Observation of electrochemical reactions at Zn electrodes in Zn-air secondary batteries

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Anode optimization strategies for aqueous zinc-ion batteries

Zinc-ion batteries (ZIBs) have received much research attention due to their advantages of safety, non-toxicity, simple manufacture, and element abundance. Nevertheless, serious problems still remain for their anodes, such as dendrite development, corrosion, passivation, and the parasitic hydrogen evolution reaction due to their unique aqueous electrolyte system constituting the main issues that must be addressed, which are blocking the further advancement of anodes for Zn-ion batteries. Herein, we conduct an in-depth analysis of the problems that exist for the zinc anode, summarize the main failure types and mechanisms of the zinc anode, and review the main modification strategies for the anode from the three aspects of the electrolyte, anode surface, and anode host. Furthermore, we also shed light on further modification and optimization strategies for the zinc anode, which provide directions for the future development of anodes for zinc-ion batteries.

Toward Long-Life Aqueous Zinc Ion Batteries by Constructing Stable Zinc Anodes

Aqueous zinc-ion batteries (AZIBs) with high safety and low cost are considered to be one of the alternatives to Li-ion batteries. In recent years, AZIBs have become a research hotspot, mainly focusing on the research of cathode, anode and electrolyte. Although many efforts have been made in cathode materials, their low specific capacity and poor cycle life remain unsolved. In fact, side reactions of zinc metal anodes, such as dendrite growth, zinc corrosion, and hydrogen evolution reactions (HER), are also the main factors restricting the electrochemical performance of AZIBs. In this review, we first discuss the fundamental of these adverse reactions. Then, the various solution strategies are summarized based on advanced materials and structural design. It includes surface modification and the internal structure optimization of Zn electrodes, the regulation of electrolytes and separators. Finally, we propose the future challenges and development prospects of zinc anode.

Zn Metal Anodes for Zn-Ion Batteries in Mild Aqueous Electrolytes: Challenges and Strategies

Over the past few years, rechargeable aqueous Zn-ion batteries have garnered significant interest as potential alternatives for lithium-ion batteries because of their low cost, high theoretical capacity, low redox potential, and environmentally friendliness. However, several constraints associated with Zn metal anodes, such as the growth of Zn dendrites, occurrence of side reactions, and hydrogen evolution during repeated stripping/plating processes result in poor cycling life and low Coulombic efficiency, which severely impede further advancements in this technology. Despite recent efforts and impressive breakthroughs, the origin of these fundamental obstacles remains unclear and no successful strategy that can address these issues has been developed yet to realize the practical applications of rechargeable aqueous Zn-ion batteries. In this review, we have discussed various issues associated with the use of Zn metal anodes in mildly acidic aqueous electrolytes. Various strategies, including the shielding of the Zn surface, regulating the Zn deposition behavior, creating a uniform electric field, and controlling the surface energy of Zn metal anodes to repress the growth of Zn dendrites and the occurrence of side reactions, proposed to overcome the limitations of Zn metal anodes have also been discussed. Finally, the future perspectives of Zn anodes and possible design strategies for developing highly stable Zn anodes in mildly acidic aqueous environments have been discussed.

A Flow-Through Membraneless Microfluidic Zinc-Air Cell

In this work, the proof of concept of a functional membraneless microfluidic Zn-air cell (μZAC) that operates with a flow-through arrangement is presented for the first time, where the activity and durability can be modulated by electrodepositing Zn on porous carbon electrodes. For this purpose, Zn electrodes were obtained using chronoamperometry and varying the electrodeposition times (20, 40, and 60 min), resulting in porous electrodes with Zn thicknesses of 3.3 ± 0.3, 11.6 ± 2.4, and 34.8 ± 5.1 μm, respectively. Pt/C was initially used as the cathode to analyze variables, such as KOH concentration and flow rate, and then, two manganese-based materials were evaluated (α-MnO₂ and MnMn₂O₄ spinel, labeled as Mn₃O₄) to determine the effect of inexpensive materials on the cell performance. According to the transmission electron microscopy (TEM) results, α-MnO₂ has a nanorod-like shape with a diameter of 11 ± 1.5 nm, while Mn₃O₄ presented a hemispherical shape with an average particle size of 22 ± 1.8 nm. The use of α-MnO₂ and Mn₃O₄ cathodic materials resulted in cell voltages of 1.39 and 1.35 V and maximum power densities of 308 and 317 mW cm^-2, respectively. The activities of both materials were analyzed through density of state calculations; all manganese species in the α-material MnO₂ presented an equivalent density of states with a reduced orbital occupation to the left of the Fermi energy, which allowed for better global performance above Mn₃O₄/C and Pt/C.

Metallic single-atoms confined in carbon nanomaterials for the electrocatalysis of oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Recent advances of single-atom-based carbon nanomaterials for the ORR, OER, HER, and bifunctional electrocatalysis are covered in this review article.

Production of a NiO/Al primary battery employing powder-based electrodes

This paper describes the use of aluminum and zinc as anodic materials for a battery employing nickel (II) oxide (NiO) as cathode. Comparison of both materials resulted in the development of a compact, cost effective, and easy to use primary NiO/Al battery employing an alkaline electrolyte. The system features electrodes composed of powder forms of the active materials on modified paper substrates that are contained in a simple multilayer design utilizing thin laminated plastic materials to provide structure and flexibility to the battery as well as a paper separator. Various concentrations of potassium hydroxide (KOH) electrolyte were examined and maximum performance was observed at 6 M KOH. A maximum current density and power density of 1.94 mA/cm² and 1 mW/cm² , respectively was achieved. This user-friendly device was able to produce a maximum capacity of 2.33 mAh/g when 2 mA/g was applied. This work demonstrates the viability of a paper-based battery featuring powder electrodes as a possible power source for microelectronic devices.

Recent Advances in Materials and Design of Electrochemically Rechargeable Zinc-Air Batteries

The century-old zinc-air (Zn-air) battery concept has been revived in the last decade due to its high theoretical energy density, environmental-friendliness, affordability, and safety. Particularly, electrically rechargeable Zn-air battery technologies are of great importance for bulk applications like electric vehicles, grid management, and portable electronic devices. Nevertheless, Zn-air batteries are still not competitive enough to realize widespread practical adoption because of issues in efficiency, durability, and cycle life. Here, following an introduction to the fundamentals and performance testing techniques, the latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018). The strategies concerning the development of Zn and air electrodes are in focus. The design of other battery components, namely electrolytes and separators are also discussed. Poor performance of O₂ electrocatalysts and the lack of the long-term stability of Zn electrodes and electrolytes remain major challenges. Finally, recommendations regarding the testing routines and materials design are provided. It is hoped that this up-to-date account will help to shape the future research activities toward the development of practical electrically rechargeable Zn-air batteries with extended lifetime and superior performance.

Enhancing the Cycle Life of a Zinc–Air Battery by Means of Electrolyte Additives and Zinc Surface Protection

The commercialization of rechargeable alkaline zinc–air batteries (ZAB) requires advanced approaches to improve secondary zinc anode performance, which is hindered by the high corrosion and dissolution rate of zinc in this medium. Modified (with additives) alkaline electrolyte has been one of the most investigated options to reduce the high solubility of zinc. However, this strategy alone has not been fully successful in enhancing the cycle life of the battery. The combination of mitigation strategies into one joint approach, by using additives (ZnO, KF, K2CO3) in the base alkaline electrolyte and simultaneously preparing zinc electrodes that are based on ionomer (Nafion®)-coated zinc particles, was implemented and evaluated. The joint use of electrolyte additives and ionomer coating was intended to regulate the exposition of Zn, deal with zincate solubility, minimize the shape change and dendrite formation, as well as reduce the hydrogen evolution rate. This strategy provided a beneficial joint protective efficiency of 87% thanks to decreasing the corrosion rate from 10.4 (blank) to 1.3 mgZn cm−1·s−1 for coated Zn in the modified electrolyte. Although the rate capability and capacity are limited, the ionomer-coated Zn particles extended the ZAB cycle life by about 50%, providing battery roundtrip efficiency above 55% after 270 h operation.

Homogeneous Coating with an Anion-Exchange Ionomer Improves the Cycling Stability of Secondary Batteries with Zinc Anodes

Limited cycling stability of secondary cells with zinc anodes arises mainly from the high solubility of oxidized zinc species in the alkaline electrolyte resulting in electrode shape change and loss of active material during repeated discharge and charge. We propose and successfully employ a homogeneous coating with an anion-exchange ionomer (AEI) on model electrodes with electron-conductive host structures to confine the oxidized zinc species. Ideally, the confinement of oxidized zinc species reduces the shape change of the electrode and keeps the active material as close as possible at its place of origin. In this work, the confinement concept for the oxidized zinc species is elucidated by means of electrochemical studies and X-ray photoelectron spectroscopy: as intended, an interlayer of zinc oxide forms between the AEI and the surface of the zinc electrode. This interlayer implies that the hydroxide ions are able to pass and react as intended, whereas the migration of oxidized zinc species into the bulk electrolyte is hindered. The coating with an AEI yields a higher amount of restored zinc during electrodeposition in comparison to an uncoated zinc electrode-applying an AEI coating increases the achievable cycle number by up to six times. We investigate the morphology of the cycled electrodes and derive thereby the needs for further material classes that might be employed in the confinement concept. This approach demonstrates the benefit of ion-selective coatings, allowing for the permeation of hydroxide ions but not of oxidized zinc species, a concept which improves rechargeable batteries with zinc anodes, such as zinc-oxygen batteries.

Zinc nanostructures for oxygen scavenging

In this work, oxidation of carbon supported Zn nanostructures was studied to elucidate their utilization as oxygen scavenging materials activated by the relative humidity in the environment. Moisture-activated nano-scavengers were produced on carbon substrates using magnetron sputtering attaining nano-islands (nanoparticles), randomly distributed on the carbon surface, with arbitrary crystallographic orientations. They possess a Zn-ZnO core-shell structure, caused by surface passivation, which provides them with a self-assembled protective layer that prevents complete oxidation of nanoparticles prior to utilization. The oxidation rate is independent of the nanoparticle size and orientation, for particles between 5 and 18 nm. The oxidation kinetics are not in complete agreement with the Cabrera and Mott theory. When exposed to a high relative humidity environment, an acceleration in the oxidation process is observed, dissolving the Zn nanoparticles and forming a layer on the carbon, which facilitates the consumption of the Zn to form ZnO. These results support the idea of its potential use in applications where high RH environments are required, such as food packaging.