Layered Structure Causes Bulk NiFe Layered Double Hydroxide Unstable in Alkaline Oxygen Evolution Reaction

Abstract

in alkaline media. The surprisingly low OER overpotential of NiFe LDH has triggered a great deal of research attentions to reveal the reaction mechanism. [4,5] Besides, lots of work have been done to further reducing the overpotential of NiFe LDH, for example, via incorporation of a third metal, [6][7][8] hybridization with carbon materials, [9,10] and applying NiFe selenide as the templating precursor. [11] Although great attention has been paid to improve the OER activity and investigate the active site of NiFe LDHs, few works actually focus on their catalytic stability despite that stability is as important as activity in practical applications. Based on literature, the OER stability of NiFe LDHs seems satisfactory. [10][11][12][13] However, the stability of NiFe LDHs was usually assessed at room temperature with current densities of tens of milliamps per square centimeter of electrode for tens of hours. The mild evaluation condition cannot reflect the long-term stability requirement under harsh conditions for practical alkaline water electrolyzers.Herein, we reveal that the layered structure of bulk NiFe LDH is detrimental to OER stability. It has been generally accepted that the edge sites of 2D electrocatalysts (e.g., MoS 2 ) are highly active in electrocatalysis, while surface sites are usually inactive. [14] We identify that the interlayer basal plane of NiFe LDH is also able to catalyze OER, while the slow diffusion of OH − into the LDH interlayers during OER in alkaline solution induces a local acidic environment within the interlayers, which thus causes dissolution of NiFe LDH. To resolve this problem, we propose to delaminate multi-layered NiFe LDH into atomically thin nanosheets, which is able to greatly improve OER stability.NiFe LDH grown on Ni foam or carbon cloth was used to investigate the deactivation mechanism of LDH in OER. Figure S1 (Supporting Information) shows the scanning electron microscopy and transmission electron microscopy (TEM) images, in which NiFe LDH nanosheets are found to intimately and uniformly cover the entire Ni foam with NiFe LDH film thickness of ≈2.5 µm and individual sheet thickness of ≈60 nm. The high-resolution TEM image ( Figure S1d, Supporting Information) shows the lattice spacing of ≈2.5 Å, close to the theoretical interplanar spacing of NiFe LDH (009). The layered structure was further confirmed by X-ray diffraction (XRD) as shown in Figure S2 (Supporting Information).NiFe-based layered double hydroxides (LDHs) are among the most efficient oxygen evolution reaction (OER) catalysts in alkaline medium, but their long-term OER stabilities are questionable. In this work, it is demonstrated that the layered structure makes bulk NiFe LDH intrinsically not stable in OER and the deactivation mechanism of NiFe LDH in OER is further revealed. Both operando electrochemical and structural characterizations show that the interlayer basal plane in bulk NiFe LDH contributes to the OER activity, and the slow diffusion of proton acceptors (e.g., OH − ) within the NiFe LDH interl...

Active Site Engineering in Porous Electrocatalysts

“…A study led by the Liu group found that the layered structures in NiFe-based electrocatalysts could limit the diffusion of proton acceptors (e.g., OH − ) into the small interlayer spaces in the materials during OER. [129] Furthermore, they indicated that the local acidic environment would cause cation dissolution, making the electrocatalyst unstable. This work provided an important explanation as to why some previously reported, highly active NiFe-based OER electrocatalysts usually had very limited catalytic lifetimes (<100 h).…”

Recent Progress on NiFe‐Based Electrocatalysts for the Oxygen Evolution Reaction

“…[81,82] The interlayer basal plane in a bulk NiFe-LDH has been reported to be conducive to the OER activity, and the tardy diffusion of proton acceptors (e.g., OH − ) within the NiFe-LDH interlayers during OER causes the NiFe-LDH to dissolve, thereby decreasing the OER activity over time. [83] To expose additional active sites and improve the NiFe-LDH stability, various valid methods have been explored for preparing ultrathin nanometer-scale sheets. For example, Gao and Yan synthesized single-unit-cell-thick (≈1.3 nm) and defect-rich NiFe-LDH nanosheets (NSs) in a simple manner ( Figure 3a); these NSs exhibited Tafel a slope of 33.4 mV dec −1 and an almost 100% faradaic efficiency.…”

Modulating Hydroxyl‐Rich Interfaces on Nickel–Copper Double Hydroxide Nanotyres to Pre‐activate Alkaline Ammonia Oxidation Reactivity

“…It is because the slow diffusionofa lkaline electrolyte into NiCu DHs interlayers are easily induce al ocal acidic environment, thus leading to the dissolution of hydroxyl groups and causing structural destruction. [14] Accordingly,a iming at boosting the conversion efficiency of ÀOH to ÀOOH and enhancing the electrochemical corrosion resistance, how to designt he distribution and retain availability of hydroxyl species in NiCu DHs becomes ac ritical problem. [15] Herein, we report the systematic synthesis of NiCu DHs with tunable intercalations and morphologies [nanotyres (T), nanoflowers (F), and nanonets (N)] through ao ne-step solvothermal approacha nd investigate their structure-function relationship towardsa lkaline AOR.…”