Designing In Situ Grown Ternary Oxide/2D Ni-BDC MOF Nanocomposites on Nickel Foam as Efficient Electrocatalysts for Electrochemical Water Splitting

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 (N=Ni, C=Co, M:F=Fe, C=Cu, and Z=Zn, 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.


Figure S1 .
Figure S1.The coupling mechanism of Ni(OH)2 as the dominant phase of NCF with Ni-BDC ligand.

Figure
Figure S4.SEM/EDS elemental mappings of a) NCF@NF, b) NCC@NF, and c) NCZ@NF; a very large area selected to observe the distribution of elements throughout the NF.

Figure S6 .
Figure S6.XRD patterns of various ternary oxides, NCM (M = F, C, and Z) stripped off from NF.

Figure
Figure S11.FT-IR spectra of ternary oxides and their corresponding nanocomposites stripped off from NF.

Figure S15 .
Figure S15.SEM images of a) NCF and c) NCF/Ni-BDC after OER tests, and b) NCC and d) NCZ/Ni-BDC after HER tests.

Figure
Figure S16.SEM/EDS elemental mappings of a) NCF and c) NCF/Ni-BDC after OER tests, and b) NCC and d) NCZ/Ni-BDC after HER tests.

Figure S17 .
Figure S17.XPS spectra after electrochemical OER and HER measurements.a and c) Ni 2p, Co 2p, Fe 2p, and O 1s for NCF and NCF/Ni-BDC after OER tests and b and d) Ni 2p, Co 2p, Cu 2p, and O 1s for NCC and NCZ/Ni-BDC after HER tests.

Figure
Figure S18.Post-electrolysis HR-TEM photographs.a) NCF/Ni-BDC after the OER durability test and b) NCZ/Ni-BDC after the HER stability test.

Table S1 .
The quantitative results of XRF analysis of ternary mixed oxides.

Table S4 .
Estimated values obtained from the EIS simulations by the ZView software.