Coordination environment evolution of Co(II) during dehydration and re-crystallization processes of KCoPO₄·H₂O towards enhanced electrocatalytic oxygen evolution reaction

Abstract

Development of efficient and stable electrodes for electrocatalytic oxygen evolution reaction (OER) is essential for energy storage and conversion applications, such as hydrogen generation from water splitting, rechargeable metal–air batteries and renewable fuel cells. Alkali metal cobalt phosphates show great potential as OER electrocatalysts. Herein, an original electrode design strategy is reported to realize an efficient OER electrocatalyst through engineering the coordination geometry of Co(II) in KCoPO₄·H₂O by a facile dehydration process. Experimental result indicated that the dehydration treatment is accompanied by a structural transformation from orthorhombic KCoPO₄·H₂O to hexagonal KCoPO₄, involving a concomitant coordination geometry evolution of Co(II) from octahedral to tetrahedral configuration. More significantly, the local structural evolution leads to an advantageous electronic effect, i.e. increased Co–O covalency, resulting in an enhanced intrinsic OER activity. To be specific, the as-produced KCoPO₄ can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Thus, this present research provides a new way of developing alkali metal transition-metal phosphates for efficient and stable electrocatalytic oxygen evolution reaction.

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1. Introduction

The oxygen evolution reaction (OER) is one of the crucial steps of energy storage and conversion applications, such as hydrogen generation from water splitting, rechargeable metal–air batteries and renewable fuel cells.^1–5 However, the intrinsic sluggish kinetics of the OER involving four-electron transfer requires an overpotential to drive the process.^6–8 Therefore, efficient electrocatalysts with high activity and high stability are desirable to realize its practical application.^9,10 To date, noble-metal oxides, such as RuO₂ and IrO₂ are considered to be the state-of-the-art electrocatalysts for the OER,^11,12 but their high cost, limited reserves and stability problem prevent their large-scale industrial application.¹³ Currently, extensive research efforts have been focused towards the development of cost-effective transition-metal based OER catalysts.^14–20

In particular, alkali metal cobalt phosphates have gained increasing attention due to their high activity and favorable kinetics, such as orthophosphate (LiCoPO₄),²¹ pyrophosphate (NaCoP₂O₇),²² and metaphosphate (NaCo(PO₃)₃).²³ The diverse orientations of phosphate ligands lead to various crystal structures, which are usually beneficial for the structural stability during OER process.^24–26 As illustrated in Fig. S2,† phosphate/pyrophosphate/metaphosphate-containing compounds have various cobalt(II) geometries including octahedral (O_h), tetrahedral (T_d) and trigonal bipyramidal (TBP), with diverse connections and low-symmetry crystal structures due to the bulky phosphate ligands. Thus, alkali metal Co phosphates with various structural configurations provide an ideal platform to design efficient and stable OER electrocatalyst. Previous research has demonstrated that the Co geometric coordination has a great impact on the OER activity of Co based phosphates. Kim et al. selected four Co based phosphates with various Co coordination environment, including Na₂CoP₂O₇ (T_d), LiCoPO₄ (O_h), NaCoPO₄ (O_h), Li₂CoP₂O₇ (O_h/TBP), as a platform to explore the relationship between the local coordination and the OER activity.²⁷ The result demonstrated that the Na₂CoP₂O₇ shows enhanced activity relative to NaCoPO₄ due to a highly distorted tetrahedral geometry. Nevertheless, Wan et al. subsequently reported that the NaCo₄(PO₄)₃ containing a rare 5 coordinated Co(II) outperformed the Na₂CoP₂O₇ (T_d).²⁸ Besides, some other reported alkali metal cobalt phosphates, such as LiCoPO₄ (O_h)²¹ and NaCo(PO₃)₃ (O_h)²³ exhibited acceptable OER activity.

Admittedly, local coordination geometry in alkali metal cobalt phosphates plays a significant role in improving their OER activity. However, there is still a lack of deep understanding of the origin of this relationship. It is well accepted that OER activity are highly dependent on both geometric configuration and electronic structure of electrocatalysts. Thus, to address this issue, there is the need for reveal the dual effect of coordination geometry of Co(II) in alkali metal cobalt phosphates. In this work, we found that the dehydration treatment of KCoPO₄·H₂O is accompanied by a structural transformation from orthorhombic to hexagonal phase, involving a concomitant coordination geometry evolution of Co(II) from octahedral to tetrahedral configuration. By this, special consideration was paid to the influence of this local structural evolution on electronic structure. Experimental result and theoretical analysis indicated that the co coordination evolution increases the Co–O covalency, resulting in an enhanced OER activity. The as-resulted KCoPO₄ exhibited an enhanced OER activity relative to the pristine KCoPO₄·H₂O. Specifically, it can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Moreover, stability test demonstrated it can hold its catalytic OER activity for 50 h at least. Besides, to the best of our knowledge, this work for the first time demonstrated the OER performance of the hexagonal KCoPO₄ containing Co(II)-T_d local coordination environment. Overall, this uniqueness of this material design not only lies in an effective strategy for local coordination evolution, but more significantly it provides an ideal platform to better understand the local coordination–activity relationship for alkali metal cobalt phosphates electrocatalysts.

2. Experimental

KCoPO₄·H₂O nanoplates were synthesized via a simple hydrothermal process. In a typical step, an aqueous solution of CoCl₂·6H₂O (0.004 mol, 30 ml) was completely added into an aqueous solution of K₂HPO₄ (0.04 mol, 30 ml) under continuous stirring. After stirred for 2 h at room temperature (25 °C), the mixture was transferred into a Teflon-lined stainless autoclave and kept at 140 °C in an electric drying box for 20 h. The resulting pink product was collected by centrifugation, rinsed with the DI water and absolute ethanol, dried at 50 °C for 12 h to obtain the KCoPO₄·H₂O nanoplates.

The coordinated water molecule in KCoPO₄·H₂O can be removed by heating at 300 °C under N₂ atmosphere for 3 h to obtain the dehydrated phase, i.e. KCoPO₄. The step involved in the dehydration process is illustrated in Fig. 1(a). The characterization techniques and the electrochemical measurements were explained in the ESI.†

Fig. 1 (a) Schematic illustration for the dehydration and re-crystallization processes of KCoPO₄·H₂O. Crystal structures of (b) KCoPO₄·H₂O, and (c) KCoPO₄. The inset shows the local environment around the Co subunit (blue) and the spin state of the Co(II).

3. Results and discussion

The XRD pattern (Fig. 2(a)) indicates the successful preparation of the KCoPO₄·H₂O. The diffraction peaks of the obtained powders perfectly matched to the orthorhombic-phase potassium cobalt phosphate monohydrate KCoPO₄·H₂O (JCPDS no. 86-0590), while the dehydrated phase shows different crystalline structure. Upon dehydration, the KCoPO₄·H₂O was transformed into hexagonal-phase KCoPO₄ (JCPDS no. 82-0762), as indicated in Fig. 2(b). Fig. 1(b and c) shows the crystal structures of KCoPO₄·H₂O and KCoPO₄, respectively. It is clearly illustrated that the dehydration is accompanied by a structural transformation and re-crystalline process. Observations by the SEM (Fig. 3(a and b)) and TEM (Fig. 3(c and e)) indicate that the as-synthesized KCoPO₄·H₂O and dehydrated KCoPO₄ exhibit plate-like nanostructures with an average size tunable from 2 to 5 μm. Furthermore, the HRTEM images show the interlayer d spacing of 0.41 nm (Fig. 3(d)) and 0.31 nm (Fig. 3(f)) nm of the two phases, which are consistent with the XRD data.

Fig. 2 XRD patterns of the as-prepared (a) KCoPO₄·H₂O (JCPDS no. 86-0590), and (b) KCoPO₄ (JCPDS no. 82-0762), respectively.

Fig. 3 SEM images of the as-prepared (a) KCoPO₄·H₂O, and (b) KCoPO₄. TEM (c and e) and HRTEM (d and f) images of KCoPO₄·H₂O and KCoPO₄, respectively.

Thermogravimetric analysis (TGA) was carried out under flowing N₂ to further demonstrate the dehydration and re-crystalline process of KCoPO₄·H₂O, as shown in Fig. 4(a). The TG curve shows a sharp weight loss of 8.61%, and is characterized by a strong endothermic DTG peak at 195.7 °C, which corresponds to the loss of coordinated water molecule in KCoPO₄·H₂O (calcd: 8.53%). There is no weight after that, indicating it transforms to be a stable KCoPO₄.

Fig. 4 (a) TGA/DTG curves of the KCoPO₄·H₂O nanoplates. (b) UV-vis spectra of KCoPO₄·H₂O and KCoPO₄. High-resolution XPS spectra for (c) O 1s, and (d) Co 2p of the KCoPO₄·H₂O and KCoPO₄, respectively.

Significantly, upon dehydration, the coordination environment around Co(II) changed from octahedral to tetrahedral configuration, which is demonstrated by a concomitant color change from pink to blue. In other words, The Co–O bond from the oxygen in the water molecule coordinated with Co(II) is lost during dehydration, leading to the coordination of the Co(II) becomes to tetrahedral. The diffuse-reflectance solid-state UV-vis spectra of the two phases was shown in Fig. 4(b). The spectrum of KCoPO₄·H₂O shows a broad band located at approximately 539 nm, which correspond to the ⁴T_1g (F) ⁴A_2g (P) transition (ν₃ band) of high-spin Co(II) in octahedral [CoO₅–(H₂O)]²⁺,²⁹ while for KCoPO₄ the bands located at about 526 nm, 575 nm, and 615 nm are assigned to the ⁴A_2g (F) ⁴T₁ (P) of high-spin tetrahedral Co(II).³⁰ Thus, the UV-vis spectra suggests a structural conversion from octahedral to tetrahedral coordination. Fig. 1(b and c) shows the local coordination environment and the spin state of Co(II) in the two phases, respectively. As mentioned above, the Co geometric coordination has a great impact on the OER activity of Co based phosphates. Therefore, the coordination environment evolution of Co(II) in the two phases motivate us to explore the corresponding change of their OER performance and the original relationship between coordination configuration with OER activity of Co phosphates. However, just knowing the structural transformation is far from enough, there is the need for further efforts to reveal the corresponding electronic effect.

The chemical compositions and bonding states in the as-prepared KCoPO₄·H₂O and dehydrated KCoPO₄ were examined by XPS, which contributes to estimate the electron states of Co in the two phases, and in turn is conductive to reveal the electronic effect of coordination environment evolution of Co(II) on OER activity. First, the full survey spectra (Fig. S3(a and b)†) indicates the presence of cobalt (Co 2p), phosphorus (P 2p), oxygen (O 1s), potassium (K 2p) from both of the two compounds. Furthermore, the O 1s spectrum of KCoPO₄·H₂O (Fig. 4(c)) fitted into two peaks situated at 530.7 eV and 532.8 eV can be ascribed to the P–O bonds and coordinated water molecule, respectively,³¹ while for KCoPO₄ only lattice oxygen can be tested, as illustrated by the peak located at about 531.0 eV. More significantly, it is found that there is a positive shift (about 0.3 eV) of binding energy for oxygen after the dehydration process, which indicates the increased valence state of oxygen in KCoPO₄. Similarly, the valence state of cobalt increases on dehydration, as confirmed by the positive shift (about 0.3 eV) of binding energy, as shown in (Fig. 4(d)). Specifically, the Co 2p spectrum of KCoPO₄·H₂O fitted into two peaks located at 796.8and 781.3 eV, while for KCoPO₄ the peaks situated at 797.1 and 781.6 eV, respectively. Besides, the Co 2p spectra of the two compounds indicate the Co²⁺ oxidation state of cobalt in both of them.³² In a word, the XPS result suggests the increased degree of Co–O hybridization, i.e. Co–O covalency in KCoPO₄.

Based on the above characterizations including crystal structure and chemical bonding states, it is clearly indicated that the dehydration process of KCoPO₄·H₂O is accompanied by a structural transformation from orthorhombic-phase to hexagonal-phase. More significantly, the local coordination geometry evolution of Co(II) from octahedral to tetrahedral configuration increases the Co–O covalency. Previous research demonstrated that the geometric configuration and electronic structure of cobalt phosphates can largely influence its OER catalytic activity.^33,34 Therefore, the local Co geometry evolution with increased Co–O covalency may imply an enhanced OER activity.

The OER performance of the as-prepared two compounds was evaluated via linear scanning voltammetry (LSV) using a conventional three-electrode system at a scan rate of 5 mV s⁻¹ in 1.0 M KOH (electrochemical measurements, see details in ESI†). Fig. 5(a) shows the LSV curves of the as-prepared KCoPO₄, together with the KCoPO₄·H₂O, and RuO₂ for comparison. As illustrated in Fig. 5(a), the KCoPO₄·H₂O requires 387 mV overpotential to reach a current density of 10 mA cm⁻². On KCoPO₄, the overpotential greatly decreased to 319 mV, which is even comparable to that of the reference sample (309 mV for noble RuO₂). Not only that, the catalytic performance of KCoPO₄ is comparable to or even superior to many recently reported transition-metal phosphate based electrocatalysts (the detailed comparison can be seen in Table S3, ESI†). Besides, a smaller Tafel slope of 61.8 mV dec⁻¹ of KCoPO₄ than that of KCoPO₄·H₂O (66.2 mV dec⁻¹) indicates the favorable reaction kinetic for OER, as shown in Fig. 5(b). It could also be inferred from the electrochemical impedance spectroscopy (EIS) result (Fig. 5(c)). An equivalent circuit model was suggested, as shown in inset of Fig. 5(c). The charge transfer resistance (R_ct), was tested to be 36.1 Ω for KCoPO₄, much lower than 139.9 Ω for KCoPO₄·H₂O. Furthermore, CV curves with different scan rates were recorded to evaluate electrochemically active surface area (ECSA) of the two compounds, as shown in Fig. 6(a and b). It can be evaluated with the electrochemical double-layer capacitance (C_dl), which is determined by current density differences plotted against various scan rates. Specifically, the C_dl of KCoPO₄ was calculated to be 14.15 mF cm⁻², which is very close to that of KCoPO₄·H₂O (13.55 mF cm⁻²), indicating an equivalent ECSA value of them. Therefore, it is proposed that the transformation of KCoPO₄·H₂O to KCoPO₄ lead to the enhanced intrinsic OER activity. In order to estimate the intrinsic activity, turnover frequency (TOF) was calculated for them. The TOF value of KCoPO₄, KCoPO₄·H₂O and RuO₂ were 0.0635 s⁻¹, 0.0413 s⁻¹, 0.015 s⁻¹, respectively, which are comparable with the previously reported values.³⁵ Such a high TOF value means a very high intrinsic activity of KCoPO₄.

Fig. 5 OER performance of the as-prepared KCoPO₄·H₂O, KCoPO₄, and RuO₂. (a) LSV curves, (b) corresponding Tafel plots, (c) EIS spectra, and (d) chronopotentiometric curves at 10 mA cm⁻².

Fig. 6 Cyclic voltammetry curves of (a) KCoPO₄·H₂O, and (b) KCoPO₄ in 1 M KOH with different scan rates. Δj of as-above electrodes plotted against scan rates, (c) KCoPO₄·H₂O, and (d) KCoPO₄.

Based on the above comparison study on structural transformation and OER performance evaluation, it is suggested that the enhanced OER activity is mainly derived from the electronic effect of local Co(II) geometry, i.e. the increased Co–O covalency.

In addition, the stability measurement for the as-prepared KCoPO₄ manifests that the long-term durability requirement can be met. The chronopotentiometry result of KCoPO₄ at a constant 10 mA cm⁻², as shown in Fig. 5(d), indicating the potential kept stable at ∼1.55 V in an OER process up to 50 h. The LSV curve after 50 h test exhibited a negligible shift, as shown in Fig. S4.† Besides, the structural characterizations, as illustrated in Fig. S5,† further demonstrated the stability.

Based on the above analysis, it is suggested that the distinct OER activity is associated with the local electronic structure modulation by coordination environment evolution of Co(II). In other words, the local Co geometry evolution from octahedral to tetrahedral configuration with increased Co–O covalency result in an enhanced OER activity. This analysis is consistent with the previous research. Recent studies demonstrated that the increased Co–O covalency promotes the interfacial charge transfer between Co and oxygen intermediates, resulting in an enhanced OER kinetics.^36–39 To further confirm the electronic effect, first-principles density functional theory plus Hubbard U (DFT+U) calculations were performed on the two compounds (computational models and methods can be seen in the ESI†). The calculated partial densities of the states (pDOS) of Co 3d and O 2p band are shown in Fig. 7(a and b). As a feasible descriptor for OER activity, the electronic parameter, i.e., O 2p band center (ε_O2p) can be extracted out from the calculated pDOS,^40–42 which also indicates the degree of Co–O hybridization, as illustrated in Fig. 7(c and d). In specific, a larger ε_O2p signify greater covalency and better OER activity. It is observed that the calculated ε_O2p of KCoPO₄ (−2.95 eV) is more positive than that of KCoPO₄·H₂O (−3.24 eV), indicating the increased Co–O covalency, which is confirmed by the XPS analysis.

Fig. 7 Computed partial electronic density of states (pDOS) of Co 3d and O 2p band of (a) KCoPO₄·H₂O, and (b) KCoPO₄. Illustration of the relationships between Co 3d and O 2p band of (c) KCoPO₄·H₂O, and (d) KCoPO₄, respectively.

4. Conclusions

In summary, KCoPO₄·H₂O nanoplates were successfully synthesized via a hydrothermal process. The coordination water molecule in KCoPO₄·H₂O can be removed by heating at 300 °C under N₂ atmosphere to obtain the dehydrated KCoPO₄. Significantly, the dehydration process is accompanied by a phase transformation, and a local coordination environment evolution of Co(II) from octahedral to tetrahedral configuration, which increases the Co–O covalency, resulting in an enhanced OER activity. In specific, the dehydrated KCoPO₄ can deliver a current density of 10 mA cm⁻² at a low overpotential of 319 mV with a small Tafel slope of 61.8 mV dec⁻¹ in alkaline electrolyte. Thus, this present research enlightens a new way of developing alkali metal transition-metal phosphates for efficient and stable water oxidation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (51602128), the Research Foundation from University of Jinan (XKY1401, XBS1508, XBH1504).

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Footnote

† Electronic supplementary information (ESI) available: Experimental, XPS, XRD, SEM, TEM and computational models and methods. See DOI: 10.1039/d0ra01813a

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