbide/platinum (TiC/Pt) composites were prepared by a simple chemical route and subsequently introduced into dye-sensitized solar cells (DSCs) as counter electrode (CE) catalysts to improve the catalytic activity and reduce the cost of DSCs. The DSC using TiC/Pt (containing 0.12 wt% of Pt) CE showed high power conversion efficiency (PCE) of 7.63%, higher than those of DSCs using TiC and Pt CEs (6.40% and 7.16%, respectively). After a long-term (one year) stability test, the PCE of the DSC using this CE retained 86.9% of its highest value, proving the outstanding durability of the TiC/Pt composite. In large-scale DSCs (55 mm × 75 mm), the DSC using TiC/Pt CE yielded a PCE of 4.98%, comparable to that of a DSC using Pt CE (4.94%). This work points out the feasibility of using TiC/Pt composite CEs in practical applications.

Molybdenum carbide nanotubes (Mo2C-NTs) were synthesized and showed remarkable catalytic activity for regeneration of an organic sulfide redox shuttle. The dye-sensitized solar cells (DSCs) using Mo2C-NTs as the counter electrode (CE) showed a high power conversion efficiency of 6.22%, which is much higher than the DSCs using a conventional Pt CE (3.91%).