Here, for a vivid description of the predicted evolution, a concept of a dynamic electrocatalyst is proposed for a rechargeable Zn–air battery. To be specific, it is predicted herein that a current-driven oxyhydroxide derived from the native materials will be generated and combined with its electrochemically unavailable bulk as a “shell-bulk” configuration the performance-dominant elements within the shell will experience periodic swing in their chemical states during cycling. The different environment may cause variances in electrochemical behaviors in comparison with the standard three-electrode system. Two key factors are worth pointing out, one is that the potentiodynamic-driven methodology applied in ORR or OER measurements is different from the continuous galvanostatic technique for battery operation, and the other is that alkaline electrolyte used in Zn–air batteries is far more concentrated, so that a lower electrochemical barrier is required for valence variation of the principle elements.
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Therefore, a systematic study is in high demand to pry into this “black box” and reveal the underlying mechanism governing the current-driven transformation of the electrocatalyst. In particular, the electrocatalyst evolution during cycling is often ignored, and limited efforts have been placed to identify the actual configuration of the electrocatalyst in the mid-way or post-cycling status. These studies expose an often-overlooked fact that directly assuming the native state of metal-based electrocatalyst as the active representation in Zn–air battery operation may lead to false correlation between material characteristics and performance, albeit the undeniable role of their native properties. Furthermore, the latest attention has been drawn toward ORR electrocatalysts, in which an alteration was also demonstrated 26. This contradiction was resolved with observations of surficial phase transformation from these compounds to their derived oxides or oxyhydroxides, which also aligns with the results on metal oxides, sulfides, and nitrides 19, 23, 24, 25. For instance, Hu and coworkers reported both nickel iron diselenide and nickel phosphide as efficient and long-lasting OER electrocatalysts, but neither of them is static stereotype under oxidation voltage in alkaline electrolyte 21, 22. Increasing reports on spectroelectrochemistry toward the half-reactions, that is, oxygen reduction (ORR) or evolution reactions (OER), in three-electrode systems suggest that an unexplored chasm is at work.
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In particular, a debate over the actual active phases during cycling of these electrocatalysts is raised recently, and conflicting results have perplexed the research community 11, 19, 20. To this day, exploration for ideal bifunctional electrocatalysts remains to be the research mainstream, but attention is shifting toward understanding the relationship between battery performance and physiochemical properties of electrocatalysts for performance breakthrough. As such, huge families of materials have been investigated, including metals, alloys, oxides, sulfides, nitrides, phosphides, and their derived composites with carbon 2, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
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Previously, limitations in the availability and rechargeability of oxygen electrocatalysts have hindered their popularization, but blooming efforts on exploring suitable and durable candidates to catalyze cathode reactions have led to the recent rejuvenation of this century-old technology 4, 5, 6. Zinc–air (Zn–air) battery is widely recognized as one of the next-generation sustainable electrochemical energy-storage systems because of its high energy density, economic, and safety merits 1, 2, 3. The revealed configuration can serve as the basis to construct future blueprints for metal-based electrocatalysts, and push zinc-air battery toward practical application. Upon maturation, zinc-air battery experiences a near two-fold enlargement in power density to 234 mW cm −2, a gradual narrowing of voltage gap to 0.85 V at 30 mA cm −2, followed by stable cycling for hundreds of hours. A dynamic picture sketching the generation and maturation of nanoscale oxyhydroxide shell is presented, and periodic valence swings of performance-dominant element are observed.
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By selecting a bimetal nitride as representation, a current-driven “shell-bulk” configuration is visualized via time-resolved X-ray and electron spectroscopy analyses. Herein, to depict the underlying behaviors, a concept named dynamic electrocatalyst is proposed. However, their electrocatalytic configuration and evolution pathway during battery operation are rarely spotlighted. Recent fruitful studies on rechargeable zinc-air battery have led to emergence of various bifunctional oxygen electrocatalysts, especially metal-based materials.