3.1. Ternary Metal Sulphides
Ever since Gratzel invented the DSSC, platinum has been the catalyst of choice on the counter electrode to facilitate the rapid reduction of the triiodide ion [
31]. Platinum, owing to its d shell composition, is unparalleled in terms of electron conductivity and catalytic ability, especially in reduction reactions [
32]. However, platinum is also very expensive. In addition, its reserves in the Earth’s crust are limited, thus leading researchers to look for other cheaper alternatives whilst not sacrificing efficiency. When Gratzel fabricated a DSSC with a binary cobalt sulphide counter electrode on ITO/PEN (indium-doped tin oxide coated polyethylene naphthalene) films which produced a 6.5% power conversion efficiency (PCE), it led to widespread effort aimed at finding the right sulphide compound which could be as effective as platinum whilst withstanding corrosion from the iodine electrolyte.
Metal sulphides possess all the necessary requirements for good counter electrode activity, namely high electron conductivity and catalytic ability. Initial research undertaken to study the influence of sulphide counter electrodes on the DSSC efficiency was done on binary sulphide compounds, including CoS [
33], NiS [
34], MoS
2 [
35], WS
2 [
36], and FeS
2 [
37]. Application of binary sulphide counter electrodes in DSSCs yielded varying results, as illustrated in
Table 1. Amongst all the binary sulphides, CoS with a reduced graphene oxide (rGO) support yielded the best efficiency parameters, as shown in
Table 1. Since cobalt and sulphur are cheap and easily accessible, CoS, if subjected to further improvement, would be deemed an ideal replacement for platinum in DSSC counter electrodes. Various methods are being used to achieve higher efficiency for binary sulphide counter electrodes; these include the use of supporting materials which enhance conductivity as well as the synthesis of hierarchical nanostructured sulphides.
Although efficiencies have inched higher, other researchers have resorted to fabricating ternary sulphide catalysts in the hopes of achieving higher efficiencies, or at the very least, equalling the platinum efficiency in counter electrodes. Ternary sulphides are believed to have more superior catalytic ability and conductivity than binary sulphides. This is attributed to the synergistic effect produced by the coexistence of two metal ions in a crystal structure, which results in greater electron conductivity and catalytic ability. Nickel-cobalt sulphides have been the most explored amongst all ternary sulphides. Most of the synthesised ternary sulphides exhibit good counter electrode properties, characterised by low charge transfer and sheet resistances, high reduction current density, and high power conversion efficiency. Initially, researchers fabricated simple ternary compounds which could be employed as counter electrode catalysts in DSSCs.
Lin et al. [
41] fabricated a highly transparent NiCo
2S
4-based counter electrode for use in a DSSC, which attained a PCE of 6.14%. Huang and associates [
42] also synthesised a NiCo
2S
4-based counter electrode which produced an 8.1% efficiency. The differences observed in PCE values for the two counter electrodes could be due to a variety of reasons, such as synthesis methods and assembly methods of the DSSCs. Therefore, to singularly determine the effect of the counter electrode on the performance of the DSSC system, analysis of its catalytic ability and conductivity are necessary. Since the counter electrode is designed to assist the reduction of the triiodide ion, thereby helping in the completion of the regeneration cycle of the dye molecules, its catalytic capability can therefore be measured by the rate at which electrons are transferred from the counter electrode surface to the electrolyte.
Determination of the rate of electron transfer or resistance to electron transfer can be achieved using several methods, which include cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarisation. Two important values from CV analysis which describe the electrocatalytic capability of the counter electrode are the reduction current density and the peak-to-peak potential difference, ∆
EPP. The higher the peak reduction current density, the greater the rate of reduction of the triiodide ion to iodide ions, whereas ∆
EPP signifies the amount of overpotential in the counter electrode. Generally, the standard electrochemical rate constant of a redox reaction is inversely proportional to ∆
EPP, and as such, its value should be minimum. Analysis of the ∆
EPP values for the two sulphide counter electrodes showed that NiCo
2S
4-1, fabricated by Lin et al. [
40], had lower over potential losses at 0.22 V, compared to 0.37 V for NiCo
2S
4-2, fabricated by Huang et al. [
41]. Consequently, the rate of reduction for NiCo
2S
4-1 proceeded faster than for NiCo
2S
4-2. Since the reduction current was higher for NiCo
2S
4-1, this means that electrons encountered lower charge transfer resistance when this counter electrode was used. This is also observable in the values of the sheet resistance
Rs and charge transfer resistance
Rct obtained from electrochemical impedance spectroscopy (EIS) analysis. PCE values for the NiCo
2S
4-1 and NiCo
2S
4-2 CEs show that despite having a lower ∆
EPP and
Rct as well as a higher open circuit voltage, NiCo
2S
4-1 (NCS1) is not as efficient as NiCo
2S
4-2 (NCS2). This signifies that the overall power conversion efficiency values of a DSSC are merely a poor indicator of the effect of the counter electrode on the DSSC. The discrepancy in PCE values could be caused by a variety of reasons, including differences in how the DSSCs were assembled, different synthesis methods, and the poor functionality of the other components of the DSSC. As clearly seen in
Table 2 the PCEs of NCS1 and NCS2 are still very low compared to that of platinum; consequently, current research has sought to fabricate sulphide counter electrodes with more highly modified structures that are believed to enhance catalytic capability.
To date, many NiCo
2S
4 nanostructured compounds have been synthesised. These include microspheres [
43], nanocomposites [
44], nanosheets [
28], nanowires [
45], and nanorods. Although most of the sulphide-based catalysts have been fabricated for use in capacitors, they have also recently found widespread use in DSSCs as counter electrodes. Nanomaterials possess many outstanding qualities, such as a large electroactive surface area, larger electrolyte infiltration area, high electron transfer rate, and good structural stability. These properties improve the catalytic capability of DSSC counter electrodes. Despite all the theoretical advantages outlined, some of the results obtained experimentally are very poor, suggesting the need for further research and perhaps implementation of better synthesis methods.
Of note, Liu et al. [
45] fabricated a NiCo
2S
4 nanowire array through an ion exchange reaction. Liu et al. [
45] achieved PCEs of 5.35% efficiency with the DSSC technology employing NiCo
2S
4 nanowire as a CE. This counter electrode produced a short current density of 14.30 mA/cm
2, compared to 13.72 mA/cm
2 for the platinum counter electrode. The higher short current density for the NiCo
2S
4 nanowire array was attributed to the high catalytic activity for
reduction, which speeds up dye regeneration at the TiO
2 photoanode. Despite having a more sophisticated structure, the PCE for the NiCo
2S
4 nanowire counter electrode was actually lower than those of most binary sulphide counter electrodes. The DSSC assembled using the pure platinum counter electrode did not fare any better, with 5.92% efficiency, which is unusually low compared to other platinum counter electrodes. Therefore, since the two PCE values were very similar for the NiCo
2S
4 nanowire and the platinum counter electrode, experimental conditions as well as the means and ways of assembling the DSSCs should also be considered as efficiency determinants.
A better-functioning nanostructured NiCo
2S
4 counter electrode was developed by Huo et al. [
43]. Together with associates, they synthesised flower-like nickel-cobalt sulphide microspheres modified with nickel sulphide. The NiCo
2S
4/NiS counter electrode was fabricated via a two-step hydrothermal method. In order to determine how the NiS support affected performance as well as compare its efficiency to platinum, separate NiCo
2S
4, NiS, and Pt counter electrodes were also developed. The DSSC assembled with the NiCo
2S
4/NiS counter electrode produced a modest 8.8% PCE.
CV analysis to determine its catalytic capability was characterised by a very low ∆
EPP, suggesting a very high rate of reduction for the triiodide ion.
Figure 2 depicts curves for the cyclic voltammetry and electrochemical impedance spectroscopy analysis. As expected, a very high reduction current density was produced, further supporting the notion of its excellent catalytic activity. Since the reduction current was high, accompanied by a low ∆
EPP, the counter electrode was expected to exhibit very low sheet and charge transfer resistance. Charge transfer resistance values from EIS analysis illustrated the good catalytic qualities of the counter electrode, with
Rct and
Rs values of 2.2 and 9.6 Ω, respectively. As clearly indicated in
Table 2 the
Rct and
Rs values for NiCo
2S
4/NiS were the lowest of the three platinum-free counter electrodes. The incorporation of NiS with NiCo
2S
4 to produce the NiCo
2S
4/NiS electrode improved the transfer of electrons from the counter electrode to the electrolyte. This was illustrated by the drop-in charge transfer resistance from 4 Ω for NiCo
2S
4 to 2.2 Ω for NiCo
2S
4/NiS, which is a 55% decline in resistance.
The increased transfer of charge is attributed to the fact that NiS increases the specific surface area of the microspheres, and hence more electrolyte is in contact with the counter electrode. The platinum counter electrode, despite having the highest reduction current density, possesses a high ∆
EPP of 504 mV, as well as a higher charge transfer resistance of 5.2 Ω. As a result, the DSSC with the NiCo
2S
4/NiS counter electrode performs better, with 8.8% PCE compared to 8.1% for platinum. As clearly shown above, the introduction of other materials could enhance the overall properties of the NiCo
2S
4 counter electrode. Most notably, carbon materials are known to be highly conductive as well as possessing a larger surface area of contact with the electrolyte.
Table 3 illustrates some of the most commonly used carbon-based materials.
The most commonly used carbon-based supporting materials include carbon nanotubes, carbon black, graphene, and graphite. Despite being touted as being highly conductive, carbon materials incorporated in DSSCs as counter electrodes have produced only moderate results, characterised by high sheet and charge transfer resistance. However, Kakiage et al. [
11] reported a high efficiency of 14.7% for a graphene nanoplatelets (GNP)-based CE, which was higher than that of the Pt-based CE DSSC (13.8%). Carbon materials have, however, found more use in DSSCs as supports for other conducive materials. One such example where the introduction of a carbon-based material enhanced the properties of the counter electrode was in the work undertaken by Anuratha et al. [
44]. In this work, Anuratha and associates undertook the fabrication of a NiCo
2S
4 counter electrode on a reduced graphene oxide support. The enhanced catalytic activity produced by the NiCo
2S
4-rGO counter electrode was attributed to the increased specific surface area of graphene oxide as well as its excellent electron conductivity. Anuratha et al. fabricated three counter electrodes via a one-step solvothermal method with ratios of NiCo
2S
4 to rGO (reduced graphene oxide) equalling 1:0.3, 1:0.6, and 1:1. The reported synthesis procedure for the counter electrodes is outlined in
Figure 3.
It was noted that the amount of graphene incorporated onto the counter electrode dictated the performance of the DSSC. The CV graph for rGO-NCS-2 produced the highest reduction current density, as shown in
Figure 4a. Analysis of EIS data from
Figure 4d shows that rGO-NCS-2 was the most catalytically active counter electrode, with the lowest charge transfer resistance of 0.37 Ω. The catalytic activity of NCS, characterised by a high
Rct of 0.85 Ω, indicates that the performance enhancement contributed by reduced graphene oxide was barely minimal. Furthermore, this work illustrates the importance of optimisation during counter electrode fabrication in order to attain the best possible performance.
At low graphene oxide levels, catalytic activity is still low, as characterised by the high
Rct followed by a subsequent increase to the optimum point as rGO content increases. After the optimum point has been attained, the catalytic activity dwindles as more reduced graphene oxide is deposited on the counter electrode. Higher charge transfer resistance for reduced graphene oxide at 28.22 Ω implies that poor catalytic activity occurs at the counter electrode for the reduction of the triiodide ion. The stability of the synthesised counter electrodes was also measured for 20 cycles without any noticeable changes. As shown in
Figure 4b–c, the shape of the cyclic voltammetry curve for NCS and rGO-NCS-2 remained virtually the same. Since photovoltaic cells normally function for a long time, it would be more ideal if the testing for stability was done for a larger number of cycles or for a lengthier period in order to ascertain their stability.
Other notable suitable sulphide counter electrodes that have been developed to replace platinum include CoNi
2S
4 [
46] and CuInS
2 [
47]. Shi et al. [
46] hydrothermally fabricated three nanostructured CoNi
2S
4 counter electrodes with different film deposition width. The most effective amongst these counter electrodes produced an efficiency of 4.61%. At lower film width, the counter electrode exhibited poor catalytic ability, and hence its cyclic voltammetry curve only had redox peaks at the positive potentials. This counter electrode consequently experienced high charge transfer resistance. The other two electrodes showed minimal charge transfer resistance characterised by low
Rct values. When more substrate is deposited above the optimal level of thickness, the charge transfer resistance begins to rise. Evaluation of charge transfer resistance values shows that the highest efficiency is attainable when the amount of substrate deposited on the FTO is optimal. The thicker the film of substrate deposited on the FTO, the more the resistance it encounters, whereas the lower the film thickness, the poor the catalytic capability it exhibits.
Using liquid impregnation and carbonisation, Wang et al. [
47] fabricated carbonised eggshell membranes (ESM) loaded with CuInS
2 nanocrystals. Photovoltaic parameters for the developed sample were rather low, with the power conversion efficiency for this counter electrode equalling 5.79%. Resistances to electron movement were obtained at 9.65 Ω and 20.49 Ω for
Rct and
Rs, respectively. Since the two resistance values were high and coupled with a low PCE, CuInS
2 would not be an ideal counter electrode. Development of quantum dot-sensitised solar cells (QDSSCs) has also emancipated the fabrication of counter electrodes. Givalou et al. [
50] synthesised novel cobalt-copper sulphide (CoS-CuS) counter electrodes for use in QDSSCs. The novel sulphide counter electrode produced a 5.03% efficiency.
Kim et al. [
51] also fabricated a CuS-NiS composite counter electrode for use in QDSSCs. The developed counter electrode exhibited low power conversion efficiency, at 4.19%.
Table 2 outlines the performance parameters for some of the ternary sulphide counter electrodes that have been developed to date. It is evident that the most actively used methods of synthesis for ternary sulphides are hydrothermal and solvothermal synthesis. Efficiency parameters of all the sulphide counter electrodes in
Table 2 are very low compared to platinum. The best-performing ternary sulphide was fabricated in a complicated three-step process, which would be very tedious for any would-be manufacturer to adopt.
Furthermore, ternary sulphides alone do not seem to possess adequate catalytic capability, and hence they require supports to enhance their catalytic ability. These supports, at times, lead to reduced catalytic activity if the thickness of the deposited film is excessive, which leads to increased sheet resistance [
55]. Thus, one of the effective ways to obtain high efficiency parameters in dye-sensitised solar cells is through optimisation of the developed counter electrodes to determine the required width of film to be deposited. Considering that sulphides are abundant and easily accessible, more research is required to develop truly efficient ternary sulphide counter electrodes that can parallel platinum’s capability. It is also particularly important that the most ideal method of synthesis be used to increase the probability of producing the required counter electrode that can exhibit the requisite properties to the fullest.
The continual fabrication of ternary sulphide CEs with efficiencies lower than the platinum counter electrode has led researchers to delve into the development of ternary transition metal alloys. Since platinum group metals have been proven to be catalytically superior in redox reactions, they could be incorporated with other transition metal elements that offer specific qualities which are needed by the counter electrode. These CEs could potentially function better than ternary sulphides, since each individual element has a specific contribution to the system. Development of these ternary alloys and their performance is discussed in the next subchapter.
3.2. Ternary Transition Metal Alloys
Transition metal elements have been known to partake in catalytic reactions with increased vigor than other elements in the periodic table. The use of platinum as the standard counter electrode catalyst for iodine reduction was borne from its ability to be one of the most effective reducing agents [
56]. Transition metal elements, especially those with lone pairs of electrons in their d orbitals, can easily partake in redox reactions with a minimal supply of energy. As outlined earlier, researchers sought to synthesise binary metallic alloys as replacements for platinum in DSSC counter electrodes. Although most of the binary alloys have performed well, their efficiency is still far below that of a pure platinum metal CE [
57,
58]. However, what can be deduced so far from the work undertaken to fabricate binary metallic alloys is that the synergistic effect of the two metals creates an environment which facilitates faster charge transfer between the counter electrode and the electrolyte. Thus, in search of materials that can offer parallel performance to platinum, the fabrication of ternary metallic alloys has been extensively explored.
Figure 5 shows the most commonly used transition metal elements for ternary alloy fabrication. The most commonly used elements reside in Group VIII to
X. These elements have lone pairs of electrons as well as empty d orbitals, making for easy electron transfer in redox reactions.
Yang et al. [
59] produced a branching NiCuPt alloy counter electrode which exhibited modest performance parameters. The NiCuPt alloy was synthesised via a three-step process, which included: (1) the electrodeposition of Ni on ZnO microrod templates, (2) the growth of Cu on the developed Ni microtubes, and (3) the galvanic displacement of Zn by Pt. Cyclic voltammetry analysis of the developed counter electrode illustrated a high reduction current density, thus good electrocatalytic activity could be expected. Furthermore, it was noted that the current density produced was dependent on the amount of galvanic displacement time the counter electrode experienced. Peak current density optimally rises to a maximum of 14.42 A within 2 h and subsequently declines for any further elongation of galvanic displacement time. The current density pattern produced in this work can be attributed to the fact that at low displacement times, not enough deposition of NiCuPt occurs. Also, after 3 h displacement time, the thickness of the deposited film becomes inhibitive to the flow of electrons, hence high
Rct values should be expected as well as low PCE. Charge transfer resistance values from EIS analysis depict the same pattern, with NiCuPt after 2 h displacement (NiCuPt-2 h) having the least resistance. Subsequently, NiCuPt-2 h yields a modest 9.66% PCE, compared to 8.22% for NiCuPt-15 min. As clearly visible from this work, optimisation is essential in order to obtain the best-functioning counter electrode. Yang et al. [
60] also developed a PtNiCo alloy counter electrode which displayed significant electrocatalytic capability. This counter electrode was developed via a three-step hydrothermal process similar to that illustrated in Reference [
60].
Figure 6 shows SEM images of the developed PtNiCo alloy.
The hexagonal ZnO microrods in
Figure 6a have an average diameter of 1.5 µm. The dissolution of the ZnO microrods in sulphuric acid and subsequent addition of Ni microspheres (
Figure 6b) and Co nanosheets (
Figure 6c) results in the formation of the PtNiCo alloy shown in
Figure 6d,e. The developed PtNiCo alloy can be described as consisting of PtCo nanosheets and PtNi nanorods. The developed counter electrode exhibited a PCE of 8.85%. CV analysis of PtNiCo produced two pairs of redox peaks with peak current density of 12.24 mA·cm
−2. The ∆
Epp produced was also low, signifying that the rate of the reduction reaction was very high. Consequently, charge transfer between the counter electrode and the electrolyte was exceptionally fast, with resistance equalling a low 0.04 Ω, compared to 0.75 Ω for platinum.
Yang et al. [
61] fabricated ternary platinum alloy counter electrodes of the composition Pt-M-Ni, where M = Co, Fe, and Pd. The three fabricated electrodes exhibited modest power conversion efficiencies, as shown here in
Table 4. The three developed counter electrodes were described as having dense stacked structures. Pt-Co-Ni was homogeneously distributed on FTO, whereas the other two CEs experienced agglomeration. As a result, Pt-Co-Ni possessed lower charge transfer resistance and higher PCE since it had more active sites for electrolyte adsorption.
Figure 7 depicts the morphological structure of the three counter electrodes. CV analysis of the three developed counter electrodes yielded high reduction current densities in the order Pt-Co-Ni > Pt-Pd-Ni > Pt-Fe-Ni. ∆
Epp values increased in the order Pt-Fe-Ni < Pt-Pd-Ni < Pt-Co-Ni. A measure of the impediment to charge transfer from the counter electrode to the electrolyte yielded a low
Rct for all, in the order Pt-Co-Ni < Pt-Pd-Ni < Pt-Fe-Ni < PtCo < PtPd < PtFe < Pt.
As clearly shown in
Table 4, ternary alloy counter electrodes possess lower resistances to the movement of electrons in the DSSC as compared to ternary sulphide counter electrodes, as described in
Table 3; consequently, their power conversion efficiencies have been reported to be higher. For example, the greater efficiency of Pt-Co-Ni compared to Pt-Pd-Ni was attributed to the matching of work functions to the redox potential of the electrolyte, leading to satisfactory catalytic activity. Matching work functions of alloy CEs are believed to induce partial electron transfer to the near surface of Pt from the transition metal atoms, leading to weakened binding energy between electrons and nucleus. These weakened electrons participate in the triiodide reduction. Impressive electrocatalytic capability and rapid electron transfer between the counter electrode and electrolyte are vital in minimising recombination reactions, thereby enhancing the overall DSSC efficiency.
Other researchers have sought to introduce nonmetallic polyaniline with binary alloy systems, since polyaniline exhibits high electrical conductivity. Polyaniline (PANI) being an electron donor, has the potential to be an excellent counter electrode material. Although the photovoltaic parameters from DSSCs employing PANI as the counter electrode are poor, its excellent electrical properties are ideal for a support material. Since PANI also exhibits good optical transparency, it can be utilised to construct bifacial DSSCs, which have the capability of generating electricity on either side. Yu et al. [
62] developed platinum alloys decorated with polyaniline. Yu et al. [
62] reported three counter electrodes, namely PANI/MoPt, PANI/CoPt, and PANI/PdPt. The fabricated DSSC devices based on these three counter electrodes had efficiencies of 8.08, 7.06, and 6.83%, respectively. In order to validate whether PANI invigorates the function of the counter electrode, photovoltaic parameters for pristine PANI were also obtained. CV measurements of the four fabricated counter electrode systems yielded the highest peak current density for PANI/CoPt, with PANI possessing the lowest current density. Charge transfer resistance from the EIS analysis followed a similar trend, with PANI/CoPt possessing the lowest resistance, whilst PANI was the most inhibitive to electron flow. The excellent catalytic capability of PANI/CoPt was attributed to the similarity between its work function and that of I/I
3. The greater the difference in work function values, the greater the resistance encountered by electrons when transferring from the counter electrode to the electrolyte.
Table 4 illustrates the wide range of synthesis methods that have been explored in ternary transition alloy fabrication. Hydrothermal synthesis of NiCuPt [
59] yielded the least charge transfer resistance, and hence produced the highest power conversion efficiency. The effects of morphology on the properties of the counter electrode are shown in
Figure 6, adopted from Reference [
61]. Since the reduction process involves the adsorption of
on the counter electrode interface, its surface area and morphology must be ideal so as to offer the maximum number of catalytically active sites, thereby enhancing the reaction speed. Despite high efficiency parameters, the continued use of platinum on counter electrodes defeats the purpose of trying to reduce costs and increase mass production. Most of the ternary metallic alloys were fabricated via tedious numerous-step processes, which would be ideal for laboratory-scale operations, but highly reprehensive for large-scale production. As outlined earlier, charge transfer from the counter electrode to the electrolyte is dependent on how their work functions match. Since the work function of the CE is dependent on the alloy, it is important that the metals constituting the alloy have matching work functions whilst also complimenting each other. Ideally, the design of ternary metal alloys should be such that the three metals complement each other. For the best-performing NiCuPt CE, platinum, being the most active, would be tasked with increasing the speed of triiodide reduction. Hence, the remaining two metals, copper and nickel, should offer the other prerequisite requirements; that is, being either highly conductive, thus increasing stability in the iodine electrolyte, or increasing the surface area of contact. Further research is, however, required to determine the effects of these metals on the outlined properties. Currently, no results have been made available on this topic.
3.3. Ternary Selenides and Oxides
Metal selenides and oxides do possess high catalytic activity and good thermal properties, as well as being cheap materials [
64]. Jiang et al. [
65] fabricated a cobalt–nickel-based ternary selenide, Co
xNi
1−xSe, with different cobalt and nickel ratios using the solvothermal method. The
x values ranged from 0 to 1, namely 0, 0.32, 0.42, 0.52, 0.74, and 1, respectively.
Figure 8a shows the CV curves of the reported synthesised counter electrodes. As can clearly be seen, Co
0.42Ni
0.58Se possessed the largest reduction current density, signifying a higher triiodide reduction intensity when it is used. ∆
Epp was also the lowest for Co
0.42Ni
0.58Se, meaning that it had a faster rate of reaction amongst all the fabricated counter electrodes. The higher electrocatalytic ability of CoNiSe was attributed to the optimised synergistic effect between Co and Ni ions. To further elucidate the electrocatalytic capability of selenide counter electrodes, EIS measurements were taken, in which Co
0.42Ni
0.58Se had the least charge transfer resistance at 2.95 Ω. Its sheet resistance
Rs was also observed to be 21.49 Ω, which was far higher than 14.78 Ω for platinum. This proves that platinum functions as a better electrical conductor than Co
0.42Ni
0.58Se. Despite its impressive electrocatalytic ability, Co
0.42Ni
0.58Se had a poor 6.15% power conversion efficiency.
Figure 8b illustrates the Nyquist plots for the synthesised counter electrodes.
Other researchers have been successful in synthesising selenide counter electrodes with higher power conversion efficiencies. Shao et al. [
66] developed ternary NiCoSe hollow microspheres which could be excellent counter electrodes in DSSC. Shao et al. [
66] fabricated four NiCoSe counter electrodes at varying temperatures via the hydrothermal method. The compositions of the four developed counter electrodes are outlined in
Table 5. From the CV curves obtained, NiCoSe-180 °C had the highest reduction current density as well as the lowest peak-to-peak difference. Charge transfer resistance from EIS analysis proved NiCoSe-180 °C was catalytically superior compared to other counter electrodes, with 1.27 Ω resistance. Consequently, NiCoSe-180 °C possessed the highest power conversion efficiency at 9.04%. From
Table 5, it can clearly be seen that catalytic performance increases as temperature increases up to 180 °C and subsequently decreases for any further increase in temperature. At temperatures below 180 °C, the formation of irregular hollow spheres occurs, whereas at temperatures above 200 °C, the hollow structures collapse and deform due to the sustained high temperatures.
At temperatures ≈ 180 °C, the formation of regular hollow structures composed of homogeneous rough nanoparticles occurs, which provide effective transport pathways for the rapid transportation of electrons and ions, leading to enhanced electrochemical kinetics.
Figure 9 illustrates how the shape of microspheres vary with temperature.
Table 5 illustrates how the amount of cobalt present in each counter electrode determines its catalytic prowess.
The power conversion efficiency trend for the four electrodes mirrors the trend in cobalt composition. Efforts to replace platinum with ternary oxides have also been explored. Du et al. [
67] hydrothermally fabricated NiCo
2O
4 for application in dye-sensitised solar cell counter electrodes. In this work, through morphology manipulation, Du and associates developed and optimised new structures with more open channels, thereby facilitating easier electron and ion movement. The newly developed structures were nanoflowers, nanosheets, and nanorods. Power conversion efficiencies for the three developed structures equalled 8.48, 3.57, and 6.48%, respectively. Charge transfer and sheet resistance were, as expected, the lowest for NiCo
2O
4 nanoflowers, at 0.35 and 4.94 Ω, respectively. These values signify that NiCo
2O
4 nanoflowers have excellent reduction capability for triiodide ions as well as good conductivity. Analysis of the effect of hydrothermal reaction time on cell efficiency shows that for shorter reaction periods, the power conversion efficiency was relatively weak, whereas after 12 h reaction time, the PCE was 8.48%. Thus, efficiency increases with an increase in reaction time. In order to improve the function of the NiCo
2O
4 counter electrodes, Wang et al. [
68] explored the combination of the ternary oxide with carbon black. This counter electrode achieved an efficiency of 6.27% to 7.38% for the platinum counter electrode. EIS analysis of the counter electrode yielded a 2.2 Ω charge transfer resistance, compared to 1.8 Ω for platinum. The NiCo
2O
4/carbon black composite counter electrode is still below the standard required to replace the platinum counter electrode. Further research is required to improve its efficiency. Perhaps the inclusion of carbon nanotubes in this composite rather than carbon black would produce satisfactory results.
Table 6 illustrates the dominance of the hydrothermal method in ternary alloy fabrication. Efficiencies obtained using this method are still very low, thus making the process less attractive since it is very expensive. Further research is required to elevate the efficiencies of ternary selenides and oxides.
The effect of the price of platinum on the large-scale use of the DSSC technology is clearly illustrated in
Table 7. The price of platinum per gram is substantially higher than for any other substance, except carbon nanotubes. Platinum’s high price, which is always fluctuating because of widespread demand, would not be sustainable for CE manufacture, and consequently, the DSSC technology could not compete with silicon-based solar cells as the best solar energy generation technology.
Table 7 also illustrates the quandary associated with carbon nanotubes, where the advanced nature and infancy of their development makes them equally expensive for use in DSSCs, Therefore, compatible solutions to eliminate the high-cost factor of platinum would be to employ cheaper sulphides or metal alloys such as NiCo or PtRu, which consist of limited amounts of platinum. To date, most of the platinum-free counter electrodes that have been developed have performed at lower efficiencies compared to the platinum counter electrode. The most vital factors determining catalytic activity are the morphology and specific area of the catalyst. As such, the development of catalysts on carbon supports, such as carbon black or graphene, which offer higher specific area will improve catalytic performance. Anuratha et al. [
44] developed a rGO-NiCo
2S
4 which had a higher PCE at 8.15%, compared to 7.37% for NiCo
2S
4. Furthermore, the atomic distribution in higher metallic alloys has a great influence on its catalytic activity. Taking PdNiCo as an example, the more catalytically active palladium should be distributed in such a manner as to have greater contact with the electrolyte, thereby influencing its catalytic ability. One possible way of achieving this is through synthesis of alloys with a core-shell structure, where the more active metal comprises the shell, or in hollow structures, which guarantee a high specific surface area-to-volume ratio.