Hierarchical ZnO/NiCo₂O₄/Nb₂C: A Novel Counter Electrode for Platinum-Free Dye-Sensitized Solar Cells

Abstract

The development of sustainable, cost-effective, and high-performance counter electrodes (CEs) remains a major challenge in dye-sensitized solar cell (DSSC) research. Although Pt remains the benchmark CE because of its excellent electrocatalytic activity and electrical conductivity, its scarcity and high cost motivate the development of efficient Pt-free alternatives. Herein, we report a novel Pt-free counter electrode based on a MOF-derived ZnO/NiCo₂O₄/Nb₂C MXene composite for DSSC applications. The counter electrode materials were synthesized by a hydrothermal method and systematically characterized to evaluate their structural, morphological, and photovoltaic properties. X-ray diffraction analysis revealed characteristic peaks at 36.3°, 34.1°, and 40.3°, confirming the formation of ZnO, NiCo₂O₄, and Nb₂C. The optimized ZnO/NiCo₂O₄/Nb₂C MXene counter electrode achieved a power conversion efficiency (PCE) of 8.08%, comparable to that of Pt-based DSSCs under standard illumination. This enhanced performance is attributed to the synergistic effects of high catalytic activity, rapid charge transfer, and robust structural integrity in the MOF-derived ZnO/NiCo₂O₄/Nb₂C MXene composite. Also, the addition of Nb₂C MXene significantly enhanced the overall electrical conductivity and accelerated electron transfer kinetics at the electrolyte–electrode interface. These findings demonstrate that the ZnO/NiCo₂O₄/Nb₂C MXene composite is a promising platinum-free counter electrode for advancing DSSCs toward practical commercialization and large-scale energy harvesting applications.

1. Introduction

Dye-sensitized solar cells, widely recognized as third-generation photovoltaic devices, offer distinctive advantages, including facile fabrication, transparency, flexibility, and adaptability, making them strong candidates for sustainable solar energy conversion. − DSSCs rely on abundant, low-toxicity materials, simple fabrication, and low processing temperatures, making them suitable for affordable and eco-friendly solar energy conversion and viable candidates for industrial-scale energy solutions. , Despite their inherent advantages, DSSCs face critical challenges such as their comparatively lower power conversion efficiency (PCE) relative to silicon-based solar cells and the sustained requirement to enhance long-term operational stability. Current research efforts aim to address these limitations through material innovations and device optimization.

The components of a DSSC comprise a conductive substrate, a semiconductor anode, a sensitizing dye, an iodine-based electrolyte, and a counter electrode. , Here, the counter electrode (CE) is a crucial component of dye-sensitized solar cells (DSSCs), accountable for collecting electrons from the external circuit and catalyzing the reduction of triiodide (I₃ ^–) to iodide (I^–) in the electrolyte, thereby completing the electrical circuit and facilitating the regeneration of the redox mediator. , Platinum (Pt) is the most widely used material for CEs due to its outstanding catalytic activity and high conductivity. , However, its high cost, scarcity, and limited stability in corrosive electrolytes restrict its large-scale application and have motivated researchers to focus on exploring earth-abundant, low-cost, and highly active substituents. − As an efficient replacement for Pt, a range of materials implemented so far includes carbon-based materials, metal sulfides, metal oxides, conducting polymers, and different hybrid composites. − Recent studies have focused on hybrids because they integrate the complementary advantages of individual components, combining high conductivity, abundant catalytic sites, and hierarchical porous sites, thereby achieving superior electrocatalytic activity, faster charge transfer kinetics, and improved long-term stability compared to single-component counterparts. ,

Metal–organic frameworks (MOFs) and their derivatives have emerged as promising alternatives to platinum (Pt) for counter electrodes (CEs) in dye-sensitized solar cells (DSSCs), owing to their unique structural features, high surface area, tunable porosity, and catalytic activity. The use of MOF-derived materials addresses the high cost and limited stability of traditional Pt electrodes, offering potential for high efficiency and low-cost DSSCs. , Carbon materials derived from ZIF-8 (a zeolitic imidazolate framework) have been applied as CEs in DSSCs, demonstrating easy fabrication, large specific surface area, and high catalytic activity for the reduction of I₃ ^– ions. These electrodes achieved a power conversion efficiency (PCE) of 7.32%, similar to Pt-based cells. N-doped carbon sheets synthesized from ZIF-8/graphene oxide composites exhibited superior electrocatalytic activity for I₃ ^– reduction, achieving a PCE of 8.2%, which surpasses that of Pt CEs (7.6%). Moreover, MOF-5, when combined with a conductive polymer binder (s-PT) and deposited on carbon cloth, forms a hierarchical electron transfer network. This composite CE achieved a PCE of 8.91%, and under dim-light conditions, efficiency reached 9.75%. However, direct carbonization of MOFs can lead to disintegration and aggregation, reducing electronic conductivity, which is one of the main concerns in integrating MOF-derived CEs.

Emerging materials such as NiCo₂O₄ and NiCo₂S₄ were also found to be used as counter electrodes for various applications. Recent studies show that NiCo₂O₄/ NiCo₂S₄ based electrodes deliver high catalytic activity toward I^–/I₃ ^– reduction, good conductivity, and significant cost advantages over Pt. However, to maximize performance, NiCo₂O₄/ NiCo₂S₄ was used in composites along with materials such as carbon allotropes and metal oxides. − Li et al. used NiCo₂O₄ with electrospun carbon nanofibers (CNFs) as CEs in DSSCs and reported an efficiency of 7.80%. Similarly, core–shell nanostructures of NiCo₂O₄@MnMoO₄ were used as CEs and reported up to 11.87% efficiency. Nukunudompanich et al. implemented NiCo₂O₄@ZnS nanorods with MWCNTs as CEs, and an efficiency of 10.03% was reported. It can be seen that mixing NiCo₂O₄ with a conducting material enhances the overall efficiency of DSSCs. MXenes are also known for exceptional conductivity along with high hydrophilicity and mechanical strength. MXene-based composites have reported superior electrocatalytic activity and lower charge-transfer resistance, often used as CEs, and the PCEs match or surpass Pt. Hu et al. used CNTs/Ti₃C₂T _x MXenes composite CEs for DSSCs. They showed low charge-transfer resistance and good electrocatalytic activity in I^–/I₃ ^– electrolyte. The PCE was very close to Pt, 5.83%. Similarly, MXene-graphene was used by Nagalingam et al. and reported a PCE of 6.4%. Furthermore, MXene/NiCo₂S₄ 2D/3D heterostructure CEs were used, and 8.76% efficiency was achieved.

From the literature, it can be seen that most studies use binary composite, and MOF-derived materials and Nb₂C MXenes have not been well explored as CEs in DSSCs. In this study, we report a novel MOF-derived ZnO/NiCo₂O₄/Nb₂C tertiary hybrid as a counter-electrode material for DSSCs. The synthesis was carried out in a hydrothermal system. The materials obtained were confirmed using XRD and FTIR. The morphological analysis was conducted using FESEM and HRTEM. The synergistic integration of MOF-derived ZnO and NiCo₂O₄ within an Nb₂C MXene matrix will create a multi-component hybrid architecture that significantly lowers the charge-transfer resistance for the I^–/I₃ ^– redox reaction. This structure provides a high density of active sites and accelerated electron pathways, leading to photovoltaic conversion efficiencies in DSSCs that are comparable to or exceed traditional Pt-based counter electrodes while offering superior long-term electrochemical stability.

2. Materials and Methods

Zinc nitrate hexahydrate (Zn­(NO₃)₂.6H₂O), 2-methylimidazole, nickel­(II) nitrate hexahydrate (Ni­(NO₃)₂.6H₂O), cobalt­(II) nitrate hexahydrate (Co­(NO₃)₂.6H₂O), urea, N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride (PVDF), and N719 dye were purchased from Sigma-Aldrich (USA) and used without further purification. The hydrothermal process was carried out using deionized water in a BAOSHISHAN 100 mL autoclave with a PTFE liner.

2.1. Synthesis of MOF-Derived ZnO

ZIF-8 was synthesized following the method reported by Mani et al. Typically, solution A comprising Zn­(NO₃)₂·6H₂O (2.376 g) in 40 mL of DI water was added dropwise to solution B, containing 2-methylimidazole (1.312 g) in 40 mL of DI water. The mixture was stirred for 4 h, after which the ZIF-8 precipitate was isolated via centrifugation and washed repeatedly with methanol and DI water. The product was dried at 70 °C for 12 h. Finally, MOF-derived ZnO was obtained by calcining the ZIF-8 crystals at 500 °C for 5 h in air.

2.2. Synthesis of MOF-Derived ZnO/NiCo₂O₄/Nb₂C Hybrid

The ZnO/NiCo₂O₄/Nb₂C hybrid was synthesized via a hydrothermal method followed by thermal annealing. Initially, Ni­(NO₃)₂·6H₂O (0.43 g, 0.03 M), Co­(NO₃)₂·6H₂O (1.1 g, 0.12 M), and urea (1.5 g, 0.5 M) were dissolved in 50 mL of double-distilled water (DDW) with stirring for 30 min. In a separate vessel, 1 g of MOF-derived ZnO and 0.56 g of Nb₂C were dispersed in 30 mL of DDW. The metal salt solution was added dropwise to the dispersion and stirred for 45 min, followed by 15 min of bath sonication to ensure homogeneity. The mixture was then sealed in a 100 mL Teflon-lined stainless-steel autoclave and maintained at 120 °C for 6 h. The resulting precipitate was collected, washed thoroughly with DDW and ethanol, and dried overnight at 80 °C. Finally, the hybrid material was obtained by annealing the dried powder at 450 °C for 2 h.

2.3. Fabrication of DSSCs

The DSSC devices were fabricated according to a previously reported procedure. To prepare the photoanode, TiO₂ was ground with four drops of N-methyl-2-pyrrolidone (NMP) for 25 min to produce a homogeneous, viscous slurry. This slurry was coated onto a fluorine-doped tin oxide (FTO) substrate and dried at 80 °C overnight. The resulting film was sensitized by immersion in a 3 mM N719 dye solution for 24 h. Separately, the counter electrode (CE) was prepared by mixing 95 mg of the synthesized CE material with 5 mg of polyvinylidene fluoride (PVDF) and NMP. This mixture was uniformly deposited onto an FTO glass substrate and dried at 80 °C for 8 h. Finally, the photoanode and CE were sandwiched together with a separator, and an iodide/triiodide (I^–/I₃ ^–) electrolyte was introduced into the active area. The photovoltaic parameters, including fill factor (FF) and power conversion efficiency (PCE), were calculated using eqs and .

$$\mathrm{FF}=\frac{V_{\mathrm{m}\mathrm{p}}\times J_{\mathrm{m}\mathrm{p}}}{V_{\mathrm{o}\mathrm{c}}\times J_{\mathrm{s}\mathrm{c}}}$$ 1

$$\mathrm{PCE}(\%)=(\mathrm{FF}\times J_{\mathrm{s}\mathrm{c}}\times V_{\mathrm{o}\mathrm{c}})/P_{\mathrm{i}\mathrm{n}}$$ 2

2.4. Characterization

The structural properties of the synthesized materials were investigated via X-ray diffraction (XRD) using a Panalytical-Empyrean diffractometer (Netherlands) equipped with Cu Kα radiation (λ = 1.54 Å) operated at 45 kV and 40 mA; diffraction patterns were recorded across a 2θ range of 10°–80°, with phase identification performed against standard ICDD database patterns. Functional group analysis was conducted through Fourier-transform infrared (FTIR) spectroscopy in ATR mode using a Shimadzu IRSpirit-X within the 400–4000 cm^–1 range. Optical properties were evaluated using a Varian Cary 4000 UV–visible spectrophotometer (200–800 nm) and photoluminescence (PL) spectroscopy (430–650 nm). The surface elemental composition and corresponding electronic valence states were analyzed using an Omicron Nanotechnology (Oxford Instruments) X-ray photoelectron spectrometer, utilizing a monochromatic Al Kα X-ray source. The spectrum was fitted employing a Gaussian functions, and peak correction were made. For device evaluation, the photovoltaic performance and electrochemical impedance spectroscopy (EIS) of the masked DSSCs were measured under AM 1.5G illumination (one sun) using a PET Photo Emission Tech SS50AAA solar simulator and a CHI660e electrochemical workstation (CH Instruments), respectively. Cyclic voltammetry was recorded in the potential range of −0.1 V to 1 V. EIS data were acquired from 1 Hz to 0.1 MHz with a 10 mV AC amplitude and subsequently analyzed using Z-fit software to correlate the interfacial charge-transfer dynamics with device efficiency.

3. Results and Discussion

X-ray diffraction (XRD) analysis was performed to determine the material formation, phase composition, and purity of the synthesized MOF-derived ZnO, NiCo₂O₄, Nb₂C MXene, binary (ZnO/NiCo₂O₄) and the ternary (ZnO/NiCo₂O₄/Nb₂C) composites. The diffraction patterns for all synthesized materials are shown in Figure a. For the MOF-derived ZnO, sharp and well-defined peaks were observed at 2θ values of 31.4° and 36.3°, which are indexed to the (110) and (101) lattice planes of phase-pure wurtzite ZnO. , These peaks correspond to a hexagonal crystal structure, showing excellent agreement with the standard JCPDS card no. 036–1451. The broadness and moderate intensity of the obtained peaks suggest nanosized crystal formation, confirming the MOF-derived nanostructured oxide ZnO. For NiCo₂O₄, the characteristic peaks were found at 31.1°, 36.7°, and 44.6°, corresponding to the planes (220), (311), and (222) of the fcc cubic spinel structure. − No other impurity peaks were observed, confirming phase-pure crystalline NiCo₂O₄. Similarly, for the Nb₂C MXene, the obtained peaks at 34.9° and 40.9° correspond to the (111) and (200) planes of the distinct cubic/fcc-like carbide diffraction profile. In the binary and tertiary composites, the peaks remain consistent with those of the pristine materials, with no observable shifts. This indicates that the structural frameworks of the individual components are preserved during composite synthesis, suggesting that the constituents are effectively integrated without inducing significant lattice distortions or chemical interactions that would alter the crystal structures. Furthermore, the absence of additional peaks confirms the high phase purity of the final composite, verifying that no secondary impurities were formed during the fabrication process. XRD results confirm the phase-pure ZnO/NiCo₂O₄/Nb₂C heterostructure formation with preserved crystallinity and interfacial coupling among the constituent phases, which are expected to be beneficial for charge transport and electrochemical performance.

1.

(a) X-ray diffraction patterns of the synthesized CE materials; (b) FTIR spectrum of the synthesized CE materials.

The crystallite size (D), microstrain (ε), and dislocation density (δ) of the synthesized materials were calculated using the standard eqs – and are summarized in Table .

$$D=\frac{k\lambda }{\beta \cos \theta }$$ 3

$$\epsilon =\frac{\beta }{4\tan \theta }$$ 4

$$\delta =\frac{1}{D^2}$$ 5

where k is the Scherrer constant (0.9), λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM) of the diffraction peaks, and θ is the Bragg diffraction angle.

1. Crystallite Size (D), Microstrain (ε), and Dislocation Density (δ) of the Synthesized Materials.

Materials	Crystallite Size D (nm)	Microstrain (ε) × 10 –3	Dislocation Density (δ) × 10 –4
ZnO	17.10	6.76	34.21
NiCo2O4	13.65	8.49	53.69
Nb2C	11.60	10.40	74.34
ZnO/NiCo2O4	10.48	10.99	91.04
ZnO/NiCo2O4/Nb2C	7.94	14.52	158.44

The pristine materials ZnO, NiCo₂O₄, and Nb₂C exhibit larger crystallite sizes of 17.10, 13.65, and 11.60 nm, respectively. Upon forming the binary hybrid, the crystallite size decreases to 10.48 nm, and further with the integration of Nb₂C MXene, the ternary composite results in a crystallite size of 7.94 nm. This decrease in crystallite size during composite formulation can be attributed to lattice mismatch induced during composite formation, interfacial strain, and growth inhibition, which broaden XRD peaks and limit crystallite growth. Also, smaller crystallite-size composites possess higher specific surface area, which improves the catalytic activity of the material. Since dislocation density has an inverse relationship with crystallite size, it increases from 34.21 × 10^–4 nm for pure ZnO to 91.04 × 10^–4 nm for the binary hybrid, and reaches 158.44 × 10^–4 nm in the ternary composite. This indicates a higher number of active surface sites and better charge separation. Furthermore, a clear upward trend is observed in the microstrain, which increases from 6.76 × 10^–3 for pristine ZnO to 10.99 × 10^–3 in the binary hybrid, and 14.52 × 10^–3 in the ternary composite. This pronounced increment in microstrain is direct evidence of lattice distortion and stress induced at the multicomponent heterojunctions, which inherently alter the local electronic band structure and facilitate faster charge-transfer kinetics at the catalyst surface.

FTIR spectroscopy was employed to further elucidate the chemical composition and the interfacial interactions within the binary and ternary composites (Figure b). The peaks around 3400 cm^–1 and 1640 cm^–1 represent O–H stretching and H–O–H bending, respectively, due to the presence of moisture in the tested material. The multiple peaks from 420 to 550 cm^–1 denote Zn–O stretching and Zn–O–Zn bending, confirming the formation of MOF-derived ZnO. Furthermore, the peak at 1600 cm^–1 is due to carboxylate COO^– stretching arising from a small amount of non-decomposed ZIF-8 residues. For the NiCo₂O₄ samples, peaks at 555 cm^–1 and 650 cm^–1 are associated with the stretching and bending vibrations of Ni–O and Co–O bonds. The additional peak at 1385 cm^–1 might be due to N–H vibrations from urea residue. For Nb₂C MXene, the characteristic peaks of Nb–C bonds were found at 520 and 550 cm^–1. , For the binary composite (ZnO/NiCo₂O₄), all peaks of the parent materials are present without any shift in position. However, for the ternary composite (ZnO/NiCo₂O₄/Nb₂C), there is a shift in the metal–oxygen stretching frequencies of ZnO and NiCo₂O₄ after forming the composite, indicating a strong interfacial interaction between the oxides and the Nb₂C MXene sheets.

The optical absorption characteristics of the synthesized materials were investigated using UV-visible spectroscopy (UV-vis). As shown in Figure a, pure MOF-derived ZnO exhibits an absorption band in the UV region and extends toward the visible region. Upon the integration of NiCo₂O₄ and Nb₂C MXene, the absorption profile of the binary and ternary composites demonstrates a significant red shift toward the visible light region up to 580 nm. This spectral shift reflects the altered electronic energy states and interfacial electronic coupling between the ZnO nanostructures, the spinel NiCo₂O₄ phase, and the high-conductivity Nb₂C MXene sheets. The bandgap (Eg) is calculated using the Tauc plot and shows that MOF-derived ZnO has a bandgap of 2.1 eV. The pristine ZnO typically exhibits a wide direct bandgap around 3.3 eV in the UV region. MOF-derived ZnO synthesized here shows a reduced optical bandgap due to the presence of in situ carbonaceous residues acting as dopants and intrinsic oxygen vacancies. Compared to MOF-derived ZnO, for binary and tertiary composites, there is a subsequent reduction in bandgap by 0.1 and 0.2 eV, respectively, due to the formation of heterojunctions.

2.

(a) UV–visible spectrum of the synthesized CE materials; (b) photoluminescence spectrum of the synthesized CE materials.

Photoluminescence (PL) spectroscopy (Figure b) was used to further examine the defect structures and electronic state interactions. The pristine ZnO exhibits two emission bands observed between 430 and 610 nm, associated with intrinsic structural defects and oxygen vacancies. The addition of NiCo₂O₄ and Nb₂C leads to significant attenuation of PL intensity. This PL quenching serves as qualitative structural evidence of non-radiative decay pathways and electronic interactions across the heterojunction interface, confirming effective material integration.

The morphological characteristics of the synthesized counter electrode materials were systematically investigated using Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM). The MOF-derived ZnO formed as hexagonal cylindrical morphology with a diameter of 1 μm (Figure a). In Figure b the NiCo₂O₄ phase is shown as spherical nanoparticles with dimensions less than 100 nm, and Figure c shows the two-dimensional (2D) sheet-like layered structure of the Nb₂C MXene. FESEM and HRTEM images in Figure d and f respectively illustrate the integration of these components into a hierarchical ternary architecture. The analysis confirms that the three architectures were well dispersed, resulting in a complex multi nanostructure. The elemental composition and spatial distribution of the synthesized ZnO/NiCo₂O₄/Nb₂C composite were investigated via Energy-Dispersive X-ray (EDX) spectroscopy and elemental mapping. The EDX spectrum (Figure e) confirms the presence of Zn, Ni, Co, Nb, C, and O, with peak intensities corresponding to the constituent phases of the ternary system. The absence of additional peaks in the EDX spectrum further validates the high chemical purity of the synthesized material, ensuring that no extraneous contaminants were introduced during the composite fabrication process.

3.

SEM images: (a) MOF-derived ZnO, (b) NiCo₂O₄ (c) Nb₂C MXene, (d) ZnO/NiCo₂O₄/Nb₂C, (e) EDX spectrum of ZnO/NiCo₂O₄/Nb₂C, and (f) HRTEM of ZnO/NiCo₂O₄/Nb₂C.

The surface electronic structure and elemental composition of the synthesized ZnO/MOF/NiCo₂O₄/Nb₂C composites were analyzed by X-ray photoelectron spectroscopy (XPS), and the resultant spectra are shown in Figure a–h. The survey spectrum of Figure a confirms the coexistence of Zn, Ni, Co, Nb, O, N, and C elements, signifying the successful formation of a hybrid architecture. The intense C 1s and N 1s signals indicate the presence of a nitrogen-enriched carbonaceous matrix, resulting from the parent MOF structure, while the characteristic Zn 2p, Ni 2p, Co 2p, O 1s, and Nb 3d peaks substantiate the successful integration of ZnO, spinel NiCo₂O₄, and Nb₂C phases.

4.

XPS spectra of the ZnO/MOF/NiCo₂O₄/Nb₂C composite: (a) survey spectrum and high-resolution spectra of (b) Zn 2p, (c) Ni 2p, (d) O 1s, (e) N 1s, (f) Nb 3d, (g) Co 2p, and (h) C 1s.

The high-resolution Zn 2p spectrum (Figure b) exhibits two distinct spin–orbit doublets located at 1021.39 and 1044.46 eV, corresponding to Zn 2p₃/₂ and Zn 2p₁/₂. The spin–orbit splitting energy of 23.07 eV corresponds to the characteristic Zn²⁺ oxidation state in hexagonal ZnO, confirming the oxidation of Zn species and formation of the ZnO crystal lattice. − The absence of satellite peaks further eliminates the presence of metallic Zn.

The Ni 2p spectrum displayed in Figure c shows two major peaks centered at 854.19 and 871.68 eV, corresponding to Ni 2p₃/₂ and Ni 2p₁/₂. Further, shake-up satellite peaks at 861.24 and 879.41 eV were observed. These intense satellite peaks are specific to Ni²⁺ species. The asymmetric broadening of the main peaks suggests the coexistence of Ni³⁺ species within the spinel framework. Such mixed-valence Ni²⁺/Ni³⁺ states are characteristic of inverse spinel NiCo₂O₄ and are known to stimulate rapid Faradaic redox reactions through enhanced electron hopping between adjacent metal centers, thereby enhancing electrochemical kinetics and electrical conductivity.

The deconvoluted O 1s spectrum of Figure d consists of two distinct oxygen environments centered at 528.72 and 531.17 eV. The low-binding-energy peak at 528.72 eV is ascribed to lattice oxygen (O^2–) coordinated with Zn, Ni, and Co atoms within the metal-oxide framework. On the other hand, the higher-binding-energy peak at 531.17 eV corresponds to surface chemisorbed oxygen species, hydroxyl groups, and defect-associated oxygen vacancies. The high relative area of this peak confirms that the MOF route successfully induced abundant surface defect sites. The relatively high intensity of the defect-related oxygen peak indicates the presence of abundant oxygen-deficient sites during thermal treatment and heterostructure formation. These oxygen vacancies can efficiently curb the local electronic structure, lower charge-transfer resistance, and offer additional electrochemically active sites, thereby facilitating electrolyte-ion diffusion and interfacial redox reactions.

High-resolution N 1s spectrum (Figure ), in which the peak located at 398.5 eV corresponds to pyridinic nitrogen species, which originates from the MOF-derived nitrogen-doped carbon framework and is usually located at defect sites. This can considerably enhance the electrical conductivity by altering the electron density distribution of adjacent carbon atoms while creating additional active sites for charge storage and surface redox reactions. The incorporated nitrogen heteroatoms can increase the surface wettability and strengthen interfacial interactions between carbon and metal oxide.

The Co 2p spectrum given in Figure g exhibits specific Co 2p₃/₂ and Co 2p₁/₂ peaks centered at 779.43 and 794.86 eV, along with a shake-up satellite feature at 802.07 eV. The obtained lower binding-energy component is associated with Co³⁺ species, whereas the satellite peak observed confirms the simultaneous presence of Co²⁺ ions within the spinel lattice. The coexistence of Co²⁺/Co³⁺ redox couples corresponds to the spinel NiCo₂O₄ and is beneficial for improving the electronic conductivity through intervalence charge-transfer mechanisms. The mixed-valence states contribute to reversible redox transitions and improved electrochemical activity during the charge-transfer process.

The Nb 3d spectrum shown in Figure f establishes the effective incorporation of niobium carbide into the composite structure. The specific peaks located at 202.6 and 206.02 eV correspond to Nb–C 3d₅/₂ and Nb–C 3d₃/₂ spin–orbit components, confirming the formation of Nb₂C. The observed spin–orbit splitting reported in the literature for transition-metal carbides. Also, a small peak located at 209.23 eV is ascribed to Nb⁵⁺ species originating from the oxidation of Nb₂C upon exposure to air. The coexistence of conductive Nb₂C and thin surface niobium oxide layers can provide synergistic advantages, where Nb₂C contributes high electrical conductivity and rapid electron transport, while the surface oxide species may introduce additional catalytic active sites.

The C 1s spectrum displayed in Figure h is deconvoluted into two major peaks at 284.4 and 285.1 eV, corresponding to graphitic C–C/CC and C–N/C–O bonding, respectively. The dominant graphitic carbon peak indicates the formation of conductive carbon domains derived from the carbonization of the MOF structure, which facilitates the interconnected electron-transport network throughout the composite. Also, the nitrogen incorporation into the carbon matrix provides additional defect-rich active sites that enhance the interfacial charge-transfer process.

The XPS results confirm the successful formation of the ZnO/MOF/NiCo₂O₄/Nb₂C heterostructure with mixed-valence metal species, conductive Nb₂C phases, nitrogen-doped carbon, and oxygen-vacancy-rich defect sites. The synergistic interaction among these components can facilitate a facile electron–ion transport pathway. Also, the structure is capable of enhancing redox kinetics and improving the overall electrochemical characteristics of the composite electrode. Furthermore, a slight shift in the binding energies of the metal oxides ZnO and NiCo₂O₄ is observed in the composite compared to the literature data for the pristine form. , This shift indicates a redistribution of electron density at the heterointerfaces, providing strong evidence of a heterostructure with electronic coupling between the metal oxides and the Nb₂C MXene sheets. This synergistic electronic interaction facilitates more efficient charge-carrier separation and migration, which contributes to the enhanced power conversion efficiency in the DSSC measurements.

The photovoltaic performance of the fabricated dye-sensitized solar cells (DSSCs) using the synthesized counter electrodes (CEs) was evaluated, and the results are presented in Figure and Table . The device fabricated using the MOF-derived ZnO CE yielded an open-circuit voltage V _oc of 0.56 V and a short-circuit current density J _sc of 7.51 mA/cm², resulting in a power conversion efficiency of 3.11%. Similarly, the NiCo₂O₄ and Nb₂C CE-based DSSCs have obtained efficiencies of 4.56% and 2.51%, respectively. In comparison, the binary and ternary composite counter electrodes demonstrated substantially superior performance. MOF ZnO/NiCo₂O₄ obtained a V _oc of 0.65 V and a J _sc of 13.58 mA/cm², and MOF-derived ZnO/NiCo ₂ O ₄ /Nb ₂ C obtained a V _oc of 0.71 V and a J _sc of 16.53 mA/cm², which correspond to enhanced efficiencies of 6.15% and 8.08%, respectively. This represents a more than 2.6 times increase in efficiency compared with the pristine ZnO counterpart, highlighting the efficacy of the ternary architecture. Also, when compared to the standard Pt counter-electrode DSSC, there is a slight improvement in the current density and a very small reduction in fill factor. However, the overall efficiency of the fabricated Pt DSSC (7.97%) remains very close to that of the MOF-derived ZnO/NiCo ₂ O ₄ /Nb ₂ C DSSC (8.08%).

5.

DSSC performance of the fabricated CEs: (a) photocurrent density versus voltage (J–V), (b) power density versus voltage (P–V), and (c) Nyquist plot with the equivalent circuit used to fit the impedance curve.

2. Photovoltaic Performance of the Fabricated DSSCs.

Counter Electrodes	J sc (mA cm –2 )	V oc (V)	FF	PCE %
MOF-derived ZnO	7.512	0.56	0.738	3.108
NiCo2O4	10.530	0.62	0.699	4.569
Nb2C	5.569	0.62	0.728	2.517
MOF-derived ZnO/NiCo2O4	13.580	0.65	0.697	6.157
MOF-derived ZnO/NiCo 2 O 4 /Nb 2 C	16.531	0.71	0.688	8.080
Pt	15.620	0.71	0.71	7.970

In previous studies ZnO/C CE-based DSSCs have reported an efficiency of 6.7%. Also, ZnO is used with PEDOT:PSS composite CEs to increase catalytic activity and improve the DSSC PCE. Similarly, NiCo₂O₄ is a widely utilized and promising Pt-free CE that delivers high catalytic activity toward I^–/I₃ ^– reduction, good conductivity, and significant cost advantages over Pt. Wang et al. achieved 6.27% using a NiCo₂O₄ carbon black composite, reporting improved electrocatalytic activity for triiodide (I₃ ^–) reduction and structural stability afforded by carbon-based synergistic effects. Similarly, studies have explored NiCo₂S₄ as a CE, yielding efficiencies up to 6.53%. In the present study, the superior efficiencies of 6.15% and 8.08% achieved by the binary and tertiary composite CEs, respectively, can be attributed to several synergistic factors. The hierarchical integration of NiCo₂O₄ and ZnO effectively optimizes the catalytic active site distribution, minimizing charge transfer resistance. Furthermore, the introduction of Nb₂C MXene significantly enhances the overall electrical conductivity and accelerates electron transfer kinetics at the electrolyte-electrode interface. The substantial improvement in V _oc and J _sc demonstrates that the ternary heterojunction effectively modulates the electrochemical potential, confirming this material as a high-performance candidate for DSSC applications. Table compares the photovoltaic performance of DSSCs with Pt-free electrodes published with the present study.

3. Comparative Analysis of the Photovoltaic Performance of DSSCs with Pt-Free Electrodes.

Counter Electrode Material	Power Conversion Efficiency (PCE, %)	Refs
NiCo2O4 with carbon black	6.27
NiCo2S4	6.53
Co-NiO	5.01
NiCo2O4@RGO nanobelts	6.17
Ti3C2T x MXene	8.68
MXene/NiCo2S4	8.76
MXene–graphene	6.4
Ti3C2T x –PEDOT nanofibers	7.1
CNTs/Ti3C2T x	5.83
V2C MXene/TiO2	1.62
Ti3C2T x –polythiophene	5.83
MOF derived ZnO	3.11	This work
MOF derived ZnO/NiCo2O4	6.15	This work
MOF derived ZnO/NiCo2O4/Nb2C	8.08	This work

To elucidate the charge-transfer kinetics at the counter electrode/electrolyte interface, electrochemical impedance spectroscopy (EIS) measurements were conducted. The Nyquist plots are presented, and an equivalent circuit was fitted (Figure c). R _s corresponds to the overall ohmic resistance, including the resistance of the FTO substrate and the contact resistance of the coating. R _ct1 and C ₁ represent the charge-transfer resistance and double-layer capacitance at the counter electrode/electrolyte interface, where the reduction of triiodide (I₃ ^–) takes place. R _ct2 and C ₂ are the charge-transfer resistance and capacitance at the photoanode/electrolyte interface, which are associated with the transfer of electrons between the photoanode and the electrolyte.

The device utilizing the MOF-derived ZnO counter electrode exhibited a relatively high R _ct (R _ct1 + R _ct2) of 47.24 Ω, indicating slower catalytic kinetics for the triiodide (I₃ ^–) reduction reaction. In contrast, the ternary ZnO/NiCo₂O₄/Nb₂C composite counter electrode displayed a significantly reduced R _ct of 13.3 Ω. Specifically, R _ct1 associated with the high-frequency charge transfer at the counter electrode/electrolyte interface reduces from 31.38 Ω to 5.65 Ω, indicates easier electron transfer at the CE/electrolyte interface. Furthermore, the series resistance (R _s) decreases significantly from 16.6 Ω for pristine MOF-derived ZnO to 4.89 Ω for the tertiary composite. This reduction confirms the enhanced electrical conductivity due to the incorporation of the highly conductive Nb₂C MXene network. The EIS characteristics are summarized in Table . This drastic decrease demonstrates the enhanced electrocatalytic activity of the composite. The substantial reduction in resistance confirms that the high-conductivity Nb₂C MXene backbone and the synergistic integration of NiCo₂O₄ effectively facilitate rapid electron transfer at the interface, thereby ultimately enhancing the overall power conversion efficiency of the DSSC.

4. EIS Characteristics of the Fabricated DSSCs.

Counter Electrodes	R s (Ω)	R ct‑1 (Ω)	R ct‑2 (Ω)
MOF derived ZnO	16.6	31.38	15.86
MOF derived ZnO/NiCo 2 O 4 /Nb 2 C	4.89	5.65	7.65

To further evaluate the catalytic activity and charge-transfer resistance, cyclic voltammetry was performed and is shown in Figure a and b. Cyclic voltammetry showed two redox couples for all prepared samples, where the first and DSSC-relevant pair corresponds to (I₃ ^– + 2e^–→3I^–) at E _pa and (I₃ ^– + 2e^– → 3I^–) at E _pc, while the second pair is associated with (2I₃ ^– → 3I₂ + 2e^–) and its reverse reaction. The peak separation was calculated as ΔE _p = E _pa – E _pc. MOF-derived ZnO exhibited E _pa =0.61902 V and E _pc = 0.39672 V, giving ΔE _p =0.22230 V; NiCo ₂ O ₄ and Nb ₂ C showed ΔE _p =0.56771 V and ΔE _p = 0.40812 V, respectively. MOF-derived ZnO/NiCo ₂ O ₄ exhibited ΔE _p =0.08265 V and the tertiary composite MOF-derived ZnO/NiCo ₂ O ₄ /Nb ₂ C showed E _pa = 0.59565 V and E _pc = 0.61218 V, resulting in the smallest absolute separation of ΔE _p =0.01653 V. The substantially smaller ΔE _p of MOF-derived ZnO/NiCo ₂ O ₄ /Nb ₂ C indicates lower electrochemical polarization, greater redox reversibility, and faster interfacial electron-transfer kinetics for triiodide reduction. Similarly, the cathodic peak current density (J _pc) and the area under the CV curve were evaluated for all tests, and the highest J _pc (0.85 mA cm^–2) and area are recorded for the MOF-derived ZnO/NiCo ₂ O ₄ /Nb ₂ C composite, indicating a higher reduction rate, greater density of active catalytic sites, and enhanced overall device current density. This confirms that the tertiary composite has a higher density of catalytically active sites, minimizing charge-transfer resistance, thereby increasing the overall performance of the fabricated DSSCs.

6.

Cyclic voltammetry analysis (a) and (b), photocurrent response (c), and long-term stability analysis (d) of the fabricated DSSCs.

To evaluate the stability of the prepared DSSCs, transient photocurrent density and long-term stability tests were performed. The transient photocurrent density for the prepared materials was evaluated for 5 cycles (10 s off/10 s on) and the results are shown in Figure c. In the absence of light, photocurrent measurements were zero. Also, the photocurrent density remained stable for all materials throughout the 5 cycles. A long-term stability test for the ternary composite DSSC was performed for up to 20 days, during which measurements were taken at regular intervals, and the device was kept in the dark after testing. The results are shown in Figure d. It can be seen that all PV performance parameters dropped slightly during the initial days. The DSSC cells were stable, and the efficiency dropped by only 1.2% in 5 days. However, there is a very high drop in J _sc after 8 days due to thermal stress and electrolyte degradation and leakage. Subsequently, the efficiency of the DSSC was also reduced from 8.08% to 3.64%.

4. Conclusion

The study successfully demonstrates the synthesis of a high-performance, platinum-free counter electrode utilizing a ternary MOF-derived ZnO/NiCo₂O₄/Nb₂C MXene composite via a hydrothermal route. Structural and morphological characterizations through XRD, FESEM, and HRTEM confirmed the successful integration of hexagonal ZnO cylinders, spherical NiCo₂O₄ nanoparticles, and conductive 2D layered Nb₂C MXene sheets, forming a robust hierarchical architecture. The optimized ternary composite achieved a remarkable power conversion efficiency (PCE) of 8.08%, which represents a significant 2.6-fold improvement over pristine MOF-derived ZnO and serves as a competitive alternative to conventional Pt-based electrodes. This enhanced performance is primarily attributed to the abundant active sites in the tertiary composite for the triiodide (I₃ ^–) reduction reaction, as confirmed by cyclic voltammetry analysis. Also, the enhanced charge kinetics from the metallic conductivity of the Nb2C MXene backbone facilitated rapid electron transport, effectively reducing the charge-transfer resistance (Rct) to 0.71 Ω. These results suggest that the ZnO/NiCo₂O₄/Nb₂C MXene architecture provides a viable pathway toward the development of sustainable, stable, cost-effective, and efficient DSSC technologies.

Glossary

Abbreviations

DSSC

Dye-sensitized solar cell

CE

counter electrode

XRD

X-ray diffraction

FTIR

Fourier transform infrared spectroscopy

UV-Vis

UV–Visible spectroscopy

PL

photoluminescence

FESEM

field-emission scanning electron microscopy

EDX

energy-dispersive X-ray spectroscopy

HRTEM

high-resolution transmission electron microscope

V _oc

open-circuit voltage

J _sc

short-circuit current density

FF

fill factor

PCE

power conversion efficiency

Pin

input power

V _mp

maximum voltage

J _mp

maximum current density

The raw/processed data required to reproduce these findings are available from the corresponding author on request.

N.C. and S.S.: conceptualization. N.C., R.K., and S.S.: methodology and data curation. N.C., R.K., and S.K.: formal analysis and validation. N.C. and R.K. writingoriginal draft. S.K. and S.S.: writingreview and editing. N.C. and S.S.: funding. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.