Insights into zinc-air battery technological advancements

Renewable and Sustainable Energy Reviews 202 (2024) 114675 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Insights into zinc-air battery technological advancements Talal F. Qahtan a, ** , Ibrahim O. Alade b , Md Safiqur Rahaman c , Tawfik A. Saleh d, * a Physics Department, College of Science and Humanities in Al-Kharj, Prince Sattam Bin Abdulaziz University, 11942, Al-Kharj, Saudi Arabia Department of Information System and Operation Management, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia c Deanship of Library Affairs, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia d Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia b A R T I C L E I N F O A B S T R A C T Keywords: Zinc-air batteries Scientometric analysis Cluster analysis electrode materials Electrocatalysts Dendrite suppression Energy storage System performance Sustainable technologies This review combines a scientometric analysis with a detailed overview of zinc-air battery (ZAB) advances. The ZAB research landscape was critically surveyed using scientometric tools like VOSviewer and Biblioshiny. This analysis covered 10,103 articles from the Web of Science database, revealing the growth evolution, citation analysis, research clusters, and countries’ collaboration networks in ZAB research. The results reveal a remarkable annual growth rate of 11.5 %, indicating a major rise in academic interest that invariably highlights ZAB’s growing relevance. The leading countries in terms of research productivity include China, the United States, South Korea, Japan, and Australia. Furthermore, the study identifies eight research clusters focusing on electrode optimization, advanced catalysis, electrochemical performance, hydrogen evolution, and the use of biomass and carbon materials, representing the critical areas of investigation in ZABs research. Importantly, the study critically reviewed essential electrochemical mechanisms governing ZABs and also provided novel perspectives on addressing the existing challenges and the mitigating strategies of ZAB components. This research serves as a helpful reference for industry professionals and policymakers looking to push the frontiers of renewable energy technology. Interestingly, the analysis is relevant to global efforts to promote sustainable energy solutions, supporting the United Nations Sustainable Development Goals and providing avenues to improve ZAB technology for greater integration into current energy systems. This study makes substantial contributions to ZAB research by outlining a path for future advancements and regulatory frameworks. 1. Introduction The depletion of fossil fuel reserves and growing concerns about environmental safety and sustainability are driving the urgent need for effective energy storage technologies [1,2]. These technologies are critical for balancing energy supply and demand, enabling electronics, incorporating renewable energy sources, and maintaining power system reliability and stability [3]. Batteries, supercapacitors, and fuel cells are among the most notable technologies, each with its own set of advantages and applications [4,5]. Batteries have received a lot of attention for their high energy density, portability, and lifespan [6]. They are essential in today’s technologically advanced world because of their ability to store and discharge electrical energy conveniently [5,6]. Numerous battery technologies, including lead-acid, nickel-metal hydride, lithium-ion [7], sodium-ion, and others, have been developed, each distinguished by its unique material characteristics and applications [7–10]. Within the domain of electrochemical storage, Metal-air batteries (MABs) are particularly noteworthy, harnessing the high energy potential of metals like magnesium, zinc, aluminum, iron, calcium, and lithium [11]. Among these, Zinc-air batteries (ZABs) are especially prominent due to their attractive attributes. Fig. 1 illustrates the substantial energy capacities of ZABs showing their competitive advantage over other battery technologies [12]. ZABs are highly regarded for large-scale energy storage applications, favored for their cost-effectiveness, broad availability, and environmental benefits [13]. These batteries are stable in both alkaline and aqueous environments and do not require specialized fabrication conditions necessary for lithium-based systems, thus positioning them as a more sustainable choice for energy storage. Moreover, ZABs are equipped with high power density, extensive shelf life, and strong safety features which makes them well-suited for high-demand applications such as powering electric vehicles, where rapid energy discharge and recharge are required [14]. The comprehensive benefits of ZABs establish them as a * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.F. Qahtan), [email protected], [email protected] (T.A. Saleh). https://doi.org/10.1016/j.rser.2024.114675 Received 25 August 2023; Received in revised form 18 May 2024; Accepted 15 June 2024 Available online 4 July 2024 1364-0321/© 2024 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies. T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Li et al. [21] conducted a comprehensive assessment of new developments in ZAB components, such as electrode types, electrolytes, and CO2 interactions. Fu et al. [26] reviewed ZABs’ effects on cellular components, response mechanisms, and battery designs. The technical limitations of ZABs were discussed by Zhang et al. [27]. Lastly, the difficulties of electrically rechargeable ZABs, including dendrite growth, bifunctional catalyst effectiveness, and electrolyte stability, were reviewed by Gu et al. [28]. These studies show that there is a high interest in ZABs because of their technological significance, which is becoming more pronounced due to the demand for cost-effective energy storage solutions. While extensive research has been conducted on various aspects of ZABs, there is a notable scarcity in the literature for analyses that integrate these individual studies into a comprehensive overview that examines the growth dynamics, keyword analysis, research clusters, key influential entities, and other scientometric dimensions. This review addresses this deficiency by employing a holistic scientometric analysis to map the current state of ZAB research, identifying emerging trends, collaboration networks, and pivotal research clusters. This methodological approach is vital for comprehensively understanding the developmental trajectories and innovation pathways within the ZAB field, providing a clearer depiction of both the existing accomplishments and promising areas for further investigation. This study contributes significantly to the advancement of ZABs by adopting a fresh analytical approach that combines scientometric analysis with a systematic overview of ZAB advances. The utilization of bibliometric tools like VOSviewer and Biblioshiny to examine over 10,000 articles from the Web of Science database has enabled a thorough analysis of the ZAB research landscape. This approach reveals a noteworthy annual growth rate of 11.5 % and also illuminates growing research trends, major contributors, and emerging collaborative networks within the ZAB field. To broaden its influence, the study identifies and discusses key research clusters such as electrode optimization, advanced catalysis, and electrochemical performance improvement. These are critical areas for tackling some of the most persistent technological issues connected with ZABs, including dendritic suppression and oxygen transport enhancement. By effectively charting quantitative growth alongside a qualitative overview of ZAB, the study provides vital strategic insights. This study will promote optimal resource allocation, foster international cooperation, and direct future research endeavors. Furthermore, by combining scientometric insights with extensive studies of technological progress, this study highlights current accomplishments and defines a route for future advancements. It supports Sustainable Abbreviations CNTs Fe–N–C HER MAB mAh/g MOFs NPs OER ORR PAM PAMC PVA SDGs ZABs Carbon nanotubes Iron-nitrogen-carbon catalysts Hydrogen Evolution Reaction Metal-Air Batteries Milliampere-hours per gram Metal-Organic Frameworks Nanoparticles Oxygen Evolution Reaction Oxygen Reduction Reaction Polyacrylamide Polyacrylamide-cellulose Poly(vinyl alcohol) Sustainable Development Goals Zinc-Air Batteries viable option not only among metal-air batteries but also in comparison to other technologies like lead-acid, lithium-ion, zinc-carbon dioxide, and iron oxide. Despite these advantages, the adoption of ZABs is hampered by the slow kinetics of oxygen reduction and evolution reactions at the air electrode, resulting in high overpotentials, poor reversibility, limited energy efficiency, and relatively low power density [15,16]. Addressing these challenges is essential to enhance ZAB’s performance and increase its market penetration [17,18]. The attractive attributes of ZABs play a crucial role in driving the development of future energy solutions. Therefore, ZABs are considered a key component in the progression of sustainable energy storage systems. Their significant energy storage capabilities and the growing recognition of their potential show why ZABs are increasingly viewed as fundamental to the future of energy technologies [19,26,27]. The development of new electrocatalysts, improved electrolytes, and nanostructured electrodes has led to substantial progress in ZABs [20]. To further improve the commercial viability and make the most of ZABs’ distinctive features requires advancements aimed at solving its technological limitations such as dendrite formation and slow kinetics of oxygen reduction [15]. In pursuit of ZAB’s advancement, several review articles and hundreds of original research papers have been published that examine the ZAB development milestones and its challenges [21–25]. For example, Fig. 1. Comparison of practical and Theoretical energy densities in various battery technologies. 2 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Development Goals (SDGs) and advances ZABs’ commercial feasibility and technological maturity, thus, making a significant contribution to the field’s progress toward a more sustainable and efficient future. The remaining sections of this work are organized as follows: Section 2 discusses material and methods, outlining the methodology used in the study. Section 3 focuses on scientometric analysis, which includes a look at publication growth, citation analysis, keyword analysis, and the topproducing authors, agencies, and countries at the forefront of ZAB advancement. Section 4 provides an overview of ZAB advances. It discusses the fundamental electrochemistry of ZAB, components’ technological problems, improvement measures, and future directions. Section 5 concludes the study. of research activity on the topic of zinc-air batteries. Another terminology used to describe scientometrics is called science mapping [32]. It graphically depicts the connections between disciplines, domains, specialties, documents, and authors. The principal bibliometric indices under the study are as follows: annual growth of literature, productive countries, and organizations, authors with a large number of publications, high citation sources, authors’ keywords, collaborative countries, funding bodies, and research themes. 2.2. Search query The bibliographic data retrieval was done using a search query in the advanced search box of the Web of Science database. The following search query was selected “zinc-air batteries” OR “zinc-air cells” OR “-zinc-air battery” to capture the possible variants of keywords used in the literature. 2. Materials and methods 2.1. Methodology This study combines a bibliometric study of zinc-air batteries (ZABs) with a systematic review of the current state of research in zinc-air batteries. The methodology employed is outlined in Fig. 2 which comprises two parts, namely, bibliometric analysis and overview of ZAB advances. Bibliometrics is a quantitative research method used to study and assess scientific literature. It is a multidisciplinary field that uses statistical and mathematical methods to measure various aspects of scholastic publications such as publication count, citation count, authorship patterns, journal influence, and collaboration networks [29, 30]. One of the bibliometrics sub-disciplines is scientometrics. It is concerned with the review of scientific publications [30,31]. For this specific analysis, a bibliographic method was utilized to study the results 2.3. Date of data extraction On March 26, 2024, the search query was conducted, resulting in the discovery of a total of 10,234 research papers relevant to the specified topic. 2.4. Inclusion and exclusion criteria Starting with the initial search, 10,234 documents relevant documents were obtained, and thereafter exclusion and inclusion criteria were applied which resulted in the exclusion of 15 documents which are related to “Retraction”, “Retracted Publication”, “Letter”, “Data Paper”, Fig. 2. Research Methodology: (A) The PRISMA flow and bibliometric analysis (B) Overview of ZAB research. 3 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 or “Note”. Furthermore, non-English articles authored in Chinese, Korean, Japanese, German, and others are excluded which are 116 in total. Therefore, after the filtering process, 10,103 research papers were found adequate and selected for the final research. the field are likely reflected in the substantial increase in citations per article that occurred between 2010 and 2012. This upward trend serves as an indicator of key developments in the subject. Conversely, with the expansion of the discipline into increasingly specialized sub-fields, the recent decrease in citation rates indicates the existence of a multitude of research subdomains. Finally, consistent with the trend toward environmentally favorable energy sources, ZAB research has emerged mostly from relative obscurity to become a significant force within the renewable energy industry. It is essential to acknowledge the study’s limitations. As mentioned in the methodology section, this study relies on the Web of Science database and English-language publications only. This could skew the analysis by excluding important non-English research which in principle could potentially underestimate ZAB’s global productivity. However, the exclusion of only 131 journals suggests that the impact of this methodological constraint is relatively limited on policymaking and scientific knowledge. To remove this limitation in future related investigations, future studies may utilize a wider database such as Scopus, Google Scholar, and Dimension and engage the services of language translators so that nonEnglish language publications are included in the analysis. 2.5. Data analysis All the 10,103 research papers were downloaded in different file formats and analyzed with bibliometric analysis tools such as VOSviewer [33], Biblioshiny [34], Histcite, Bibexcel [35], and Microsoft Excel. 2.6. Limitations and potential sources of Error This study utilizes the Web of Science database and focuses exclusively on research papers written in English, leading to the elimination of a total of 116 publications. However, this has a negligible impact on the final result given the large number of articles analyzed. Furthermore, employing the translation services for non-English articles may improve the analysis’s accuracy and depth thus resulting in more informed policy and technology advances. To enhance our understanding of global advancements in ZAB technology, it is recommended that future research utilize a more extensive database, such as Scopus, Google Scholar, and Dimension which encompasses a broader range of papers. 3.2. Types of research publications in zinc-air batteries Fig. 4 classifies the various publication types that contribute to ZAB research, emphasizing their publication numbers, citations, citations per article, and h-index scores. Articles are the foundation of ZAB literature, with 8462 items and a considerable citation count of 435,124, boasting a high h-index of 260. This emphasizes their critical significance in spreading new research results. Reviews follow suit, with 1035 publications receiving 135,223 citations and a noteworthy h-index of 178, indicating their complete synthesis of current literature and significance in guiding the course of research on the subject. Proceedings papers, which document contributions to conferences and symposia, account for 234 publications with a total of 4799 citations and an h-index of 48. Articles under Early Access, indicating the urgency of continuing research, total 189 with 564 citations and a growing h-index of 13. News Items, Corrections, Early Access Reviews, Editorial Materials, and Meeting Abstracts, despite their lower volume and citation metrics, contribute to the scholarly discourse by offering timely updates, corrections, and editorials that enhance the academic narrative. These 3. Result and discussion 3.1. Yearly growth of publications The progression of ZAB research over nearly six decades is depicted in Fig. 3. It is noted that the publication and citation per paper indicate a substantial increase in scholarly attention. The remarkable growth of ZAB from a single publication in 1966 to 1399 papers by 2023 is evidence of its expanding influence. The average annual growth rate for publication within this period is 11.5 %. This increase in publications, especially since 2004, demonstrates the growing attention towards renewable energy sources and the importance of ZABs in the process of energy transition. The average number of citations per paper provides insight into the study’s long-term influence. Significant advancements in Fig. 3. Trend in total publications and average citation per article. 4 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 4. Types of research publications in Zinc-air batteries (a) total publication (b) total Citation (c) total citation per total publication (d) h-index. varied sorts of publications, as indicated in Fig. 4, constitute the full body of information contributing to the growth of ZAB research. number of publications. As an illustration, the scholarly journal “Advanced Materials” distinguishes itself by its substantial citation average of 204.45 per publication and 177 publications. Similarly, despite having fewer publications, “Angewandte Chemie-International Edition” demonstrates a significant impact with a high citation average. In contrast, “Journal of Materials Chemistry A″ is the most prolific journal in terms of volume, having published 560 articles. 3.3. Most relevant sources in zinc air batteries As a function of publication and citation per article, Fig. 5 depicts the top 20 journals in ZAB research in terms of their research impact and Fig. 5. Top 20 most relevant sources in Zinc Air batteries research. 5 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 However, the average number of citations per paper is comparatively moderate at 56.05, indicating that the journal covers a wide range of topics but receives fewer citations. In contrast to its peer journals, “Journal of Alloys and Compounds” has a comparatively modest impact per article, as evidenced by its 14.99 citation rate per paper and respectable 206 publications. This analysis revealed the leading journals in ZAB research. 3.5. Productive countries in zinc air batteries research The country-wise distribution of research output and its impact from among the top 20 countries are illustrated in Fig. 7. China maintains the lead in total publications (TP) with 6,381, which signifies a significant investment in this domain. However, its average citations per publication (TC/TP) is only 51.14, indicating a moderate impact despite its extensive influence. On the contrary, the United States, which has 1435 TPs, exhibits a notably higher TC/TP ratio of 118.93, which signifies not only substantial scholarly involvement but also a high influence within the academic sphere. South Korea and Japan, with TPs of 793 and 520, respectively, make substantial contributions to ZAB research and exhibit a substantial impact score, which signifies the caliber of their research contributions in furthering the technology. Australia and Canada demonstrate robust involvement and impacts that surpass the average, as evidenced by their respective TC/TP values of 83.89 and 92.59, which underscore the worldwide dedication to improving ZAB technology. European nations, including Germany, England, and France, make substantial contributions to the overall volume of research and exert a notable influence, as evidenced by France’s research impact being particularly high at 119.69 TC/TP. These results underscore a heterogeneous and cooperative global endeavor to promote ZAB research, which is reflected in the magnitude and caliber of scientific contributions. 3.4. High Yield institutions in zinc air batteries research The research impact and scholastic output among the leading 25 institutions conducting ZAB research globally are depicted in Fig. 6. The data reveals that research output is concentrated in prominent academic institutions in China, led by the Chinese Academy of Sciences, which has an average of 70.36 citations per publication and 842 publications to its credit. Nankai University, with 208 publications, attains a notable citation impact of 102.56 among the top institutions, whereas Tsinghua University has 235 publications and a citation impact of 97.91. The substantial contributions made by these organizations demonstrate their commitment to advancing battery technology and their critical role in influencing the trajectory of ZAB research. The scholarly influence and international recognition of scientists from China are further demonstrated by the University of Science & Technology of China, Tianjin University, and the University of Chinese Academy of Sciences, which each have citation impacts of 79.89, 76.99, and 75.27, respectively. The significance of the institution’s contribution to both the generation of research volume and the effective dissemination of ZAB knowledge is highlighted by the collective output shown in Fig. 6. The ongoing improvement and expansion of energy storage solutions depend upon this collaboration and contribution. 3.6. Prolific authors in ZABs research The top twenty-five scholars engaged in ZABs research are provided in Table 1. A notable individual is Wang Y who is affiliated with Qingdao University. With a remarkable average of 59.34 citations per publication, his 199 publications attest to a prolific and sustained research engagement in energy storage. Wang J, an individual affiliated with the Fig. 6. Top 25 most productive institutions in ZAB. 6 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Table 1 Leading authors and their contributions to ZABs research. Author Affiliations Country TP TC TC/TP WANG Y WANG J Qingdao University Chinese Academy of Sciences Heilongjiang University The University of Sydney Chinese Academy of Sciences Qingdao University China China 199 173 11808 13415 59.34 77.54 China Australia China 169 168 138 8546 8223 6996 50.57 48.95 50.70 China 137 10754 78.50 Tsinghua University China 125 12812 102.50 Qingdao University of Science and Technology Xinjiang University China 125 9214 73.71 China 114 5624 49.33 China 111 4823 43.45 China China 111 107 3822 6712 34.43 62.73 Australia China 102 97 11596 8843 113.69 91.16 China Canada 96 91 2856 11732 29.75 128.92 China USA China Singapore 89 89 88 86 6189 5832 3689 5372 69.54 65.53 41.92 62.47 Australia 84 5876 69.95 China 82 4423 53.94 China 81 6786 83.78 Australia China 81 81 4705 3629 58.09 44.80 WANG L LI J ZHANG Y ZHANG J ZHANG Q LIU J ZHANG L WANG H LIU Y WANG X CHEN J LI L LI Y CHEN ZW LIU X YANG Y YANG J ZHANG H SHAO ZP CHEN X Fig. 7. Top 20 most productive countries in ZAB research by (a) total publication and; (b) total citations per paper. Chinese Academy of Sciences, exhibits a substantial influence in the field, as evidenced by the 77.54 citation impacts and 173 articles that his work has amassed. An additional notable contributor is LI J from the University of Sydney, who has 168 publications and a TC/TP ratio that substantiates the impact of his research contribution. Wang L, affiliated with Heilongjiang University, has authored over 169 publications. An illustration of high-caliber research being generated is the exceptional mean citation impact of 113.69 exhibited by Chen J from the University of Wollongong. It is important to mention that Table 1 was extracted from the Web of Science database. There could be some variations in these numbers depending on the database. The main essence of Table 1 is to highlight influential authors working in ZAB research. As demonstrated in Table 1, the collaborations of these individuals and their respective organizations constitute a global network that plays a pivotal role in the swift advancement of ZAB technology. ZHANG X CHEN Y WANG Q Qingdao University of Science and Technology Zhengzhou University Qingdao University of Science and Technology University of Wollongong University of Shanghai for Science & Technology Tsinghua University University of Waterloo Shihezi University University of Central Florida Tianjin University National University of Singapore Curtin University Chinese Academy of Sciences Chinese Academy of Sciences The University of Sydney Chinese Academy of Sciences directly affects the speed and direction of scientific progress in the subject. 3.8. Analysis of co-occurrence of the keyword Author keywords are important in scientometric investigations because they provide insight into research issues, trends, and collaborations [36,37]. Clustering approaches use bibliographic information to categorize similar articles, authors, organizations, or research fields. Research trends, subfields, and communities are revealed using clustering techniques such as clustering or hierarchical k-means, as well as a similarity matrix [38]. Clustering algorithms are used to group similar publications, authors, organizations, or research topics according to bibliographic information or citation patterns [36]. This technique helps study the growth and development of scientific publications. The keywords utilized by the authors to reproduce the content and perspective of the work are analyzed in Fig. 9. The analysis takes into account keywords that appear a minimum of 20 times out of a total of 10,519. As a result, 149 keywords satisfy the specified criterion. The prevalence of the keywords is visually depicted in Fig. 9 using the sizes of the circles and spheres, which signify the frequency and intensity of link occurrences. By indicating the publications that contain the provided keywords through the strength of the association between the nodes, additional classification of the 149 keywords into eight subgroups, each represented by a unique color is performed. Keyword clusters for ZAB research encompass a wide range of issues, each with its focus which are described as follows: Cluster 1 (Core Components and Technologies) focuses on the many components of 3.7. Most Influential Funding Agencies in ZAB research A summary of the funding organizations most active in the ZAB research is shown in Fig. 8. Standing out clearly with an impressive 4751 publications, the National Natural Science Foundation of China (NSFC) demonstrates its leading position in funding research in ZAB. The Fundamental Research Funds for the Central Universities rank second with 691 publications. Other noteworthy organizations that actively support research include the US Department of Energy (DOE) with 387 publications and the China Postdoctoral Science Foundation with 433 publications. There is a strong global effort towards ZAB development, as shown by lesser but no less significant participation from groups like the National Science Foundation (NSF) in the United States with 323 publications and the National Research Foundation of Korea with 263. As a reflection of their objectives and the value they put into developing ZAB technology, Fig. 8 highlights the disparate amounts of research engagement by various nations and organizations. This figure offers a vital window into the global distribution of research funding, which 7 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 8. Top 10 most Influential Funding Agencies in ZABs research. Fig. 9. Analyzing and visualizing the co-occurrence of Author Keywords. such as flexible ZABs and hydrogen evolution demonstrates the cluster’s broad approach to understanding the many aspects of battery technology and proposes avenues for enhancing critical parts to improve battery efficiency and durability. Cluster 2 (Advanced Catalysis) focuses on ZABs, such as electrode materials (e.g., air cathode, zinc anode), electrolytes (e.g., alkaline, gel polymer), and catalysts. This cluster also investigates the performance and characterization of batteries, such as using electrochemical impedance spectroscopy. The inclusion of issues 8 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 air batteries and how to improve their performance utilizing materials such as carbon nanofibers and metal-organic frameworks, with a particular emphasis on their involvement in bifunctional oxygen electrocatalysis and total water splitting. This cluster seeks to increase the stability and efficiency of rechargeable ZABs using innovative material applications. Cluster 3 (Material-Driven Electrocatalysis) focuses on electrocatalysis, including materials like carbon nanotubes and cobalt oxide employed in energy conversion and storage. This cluster emphasizes the crucial importance of these materials in processes such as oxygen evolution and reduction, which are required for efficient electrocatalysis in energy systems. Cluster 4 (Materials and Processes for Energy Conversion) focuses on batteries, catalysis, and energy conversion, investigating materials and procedures that improve battery technology, such as graphene and heterogeneous catalysis. This cluster’s research focuses on improving the performance and energy storage capacities of batteries, namely metal-air batteries, fuel cells, and supercapacitors. Cluster 5 (Optimization of Electrocatalytic Materials) focuses on improving electrocatalytic materials for oxygen evolution and reduction processes (OER and ORR), which are critical for energy storage and conversion systems. Materials such as nitrogen-doped graphene and perovskites are being studied for their capacity to improve electrocatalytic activity and reliability. Cluster 6 (Active Site and Catalyst Design) centers on the optimization of active sites within catalysts to improve electrocatalytic activity in OER and ORR. It focuses on materials such as manganese oxide and nanosheets to build extremely active and stable single-atom catalysts. Cluster 7 (Sustainable Energy Materials) examines biomass and carbon materials, particularly their application in energy processes such as ORR, and how to improve their performance using techniques such as heteroatom doping. Cluster 8 (Hydrogen Evolution Mitigation) focuses on the hydrogen evolution process (HER) in ZABs, with the goal of understanding and mitigating its influence to reduce energy loss and enhance battery performance via better catalysts and battery designs. Each cluster focuses on different aspects of ZAB research, resulting in a greater understanding and development of more efficient and sustainable battery technology. 3.9. Visualizing research keywords by word cloud of titles Fig. 10 depicts the top 100 keywords retrieved from the titles of research publications on ZABs that were examined using word clouds. The word cloud depicts the most commonly recurring terms in the titles. It provides a concise summary of the major themes or issues addressed in the studied set of titles [39,40]. The larger and bolder words appear more frequently, whereas the smaller words occur less frequently. This word cloud is a useful tool for detecting repeating themes, popular issues, or key areas of concentration in the dataset [41]. It can help to grasp the core interests or subjects of the titles examined, offering insights into significant areas of discussion or study [41]. The most common terms in the list are battery kinds such as “rechargeable,” “zinc-air,” “lithium-air,” and “metal-air” batteries. These batteries have lately gained popularity due to their high energy density and long cycle life. The phrases “oxygen,” “reduction,” and “evolution” are also prominently included in Fig. 10, indicating the significance of oxygen electrochemistry in the field of batteries and electrochemistry. These phrases refer to ORR and OE are required for the operation of many types of batteries and fuel cells. The terms “carbon,” “graphene,” and “nanoparticles” appear often in Fig. 10, showing an interest in employing carbon-based materials like nanoparticles and graphene in battery applications [42,43]. These materials have a wide surface area and strong conductivity, making them ideal for battery electrode materials. Fig. 10 also includes several battery-related terminology, including “cathode,” “anode,” “electrolyte,” and “metal.” These words emphasize the need to create novel materials with higher performance and stability for use in batteries. Overall, Fig. 10 is a valuable snapshot of current research trends in batteries and electrochemistry. Future studies can acquire insights into the most active areas of study and the significant difficulties confronting the field by evaluating the most frequently used keywords in research paper titles. 3.10. Top 10 Keywords Driving Zinc-Air Battery Innovation Fig. 11 shows the top ten keywords from ZAB’s research, along with their frequency distributions. The most often appearing term “batteries” (4986 times) is expected since it embodies the essence of what is being Fig. 10. Frequency-based word cloud generated from the top 100 titles in zinc-air battery research. 9 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 11. Top 10 keywords driving zinc-air battery Innovation. investigated, demonstrating a general interest in energy storage devices. The second-most commonly appearing term “oxygen” (3953) indicates how important oxygen is in the chemistry of ZABs. Furthermore, the prevalence of “zinc air” (2956 times) shows that this sort of system is now receiving significant attention. The term “carbon,” which has been referenced 2381 times, is most likely related to the investigation of carbon-based materials, such as carbon nanotubes or graphene, for use in ZAB electrodes. This emphasizes the material science aspect of the discipline, in which carbon’s flexibility and electrical capabilities are used to improve performance. The keyword “electrocatalysts,” which appears 2204 times, indicates an intensive study on improving the efficiency of electrochemical processes within the battery. The keyword “reduction” (2074 times) most likely relates to the term oxygen reduction reaction (ORR) in the battery anode. The keyword “rechargeable” (1569 times) is also in the top ten, indicating that there is a great interest in developing rechargeable ZAB. The keyword “bifunctional,” used 1553 times, refers to research in catalysts that are effective in both oxygen evolution and reduction processes, which is crucial for rechargeable zinc-air batteries. This indicates a tendency toward creating catalysts that can function efficiently in both the charging and discharging stages. These keywords indicate that research on ZABs combines material innovation, process improvement, and a need for rechargeability. The frequency with which these keywords appear in the literature indicates that they are at the forefront of current research efforts and will continue to be crucial to the growth of ZAB technology. visible line between China and the United States represents the most research collaboration. Thinner lines linking China to Singapore, Japan, the United Kingdom, and Germany show lower collaboration activity, as demonstrated by frequencies of 210, 175,156, and 118. Fig. 12 depicts the number of shared research projects, as well as the breadth of scientific interchange, innovation, and information transfer across borders in the field of ZABs. The map portrays the global scope of scientific advancement, with the thickness of the lines indicating the deep connections and interdependence of the international research community. 4. Overview of ZAB This section examined the details of ZABs focusing on recent advances in key components of ZABs such as zinc electrode, air cathode, separators, and electrolytes. Included in this discussion are the challenges confronting ZABs and the various improvement strategies that are being implemented in furtherance of the ZAB technology. 4.1. Fundamental of zinc-air batteries Fig. 13 depicts the fundamental electrochemical reactions of ZAB. Rechargeable ZABs use a reversible electrochemical process that happens throughout both the discharge and charging stages. This process is based on redox interactions between zinc and oxygen from the air, which result in the formation and dissolution of zinc oxide [25,44,45]. As shown in Fig. 13, the operating mechanism of the ZAB operating under an alkaline medium may be split down into two steps: the discharge and charging process, which are briefly illustrated as follows: Discharge process: Anode Reaction. During the discharge cycle, zinc metal at the anode reacts with hydroxide ions to form zincate ions while releasing electrons into the external circuit. This oxidation process is fundamental for driving the electrical current. The reactions occurring at the anode are summarized as follows [12]: 3.11. Country collaboration in producing zinc air batteries research Fig. 12 depicts a ZAB-based collaborating network among major countries. It demonstrates the level of collaboration among the leading nations involved in ZAB research. The strength and thickness of the lines that connect nations indicate the intensity and frequency of their collaborative efforts. China’s collaborative publications with the United States, Australia, and Canada are the most numerous, with 510, 365, and 221 publications, respectively. According to the data chart, the most 10 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 12. Global Collaborative Networks in Zinc-Air Battery Research. The thickness of the lines indicates the frequency of collaboration, with a uniform color representing all connections for clarity. − Zn + 4OH− → Zn(OH)2− 4 + 2e (1) − Zn(OH)2− 4 → ZnO + H2 O + 2OH (2) Cathode Reaction. At the cathode, oxygen from the air is reduced by water and electrons from the external circuit to produce hydroxide ions. This oxygen reduction reaction (ORR) is crucial for maintaining the flow of electrons through the battery. The governing equation is described as follows O2 + 2H2 O + 4e− → 4OH− (3) Overall Discharge Reaction. The overall reaction for the discharge process combines the anode and cathode reactions, showing the transformation of zinc and oxygen into zinc oxide, thereby providing electrical energy: 2Zn + O2 → 2ZnO (4) Charging process. Anode Reaction. Upon recharging, zinc oxide at the anode is converted back to zincate in the presence of water and hydroxide ions. This reaction reverses the anode process during discharge, preparing the anode for future cycles. The reactions occurring at the anode are summarized as follows. Fig. 13. Schematics of the zinc-air battery illustrating the discharge and charging reactions. 11 ZnO + H2 O + 2OH− → Zn(OH)2− 4 (5) − Zn(OH)2− → Zn + 4OH− 4 + 2e (6) T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Cathode Reaction. During the charging phase, hydroxide ions at the cathode are oxidized to form oxygen and water, releasing electrons back to the external circuit. This oxygen evolution reaction (OER) reverses the cathodic process observed during discharge: 4OH− → O2 + H2 O + 4e− efficiency and cycle life. These challenges depicted in Fig. 14 include dendrite formation, corrosion, passivation, and hydrogen evolution [51–54]. It is crucial to address these obstacles to advance the development of zinc-based battery systems that are sustainable. A comprehensive analysis of the primary obstacles associated with zinc electrodes is described as follows. (7) 4.2.1. Zinc dendrite formation Zinc dendrites formation is a common limitation of ZABs. They are structurally sharp, metallic spikes that have a needle-like structure that occurs during ZAB’s charging and discharging cycles [55–57]. Fig. 15a-d presents the various stages of zinc dendrites formation. Initially, the zinc anode is a smooth and flat surface with no apparent irregularities. In the process of discharging, zinc ions are oxidized, which dissolves into the electrolyte and leads to an uneven distribution of the ion as reflected in Fig. 15b. This condition is conducive to dendrite development, which is realized in the charging cycle. When the zinc in the electrolyte is re-deposited onto the anode as shown in Fig. 15c, it tends to deposit mostly on the high point surface, appearing to be the initial formation of the dendritic structure. The dendrites formation continues throughout the charging phase until fully developed when the visual state appears, as shown in Fig. 15d. The length of the dendrite may result in a condition where the dendrite becomes a possible short-circuit bridge between the anode and the cathode [55]. Different strategies are employed for preventing the formation of zinc dendrites. These include the use of electrolytes, anode surface modifications, and the use of coatings [58,59]. These approaches principally intend to influence uniform zinc deposition, which reduces dendrite formation [60-62]. Overall Charging Reaction. The overall reaction for the charging cycle regenerates zinc and releases oxygen, highlighting the battery’s capability to reverse the discharge reactions and prepare for subsequent energy output: 2ZnO → 2Zn + O2 (8) This equation summarizes the net effect of the charging process, highlighting the restoration of zinc and the evolution of oxygen [46]. This process regenerates zinc metal at the anode and produces oxygen at the cathode. ZABs may be charged and discharged several times, making them a feasible alternative for energy storage applications such as electric cars, renewable energy integration, and portable devices. The great energy density of these batteries and their ability to be recharged make them very useful for these applications [47–49]. 4.2. Zinc electrode in ZABs In ZABs, the zinc electrode is a fundamental component. The appeal of ZABs as a feasible energy storage device primarily stems from the abundant availability and economical cost of the zinc electrode, in addition to its relatively high energy storage capacity [50]. In addition, its high reduction potential renders zinc an excellent candidate for electrochemical reactions. Furthermore, considering that ZABs rely on unlimited oxygen, a significant portion of the capacity limitation is imposed on the zinc electrode. Effective electrodes must have a sizeable active material, exhibit high rechargeability, and endure numerous cycles of charging and discharging. Critical to performance and, by extension, battery efficacy as a whole is the structure and morphology of the zinc electrode. By augmenting the surface area of the zinc electrode, the reaction kinetics are enhanced, consequently leading to a more substantial power density of the battery as a whole. There are several challenges that zinc electrodes must overcome to achieve maximum 4.2.2. Zinc anode corrosion Zinc anode corrosion is an unavoidable electrochemical process in which zinc gradually degrades due to its inherent reactivity with oxygen and moisture [63]. This process has the potential to result in the accumulation of zinc oxides or carbonates in the form of white powder. Such a buildup may undermine the anode’s structural and electrical integrity, thereby diminishing the battery’s overall capacity and lifespan [63].To mitigate this, it is necessary to employ a multifaceted strategy. To ensure the anode’s longevity and prevent direct contact with corrosive elements, protective coatings are implemented over the zinc [64] Furthermore, the integration of metals such as magnesium or aluminum into zinc alloys can result in the formation of a durable protective coating on the anode, thereby augmenting its resistance to corrosion [65–67]. Additionally, the corrosion process can be slowed by optimizing the electrolyte’s composition, ensuring a proper pH balance, and adding corrosion inhibitors [68]. Cathodic protection, an additional efficacious approach, operates by employing a consistent and modest electric current to counteract the adverse electrochemical reactions that give rise to corrosion [63,69]. As a result of the combined implementation of these strategies, the durability and effectiveness of zinc anodes are improved, thereby aiding in the advancement of more resilient and extended battery systems. 4.2.3. Passivation When insoluble zinc compounds build up on the anode during discharge in ZABs, a barrier that prevents electrical flow and reduces the cell’s efficiency results [15]. The electrochemical interactions between the zinc and the electrolyte are what mostly cause these phenomena, especially when zinc changes into its oxide or hydroxide state and produces a non-conductive coating that sticks to the surface of the anode [15]. Carefully textured zinc anode surfaces can introduce microstructural differences that prevent the passivation layer from forming uniformly, therefore addressing this problem [68]. Furthermore, careful application of electrolyte additives might have two purposes: certain additives are intended to stop the passivation layer from forming at all, while others try to keep any layer that does form electrically conducting [70]. By periodically reversing its formation, a pulsed charging regimen Fig. 14. Key challenges in zinc anode for zinc-air batteries. 12 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 15. Dendrite formation Process in Zinc-Air Battery. also prevents the passivation layer from growing, maintaining the anode’s surface open to ion exchange and guaranteeing long-term battery performance [71,72]. By reducing the negative consequences of passivation, these focused actions protect the energy production and operational life of ZAB. 4.2.4. Hydrogen evolution reaction The Hydrogen Evolution Reaction (HER) in ZAB chemistry is an unwelcome competitor to the main energy-producing reactions [73,74]. The discharge of hydrogen gas at the end of this reaction not only saps energy but also increases the possibility of expanding or even breaking the battery container [75,76]. Solving this problem calls for a multidimensional strategy. Carefully chosen catalysts will drive the primary processes while limiting the synthesis of hydrogen [15]. Technological developments in electrolyte solutions are equally important since novel formulations are designed to lower the risk of HER [76,77]. Lastly, batteries have devices built in to control pressure by properly releasing hydrogen gas, therefore guaranteeing safety and preserving structural integrity. These strategies show a deliberate attempt to improve zinc-air battery technology, therefore raising its general effectiveness and dependability. Fig. 16. Sectional view of air cathode in zinc-air battery [78]. noteworthy catalytic stability, however, they have higher prices relative to carbon-based material and a propensity for material degradation [79]. The development of bifunctional materials capable of catalyzing both oxygen reduction and evolution processes marks a significant step towards rechargeable devices. Nonetheless, long-term stability and the identification of such active materials continue to be top research priorities [80–82]. Noble metals have long dominated the field of catalytic efficiency and kinetics; however, cost, availability, and susceptibility to contaminants limit their applicability [83]. Other cathode materials include composites and polymer-based cathodes. These choices are less expensive, but they fall short of their counterparts in terms of conductivity and catalytic activity. The development of composite materials tries to capitalize on synergistic effects to increase activity and durability. However, this increases fabrication complexity and presents material distribution challenges [84]. Table 3 shows the key performance metrics of various novel electrocatalysts used in ZABs. This table contains information on electrolyte composition, energy and power densities, and durability over cycles. The data provides a foundational comparison for assessing the effectiveness and potential practical applications of different electrocatalyst formulations. Complementing this information, Fig. 17 provides insights into the various factors that influence the performance and stability of zinc and air electrodes in ZABs. It identifies key issues such as dendrite 4.3. Air cathode The air cathode is an important component in the effectiveness of ZABs because it supports the redox processes that promote energy conversion. It is characterized by porosity and electrocatalytic robustness, which are critical in the media in which ZABs function, hence improving the oxygen reduction reaction required for battery power generation. Fig. 16 depicts the cross-sectional view of the air cathode’s multi-layered structure. This structure includes the oxygen-permeable top layer, gas diffusion layer, catalytic layer, and current collector all of which are essential to the cathode’s functionality. Material science plays an important part in the progress of air cathode technology. Table 2 lists the use of a variety of materials for air cathode, each with its own set of benefits and disadvantages. Carbon-based materials, such as activated carbon and graphene, have higher electrical conductivity and manufacturing simplicity but are limited by poor catalytic activity and corrosion susceptibility [78]. Metal-based compounds, such as manganese and iron oxides, have 13 T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 transport between the zinc anode and the air cathode [103]. The choice of electrolyte significantly affects the battery’s energy capacity, efficiency, and lifespan. As shown in Fig. 18, electrolytes are classified into three main categories: aqueous, solid, and non-aqueous, each with distinct advantages and challenges that influence their suitability for different applications. Aqueous electrolytes are modified with additives like organic compounds and surfactants to enhance chemical stability and viscosity [104–107]. Non-aqueous electrolytes, which include organic compounds, deep eutectic solvents, and room-temperature ionic liquids (RTILs), offer excellent solvation and conductivity [108]. Solid-state electrolytes, though generally less conductive than liquid types, provide increased safety and stability and can achieve higher conductivity under certain conditions [109–111]. This classification aids in understanding the diverse electrolyte chemistries and their impact on enhancing battery performance, focusing on improving conductivity, stability, and environmental sustainability. The detailed discussion of each type of electrolyte is explained further in the subsequent sections. Table 2 Comparative analysis of air cathode materials: Benefits and challenges. Types of Air Cathode Material Benefits Challenges References Carbon Based Material (Activated carbon, carbon black, graphene, carbon nanotubes) Metal oxides Materials (Manganese oxide, iron oxide High electrical conductivity, low cost, easy fabrication Limited catalytic activity, carbon corrosion [88] Abundant in natural ores, minimally toxic, cost-effective, and environmentally benign. Catalyzes both ORR and OER, essential for rechargeability Potential for degradation [89,90] Active material discovery, longterm stability [22] High cost, scarcity, susceptibility to contaminants Fabrication complexity and material distribution issues [91] Bifunctional Material (Transition metal oxides, perovskites) Noble Metal (Platinum, palladium on carbon supports) Composite (Carbon with metal oxides, polymers with metal nanoparticles) High catalytic activity, excellent kinetics Synergistic effects can enhance activity and durability 4.4.1. Aqueous electrolytes Aqueous electrolytes are essential for ZABs because of their low cost, high ionic conductivity, and compatibility with zinc electrodes [112]. Potassium hydroxide (KOH) is especially popular in alkaline solutions due to its remarkable conductivity. However, difficulties such as electrode corrosion and electrolyte passivation restrict their usefulness, necessitating extensive study into improving conductivity and chemical stability to increase battery efficiency and longevity [112]. Table 4 presents an evaluation of the performance metrics exhibited by different aqueous electrolytes employed in zinc-air batteries. The comparison encompasses significant attributes of each configuration, including specific capacity, power density, durability, electrolyte composition, and electrode materials. By illustrating the wide range of electrolyte formulations and their respective effects on battery performance, this table emphasizes how customized electrolyte compositions can significantly improve the durability and effectiveness of zinc-air batteries. Optimal KOH concentrations, usually between 20 and 30 wt%, are critical because they improve conductivity and zinc oxide solubility. [92] growth, corrosion, and sluggish kinetics and explores mitigation strategies such as the use of polymer electrolytes, bifunctional catalysts, and a range of catalyst technologies (PGMs-dependent, PGMs-free, carbonfree) [85–87]. The combination of Table 3 and Fig. 17 helps deepen the understanding of the complex interactions and challenges within zinc-air battery systems. 4.4. Electrolytes in zinc-air batteries Electrolytes are crucial for the operation of ZABs as they facilitate ion Table 3 Recent studies on electrocatalyst performance metrics in zinc-air battery. Electrocatalysts Electrolyte Used Energy Density Power Density Durability (Cycle Life) Notable Features Reference Nitrogen, Phosphorus, and Sulfur Co-doped Graphene (NPS-G-2) 6.0 M KOH with 0.2 M zinc acetate 805 (normalized to Zn) [93] 0.1 M KOH – 6 M KOH/0.2 M zinc acetate – 67.9 mW cm−2 Nickel-iron layered double hydroxides interlinked by Ndoped carbon network (NiFeCNTs-rGO) CoFe@NCNT 6.0 M KOH 793 mA h g−1 at 10 mA cm−2 (normalized to Zn) 87 mW cm−2 Over 100 h of stable operation Superior ORR activity, temperature adaptability from −10 ◦ C to 40 ◦ C, high mechanical flexibility Enhanced ORR and OER performance, superior to noble metal catalysts, good cycling efficiency Superior bifunctional activity for ORR and OER, excellent cycling stability, high specific capacity [94] Core-shell-structured NiCo2O4 with GO/C Very stable, minimal loss after 10,000 cycles Over 650 h of stable charging and discharging Over 200 h with excellent stability High ORR activity, methanol tolerance, excellent electrochemical stability Mn-modulated Co–N–C oxygen electrocatalyst (Co4Mn1@NC) 151 (0.151 W/cm2) 163.9 mW cm−2 Mixed solution of 6.0 M KOH and 0.2 M Zn(OAc)2 6.0 M KOH, 0.1 M Zn(Ac)2 KOH, Zn(Ac)2 795.0 mAh/g 194 mW cm−2 900 cycles at 5 mA/cm2 808.6 mAh/g 178 mW/ cm2 22.4 mW/ cm2 60 mW/ cm2 144 mW cm−2 138 mW/ cm−2 Stable after 1000 h CoFe–Co5.47N@NC Mn–N–C ZnO–N–C-600 Co–N co-doped carbon nanosheets 6 mol/L KOH + 0.2 mol/L Zn(Ac)2 0.1 M KOH NiCo-LDH supported on GNF 0.5 M KOH 763.9 mAh/g – 821.50 Wh/kg 806 mAh g−1 14 1000 h cycle life 800 h at 0.5 mA cm2 430 cycles at 5 mA/cm2 950 cycles High ORR/OER activity, superior kinetics, outstanding discharge stability, and excellent durability High ORR and OER activity, good stability in electrolyte High ORR activity, excellent catalytic stability, effective Mn dispersion Excellent reversibility, dendrite suppression, enhanced electron transport Superior ORR performance, excellent stability, methanol resistance Superior activity in oxygen reduction and evolution reactions, stable performance up to 950 cycles, high specific capacitance (806 mAh/g). [95] [96] [97] [98] [99] [100] [101] [102] T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 Fig. 17. Zinc-air challenges and improvement methods. These electrolytes are often composed of organic solvents such as propylene carbonate or ionic liquids such as 1-ethyl-3-methylimidazolium tetrafluoroborate, and they are chosen for their low freezing temperatures and strong electrochemical stability [122,123]. While they improve battery safety and reduce risks such as hydrogen gas evolution and dendrite formation, they also face challenges such as lower ionic conductivity than aqueous solutions, volatility, flammability, and compatibility issues with zinc electrodes, all of which can jeopardize battery structure and efficiency [124]. Ongoing research intends to enhance these electrolytes using new solvent combinations and stability-enhancing additives, hence improving stability and prolonging the life of ZABs [113,125]. Despite these advances, the use of non-aqueous electrolytes in ZABs continues to evolve, matching the considerable development observed in lithium-ion batteries and showing promise in enhancing the electrochemical stability and performance of zinc-air batteries. 4.4.3. Solid state electrolyte Solid-state electrolytes, particularly solid-polymer electrolytes (SPEs) and gel-polymer electrolytes (GPEs) are becoming important in improving the performance and durability of ZABs [109]. SPEs, which are made up of ionic conductive macromolecules, offer various advantages, including increased mechanical strength, the capacity to produce thin films, reduced electrode corrosion, and the removal of leakage concerns, consequently improving battery life and operating stability [126]. Despite their benefits, SPEs have several obstacles, including low ionic conductivity, restricted zinc salt solubility, the creation of passive layers, and high interfacial resistance, which can impede ion transport and increase operating overpotentials [126]. GPEs, on the other hand, improve ionic conductivity by integrating liquid electrolytes within a polymer gel matrix, making them ideal for ZAB applications [110]. These electrolytes are often created utilizing techniques such as phase inversion, solvent casting, and in-situ polymerization. Poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA), poly (acrylonitrile) Fig. 18. Classification of electrolytes for zinc-air batteries (ZABs). Concentrations above 6–8 M can increase viscosity and stimulate the production of Zn(OH)₄2⁻, resulting in electrode passivation. Modifying the zinc electrode structure and using chemicals such as K2CO3 can help to improve electric potential, energy density, and stability [112]. Additives such as sodium dodecylbenzene sulfonate (SDBS) and polyethylene glycol (PEG) are particularly important because they limit dendrite growth and hydrogen evolution, which improves battery safety and performance [113]. 4.4.2. Non-aqueous electrolyte Non-aqueous electrolytes are becoming more important for ZABs, as they improve performance across a wide temperature range [121]. 15 Renewable and Sustainable Energy Reviews 202 (2024) 114675 T.F. Qahtan et al. Table 4 Performance comparison of aqueous electrolytes in Zn-air batteries. Electrolyte Composition Electrode Materials Specific Capacity (mAh g−1) Power Density (mW cm−2) Durability Notable Features References 6 M KOH Zinc plate//Mn–N–C 816.6 140.0 Zinc plate//Fe/CoNx-C 798.25 134.97 Zinc plate// NiCo2O4@GO/C 774.2 67.9 Over 200 h 7 M KOH þ saturated ZnO þ 0.35 M KI Zinc plate//Fe-CoNx – 141 21 h of stable operation 6 M KOH þ 0.2 M Zn (OAc)2 6 M KOH þ 0.2 M Zn (OAc)2 6 M KOH þ 0.2 M Zn (OAc)2 Zinc plate//Fe/Ni–NC – 148.5 Over 150 cycles – 217.7 – 150.1 Exceptional stability over 459 h Over 4000 cycles Abundant Mn–Nx active sites, enhanced ORR activity, long-life cycle Hierarchical porous structure enhances electrocatalytic performance Enhanced catalytic activity and stability with core-shell structure integrating NiCo2O4 and GO/C The addition of KI enhances stability and lowers charging voltage, improving roundtrip efficiency. High bifunctional ORR/OER activity with ΔE of 0.75 V Superior ORR activity due to synergistic effects and vacancy engineering Integration of N/S co-doped graphene enhances ORR and OER activities [114] 6 M KOH þ 0.2 M Zn (OAc)2 6 M KOH þ 0.2 M Zn (OAc)2 1000 h at 10 mA cm⁻2 80 h at 10 mA cm⁻2 Zinc plate//Co/ MnO@NC Zinc plate//Lamellarstacked Co3O4AS/NSG (PAN), and poly (vinylidene fluoride) (PVDF) are common polymers utilized in GPEs, along with aprotic solvents [127]. Continuous developments in SPE and GPE technologies are critical for increasing ZAB capabilities [110]. The latest developments in zinc-air batteries utilizing solid-state hydrogel electrolytes are presented in Table 5. The table provides comprehensive information on the compositions of the electrolytes, electrode materials, specific capacities, power densities, and durability metrics. The table underscores the critical significance of hydrogel electrolytes in augmenting the mechanical characteristics, ionic conductivity, and overall efficacy of solid-state zinc-air batteries. This is especially true for applications that demand adaptability and sustained stability. [115] [116] [117] [118] [119] [120] characterized by their favorable ionic permeability and chemical stability [137]. In contrast, ceramic separators are favored for use in applications involving high temperatures because of their improved thermal stability [138]. By combining polymeric materials with ceramic or other additives, the composite separators increase mechanical strength and ionic conductivity. Maintaining a sufficient ionic flow while avoiding deterioration and leakage—both of which can reduce the performance and longevity of batteries—is one of the major issues with separators. To improve safety and efficiency, new nanostructured materials have been developed that boost ionic conductivity while decreasing thickness [137,138]. Other promising innovations include bio-based materials, which provide competitive performance without sacrificing environmental sustainability [139]. In the future, it is desirable to design separators that are even more structurally sound and efficient in transporting ions by incorporating novel nanocomposite materials and functional coatings. Improvements in these areas may pave the way for ZABs that are more suited to meet the energy storage demands of the future [135]. 4.5. Air separators in zinc-air batteries Although ionic transfer is required for electrochemical reactions, separators in ZABs are of critical importance in preventing physical contact between the anode and cathode [135]. In addition to preventing internal short circuits, these components are vital for preserving the battery’s efficiency and internal stability. ZABs employ a variety of separator types, which are categorized primarily by the materials employed [136]. As an illustration, polymeric separators composed of polyolefin compounds (e.g., polyethylene or polypropylene) are 4.6. Future outlook The scientometric study and extensive overview highlight ZABs as a promising sustainable energy storage technology. This section discusses Table 5 Recent advances in zinc-air batteries using solid-state hydrogel electrolytes. Electrolyte Composition Electrode Materials Specific Capacity (mAh g−1) Power Density (mW cm−2) Durability Notable Features References Alkaline PAM hydrogel NiCo layered double hydroxide@NiCo2S4 – 88.4 NiFe alloy embedded in NC nanotubes – 114.7 Sodium polyacrylate (PAANa) hydrogel Zinc plate anode//MnO2/C cathode – 100.7 183 cycles at 10 mA cm2 Bioinspired tough solidstate electrolyte (MC/ PAM-PDMC) Zinc foil anode//Carbon cloth air cathode 758 148 Ultralong cycling stability of 320 h KOH doped PVA gel Zinc foil//Co3O4/Co(OH)2 hetero quantum dots Zinc anode//Mn-based spinel (CoMn1.5Ni0.5O4) cathode Zinc foil//Pressed nickel foam cathode 808 103.4 816.4 147.4 Stable voltage gap over 70 h High durability 476.8 63 68 cycles Enhanced charge transfer kinetics, optimized energy-level configuration, excellent stability in solid-state ZABs High performance in flexibility and electrochemical metrics, suitable for wearable applications High conductivity and excellent mechanical properties; suitable for flexible electronics High ion conductivity, excellent mechanical properties, and outstanding flexibility suitable for wearable applications Superior reversibility and performance in wearable applications High ionic conductivity, excellent tensile strength Flexibility suitable for wearable electronics [128] Dual network hydrogel electrolyte High round-trip efficiency of 74.3 % 500 h of operation PAMC gel (Polyacrylamidecellulose) Alkaline PVA gel (PVA/ KOH) 16 [129] [130] [131] [132] [133] [134] T.F. Qahtan et al. Renewable and Sustainable Energy Reviews 202 (2024) 114675 future research directions targeted at increasing ZABs’ global influence. Moving ZABs to the forefront of next-generation energy technologies necessitates significant advancement across a variety of technical areas. Identified here are key areas for new research that could considerably improve ZAB capabilities and deployment, which is critical for overcoming current constraints and realizing full promise in energyintensive applications. 2. The scientometric analysis revealed that China, the United States, South Korea, Japan, and Australia are the top 5 leading contributors to ZAB research. The total publications (TP) from these countries during the time under review are 6,381, 1435, 793, 520, and 496, respectively. Regarding research impact measured by total citations per total publication record (TC/TP), the United States leads with a TC/TP of 118.93. The TC/TP for China is 96.85, and for South Korea, it is 52.56. 3. The scientometric analysis identified eight research clusters. These highlight the critical areas of focus for future technological advancements in ZABs. These research clusters are “Core Components and Technologies”, “Advanced Catalysis”, “Material-Driven Electrocatalysis”, “Materials and Processes for Energy Conversion”, “Optimization of Electrocatalytic Materials”, “Active Site and Catalyst Design”, “Sustainable Energy Materials”, and “Hydrogen Evolution Mitigation”. Each cluster represents a vital aspect of research that contributes to the ongoing development and enhancement of ZAB technologies. 4. The collaboration network analysis shows strong international partnerships between countries. Chinese institutions collaborated more with the leading countries than with any other developed country. Such collaborations are vital for pooling resources, sharing expertise, and accelerating the development of ZABs technology. 5. This study also highlights continuing hurdles with ZABs, including dendrite growth, oxygen diffusion issues, and electrolyte stability. To address these issues, this research also discusses strategies like enhanced electrode material formulations, optimized electrolyte compositions, and novel air cathode designs. These strategic interventions are intended to improve the functional stability and overall performance of ZABs, which can unlock the path for practical deployment and commercial scalability. 6. Notably, this study has a few limitations stemming from its primary reliance on English-language publications and the Web of Science database. In total, 131 non-English ZAB-based studies were excluded, which could suggest that some important publications were not used in the analysis. The authors believe that the final analysis is not affected significantly as a result of this exclusion given that the study involved over 10,000 articles. Nonetheless, this restriction emphasizes the necessity for future studies to use more inclusive databases, such as Scopus, Google Scholar, or Dimension, and to include all publications, including non-English articles, for a more precise analysis. 1. It is vital to develop low-cost, high-efficiency bifunctional oxygen electrocatalysts that can support both oxygen reduction and oxygen evolution events. These catalysts must work well in a wide range of environmental conditions and scale effectively for industrial use, ensuring good performance in commercial applications. 2. The development of multidimensional air electrodes (1D, 2D, and 3D) that improve electron transport and mass diffusion is necessary. This entails creating designs that mix mesoporous and macroporous architectures to maximize active surface area and promote reactant/ product transport while maintaining mechanical stability and direct integration with current collectors. 3. It is critical to focus on producing zinc anodes that can sustain multiple recharge cycles without deterioration. This comprises novel materials or structures that limit dendrite growth and hydrogen evolution, both of which are crucial for flexible battery applications. 4. There is a significant opportunity to develop electrolyte technology by combining the advantages of aqueous and non-aqueous solutions. This hybrid technique should account for environmental interactions like CO2 and humidity, improve long-term durability, and optimize the electrolyte-electrode contact. 5. Designing cell assemblies that meet the growing demand for portable devices is critical. This includes not just improving individual battery performance but also efficiently organizing several cells inside a battery pack to maximize energy density and operational efficiency. 6. The connection between ZAB research and the Sustainable Development Goals (SDGs) is becoming increasingly important. Future research should investigate how ZABs can improve renewable energy access in underserved areas, reduce carbon emissions, and promote sustainable industrial development, in line with SDGs 7 (affordable and clean energy), 12 (responsible consumption and production), and 13 (climate action). 7. Future studies should investigate the environmental, social, and governance impacts of ZAB adoption. This involves research on the social effects of widespread ZAB adoption, such as job creation and equitable energy distribution, as well as the environmental consequences of resource usage and disposal at the end of their lives. To summarize, the progress of ZABs is dependent on both breakthroughs in individual components and a comprehensive approach that incorporates cutting-edge research from multiple disciplines. As ZAB technology advances, its alignment with environmental and sustainability goals will strengthen its role in the transformation of renewable energy storage systems. In sum, this review emphasizes the critical importance of ZABs in future energy storage, as seen by annual research growth, notable journal contributions, and strong international collaborations. This emerging technology has the potential to transform sustainable energy by overcoming current hurdles and capitalizing on new opportunities. Strategic collaboration between academic institutions and industries is required to advance this technology toward commercial feasibility and environmental benefits. 5. Conclusion Declaration of competing interest This study presents the growth dynamics, top contributing entities (countries, institutions, and authors), keyword analysis, research clusters, technological challenges, and advancement of zinc-air batteries (ZABs) through a thorough scientometric and systematic overview of ZABs. The key findings in this study are highlighted as follows. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability No data was used for the research described in the article. 1. A consistent increase in research productivity is observed, particularly from the early 2000s onwards. The annual growth rate of ZAB research is approximately 11 percent. This trend shows the growing recognition of ZAB as a viable solution for sustainable energy storage, which has an essential role in the global drive for renewable energy technologies. 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