Figure 1. Schematic illustration showing Pt dissolution and redeposition during the HER experiment if Pt is applied as the counter electrode.

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...

Well-defined star-like amphiphilic polymers composed of a β-cyclodextrin core, from which 21 hydrophobic poly(lactic acid) arms and hydrophilic poly(ethylene glycol) arms are grafted sequentially, form robust and uniform unimolecular micelles that are biocompatible and efficient in the delivery of anticancer drugs.

The orthogonal sulfur–fluoride exchange reaction (SuFEx) and copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) are employed to synthesize sequence‐regulated synthetic polymers. The high efficiency and broad tolerance of SuFEx and CuAAC to diverse chemical functionalities enable the one‐pot synthesis of polydispersed sequence‐controlled polymers by step‐growth copolymerization in high yield and sequence complexity. Furthermore, iterative SuFEx and CuAAC coupling reactions on a solid support, without the need of protecting groups, afford monodispersed sequence‐defined oligomers. The use of this orthogonal pair of click reactions provides new opportunities to facilely access sequence‐regulated synthetic polymers with a high degree of structural diversity.

Sequence-controlled polymers are an emerging class of synthetic polymers with a regulated sequence of monomers. In the past decade, tremendous progress has been made in the synthesis of polymers, with the sophisticated sequence control approaching the level manifested in biopolymers. In contrast, the exploration of novel functions that can be achieved by controlling synthetic polymer sequences represents an emerging focus in polymer science. This Viewpoint will survey recent advances in the functional applications of sequence-controlled polymers and provide a perspective on the challenges and outlook for pursuing future applications of this fascinating class of macromolecules.