Mezo-gözenekli M2P2O7 (M: Co(II), Ni(II), ve Mn(II)) ve Mn2-xMxP2O7 (M: Co(II) And Ni(II)) ince filmlerinin sentezinde çözücünün rolü ve oksijen evrim reaksiyonu performansları

Mesoporous transition metal pyrophosphates (M2P2O7) have attracted considerable attention due to their diverse structural features and wide range of potential applications in catalysis, ion exchange, and energy storage. This study presents a novel synthesis route for high surface area and porous M2P2O7 materials, focusing on the influence of solvent selection on their structural and electrochemical properties. Co2P2O7, Ni2P2O7, Mn2P2O7, and binary metal pyrophosphates have been synthesized via a molten salt-assisted self-assembly (MASA) method using pyrophosphoric acid (PPA, H4P2O7), salt [Mn(H2O)4](NO3)2, [Co(H2O)6](NO3)2, and [Ni(H2O)6](NO3)2), surfactant (pluronic, P123), and solvents (ethanol, butanol, and methanol). Sol-gel process takes place both in ethanol and butanol, while methanol-based solutions remained cloudy. After centrifugation, the solution, supernatant, and precipitate parts are coated onto substrates to form lyotropic liquid crystalline (LLC) mesophases, followed by calcination at different temperatures to obtain mesoporous metal pyrophosphates. Structural evolution was monitored by small-angle XRD and ATR-FTIR techniques. Aging of the supernatants at room temperature produce semi-solid mesostructures via polymerization between metal species and PPA. Calcination between 300 and 700 ⁰C yields highly porous M2P2O7 materials with BET surface areas of 145, 410, and 83 m2/g for Co(II), Ni(II) and Mn(II) pyrophosphates, respectively. XRD analysis shows that Ni2P2O7 crystallizes at 700⁰C, regardless of the solvent, while the Co2P2O7 and Mn2P2O7 samples crystallize at lower temperatures. Supernatant-derived samples mainly form M3(PO4)2, metal oxides, and some undefined impurities. High crystallization temperatures results in the pores collapse and surface area loss. Electrodes are fabricated on FTO and graphite rod (GR) substrates; between these two electrodes, the GR coated electrodes have a better electrochemical stability. Electrochemical measurements (cyclic voltammetry (CV), multi-step chronoamperometry (MCA), and multi-step chronopotentiometry (MCP)) were performed in 1 M KOH using a three-electrode system. Post electrochemical analysis reveales that M2P2O7 transforms into metal hydroxide in alkaline media. To evaluate the intrinsic activity, CVs were also conducted in a neutralized electrolyte (2.4 M KOH + 0.6 M H4P2O7) solution, where the metal hydroxides exhibite superior OER performance. The Mn2P2O7 electrodes are unstable due to disproportionation of Mn(VI) to MnO4- and MnO2 during OER. To improve performance, Mn2-xMxP2O7 (M=Co(II), Ni(II); x=0.05,0,10, 0,25) binary electrodes are also fabricated, exhibiting higher surface areas and better OER activity. Notably, ethanol- derived MnNi25% (with 25% Ni(II)) and butanol-derived MnCo10% (10% Co(II)) precipitates, calcined at 300⁰C, display high surface areas of 202 and 144 m2/g, respectively. These electrodes display high stability in OER and have overpotentials of 280-300 mV at 1 mA/cm2 current density.