In‐situ Growth of a Bimetallic Cobalt‐Nickel Organic Framework on Iron Foam: Achieving the Electron Modification on a Robust Self‐supported Oxygen Evolution Electrode

Abstract

As a clean energy source, hydrogen may be obtained via overall water splitting. However, the reaction is limited by the anodic oxygen evolution reaction (OER) in the catalytic field. In order to overcome the limitation in OER catalyst activity, this paper attempts to synthesize and develop a series of three-dimensional metal-organic frameworks (MOFs) electrocatalytic materials. The structure and morphology of bimetallic cobalt and nickel MOFs are characterized by FT-IR, Raman, SEM, TEM, and XPS. It is found that the bimetallic cobalt and nickel MOFs have a columnar micro-structure assembled by nanosheets. Further in-situ growth on the iron (Fe) foam to prepare an integrated nanoarray self-supported electrode did not significant change their micro-structure. The obtained bimetal electrocatalysts exhibit significantly enhanced OER activity as compared to the single-metal MOFs counterpart. The cobalt and nickel doubledoped materials in-situ grown on Fe foam exhibited an excellent electrocatalytic activity and remarkable durability with a current density of 10 mA cm À 2 in a 0.1 M KOH solution (with the overpotential of 264 mV and the Tafel slope of 54.3 mV dec À 1 ). There is a significant improvement compared with the material on the glassy carbon electrode. The introduction of the bimetal species (Co and Ni) into the ordered MOFs provides a synergistic effect of the active centers resulting in an enhanced electron transfer and improved electrochemical performance. This work enriches the MOFs electrocatalytic electrode and pave the way for their practical applications.[a] Dr.Reagent: All reagents and materials in this experiment were obtained commercially and were used as received without any further purification. 1,2,4,5-Benzenetetracarboxylic acid (H 4 btec 98 %, AR), NiCl 2 ·6H 2 O (98 %, AR), CoCl 2 ·6H 2 O (98 %, AR), NaOH (96 %, AR), KOH (96 %, AR) and the Nafion solution (5 wt.%) were purchased from J&K Chemical Co. The ultrapure water (18 MΩ cm) was supplied by a Millipore System (Millipore Q). The thickness of iron foam was 0.5 mm. Synthesis of [M 2 (btec)(H 2 O)4]·2H 2 O (M = Ni and Co):The [M 2 (btec) (H 2 O) 4 ]·2H 2 O was synthesized via a hydrothermal process, as previously reported and with minor modifications. [44] The specific synthesis process is as follows: 1.4 mmol of MCl 2 ·6H 2 O (M: Co, Ni, or Co/Ni mixed with 1 : 1 in at.%) and 0.94 mmol of H 4 btec were dissolved in 10 mL ultrapure water, then the pH was adjusted by 2.2 mmol of NaOH. Afterwards, the as-prepared mixture solution was transferred into a 25 mL Teflon stainless-steel autoclave, and heated at 110°C for 72 h. After that, the autoclave was cooled to room temperature, the as-prepared three-dimensional (3D) MOFs were collected after being washed with ultrapure water and dried at 60°C in an oven. The as-obtained red crystals of 3D-CoMOFs were obtained with a yield of 78.6 %, the green crystals of 3D-NiMOFs with a yield of 85.2 %, and the brown crystals of 3DÀ Co/ NiMOFs with a yield of 80.6 %, respectively ( Figure S1). In-situ synthe...

Surface‐Adsorbed Carboxylate Ligands on Layered Double Hydroxides/Metal–Organic Frameworks Promote the Electrocatalytic Oxygen Evolution Reaction

“…[24] Tw opeaks at 1576 and 1498 cm À1 are clearly seen and are ascribed to n as (-COO-) and n s (-COO-) of carboxylate, [25] respectively.Notably,the peak at Angewandte Chemie 1668 cm À1 corresponds to the bending vibrations of -COO-, [26] demonstrating the existence of CLs.Compared with the peaks of metal-oxygen bonds of NiFe-LDHs,anegatively-shifted and strong peak appears at % 500 cm À1 for NiFe-LDHs/CLs, indicating ac oordination between Ni/Fem etal sites and CLs. [27] Theabove results lead us to think about why and how CLs promote OER electrocatalytic activity of NiFe-LDHs or Angewandte Chemie other catalysts.H erein, as eries of DFT calculations were conducted to further answer this question. As shown in Figure S7, we firstly investigated the adsorption energy (E ads ) of terephthalate on Ni and Fe sites.O bviously,t he E ads of terephthalate on Ni site (À2.837 eV) is lower than that of Fe site (À1.085 eV), indicating the adsorption of terephthalate on Ni site is easier than that on Fe site.T he E ads of CL with different contents on the surface of NiFe-LDHs was calculated including one,t wo and three CL(s) (Figure S8).…”

Surface‐Adsorbed Carboxylate Ligands on Layered Double Hydroxides/Metal–Organic Frameworks Promote the Electrocatalytic Oxygen Evolution Reaction

“…Compared with the peak of metal-oxygen bonds of NiFe-LDHs, a strong and shifted peak appears at ~ 500 cm − 1 for NiFe-LDHs/CLs, indicating a coordination between Ni/Fe metal sites and CLs. 28 The above results lead us to think about why and how do CLs promote OER electrocatalytic activity of NiFe-LDHs or other catalysts. Herein, a series of DFT calculations were conducted to further answer this question.…”

“…Notably, the peak at 1668 cm −1 corresponds to the bending vibrations of ‐COO‐, [26] demonstrating the existence of CLs. Compared with the peaks of metal‐oxygen bonds of NiFe‐LDHs, a negatively‐shifted and strong peak appears at ≈500 cm −1 for NiFe‐LDHs/CLs, indicating a coordination between Ni/Fe metal sites and CLs [27] …”

Modulating Metal–Organic Frameworks as Advanced Oxygen Electrocatalysts

“…Similarly, in some other literatures, the metal ions not only come from the prefabricated metal‐based nanoarrays, but also from the Ni foam substrates, which help the formation of Ni containing mixed metal MOFs on Ni foam. [ 111–115 ] Some other substrates, such as Fe foam, [ 116 ] Cu foam [ 117 ] and carbon cloth, [ 118,119 ] can also be employed as conductive substrates to support MOFs so as to get excellent OER activities.…”