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Highly efficient and robust electrocatalysts intended for the oxygen evolution reaction (OER) are essential for energy conversion devices; the structural and morphological fractions of the electrocatalyst can also greatly influence the OER performance. Therefore, developing a high-performing electrocatalyst with desired properties is crucial through a simple and cost-effective chemical process. So, the binder-free, hydrous cobalt phosphate (Co3(PO4)2.nH2O) thin film electrocatalysts are prepared via the successive ionic layer adsorption and reaction (SILAR) method onto stainless steel (SS) substrates at ambient temperature. Additionally, the impact of annealing on the OER efficiency of thin film electrodes made of hydrous cobalt phosphate was observed by subjecting the electrocatalysts to different temperatures (200°C and 400°C). The SILAR synthesized hydrous Co3(PO4)2.nH2O with the short-range ordered agglomerated particles transformed into discrete nanoparticles with an annealing temperature. The as-prepared hydrous cobalt phosphate (CP) demonstrated outstanding OER performance with the least overpotential (η) of 265 mV at 10 mA cm-2 current density and the lowest Tafel slope of 37 mV dec-1, and the overpotential ( η 10 ) increased upon the annealing of catalysts (CP 200 and CP 400). Moreover, the as-prepared electrocatalyst demonstrated overall water splitting at the lowest potential of 1.56 V (@10 mA cm-2) in the alkaline electrolysis system (CP//Pt). The present study reveals that the electrocatalytic performance of the as-prepared cobalt phosphate thin film catalyst is significantly associated with the hydrous content present in catalysts and demonstrates the practical applicability of SILAR-synthesized binder-free cobalt phosphate thin film electrocatalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microflower to sheet evolution observed in Ni 3-x Co x (PO 4) 2 with Ni:Co ratio variation. • The optimum composition (∼1:1) of nickel and cobalt delivers highest capacitance. • HAAS device delivers 42.3 Wh kg−1 energy density at 1 kW kg−1 power density. • HASS device exhibits energy density of 36.2 Wh kg−1 at 160 W kg−1 power density. A rational combination of metal cations is necessary to obtain a synergistic effect leading to improved electrochemical energy storage performance of bimetal compounds. So, the effect of concentration variation of reactants (nickel and cobalt ratio) in nickel cobalt phosphate material and their influence on physicochemical properties and electrochemical capacitive performances are investigated. The morphological evolution from microflowers (nickel phosphate) to microsheets (cobalt phosphate) is observed with increasing cobalt content in nickel cobalt phosphate thin films. The optimum nickel and cobalt composition (∼1:1) in nickel cobalt phosphate (Ni 1.38 Co 1.62 (PO 4) 2 ·8H 2 O) thin film electrode demonstrate maximum specific capacitance (capacity) of 1116 F g−1 (446 C g−1) at 0.5 A g−1 current density. Also, a fabricated hybrid asymmetric aqueous device consists of nickel cobalt phosphate thin film as cathode and rGO as an anode in 1 M KOH electrolyte delivers a higher specific capacitance of 120 F g−1 with a high energy density of 42.3 Wh kg−1 at a power density of 1 kW kg−1. Moreover, a hybrid asymmetric solid-state device fabricated using PVA-KOH gel electrolyte exhibits a maximum specific capacitance of 102 F g−1 with an energy density of 36.2 Wh kg−1 at 160 W kg−1 power density and 83.7 % initial capacitance retention after 4000 GCD cycles. Such excellent electrochemical capacitive performance of hybrid asymmetric devices is attributed to the synergy between nickel and cobalt species in nickel cobalt phosphate thin film electrodes, suggesting its potential application as a cathode in hybrid energy storage devices. [ABSTRACT FROM AUTHOR]

Efficient and durable electrocatalysts are crucial for energy conversion devices that perform the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The structural and morphological characteristics of the electrocatalyst can significantly impact the HER/OER performance. Therefore, it is essential to develop a high-performing electrocatalyst with desired properties using a simple and cost-effective chemical process. So, herein, successive ionic layer adsorption and reaction (SILAR) deposited amorphous, hydrous cobalt iron phosphate (CFP) thin film electrocatalysts are implemented toward electrochemical water splitting. Moreover, in the present work, the molar proportions of cobalt and iron were streamlined to study their synergistic effect on electrochemical HER and OER performance. The electrode of best-performing (CFP–S2) requires the lowest overpotentials of 242 mV for OER and 67.9 mV for HER at a current density of 10 mA/cm2, which maintains its activity after 24 h. The alkaline water splitting into a similar electrolytic bath using two electrode systems was demonstrated for 100 h with the lowest overpotential of 1.72 V. The remarkable electrochemical performance and postmortem analysis unambiguously demonstrate that CFP electrodes are a highly promising and robust option for long-duration water-splitting devices, and the facile SILAR method for scalable CFP electrode synthesis indicates enormous potential for commercial applications. The facile chemical synthesis successive ionic layer adsorption and reaction (SILAR) method employed in this study yields promising results, highlighting the scalable binder-free synthesis and potential for durable electrochemical overall water splitting for clean fuel production. [Display omitted] • Novel synthesis of cobalt iron phosphate (CFP) by SILAR method. • Lower overpotentials for OER (242 mV) and HER (67.9 mV). • Overall water splitting with low potential (1.72 V). • Robustness of material for more than 100 h overall water splitting. • Lab-made device performance with the volume ratio 1:2 for O 2 and H 2 evolution. [ABSTRACT FROM AUTHOR]