The security of future energy, hydrogen, is subject to
designing
high-performance, stable, and low-cost electrocatalysts for hydrogen
and oxygen evolution reactions (HERs and OERs), for the realization
of efficient overall water splitting. Two-dimensional (2D) metal–organic
frameworks (MOFs) introduce a large family of materials with versatile
chemical and structural features for a variety of applications, such
as supercapacitors, gas storage, and water splitting. Herein, a series
of nanocomposites based on NCM/Ni-BDC@NF (NNi, CCo,
M:FFe, CCu, and ZZn, BDC: benzene dicarboxylic
acid, NF: nickel foam) were directly developed on NF using a facile
yet scalable solvothermal method. After coupling, the electronic structure
of metallic atoms was well-modulated. Based on the XPS results, for
the NCF/Ni-BDC, cationic atoms shifted to higher oxidation states,
favorable for the OER. Conversely, for the NCZ/Ni-BDC and NCC/Ni-BDC
nanocomposites, cationic atoms shifted to lower oxidation states,
advantageous for the HER. The as-prepared NCF/Ni-BDC demonstrated
prominent OER performance, requiring only 1.35 and 1.68 V versus a
reversible hydrogen electrode to afford 10 and 50 mA cm–2 current densities, respectively. On the cathodic side, NCZ/Ni-BDC
exhibited the best HER activity with an overpotential of 170 and 350
mV to generate 10 and 50 mA cm–2, respectively,
under 1.0 M KOH medium. In a two-electrode alkaline electrolyzer,
the assembled NCZ/Ni-BDC (cathode) ∥ NCF/Ni-BDC (anode) couple
demanded a cell voltage of only 1.58 V to produce 10 mA cm–2. The stability of NCF/Ni-BDC toward OER was also exemplary, experiencing
a continuous operation at 10, 20, and 50 mA cm–2 for nearly 45 h. Surprisingly, the overpotential after OER stability
at 50 mA cm–2 dropped drastically from 450 to 200
mV. Finally, the faradaic efficiencies for the overall water splitting
revealed the respective values of 100 and 85% for the H2 and O2 production at a constant current density of 20
mA cm–2.