Phosphorous acid-mediated novel synthesis of MnPO₄·H₂O and Mn₂P₂O₇ with a unique flower-like morphology: a new approach to manganese phosphate supercapacitors

Abstract

Manganese phosphates in various forms have garnered substantial interest because of their applications in energy storage, catalysis, and materials science. In this study, we present a novel phosphorous acid (H₃PO₃)-mediated approach for the facile synthesis of MnPO₄·H₂O and Mn₂P₂O₇ with a unique flower-like morphology. As precursors, KMnO₄ and H₃PO₃ were chosen to hydrothermally synthesize MnPO₄·H₂O. The crystalline Mn₂P₂O₇ was produced by heating MnPO₄·H₂O for two hours at 700 °C. XRD, FTIR, FESEM, TEM, and other techniques were used to characterize the materials. The monoclinic structure of MnPO₄·H₂O and Mn₂P₂O₇ was confirmed by XRD. A possible formation mechanism is proposed, highlighting the role of H₃PO₃ as both a phosphorus source and a structure-directing agent. The electrochemical study showed that Mn₂P₂O₇ has a specific capacitance of 169 F g⁻¹, whereas that of MnPO₄·H₂O is 63 F g⁻¹ at a current density of 1 A g⁻¹. This approach provides a straightforward, economical, and scalable pathway to manganese phosphates with a unique morphology, paving the way for a potential future material with enhanced performance in electrochemical and catalytic applications.

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1. Introduction

Manganese phosphates, encompassing compounds such as MnPO₄·H₂O and Mn₂P₂O₇, represent a versatile class of materials that have gained substantial attention recently for their promising roles in energy storage systems, catalytic processes, and advanced materials engineering.¹ Transition metal phosphate materials exhibit intriguing characteristics due to their extensive use in electric, magnetic, dielectric, ceramic, laser host, and catalytic procedures.^2,3 A large number of studies have documented the ion exchange properties of transition metal phosphates.^3–5 Transition metal phosphates are extensively used as adsorbents, ionic conductors, catalysts, ion exchangers, catalyst carriers and catalysts due to their diverse structures.⁶ These materials have found applications in supercapacitors, magnetic materials, heat resistant materials, friction materials, and molecular recognition, because of their exceptional physical and chemical characteristics.⁷ As a result, transition metal phosphates have recently gained popularity as a materials science research area.^4,8,9

The phosphate block unit is used to separate the metal phosphate compounds into PO₄³⁻, HPO₄²⁻, H₂PO₄⁻, P₂O₇⁴⁻, and P₄O₁₂⁴⁻ units.¹ Metal(III) phosphates are generally insoluble in water and commonly occur in amorphous, crystalline, or partially crystalline forms. These metal phosphates have varying levels of crystallinity and pH-dependent surface charge characteristics.⁶ The use of MnPO₄·H₂O as a functional material, such as a cathode material for lithium-ion batteries, is made easier by manganese's various oxidation states.^6,8,10 Notably, the morphology of these phosphates plays a critical role in enhancing their performance; hierarchical structures, such as flower-like architectures, offer increased surface area, improved ion diffusion pathways, and better electrochemical accessibility compared to bulk forms. It is crucial to look for a low-cost, straightforward synthesis approach for the bulk production of MnPO₄·H₂O to increase its use.^6,8,11–13 Christensen et al.¹⁴ initially prepared MnPO₄·H₂O by combining manganese(III) nitrate with phosphoric acid and nitric acid, as well as by oxidizing manganese(II) nitrate with phosphoric acid. Other researchers used manganese(II) carbonate as a source and oxidized it by the action of nitric acid and phosphoric acid as part of an additional preparation technique.^15–17 Zhang et al. used Mn(NO₃)₂ with H₃PO₄ to synthesize hydrothermally controlled MnPO₄·H₂O, at 130 °C in 16 h.¹⁸ Hu et al. synthesized microporous manganese phosphate using organophosphonic acid from bulk manganese oxide via a soft templating method.¹⁹ These previously documented techniques involved the evolution of harmful gases, including NO₂ and CO₂, and were time-consuming at high temperatures.²⁰

The precipitation, sol–gel, and hydrothermal processes used to prepare manganese phosphate hydrate allowed for a close mixing of the solution's constituent elements, which facilitated quick homogenous nucleation, producing finer particles and higher-purity materials.^21–24 Mixed orthophosphates, like LiMPO₄ (where M can be Fe, Co, Ni, or Mn), provide good electrochemical performances in lithium-ion battery cathode materials and are another area of interest for solution-based approaches.^17,20–23 Manganese phosphate, when prepared using the typically high-temperature process and using concentrated acid (conc. HNO₃), causes particle aggregation and sintering, which improves its electrochemical performance.²⁰

Many methods were used to prepare MnPO₄·H₂O, and each had its own pros and cons. The precipitation procedure was often used to produce MnPO₄·H₂O. This method involved mixing soluble manganese salts with phosphate sources in water. Although this method is easy to use and reasonably economical, it does not give precise control over crystal size, shape, and purity. The wide size range of the particles produced affects the material's properties and performance.^21–24

Researchers have also used the sol–gel method, which involves breaking down and combining metal alkoxides or salts to create a sol. This sol is then converted to manganese phosphate monohydrate via gelation. While the sol–gel method allows for better control over the composition and consistency of the final product, it is often slow and requires high temperatures during the gelation process.^16,24,25 Researchers have studied microwave irradiation to encourage the nucleation and growth of cobalt manganese phosphate crystals.²⁶ This method reduces the time of synthesis and improves control over particle size. But it is important to carefully manage the synthesis conditions to prevent overheating and ensure consistent microwave energy distribution, as this could cause unwanted side effects.^26,27 The hydrothermal method was chosen for manganese phosphate monohydrate because it produces high-purity, well-defined crystals with better properties. Its scalability, energy efficiency, and eco-friendliness made it a good option for large-scale industrial production of MnPO₄·H₂O.^28,29 In the hydrothermal synthesis method, the reaction takes place in a closed vessel under high temperatures and high pressures.^28,29

For the synthesis of MnPO₄·H₂O, similar challenges persisted despite using various synthesis techniques in the past. These challenges included complex or slow synthesis processes, difficulties in achieving high purity, trouble in controlling crystal size and shape, and the possible use of harmful chemicals or conditions in certain methods.^20,30 To overcome these limitations and boost the production of MnPO₄·H₂O with better properties for various applications, such as environmental clean-up, battery technology, and catalysis, scientists have previously explored various synthesis methods.^20,30 Furthermore, one-pot syntheses that integrate reduction and phosphorylation steps without additional oxidants or pH regulators are underrepresented, particularly for achieving hierarchical flower-like structures in a facile manner. Traditional reductive approaches often employ harsh agents, leading to byproduct formation and purification challenges, while morphology tuning typically demands multi-step processing or specialized precursors.

The present work introduces a phosphorous acid-mediated reductive pathway, which differs fundamentally in reaction chemistry from conventional phosphate-based methods that use Mn(II) or Mn(III) salts as precursors, and enables the formation of unique flower-like morphologies without external templating agents. Here we present hydrothermal synthesis of nanocrystalline MnPO₄·H₂O at 150 °C using only KMnO₄ and H₃PO₃ as precursors. In this approach, H₃PO₃ acts both as a reducing agent and a phosphate source, enabling a simple, rapid, low-cost, and eco-friendly one-pot process that avoids the use of additional manganese salts and does not release any harmful gases. Subsequent calcination at 700 °C conveniently converts MnPO₄·H₂O into Mn₂P₂O₇. To the best of our knowledge, this is the first reported synthesis of manganese phosphates via reduction with phosphorus acid (H₃PO₃). The simplicity of the process, combined with the high phase purity and unique morphology achieved, offers a significant advancement over conventional multi-reagent routes and provides an efficient pathway for producing these materials for potential applications in catalysis, supercapacitors, and battery materials.

2. Experimental

2.1. Materials and chemicals

KMnO₄ (AR grade) and H₃PO₃ (AR grade) were purchased from Fisher Scientific, India. The Teflon-lined stainless steel autoclave (250 mL) was purchased from Shilpa Enterprises, Nagpur, Maharashtra.

2.2. Synthesis

2.2.1. Synthesis of MnPO₄·H₂O. In a beaker, 13.01 g (0.0822 M) KMnO₄ was dissolved in 150 mL of distilled water and stirred for 1 hour (solution A). In another beaker, 13.50 g (0.1644 M) H₃PO₃ was added to 50 mL of distilled water (solution B). The as-prepared solution B was slowly added to solution A under continuous stirring, and the pH of this solution was 1.1. The solution was further stirred for 2 hours under ambient conditions, and the pH of the solution changed to 1.5. The mixture was then hydrothermally treated at 150 °C for 16 hours in a Teflon-lined stainless steel autoclave, followed by filtration. The obtained product was washed multiple times with distilled water to neutralize any remaining acid. The as-synthesized product was dried in an oven at 100 °C for 4 hours to obtain MnPO₄·H₂O nanoparticles.

2.2.2. Synthesis of Mn₂P₂O₇. The above prepared MnPO₄·H₂O was calcined in a muffle furnace at 700 °C for two hours in air to obtain Mn₂P₂O₇. Fig. 1 shows the schematic of the synthesis of MnPO₄·H₂O and Mn₂P₂O₇.

Fig. 1 Schematic illustration of the synthesis of MnPO₄·H₂O and Mn₂P₂O₇.

2.3. Characterization

MnPO₄·H₂O and Mn₂P₂O₇ were characterized using Fourier transform infrared (FTIR) spectroscopy (PerkinElmer Frontier FTIR). FTIR was used to identify the surface functional groups present in MnPO₄·H₂O and Mn₂P₂O₇. X-ray diffraction (XRD) (Bruker D8-Advance Eco) was utilized to analyze their crystalline structures. The surface morphology and structural features of MnPO₄·H₂O and Mn₂P₂O₇ were examined using a JEOL JSM-7600PLUS field emission scanning electron microscope (FE-SEM). High-resolution transmission electron microscopy (HRTEM) images were acquired using a JOEL JEM2100 HRTEM, and the images were obtained using an accelerating voltage of 300 kV. The thermal stability of the materials was studied using thermogravimetric analysis (TGA2 System, Mettler Toledo) under N₂ flow at a heating ramp of 10 °C per minute. The specific surface area was measured by the Brunauer–Emmett–Teller (BET) method using a Microtrac BELSORP MAX G instrument.

2.4. Electrochemical study

Electrochemical charge storage experiments were conducted using a Bio-Logic SP-50e potentiostat in a three-electrode configuration. The working electrode was fabricated by depositing the active material onto carbon paper (1 cm × 3 cm). For electrode preparation, 8 mg of Mn₂P₂O₇ or MnPO₄·4H₂O powder was dispersed with 1 mg of conductive carbon and 1 mg of Nafion binder in 1 mL of ethanol. Following 10 minutes of sonication, the homogeneous suspension was drop-cast onto the carbon paper substrate. The final deposited mass of the sample was 1 mg. Prior to electrochemical testing, the prepared electrodes were dried under an IR lamp. A graphite counter electrode and Hg–HgO reference electrode were utilized, with measurements conducted in 1.0 M KOH electrolyte at ambient temperature.

3. Results and discussion

3.1. Characterization

3.1.1. X-ray diffraction. The XRD pattern of the sample without hydrothermal treatment revealed an amorphous material without any characteristic peaks of manganese phosphate, as shown in Fig. 2(a). The XRD patterns of the hydrothermally treated sample and the calcined sample are shown in Fig. 2(b and c). Lattice parameters of the synthesized samples were calculated from XRD data using MDI JADE 6.0 software by performing peak analysis, phase matching, and refinement. The peaks in Fig. 2(b) were matched with the MnPO₄·H₂O structure (JCPDS# 440071). These findings indicate that MnPO₄·H₂O possesses a monoclinic crystal structure with the space group C2/c. X-ray line broadening was used to calculate the average crystallite size. The average crystallite size for MnPO₄·H₂O was 56.17 nm, calculated from Scherrer's formula (i.e. D = 0.89λ/β cos θ), where λ is the wavelength of Cu-K_α radiation, D is 0.89 (constant), θ is the diffraction angle, and β is the full width at half maximum (FWHM).²⁰ The reflections at 2θ values 18.2(110), 19.1(111̄), 23.8 (020), 25.3(111), 27.2(112̄), 30.2(202̄), 35.6(022), 41.0(311̄), 42.9(221), 47.6(313̄) and 56.2(224̄) match perfectly with the peaks of MnPO₄·H₂O. The lattice parameters of MnPO₄·H₂O closely match those reported in the standard reference JCPDS# 440071. The peaks in Fig. 2(c) at 2θ values 17.1(1̄10), 20.0(001), 28.7(111), 28.9(021), 30.4(2̄01), 34.7(2̄20), 41.6(131), 43.5(221), 49.2(2̄22), and 56.9 (400) were matched with the Mn₂P₂O₇ (JCPDS# 290891). These results indicate that Mn₂P₂O₇ exhibits a monoclinic crystal structure with the space group C2/m. The average crystallite size of 30.82 nm for the Mn₂P₂O₇ sample was calculated using the Scherrer equation.²⁰ The lattice parameters of Mn₂P₂O₇ match those of the standard data (JCPDS# 290891). The XRD data confirm the phase purity of the synthesized material (Table 1).

Fig. 2 (a) Amorphous manganese phosphate, (b) MnPO₄·H₂O and (c) Mn₂P₂O₇ XRD patterns.

Table 1 Average crystallite sizes and structural parameters of MnPO₄·H₂O and Mn₂P₂O₇ calculated from the XRD data

Lattice parameters	MnPO4·H2O			Mn2P2O7
	This work	PDF#440071	Difference (this work-PDF)	This work	PDF#290891	Difference (this work-PDF)
a (Å)	6.947	6.928	0.019	6.657	6.636	0.021
b (Å)	7.483	7.478	0.005	8.592	8.584	0.008
c (Å)	7.386	7.372	0.014	4.540	4.545	−0.005
β (°)	112.28	112.30	−0.020	102.86	102.78	0.080
Average crystallite size	56.17 nm			30.82 nm

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3.1.2. FTIR. The FTIR spectrum (Fig. 3) provides additional evidence for the generation of the MnPO₄·H₂O structure. The fundamental vibrating units, PO₄³⁻ ions and H₂O molecules, are used to identify the FTIR bands. The water bending and stretching vibrations are responsible for the bands that were seen at about 1644 and 3436 cm⁻¹, respectively.¹⁷ Three bands show up in the hydroxyl stretching (3436, 3090, and 2921 cm⁻¹).¹⁷ P–OH stretching is responsible for the primary absorption band, which is centered at 3090 cm⁻¹ and characteristic of hydrogen phosphates.³¹ On the other hand, it is also conceivable that the overtones υ₂(H₂O) and υ₁(H₂O) are assigned to the potential Fermi resonance.³² The band at 1384 cm⁻¹ was observed due to the residual phosphite group, which originates from the incomplete oxidation of H₃PO₃. The band in the region 1300–1400 cm⁻¹ was due to the P–H bending and P–O stretching vibrations of phosphite ions.³³ The other two bands at 3436 and 2921 cm⁻¹ appear as shoulders. When the water interacts effectively through the hydrogen bonds, the first band is attributed to υ(H–O–H). It is possible to attribute the second shoulder to υ(MnO–H), υ(H₃O⁺), or both.^17,31 The doubly degenerate band at 1096 cm⁻¹ and non-degenerate band at 1026 cm⁻¹ appeared due to the splitting of the solitary PO₄³⁻ tetrahedron's triply degenerate asymmetric stretching vibrations into two bands. The peaks at approximately 835, 670, 602, 536, and 406 cm⁻¹ wavenumbers were due to vibrations of υ(P–OH) stretching, δ(Mn–O–H) bending, δ(P–O–Mn), υ4(PO₄³⁻), and υ(Mn–OP), respectively.¹⁷ The two peaks at 835 and 670 cm⁻¹ disappeared in Mn₂P₂O₇ due to the calcination at high temperature of MnPO₄·H₂O.¹⁷ The characterization and analysis presented above confirm the characteristic vibrations of MnPO₄·H₂O.

Fig. 3 FTIR spectra of MnPO₄·H₂O and Mn₂P₂O₇.

The FTIR spectra of Mn₂P₂O₇ shown in Fig. 3 indicate the fundamental vibrations of the P₂O₇⁴⁻ anion. Among the spectrum's most notable characteristics are the strong bands located at 1099, 939, 569, 527, and 422 cm⁻¹. υ_as (PO₃), υ_as (P–O–P), δ (PO₃), δ (PO₃), and ρ (PO₃) can all be attributed to these bands.¹ The symmetric and asymmetric stretching vibrations of the P–O–P bridge (υ_as POP and υ_s POP) observed in the 700–970 cm⁻¹ range confirm P₂O₇⁴⁻ anions with bent P–O–P angles.¹ Comparing the two spectra, it can be clearly seen that the peaks at 835 and 602 cm⁻¹ in MnPO₄·H₂O disappear completely in Mn₂P₂O₇.¹⁷ These two peaks are assigned to the stretching vibrations of P–OH and the bending vibration of Mn–O–H, respectively, which disappear after calcination when the structure is converted to Mn₂P₂O₇.^1,17

3.1.3. SEM. The SEM images show the flower-like morphology of MnPO₄·H₂O and Mn₂P₂O₇ (Fig. 4). MnPO₄·H₂O exhibited uniform polycrystals, consisting of nanoparticles with a size distribution ranging from small particles (200–400 nm) to larger ones (>400–600 nm). This morphology suggests the presence of nucleation sites that lead to nanocrystal growth during hydrothermal treatment, although the exact mechanisms remain unclear. Mn₂P₂O₇ displays a porous structure with agglomerated small particles measuring 80–100 nm. The porous surface structure observed on the particles likely results from the decomposition process.

Fig. 4 SEM micrographs of (a and b) MnPO₄·H₂O and (c and d) Mn₂P₂O₇.

3.1.4. TEM. The TEM micrograph of MnPO₄·H₂O, as shown in Fig. 5(a and b), confirms the nanorod-like structure of the particle. The crystalline nature of the as-synthesized MnPO₄·H₂O and the calcined Mn₂P₂O₇ was primarily confirmed by powder XRD, which showed sharp diffraction peaks indexed to the monoclinic phases. The SAED pattern of Mn₂P₂O₇ aligns well with the XRD data, which confirms its monoclinic crystal structure (space group C2/m). The key d-spacings include 3.08 Å (021), 2.94 Å (2̄01), 2.58 Å (220), and 2.17 Å (131), which would produce rings at corresponding reciprocal distances. The observed pattern's ring distribution and spotty nature are consistent with nanocrystalline Mn₂P₂O₇, often seen in derivatives from thermal decomposition of precursors like manganese phosphates.³⁴

Fig. 5 TEM micrographs of MnPO₄·H₂O (a) and (b) and Mn₂P₂O₇ (d) and (e), and SAED patterns of (c) MnPO₄·H₂O and (f) Mn₂P₂O₇.

In comparison, MnPO₄·H₂O has a distinct monoclinic structure (space group C2/c). Its prominent d-spacings are larger overall, such as 4.86 Å (110), 4.64 Å (111̄), 3.51 Å (111), 3.32 Å (112̄), 2.95 Å (202̄), and 2.52 Å (022). An SAED pattern for MnPO₄·H₂O would thus feature inner rings at smaller reciprocal distances (larger d-values) compared to Mn₂P₂O₇, reflecting its different unit cell and bonding (orthophosphate PO₄³⁻ groups with coordinated water vs. pyrophosphate P₂O₇⁴⁻ in Mn₂P₂O₇).

3.1.5. Thermogravimetric analysis. Thermogravimetric analysis (TGA) of MnPO₄·H₂O and Mn₂P₂O₇ reveals distinct thermal decomposition stages, as shown in Fig. 6. The TGA graph of MnPO₄·H₂O indicates an initial weight loss of approximately 6% from 100 °C to 320 °C, primarily attributed to the dehydration of the monohydrate, where the coordinated water molecule is released, forming anhydrous MnPO₄. A subsequent, more significant weight loss of 18% occurs between 320 °C and 510 °C, likely due to the decomposition of the phosphate structure, possibly involving the release of volatile components or structural rearrangement, reducing the sample to 84 wt%.¹³ Beyond 510 °C, the weight stabilizes at approximately 83%, indicating the formation of a thermally stable phase of Mn₂P₂O₇, with no further significant mass loss. The TGA graph of Mn₂P₂O₇ shows no significant weight loss until 900 °C, indicating a thermally stable material.

Fig. 6 TGA plot of MnPO₄·H₂O and Mn₂P₂O₇.

3.1.6. BET surface area. The surface area of manganese pyrophosphate (Mn₂P₂O₇) and manganese phosphate monohydrate (MnPO₄·H₂O) was determined using the Brunauer–Emmett–Teller (BET) method. Mn₂P₂O₇ showed a typical type IV isotherm, whereas MnPO₄·H₂O showed an open hysteresis loop due to slow kinetics in the micropores or structural changes. Mn₂P₂O₇ exhibited a moderate BET surface area of 25.0 m² g⁻¹, indicating a reasonably porous structure suitable for applications requiring good accessibility of the active sites. In contrast, MnPO₄·H₂O showed a significantly lower BET surface area of 8.1 m² g⁻¹, suggesting a more compact or less porous morphology with reduced external surface available for interaction. These differences in surface area between the two manganese phosphate compounds can influence their performance in catalytic, electrochemical, or adsorption-related processes (Fig. 7).

Fig. 7 BET N₂ adsorption isotherm of MnPO₄·H₂O and Mn₂P₂O₇.

3.2. Plausible mechanism of MnPO₄·H₂O and Mn₂P₂O₇ formation

The synthesis of manganese phosphate monohydrate (MnPO₄·H₂O) using phosphorous acid (H₃PO₃) and potassium permanganate (KMnO₄) as the sole manganese source involves a redox-driven precipitation reaction under controlled conditions. KMnO₄ is a strong oxidizing agent in aqueous solution, where it dissociates to form permanganate ions (MnO₄⁻). The manganese in MnO₄⁻ is in the +7 oxidation state (Mn(VII)). In the presence of a reducing agent, MnO₄⁻ can be reduced to lower oxidation states, such as Mn(III), which is required for the formation of MnPO₄·H₂O, where manganese exists as Mn³⁺.³⁵ The progress of the reaction was monitored by UV-vis spectroscopy (Fig. S1) and the colour change of the solution. The colour of the solution changed from purple (Mn⁷⁺) to brown pink (Mn³⁺), which was also indicated by the decrease in absorbance of typical peaks of MnO₄⁻ species (490–570 nm) as H₃PO₃ was added to the KMnO₄ solution. Possibly, some Mn²⁺ also forms transiently or as an intermediate during the initial reduction step. However, the hydrothermal treatment oxidises any remaining Mn²⁺ phase (or dissolved Mn²⁺) to the Mn³⁺ product, because of the presence of MnO₄⁻.

Phosphorous acid (H₃PO₃) serves a dual purpose. First, it acts as a reducing agent, capable of reducing Mn(VII) in MnO₄⁻ to Mn(III). Second, upon oxidation, H₃PO₃ is converted to phosphate ions (PO₄³⁻), which are essential for forming the MnPO₄·H₂O product. The oxidation of H₃PO₃ to H₃PO₄ (phosphoric acid) provides the phosphate groups necessary for the precipitation reaction. H₃PO₃ plays another significant role as a structure-directing agent. The presence of PO₃³⁻ ions in the solution helps in the development of a unique flower-like morphology as was evidenced in the FESEM images.^36,37

Redox reaction: The synthesis begins with the redox reaction between MnO₄⁻ and H₃PO₃ (eqn (1)). In an acidic aqueous medium, the permanganate ion is reduced, and H₃PO₃ is oxidized. The simplified redox reaction can be represented as follows:

Reduction half-reaction:

MnO₄⁻ + 8H⁺ + 4e⁻ → Mn³⁺ + 4H₂O (1)

Here, Mn(VII) is reduced to Mn(III).

Oxidation half-reaction:

H₃PO₃ + H₂O → H₃PO₄ + 2H⁺ + 2e⁻ (2)

H₃PO₃ is oxidized to H₃PO₄, releasing phosphate ions (PO₄³⁻) upon dissociation of H₃PO₄ in solution (eqn (2)).

The overall redox reaction can be written as

MnO₄⁻ + H₃PO₃ + 4H⁺ → Mn³⁺ + H₃PO₄ + 2H₂O (3)

Formation of MnPO₄·H₂O: once Mn³⁺ and PO₄³⁻ ions are generated in the solution, they combine to form the insoluble MnPO₄·H₂O precipitate (eqn (4)). The reaction can be written as:

Mn³⁺ + PO₄³⁻ + H₂O → MnPO₄·H₂O (s) (4)

The monohydrate form incorporates one water molecule per formula unit, which is typical for manganese phosphate synthesized under aqueous conditions. The reaction conditions, such as pH, temperature, and concentration, are critical to ensure the formation of the monohydrate phase with high crystallinity and purity.

The synthesis typically occurs in an aqueous medium under mild conditions resulting in the formation of an amorphous manganese phosphate. Treatment of this amorphous manganese phosphate by the hydrothermal method at 150 °C results in a crystalline MnPO₄·H₂O phase as confirmed by X-ray diffraction analysis. The pH of the solution influences the dissociation of H₃PO₄ and the stability of the Mn³⁺ ions. An acidic (pH ∼ 1.5) environment is maintained to facilitate the redox reaction and prevent the formation of undesirable manganese oxides or hydroxides.

Upon heating to 700 °C, the anhydrous MnPO₄ undergoes a condensation reaction, where two orthophosphate units combine, releasing oxygen to form manganese pyrophosphate (Mn₂P₂O₇) (eqn (5)).³⁸ The reaction can be written as:

2MnPO₄ → Mn₂P₂O₇ + ½O₂ (5)

Mn₂P₂O₇, which is a thermally stable pyrophosphate phase, consists of P₂O₇⁴⁻ anions (pyrophosphate groups) coordinated with Mn²⁺ cations, forming a crystalline lattice. The transformation of MnPO₄·H₂O to Mn₂P₂O₇ upon calcining at 700 °C proceeds through dehydration to form MnPO₄, followed by thermal decomposition and condensation of the phosphate units, resulting in the stable pyrophosphate phase. The TGA data support this, with the observed weight losses aligning with the loss of water and structural reorganisation, resulting in the formation of Mn₂P₂O₇.

H₃PO₃'s dual role as a reducing agent and phosphate source further streamlines the process, reducing the number of reagents and minimizing byproducts. The redox-driven mechanism enables precise control over the formation of MnPO₄·H₂O, ensuring high phase purity with unique morphology. This synthesis method for MnPO₄·H₂O under appropriate aqueous conditions and subsequent calcination to produce Mn₂P₂O₇ offers an efficient route to this material for applications in catalysis, supercapacitors, battery materials, and other fields.

3.3. Electrochemical performance of Mn₂P₂O₇

To study the electrochemical properties of Mn₂P₂O₇ and MnPO₄·H₂O, a three-electrode configuration was employed with 1 M KOH electrolyte, a reference electrode of Hg–HgO, and a counter electrode made of graphite. Cyclic voltammetry (CV) measurements of Mn₂P₂O₇ conducted at various scan rates revealed that the capacitance contribution predominantly occurs in the positive potential window (−0.04 V to 0.67 V vs. Hg–HgO), as shown in Fig. 8. The CV curve of Mn₂P₂O₇ at 5 mV s⁻¹ exhibited two distinct redox peaks at approximately 0.38 V and 0.47 V (vs. Hg–HgO).

Fig. 8 (a) CV curves of Mn₂P₂O₇ at various scan rates, (b) GCD curves of Mn₂P₂O₇ at different current densities, (c) specific capacitances of Mn₂P₂O₇ vs. current density, (d) CV curves of MnPO₄·H₂O at various scan rates, (e) GCD curves of MnPO₄·H₂O at different current densities, and (f) specific capacitances of MnPO₄·H₂O vs. current density.

Similarly, MnPO₄·4H₂O displayed capacitive behaviour in the positive potential range (−0.26 V to 0.8 V) at 10 mV s⁻¹ scan rate. The CV profile of MnPO₄ showed characteristic peaks at 0 V and 0.6 V. To further assess the charge storage capabilities of these materials, galvanostatic charge/discharge (GCD) measurements were performed by varying the current density from 1 to 20 A g⁻¹.

The following formula was used to calculate specific capacitance (C_s, F g⁻¹) from the GCD curves:

C_s = 2I∫Vdt/m(ΔV)²

where I represents the applied current (mA), m is the active material mass (g), ∫Vdt denotes the area under the discharge curve in a GCD, and ΔV is the potential window.

GCD analysis revealed that Mn₂P₂O₇ showed a specific capacitance value of 169 F g⁻¹ at 1 A g⁻¹ current density. Although the specific capacitance decreased upon increasing current density, Mn₂P₂O₇ still retained 40% of its initial specific capacitance value even at a high current density. In contrast, MnPO₄ exhibited a lower specific capacitance value of 63 F g⁻¹ at 1 A g⁻¹ and showed a drastic decrease in specific capacitance (5.5%) at higher current density (20 A g⁻¹), yielding only 3.5 F g⁻¹.³⁹

When compared with MnPO₄·H₂O, Mn₂P₂O₇ exhibits a markedly superior electrochemical performance. At a current density of 1 A g⁻¹, Mn₂P₂O₇ delivers a specific capacitance of 169 F g⁻¹, which is nearly 2.7 times higher than that of MnPO₄·H₂O (63 F g⁻¹). This significant enhancement can be attributed to the distinct phosphate framework and higher density of electrochemically active manganese sites in Mn₂P₂O₇, which facilitate more effective faradaic redox reactions (Mn²⁺/Mn³⁺). Additionally, the pyrophosphate structure is likely to provide improved structural robustness and better ion diffusion pathways compared to the hydrated orthophosphate phase, thereby contributing to enhanced charge-storage capability. The relatively lower capacitance of MnPO₄·H₂O may arise from its more compact crystal structure and the presence of coordinated water molecules, which can impede electron transport and limit the utilization of the active material. The electrochemical performance of Mn₂P₂O₇, with a specific capacitance of 169 F g⁻¹ at a current density of 1 A g⁻¹, highlights its potential as a promising pseudocapacitive electrode material for supercapacitor applications. This moderate yet significant capacitance, achieved in a pure and unmodified form, reflects the intrinsic redox activity of manganese species and the reversible faradaic charge-storage mechanism of the phosphate framework. Owing to its structural stability, earth abundance, and environmentally benign nature, Mn₂P₂O₇ is particularly attractive for cost-effective and sustainable energy storage systems. While the relatively low electrical conductivity of phosphate-based materials may limit its performance, Mn₂P₂O₇ can serve as an efficient base material for further performance enhancement through nanostructuring or hybridization with conductive matrices. Therefore, Mn₂P₂O₇ holds considerable potential for use in next-generation supercapacitors, especially in composite or asymmetric device configurations where improved power and energy densities are desired.³⁹

Mn₂P₂O₇ exhibited excellent specific capacity retention (100%) over 250 cycles at a current density of 5 A g⁻¹ as shown in Fig. S3. Cyclic voltammetry (CV) at varying scan rates (5 to 100 mV s⁻¹), was conducted within the selected potential window (0 V to 0.4 V), to calculate the electrochemically active surface area. The ECSA values are found to be 0.41 cm² for the Mn₂P₂O₇ sample.

BET surface area calculations showed that Mn₂P₂O₇ possessed a higher surface area (25.0 m² g⁻¹) than MnPO₄·H₂O (8.1 m² g⁻¹). Since capacitance directly correlates with the electrode surface area, MnPO₄·H₂O's limited surface area likely hindered its charge storage performance. This structural limitation explains why MnPO₄·H₂O exhibited substantially lower capacitance than Mn₂P₂O₇, particularly at higher charging rates, where efficient charge distribution becomes crucial.

Although the specific capacitance of our Mn₂P₂O₇ (169 F g⁻¹ at 1 A g⁻¹) is lower than certain nanostructured or composite Mn₂P₂O₇ electrodes reported recently, it remains competitive among pure-phase materials synthesized via simple routes (Table 2). The performance gap relative to binder-free Ni-foam or polymer–composite electrodes is primarily attributed to differences in electrode configuration, conductivity, and mass loading. The flower-like morphology obtained through our H₃PO₃-mediated approach provides a good ion-accessible surface area while maintaining structural stability after calcination. Our future plans include making nanocomposites of Mn₂P₂O₇ with other conductive materials to improve the supercapacitor performance.

Table 2 Comparison of the specific capacitance of Mn₂P₂O₇ (and the composites) reported in recent literature

Material	Specific capacitance	Current density (A g^−1)	Electrolyte (M KOH)	Electrode configuration	Ref.
Flower-like Mn2P2O7	169 F g^−1	1	1	Drop-cast on carbon paper	This work
Mn2P2O7 nanosheets (freeze-dried)	∼140–180 F g^−1	1	3	Ni foam	40
Mn2P2O7 nanoclusters	∼120–160 F g^−1	1	1	Three electrode	41
Mn2P2O7 (from Mn-organic phosphate, calcined at 550 °C)	230.9 F g^−1	0.5	3	Three electrode	39
Mn2P2O7@polypyrrole (sunflower-like composite)	658 F g^−1	1	1	Ni foam	42
Mn2P2O7 (in PANI hybrid study)	750 C g^−1	1	4	Three electrode	43

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4. Conclusion

In conclusion, this study demonstrates a novel and efficient hydrothermal synthesis of MnPO₄·H₂O using KMnO₄ and H₃PO₃ as precursors, followed by thermal treatment at 700 °C for 2 hours to yield crystalline Mn₂P₂O₇. Unlike conventional methods that often require multiple manganese salts or complex phosphate sources, this approach streamlines the reaction by leveraging the strong oxidizing properties of KMnO₄ and the dual functionality of H₃PO₃. This reduces the number of reagents, minimizes byproducts, and enhances phase purity, as confirmed by XRD analysis showing pure monoclinic phases for both compounds. These findings contribute to the advancement of the synthesis methodologies for manganese-based materials, with potential applications in catalysis, energy storage, anti-corrosion coatings, etc., paving the way for further exploration of tailored reaction conditions to optimize crystallite size and phase purity. Mn₂P₂O₇ demonstrated a specific capacitance of 169 F g⁻¹ at 1 A g⁻¹, highlighting Mn₂P₂O₇ as a promising manganese phosphate-based electrode material and a viable platform for further optimization toward high performance supercapacitor applications.

Author contributions

Prakash Bobde: writing – review and editing, writing – original draft, validation, methodology, and investigation; Kajal Uphade: investigation; Sharath Kandambeth: investigation; Shikha Wadhwa: writing – review and editing, and resources; Ranjit Kumar: writing – review and editing, writing – original draft, resources, and conceptualisation.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors declare that all the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5ma01523e.

Acknowledgements

We acknowledge the characterization facility at the Indian Institute of Petroleum, Dehradun, for FESEM and TEM analysis. We would also like to thank the central characterization facility at UPES, Dehradun, for this study.

References

1. B. Boonchom, R. Baitahe, Z. Joungmunkong and N. Vittayakorn, Grass blade-like microparticle MnPO₄·H₂O prepared by a simple precipitation at room temperature, Powder Technol., 2010, 203(2), 310–314,  DOI:10.1016/J.POWTEC.2010.05.022.
2. H. Zhao and Z. Y. Yuan, Insights into Transition Metal Phosphate Materials for Efficient Electrocatalysis, ChemCatChem, 2020, 3797–3810,  DOI:10.1002/cctc.202000360.
3. S. Karafiludis, T. W. Ryll, A. G. Buzanich, F. Emmerling and T. M. Stawski, Phase stability studies on transition metal phosphates aided by an automated synthesis, CrystEngComm, 2023, 25(30), 4333–4344,  10.1039/d3ce00386h.
4. G. Renaudin, S. Gomes and J. M. Nedelec, First-row transition metal doping in calcium phosphate bioceramics: A detailed crystallographic study, Materials, 2017, 10(1), 92,  DOI:10.3390/ma10010092.
5. X. Li, X. Xiao, Q. Li, J. Wei, H. Xue and H. Pang, Metal (M = Co, Ni) phosphate based materials for high-performance supercapacitors, Inorg. Chem. Front., 2018, 5(1), 11–28,  10.1039/C7QI00434F.
6. G. Qiu, Z. Gao, H. Yin, X. Feng, W. Tan and F. Liu, Synthesis of MnPO₄·H₂O by refluxing process at atmospheric pressure, Solid State Sci., 2010, 12(5), 808–813,  DOI:10.1016/j.solidstatesciences.2010.02.014.
7. G. Tatrari, M. Karakoti, C. Tewari, S. Pandey, B. S. Bohra and A. Dandapat, et al., Solid waste-derived carbon nanomaterials for supercapacitor applications: a recent overview, Mater. Adv., 2021, 2(5), 1454–1484,  10.1039/D0MA00871K.
8. N. Stegmann, C. Ochoa-Hernández, K. N. Truong, H. Petersen, C. Weidenthaler and W. Schmidt, The Mechanism and Pathway of Selective Partial Oxidation of n-Butane to Maleic Anhydride Studied on Titanium Phosphate Catalysts, ACS Catal., 2023, 13(24), 15833–15840,  DOI:10.1021/acscatal.3c03172.
9. C. Nithya, Transition Metal Phosphates/Phosphonates for Lithium-Ion Batteries, in Engineering Materials, 2023, pp. 283–299 DOI:10.1007/978-3-031-27062-8_16.
10. T. Liu, C. Chen, S. Liu, Z. Chen, Z. Pu and Q. Huang, et al., Transition metal phosphides as noble-metal-alternative co-catalysts for solar hydrogen production, Coord. Chem. Rev., 2024, 521, 216145,  DOI:10.1016/j.ccr.2024.216145.
11. N. L. W. Septiani, Y. V. Kaneti, K. B. Fathoni, K. Kani, A. E. Allah and B. Yuliarto, et al., Self-assembly of two-dimensional bimetallic nickel-cobalt phosphate nanoplates into one-dimensional porous chainlike architecture for efficient oxygen evolution reaction, Chem. Mater., 2020, 32(16), 7005–7018,  DOI:10.1021/acs.chemmater.0c02385.
12. D. Zhang, L. L. Hu, Y. G. Sun, J. Y. Piao, X. S. Tao and Y. S. Xu, et al., Construction of uniform transition-metal phosphate nanoshells and their potential for improving Li-ion battery performance, J. Mater. Chem. A, 2018, 6(19), 8992–8999,  10.1039/c8ta01320a.
13. Y. Wang, G. Hu, Z. Peng, K. Du, B. Zhang and Y. Cao, Self-assembly synthesis of dumbbell-like MnPO₄·H₂O hierarchical particle and its application in LiMnPO₄/C cathode materials, Ceram. Int., 2021, 47(14), 19687–19693,  DOI:10.1016/J.CERAMINT.2021.03.306.
14. O. T. Christensen, Beiträge zur Kenntniss der Oxyde des Mangans, J. Prakt. Chem., 1883, 28(1), 1–37,  DOI:10.1002/PRAC.18830280101.
15. X. Bao, W. Bin Zhang, L. Zhang, Y. W. Guo, X. Zhou and X. L. Zhang, et al., Alkali cation intercalation manganese phosphate hydrate boosting electrochemical kinetics for pseudocapacitive energy storage, J. Materiomics, 2022, 8(4), 833–842,  DOI:10.1016/j.jmat.2022.01.005.
16. Y. Wang, G. Hu, Z. Peng, K. Du, B. Zhang and Y. Cao, Self-assembly synthesis of dumbbell-like MnPO₄·H₂O hierarchical particle and its application in LiMnPO₄/C cathode materials, Ceram. Int., 2021, 47(14), 19687–19693,  DOI:10.1016/j.ceramint.2021.03.306.
17. B. Boonchom, S. Youngme, S. Maensiri and C. Danvirutai, Nanocrystalline serrabrancaite (MnPO₄·H₂O) prepared by a simple precipitation route at low temperature, J. Alloys Compd., 2008, 454(1–2), 78–82,  DOI:10.1016/J.JALLCOM.2006.12.064.
18. Y. Zhang, Y. Liu, S. Fu, F. Guo and Y. Qian, Hydrothermally Controlled Growth of MnPO₄·H₂O Single-Crystal Rods, Bull. Chem. Soc. Jpn., 2006, 79(2), 270–275,  DOI:10.1246/bcsj.79.270.
19. Y. Hu, Y. Zhang, C. Li, L. Wang, Y. Du and G. Mo, et al., Guided Assembly of Microporous/Mesoporous Manganese Phosphates by Bifunctional Organophosphonic Acid Etching and Templating, Adv. Mater., 2019, 31(25), 1–8,  DOI:10.1002/adma.201901124 , PubMed PMID: 31062894.
20. B. Boonchom, S. Youngme, S. Maensiri and C. Danvirutai, Nanocrystalline serrabrancaite (MnPO₄·H₂O) prepared by a simple precipitation route at low temperature, J. Alloys Compd., 2008, 454(1–2), 78–82,  DOI:10.1016/j.jallcom.2006.12.064.
21. S. Yang, X. Li, Y. Li, Y. Wang, X. Jin and L. Qin, et al., Effect of Proton Transfer on Electrocatalytic Water Oxidation by Manganese Phosphates, Angew. Chem., Int. Ed., 2023, 62(1), e202215594,  DOI:10.1002/anie.202215594 , PubMed PMID: 36342503.
22. S. Yang, K. Yue, X. Liu, S. Li, H. Zheng and Y. Yan, et al., Electrocatalytic water oxidation with manganese phosphates, Nat. Commun., 2024, 15(1), 1410,  DOI:10.1038/s41467-024-45705-1 , PubMed PMID: 38360868.
23. J. Yang, T. Yu, X. Jiang, X. Zhang, J. Guo and Y. Chen, et al., Hydrated manganese hydrogen phosphate coated membrane with excellent anticrude oil-fouling property for separating crude oil from diverse wastewater, Surf. Coat. Technol., 2023, 454,  DOI:10.1016/j.surfcoat.2022.129215.
24. S. Munkaila, R. Dahal, M. Kokayi, T. Jackson and B. P. Bastakoti, Hollow Structured Transition Metal Phosphates and Their Applications, Chem. Rec., 2022, 22(8), e202200084,  DOI:10.1002/tcr.202200084 , PubMed PMID: 35815949.
25. A. Bouddouch, E. Amaterz, R. Haounati, Y. Naciri, A. Taoufyq and B. Bakiz, et al., Synthesis, characterization and luminescence properties of manganese phosphate Mn₃(PO₄)₂, Mater. Today: Proc., 2020, 16–21,  DOI:10.1016/j.matpr.2019.08.058.
26. J. Cherusseri, S. A. Thomas, A. K. Pandey, M. A. Zaed, N. K. Farhana and R. Saidur, Rapid synthesis of cobalt manganese phosphate by microwave-assisted hydrothermal method and application as positrode material in supercapatteries, Sci. Rep., 2024, 14(1), 1–18,  DOI:10.1038/s41598-024-77278-w , PubMed PMID: 39489744.
27. S. H. Jhung, J. S. Chang, J. W. Yoon, J. M. Grenèche, G. Férey and A. K. Cheetham, Synthesis of transition-metal-incorporated nickel phosphate molecular sieves TMI-VSB-1, Chem. Mater., 2004, 16(26), 5552–5555,  DOI:10.1021/cm049081e.
28. J. Zhao, Q. Jing, T. Zhou, X. Zhang, W. Li and H. Pang, Controllable Synthesis of Manganese Organic Phosphate with Different Morphologies and Their Derivatives for Supercapacitors, Molecules, 2024, 29(17), 4186,  DOI:10.3390/molecules29174186 , PubMed PMID: 39275034.
29. Y. Zhang, Y. Liu, S. Fu, F. Guo and Y. Qian, Hydrothermally controlled growth of MnPO₄·H₂O single-crystal rods, Bull. Chem. Soc. Jpn., 2006, 79(2), 270–275,  DOI:10.1246/bcsj.79.270.
30. H. Uchiyama, I. Okumura, S. Inoue, M. Kato and M. Wakahara, Aqueous Synthesis of Manganese Phosphate Hydrate Crystals for Creating Inorganic Pigment Materials, Inorg. Chem., 2021, 60(19), 14779–14785,  DOI:10.1021/acs.inorgchem.1c02008 , PubMed PMID: 34553908.
31. M. A. G. Aranda and S. Bruque, Characterization of manganese(III) orthophosphate hydrate, Inorg. Chem., 2002, 29(7), 1334–1337,  DOI:10.1021/IC00332A009.
32. G. Herzberg and L. C. Bryce, Infrared and Raman Spectra of Polyatomic Molecules, J. Phys. Chem., 2002, 50(3), 288,  DOI:10.1021/J150447A021.
33. M. Tsuboi, Vibrational Spectra of Phosphite and Hypophosphite Anions, and the Characteristic Frequencies of PO₃- and PO₂- Groups, J. Am. Chem. Soc., 2002, 79(6), 1351–1354,  DOI:10.1021/JA01563A026.
34. Y. Huang, N. A. Chernova, Q. Yin, Q. Wang, N. F. Quackenbush and M. Leskes, et al., What Happens to LiMnPO₄ upon Chemical Delithiation?, Inorg. Chem., 2016, 55(9), 4335–4343,  DOI:10.1021/acs.inorgchem.6b00089.
35. Y. Song, J. Jiang, J. Ma, S. Y. Pang, C. Luo and W. Qin, Oxidation of inorganic compounds by aqueous permanganate: Kinetics and initial electron transfer steps, Sep. Purif. Technol., 2017, 183, 350–357,  DOI:10.1016/j.seppur.2017.04.015.
36. Y. Zhu, G. Hasegawa, K. Kanamori and K. Nakanishi, Comprehensive studies on phosphoric acid treatment of porous titania toward titanium phosphate and pyrophosphate monoliths with pore hierarchy and a nanostructured pore surface, Inorg. Chem. Front., 2018, 5(6), 1397–1404,  10.1039/C8QI00146D.
37. J. C. Munyemana, H. He, S. Ding, J. Yin, P. Xi and J. Xiao, Synthesis of manganese phosphate hybrid nanoflowers by collagen-templated biomineralization, RSC Adv., 2018, 8(5), 2708–2713,  10.1039/c7ra12628j.
38. B. Boonchom, S. Youngme, S. Maensiri and C. Danvirutai, Nanocrystalline serrabrancaite (MnPO₄·H₂O) prepared by a simple precipitation route at low temperature, J. Alloys Compd., 2008, 454(1–2), 78–82,  DOI:10.1016/J.JALLCOM.2006.12.064.
39. J. Zhao, Q. Jing, T. Zhou, X. Zhang, W. Li and H. Pang, Controllable Synthesis of Manganese Organic Phosphate with Different Morphologies and Their Derivatives for Supercapacitors, Molecules, 2024, 29(17), 4186,  DOI:10.3390/molecules29174186 , PubMed PMID: 39275034.
40. H. Chevulamaddi and V. R. Kalagadda, Impressive electrochemical characteristics of freeze-dried nanosheet structured Mn₂P₂O₇ for affordable electrochemical capacitor, Surf. Interfaces, 2024, 55, 105340,  DOI:10.1016/J.SURFIN.2024.105340.
41. H. Chevulamaddi and V. R. Kalagadda, Redox Mediator–assisted Electrochemical Behaviour of Mn₂P₂O₇ Nanoclusters for Symmetric Supercapacitors, Fuel, 2025, 398, 135543,  DOI:10.1016/J.FUEL.2025.135543.
42. R. BoopathiRaja, S. Vadivel, M. Parthibavarman, S. Prabhu and R. Ramesh, Effect of polypyrrole incorporated sun flower like Mn₂P₂O₇ with lab waste tissue paper derived activated carbon for asymmetric supercapacitor applications, Surf. Interfaces, 2021, 26, 101409,  DOI:10.1016/J.SURFIN.2021.101409.
43. E. A. Alabdullkarem and J. Khan, Mn₂P₂O₇–polyaniline hybrid composite as a promising electrode material for advanced symmetric supercapacitors, RSC Adv., 2025, 15(30), 24831–24843,  10.1039/D5RA04149J.

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