Unraveling the efficacy of defect engineered mesoporous Ni–Co spinel oxide nanowires as an energy efficient electrocatalyst for the oxygen reduction reaction and fuel cell applications

Abstract

Defect engineering and morphology control are crucial in designing advanced transition metal oxide (TMO)-based electrocatalysts with superior activity towards the oxygen reduction reaction (ORR). Following this blueprint, one-dimensional mesoporous Ni–Co oxide spinel (NCO) nanowires were synthesized using a simple solvothermal method, where defects were introduced by modulating the intrinsic metal ion ratio. Extensive structural, morphological, and compositional characterization of the nanowires was performed using various experimental techniques. The nanowires exhibited appreciably large surface area and porous architecture. X-ray absorption and positron annihilation spectroscopic studies revealed an exclusive Co-ion vacancy in Co-deficient nanowires and oxygen vacancies in Co-rich and -deficient nanowires. X-ray photoelectron spectroscopic studies unraveled preferential surface enrichment of the Co-rich NCO catalyst with Co³⁺ and Ni²⁺ ions. Co-excess NCO catalysts showed outstanding electrocatalytic activity towards the ORR, with a Tafel slope as low as ∼50 mV per decade with an overpotential of ∼0.35 V, comparable to benchmark noble metal-based catalysts, and periodic DFT studies delineated the crucial role of excess Co on the catalyst surface for demonstrating such superior activity. The NCO catalyst with a Co/Ni content of 2.6 : 1 exhibited the highest selectivity towards O₂/H₂O conversion, and it was further deployed in a real-life operational alkaline anion exchange membrane fuel cell (AEMFC) at room temperature. This study successfully demonstrates practical utilization of judiciously designed NCO catalysts as suitable alternatives to noble metal-based catalysts in sustainable energy applications.

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1. Introduction

Over the last few decades, with the gradual depletion of conventional energy resources such as fossil fuel and their consequent harsh environmental impact leading to global warming, renewable energy resources with technologies like regenerative fuel cells, high performance metal–air batteries and supercapacitors have gained paramount attention in the scientific community.^1–4 The heart of these energy storage devices lies in the efficiency of utilization of the water-electrolysis principle, i.e., storing energy by splitting water into its components via oxygen evolution reaction (OER)/hydrogen evolution reaction (HER) combination and the release of stored energy through the oxygen reduction reaction (ORR)/hydrogen oxidation reaction (HOR) couple.⁵ However, the inherent complexity of the ORR reaction mechanism makes it kinetically sluggish, which significantly impedes the performance of fuel cells or batteries.^6,7 In this context, it is worth mentioning that to achieve the overall efficacy of the energy conversion system, the four-electron transfer pathway of the ORR is highly preferred instead of the competitive two-electron transfer pathways.⁶ Noble metal Pt-, Ru- and Ir-based catalysts are recognized as the most efficient ORR catalysts to date, but their scarcity and high cost restrict their appropriate commercialization.⁸ To circumvent this issue, transition metal oxide (TMO)-based catalysts have emerged as a suitable alternative to noble metal catalysts for sustainable energy applications by virtue of their greater abundance, cost effectiveness, and environment-friendly nature.^9,10

To design an efficient, cost-effective, and highly durable TMO catalyst, it is imperative to acquire a consolidated picture of the various factors governing its efficacy as a catalyst for a targeted reaction and to master the way of fine tuning those matrices to achieve the desired functionality. Regulation strategies such as reduction in the particle size to generate a larger number of surface-active sites and meso-scale modulation such as surface reconstruction and hetero-structure formation have been well-sought approaches for enhancing the performance of TMO catalysts.¹¹ In this context, strategies such as defect engineering at the atomistic level either by heteroatom doping or by stoichiometric manipulation as well as morphology control have evolved as remarkably efficient and effective tools to enhance catalytic performance by tuning their surface activity and transport properties. Among various TMOs, Ni- and Co-based binary and ternary oxides have been widely explored in the field of water-electrolysis and in other green energy applications.^12–14 Previous reports have demonstrated that nickel cobaltite spinel (NiCo₂O₄; henceforth referred to as NCO) has higher electrical conductivity and lower electron transport activation energy than their corresponding binary counterparts, viz., NiO and Co₃O₄.¹⁴ To enhance OER catalytic activity of NCO, Zheng and his group engineered Co defects in NCO by Al-doping, followed by subsequent plasma treatment.¹⁵ However, Devaguptapu and co-workers explored the morphology dependence of NCO on its bifunctional electrocatalytic behavior towards both ORR and OER.¹⁶ Y. Xiao et al. investigated the effectiveness of three-dimensional macroporous nanosheets of NCO towards ORR.¹⁷ Moreover, compared to two-dimensional and three-dimensional nanostructures, one-dimensional nano-wires of NCO are expected to display improved activity with more interfacial active sites along the long dimension and better electronic conductivity along the short dimension, highly suitable for enhanced catalytic performance towards water-splitting reactions.¹⁸ Hybrid catalysts combining NCO with other supportive catalysts, such as NiCoSe₂, Co₃O₄, MnCo₂O₄, and reduced graphene oxide, are also reported for battery and supercapacitor applications.^12,19,20

Apart from electrocatalysis, Chen and Wang explored the versatility of NCO as a useful catalyst for ozonation of a typical sulfonamide antibiotic.²¹ Fang and his coworkers demonstrated the applicability of NCO as an easily recoverable magnetic catalyst for Knoevenagel condensation reaction with 99% conversion efficiency.²² Dai et al. investigated unusual bowtie-shaped NCO nano-structures as an efficient catalyst for low-temperature methane combustion using Temperature Programmed Oxidation (TPO).²³ The effect of the Co to Ni ratio in cobalt–nickel mixed oxide catalysts on methane combustion was investigated by Lim and co-workers.²⁴ The effect of variation in the Co to Ni ratio on the electrocatalytic activity of NCO towards oxygen reduction has not been explored enough although some studies have been reported.²⁵ Systematic studies delineating the synergistic role of defects, morphology, and tunability of metal ion oxidation states on the surface activity and their correlation with ORR catalytic activity are also few.

In this work, we synthesized mesoporous NCO catalysts with well-defined nanowire morphology and appreciably high surface area using a simple solvothermal method starting from nitrate salts of abundant Ni and Co. Variation in the Co to Ni ratio was utilized to engineer defects in the system. The phase purity of the catalysts was confirmed by applying the powder X-ray Diffraction (pXRD) technique. Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM) studies were performed to gain insight into shape and morphology, and Scanning Transmission Electron Microscopy coupled with High Angle Annular Dark Field Imaging (STEM-HAADF) was performed to ensure the homogeneity of the elemental distribution. The elemental bulk composition of the catalysts was checked using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis. The surface area and porosity measurements were carried out using an N₂ adsorption–desorption study. The local structure around the transition metal ions was investigated using X-ray Absorption Spectroscopic Studies comprising both Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES). The elemental distribution of metal ions present at specific oxidation states at the surface was studied using X-ray Photoelectron Spectroscopy (XPS). Positron Annihilation Lifetime Spectroscopy (PALS) was implemented to understand the evolution of defects in the system produced by stoichiometric variation in the relative metal content. The activity of the NCO nano-catalysts was further examined for ORR, and mechanistic insights pertaining to peroxide formation have been acquired. Furthermore, periodic Density Functional Theory (DFT) studies were implemented to understand the performance of NCO catalysts. An Alkaline Anion Exchange Membrane Fuel Cell (AEMFC) was constructed using the best-performing catalyst with optimum catalytic response coupled with the improved selectivity towards H₂O formation as the cathode. The systematic development of the catalyst, followed by its relevant application in a device, highlights the potential pathway for utilizing low-cost and efficient TMO-based fuel cells for sustainable energy applications.

2. Experimental

The details of the synthesis procedure, characterization techniques, defect studies, electrochemical measurements and computational details are presented in S1 of the (ESI) file along with Fig. S1–S3.†

3. Results and discussion

3.1. Structural, morphological and compositional investigations

NCO crystallizes in the inverse spinel structure where Ni atoms occupy one-fourth of the O_h voids, while Co atoms are distributed in one-fourth of the O_h voids as well as one-eighth of the T_d voids in contrast to the normal spinel structure where the O_h voids are solely occupied by the trivalent ion. As evident from the powder XRD patterns shown in Fig. 1a, all the calcined NCO samples with varied metal ion ratios perfectly agree with the standard pattern of the Fd3̄m space group [JCPDS File no.: 20-0781]. Moreover, the broadened feature of the XRD peaks indicates the nano-crystallinity of the synthesized NCO samples. Synchrotron-based powder XRD (SXRD) patterns of certain select catalysts are shown in Fig. S4 in ESI.† The powder SXRD patterns beautifully portray the characteristic pattern of the cubic spinel structure, which is a bit suppressed by a relatively high background in the Cu K-α lab source-based XRD.

Fig. 1 (a) Cu K-alpha (laboratory) source-based powder XRD and (b) background corrected FTIR patterns of the NCO catalysts with different Co to Ni ratios.

From the background-corrected FTIR patterns of the NCO nanowires, as depicted in Fig. 1b, the characteristic metal–oxygen stretching frequencies were observed in the wavenumber range of 500–700 cm⁻¹. Based on the literature, the peak around 636 cm⁻¹ is ascribed to the Co–O stretching vibrational mode, and Ni–O bond stretching appears as a broad band around 541 cm⁻¹.²⁶ The Co–O stretching is blue shifted and sharper with increasing Co fraction, indicating an increase in Co–O bond strength. Considering the crystal field stabilization energy, an O_h field of ligands around the central atom leads to a stronger M–L interaction compared to the T_d field of ligands. This indirectly suggests the preferential occupancy of O_h sites over T_d sites by Co cations when present in excesses.

Fig. 2(a) and (b) show the FE-SEM images of NCO nanocrystals at two different magnifications. A similar nano-wire shape was observed in the rest of the samples, confirming that morphology remains unaltered upon variation of the relative conc. of metals (as shown in Fig. S5 in ESI†). Unlike the findings of Dai and coworkers, where variation in the Ni/Co ratio also resulted in a change in the morphology of the metal-hydroxide precursor from bowtie shaped to irregular thin plates with increasing Co content; here, nanowire morphology of the calcined NCO powder remained same.²³

Fig. 2 (a) and (b) FE-SEM micrographs of NCO-2:1 nanowires at two different magnifications.

The nanowire morphology is further evident from the TEM images of the NCO catalyst (Fig. 3a and b). The TEM micrographs and SAED pattern (Fig. 3c) clearly depict that each NCO nanowire is composed of small crystalline grains, leading to the evolution of a mesoporous architecture, which is extremely desirable for effective penetration of electrolyte onto the surface of nanowires so that rapid charge transfer reactions can occur due to the short ion diffusion paths.²⁷ STEM-HAADF images (Fig. 3d–f) indicate the homogeneous distribution of the constituent elements in each single nano-wire. TEM micrographs and HAADF images of two more representative catalysts (one Co deficient and another Co excess) are shown in Fig. S6 and S7 in the ESI.†

Fig. 3 (a) and (b) HRTEM micrographs at two different magnifications; (c) SAED pattern; (d)–(f): HAADF images of the NCO-2:1 nanowire catalyst.

The linear correlation plot of the relative metal ratio obtained from ICP-AES analysis with respect to the nominal concentration ratio is shown in Fig. S8 of ESI.†

3.2. Surface area and porosity analysis using N₂ adsorption–desorption studies

The surface area, porosity and texture properties of the nano-wires were examined from N₂ adsorption–desorption studies performed at 77 K. The nitrogen adsorption and desorption isotherms of few selected samples are illustrated in Fig. 4a and of all the samples are illustrated in Fig. S9a of ESI.† The sorption isotherms exhibited Type-IV (a) behaviour along with an H3 type hysteresis loop characteristic of meso-porous architecture suggesting multilayer adsorption accompanied by capillary condensation at mesopores (Fig. 4a), which are presumed to be present within a nanowire.²⁸ Specific surface area was calculated from the isotherm by applying Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.35, while pore size distribution and pore volume were acquired from the analysis of the desorption branch of the isotherm using Barrett–Joyner–Halenda (BJH) algorithm. Fig. 4b shows the calculated surface area of the nanowires ranging from 109 to 225 m² g⁻¹, which was found to be appreciably higher than previously reported values of 69.4 (ref. 14) and 72.6 (ref. 29) m² g⁻¹ of NCO catalysts. Substantially high surface area is extremely desirable for providing enough surface-active sites to promote the catalytic activity towards ORR. The pore size distribution plot (Fig. 4c and S9b†) depicts the abundance of mesopores ranging from the ∼2 to ∼8 nm size regime, with ∼3 nm sized pores being the predominant ones. The total specific pore volume of the nanowires was evaluated in the range of ∼0.15 to ∼0.21 cm³ g⁻¹ (Fig. 4d), which can provide transport pathways for oxygen supply to the interior part of the cathode and provide the “tri-phase region” required for ORR.³⁰

Fig. 4 (a) N₂ adsorption–desorption isotherms, (b) specific surface area, (c) pore size distribution, and (d) specific pore volume of the different NCO catalysts. Uncertainties with respect to the pore size and surface area are within 5%.

3.3. Elemental analysis using X-ray photoelectron spectroscopic (XPS) study

To obtain an in depth understanding of the composition and oxidation state of the metal ions on the surface of the NCO catalysts, an XPS study was carried out. Fig. S10a and S10b† show the survey scans for the parent stoichiometric NCO and the best performing catalyst, (as discussed in the subsequent sections) with characteristic features of Ni, Co, C and O indicating the absence of any other impurity. Fittings of the XPS spectra for the samples are shown in Fig. 5 and S11.†

Fig. 5 (a) Co-2p, (b) Ni-2p and (c) O-1s XPS spectra of the NCO (Co : Ni = 2 : 1) catalyst fitted using a combination of the Gaussian–Lorentzian method.

Implementing a combination of Gaussian and Lorentzian fitting methods, the high resolution Co-2p spectrum could be deconvoluted into two spin orbit doublets and two shake-up satellite peaks (denoted by S in the spectrum, as shown in Fig. 5a). Based on the literature, peaks at 778.62 and 793.89 eV are assigned to Co³⁺, while peaks at 780.14 and 795.34 eV are ascribed to Co²⁺.³¹ In a similar way, the Ni-2p spectrum could be fitted with two spin–orbit doublets and two satellite peaks, as demonstrated in Fig. 5b. Based on reported data, peaks at 853.13 and 870.67 correspond to Ni²⁺, and the peaks present at 854.83 and 872.32 eV are ascribed to the +3 oxidation state of Ni.³¹ The relative fraction of the metal ions at particular oxidation states was evaluated from the fractional area under the corresponding peaks. Interestingly, it should be noted here that, although in an ideal spinel crystal structure, the divalent and trivalent ions obey specific site occupancy, and the presence of the tunable oxidation state of the constituent metals makes the scenario more fascinating. Such intriguing behavior is harnessed in these NCO catalysts with the interplay of the +2 and +3 oxidation states of Ni and Co ions, respectively. On the other side, the O-1s (Fig. 5c) spectrum is deconvoluted into three peaks located at the binding energy maxima of 528.76 (O1), 530.12 (O2) and 531.44 (O3) eV, corresponding to metal–oxygen, oxygen in surface adsorbed hydroxyl species and oxygen vacancies, respectively.^13,32

XPS spectra were also recorded at the post catalysis stage for the best performing catalyst. The fractional area corresponding to the oxygen defects (O3) was found to be higher in Co excess catalyst NCO-2.6:1 compared to the parent stoichiometric NCO-2:1. Interestingly, the relative contribution of the O2 peak was also observed to be increased in the recovered NCO-2.6:1 catalyst compared to its pristine state, which consequently indicates surface adsorption of the hydroxyl species at the post-catalysis stage. The details are further discussed in a subsequent section (Section 3.6).

3.4. Understanding local structure using X-ray absorption spectroscopy (XAS)

XANES and EXAFS characterization were conducted to understand the local geometric structure of the NCO catalysts and the existence of defects in them. XANES is sensitive to chemical states and local atomic symmetry. The normalized Ni K-edge and Co K-edge XANES spectra are shown in Fig. S12a and S12b, respectively, in ESI.† The position of the rising absorption edge as well as the peak maxima for Ni is slightly shifted towards the higher energy side compared to the NiO standard, indicating the presence of Ni in an oxidation state higher than +2.³¹ The XANES spectra of the three NCO catalysts followed a similar profile to that of the NiO standard, where all the Ni atoms are octahedrally coordinated, suggesting that the catalysts maintained a similar geometric structure. The k²-weighted Fourier transformed spectra of the Ni K-edge EXAFS (Fig. 6a) demonstrate that the peak at 1.57 Å corresponding to the Ni–O bond moves to a lower value (∼1.44 Å) with increasing relative Co content in the catalyst. This interesting observation of Ni–O bond contraction indicates that the incorporation of excess Co leads to a mild densification/compaction of the system. Contraction of the bond length could also be an indication of the presence of a higher oxidation state of Ni in the catalysts.³³ A similar effect is not observed in the Fourier-transformed spectra of the Co K-edge EXAFS (Fig. 6b). Normalized Co K-edge XANES spectra of the NCO catalysts (Fig. S12b†) showed a similar profile to that of the Co₃O₄ standard, where Co atoms are distributed between O_h and T_d sites along with the presence of the mixed valence states of +2 and +3. The presence of a pre-edge at 7709 eV corresponding to the electronic transition from 1s to 3d orbital also supports the occupancy of Co atoms at T_d sites with a lack of inversion symmetry.¹⁵

Fig. 6 Fourier transformed EXAFS: (a) Ni K-edge and (b) Co K-edge. (c) Ni K-edge and (d) Co K-edge WT EXAFS plots of the NCO catalysts with different Co : Ni ratios.

To obtain atomistic insights into the defect present around the metal ion centres, Wavelet Transform (WT) analysis of the EXAFS spectra was performed, and the corresponding WT plots are shown in Fig. 6c and d. The intensity maximum of the Ni K-edge around 2.6 Å of the parent NCO-2:1 catalyst, as shown in Fig. 6c, corresponds to the Ni–Co bond. The extent of the lobe corresponding to the Ni–Co bond decreases in the Co deficient catalyst, suggesting the creation of Co vacancy. A similar reduction in the spatial appearance of the metal–metal coordination lobe reflecting the presence of cationic vacancy sites was earlier demonstrated by Liu et al. in 2D MnO₂ nanobelts.³⁴ However, in the Co K-edge WT plots (Fig. 6d), the spread and intensity of the lobe around 1.5 Å attributed to metal–oxygen coordination are appreciably reduced in the Co excess catalyst compared to parent stoichiometric compound, which reveals the existence of oxygen vacancies.³⁵ Full scale WT spectra are shown in Fig. S13.†

3.5. Investigation of defects using positron annihilation lifetime spectroscopy (PALS)

The positron annihilation lifetime spectra are multi-exponential in nature following the decay equationwhere F(t) = the number of counts at time t, k = the number of exponential decay components, τ_i = the lifetime of the ith component, and I_i = the corresponding intensity. The lifetime spectra of all the NCO catalysts could be fitted as a sum of three-lifetime components, where the third lifetime component (τ₃) in the range of ns is attributed to the pick-off annihilation of ortho-positronium (o-Ps) at the mesopores, grain boundaries and inter-crystalline voids.³⁶ Surprisingly, the magnitude of τ₃ was found to be quite small along with negligibly small intensity compared to those values observed in other porous materials, such as zeolites and metal–organic frameworks. This can be illustrated based on the quenching effect on o-Ps by the unpaired electron present in Ni²⁺ (d⁸ system) and Co²⁺ (d⁷ system). Moreover, Co³⁺ and Ni³⁺ behave as strong Lewis acid centres that can suppress o-Ps formation, which is reflected in the substantially reduced I₃.³⁷ Fitted positron lifetime spectra of all the NCO catalysts are shown in Fig. S14 in the ESI.†

The first lifetime component τ₁ is attributed to the presence of small vacancy defects inside the bulk, while the second lifetime component τ₂ is ascribed to the larger vacancy clusters present on the surface or sub-surface. I₁ and I₂ respectively denote the corresponding intensities of the first two-lifetime components, and τ_av represents the intensity-weighted average positron lifetime, which reflects the overall defect density of the system. Deeloed et al. theoretically calculated the bulk positron lifetime in the Co₃O₄ system as 145.5 ps.¹³ Positrons do not feel much affinity towards positively charged oxygen vacancies; however, neutral oxygen vacancies trap positrons owing to the effective lowering in Coulomb potential, and the enhancement from the bulk lifetime solely owing to oxygen vacancies is also small, such as 147.8 ps. Moreover, negatively charged cation vacancies, such as Co vacancies in Co₃O₄, act as deep positron traps, and the calculated positron lifetime corresponding to two crystallographically non-equivalent Co vacancies are 174.8 and 217.6 ps. The calculated lifetime for V_Co–V_O vacancy associates is reported to be 249.7 ps. In a similar context, it can be suggested that τ₁ in the present study is contributed by Co vacancy and oxygen vacancy associates in Co deficient samples. In a similar iso-structural spinel MgAl₂O₄ system, the first lifetime component is proposed to reflect microstructural specifics of spinel ceramics with characteristic octahedral and tetrahedral vacant cation sites along with a contribution from the annihilation of p-Ps atoms.³⁸ It can be clearly observed from Fig. 7 that the variation in the lifetime components and in their corresponding intensities does not follow a monotonic trend upon variation in the relative metal ratio. Fig. 7a shows that τ₁ is higher in all Co-deficient samples and in one Co excess sample (2.6 : 1) compared to the parent NCO. Comparing τ₂ and I₂ (Fig. 7b and c), it can be further stated that, in Co-excess catalysts, a smaller number of larger vacancy clusters are present on the surface of the catalyst. Particularly, in the catalyst with a Co to Ni ratio of 2.6 : 1, cation vacancies form a relatively larger complex with oxygen vacancy inside the bulk, and oxygen vacancies are agglomerated, forming substantially larger clusters at the surface. However, the overall defect density is lower in Co-excess catalysts compared to Co-deficient catalysts, as depicted in Fig. 7d.

Fig. 7 Variation in (a) and (b) positron lifetime components; (c) corresponding intensities and (d) intensity-weighted average positron lifetime with variation in Co : Ni ratio.

3.6. Electrochemical and computational studies for evaluating catalytic activity towards ORR

The linear sweep voltammograms (LSVs) of the NCO nanowire catalysts were recorded initially for a cathodic scan without any external movement of the solution (Fig. 8a) for the assessment of their intrinsic electrocatalytic activity without the influence of enhanced mass transport. The corresponding half wave potential (E_1/2) values recorded during this experiment are listed in Table 1. It is observed that excess Co-containing NCO catalysts exhibit superior performance in terms of both limiting current density and E_1/2 compared to the Co-deficient nanowires. It is worth mentioning that E_1/2 is measured as the potential where the half-maxima of the catalytic response is observed. This value directly links to the energy efficiency of the catalysts as the overpotential of a particular electrocatalytic reaction is defined as the difference between the respective reaction's equilibrium potential and E_1/2. The highest current density was observed for the NCO catalyst with the Co to Ni ratio as 2.2 : 1, which also showed maximum abundance of smaller-sized surface vacancy clusters in earlier PAS studies. The next best response in terms of current density was displayed by the catalysts with a Co : Ni ratio ranging from 2.8 : 1 to 2.6 : 1. However, E_1/2 bears the signature of the efficiency of the catalyst to reduce the overpotential of the ORR process arising from various contributions, such as activation and concentration polarization and ohmic drop. As reflected from the E_1/2 values in Table 1, Co-excess NCO nanowires exhibit high current density and possess remarkably low overpotential compared to the previously reported pristine, heteroatom-doped and composite versions of NCO catalysts.^39–42

Fig. 8 (a) ORR polarization curves of all the synthesized catalysts in O₂-saturated 1 M KOH solution at room temperature at a scan rate of 5 mV s⁻¹. (b) Tafel plots of all the NCO catalysts. (c) Response of disk current and ring current with the rotation rate ranging from 0 to 2500 rpm at a scan rate of 10 mV s⁻¹ for the NCO-2.6:1 catalyst. (d) Number of electrons transferred with % of H₂O₂ formation of the NCO-2.6:1 catalyst in 1 M KOH solution. (e) Long-term stability test over a wide range of current densities of the NCO-2.6:1 catalyst in the same solution.

Table 1 ORR parameters measured for the NCO catalysts having varying Co/Ni ratios under alkaline conditions

Co : Ni ratio in the NCO catalyst	Half wave potential (E1/2 in V vs. RHE)	Overpotential (V)	Tafel slope (mV per decade)
2.8 : 1	0.876	0.354	51
2.6 : 1	0.882	0.348	54
2.4 : 1	0.886	0.344	51
2.2 : 1	0.868	0.362	61
2 : 1	0.876	0.354	56
1.7 : 1	0.878	0.352	96
1.5 : 1	0.850	0.380	130
1.25 : 1	0.753	0.477	224
1 : 1	0.790	0.440	186

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To further evaluate the kinetics of the reduction process, the Tafel slope from the corresponding LSV plots was calculated in the potential range of 0.9 V–0.8 V (vs. RHE). The Tafel plots are shown in Fig. 8b, and the Tafel slope values are listed in Table 1. Here, it was observed that electrocatalytic ORR is highly facilitated in Co-excess samples with appreciably lower Tafel slope values. As observed previously, the NCO (Co : Ni = 2.2 : 1) catalyst showed the best response in terms of current density; however, the NCO (Co : Ni = 2.4 : 1) and NCO (Co : Ni = 2.8 : 1) catalysts exhibited the lowest Tafel slope of 51 mV per decade. Moreover, the catalyst with a Co to Ni ratio of 2.6 : 1 demonstrated an optimum combination of all the three parameters, viz. current density, E_1/2 as well as Tafel slope. The kinetics observed in these Co-excess NCO nanowires are exceptionally faster than those of previously reported NCO nanowire arrays synthesized by the co-precipitation route,⁴³ mixed spinel Ni–Co oxides¹³ and hydroxylated mesoporous NCO.²⁵ In fact, such remarkably low values of Tafel slopes are comparable to some of the previously reported noble metal-based state-of-the-art catalysts^44,45 while superseding a few of them.^46,47 The improved ORR kinetics can be attributed to the enhanced electronic interaction among the metals and the presence of defects on the surface.¹³ However, the ORR kinetics become poorer with an increase in the Ni-content in NCO catalysts, as indicated by higher Tafel slopes (Table 1).

The RRDE voltammograms recorded for the Co-excess catalysts at various rotation rates are illustrated in Fig. 8e and S15.† A higher responsive current recorded in the Pt ring (known as ring current, I_R) represents the formation of hydrogen peroxide (H₂O₂), which is the undesirable side product of the two-electron reduction of oxygen. According to the Koutecky–Levich relation, a higher electrode rotating speed leads to better mass diffusion and a higher disk current (I_D), which is evident from the RRDE experiment with NCO samples. The I_R responses of the NCO catalysts at a given rotation speed differed appreciably, unveiling their varied preference for the undesirable H₂O₂ production. Among all the Co-excess catalysts, the one with a Co to Ni ratio of 2.6 : 1 showed the least H₂O₂ formation by displaying minimal I_R response as observed in Fig. 8c and S16.† Further analysis exhibited ∼4 electron transfers during electrocatalytic ORR with negligible H₂O₂ formation for this NCO catalyst (Fig. 8c). Furthermore, the stability of the best performing catalyst was tested under variable current densities of 1–10 mA cm⁻², where the catalysts displayed appreciable stability (Fig. 8d). The results highlight that the stoichiometric manipulation coupled with morphology control can be instrumental for achieving the desired catalytic selectivity towards water formation rather than peroxide formation.

To further introspect the outstanding performance of the Co-excess NCO nanowires, ORR on the TMO catalyst surface was considered, and periodic DFT studies were carried out. In the optimized minimum energy structure of bulk NCO, all the 8 octahedral Co(III) were found to be in a low spin state with zero unpaired electrons per site, while the Co(II) in the T_d site was found to have three unpaired electrons per site. Ni atoms occupying the O_h sites are found to have one unpaired electron per Ni. From the optimized NCO unit cell, a (100) surface was generated to study the ORR, and the optimized structure of the (100) surface is shown in Fig. S17a.† The surface is found to have two Co and two Ni sites and is five coordinated owing to the surface termination. For the optimized (100) surface, the work-function was calculated as (eΦ_vacuum − μ_Fermi), where Φ_vacuum is the planar averaged electrostatic potential along the normal direction to the surface and the calculated work-function was found to be 5.619 eV. To study the NCO with a higher Co concentration, two Ni sites (one on the surface and one from the sub-surface) are replaced with Co, and the corresponding surface was optimized, as shown in Fig. S17b.† The work-function for the corresponding surface was found to be 5.593 eV. The lower work-function of the Co excess NCO compared to the mother stoichiometric NCO-2:1 indicates a better reducing power of the Co excess catalyst. From the spin density iso-surface plots shown in Fig. S17c,† it can be observed that the surface Co sites have more spin density compared to those on the surface Ni sites, indicating more possible charge transfer from the Co site compared to the Ni site.

To study the ORR over these surfaces, the adsorption of molecular O₂ over both pristine and Co excess NCO surfaces was explored. O₂ adsorbed along (parallel) the Co–Ni/Co–Co bonds was found to be the minimum energy configuration, as shown in Fig. S18a and S18b.† Calculated O₂ adsorption energy over the pristine NCO surface was found to be −0.714 eV, while it was −1.22 eV in the Co excess catalyst surface. The O–O bond distances in the O₂ bound over these two surfaces were measured to be 1.301 Å and 1.337 Å, which are consistent with the order of binding energies. Both the O₂ binding energy and O–O bond elongation results clearly indicate a possible enhancement in ORR activity on Co excess NCO catalyst surface. To understand the charge transfer between the O₂ molecule and the catalyst surface, Bader charge density analysis was carried out, and the atomic charges were measured. From the measured charges, the O₂ molecule bound to the pure NCO surface acquired a negative charge of 0.358e while the amount of charge transfer was 0.485 in the case of the Co-doped NCO surface. Charge transfer from the catalyst to the antibonding states of the O₂ molecule is further supported by the charge density difference iso-surface plots shown in Fig. S18c.† To further obtain a detailed mechanistic insight into the ORR pathway, the adsorption of all the reaction intermediates described in both the associative and dissociative mechanisms of ORR over both pristine NCO and Co-excess NCO surfaces was optimized at the PBE + D3 level of theory. The individual steps of the associative and dissociative mechanisms of the reaction pathway are elaborated in Subsection S1.4 of ESI.† Using the energy of the minimum energy structures, the free energy change associated with each step is calculated, and the corresponding free energy profile for both routes of ORR on pristine and Co excess NCO systems is shown in Fig. 9. It is observed that the free energy changes for the intermediate steps are more negative for the Co excess NCO system compared to the pristine NCO along both the associative and dissociative pathways. In the case of the dissociative mechanism, the reaction step involving the dissociation of O₂ to 2*O is energetically uphill, making this route unfavorable; here, it is more endothermic in the parent stoichiometric NCO system compared to the Co excess system. Thus, in both the parent NCO and Co-excess NCO systems, the associative mechanism is the preferred route for oxygen reduction. From these free energy plots, it can be observed that the Co excess system improved the ORR performance through both dissociative and associative paths.

Fig. 9 Free energy plot of ORR on (100) plane via (a) dissociative mechanism and (b) associative mechanism.

As per the most plausible mechanisms proposed for ORR on the TMO surface in the literature, the catalytic activity is highly influenced by the overlap integral between the e_g orbitals of TM and the sp_σ orbital of oxygen.⁶ It was proposed that strong metal-to-oxygen interaction is established by the overlapping of the π orbitals of O₂ with the d_z2 orbitals of a TM element along with back-bonding from at least partially filled d_xz or d_yz orbitals of TM to the π* orbitals of O₂. The stronger the TM–O interaction, the weaker the O–O bond, subsequently leading to the dissociative adsorption of O₂ and probably simultaneous protonation and valence change in TM. Hence, it can be further suggested that the low-spin Co³⁺ present at the octahedral sites can facilitate the initial cleavage of the O–O bond. Han et al. previously demonstrated similar evidence of the crucial role played by octahedrally coordinated Co³⁺ expediting adsorption, dissociation, and desorption of oxygen species, thereby impacting its overall electrochemical activity.⁴⁸

To probe the interplay of the surface metal ions and their variable oxidation state during the ORR driven by the NCO-2.6:1 catalyst, XPS measurements were carried out before and after electrolysis. The XPS spectra of the best performing catalyst are shown in Fig. S11.† The relative metal ion distribution at the two different oxidation states is shown in Fig. 10a, and it clearly depicts that the surface of the catalyst preferentially becomes enriched with Co³⁺ and Ni²⁺ in the presence of excess Co in the catalyst. As evident from the comparative histogram of the relative metal ion distribution, the oxidation state of the Ni ions at the surface of the Co excess catalyst (NCO-2.6:1) is substantially affected during catalysis. The XPS data suggest that both the Co and Ni centers possibly participate in catalysis, where Co sites act as the initial site of water adsorption. Moreover, the electron transfer during ORR is supported by the surface Ni atoms, which are subsequently converted to the +3 state from the +2 state.

Fig. 10 (a) Comparison of the Co³⁺/Co²⁺ and Ni²⁺/Ni³⁺ ratios in the NCO-2:1 and NCO-2.6:1 catalysts, and (b) polarization curves (black) and power density (red) curve for the NCO-2.6:1 catalyst in 1 M KOH solution at room temperature in the AEMFC.

3.7. An operational fuel cell

To evaluate the performance of the synthesized catalysts in real life applications, an anion exchange membrane fuel cell (AEMFC) was assembled. Since the practical application of fuel cell demands harsh alkaline conditions, the earlier ORR measurements were carried out using 1 M KOH as an electrolyte. Here, the catalyst (NCO-2.6:1) coated gas diffusion layer (GDL) with a variable loading of 0.5–10.0 mg cm⁻² and a Pt/C coated GDL with a loading of 0.25 mg cm⁻² were used as cathode and anode, respectively. A double serpentine flow field designed-graphite with an active area of 2.2 × 2.2 cm² and copper plate was used as a bipolar plate and current collector, respectively. Several combinations of catalyst loading were tested, and better performance was achieved in the case of 5 mg cm⁻² loading at the cathode. The humidified gaseous reactants, H₂ and O₂ were supplied to the anode and cathode sides of the fuel cell, respectively, with flow rates of 100 mL min⁻¹ at room temperature. The I–V polarization curves were recorded using Metrohm-Autolab potentiostat/galvanostat (PGSTAT302N connected to a 10A booster), within the potential region of 0.9–0.1 V and a scan rate of 5 mV s⁻¹. Under this circumstance, an Open Circuit Voltage (OCV) of 0.89 V, and a peak power density of 80 mW cm⁻² were achieved at a 250 mA cm⁻² current density (Fig. 10b). This setup reached a current density of 450 mA cm⁻² at the mass transport region, as shown in Fig. 10b. A comparison of the performance of the NCO-2.6:1 catalyst for fuel cell applications with different classes of reported advanced catalysts is presented in Table 2. It is observed from the table that the performance of the catalyst reported here is based on other reported works, making this low-cost NCO catalyst a promising catalyst for the ORR operating in 1 M KOH solution. The performance of the fuel cell can be further improved by increasing the flow rate of the gases, increasing the temperature, changing the humidity, and ionomer distribution.⁴⁹ Hence, the NCO catalysts can be successfully deployed for fuel-cell application in an AEMFC setup.

Table 2 Comparative study of various ORR parameters and fuel cell characteristics of the NCO-2.6:1 catalyst with other reported advanced catalysts

Catalyst and electrolyte conditions	Onset potential (V vs. RHE)	Half wave potential, (V vs. RHE)	Tafel slope (mV dec^−1)	No. of e^−s transferred	Peak power density, PPD (mW cm^−2)	Current density at PPD (mA cm^−2)	Ref.
Co/SiOC (ORR in 0.1 M KOH and AFC in 1 M KOH)	0.87	0.77	76	3.7	54	100	50
Cr–Ni–V (1 M KOH)	0.83	0.73	—	2.8	22	50	51
NiCo/NCNT (ORR in 0.1 M and AFC in 1 M KOH)	0.98	0.88	81	3.98	65	130	52
Co/N/CDC (0.1 M KOH)	0.96	0.80	60	4.0–4.2	78	130	53
PVP-ZIF-8 @CNT-900 (ORR in 0.1 M KOH and AFC in 1 M KOH)	0.96	0.80	67	3.8–3.9	45	88	54
N–F/PGPC (0.1 M KOH for the ORR and AFC in 2 M KOH)	0.92	0.87	71	3.90–3.98	22	67	55
N–F co-doped GNF (GNF-H/N–F) (0.1 M KOH)	1.02	0.88	94	3.97	51	130	56
Metal oxide-carbon (TMO-MOx-C) α-MnO2/C (O.1 M KOH for the ORR and AFC in 1 M KOH)	0.83	0.67	—	—	45.2	75	57
Fe-MBZ (O.1 M KOH for the ORR and AFC in 1 M KOH)	0.85	0.74	—	—	90	223	58
Cu–N/B–C-800 (0.1 M KOH)	0.95	0.84	—	3.80–3.85	80	250	59
NFC@Fe/Fe3C@C (O.1 M KOH for the ORR and AFC in 1 M KOH)	0.97	0.87	53 (low)	4.07	96	250	60
			84 (high)
NCO-2.6:1 (1 M KOH)	0.93	0.88	54	3.96	81.7	245	This work

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4. Conclusion

In this work, several samples of defect-engineered one-dimensional mesoporous Ni–Co oxide spinel (NCO) nanowires with appreciably high surface area were synthesized using the solvothermal method, where the Co/Ni ratio varied between 2.8 and 1 : 1. The initial structural studies on these materials were determined using several methods, including powder XRD, FTIR, FESEM, HRTEM and ICP-AES, to unravel their morphology at a microscopic level. The local structural arrangement was probed with XAS, demonstrating the existence of Co vacancy sites in low-Co NCO nanowires, while oxygen vacancies were observed in both the Co-excess and deficient catalysts. PALS studies unraveled the presence of cationic vacancies in the Co-deficient catalysts while indicating the creation of a relatively smaller number of large-sized oxygen vacancy clusters in Co-excess catalysts. NCO catalysts containing excess Co ions exhibited excellent ORR catalytic activity, operating at overpotential values ∼0.35 V with a Tafel slope ∼50 mV per decade. Such fast and energy-efficient catalysis is rarely reported for any leading ORR electrocatalysts, including noble metal-based materials. The detailed electrochemical studies depicted that the better performing Co-excess variants of this genre of NCO catalysts drive the ORR almost exclusively via a four-electron O₂/H₂O reaction pathway along with negligible production of the side product hydrogen peroxide. Computational studies revealed a lower work function thereby greater reducing power of the (100) surface of Co excess NCO catalyst as compared to pristine NCO which was further supported by higher binding energy and greater O–O bond elongation over the same. The optimized NCO, containing Co/Ni at a ratio of 2.6 : 1, was directly deployed in an AEMFC setup, where it performed flawlessly along with the Pt/C anode partner. This NCO‖Pt/C-based AEMFC operated with an OCP of 0.89 V, and it reached a peak power density of 80 mW cm⁻² at a current density of 250 mA cm⁻² at room temperature. This result shows that this uniquely designed NCO material can excel in the role of an ORR electrocatalyst under alkaline conditions and can replace the noble metal-based cathodic materials to operate in a real-life AEMFC setup.

Data availability

Raw data are available from the authors and can be provided on request. Other relevant data are included in the electronic ESI file.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors convey sincere thanks to Mr Nitin Gumber for his kind support in the N₂ adsorption–desorption studies. The authors are thankful to Mr Manavendra Narayan Singh, Indus Synchrotron Utilization Division, RRCAT, Indore, for his active support in SXRD experiment. The authors are grateful to Dr H. S. S. Ramakrishna Matte and Dr Vijay Kumar, Centre for Nano and Soft Matter Sciences, Bangalore, for their kind support in the TEM measurements. The authors pay thanks to the ICP-AES facility in-charge, IIT Bombay, and Supercomputing facility of BARC, Mumbai. The authors would like to acknowledge Dr Biplab Ghosh, Beamline Development and Applications Section, Bhabha Atomic Research Centre, Mumbai, India, for his kind assistance in the XAS measurements. The authors also thank Dr Tarik Aziz, IIT Bombay, for his valuable insights in the electrocatalytic study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04454a

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