Preferential Adsorption of Hydroxide Ions onto Partially Crystalline NiFe-Layered Double Hydroxides Leads to Efficient and Selective OER in Alkaline Seawater

A variety of compounds, including Ni-, Fe-, and Co-containing layered double hydroxides (LDHs), have been explored as catalysts for the oxygen evolution reaction (OER). However, few can meet the industrially mandated overpotential of 0.30 V at 500 mA/cm2 and cell voltage of 1.60 V, let alone be applied to electrolysis of seawater. We synthesized a nickel foam (NF)-supported NiFe-LDH whose OER overpotential is only 0.257 V at 500 mA/cm2 in an alkaline saline solution and requires a cell voltage of 1.54 V for the same current density when coupled with a MoNi4/MoO2/NF cathode for electrolyzing alkalized seawater. The NiFe-LDH catalyst comprises numerous nanometer-sized crystalline facets surrounded by an amorphous phase, in contrast to its highly crystalline counterpart. X-ray photoelectron spectroscopy reveals that the boundaries separating crystalline facets and amorphous phase contain more Ni3+ than other areas. Anion chromatographic analysis indicates that OH– adsorbs preferentially over Cl– onto the sites of Ni3+ of both partially and highly crystalline NiFe-LDHs, whereas Cl– adsorbs more extensively onto the crystalline planes or facets. These adsorption behaviors and the resultant different catalytic activities at high current densities can be readily rationalized by the Pearson's hard–soft acid–base principle. Because more boundaries exist in the partially crystalline NiFe-LDH, the partially crystalline NiFe-LDH catalyst is not only more catalytically efficient than its highly crystalline counterpart and other catalysts reported up to the present, but it is also stable in alkalized seawater and unaffected by Cl– adsorption.