Higher-voltage asymmetric-electrolyte metal-air batteries

Abstract

Asymmetric-electrolyte metal-air batteries (AMABs) with high operating voltage and energy density are highly appealing, but some challenges remain. In the April 7, 2021 issue of Matter, Jung-Ho Lee and co-workers first reported a novel separator and an atomically dispersed catalyst to achieve high voltage and long stability of AMABs. Asymmetric-electrolyte metal-air batteries (AMABs) with high operating voltage and energy density are highly appealing, but some challenges remain. In the April 7, 2021 issue of Matter, Jung-Ho Lee and co-workers first reported a novel separator and an atomically dispersed catalyst to achieve high voltage and long stability of AMABs. The increasing concerns about the increasing energy demand and environmental crisis have led to an extensive search for a new and efficient energy conversion device. Among the developed energy storage devices, aqueous metal-air batteries (MABs) are particularly promising due to high theoretical energy density, high safety, eco-friendliness, as well as high tolerance to moisture. They show considerable potential in portable and large-scale power supply and storage applications.1Liu Q. Pan Z. Wang E. An L. Sun G. Aqueous metal-air batteries: Fundamentals and applications.Energy Stor. Mater. 2020; 27: 478-505Crossref Scopus (86) Google Scholar,2Wang H.-F. Xu Q. Materials Design for Rechargeable Metal-Air Batteries.Matter. 2019; 1: 565-595Abstract Full Text Full Text PDF Scopus (188) Google Scholar However, the major obstacles to the large-scale implementation of MABs are CO2 poisoning issues and sluggish air electrode reactions, namely oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In view of the above tough issues, Yu and Manthiram3Yu X. Manthiram A. Electrochemical Energy Storage with Mediator-Ion Solid Electrolytes.Joule. 2017; 1: 453-462Abstract Full Text Full Text PDF Scopus (17) Google Scholar first proposed the concept of asymmetric-electrolyte MABs (AMABs) in 2017, which fundamentally solved the problem of continuous CO2 ingression on the side of air electrode. More importantly, the higher ORR potential (1.23 V versus SHE) delivered in the cathodic acidic electrolyte (e.g., H2SO4) of AMABs increases the output voltage and power output of practical devices. Although significant progress has been made in improving the electrochemical performance and long-term stability of AMABs in recent years,4Yu J. Li B.-Q. Zhao C.-X. Liu J.-N. Zhang Q. Asymmetric Air Cathode Design for Enhanced Interfacial Electrocatalytic Reactions in High-Performance Zinc-Air Batteries.Adv. Mater. 2020; 32: e1908488Crossref PubMed Scopus (50) Google Scholar, 5Li L. Manthiram A. Long-Life, High-Voltage Acidic Zn-Air Batteries.Adv. Energy Mater. 2016; 6: 1502054Crossref Scopus (77) Google Scholar, 6Yu X. Gross M.M. Wang S. Manthiram A. Aqueous Electrochemical Energy Storage with a Mediator-Ion Solid Electrolyte.Adv. Energy Mater. 2017; 7: 1602454Crossref Scopus (18) Google Scholar the emerging AMABs still face several key obstacles and drawbacks that still need to be overcome. The first obstacle is the high demand for cost-effective separator: the currently used bipolar polymer films and ceramic solid-state membrane show ultrahigh cost ($3,200 m−2 for bipolar polymer membrane and $6,000/kg for ceramic solid-state membrane),7Cai P. Li Y. Chen J. Jia J. Wang G. Wen Z. An Asymmetric-Electrolyte Zn-Air Battery with Ultrahigh Power Density and Energy Density.ChemElectroChem. 2018; 5: 589Crossref Scopus (32) Google Scholar limited ionic conductivity, and the unavoidable crossing between H+ and OH−. Another obstacle is the lack of low-cost, highly efficient, and stable bifunctional ORR/OER catalysts in the acidic electrolyte.8Zhang P. Zhao Y. Zhang X. Functional and stability orientation synthesis of materials and structures in aprotic Li-O2 batteries.Chem. Soc. Rev. 2018; 47: 2921-3004Crossref PubMed Google Scholar To address the above bottleneck issues, in the April 7, 2021 issue of Matter, the new materials design reported by Lee and coworkers,9Lin C. Kim S.-H. Xu Q. Kim D.-H. Ali G. Shinde S.S. Yang S. Yang Y. Li X. Jiang Z. et al.High-voltage asymmetric metal-air batteries based on polymeric single-Zn2+-ion conductor.Matter. 2021; 4: 1287-1304Abstract Full Text Full Text PDF Scopus (8) Google Scholar includes two important breakthroughs: the novel separator fabrication and atomically dispersed catalyst design to achieve the high-voltage and long-term stability AMABs. The first, it is the first reported the cost-effective polyacrylonitrile (PAN)-based single-Zn2+-ion conductor to selectively transport Zn2+ ions and inhibit the crossover of H+ and OH– ions. Specifically, Zn2+ ion-selective transport membrane (ZnSTM) (Figure 1A) was formed from a mixture of Zn(OAc)2 and PAN heated at 140°C in the air by cross-linking polymerization between adjacent molecules. As-obtained ZnSTM possesses good mechanical stability and flexibility, and its tensile strength is 0.8 Ma. X-ray diffraction (XRD), Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy clearly reveal that the typical Zn2+ anchoring ladder structure in ZnSTM is formed by a cyclization reaction, which transforms the C≡N in the PAN into C=N (Figure 1B). The microstructure of ZnSTM was further investigated using transmission electron microscopy (TEM) (Figure 1C). The microphase separation structure presented in ZnSTM consists of a hydrophobic polymer backbone (bright region) and a hydrophilic domain (dark region) generated by Zn2+ ions aggregated around the N-related groups at the edge of the carbon backbone. The hydrophilic domains with a Zn2+ ionic conductivity of 1.9 × 10−4 S cm−1 at 313 K provided a fast transport channel for Zn2+ ions. The Arrhenius plot shows that the calculated activation energy of ZnSTM is about 0.2 eV (<0.4 eV), indicating that the migration of Zn2+ in ZnSTM follows the Grotthuss jump mechanism (Figure 1D). According to the Bruce-Vincent-Evans equation, the migration number of Zn2+ ion is determined to be 0.8, suggesting that the ion conduction mainly comes from the mobility of the cation (Zn2+) in ZnSTM. In addition, the self-made diffusion cell experiment confirmed that ZnSTM can effectively inhibit the cross of H+ and OH−. Computational calculation reveals that the attractive force between Zn2+ sites and guest ions (H +/OH− ions) inhibits the transport of H + and OH− ions. On the contrary, the repulsive force between Zn2+ binding sites and guest Zn2+ ions is conducive to the migration of Zn2+ ions.Figure 1Structure and properties of ZnSTM, Co/NS, and the assembled AZnAB based on ZnSTM separator and Co/NS catalystShow full caption(A) Optical images of ZnSTM.(B) XRD pattern of ZnSTM. Inset shows its chemical structure.(C) TEM image and corresponding SAED pattern (inset) of ZnSTM.(D) Arrhenius plots of ZnSTM.(E) ORR polarization curves of NS, Co/NS, Co-NPs/NS, and Pt/C in O2-saturated 0.1 M HClO4.(F) OER polarization curves of NS and Co/NS in O2-saturated 0.1 M HClO4 with rotating rate of 1,600 rpm.(G) Comparison of in situ EXAFS obtained on Co/NS before and after in situ exposure to periodic changed ORR and OER potentials.(H) Schematic of the theoretical voltage and configuration of aqueous AZnAB.(I) Comparison of energy densities of the AZnAB and previously reported ZnAB at various current densities.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A) Optical images of ZnSTM. (B) XRD pattern of ZnSTM. Inset shows its chemical structure. (C) TEM image and corresponding SAED pattern (inset) of ZnSTM. (D) Arrhenius plots of ZnSTM. (E) ORR polarization curves of NS, Co/NS, Co-NPs/NS, and Pt/C in O2-saturated 0.1 M HClO4. (F) OER polarization curves of NS and Co/NS in O2-saturated 0.1 M HClO4 with rotating rate of 1,600 rpm. (G) Comparison of in situ EXAFS obtained on Co/NS before and after in situ exposure to periodic changed ORR and OER potentials. (H) Schematic of the theoretical voltage and configuration of aqueous AZnAB. (I) Comparison of energy densities of the AZnAB and previously reported ZnAB at various current densities. The second, an atomically dispersed Co electrocatalyst supported on nitrogen-doped carbon nanosheet (Co/NS) was developed as the cathodic catalyst for AMABs, which has high activity and stability for both ORR and OER in acidic medium. In this work, Co/NS was prepared by pyrolysis of imidazolate frameworks of bimetallic ZnCo zeolitic encapsulated with sodium chloride crystals ([email protected]). The coexistence of nanocluster with an average diameter of ~1 nm, lattice fringes of 0.2 nm, and single atoms in the synthesized Co/NS sample were confirmed by the aberration-corrected atomic-resolution high-angle annular dark-field scanning TEM (AC-HAADF-STEM) image. In line with the AC-HAADF-STEM, X-ray absorption spectrum also revealed co-existence of the nanoclusters with Co-Co coordination number of 2 and Co species of Co-Nx (x = 1–4) in the Co/NS sample. Through the test of catalytic performance, the atomically dispersed Co/NS catalyst shows excellent ORR and OER performance in O2-saturated 0.1 M HClO4. The results showed that the half-wave potential of ORR of as-prepared Co/NS was 0.71 V, and the Tafel slope was 82.1 mV dec−1 (Figure 1E). The overpotential of OER for Co/NS at 10 mAcm–2 was 605 mV after 2,000 consecutive cyclic voltammetry cycles, and no active degradation was observed after 20 h of electrocatalysis at 10 mAcm−2 (Figure 1F). In situ extended X-ray absorption fine structure (EXAFS) data (Figure 1G) confirmed that the bond length of Co–N varies significantly between ORR and OER potentials, and periodically decreases first and then increases, suggesting that the active sites of ORR and OER are Co–Nx species. To demonstrate the practical potential, the asymmetric Zn-air battery (AZnAB) was assembled on the basis of the prepared novel ZnSTM separator and atomically dispersed Co/NS catalysts (Figure 1H). Remarkably, the AZnAB delivered an operating voltage of 1.96 V, an ultrahigh specific energy density of 1,354 Wh kgZn−1, and long-term stability (~100 h) with a round-trip efficiency of 76.6%. The specific energy density of 1,354 Wh kgZn−1 is higher than those of recently reported conventional ZnABs (Figure 1I) and even approaches the theoretical value of the alkaline electrolyte-based ZnABs. And after 300 cycles over 100 h, an increased round-trip efficiency of 70.8% can be obtained, which is comparable with the state-of-the-art Pt/C+RuO2-based AZnAB (75.1%). Moreover, the smart combination strategy is also applicable to the other type AMABs, including ASiAB and ASnAB. Impressively, both ASiAB and ASnAB have demonstrated enhanced operating voltage, power density, and excellent rate performance over the recently reported SiABs and SnABs. This work not only opens up a new way for the design of new separators that can selectively transport heavy multivalent ions in AMABs but also makes an important step for the practical application of AMABs. However, AMABs still have room for improvement in the formation of dendrites in the anode in alkaline electrolytes, the internal resistance of the cathode, and the material science and engineering of the electrode. In addition, this work provides a general strategy for AMABs, which is also applicable to the other-type assembled asymmetric MABs (metals, Si, Sn), and has guiding significance for the design of next generation metal-air batteries with large current and long-term energy storage applications. High-voltage asymmetric metal–air batteries based on polymeric single-Zn2+-ion conductorLin et al.MatterJanuary 29, 2021In BriefThe development of asymmetric-electrolyte metal–air batteries (AMABs) has been hindered by lack of polymer separator that can selectively transport metal ions and cost-effective bifunctional electrocatalyst. Here, we report breakthroughs in both key components by constructing a polyacrylonitrile membrane that can selectively transport Zn2+ ions, and an atomically dispersed Co electrocatalyst that can stably catalyze ORR and OER in acid. The assembled AMABs (metals Zn, Si, Sn) showed stable battery performance with enhanced operating voltage, power densities, and excellent rate performance that surpass those of recently reported MABs. Full-Text PDF Open Archive