Hierarchical NiFe@NiFe Layered Double Hydroxides for Efficient Solar-Powered Water Oxidation

The development of green hydrogen is crucial for achieving zero CO2 emissions and sustainable energy production. Solar-powered electrocatalysis, also known as photovoltaic-electrolysis (PV-EC), is a promising technology that has the potential to efficiently convert solar energy into hydrogen with high efficiency and durability. However, the sluggish kinetics of the oxygen evolution reaction (OER) pose a significant challenge to efficient water splitting. To overcome this, there is a pressing need to develop cost-effective catalysts with high electrochemical activity and stability, as well as scalable synthetic processes for large-scale production. Layered double hydroxides (LDHs) based on transition metals have garnered considerable attention as promising candidates due to their abundant availability, porous morphologies, and adjustable compositions.<br/>In this study, a hierarchical heterogeneous Ni²⁺Fe³⁺@Ni²⁺Fe²⁺ LDH was successfully synthesized using a sequential electrodeposition technique. The synthesis process involved the use of separate electrolytes containing iron precursors with different valence states (Fe²⁺ and Fe³⁺). By utilizing Fe²⁺ precursors, a well-defined nanosheet array of NiFe LDHs with high crystallinity, referred to as Ni²⁺Fe²⁺ LDH, was synthesized. On the other hand, employing Fe³⁺ precursors led to the formation of a thin layer of NiFe LDH with a relatively higher Ni content and lower crystallinity, denoted as Ni²⁺Fe³⁺ LDH. Different deposition behaviors depending on the oxidation state of the Fe precursor ions was attributed to their different solubility product constants (<i>K</i>_sp). The amorphous Ni²⁺Fe³⁺ LDH exhibited superior OER performance compared to its Fe²⁺-based counterpart. The heterogeneous hierarchical OER catalyst electrode was prepared by a simple two-step electrodeposition process using the Ni²⁺Fe²⁺ LDH nanosheet arrays with large surface area. As a result of the synergetic effect between the large dual advantages of large surface area of the Ni²⁺Fe²⁺ LDH nanosheet arrays and excellent OER activity of Ni²⁺Fe³⁺ LDH, the Ni²⁺Fe³⁺@Ni²⁺Fe²⁺ LDH exhibited outstanding OER activity with overpotentials of 218 and 265 mV required to achieve current densities of 10 and 100 mA cm⁻², respectively. Moreover, it demonstrated exceptional long-term stability, sustaining efficient performance for 30 hours even at a high current density of 500 mA cm⁻². In a water splitting system, the electrolyzer, using a Sn₄P₃/CoP₂ electrocatalyst as the cathode, only required a cell voltage of 1.55 V to achieve a current density of 10 mA cm^-2. Furthermore, the solar-powered overall water splitting system, comprising our electrolyzer and a perovskite/Si tandem solar cell, exhibited a high solar-to-hydrogen conversion efficiency of 15.3%.