Non-precious macrocycle embedded hybrid nanocomposites for efficient water oxidation

Abstract

The development of efficient electrocatalysts for OER remains a challenge owing to issues such as unfavourable mass transport, reaction thermodynamics, and kinetics. In this study, we present a novel bio-inspired polymeric cobalt(II) phthalocyanine (Poly CoTAPc) as an effective OER catalyst. Mimicking the active sites of metalloenzymes involved in biological processes, Poly CoTAPc demonstrated exceptional catalytic activity and stability. To enhance the conductivity and maximize the exposure of active sites, Poly CoTAPc was combined with Ketjen black (KB) nanoparticles. The resulting Poly CoTAPc/KB composite was subsequently coated onto a nickel foam current collector. The fabricated Poly CoTAPc/KB/Ni electrode exhibited superior electrocatalytic performance compared with that of the benchmark catalyst IrO₂ under alkaline conditions. Specifically, it achieved an overpotential of 306 mV, a low Tafel slope of 81 mV dec⁻¹, and enhanced mass activity (22.72 A g_metal⁻¹ at 1.58 V) and turnover frequency (TOF) (0.0122 s⁻¹) for the OER. Additionally, the electrode demonstrated exceptional stability and maintained performance for over 30 hours. The Poly CoTAPc/KB electrocatalyst exhibited better electron transfer capacity and good stability. Moreover, its cost-effectiveness, use of very low amount of non-precious Co metal, and eco-friendliness make this catalyst highly efficient and promising compared with traditional metal-based catalysts in advancing OER catalysts for sustainable energy applications.

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Introduction

The rapid growth in the global population and rising energy demands call for urgent sustainable energy solutions to reduce the heavy reliance on fossil fuels.^1–3 Scientists from the Global Carbon Budget (GCB) analysed that CO₂ emissions increased by 1.1% in 2023 compared with that in the previous year (2022), bringing the total release of CO₂ into the atmosphere to 36.8 billion metric tons.⁴ This alarming trend underscores the need for environmentally friendly alternative energy sources, such as fuel cells, water electrolysers, and metal–air batteries, to build a stable and sustainable world.

The oxygen evolution reaction (OER) is a key process for the above-mentioned electrochemical technologies. However, the OER is a sluggish oxidative half reaction, involving a four-electron process. The OER reaction is generally more favourable in alkaline electrolytes than in the acidic medium because the basic medium has higher conductivity, which facilitates the transport of electrolyte ions to and from the electrode surface.⁵ In basic media, O₂ gas is produced along with H₂O and electrons (e⁻) via the oxidation of hydroxyl ions (OH⁻). However, this complexity demands a larger overpotential than the thermodynamic equilibrium potential (theoretical potential 1.23 V vs. RHE).⁶ Currently, precious noble metal oxides such as RuO₂ and IrO₂ are heavily used in commercial OER systems. Nevertheless, these materials are costly, scarce, and less stable.^7,8 To address these challenges from a commercial standpoint, it is crucial to find suitable OER electrocatalysts that are non-precious, cost-effective, eco-friendly, and capable of reducing the overpotential, have abundant resources, and offer better stability.

Chemical materials composed of first-row transition metals, such as manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni), have been identified as promising OER catalysts. Among these, cobalt stands out as the most efficient one owing to its excellent intrinsic activity, optimal electronic structure, and ability to facilitate oxygen evolution at lower overpotential.^9,10 Volcano plot analysis indicated that cobalt oxide was positioned near the benchmark catalysts IrO₂ and RuO₂ in terms of performance.¹¹ However, the effectiveness of cobalt oxides is limited by their electronic structure, which results in fewer active sites and lower conductivity. Significant efforts are being made to enhance the stability and catalytic performance of cobalt-based compounds for the OER. The advancement of cobalt–ligand-based coordination complexes, including porphyrins,¹² salens,¹³ bipyridine,¹⁴ and phthalocyanines,^15,16 has garnered researcher's attention because these complexes involve sufficient electron density transfer between the ligand and central metal atoms, resulting in stable structures along with greater catalytic responses.

Phthalocyanines (Pc) are promising electrocatalysts due to their well-defined macrocyclic structure and versatile electronic properties.¹⁷ Their planar conjugated system facilitates efficient charge transfer, whereas the metal center (cobalt, iron, or nickel) plays a crucial role in catalysing redox reactions by switching between oxidation states.^18–20 Jun Chen et al.²¹ studied how the geometric and electronic structures of phthalocyanine catalysts and intermediates affect the OER. Their research revealed that FePc and binuclear-CoPc exhibit robust stability against dissolution and possess low OER potential energy, describing their promises for use in alkaline OER processes. Furthermore, the large macrocyclic ligand present in bi-MPc and the increase in the number of metal active centres enhance its ability to store holes and facilitate efficient electron transfer between its metal sites. These attributes stabilize OER intermediates, thereby reducing the energy required for the rate-determining step (RDS). Although various studies have been conducted, the exact mechanism of the OER at the cobalt phthalocyanine electrode is not fully understood. However, it was confirmed from earlier studies that the functional groups attached to the phthalocyanine edges would make the electrocatalysts more active. A key challenge with these materials is their limited conductivity, which can be improved by the introduction of conductive carbon nanoparticles. In this regard, Zhou Z.-P. et al.²² concluded that the addition of conductive carbon to MPc causes the bending of N–metal–N (N–M–N) angles of MPc. As the increase in the bending increases, there will be an increase in the diameter of the conducting carbon, which leads to more adsorption kinetics of intermediates to the MPc complex, resulting in a lesser overpotential of OER catalytic action. The addition of conducting carbon increases the density and surface area of the active catalyst, whereas the porous nature of carbon materials facilitates the diffusion of reactants and products to and from the catalytic sites, improving mass transport kinetics and minimizing diffusion limitations. This flexibility enables Pc to effectively catalyze key reactions such as the oxygen evolution reaction (OER). Additionally, their robust chemical and thermal stability allows them to operate in harsh electrochemical environments, making them durable and effective electrocatalysts for energy conversion and storage applications.^23–25 Another important aspect is the selection of a suitable substrate for the fabrication of the working electrode. Here, we selected a Ni foam substrate because of its numerous beneficial properties over other substrates such as glassy carbon (GC) and Cu foil. Initially, the Ni foam provided greater electrical conductivity and mechanical stability, which are the major parameters required for effective charge transport under harsh electrolytic conditions.²⁶ In addition, the highly porous 3D architecture of the Ni foam improves its integration with the catalyst, providing a solid framework for catalyst deposition. In addition, the higher surface area of the Ni foam enables homogeneous dispersion and effective attachment of the catalyst material, leading to improved mass transport. Furthermore, importantly, the Ni substrate forms a NiOOH surface layer during water oxidation at initial potentials, which further boosts the OER activity.^26,27 Lastly, the wide usage of Ni substrates in industrial applications for alkaline water electrolysis makes Ni foam a relevant choice for studying electrocatalysts in real-world applications, particularly in energy conversion devices.²⁸

However, major advancements in Co–ligand-based complex systems still suffer from many challenges, such as poor conductivity, limited active site accessibility, and insufficient long-term durability. To address this issue, we synthesized a novel, eco-friendly, and cost-effective polymeric cobalt-based phthalocyanine (Poly CoTAPc) electrocatalyst using a simple two-step process involving oxy-bridging and tetramerization. The working electrode was prepared by combining Poly CoTAPc with Ketjen black (KB) in an optimized 4 : 1 ratio and was tested for OER. The Poly CoTAPc + KB hybrid immobilized on the nickel foam showed excellent OER performance with a low overpotential of 306 mV at 10 mA cm⁻², a low Tafel slope (81 mV dec⁻¹), and high stability over 30 h in 1 M KOH. The enhanced performance of the composite material can be attributed to the synergistic interaction between poly Pc and conductive KB. The –COOH groups from the tartaric acid ligand attached to the Pc polymer play a crucial role in improving the hydrophilicity of the catalyst, promoting a supramolecular arrangement that facilitates ion and water transport. These groups also contribute to more efficient charge transfer and effective proton coupling. Additionally, the π–π stacking environment in the polymeric structure enables electron delocalization, further boosting the catalytic activity. The porous structure of Ketjen black significantly increases the overall surface area of the catalyst, providing more active sites for the catalytic reactions. The combination of these properties, such as improved hydrophilicity, enhanced charge transfer, and an expanded surface area, results in a more effective catalytic system. This study proposes a bio-inspired strategy for designing efficient and durable cobalt-based electrocatalysts for water oxidation.

Experimental details

Materials and methods

The chemicals and reagents used for the synthesis and electrochemical experiments, as well as the instruments and their specifications for physical characterizations, are detailed in the ESI.†

Synthesis methodology

Synthesis of 2,3-bis(3,4-dicyanophenoxy)succinic acid, (3). 2,3-Bis(3,4-dicyanophenoxy)succinic acid (3) was produced by combining tartaric acid (2 g, 0.0133 mol) (1) and 4-NPN (4.605 g, 0.0266 mol) (2) with 50 mL of DMF solvent in a 100 mL round-bottom flask. The reaction mixture was initially stirred for 10 minutes while simultaneously adding potassium carbonate (6.433 g, 0.0465 mol). The solution was then agitated for 72 hours at room temperature under a nitrogen atmosphere. After confirming product formation using the TLC method, the product was precipitated by introducing the reaction solution into ice-cold water. The precipitate was filtered, washed with hot water, dried, and recrystallized in ethanol.

Yield: 82% (5.4 g). Melting point: 145 °C. Molecular formula: C₂₀H₁₀N₄O₆; molecular mass: 402.3; elemental composition: theoretical-C, 59.71%; H, 2.51%; N, 13.93%; O, 23.86% and experimental-C, 71.58%; H, 2.39%; N, 9.54%; IR absorption bands (cm⁻¹) (KBr pressing pellet method): 3400, 2240, 3089 and 1710 cm⁻¹; mass (m/z): 404.2 (M²⁺).

Synthesis of polymeric cobalt phthalocyanine (Poly CoTAPc) (4). Addition of the above-synthesized ligand (3) (0.5 g, 0.00124 mol) to a 100 mL RB flask containing 20 mL of pentanol solvent. The cobalt chloride salt (0.0737 g, 0.00031 mol) and DBU catalyst (about 2 mL) were then added to the same RB flask. The mixture was refluxed with continuous agitation at 138 °C for 24 hours. The resulting blue-coloured crude mixture was then cooled, filtered, and washed repeatedly with ethanol and distilled water. The pure product was dried in an oven at 100 °C to obtain a bluish solid.

The yield of Poly CoTAPc was 76% (0.43 g). Molecular formula: (C₄₈H₃₂CoN₈O₂₄)_n; molecular mass: 1163.75 amu; FTIR (KBr pellet, cm⁻¹): 3420, 3092, 1710, 680, 733, 819, 1096, 1149 and 1138 cm⁻¹; UV-visible spectra (DMSO, nm): 290, 661 and 625 nm; mass (m/z): 1183.2 [M + H₂O]²⁺.

Preparation of the hybrid composite (Poly CoTAPc + KB)

The hybrid composite Poly CoTAPc + KB was prepared by mixing pre-characterized Poly CoTAPc with commercial KB in different weight ratios, such as 4.5 : 1.5, 4.0 : 1.0, and 3.5 : 1.5. The specified weights (in mg) of the materials were thoroughly ground using a mortar and pestle and then stored in 2 mL vials.

Fabrication of working electrodes

The OER activity of individual electrodes and composite electrodes with different ratios was analysed using electrochemical methods because oxygen generation at the electrode surface is directly correlated with electron transfer kinetics. To fabricate each modified working electrode catalyst, a finely dispersed ink solution was prepared. In brief, 5 mg of synthesized Poly CoTAPc was transferred to a 2 mL vial containing 0.5 mL of IPA solvent. Then, 10 μL of a 5% Nafion solution was added as a binder to enhance the adhesion and reduce the catalyst leaching from the electrode surface. The mixture was sonicated for half an hour to obtain a homogeneous, uniform ink solution. Nickel foam was used as the conducting substrate and pretreated in a KOH solution. The well-dispersed ink was drop-coated onto the Ni foam surface and then dried in an oven for 10 minutes. The electrodes with different compositions of hybrid catalysts were prepared by varying the weight (in mg) of Poly CoTAPc and KB in ratios of 4.5 : 0.5, 4.0 : 1.0, and 3.5 : 1.5, respectively. Additionally, other electrodes modified with KB and IrO₂ were produced by following the same process.

Electrochemical measurements

The electrochemical characterization of both the bare Ni foam and the electrocatalyst-modified Ni foams was conducted using a standard three-electrode electrochemical setup with a CHI 6005E potentiostat (CH Instruments, USA). All electrochemical experiments were conducted in a 1 M KOH electrolyte using a catalyst-coated nickel foam with dimensions of 0.25 cm² (0.5 cm × 0.5 cm) as the working electrode, Hg/HgCl₂ (calomel electrode), and platinum spring (0.5 mm) as the reference electrode and counter electrode, respectively. Linear sweep voltammetry (LSV) was used to capture all the LSV polarization curves at a scan rate of 5 mV s⁻¹ and presented without IR compensation. The kinetic mechanism and activity of the catalysts were assessed by extracting the overpotential and Tafel slopes from the LSV polarization curves. Electrochemical impedance spectroscopy (EIS) was conducted with an amplitude of 5 mV over a potential range of η₁₀vs. RHE to understand the conductivity/resistance characteristics of the catalysts at the electrode/electrolyte interface. Electrochemical double-layer capacitance (C_dl) measurements were conducted by cycling the bare Ni and fabricated Ni foam electrodes in the non-faradaic region across various scan rates from 10 to 100 mV s⁻¹. The recorded potentials were referenced to reversible hydrogen electrode (RHE) values using the Nernst equation given below (eqn (1)).where E_(RHE) is the determined potential vs. RHE, is the experimental potential vs. Hg/HgCl₂ reference electrode, is the standard reference electrode potential, such as 0.244 V at room temperature, pH is found as 14 for the electrolyte 1 M KOH.^29,30

Results and discussion

The synthesis of polymeric cobalt phthalocyanine (Poly CoTAPc) (4) involves two straightforward steps, as outlined in Scheme 1. In the first step, the dioxybridged ligand (3) was prepared through a nucleophilic substitution reaction between two –OH groups of tartaric acid (1) and –NO₂ group of 4-nitrophthalonitrile (4-NPN) (2) in dry DMF solvent, at a molar ratio of 1 : 2. The hydroxyl groups of tartaric acid roll as nucleophiles, displacing the nitro groups of 4-NPN to form ether linkages (oxy-bridging linkage). This step was catalysed by the mild inorganic base KNO₃, which helps to deprotonate the –OH groups and promotes nucleophilic attack. This step was performed at room temperature, resulting in the generation of the product via the elimination of nitrous acid.³¹ The reaction progress was continuously monitored by TLC. In the second step, the cyclotetramerization of the phthalonitrile groups of the oxybridged ligand (3) was carried out in the presence of a CoCl₂ salt as the metal source. The reaction was conducted using a high-boiling n-pentanol solvent (bp ∼ 138–140 °C), which accelerates the essential thermal cyclization to form the N₄-macrocyclic core of phthalocyanine. A catalytic amount of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) was added as a strong non-nucleophilic base to boost the rate of formation of the N₄ macrocycle through nitrile groups.³² The reaction mixture was then refluxed for 24 hours to confirm complete tetramerization and insertion of Co²⁺ metal ions into the core structure of the porphyrin ring. Finally, the reaction yielded the desired blue-coloured Poly CoTAPc complex (4). The complex was then filtered and washed with hot water, ethanol, and acetone to remove soluble impurities and purify the compound. The resulting Poly CoTAPc complex (4) is rigid and insoluble in aqueous solvents and dissolves only in aprotic solvents such as DMSO, DMF, and concentrated H₂SO₄ (conc. > 28 N).

Scheme 1 Schematic for the synthesis of polymeric cobalt phthalocyanine (Poly CoTAPc). (1) Tartaric acid and (2) 4-nitrophthalonitrile (NPN).

Structural characterization

Various spectro-analytical methods were used to confirm the purity of the synthesized ligand (3) and polymeric complex (Poly CoTAPc), as described below.

FT-IR spectroscopy

The confirmation of different functional moieties in the synthesized compounds was confirmed using FTIR spectroscopy, scanning in the wavenumber range of 400 to 4000 cm⁻¹ using the KBr pressing technique. The obtained spectra are shown in Fig. 1a. The oxybridged ligand (3) exhibited a prominent peak at 2238 cm⁻¹, mainly attributed to the nitrile group (–C≡N), suggesting the formation of ligand (3). In contrast, this –C≡N peak is absent in the Poly CoTAPc spectra, confirming that all –CN groups underwent tetramerization to form the polymeric complex (4). The standard peaks observed at 3402 (±2), 3089 (±2), and 1714 (±4) cm⁻¹ in both compounds correspond to the –OH of carboxylic acid, Ar–CH stretching, and –C=O functional groups, respectively. Additionally, Poly CoTAPc exhibits characteristic skeletal vibration peaks of phthalocyanine at 680, 733, 819, 1096, 1149, and 1138 cm⁻¹ in the spectra, further supporting the confirmation of polymeric Pc.^33,34

Fig. 1 (a) FT-IR spectra of (black color) oxybridged ligand and (red color) Poly CoTAPc. (b) UV-visible, (c) TGA and (d) XRD spectra of the synthesized Poly CoTAPc. (e) BET surface analysis and (f) size distribution of Poly CoTAPc and Poly CoTAPc + KB.

On the other hand, the high-resolution mass spectrometric (HRMS) results depicted in Fig. S1† illustrate that the synthesized ligand (3) exhibits an m/z peak at 404.2, assigned to the [M²⁺] molecular ion peak with respect to the theoretical mass of the ligand as 402.3 amu, which confirms the production of the ligand. The MALDI mass spectrum of Poly CoTAPc in Fig. S2† shows an m/z peak at 1183.2 amu, which is attributed to [M + H₂O]²⁺.

UV-visible spectroscopy

To study the electronic transitions of Poly CoTAPc, UV-visible spectroscopic analysis was conducted in DMSO solvent, scanning in the wavelength range from 250 to 800 nm. The absorption spectra depicted in Fig. 1b reveal two significant characteristic peaks of phthalocyanine. The first peak, appearing in the lesser wavelength region at 290 nm, is known as the Soret band or B-band, and it contributes to the deeper π → π* transition of the a_2u–e_g levels of unoccupied molecular orbitals (LUMO). The second band observed at a longer wavelength of 661 nm is the Q-band associated with the π → π* transition of the (a_u–e_g) level of the Pc ring structure. Additionally, the peak at 625 nm, which is slightly merged with the Q-band, is referred to as a shoulder peak and is likely due to the presence of oligomers in the solution.³⁵

Thermal stability, X-ray diffraction and BET studies

The thermal stability and decomposition properties of the prepared Poly CoTAPc were investigated using a thermogravimetric analyzer (TGA) in the temperature range of 30–700 °C, and the resulting plot is shown in Fig. 1c. It is evident from the figure that the synthesized polymeric complex undergoes decomposition in three main steps. Initially, volatile solvents, moisture, and other impurities adsorbed on the material surface are evaporated below 150 °C. In the second step, a 48% mass loss corresponds to the carbonization of the core structure of the macrocyclic ring and its elements between 200 and 350 °C, and in the final step, the 14% mass loss above 400 °C is mainly attributed to detachment of all the functional groups of the ring and the formation of metal oxides.³⁶ The X-ray diffraction (XRD) graph of the synthesized Poly CoTAPc is shown in Fig. 1d, displaying a broad, noisy humph in the 2θ range between 15° and 35°, suggesting an amorphous nature because of its disordered polymeric structure. However, some less intensive bands obtained at 2θ = 17°, 26° and 31°, which were due to short-range ordering originating from π–π stacking of the Pc units.^37,38 To investigate the effect of KB on the composite surface area, BET (Brunauer–Emmett–Teller) measurements were performed for both pristine Poly CoTAPc and Poly CoTAPc + KB. As shown in Fig. 1e, both adsorption isotherms demonstrate an H3 hysteresis loop with a type IV character, as per the IUPAC classification.³⁹ The pristine Poly CoTAPc demonstrates a specific surface area of 39.9 m² g⁻¹, whereas the composite material (Poly CoTAPc + KB) exhibits an increased surface area of approximately 148.75 m² g⁻¹ after integration with KB. In addition, Fig. 1f results that the materials exhibit a mesoporous nature, with an average pore diameter size between 1 and 20 nm. These BET results indicate that the significant enhancement is due to the well-dispersion of Poly CoTAPc on KB, which increases the specific surface area, facilitates greater exposure of active sites and leads to higher electrochemical activity.

Surface morphology characterization

FESEM and EDX studies. The morphological structure and elemental composition of the designed electrocatalysts were analysed by FESEM, TEM and EDAX methods. The FESEM images of Poly CoTAPc (Fig. 2a) and KB (Fig. 2b) exhibit a randomly distributed small rod and quasi-spherical-like shape on the surface, respectively. Whereas the hybrid composite Poly CoTAPc + KB, the FESEM image represented in Fig. 2c shows polymeric molecules agglomerated on each other with a highly conductive KB matrix (it can be visible in TEM images), which results in a further increase in the active surface area of the hybrid composite and its conductivity too. The presence of trace amounts of each element and their composition in the hybrid composite were determined using the EDAX technique, as shown in Fig. 2d. The elemental compositions of the composite material were found to be 67.90% C, 11.52% N, 16.96% O, and 3.62% Co, which slightly deviate from the theoretically calculated values of monomeric units of Poly CoTAPc (49.54% C, 9.63% N, 33.00% O, and 5.06% Co). This deviation is mainly attributed to the addition of conductive carbon (KB), which significantly increases the carbon content in the composite and suppresses the other elemental content. The TEM images in Fig. 2e and f show that the conducting carbon KB exhibits a spherical-like structure with interconnections between multilayered carbon atoms with each other. The images also suggest the higher porosity nature of KB. The TEM images of the composite (Poly CoTAPc + KB) in Fig. 2g reveal that the rod-like structured phthalocyanine molecules are randomly distributed on the agglomerated spherical-shaped high porous KB matrix, which significantly increases the surface area of the hybrid composite and allows a greater number of electrolyte ions for electron transfer kinetics in the OER reaction. Overall, the FESEM and TEM agreed with each other. The uniform distribution of desired elements (Co, N, O and C) in the hybrid composite is displayed in the colour mapping image in Fig. 2h, and all elements are in good agreement with the EDAX results. To provide a clearer insight, each element (such as C, N, O and Co) mapping image is shown in Fig. S3.† It is well observed from the EDAX and elemental mapping images that the higher carbon content in the composite material is attributable to the substitution of the external carbon content in the KB. In addition, the Selected Area Electron Diffraction (SAED) patterns shown in Fig. 2i do not exhibit any lattice fringes around the diffused rings, indicating that the prepared hybrid composite is amorphous, which is well consistent with the XRD pattern results.

Fig. 2 FESEM image of (a) Poly CoTAPc, (b) KB and (c) Poly CoTAPc + KB and EDAX spectrum of (d) Poly CoTAPc + KB. TEM images of (e and f) KB and (g) Poly CoTAPc + KB. (h) Elemental mapping image of Poly CoTAPc + KB. (i) SEAD pattern of Poly CoTAPc + KB.

XPS analysis. Further investigation of the elemental composition and the valency state of an atom on the surface of a hybrid composite (Poly CoTAPc + KB) was carried out by the XPS method. The data were deconvoluted using XPSPEAK 4.1 software, and the resulting plots are shown in Fig. 3a–e. The survey scan (Fig. 3a) suggests that a total of four elements, such as C, O, N and Co, are present on the surface of the composite. The deconvoluted C 1s spectrum (Fig. 3b) of Poly CoTAPc + KB reveals four peaks in-between the binding energy range of 280–290 eV which are responsible for the carbon binding of sp² C=C (284.2 eV), sp³ C–C (284.8 eV), C–N (285.8 eV) and C=O (289.3 eV), respectively.⁴⁰ The deconvolution of the N 1s spectrum (Fig. 3c) yields two superimposable peaks, such as pyrrolic nitrogen (–C–N=C) at 398.9 eV and metallic nitrogen (Co–N) at 399.7 eV. Similarly, the O 1s spectrum (Fig. 3d) demonstrates two intense bands at binding energies of 532.4 eV and 533.8 eV, which are correlated with O–C and –O=C bonds, respectively. The cobalt metal spectrum (Co 2p) obtained in Fig. 3e affords two major peaks at 780.9 eV and another at 796.1 eV, which mainly correspond to Co 2p_3/2 and Co 2p_1/2, respectively, suggesting that the cobalt metal exists in a high-spin Co²⁺ oxidation state, as strongly indicates Co bonding with nitrogen atoms.^40,41

Fig. 3 XPS spectrum of the hybrid composite Poly CoTAPc + KB: (a) survey scan and (b–e) C 1s, N 1s, O 1s and Co 2p core spectrum.

Electrochemical activity

Solution cyclic voltammetric analysis. The electrochemical properties of the synthesized polymer Poly CoTAPc were investigated through cyclic voltammetry (CV) by immersing a bare glassy carbon electrode (GCE) in 10 mL of a deoxygenated dimethyl sulfoxide (DMSO) electrolyte. The CV experiment was performed both with and without the addition of 4 mg of the Poly CoTAPc complex, using 10 mM tetra butyl ammonium perchlorate (TBAP) as a supporting electrolyte and scanned at a rate of 50 mV s⁻¹versus Ag/AgCl, as evidenced in Fig. S4.† The pristine GCE in DMSO without Poly CoTAPc did not exhibit any peaks. However, the electrolyte containing Poly CoTAPc displayed two distinct redox peaks, indicating the electroactive nature of the synthesized Poly CoTAPc. The first redox couple observed in the potential sweep of −0.3 to −0.41 V is referred to as the redox properties (Pc²⁻/Pc¹⁻) of the phthalocyanine ring. The second redox peak, occurring in the positive potential array from 0.35 to 0.5 V, was due to the redox behaviour of the central cobalt metal ion (Co²⁺/Co¹⁺).⁴²

Electrochemical OER activity. The OER catalytic activity of the prepared Bare Ni foam, IrO₂, Poly CoTAPc, and hybrid composite Poly CoTAPc + KB catalysts on the Ni foam was investigated in a 1 M KOH electrolyte with a scan rate of 5 mV s⁻¹. For comparison, the OER activity of commercially procured IrO₂ electrocatalysts was also studied under similar experimental conditions. A higher electrolyte concentration (1 M KOH) was preferred over lower KOH concentrations (0.1 and 0.5 M KOH) due to the significant effect of ion concentration on OER activity. Higher ion concentration increases the electrolyte's conductivity and ion mobility, thereby enhancing the electron transfer kinetics during the OER process. Additionally, a higher ion concentration provides greater chances for collisions of reactant particles with increased ionic strength near the electrode surface, leading to higher activity and reduced ohmic resistance.^43,44

The LSV polarization curves of each electrode are shown in Fig. 4a. The catalysts on the working electrode exhibited distinct OER anodic peaks for water oxidation. As shown in the graph, the Poly CoTAPc electrode enhanced the OER activity by shifting the overpotential towards the negative region. Poly CoTAPc activity was further improved by fabricating a composite with KB as the conducting material. The optimization of Poly CoTAPc with KB as a composition for better OER activity was confirmed by the LSV graph, and the extracted overpotential bar graph is shown in Fig. S5a and b.† In this figure, all three compositions on Ni foam exhibited significant responses. Notably, the electrode with a 4 : 1 ratio exhibited a lower overpotential and a better current response than the others; thus, this ratio was maintained for all further experiments. In contrast, the hybrid ratio of 3.5 : 1.5 resulted in a similar overpotential as the 4 : 1 ratio but lower current density, possibly due to the higher amount of KB hindering the OER activity.

Fig. 4 (a) LSV curves of bare Ni foam, Poly CoTAPc/Ni, KB/Ni, Poly CoTAPc + KB/Ni and IrO₂/Ni in an alkaline electrolyte (1 M KOH) at a scan rate of 5 mV s⁻². (b) Bar graph indicating overpotential for OER of distinct electrodes. (c) Tafel slope values for different catalysts and (d) long-time durability study of Poly CoTAPc/Ni at various current densities using chronoamperometry.

Among all the modified electrodes shown in Fig. 4a, the hybrid composite exhibited better OER activity than the individual catalysts. The overpotential needed for water oxidation at a current density of 10 mA cm⁻² was extracted from the LSV curves (from Fig. 4a) and shown in a bar graph (w.r.t. overpotential, η₁₀ = E_RHE − 1.23), as in Fig. 4b. The bar graph reveals that the hybrid composite modified on Ni foam (Poly CoTAPc + KB/Ni) displayed an extraordinarily lower overpotential of 306 mV, whereas the bare Ni foam, Poly CoTAPc/Ni, KB/Ni, and benchmark IrO₂/Ni exhibited 574 mV, 386 mV, 418 mV, and 330 mV at 10 mA cm⁻² current density, respectively. The enhancement in OER catalytic activity on the Poly CoTAPc + KB/Ni electrode is attributed to the synergistic intra- and inter-molecular transitions of the central metal ion (Co^II) from the D-band centre as well as the M–N–C skeleton of Poly CoTAPc with high-conducting carbon nanomaterial KB, which increases the electron density, the number of active sites on catalysts and improves π–π interactions.^45,46 The activity of the designed hybrid catalyst is claiming its superiority compared to existing catalysts, as listed in Table S1.†

To obtain greater insight into the mechanisms and reaction kinetics of the fabricated electrodes, further studies were conducted to analyse their Tafel slopes and exchange current density (ECD). The Tafel slopes were derived from linear sweep voltammetry (LSV) polarization curves by plotting the overpotential against the logarithm of the current density, as illustrated in Fig. 4c. A lower Tafel slope indicates superior electrocatalytic performance and more efficient reaction kinetics.^47,48 Among all the catalysts tested, the hybrid composite catalyst (Poly CoTAPc + KB/Ni) demonstrated the lowest Tafel slope of 81 mV dec⁻¹ and the highest ECD of 2.88 × 10⁻⁴, whereas the benchmark catalyst IrO₂/Ni exhibited a slightly higher Tafel slope of 85 mV dec⁻¹ with an ECD of 2.1 × 10⁻⁴. The Tafel slopes and ECD results for the other catalysts are presented in Table 1. To further study the catalytic properties of the modified electrodes, key metrics, such as the mass activity and turnover frequency (TOF), were calculated. These matrices are crucial for understanding the catalytic performance. The mass activity and TOF values were calculated at 1.58 V vs. RHE (η = 350 mV) by assuming that all the cobalt metal atoms in the catalysts were active sites in the coated catalysts. The mass activity (Fig. S7a†) of the hybrid composite Poly CoTAPc + KB/Ni was 22.72 A g_metal⁻¹, which is higher than that of individual electrodes, including benchmark IrO₂, as illustrated in Table 1. The TOF values presented in Fig. S7b† also indicate that Poly CoTAPc + KB/Ni yielded a higher TOF value of 0.0122 s⁻¹ compared to other electrodes such as Poly CoTAPc (0.006 s⁻¹), KB (0.0002 s⁻¹) and IrO₂ (0.0075 s⁻¹). These findings highlight that the hybrid composite catalyst facilitates superior reaction kinetics for OER activity by generating a greater amount of O₂ than the other tested catalysts.

Table 1 Electrocatalytic activity of various designed electrodes towards OER

Electrocatalysts	Overpotential (at 10 mA cm^−2)	Tafel slope (mV dec^−1)	ECSA (cm^2)	ECD (A cm^−2)	Mass activity (A gmetal^−1 at 1.58 V)	Roughness factor
Ni foam	574	136	7.5	—	—	30
Poly CoTAPc	386	103	27.5	1.99 × 10^−5	5.194	110
KB	418	116	—	1.25 × 10^−4	—	—
Poly CoTAPc + KB	306	81	104.5	3.98 × 10^−4	22.72	416
IrO2	330	85	73	2.6 × 10^−4	15.99	288

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The long-term durability and stability of electrocatalysts are essential for the commercial viability of sustainable energy devices. To assess the long-term stability of the designed hybrid composite (Poly CoTAPc + KB/Ni) for the OER, a chronoamperometric (CA) technique was employed at varying current densities of 10, 25, and 50 mA cm⁻², maintaining their respective potentials. The resulting data (Fig. 4d) demonstrate that the hybrid polymeric composite electrode exhibits better stability and maintains a consistent performance for over 30 hours. The rate of current loss as a metric of catalyst degradation was measured and was relatively low at 0.12, 0.15 and 0.2 mA cm⁻² h⁻¹ for applied current densities of 10, 25 and 50 mA cm⁻² respectively. This outstanding stability can be attributed to the extended conjugation, the increased density of cobalt active centres, and the strong synergy of π–π interaction between polymeric phthalocyanine (Poly Pc) and KB nanocarbon, which together prevent the leaching of electrocatalysts from the electrode surface. Furthermore, the minor current loss observed at higher current densities was primarily attributed to the loss of a small amount of catalyst material during the significant oxygen evolution. Additionally, the stability of the Poly CoTAPc + KB/Ni composite before and after CA stability towards the OER was evaluated through repetitive LSV cycles, as depicted in Fig. S8a and b,† using a 1 M KOH solution. Throughout the 30 h period, there was negligible change between the curves obtained before and after the LSV test, with only a slight deviation in current density. This minor variation can be attributed to the previously mentioned factors, such as the leaching of small catalysts with stronger evolution of O₂ bubbles. The superior activity is due to the strong synergy between polymeric phthalocyanine (Pc) and KB, and the abundance of cobalt active sites. Following these long-term stability tests, the composite electrode surface material was examined using XPS, HRTEM and EDX, as shown in Fig. S9a–f,† to investigate structural changes during the experiment. Remarkably, the electrode surface displayed similar morphological characteristics after stability studies compared to its initial state, indicating that the Poly CoTAPc + KB/Ni composite maintains its structural integrity. The deconvolution of the XPS spectra indicates the same atomic state with negligible shifting, and the EDAX image demonstrates that the elemental content was also aligned with the initial one. These characterization measurements confirmed the stability of the Poly CoTAPc + KB/Ni composite over extended periods.

ECSA and EIS studies

ECSA. The presence of active centres on the electrode surface is highly significant for the boosting catalytic activity, which subsequently affects the electrode's effectiveness and reaction kinetics. To gain a deeper understanding of the number of active sites on the electrode surface, electrochemical surface area (ECSA) measurements were conducted on the designed electrodes. The ECSA values of the fabricated electrodes were determined using eqn (2).

ECSA = C_dl/C_s (2)

where C_dl is the electrochemically measured double-layer capacitance (C_dl) and C_s is the specific capacitance of the electrode surface in 1 M KOH.⁴⁹

The C_dl values of the electrodes were determined by scanning each electrode at different scan rates from 10 to 100 mV s⁻¹ in the non-faradaic potential range (1.05 to 1.1 V vs. RHE) in a 1 M KOH electrolyte. The resulting cyclic voltammograms are shown in Fig. 5a–d. From these curves, linear plots were created (Fig. 5e) by extracting the charging currents at various scan rates. The slopes of these linear plots were taken as the C_dl values of the respective electrodes. It is assumed that C_dl values are directly proportional to ECSA, indicating that higher C_dl values correspond to greater ECSA and more active sites.

Fig. 5 (a–d) Cyclic voltammograms for prepared electrodes at different scan rates in the non-faradaic region in 1 M KOH. (e) Corresponding current vs. scan rate plots to calculate double-layer capacitance values and (f) EIS Nyquist plots and equivalent circuits (inset) for the designed electrodes.

As shown in Fig. 5e, among all the designed electrodes, the hybrid CoTAPc + KB/Ni composite exhibits the highest C_dl value of 4.1 mF, compared to the benchmark catalyst's 2.91 mF and other individual electrodes. Subsequently, the ECSA values of each electrode were calculated using their C_dl data viaeqn (2), and the resulting values are summarized in Table 1. The higher ECSA value of CoTAPc + KB/Ni (104.5 cm⁻²) suggests that its surface area can accommodate a larger number of active centres, leading to higher OER activity. The roughness factor (R_f) of each modified electrode was calculated, as listed in Table 1. Among all electrodes, CoTAPc + KB/Ni exhibited the highest R_f value. These ECSA and roughness factor results agree well with the lower overpotential and Tafel slopes of the hybrid composite. Further studies were conducted to evaluate the intrinsic properties of the fabricated electrode by plotting ECSA-normalized LSV curves (Fig. S10†). The composite electrode CoTAPc + KB/Ni exhibits a lesser overpotential (about 390 mV) than the other measured electrodes, indicating its superior intrinsic behaviour towards OER activity.

Electrochemical impedance spectroscopy (EIS) was utilized to gain a more profound comprehension of the charge transfer kinetics at the electrode–electrolyte interface, which is an important parameter for assessing resistivity and reaction kinetics in electrode/electrolyte systems.⁵⁰ EIS measurements were performed at a constant potential of η₁₀vs. RHE with a 5 mV amplitude across frequencies ranging from 0.1–10⁴ Hz. The resulting EIS Nyquist plot and its equivalent circuit (inset) are shown in Fig. 5f. All catalysts produced semicircular curves with different resistivity characteristics, as summarized in Table S2.† The circuit parameters are R_s (solution resistance), R_ct (charge transfer resistance), R_c (coating resistance), and Q (constant phase element).⁸ Among all the electrode catalysts, the hybrid composite Poly CoTAPc + KB/Ni demonstrated higher conductivity with lower R_ct values, indicating that this composite electrode facilitates significantly more rapid electron transfer kinetics than the other catalysts. The EIS values followed the observed overpotential and Tafel slopes for Poly CoTAPc + KB in the OER.

Mechanism

The OER is a pivotal and difficult step in water electrolysis, characterized by its intrinsically slow anodic kinetics involving four electron/proton coupled reactions. Density functional theory (DFT) studies provide the information that the arrangement of the electronic structure of phthalocyanine with incorporated conductive carbons can affect the overpotential values in OER.²² The strength of oxygen binding to the surface of the active site plays a very important key role, which determines the overpotential of electrocatalysts, which is aligned with Sabatier's principle.⁵¹ This principle suggests that for optimal performance, the catalyst–oxygen binding should balance adsorption and desorption to minimize overpotential. The electron-withdrawing groups at the corners of polymeric CoPc enhance π–π interactions between Pc molecules with conductive KB, which reduces the electron density and provides a space for the OH⁻ ion binding on the cobalt active centre.²¹

Considering these earlier theoretical and experimental research,^21,22 the prepared polymeric CoTAPc exhibited multiple cobalt active sites with a higher surface area supported by KB, boosting the storage of a greater number of intermediates and resulting in a lower overpotential compared with other electrodes. With the help of previous findings and DFT studies, the possible adsorption and desorption of reaction intermediates can occur in four steps, as shown in Fig. 6. The first step involving adsorption of OH⁻ to the metal active site (cobalt) and formation of a Co–OH intermediate with the release of an electron is considered the rate-determining step (RDS). This is also supported by the Tafel slope of the composite electrode at 81 mV dec⁻¹ and aligns well with earlier results on cobalt phthalocyanine complexes, where steric hindrance and the ligand environment limit the kinetics of the initial OH⁻ adsorption.^52,53 In the second step, the adsorbed OH⁻ groups on the cobalt active site react with another OH⁻ ion of the electrolyte, undergo oxidation and release H₂O molecules with an e⁻, with the production of a cobalt oxide intermediate. In the third step, the as-formed cobalt oxide intermediate then reacts with another OH⁻ ion produces the peroxide intermediate on Co metal, which again reacts with another OH⁻ undergo reduction and forms oxygen molecules with water and e⁻ and the reduced forms of cobalt is again active for the subsequent OER cycles.^54,55

Fig. 6 Possible OER mechanism of Poly CoTAPc + KB in alkaline media.

Conclusion

In conclusion, a bio-inspired, simple, low-cost and environmentally friendly novel Poly CoTAPc complex was synthesized using a two-step method. The synthesized materials were successfully characterized using various spectro-analytical techniques for physical and surface analysis. Subsequently, a hybrid composite of Poly CoTAPc was developed by incorporating carbon nanoparticles (KB), resulting in a significant enhancement in the field of electrocatalysis for water oxidation by exhibiting a lesser overpotential of 306 mV at a current density of 10 mA cm⁻² and lower Tafel slopes. The enhancement in the OER activity is attributed to the synergetic effects between Poly CoTAPc and KB, which include the greater hydrophilicity, electron-withdrawing acidic groups, a higher number of Pc corners, and an improved electrochemical surface area by higher porosity of KB and π–π interactions. These findings highlight the potential of bio-inspired polymeric CoPc composites as efficient and effective electrocatalysts for water oxidation, paving the way for future research and development in sustainable energy solutions. The integration of carbon nanoparticles not only enhances the catalytic properties and provides a scalable and economically viable pathway for the development of advanced water oxidation electrocatalysts.

Data availability

Data will be shared upon request.

Author contributions

Giddaerappa: conceptualization; methodology; data curation; formal analysis; investigation; validation; original draft. Sundarraj Sriram: investigation; validation. Abdul Junaid P.: investigation. Lokesh Koodlur Sannegowda: investigation and review. M. H. Naveen: investigation and review. K. Sudhakara Prasad: data curation; funding acquisition; supervision; review.

Conflicts of interest

The authors declare that they have no competing financial interests.

Acknowledgements

The author Giddaerappa greatly acknowledge the Anusandhan National Research Foundation (ANRF), India, for providing the financial assistance under SERB-NPDF grant (ANRF/F/11090/2024-2025). KSP is thankful for the support of Yenepoya Technology Incubator through BIRAC-ETA, India (YTI/ETA/SL/PR009) and Yenepoya Technology Incubator MedTech rapid prototyping facility supported by National Biopharma Mission (NBM), DBT-BIRAC, Government of India.

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Footnote

† Electronic supplementary information (ESI) available: Chemicals, reagents, and characterization methods; mass spectrum of the precursor (3) and Poly CoTAPc (4); elemental mapping of carbon, nitrogen, oxygen and cobalt in Poly CoTAPc; solution cyclic voltammograms of bare GCE, Poly CoTAPc + KB/GCE in DMSO; LSV polarization curves and overpotential bar graph of different composition of Poly CoTAPc + KB; exchange current density of modified electrodes; mass activity and TOF value of catalysts modified electrodes; LSV polarization curves for OER before and after chronoamperometric (CA) study carried with Poly CoTAPc + KB/Ni in 1 M KOH at 5 mV cm⁻¹ scan rate; post characterization of Poly CoTAPc + KB/Ni electrode after long term experiment; and comparison of OER activity of Poly CoTAPc + KB with other materials exist in literature and analytical values for equivalent circuit parameters for fitting the Nyquist plots. See DOI: https://doi.org/10.1039/d5se00434a

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