Alkali‐Etched Ni(II)‐Based Metal–Organic Framework Nanosheet Arrays for Electrocatalytic Overall Water Splitting

Abstract

However, only limited MOFs were explored for electrocatalytic water splitting, mainly due to the poor structure stability of most MOFs in water system, especially under acidic/alkaline conditions. [11-13] Most importantly, metal nodes in most MOFs are coordinately saturated with ligand/ solvent molecules, [14,15] which are inert in electrocatalysis, even if these metal sites are highly exposed by rich pores. [16] For instance, Zhang and co-workers reported a new metal azolate framework [Co 2 (µ-Cl) 2 (bbta)], [12] but showing poor OER activity. The metal sites became active only on the functionalization by hydroxide ligands, and then exhibited efficient electrocatalysis performance. In addition, the low electrical conductivity for most MOFs also affects the electrocatalytic activity. [17] The principal reason is that the larger inner impedance of most organic ligands and rigid pore structures are not conducive to long-range charge transport. [18,19] Despite few of efforts including synthesis of ultrathin 2D layers or introduction of conductive materials in channels for exposing more active sites or improving their conductivity, the electrocatalytic activity enhancement is still not prominent. [20,21] Thus, it is desired to rationally design or optimize the structure and composition of MOFs to improve water spitting activity. Herein, we propose that the construction of defects into MOFs could be a feasible strategy to create highly active sites and improve conductivity for optimizing electrocatalytic properties. As a proof of concept, a Ni(II)-based MOF [Ni 2 (OH) 2 (BDC); BDC = 1,4-benzenedicarboxylate] with intrinsic 2D metal-oxide layers connected by BDC ligands [22] was selected and its defectrich nanosheet array (as D-Ni-MOF NSA) was prepared via a simply alkali-etched strategy (Figure 1). The intrusion of KOH leads to partial breaking of NiO bonds in the MOF, accompanied with the generation of open unsaturated Ni sites and the introduction of extra-framework K cations. Subsequently, the unsaturated Ni sites were attacked by the OH − ions, leading to the exposed active site NiO(OH), while the original framework topology is still well maintained. Theoretical/experimental results reveal that the above features could endow the D-Ni-MOF with richer active sites and largely improved conductivity, compared with pristine Ni-MOF. Finally, the assembled D-Ni-MOF||D-Ni-MOF electrolyte cell could achieve comparable electrocatalytic performance with that of the noble metal-based benchmark catalysts, demonstrating a promising bifunctional electrocatalyst for overall water splitting. The exploration of efficient electrocatalysts is the central issue for boosting the overall efficiency of water splitting. Herein, pertinently creating active sites and improving conductivity for metal-organic frameworks (MOFs) is proposed to tailor electrocatalytic properties for overall water splitting. An Ni(II)-MOF nanosheet array is presented as an ideal material model and a facile alkali-etched strategy is developed to break its NiO b...

Atomically-dispersed NiN4–Cl active sites with axial Ni–Cl coordination for accelerating electrocatalytic hydrogen evolution

“…Meanwhile, the same conclusion can be drawn from the rst derivative curve (Fig. 2b), 33,42 24,25 The wavelet transforms (WT) was resorted to demonstrate the existence of single-atom dispersed Ni species in the samples. 42,43 As shown in Fig.…”

“…S5b, † a noticeable shi to the higher binding energy of Ni 2p spectra can be observed from NiN 4 SAs/ N-C to NiN 4 -Cl SAs/N-C, revealing the increasing oxidation state of Ni with the coordination of Cl. 26,33 Such interaction is also reected in the Cl spectrum shown in Fig. 1g and the binding energy at 197.5 eV is assigned to the Ni-Cl bond, indicating the coordination of Cl can effectively regulate the electronic structure of Ni atoms.…”

“…1d). 33,34 As shown in Table S1, † the Ni loading in Ni (I) SAs/N-C, NiN 4 SAs/N-C, NiN 4 -Cl SAs/N-C are 0.48 wt%, 1.38 wt% and 1.37 wt%, respectively. This is because Ni (I) SAs/N-C was synthesized by a one-step encapsulation strategy while NiN 4 SAs/N-C and NiN 4 -Cl SAs/N-C were prepared by the secondary adsorption process increasing the loading content of Ni.…”

Avoiding Pyrolysis and Calcination: Advances in the Benign Routes Leading to MOF‐Derived Electrocatalysts

“…Accordingly, some latest works have reported an alkaline hydrolysis strategy for delivering MOF‐derived OER electrocatalysts [51a,53] . As an example, Li and co‐workers developed a facile alkali‐etched method to obtain the defect‐rich ultrathin Ni‐MOF nanosheet array (D−Ni−MOF NSA) for OER [54] . In alkaline electrolyte, partial coordinated Ni−O bonds of 2D MOFs could be broken due to KOH invasion, producing open unsaturated Ni(II) sites (Figure 3a) [55] .…”

Mechanistic Insight into Charge and Energy Transfers of Luminescent Metal–Organic Frameworks Based Sensors for Toxic Chemicals

“…Therefore, combinations of different methods are necessary to measure hazardous chemicals or ions at low level (Table 1). [101][102][103][104][105] Basic unresolved issues are related to the amount, speciation, location, mechanisms of changes upon detection, environmental changes, or onset of disease. Accordingly, studies related to the detection of hazardous chemicals present in solutions at low levels could be explored for the detection of various analytes in future.…”