Phosphate-based polyanionic insertion materials for oxygen electrocatalysis

Abstract

Electrocatalyst-based energy storage technologies such as alkali metal–air batteries, fuel cells, and water splitting devices are the new holy grail in the next-generation energy storage landscape as they deliver higher energy densities than Li-ion/Na-ion batteries (LIBs/SIBs). The new chemistries of energy storage such as metal–air batteries under aqueous or non-aqueous conditions will complement existing LIBs/SIBs owing to the increasing requirement for batteries with high energy density in the present era. Phosphate-based polyanionic frameworks have long been known for their ability to (de)intercalate alkali metal ions. Because of their innate oxygen electrocatalytic activity, these insertion cathode materials have lately emerged as air electrodes in metal–air battery systems. In this review, the present status of phosphate-based polyanionic insertion materials for oxygen reduction and oxygen evolution reaction (ORR and OER) electrocatalysis is summarized. Factors influencing electrocatalytic activity in these materials, such as the presence of different types of alkali metal cations, transition metals, and the type of ligand/mixed anion as well as coordination around the transition metals are discussed. Finally, the development of metal–air batteries derived from phosphate-based polyanionic insertion materials as air electrodes is discussed.

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1. Introduction

Energy storage from lithium-ion batteries (LIBs) needs no introduction and validation in the present era as they are ubiquitous in the modern world. The ever-growing consumption of LIBs has triggered the pressure to overcome the existing supply limitations and has opened more markets and industries for other analogues of lithium-based energy storage devices. In this era of post-Li-ion batteries, Li–O₂ batteries form a potential candidate as they exhibit the highest energy density, capable of achieving the ultimate goal of batteries for long-range electric vehicles (EVs). For this application, O₂ gas should ideally be replaced with air from Earth's ambient atmosphere.¹ In 1996, Abraham et al. introduced the first Li–O₂ battery employing an organic polymer electrolyte as an O₂ permeable membrane.² Since then, numerous advancements have been made at the architecture level to explore efficient Li–air batteries. However, Li–O₂/air battery technologies are still in their infancy because of poor cyclability, large overpotential during the charging process, and the existence of other side reactions forming parasitic products owing to either unstable electrolytes or inefficient electrocatalysts.^1,3–8

Practically, perovskite oxide-based electrodes are the most active and widely studied catalysts to perform oxygen electrocatalytic reactions (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)).^9–11 Exhibiting efficient O₂ electrocatalysis, the transition metal phosphate (PO₄)³⁻ class of polyanionic materials has also emerged as a potential candidate for possible application as cathodes in metal–air batteries.¹² Despite respectable current densities and promising overpotentials delivered by these (PO₄)³⁻-based catalysts, these materials suffer from poor stability in highly alkaline aqueous electrolytes. To the best of our knowledge, there is no literature covering different types of transition metal (PO₄)³⁻ based polyanionic materials showing intercalation chemistry and acting as catalysts for oxygen electrocatalysis. In light of this, this review offers a thorough summary of the research work to date on transition metal phosphate-based polyanionic materials for electrocatalytic oxygen reduction, oxygen evolution, and metal–air battery applications.

Electrocatalysts are known to lower the activation energy of oxygen reduction and water-splitting reactions. In principle, ORR/OER processes show different reaction pathways in acidic and alkaline aqueous environments. These pathways are represented as follows:¹³

The oxygen half-cell reaction at alkaline pH:

The oxygen half-cell reaction at acidic pH:
Based on linear free energy relationships (LFERs) phenomenologically employed by ab initio computations to forecast catalytic activity trends of reaction on metal surfaces, the four-proton coupled electron transfer steps for ORRs/OERs at the surface site (marked by * symbol) can be expressed as follows:

Reactions at alkaline pH:

Reactions at acidic pH:



Here, OOH*, O*, and OH* are the reaction intermediates undergoing desorption/adsorption. Due to the intrinsic stability of phosphates against temperature and pH, the electrocatalytic properties of most of the PO₄-based polyanionic insertion materials investigated in an alkaline medium make the reaction pathways under alkaline pH more important for discussion in the later sections.

On another note, to realise a sustainable circular energy economy, it is highly important to recycle present batteries such as EV batteries (containing LiFePO₄, LiCoO₂, etc., as cathodes) when they reach their end of life. Recycling the electrode materials in the present battery market is of utmost importance. Instead of subjecting the coated electrode materials to toxic acid treatment to extract the remnant precursor for further synthesis, the recycled materials can directly be used in various applications. In this spirit, this review will also briefly highlight a different approach of using the cycled cathodes from the dead batteries towards the application in energy storage devices based on ORRs and OERs. This review outlines various oxide and polyanionic insertion materials demonstrating electrocatalytic activity with emphasis on phosphate-based polyanionic battery insertion compounds. It will begin with the discussion on environmental benignity, materials economy, and efficient scalable synthesis of PO₄-based materials. Highlighting oxygen electrocatalysis chemistry (ORR and OER) at alkaline pH, the specific discharge capacity of a variety of PO₄-based polyanionic insertion materials will also be stressed. The electrocatalytic activity (ORR and OER) is found to be greatly influenced by the type of transition metal (Mn, Fe, Co and Ni) present as well as the neighbouring alkali cation and PO₄ units. We also incorporated fluorophosphates, which have an F-atom bonded to the transition metal centre, showing superior electrochemical activity. Eventually, all these catalysts (oxides or phosphates) cause electrodeposition of metal-oxides/oxy(hydroxide)/phosphate at the surface that cover the electrocatalyst. The underlying mechanism was correlated with the molecular orbital (MO) concepts, e_g filling theory and metal-oxygen covalency followed by transition metal coordination and its connection with PO₄-units (corner-shared or edge-shared).⁹

Phosphate-based materials with bifunctional electrocatalytic activities are further extended to the application encompassing hybrid Na–air batteries and Zn–air batteries. These batteries employ PO₄ materials as air cathodes and function based on the underlying ORR and OER mechanisms to store energy. These materials can open up a hitherto untapped route for advancement in the research and development of Li–O₂/Li–air batteries, which are still in their infancy marred with several challenges.¹⁴ Similar to hybrid Na–air batteries and Zn–air batteries, in the case of Li–O₂/Li–air batteries, the cathode reaction overpotentials are much higher than those of the anode reaction.^15,16 Various groups have reported the use of carbonaceous materials, precious metals/metal-oxides and composite materials as cathodes for non-aqueous Li–O₂/Li–air batteries, which may require sophisticated synthesis. In this scenario, the PO₄-based insertion materials, with their attractive material/process economy, can work as potential cathodes.^9,11–19

2. Why phosphate-based polyanion materials?

Mother Nature has already considered and proven the major significance of phosphates in the vital events of life. We are all aware of the critical role that phosphorous plays in every biological or physiological event of energy gain or loss.²⁰ The maxim by Alexander Todd, who won the Nobel Prize in 1958, “Where there is life, there is phosphorus” is unquestionably true. The vital roles of phosphates, enlisted by Bowler et al., also showed phosphate as the most important entity from genome stability and energetics to regulation and signalling.²¹ Inspired by the article by Knouse et al. and highly encouraged to choose phosphates, P(5+) oxidation state by chemists, the next-generation energy-driven materials can also be phosphate-based systems offering ample room for exploration targeting continuously growing demands of energy storage devices.^22,23 Since the recognition of LiFePO₄ as a cathode material, it has attracted the attention of electric vehicle (EV) industries due to its cheapness and thermal stability. In the beginning, major car industries chose LiCoO₂/layered oxides as cathodes like major portable electronics sector, causing significant price rise for Co-precursors. Owing to the abundance of Fe-based resources, currently LiFePO₄ has enabled low-cost consumer electronics and electric vehicles.²⁴ An imaginary scheme of energy storage in a biological system is compared with the physical world's energy storage employing phosphate-based cathode materials that are synthesized in industries propelling several energy storage devices (Fig. 1).

Fig. 1 Design of phosphate-based materials for energy storage and hereditary carriers inside living beings at the biological level and industrial-scale man-made cathode materials such as LiFePO₄ for sustainable batteries at the industrial level.^25,26

Perovskites (ABX₃, A = cation⁺, B = transition metal⁺, X = anion⁻) are known to catalyse various chemical and electrochemical reactions such as oxidation of CO, hydrocarbons and NO_x, reduction of O₂, N₂, and CO₂ and photo/electrochemical splitting of H₂O.⁹ One of the key difficulties is to rationalise the design of nanostructured perovskite-type oxide-electrocatalysts from non-precious transition metals with least overpotentials for ORR and OER processes.¹³ The central challenge for oxygen electrochemical technologies is to achieve descent long-term stability of catalysts due to the dissolution of the catalyst surface during the ORR or OER. Here, PO₄-based materials can offer stable operation during oxygen electrocatalysis.^27,28 Among several reported materials for catalysing O₂ (either ORRs or OERs or both), recently the number of papers based on phosphates has dramatically increased. This review primarily focuses on phosphate-based polyanion materials for oxygen electrocatalysis. Following, the major advantages of phosphate compounds are assessed and discussed in detail.

2.1. Environmentally benign

Currently, no systematic experimental techniques are used across the community to compare the electrocatalytic activities based on materials rather than those that evaluate the performance of the overall device.^29,30 However, if we consider only the best-performing oxygen electrocatalysts, IrO₂ and RuO₂ for OERs and Pt/C for ORRs are the best-known conventional state-of-the-art catalysts whose activity is highly influenced by the pH value. Further, they are very expensive and may not be suitable for large-scale practical applications such as in metal–air batteries or other devices. Additionally, La_xSr_yMn_1−yO₃ perovskite type electrocatalysts are widely used in the OER-driven next-generation energy storage systems.¹¹ However, the environmental cost of these cutting-edge technologies will be significantly impacted if the substance comprises rare earth metals such as La/Sr or other 4d elements.^31,32 Since Li-rich metal-oxides undergo anion redox reactions at high operating potentials, polyanionic materials based on phosphate are both much safer and less harmful to the environment than oxides if comprised of 3d-tranistion metals.³³ Because of strong covalent P–O bonds in phosphate-based polyanionic materials, they have the least possibility of O₂ gaseous evolution caused due to anion redox process at higher potentials.^34,35 Thus, the limiting nature of oxide cathode materials has stimulated the search of new phosphate-based polyanionic materials to cater the demand of high energy density vs. cost equilibrium while offering operational safety at a higher voltage.

2.2. Direct synthesis

Typically phosphate-based intercalation materials can be simply prepared via a low-cost, solvothermal ‘splash combustion synthesis route’, which is the most straightforward method for achieving carbon-coated nano-range particle sizes.^36,37 In general, solution combustion is sufficiently adapted as an unsophisticated technique for nanoscale material synthesis to satisfy the requirement of charge conductivity and pulverization inhibition of the electrodes for energy conversion and storage devices.^38,39 For instance, Bashir et al. have summarised the unique properties or combinations of properties offered by nanoscale electrodes for various energy storage devices.⁴⁰ They emphasized the merits of size reduction, such as how nanoparticulate electrodes or electrocatalysts increase electrode/electrolyte contact areas, spatial confinement and surface contribution, ultimately increasing electrode reaction rates, which are crucial for advanced energy storage devices.

There have been several reports where the electrocatalysts require complex strategies such as CVD (chemical vapour deposition) and AVD (atomic vapour deposition) methods for the synthesis of catalysts.^41–43 Nonetheless, it is cumbersome to implement them to increase the loading/structuring/active site of catalysts as well as enhanced intrinsic activity of each active site, which is necessary to achieve better material performances at the device level.⁴⁴ Mohamed et al. showed the synthesis of ternary spinel MCo₂O₄ (M = Mn, Fe, Ni, and Zn) porous nanorods by a hydrothermal reaction exhibiting bifunctional (ORR and OER) activity, suitable for Li–O₂ batteries.⁴⁵ For the synthesis of precious metal oxides or highly crystalline multimetallic nanocrystals, as being highly efficient for OER or ORR activity, multistep synthesis has been opted with the use of highly toxic oleylamine solvent as a stabilizing agent.^46–48 Thus, despite aforementioned complex strategies, it would be encouraging to obtain cost-efficient and durable phosphate-based materials prepared via a ultrafast facile combustion route, where the proposed catalysts would also be carbon coated and suitable for high-performance electrocatalysis devices.^49–51

2.3. Recycling

Recycling old batteries is currently a critical necessity. Irrespective of the purity at the end of the life of a battery, the cathode materials are primarily subjected to metallurgical processes as well as direct repair processes for the reclamation of materials.⁵² Recycling will not only reduce the battery waste, but also mitigate the harmful effects of mining activities as well as critical metal sustainability in our high-tech society.^53–55 The metallurgical way to recycle spent cathodes such as pyrometallurgical and hydrometallurgical approaches demand acid/alkaline leaching and cause huge greenhouse gas emissions due to high temperature and energy to smelt transition metals with toxic by-products. However, the direct repair approach is straightforward to use the cycled cathodes as such irrespective of the impurity. Liu et al. provided an innovative solution to avoid the use of corrosive acids showing acid-free mechanochemically induced isomorphic substitution of Na in LiFePO₄, forming NaFePO₄ and recreating Li metal precursor (Li₂CO₃).⁵⁶ Phosphate-based intercalation materials have the potential to be employed in electrocatalytic devices because of their promising stability brought about by a strong P–O bond, in contrast to the instability of Pt/C electrocatalysts in other types of energy storage devices.^27,57 In order to fabricate air-electrodes for catalysis-driven energy storage devices, it is simple to reclaim the used insertion cathode materials from Li/Na-ion batteries.

3. Battery insertion materials as oxygen electrocatalysts

First-row transition metal oxides, whether they are spinels or perovskites having edge-shared or corner-shared octahedra or tetrahedra, can exhibit comparable OER and ORR catalytic activities to those of RuO₂ and IrO₂.⁵⁸ In addition to their usage in oxygen electrocatalysis-based devices, capable of reversible Li⁺ (de)intercalation into their frameworks, the first-row transition metal oxides (Mn, Fe, Co, and Ni) have also embarked a remarkable history in the development of LIBs.⁵⁹ Moving from the layered oxides to a three-dimensional polyanionic framework system, LiFePO₄ triggered the Li⁺ ion storage in olivine frameworks, where transition metal oxides (T_M–O₆) have edge sharing as well as corner sharing with phosphate (PO₄) units. Shao-Horn group for the first time examined and compared the OER activities, at pH 7 and 13, of well-known LIB cathode insertion materials: oxides vs. olivine.⁶⁰ They demonstrated superior activity in Co-based materials, particularly LiCoO₂ and LiCoPO₄ (Table 1 and Fig. 2a). Whether it is oxide or phosphate, it involves the formation of an amorphous surface layer on the catalyst. However, in case of phosphates, these amorphous surface layers are thicker at a higher pH than that of oxides due to more P being leached from the material, as shown in Fig. 2b.

Table 1 Overview of ORR and OER (vs. RHE) activities for crystalline phosphate-based-polyanionic intercalation materials in alkaline solutions

Materials	Crystal structure (space group)	Intercalation-based electrochemical activity			Electrocatalytic activity								Electrolyte solution	Ref.
					ORR				OER
		Specific capacity (mA h g^−1)	Working potential (V)	Ref.	E onset (V)	E 1/2 (V)	n ^e− (no. of electrons)	Tafel slope (mV dec^−1)	E onset (V)	η@10 mA cm^−2 (mV)		Tafel slope (mV dec^−1)
With no alkali metal-cation
Co2P2O7/NPGA	B121/c1	Inactive			0.91	0.80	3.72	—	1.45	340		96	0.1 M KOH	65
Co3(PO4)2/NC	P121/c1	Inactive			0.96	0.83	3.99	56	—	296		53	0.1 M KOH	89
ZnCo2(PO4)2	P21/n	Inactive			0.87	0.73	3.90	—	—	450		—	0.1 M KOH	66
ZnCoP2O7	P21/n	Inactive			0.86	0.72	3.82	—	—	460		—	0.1 M KOH	66
Li-cation based
LiFePO4	Pnma	162	3.45	90	N.A.				1.52	—		—	0.1 M KH2PO4	60
LiMnPO4	Pnma	∼91	4.10	91	0.85	0.89	—	—	OER inactive				0.1 M KOH	92
LiCoPO4	Pnma	∼93	∼4.8	91	N.A.				1.60	—		60	0.1 M KOH	60, 63 and 70
					N.A.				1.68	—		120	0.1 M KH2PO4	60
Li0.7Co0.75Fe0.25PO4/rGO	Pnma	—	3.4 & 4.8	69	N.A.				1.61	—		53	0.1 M KOH	69
LiCo0.60Fe0.40PO4	Pnma	∼135	3.4 & 4.8	68	N.A.				1.47	345		44	0.1 M KOH	68
LiNiPO4	Pnma	∼120	4.93 & 5.27	71	N.A.				1.65	—		—	1 M KOH	67
LiNi0.75Fe0.25PO4/rGO	Pnma	N.A.			N.A.				1.45	295		47	1 M KOH	67
Li2CoP2O7	P21/c	∼85	4.93 & ∼4.9	93	N.A.				1.81	—		84	0.5 M Na2HPO4	63
Na-cation based
NaCoPO4	Pnma	Inactive		94	0.77	0.69	-	109	N.A.				0.1 M NaOH	63
					0.85	0.73	3.58	87	1.55	390	52		1 M NaOH	73
NaFePO4	Pnma	∼100	2.88 & 3.02	95	1.04	0.79	3.87	∼94	OER inactive				0.1 M KOH	96
NaFe(PO3)3	Pa3̄	∼22	2.8	97	ORR Inactive				—	550		90	1 M KOH	78
NaMn(PO3)3		N.A.			ORR Inactive				—	—		189
NaCo(PO3)3		∼56	3.2	77	—				1.54	340		76
					0.83	—	—	60	1.52	—		77	0.1 M NaOH	79
NaCoFe2(PO4)3	C2/c	55	∼3.0	80	0.75	—	—	108	1.55	—		—	0.1 M KOH	80
NaCo4(PO4)3	P21/n	N.A.			N.A.				—	520		121	Phosphate Buffer	98
Na2FePO4F	Pbcn	100	∼3.05	99	0.89	0.77	—	119	1.69	570		130	0.1 M KOH	83
Na2MnPO4F	P21/c	∼60	∼3.7	100	0.91	0.84	—	137	1.72	—		250	0.1 M KOH	83
Na2CoPO4F	Pbcn	55	4.2	84	0.90	0.83	—	93	1.61	500		110	0.1 M KOH	83 and 84
					0.85	0.79	3.7	105	1.59	420		—	0.1 M NaOH
β-Na2MnP2O7	P1̄	80	∼3.6	101	0.87	0.84	∼3.5	118	1.56	470		64	0.1 M KOH	76
α-Na2MnP2O7	P1	N.A.			0.87	0.83	4.0	131	1.58	—		145
Na2CoP2O7	Pna21	∼80	∼3.0	102	0.78	—	—	115	N.A.				0.1 M NaOH	73
					0.86	0.74	—	96	1.50	—		51	1 M NaOH
					0.87	—	3.50	82	N.A.				0.1 M KOH	75
					N.A.				—	339		79	1 M KOH
	P1̄	∼80	∼4.3	103	0.87	—	3.90	78	N.A.				0.1 M KOH
					N.A.				—	374		64	1 M KOH
K-cation based
KFePO4	P21/c	∼25	∼2.8	85	1.01	—	3.84	104	N.A.				0.1 M KOH	96
KCo(PO3)3	P6̄c2	N.A.		104	∼0.81	—	—	85	1.52	—		156	0.1 M NaOH	79
K2CoP2O7	P42/mnm	Inactive		87	0.87	0.78	3.84	114	1.48	370		106	0.1 M KOH	87
					0.95	0.82	—	109	N.A.				Phosphate Buffer
K2Co(PO3)4	Cc	N.A.			0.83	0.87	—	59	1.44	1.57		80	0.1 M NaOH	88

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Fig. 2 OER activity of LiCoPO₄, a commonly used cathode material for lithium-ion battery devices under (a) neutral conditions and (b) post-mortem analysis of cycled catalysts in 0.1 M KPi (pH 7) and 0.1 M KOH (pH 13). HRTEM images (left) and FFTs (right) of the LiCoPO₄ surface showing the evolution of the amorphous region, i.e. change in surface morphology at alkaline pH, which is distinguished from crystalline regions with the white dashed lines. Reproduced from Lee et al.,⁶⁰ with permission from the American Chemical Society. The OER activity of sodium ion battery's cobalt phosphate-based cathode (Na₂CoP₂O₇ and NaCoPO₄) vs. lithium-ion battery's cobalt phosphate-based cathode (Li₂CoP₂O₇ and LiCoPO₄). (c) CV curves of 1st and 100th cycles at neutral pH from 0.7 to 1.5 V (versus NHE), with the thermodynamic potential for water oxidation at 0.816 V (versus NHE). (d) Bulk electrolysis of Na₂CoP₂O₇, NaCoPO₄, Li₂CoP₂O₇ (inset), and LiCoPO₄ (inset) performed under a constant potential of 1.4 V (versus NHE). Reproduced from Kim et al.,⁶³ with permission from Springer Nature.

Later, Manthiram group characterized and compared the OER activity in layered oxide battery materials containing various compositions of Mn, Co and Ni in LiT_MO₂. They observed that Ni-rich compositions exhibited the highest OER activity, while Mn-rich compounds were found to suppress the OER activity.⁶¹ The root cause is related to the singly occupied e_g orbital along with an oxidation state of +3 for the transition metal as well as surface atomic arrangements, which is explained in detail in the following sections.^61,62 Certainly, fresh or spent battery insertion materials can be put on anvil to design economic electrocatalysts showing ORRs, OERs, or bifunctional activities, suitable for metal–air batteries. In this spirit, the recent advances in the exploration of various phosphate-based battery insertion cathodes are elaborated in the following sections.

4. Phosphate-based polyanionic insertion materials as catalysts

Before enlisting the significant phosphate-based battery materials exhibiting electrocatalysis until now, at first, consideration should be given to how phosphates gained interest in oxygen electrocatalysis. The story begins with in situ-generated Co–Pi (phosphate in an approximately 1/2 ratio with cobalt), reported by Kanan and Nocera in 2008 exhibiting remarkable OER activity in earth-abundant catalysts at neutral pH under ambient conditions.⁶⁴ Later Ren et al. developed N, P codoped reduced graphene-oxide-aerogel-supported (rGOA-supported) Co₂P₂O₇ (CoPi) particles (CoPi/NPGA) via a simple hydrothermal route followed by pyrolysis route delivering excellent bifunctional activities in these Co–Pi materials.⁶⁵ Taking advantage of cobalt phosphate, Baby et al. modified the catalytic activity of Co by a divalent cation (Zn) substitution to realize robust bifunctional activity in ZnCo₂(PO₄)₂ and ZnCoP₂O₇ (pyro)phosphates.⁶⁶ The bifunctional oxygen electrocatalytic performance of Zn-substituted Co phosphate and pyrophosphate is compared with non-substituted Co-phosphate Co₂P₂O₇/NPGA and Co₃(PO₄)₂/NC (Table 1). During the ORR activity, the onset potential (E_onset) for Co₂P₂O₇/NPGA, Co₃(PO₄)₂/NC, ZnCo₂(PO₄)₂ and ZnCoP₂O₇ was found to be 0.91, 0.96, 0.87 and 0.86 V (vs. RHE), respectively, with the increasing order of half-wave potential (E_1/2): ZnCoP₂O₇ (0.72 V) < ZnCo₂(PO₄)₂ (0.73 V) < Co₂P₂O₇/NPGA (0.80 V) < Co₃(PO₄)₂/NC (0.83 V) in 0.1 M KOH. The LSV curve showed a higher saturation current for Zn-substituted Co catalysts, which is comparable to that of the Pt/C benchmark, which yielded 5.98 mA cm⁻² exchange current density. In case of Co₂P₂O₇/NPGA, Co₃(PO₄)₂/NC, the untreated catalysts with no N and P doping, exhibited inferior ORR activity. For the OER activity, at a current density of 10 mA cm⁻², the overpotential from 1.23 V (vs. RHE) for these catalysts was reported to be 340 mV, 296 mV, 450, and 460 mV for Co₂P₂O₇/NPGA, Co₃(PO₄)₂/NC, ZnCo₂(PO₄)₂ and ZnCoP₂O₇ respectively. It is worth mentioning that without any further addition of N or P or reduced graphene oxide into the structure, ZnCo₂(PO₄)₂ and ZnCoP₂O₇ showed comparable ORR and OER activities. Consequently, it can be claimed that Zn-substituted Co-phosphates increased the catalytic activity of untreated Co-phosphates.^65,66

4.1. Phosphate (PO₄)

As mentioned above, besides LiFePO₄, LiCoPO₄ and LiMnPO₄ olivine structures containing only one transition metal, the OER activity in olivines containing mixed divalent cations has also been investigated by Ma et al. in 2016 and Wu et al. in 2020.^67,68 Apart from the partial substitution of Co/Ni or Fe in the host structure, Gershinsky et al. achieved lithium sub-stoichiometric Li_0.7Co_0.75Fe_0.25PO₄ compound by the direct synthesis having an olivine structure and identified excellent electrochemical water oxidation.⁶⁹ These findings coincide with those of Cui group's work demonstrating increased OER activity in electrochemically extracted olivine-type lithium transition metal phosphates.⁷⁰ For LIB cathodes Li_0.7Co_0.75Fe_0.25PO₄/rGO, LiCo_0.60Fe_0.40PO₄, LiNiPO₄ and LiNi_0.75Fe_0.25PO₄/rGO showing a specific discharge capacity ≥120 mA h g⁻¹, the OER activity is presented in Table 1, showing a clear influence of transition metals on water oxidation properties into the host LiT_MPO₄ olivine-type structure.^68,69,71 The onset potential for the OER activity of LiCoPO₄, Li_0.7Co_0.75Fe_0.25PO₄/rGO and LiCo_0.60Fe_0.40PO₄ was found to be 1.60, 1.61 and 1.47 V (vs. RHE), respectively, in 0.1 M KOH, while for LiNiPO₄ and LiNi_0.75Fe_0.25PO₄/rGO, it showed E_onset of 1.65 and 1.45 V (vs. RHE), respectively (Table 1).^67,69 In all these cases, the activities of the end member/undoped LiT_MPO₄ were enhanced because of the synergistic effect between Fe and Co or Ni.^67,68

4.2. Pyrophosphate (P₂O₇)

Kim et al. comprehensively evaluated the influence of PO₄vs. P₂O₇ units on OER activity using PO₄-chemistry in one formula unit (f.u.) in its crystal structure. For this study, LiCoPO₄, Li₂CoP₂O₇, NaCoPO₄ and Na₂CoP₂O₇ cathode materials were taken into consideration, showing a specific discharge capacity ≤90 mA h g⁻¹ for LIBs/SIBs. The four chosen Co-based phosphate catalysts exhibited a range of catalytic activity, championed by Na₂CoP₂O₇ exhibiting the highest exchange current during water oxidation. It clearly explained the crucial role of metal coordination, which serves as a platform to fully comprehend the catalytic activity of these materials under neutral conditions. Referring to Fig. 3a, the crystal structures of LiCoPO₄, Li₂CoP₂O₇, NaCoPO₄ and Na₂CoP₂O₇ revealed the presence of different networks between Co polyhedra and various Co coordination, resulting in different catalytic activities with the increasing trend of LiCoPO₄ < Li₂CoP₂O₇ < NaCoPO₄ < Na₂CoP₂O₇ (Fig. 2c and d). The distorted local Co–O₄ geometry in Na₂CoP₂O₇ displayed the highest OER activity compared to others, where the rotation of the flexible pyrophosphate ligands allowed Co–O₅i.e. an additional Co–O bond at the surface due to water adsorption and further water oxidation catalysis at neutral pH.^63,72 Furthermore, Gond et al. evaluated the increased specific OER and ORR activities for Na₂CoP₂O₇ denting this material as a bifunctional catalyst in an alkaline pH environment.⁷³ They also investigated the importance of interplay between the T_M and P₂O₇ units, showing polymorphism vs. bifunctional electrocatalysis in Na₂CoP₂O₇ and Na₂MnP₂O₇. Pyrophosphates are likely to exhibit polymorphism, providing a variety of crystal structures acting as potential cathodes for LIBs and SIBs.⁷⁴ Later, realizing these materials for ORRs and OERs, different polymorphs of Na₂CoP₂O₇, such as orthorhombic phase o-Na₂CoP₂O₇ (Pna2₁) and triclinic phase t-Na₂CoP₂O₇ (P1̄) and triclinic polymorphs of Na₂MnP₂O₇ like β-Na₂MnP₂O₇ (P1̄) and α-Na₂MnP₂O₇ (P1) have been synthesized at low and high temperatures, respectively, as shown in Fig. 3b.^75,76 Acting as insertion materials for the sodium-ion batteries, o-Na₂CoP₂O₇, t-Na₂CoP₂O₇, and β-Na₂MnP₂O₇ delivered a specific discharge capacity of ∼80 mA h g⁻¹ at 3.0 V, 4.3 V, and 3.6 V, respectively. In the case of Na₂CoP₂O₇, the trend for ORR exchange current density was o-Na₂CoP₂O₇ < t-Na₂CoP₂O₇, while for OER the trend was opposite; t-Na₂CoP₂O₇ < o-Na₂CoP₂O₇. However, in case of Na₂MnP₂O₇, the bifunctional activity of β-Na₂MnP₂O₇ was found to be superior to that of α-Na₂MnP₂O₇ (the data are summarised in Table 1).

Fig. 3 Crystal structure of selected phosphate-based polyanion insertion materials with the mobile Na/Li atom (yellow/green) in the 3D frameworks. (a) Cobalt crystal field of Co-phosphate catalysts: NaCoPO₄, LiCoPO₄, Na₂CoP₂O₇, and Li₂CoP₂O₇ showing their local environment around the Co-subunit (blue for Co–O₄ or Co–O₆, grey for PO₄ unit) and the corresponding spin state of Co atoms. The presented image is reproduced from Kim et al.,⁶³ with permission from Springer Nature. (b) Structural comparison of two polymorphs of Na₂CoP₂O₇ and Na₂MnP₂O₇ w.r.t. temperature, where Co–O, Mn–O, and P–O subunits are shown in blue, brown and grey, respectively. The bridging of pyrophosphate (P₂O₇) units and Co–O octahedra in P1̄ Na₂CoP₂O₇ are edge shared and corner shared, while in Pna2₁ Na₂CoP₂O₇, the P₂O₇ units and Co–O tetrahedra are only corner shared. Reproduced from Gond et al.⁷⁵ with permission from the American Chemical Society. In Na₂MnP₂O₇, both polymorphs P1 and P1̄ have only corner shared Mn-O₆ and P₂O₇ subunits. Reproduced from Gond et al.⁷⁶ with permission from the Royal Society of Chemistry. (c) Na₂FePO₄F (pink for Fe–O₄F₂ and green for PO₄ unit), Na₂CoPO₄F (blue for Co–O₄F₂ and grey for PO₄ unit), and Na₂MnPO₄F (olive green for Mn–O₄F₂ and brown for PO₄ unit) crystallize in orthorhombic, orthorhombic, and monoclinic systems respectively, where all the transition metal octahedra are corner shared to PO₄ tetrahedra. Reproduced from Sharma et al.⁸³ with permission from the Royal Society of Chemistry.

4.3. Metaphosphate (P₃O₉)

Pursuing the story further, Gond et al. for the first time reported excellent OER activity in NaCo(PO₃)₃, which is a 3.2 V cathode material for SIBs.^77,78 This framework is built from the interlinked PO₄ tetrahedra connecting the CoO₆ octahedra. The Co-analogue showed superior electrocatalytic activity to the Fe and Mn analogues in metaphosphate NaT_M(PO₃)₃. Recently, Murugesan et al. have reported the bifunctional activity of NaCo(PO₃)₃ in 0.1 M NaOH and extended it to KCo(PO₃)₃ witnessing metaphosphate as a bifunctional catalyst for metal air-battery applications.⁷⁹ The LSV curves showed ORR onset potentials of 0.83 V and ∼0.81 V for NaCo(PO₃)₃ and KCo(PO₃)₃, respectively. However, both materials delivered similar OER activity with an onset potential of 1.52 V.

4.4. Alluaudite [(PO₄)₃]

The study further extended to the phosphate-based alluaudite class of materials in particular NaCoFe₂(PO₄)₃. This material works as a ∼3.0 V cathode without any material optimization delivering a discharge capacity of 60 mA h g⁻¹ at C/20.⁸⁰ Exploiting the Co species, it showed bifunctional activity with onset potentials of 0.75 V and 1.55 V (vs. RHE) for ORR and OER processes respectively.

4.5. Fluorophosphates (PO₄F)

Another interesting class of insertion materials comprises fluorophosphates having a general formula Na₂T_MPO₄F (T_M = Fe, Mn, and Co; crystal structure is represented in Fig. 2c), which tend to exhibit high working potentials concerning their phosphate analogues.⁸¹ In principle, the participation of fluorine in a Na₂T_MPO₄F material possesses a larger ionicity of the T_M–F bond than that of the T_M–O one (occurring in PO₄ analogues). Due to the higher electronegativity of the F atom, fluorophosphates show a higher redox potential for the T_M redox activity.⁸² The catalytic performance of this high-voltage battery cathode materials was unknown before our group explored the oxygen bifunctional electrocatalytic capabilities of fluorophosphate containing Fe, Mn, and Co transition metals (Table 1).⁸³ Among them, Na₂CoPO₄F was found to deliver excellent bifunctional properties exhibiting onset potentials of 0.854 V and 1.591 V (vs. RHE), for ORR and OER activities respectively in 0.1 M NaOH.⁸⁴

In parallel, interest in studying the electrocatalytic properties of KIB cathode materials has recently grown. Lack of adequate electrode materials that can reversibly intercalate K⁺ with high capacity, nominal voltage, rate kinetics, and stable cycle life makes this field challenging than the Li/Na-analogues.⁸⁵ One reason lies in their relative cationic size (0.76 Å for Li⁺ < 1.02 Å for Na⁺ < 1.38 Å for K⁺). Considering one of the simplest phosphate-based cathode materials, KFePO₄ was found to yield a reversible discharge capacity of ∼47 mA h g⁻¹ in the amorphous form. However, its crystalline form showed a sloping voltage profile with very low specific capacity originating from electric double layer capacitance instead of any efficient intercalation.⁸⁶ Murugesan et al. reported the ORR activity of monoclinic KFePO₄ with an onset potential of 1.01 V (Pt/C E_onset = 0.79 V vs. RHE). Its pyrophosphate derivative, K₂CoP₂O₇, also worked as a potential electrocatalyst showing excellent bifunctional properties with onset potentials of 0.87 V and 1.48 V for ORR and OER activities, respectively (Table 1).⁸⁷ Similarly, K₂Co(PO₃)₄ exhibited bifunctional properties with unavailable charge–discharge intercalation properties of K-ions, where onset potentials of 0.83 V and 1.44 V for ORR and OER activity, respectively, were obtained in 0.1 M NaOH.⁸⁸

4.6. Influence of transition metals (T_M = Co, Ni, Mn and Fe)

In Li⁺ intercalating cathode materials, the operating voltages normally increase with the higher amount of d electrons (up to Ni). In 2004, Ceder group estimated the working potential for LiT_MPO₄ (T_M = Fe, Mn, and Co), which was consistent with the previous experimentally measured operating potential.¹⁰⁵ The obtained potentials for the redox activity of T_M based on both the methods merged to one simple conclusion explained with ionic molecular-orbital theory (MOT). It is based on the band structure of the material, which splits 3d-orbitals into three lower t_2g and two upper bands in case of octahedral oxygen coordination. Typically, for the early T_M in LiT_MPO₄, the valence bands are mainly well separated from metal-deprived bands with oxygen-p character, while later as the d electron increases, the t_2g and drop in energy and the former mix with the oxygen-p bands. Until Co, the M–O bond length noticeably decreases, due to the increase in nuclear charge on the metal, leading to an effectively smaller metal ion. However, afterwards for Ni, the M–O bond length increases due to the filling of the antibonding bands. Towards the end (for Cu and Zn), because of the increase in the nuclear charge of the metal ion, the metal-d band character moves to a lower energy and virtually intermixed with the oxygen-p bands, resulting in higher average intercalation voltage that might be limited by unstable electrolytes at that potential.¹⁰⁶

Using the same MOT approach, the designing and discovery of new cost-effective highly active catalysts for electrochemical energy conversion and storage can be estimated. Motivated by Nørskov, Shao-horn group proposed a d-band theory activity descriptor for metal surfaces to explain the mechanisms for oxygen electrocatalysis in the transition metal oxides in perovskite-type structures. This landmark contribution accurately defined the transition metal oxides such as ABO₃ perovskites composed of rare and alkaline earth (A) and 3-d T_M cations (B) exhibiting ORR/OER activities comparable to the state-of-the-art catalysts IrO₂, RuO₂ and Pt/C.^9,62,107,108 They developed the descriptor for ORRs, revealing M-shaped relationship with the increase in d-electron (for B site T_M cations), with the maximum activity for d⁴ and d⁷, resulting in a volcano shape plot as a function of e_g-filling of T_M cations at B site in the case of perovskites. A similar volcano shape trend was achieved for OERs with e_g-filling vs. potential at a particular current density.^62,108 According to them, the identification of e_g filling (of antibonding states) of surface transition metal cations is a universal activity descriptor for the oxides. As per the catalysis surface science based on the reaction mechanism (alkaline pH in the introduction section) for T_M in the octahedra geometry, the σ-bonding e_g orbital has a stronger overlap with the oxygen-related adsorbate, in particular, the OOH* intermediates than does the π-bonding t_2g orbital. The reported e_g filling close to unity experimentally facilitates the highest ORR and OER activity, due to more efficient electron transfer between surface cation and adsorbed reaction intermediates.⁶²

Inspired by this, the oxygen electrocatalytic activity in selected phosphate-based catalysts containing different transition metals, where simply the number of electrons in the 3d orbital and e_g (antibonding σ*-orbital; T_MO₆ and bonding σ-orbital; T_MO₄) orbital is represented during ORRs and OERs, respectively, is shown in Fig. 4 and the related data are tabulated in Table 2. Fig. 4a and b are recreated from Suntivich et al. work for ORR and OER activities, respectively,^62,107,108 providing deep insights into the obtained activities in LiCo_0.60Fe_0.40PO₄,⁶⁸ LiNi_0.75Fe_0.25PO₄/rGO,⁶⁷ NaCoPO₄,⁶³ NaFe(PO₃)₃,⁷⁸ NaCo(PO₃)₃,⁷⁸ NaCo₄(PO₄)₃,⁹⁸ Na₂FePO₄F,⁸³ Na₂CoPO₄F,⁸³ β-Na₂MnP₂O₇,⁷⁶o-Na₂CoP₂O₇,⁷⁵t-Na₂CoP₂O₇,⁷⁵ KCo(PO₃)₃,⁷⁹ K₂CoP₂O₇,⁸⁷ and K₂Co(PO₃)₄⁸⁸ materials. Respective crystal structure, valence or oxidation state of transition metal, causing the number of electrons in 3d-orbitals, its spin state, and e_g filling state (antibonding σ*-orbital for octahedra and bonding σ-orbital for tetrahedra geometry) are tabulated against potential in V (vs. RHE) at −3.0 mA cm⁻² and 10.0 mA cm⁻² for ORR and OER activities, respectively. Among all the known phosphate-based catalysts displaying intercalation chemistry, most of the articles are based on Co-based phosphate materials for ORR and OER electrocatalysis (Fig. 4 shows the predominant Co-based catalysts in the plot).

Fig. 4 ORR and OER activities of phosphate-based polyanionic intercalation materials in alkaline solutions. (a) ORR activity: potential at a current density of 3 mA cm⁻² of the selected phosphate-based catalysts with the respective transition metal centre d-electron, following the M-shaped relationship, as reported for perovskite oxides from Shao-horn et al.¹⁰⁸ (b) OER activity: potential at a current density of 10 mA cm⁻² of phosphate-based electrocatalysts with the respective e_g electrons in d-orbital of transition metal responsible for the activity, the dashed volcano lines are shown only for guidance from the literature Shao-horn et al. study for perovskite oxides.^9,62 Data symbols are as follows: brown triangles for Mn, blue triangles for Co, red triangles for mixed transition metal, and green open circles for Fe-based phosphate-containing electrocatalysts. The data presented are replotted and were originally reported as: LiCo_0.60Fe_0.40PO₄,⁶⁸ LiNi_0.75Fe_0.25PO₄/rGO,⁶⁷ NaCoPO₄,⁶³ NaFe(PO₃)₃,⁷⁸ NaCo(PO₃)₃,⁷⁸ NaCo₄(PO₄)₃,⁹⁸ Na₂FePO₄F,⁸³ Na₂CoPO₄F,⁸³ β-Na₂MnP₂O₇,⁷⁶o-Na₂CoP₂O₇,⁷⁵t-Na₂CoP₂O₇,⁷⁵ KCo(PO₃)₃,⁷⁹ K₂CoP₂O₇.⁸⁷ The electrocatalytic activity of the aforementioned catalysts as well as the spin state of the catalytic transition metal centre are detailed in Table 2.

Table 2 Summary of the literature studies on the spin states (H.S.; high spin, L.S.; low spin, I.S.; intermediate spin) of the selected phosphate-based electrocatalysts based on ORR and OER performance

Materials	Crystal structure (space group)	Valence	electrons in 3d-orbital	Spin state	eg filling assignment	Potential, V@−3 mA cm^−2 (ORR)	Potential, V@10 mA cm^−2 (OER)	Ref.
LiCo0.60Fe0.40PO4	Pnma	Co^+2/+3	d^7 and d^6	H.S./I.S.	t^52g e^2g (d^7) and t^4.52g e^1.5g (d^6)	N.A.	1.57	69 and 109
		Fe^+2	d^6	L.S.	t^62g e^0g (d^6)
LiNi0.75Fe0.25PO4/rGO	Pnma	Ni^+2	d^8	L.S.	t^62g e^2g (d^8)	N.A.	1.52	67
		Fe^+2/+3	d^6 and d^5	H.S.	t^42g e^2g (d^6) and t^32g e^2g (d^5)
NaCoPO4	Pnma	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.69	1.61	63
NaFe(PO3)3	Pa3̄	Fe^+2	d^6	H.S.	t^42g e^2g (d^6)	Inactive	1.78	97
NaCo(PO3)3	Pa3̄	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.72	1.57	78
NaCo4(PO4)3	P21/n	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	—	1.75	98
Na2FePO4F	Pbcn	Fe^+2	d^6	H.S.	t^42g e^2g (d^6)	0.67	1.80	83
Na2CoPO4F	Pbcn	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.64	1.73	84
β-Na2MnP2O7	P1̄	Mn^+2	d^5	H.S.	t^32g e^2g (d^5)	0.68	1.70	76
t-Na2CoP2O7	P1̄	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.71	1.60	75
o-Na2CoP2O7	Pna21	Co^+2	d^7	H.S.	e^4 t2^3 (d^7)	0.68	1.57	75
KCo(PO3)3	P6̄c2	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.66	—	79
K2CoP2O7	P42/mnm	Co^+2	d^7	H.S.	e^4 t2^3 (d^7)	0.70	1.60	87
K2Co(PO3)4	Cc	Co^+2	d^7	H.S.	t^52g e^2g (d^7)	0.72	1.57	88

---

For the ORR activity, the catalyst exhibiting a current density ≥ −3.0 mA cm⁻² is considered and shown in Fig. 4a. Such catalysts are enlisted with the potential (written next to the material) at a current density of −3.0 mA cm⁻²vs. the respective 3d-electron in T_M. Based on this potential, onset potential and Tafel slope from Tables 1 and 2, the decrease in ORR activity trend is shown here with potential in V (vs. RHE), at −3.0 mA cm⁻² in brackets: K₂Co(PO₃)₄ (0.72 V) ≥ NaCo(PO₃)₃ (0.72 V) > t-Na₂CoP₂O₇ (0.71 V) > K₂CoP₂O₇ (0.70 V) > NaCoPO₄ (0.69 V) > o-Na₂CoP₂O₇ (0.68 V) ≥ β-Na₂MnP₂O₇ (0.68 V) > Na₂FePO₄F (0.67 V) > KCo(PO₃)₃ (0.66 V) > Na₂CoPO₄F (0.64 V). The observed trend clearly shows the validation of the M-shaped relationship from the Suntivich et al. descriptor for ORRs, implying that the increase in d-electron enhances the activity, with the maximum ORR activity for d⁴ (the present review study deployed of d⁴ transition metals for ORR) and d⁷. The same descriptor of e_g-filling stands well to explain the trend for OER activity in Mn, Fe, Co and Ni containing phosphate-based intercalation cathodes studied so far as catalysts for oxygen evolution chemistry in alkaline pH. Fig. 4b represents such cathodes whose potential (E₁₀ in V) at 10.0 mA cm⁻² is shown vs. the number of electrons in the e_g orbital except for o-Na₂CoP₂O₇ and K₂CoP₂O₇, which possess T_M–O₄, thus it represents e (bonding σ-orbital, will be overfilled >2 for Co²⁺). The spatial mismatch between the oxygen ligand of T_M–O₄ and the d-orbitals provided by transition-metal cations on the tetrahedral sites suggests weak interaction with molecular oxygen and oxygenated species.

The declining trend of OER activity is likely to be shown here, representing potential in V (vs. RHE), at 10.0 mA cm⁻² in brackets: LiNi_0.75Fe_0.25PO₄/rGO (1.52 V) > LiCo_0.60Fe_0.40PO₄ (1.57 V) ≥ NaCo(PO₃)₃ (1.57 V) > o-Na₂CoP₂O₇ (1.57 V) > K₂Co(PO₃)₄ (1.57 V) > K₂CoP₂O₇ (1.60 V) > t-Na₂CoP₂O₇ (1.60 V) > NaCoPO₄ (1.61 V) > β-Na₂MnP₂O₇ (1.70 V) > Na₂CoPO₄F (1.73 V) > NaCo₄(PO₄)₃ (1.75 V) > NaFe(PO₃)₃ (1.78 V) > Na₂FePO₄F (1.80 V). In summary, for ORR as well as OER processes, the Co-containing phosphates are found to be superior due to the appropriate number (d⁷ and e_g-filling close to ≥1) of electrons in the e_g orbital that causes efficient bonding of O–Co–OOH* oxygen-related intermediates. In particular, LiCo_0.60Fe_0.40PO₄ showed e_g-filling close to 1.2 (in Co), laid on the imaginary volcano plot in Fig. 4b. Moreover, the Fe-doped Co/Ni-phosphates (LiCo_0.60Fe_0.40PO₄ and LiNi_0.75Fe_0.25PO₄/rGO) exhibited dramatically improved OER activity compared to the pure phases, due to the synergistic coupling effects for the favorable electronic structure of the surface Co, improving the rate-limiting step which is the formation of OOH* derived from O* during the OER.^67,68 The metal cations and ligands (here phosphates) also affect the kinetics of intermediate adsorption/desorption and overall physicochemical processes governing catalytic activities as discussed below.

4.7. Influence of alkali cations

Besides establishing a correlation between 3d-electrons and e_g occupancy with the oxygen chemistry as an activity descriptor, metal-oxygen covalency is also a very important activity descriptor. Based on the study conducted by Wei et al. for spinels with Co or Ni as active sites (e.g. Co₃O₄, ZnCo₂O₄, and NiCo₂O₄), because of different cations at the tetrahedra site, the ORR and OER activities in these materials deviated from the e_g-filling theory, occurring due to the more covalent metal-oxygen bond nature at the octahedral site.¹¹⁰ Further, sticking to one particular cation (La), in LaT_MO₃ oxide (where T_M = Mn, Co and Ni), the potential to obtain 25 μAcm⁻² during the ORR follows the trend LaNiO₃ > LaCoO₃ > LaMnO₃ with values of 908 (±8) mV, 847 (±3) mV and 834 (±24) mV (versus RHE) respectively. It witnesses superior ORR activity in Ni not only due to e_g-filling theory, but also because of the higher covalent nature of the T_M–O bond due to the higher electronegativity of the metal cation.¹⁰⁸ Similarly, changing cations (M) in M_xT_M(PO₄)_y will greatly influence the overpotential linked to the electronegativity of the T_M–O bond. Interestingly, a comparison between the ORR and OER activities reveals such influence supporting the truth of the covalent T_M–O bond regardless of the e_g-filling theory, as shown in Fig. 5 and Table 2. Considering Na₂CoP₂O₇ and K₂CoP₂O₇ having Pna2₁ and P4₂/mnm space groups, respectively, they possess Co–O₄ tetrahedra in both cases with the only difference of metal cations (Na and K). It establishes evidence of the Co–O bond covalency when the electronegativity of neighbouring metal cation differs.¹¹¹ A careful examination of Fig. 5 showing LSV curves of different Co-containing phosphate-based catalysts will reflect two types of Na₂CoP₂O₇ of Pna2₁ named o-Na₂CoP₂O₇ and o-Na₂CoP₂O₇*, where the one labelled with * has improved oxygen electrocatalytic activity due to the synergistic effect between carbon and o-Na₂CoP₂O₇, obtained after the pre-treatment of the catalyst with Super P. Still, it can be noted that K₂CoP₂O₇ performs better than Na₂CoP₂O₇ for total oxygen electrocatalysis (ORR and OER activities in Fig. 5a and b, respectively). In particular, comparing them in 0.1 M KOH, the overall oxygen electrocatalysis is seen to be better in K₂CoP₂O₇ than in Na₂CoP₂O₇. In support of this statement, comparing the ORR activity in 0.1 M KOH for both catalysts exhibits a similar onset potential (0.87 V vs RHE), but K₂CoP₂O₇ ended up displaying a large limited current density (−4.23 mA cm⁻²) than Na₂CoP₂O₇ (−3.53 mA cm⁻²) at 0.3 V vs RHE (Fig. 5a). It demonstrates a more efficient mass transfer among K₂CoP₂O₇ due to the presence of K-cations (electronegativity trend is K < Na) in the electrocatalyst. A similar superior influence of K-cation is observed for the OER activity, where the achieved current density at 1.7 V (vs. RHE) was found to be just 2.79 mA cm⁻² for Na₂CoP₂O₇, while a ∼6-fold higher current density (ca. 16.82 mA cm⁻²) was achieved for K₂CoP₂O₇ (Fig. 5b). It clearly witnesses the positive impact of T_M–O bond covalency happening due to less electronegative alkali metal cations in alliance with the transition metals.

Fig. 5 Linear sweeping voltammograms (LSVs) for different Co-containing phosphate-based-polyanionic intercalation materials in alkaline aqueous electrolytes (pH 13–13.7) performed at a scan rate of 10 mV s⁻¹ with a rotation of 1600 rpm. (a) Oxygen reduction activity compared with Vulcan carbon XC (purple dashed line) and Pt/C (Pt is 20 wt%, grey dashed line) in O₂-saturated 0.1 M KOH. (b) Oxygen evolution activity compared with state-of-the-art RuO₂ catalyst in Ar-saturated 0.1 M and 1 M KOH. Presented data are replotted together for a better comparison using the originally reported data as follows: NaCoPO₄,⁶³ NaCo(PO₃)₃,⁷⁸ NaCoFe₂(PO₄)₃,⁸⁰ Na₂CoPO₄F,⁸³o-Na₂CoP₂O₇,⁶³o-Na₂CoP₂O₇*,⁷⁵t-Na₂CoP₂O₇,⁷⁵ and K₂CoP₂O₇.⁸⁷

4.8. Influence of phosphate units as ligands

Continuing the above discussion, it is not only the type of cations that influences the metal covalency affecting the affinity towards the oxygen intermediates responsible for oxygen electrocatalysis, but the neighbouring ligands also determine the spatial arrangements of these intermediates binding temporarily to T_M–O and ease of reaction. Structural conformations of PO₄-ligands highly influence the activity. Explicitly considering the case of simple PO₄, P₂O₇ and PO₄F, to complex multiple units of interlinked PO₄ as in metaphosphates or alluaudite phosphates, the bifunctional oxygen electrocatalysis was found to be different. It is described in detail for NaCoPO₄,⁶³ NaCo(PO₃)₃,⁷⁸ NaCoFe₂(PO₄)₃,⁸⁰ Na₂CoPO₄F⁸³ and o-Na₂CoP₂O₇⁶³ in the present section. Their LSV profiles for ORRs and OERs reactions are presented in Fig. 5.

LSVs from the ORR activity conducted in 0.1 M KOH showed the highest limited current density for NaCoFe₂(PO₄)₃ with a value of −4.32 mA cm⁻² at 0.3 V (vs RHE) (Fig. 5a) close to −4.48 mA cm⁻² of commercial Pt/C, followed by Na₂CoPO₄F, o-Na₂CoP₂O₇ and NaCoPO₄ which achieved −3.29, −2.12, and −1.77 mA cm⁻², respectively.^63,80,83o-Na₂CoP₂O₇* and other polymorphs labelled as t-Na₂CoP₂O₇ were treated with Super P carbon, where the as-synthesised phases were ball milled with conductive carbon exhibiting enhanced activity when compared to the untreated o-Na₂CoP₂O₇ (Fig. 5a).⁶³ In particular, ignoring the bimetallic NaCoFe₂(PO₄)₃ electrocatalyst which showed superior limited current density, the effect might be an outcome of the synergistic effect of Fe and Co as reported for LiCo_0.60Fe_0.40PO₄.⁶⁹ Thus, based on the ORR activity trend of Na₂CoPO₄F > o-Na₂CoP₂O₇ > NaCoPO₄, in Na₂CoPO₄F, the presence of the F ligand in PO₄-based materials forms F₂–Co–O₄ octahedra, as compared with NaCoPO₄, which has CoO₆ units (Fig. 3). The active centre with CoO₄F₂ showed superior activity to CoO₆, which could be due to the presence of most electronegative F atoms in the vicinity of the Co metal, that is going to interact with the oxygen intermediates OOH* (as explained in the introduction section).⁸³ However, metal coordination varies in the case of P₂O₇vs. PO₄, where CoO₄ is corner shared with the P₂O₇ unit, while CoO₆ is edge shared with the PO₄-unit. In this case, P₂O₇ offers more degrees of spatial rearrangements when the oxygen intermediate forms a temporary bond with an active metal centre. Nevertheless, the e_g-filling theory is more reasonable when it comes to the ORR activity while comparing two polymorphs of Na₂CoP₂O₇. Here, regardless of CoO₄ corner shared with the P₂O₇ unit in o-Na₂CoP₂O₇* (Fig. 5a) possessing more possibility of spatial rearrangements, CoO₆ which is edge shared with the P₂O₇ unit in t-Na₂CoP₂O₇ showed a higher exchange current density.⁷⁵

Furthermore, from Fig. 5b, the OER activity results conducted at pH ∼ 13 showed current densities of 2.79, 3.20, 4.87, 7.09 and 8.03 mA cm⁻² (at 1.7 V vs. RHE) for o-Na₂CoP₂O₇, NaCoPO₄, NaCoFe₂(PO₄)₃, Na₂CoPO₄F, and commercial RuO₂, respectively. In principle, based on Kim et al.'s report, the onset potential for the OER activity and exchange current density for Na₂CoP₂O₇ is superior to that of NaCoPO₄ at neutral pH or alkaline pH. Our results also overlapped with the reported findings proving favourable binding of water molecules in Na₂CoP₂O₇ as compared to NaCoPO₄ due to flexible local cobalt coordination in Na₂CoP₂O₇.⁶³ However, in 0.1 KOH, the current density at 1.7 V and onset potentials are quite comparable as mentioned above, while the onset potential for o-Na₂CoP₂O₇ outperforms at a more basic alkaline pH. Due to the compositional mixing of Co and Fe in NaCoFe₂(PO₄)₃ alluaudite-type structure, the bulk electronic structure of CoO₆ (metal 3d and O 2p states greatly influenced) lowered surface adsorption energetics and contributed a higher current density at 1.7 V (vs. RHE) than o-Na₂CoP₂O₇ and NaCoPO₄.^9,111 Similar to the superior ORR activity in CoO₄F₂ type of T_M active centres, Na₂CoPO₄F exhibits a current density (7.09 mA cm⁻²) very similar to commercial RuO₂ (8.03 mA cm⁻²) at pH 13. The structure of Na₂CoPO₄F, which is more electrophilic in nature due to the F-atom, allows for the minimum binding of water molecules and promotes greater activity than that of o-Na₂CoP₂O₇, NaCoPO₄ and NaCoFe₂(PO₄)₃, demonstrating the impact of the ligands on electrocatalysis. Comparing the OER activity in 1 M KOH, NaCo(PO₃)₃ showed an exceptionally high current density of 89.72 mA cm⁻² at 1.7 V (vs. RHE) that is way higher than that of RuO₂ (55.43 mA cm⁻²). The explanation behind the superior activity in NaCo(PO₃)₃ again suggests the importance of local Co-coordination and preference of corner-sharing to the neighbouring PO₄-units. For clarity, it is important to mention that the current densities at 1.7 V (vs. RHE) for t-Na₂CoP₂O₇ and o-Na₂CoP₂O₇ polymorphs are 49.81 and 69.43 mA cm⁻², respectively. Thus, for the material possessing CoO₆ as an active metal centre, the trend for OER activity is NaCo(PO₃)₃ > t-Na₂CoP₂O₇, as in both the cases the active metal centre e_g-orbital directly interacts with incoming intermediates, but the main difference is the edge-shared and corner-shared octahedra with the pyrophosphate unit [(P₂O₇)⁴⁻]. The isolated CoO₆ active centre in NaCo(PO₃)₃ facilitates efficient surface binding properties and allows all possible spatial rearrangement required during electrocatalysis. Due to the edge-shared octahedra in t-Na₂CoP₂O₇, despite the direct interaction between the e_g-orbital and the incoming intermediates, the system undergoes some stress. o-Na₂CoP₂O₇* showed superior OER activity to Co–O₄ as the catalytic centre (Fig. 5b).

5. Metal–air batteries using phosphate-based materials

Inspired by the demonstration of low-cost bifunctional electrocatalysts using PO₄-based battery insertion materials, they have been implemented in (hybrid) metal–air batteries.^112,113 Real-time pictures, schematic illustrations and galvanostatic cycling results of two such devices are shown in Fig. 6. Promising Na⁺-ion intercalation into the host structures of Na_0.5Co_0.5Mn_0.5O₂ and NaCoPO₄F promoted their potential towards sea water batteries and hybrid Na–air batteries, respectively. K₂Co(PO₃)₄, another potential cathode material for K-ion batteries, could also be implemented as air-cathode for hybrid Na–air battery applications. The application of bifunctional electrocatalysts for rechargeable Zn–air batteries (ZABs) is also widely known, where Co₂P₂O₇/CoPi, ZnCoP₂O₇, t-Na₂CoP₂O₇, o-Na₂CoP₂O₇, and K₂CoP₂O₇ have been used as cathodes for the fabrication of such storage devices.

Fig. 6 Rechargeable secondary devices made of selected phosphate-based materials. Hybrid Na–air battery: (a) digital image of a hybrid Na–air battery illuminating a green LED with the Na₂CoPO₄F air cathode, containing a pouch cell (made of sodium metal dipped in an organic non-aqueous electrolyte and a solid membrane, NASICON, which is exposed outside) dipped in a beaker-type cell filled with aqueous electrolytes. (b) Schematic illustration of a hybrid Na–air battery describing the processes involved at the air electrode, cathode (Na₂CoPO₄F⁸⁴) during charge and discharge. (c) Corresponding hybrid Na–air battery performance at different current rates. Reproduced from Sharma et al.⁸⁴ with permission from the American Chemical Society. Zn–air battery: (d) image of the Zn–air battery configuration with the catalyst, anode, and simple set-up using a 20 mL plastic box. (e) Schematic diagram of a Zn–air battery describing the processes involved at the air electrode, in particular, with Na₂CoP₂O₇⁷⁵ as the cathode and Zn metal as the anode, submerged in 6.0 M KOH containing 0.2 M Zn acetate as the additive. (f) Galvanostatic (current density, I_d = 4 mA cm⁻²) discharge–charge cycling curves for two polymorphs of Na₂CoP₂O₇, rechargeable Zn–air batteries. Reproduced from Gond et al.⁷⁵ with permission from the American Chemical Society.

Sea water batteries and hybrid Na–air batteries comprise sodium (Na) metal as the anode and bifunctional electrocatalysts as the air cathode separated by a selective ionic conducting membrane, e.g. NASICON-type Na₃Zr₂SiPO₁₂. Such secondary storage systems are unique combinations of non-aqueous, i.e. organic solvent-based and aqueous electrolytes (seawater in case of seawater battery) at the anode and cathode sides, respectively, as shown in Fig. 6a and b. A typical charge–discharge plot for sea water or hybrid Na–air batteries is shown in Fig. 6c. A suite of materials such as Na_0.5Co_0.5Mn_0.5O₂, NaCoPO₄F and K₂Co(PO₃)₄ showing bifunctional activities have been employed as air-cathodes for sea water/hybrid Na–air battery applications.^84,88,114 Manikandan et al. demonstrated an outstanding electrochemical performance of P2-type layered Na_0.5Co_0.5Mn_0.5O₂ oxide electrocatalyst as the cathode for rechargeable seawater battery applications, where the half-cell battery showed a voltage gap of ∼0.78 V with an 80% voltage efficiency for 50 cycles (5 h charge and 5 discharge) at 0.1 mA current. Similarly, NaCoPO₄F and K₂Co(PO₃)₄ showed a potential gap (overpotential) of ∼0.40 and 0.70 V, respectively, in the case of hybrid Na–air batteries. The voltage efficiencies in the case of NaCoPO₄F and K₂Co(PO₃)₄ as the cathode were found to be 88% and 72% at current densities of 10 and 20 μA cm⁻², respectively (Table 3).

Table 3 Metal–air battery performance using selected phosphate-based battery insertion materials as air cathodes (round-trip efficiency calculation based on charge–discharge voltage plateaus)

Electrocatalysts	Electrolyte	Type of metal–air battery	Current density (mA cm^−2)	Open-circuit voltage (OCV in V)	Overpotential gap (V)	Round trip efficiency (%)	Cycles	Ref.
Na0.5Co0.5Mn0.5O2	Sea water	Sea water	0.05	3.10	0.78	80	50	114
NaCoPO4F	0.1 M NaOH	Hybrid Na–air	0.01	3.10	0.40	∼88	30	84
K2Co(PO3)4	0.1 M NaOH	Hybrid Na–air	0.02	2.85	0.70	∼72	14	88
Co2P2O7/CoPi	6.0 M KOH	Zn–air	10	1.41	0.70	∼65	20	65
ZnCoP2O7	6.0 M KOH + 0.2 M Zn(Ac)2	Zn–air	1.0	1.50	0.80	60	25	66
t-Na2CoP2O7	6.0 M KOH + 0.2 M Zn(Ac)2	Zn–air	4.0	1.34	1.13	∼42	20	75
o-Na2CoP2O7	6.0 M KOH + 0.2 M Zn(Ac)2	Zn–air	4.0	1.33	0.93	∼53	20	75
K2CoP2O7	6.0 M KOH + 0.2 M Zn(Ac)2	Zn–air	1.7	1.35	<0.81	∼52	112	87

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Similarly, Zn–air battery fabrication is another sustainable approach towards energy storage employing bifunctional electrocatalytic active materials as cathodes, as illustrated in Fig. 6d. Zn–air batteries comprise Zn–metal as the anode and bifunctional electrocatalysts as the air cathode dipped in 6.0 M KOH electrolyte with or without 0.2 M Zn acetate as an additive. It is schematically shown in Fig. 6e, followed by the typical galvanostatic discharge–charge plot in Fig. 6f. The metal–air battery performance achieved by selected phosphate-based insertion materials as air cathodes is summarised in Table 3, where round trip efficiency was calculated from charge–discharge voltage plateaus. The assembled Zn–air batteries based on phosphate-based non-intercalation materials such as Co₂P₂O₇/CoPi and ZnCoP₂O₇ as air cathodes exhibit high open-circuit voltages (OCVs) of 1.41 V and 1.50 V, respectively, compared to other phosphate-based intercalation materials such as t-Na₂CoP₂O₇o-Na₂CoP₂O₇, and K₂CoP₂O₇ as air cathodes showing OCVs of 1.34 V, 1.33 V and 1.35 V, respectively.^65,66,75,87 The ratio of the discharge to charge voltages at the end of the cycle gives the round-trip efficiency. At current densities of 10, 1.0, 4, and 1.7 mA cm⁻², for Co₂P₂O₇/CoPi, ZnCoP₂O₇, Na₂CoP₂O₇, and K₂CoP₂O₇, respectively, towards the end of the discharge–charge cycling, an overpotential gap of 0.70/0.80, 1.13, 0.93, and <0.81 V was achieved. The roundtrip efficiencies of ∼65, 60, ∼42, ∼53 and ∼52% were obtained for Co₂P₂O₇/CoPi (end of 20 cycles), ZnCoP₂O₇ (end of 25 cycles), t-Na₂CoP₂O₇ (end of 20 cycles), o-Na₂CoP₂O₇ (end of 20 cycles), and K₂CoP₂O₇ (end of 100 cycles), respectively.

6. Summary and outlook

In summary, we have broadly overviewed phosphate-based polyanionic battery insertion materials, which exhibit bifunctional (ORR and OER) electrocatalytic activity. Phosphorus plays vital roles in life, including genome stability, energetics, regulation, and signalling. In the energy storage sector, phosphate-based polyanion materials (e.g. LiFePO₄ and Na₃V₂(PO₄)₂F₃) have realized commercial battery applications combining their low cost and chemical/thermal stability.^24,115 Moreover, exploiting the structure and transition metal centre, many PO₄-based (non) intercalation compounds can work as economic electrocatalysts. Using these compounds as oxygen electrocatalysts, (hybrid) metal–air batteries can be harnessed for energy storage.

Metal–air batteries, like Li–air/O₂ batteries, require advanced electrocatalytic materials that are not expensive and environmentally friendly. Current conventional catalysts such as IrO₂ and RuO₂ are expensive, limiting their practical applications. Polyanionic phosphate-based materials offer a variety of polymorphs that are safer and less harmful to the environment than the La_xSr_yMn_1−yO₃ perovskite-type electrocatalyst, with the highest OER activity known to date. These phosphate-based electrocatalysts can be easily prepared via conventional solid-state and wet chemical routes like a solution-combustion method. Depending on the synthesis conditions (e.g. annealing temperature), various polymorphs with diverse structures can be realized exhibiting electrocatalytic properties. The nanoscale electrodes synthesized via the solution-combustion approach showed superior electrocatalytic properties due to the increased electrode/electrolyte contact area, spatial confinement, one-pot carbon coating, and surface contribution towards adsorption and desorption of intermediates.

The electrochemical properties such as existing working voltage and specific discharge capacity of the polyanionic phosphate-based materials are briefly discussed, followed by their electrocatalytic properties. There are many PO₄-based cathode materials (e.g. Na₂CoPO₄F), which cannot be used in batteries as their electrochemical activity is limited by the unavailability of stable high-voltage electrolytes. Nonetheless, exploiting their bifunctional electrocatalytic behavior, they can be alternately employed in metal–air batteries for energy storage. In addition, intrinsic oxygen electrocatalytic properties of cathode materials can enable spent cathodes to have an additional life after death. For example, the LiNi_1−x−yMn_xCo_yO₂ (NMC)-type oxide cathodes that are commonly used in LIBs can be directly extracted from the dead LIBs and can be converted into nanosized catalysts to improve their bifunctional (ORR/OER) catalytic activities, enabling their application as air-cathode components in zinc-air batteries.¹¹⁶

Recent studies on electrocatalysis have revealed that the underlying electrocatalytic activity of polyanionic phosphate-based materials can be tuned by various factors such as the type of transition metal centre (Co, Ni, Mn, and Fe), alkali cation, and conformation of the phosphate unit as the ligand present in the phosphate framework structures. Their role and impact on the ORR and OER activity have been discussed in this review. Herein, we also summarized the mechanism underlying the variation in activity arising due to the change in T_M, alkali cation and PO₄-unit, which is a correlation between 3d-electrons, e_g occupancy and metal-oxygen covalency. They can be used as an activity descriptor, affirmatively extended from the d-band theory activity descriptor, to explain the mechanisms for oxygen electrocatalysis.

Phosphate-based polyanionic materials offer a rich playground for material discovery with wide structural diversity and polymorphism. The local structure can be further tuned by iso/aliovalent doping, which can affect the final electrocatalytic performance. The nanosizing and degree of crystallinity can be altered to improve their performance. They form economic and eco-friendly candidates for oxygen electrocatalysis, which can be readily employed in (hybrid) Na–air batteries and Zn–air batteries. Most importantly, exploiting the bifunctional activity and integrating the carbon matrices into polyanionic materials during combustion followed by Ar-annealing can have an interesting aspect towards carbon-free air-electrode. It can be an effective strategy to improve the amount of charge storage and prolonged cycle life in metal–air batteries with high active material loadings. With suitable material advancement, these polyanionic phosphates can ideally be implemented in Li–air/O₂ battery applications, which offer a vast area for future research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The current work was financially supported by the Technology Mission Division (Department of Science and Technology, Govt. of India) under the aegis of Materials for Energy Storage (MES-2018) program (DST/TMD/MES/2K18/207), and the Swedish Energy Agency (P2020-90216 and 50674-1). PB thanks the Alexander von Humboldt Foundation (Bonn, Germany) for a 2022 Humboldt fellowship for experienced researchers.

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