Visualization of energy-environment-economy system research characteristics and hotspots evolution trends based on CiteSpace

English research from the Web of Science (hereafter referred to as the WOS database) core database. The citation index was set to SCI-EXPANDED and SSCI, the language was English, and the search conditions were: Subject: (Energy) AND theme: (Environment) AND theme: (Economy). The retrieval time span: 2001–2020, the retrieval time was December 27, 2020, and a total of 3,501 documents were retrieved. To eliminate the interference of irrelevant conference papers, reviews, and other documents, the document type was set to Article, and the number of documents after screening was 3,007. After manual screening is performed, the remaining documents are totally unrelated to the topic and deduplicated by CiteSpace, amounting to 2,947.

Chinese research is from the China National Knowledge Infrastructure (hereinafter referred to as CNKI) database. In the advanced search, theme = Energy-Environment-Economy is used as the search scope to search, the document type is set to journal, and the document source category is restricted to core journals, CSSCI and CSCD, the search period is 2001–2020, and the search time is December 26, 2020. A total of 252 results were obtained in this study. Through manual screening, the documents that did not meet the criteria such as reviews, interviews, soliciting papers, and notifications were eliminated, and 251 documents were finally obtained. In this paper, the analysis using the WOS database as the data source is called the English literature analysis, and the analysis using the CNKI database as the data source is called the Chinese literature analysis.

The graph of scientific knowledge is a graphic structure that can reflect the hidden complex relationships between knowledge units and knowledge groups, such as network relationships, structural characteristics, and evolutionary trends. It is often used in the field of knowledge management to present the development context and structure of scientific knowledge (Li and Chen 2016, Sun et al 2019, Wang et al 2019). The scientific knowledge map can give full play to the advantages of quantitative statistics and intuitive visualization, helping researchers to accurately focus on research hotspots and development trends from the massive research information, and presenting knowledge research trends more intuitively and clearly.

This article uses the most commonly used tools in the field of bibliometrics, and the graph was drawn by Dr Chen Chaomei's application software CiteSpace 5.7.R5 based on Java scenarios. Through 3E system research in the past 20 years, the number of articles issued, the cooperation relationship between research institutions and research members, and the co-occurrence of keywords, analyze the current status, research hotspots, and evolutionary trends of 3E system research.

Through the acquisition of literature data, it is known that the number of articles and citations of the 3E system from 2001 to 2020 has been increasing. This number indicates that the academic community's attention to and research on the 3E system has increased, and scholars in this field have also received wide attention. In terms of the number of papers, there was a slow upward trend until 2009, and fluctuated from 2010 to 2015. After 2015, the number of papers has shown a substantial growth trend, and the growth rate has increased significantly. In 2017, the rate exceeded 250. In 2019, over 450 articles were published annually. Chinese studies generally show an upward trend in volatility. Before 2010, the number of published articles continued to rise, reaching 23 in 2010. After a slight decline, there was a gradual upward trend. The number of articles published in 2014 reached 21, and there was an upward trend again. It continued to rise thereafter.

Based on the number of publications, the number of documents included in the WOS database was far greater than that of the CNKI database. The WOS database contains research results from scholars worldwide. The documents originating from the CNKI database are the research results of Chinese scholars. In contrast, there are fewer Chinese research documents. The overall publication trend shows that the research on 3E systems is on the rise, indicating that 3E systems are still a hot topic of research at present and there is much room for development in the research on 3E systems.

By analyzing the volume of articles in the 3E system research by analyzing the data from the WOS sources, the top four journals accounted for 21% of the articles, and the top seven journals accounted for more than 29% of the articles. The top journals are Journal of Cleaner Production, Sustainability, Energy Policy, Environmental Science & Technology, Energy, Energies and Applied Energy, the research found that the types of journals are mainly concentrated in the field of energy and environment, and the quality of journals There are big differences. The above-mentioned high-volume journals are mostly high-quality top-level journals, but there are also those that are dynamically included in the International Journal Alert List (Implementation), which is a widely influential scientific research evaluation standard obtained by the Scientometrics Center of the Chinese Academy of Sciences Literature and Information Center based on big data algorithms, using scientometric methods, data mining, complex networks, knowledge graphs and other methods. The Journal Alert is not an evaluation of papers, nor is it a denial of the results published by the alert journals. The list of alert journals is updated once a year to remind researchers to choose their publication platforms carefully and to prompt publishers to strengthen the quality management of journals.

Through the analysis of the CNKI database publication volume, the top four journals with publication volume accounted for more than 5% of the publication volume, which are China Population, Resources and Environment, Economic Geography, and Journal of Xi'an Jiaotong University (Social Sciences), Economic Research Journal. The journal types are all source journals of the Chinese Social Science Citation Index (2019–2020), the disciplines are concentrated in environmental science and resource utilization, industrial economy, economic system reform, macroeconomic management, and sustainable development direction. Fund support mainly comes from the National Social Science Foundation and the National Natural Science Foundation of China.

To clarify the situation of international publication, the WOS documents were processed through CiteSpace to obtain the knowledge map of the research country distribution, as shown in figure 1. The size of the nodes and the thickness of the lines in figure 1 indicate the number of posts and the closeness of the connections, respectively. As shown in table 1, in the field of 3E system research, China ranked first with 1,067 articles, the United States and the United Kingdom ranked second and third with 437 and 240 articles, respectively. The volume of Chinese publications is 2.4 and 4.4 times that of the United States and the United Kingdom, respectively. Table 1 also shows that Australia, Italy, Germany, Canada, Spain, and Turkey have also posted more than 100 articles. With the exception of Brazil and Poland in the top 20 cumulative posts, all other countries posted more than 50 cumulative posts. The density of the map network was 0.1114, and the connection was relatively close, indicating strong cooperation between the countries.

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A total of 105 countries and regions were covered in the 2,947 documents analyzed. Table 1 lists 21 countries and regions that rank among the top 20 in terms of publication volume (two countries ranked twentieth). It can be seen from the quantitative indicators in the table: First, the study area and country are widely distributed. Among the top 20 countries, scholars from European and Asian countries have conducted the most research on the 3E system. America, Canada, Brazil, and Australia, the largest countries in Oceania, are among the top 20. Generally, research countries are widely distributed all over the world, with most being developed countries. Second, the number of papers issued by research countries is quite different. Chinese scholars have conducted the most research on 3E system issues, ranking first in statistics, with the United States and the United Kingdom occupying the second and third places, and there is a large gap in terms of total quantity. Third, the overall difference in the influence of publications was relatively small. The United States has the highest total citations of research results, followed by China, the United Kingdom, and Australia. Among the four, Australia had the highest average citations, followed by the United States, the United Kingdom, and Canada. Fourth, the overall impact of cooperation was not strong. Based on the statistics of the intermediary centrality of the top 20 countries with the volume of publications, the intermediary centrality of the United States, China, Germany, and Spain is relatively high. High intermediary centrality indicates that this country has a strong influence on international cooperation. Although the United Kingdom, Australia, and Italy have a relatively high volume of publications, their international cooperation influence is relatively weak. Among them, the intermediary center of the United States has reached 0.22, indicating that the United States has a strong influence on cooperation.

According to the analysis of WOS source data, there are 19 Chinese institutions and 1 British institution among the top 20 institutions. As shown in table 2 and figure 2, the Chinese Academy of Sciences ranked first with 101 articles issued, accounting for 3.43% of the total number of articles issued. North China Electric Power University, Beijing Institute of Technology, and Tsinghua University are ranked 2nd and 3rd respectively. The University of Cambridge in the UK and Soochow University in Taiwan are ranked 11th and 20th, respectively, with 18 and 13, respectively. The top ten institutions had a total of 414 articles, accounting for 14.05% of the total number of articles posted. This shows that China has many powerful scientific research institutions engaged in 3E system research, and it also shows that Chinese academia pays great attention to 3E system research and has contributed many research results to 3E system research. The institutions with the highest total citations are the Chinese Academy of Sciences, Beijing Institute of Technology, and Tsinghua University. The institutions with the highest average citations are Chongqing University, Beijing Institute of Technology, and the Chinese Academy of Sciences, indicating that compared with other countries, China's research results have a stronger influence.

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As shown in figure 3, by analyzing CNKI sample data, the top four publications are the Chinese Academy of Sciences, China University of Mining and Technology, Qingdao University, and Beijing University of Technology. There are few 3E system research documents published in Chinese databases, and the Chinese Academy of Sciences ranks first in terms of the number of publications. Through the analysis of CNKI platform institutions, it can be concluded that the overall level of publication of various institutions is low, the cooperation relationship is relatively simple, and the number of literature citations is low.

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The comparison of research institutions shows that 3E systems are receiving attention from scholars around the world, and research results are mostly published in international journals. Chinese research institutions are among the top in terms of attention and research results in the field of 3E, with the Chinese Academy of Sciences being the most representative research institution in the field of 3E systems. Chinese research institutions that have published numerous research results in both Chinese and international journals include the Chinese Academy of Sciences, China University of Mining and Technology, North China University of Electric Power, and Xiamen University. Beijing University of Technology, Tsinghua University, Beijing Normal University and Peking University have outstanding results, mainly published in international journals.

Through a visual analysis of the source documents of the WOS database for high-yield authors, figure 4 can intuitively reflect the overall dispersion of 3E system research in the world, with a wide range of research authors, a large number, and a small number of high-yielding authors and weak cooperation. The most representative mainstream co-authors are Boqiang Lin, Malin Song, Zhijie Jia, Andrew Adewale Alola, and Muhammad Hafeez. Chinese scholar Boqiang Lin's research focuses on policy changes, efficiency calculations, and industrial transformation that considers the environment and green development (Lin and Jiang 2010, Wesseh and Lin 2016, Liu and Lin 2016, Li and Lin 2017, Lin and Oluwasola 2017, Lin and Zhu 2018, Lin and Chen 2018, Wesseh and Lin 2018, Du et al 2019, Lin and Zhu 2019). The team represented by him and Zhijie Jia are committed to emissions trading rights, carbon emissions trading markets, carbon taxes, and carbon prices on the environment, economy, and industry (Li and Jia 2017, Li et al 2017, Lin and Jia 2018a, 2018b, Lin and Jia 2019a, 2019b, 2019c, 2019d). The team with Malin Song as the core pays more attention to the measurement of environmental and industrial eco-efficiency and the measurement of corporate efficiency under the constraints of green and energy (Song et al 2013, Song and Guan 2014, Xiong et al 2017, Wang et al 2018, Song et al 2019). The research perspectives of the above scholars and teams are more macro-oriented. Muhammad Hafeez's research focuses on the environmental impact of energy and the relationship between carbon footprints and finance in the Belt and Road region, with a more meso perspective (Hafeez et al 2019a, 2019b, Peng et al 2019, Hafeez et al 2020). Muhammad Shahbaz et al (Badeeb et al 2020, Chen et al 2020, Shahbaz et al 2020, Wang et al 2020, Leal et al 2021) focused on the study of the coupling and interaction between the environment, energy and economic systems such as carbon emissions.

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The high-yield authors are visually analyzed through the source documents of the CNKI database, and the author's cooperation map can be analyzed. The figure 5 shows that the total number of authors is relatively large and the distribution is relatively scattered, indicating that the research volume of unit authors is relatively small. Through the threshold setting, the authors with 3 or more publications were visually displayed. There is a big gap between the authors of the Chinese studies in this field and the international ones, and the studies of the authors of the Chinese studies mainly focus on the economic and environmental impacts of traditional fossil energy from different research perspectives.

The team represented by Wei Xiaoping, Cang Dingbang and Yan Xiaofei in the field of 3E systems mainly focuses on energy, mineral resources and coal perspectives, such as the perspective based on energy substitution and environmental pollution management, the perspective based on new energy substitution and energy technology progress, the distribution of mineral resources benefits based on government and enterprise perspectives, optimal mining of mineral resources and coal consumption prediction and withdrawal (Guo and Wei 2014, Yan and Wei 2015, Zhang et al 2015, Zhou et al 2015, Wang et al 2016, Cang and Wei 2017, Cang et al 2017, Cang et al 2019, Cang et al 2020a, 2020b). Wei's team's research is characteristic of the era of Energy-Environmental-Economic research in China, i.e. coal reduction and low carbon development based on energy restructuring, and is dedicated to the study of economic growth under China's long-term coal-dominated energy structure and the current development goal of coal reduction and low carbon.

Lu Jin, Chang Hong, etc studied the 3E system from the perspective of environmental protection and low-carbon, such as the impact of urban environmental pollution control, human capital, technological innovation and new energy demonstration cities on regional environmental pollution and governance (Chen et al 2019, Zhen and Wang 2019, Lu et al 2020a, 2020b, Zhao and Lu 2020). In terms of low-carbon, the team conducted research on the impact of low-carbon industrial structure upgrading and technological innovation (Lu and Wang 2019, Lu et al 2020a, 2020b), focusing on the coupling and evolution mechanism of regional and provincial energy, environment and economy (Liu et al 2016, Lu et al 2016, Liu et al 2017, Chang and Lu 2018).

Zhang Lei studied the impact of fiscal policy on the 3E system (Zhang 2020). Because of the externality characteristics of environmental and economic issues, environmental policies, energy policies, and fiscal policies are introduced as constraints. Boqiang Lin's team, who has achieved representative results in policy, has earlier studied the current status and implementation of China's carbon emissions trading rights (Lin 2016). The Xiamen University research team, represented by him, has achieved many results in the study of 3E system policy constraints. Jiang Yue, Shuihua Han and others have studied the impact of carbon tax regulation on the supply chain, retail industry, and supplier companies (Jiang and Han 2016, Jiang and Han 2017, Jiang et al 2019), Peng Shuijun and others have conducted research on issues such as carbon emission reduction programs and carbon leakage (Peng and Yu 2017, Yang and Peng 2017, Peng and Wang 2020).

Based on the WOS database source literature, visual analysis was carried out based on keywords, and tags were extracted for clustering using the log-likelihood method, which is a clustering algorithm in Citespace. The larger the LLR, the better the representation of the clusters, and the highest reliability of the tags obtained by the LLR algorithm (Chen et al 2010). Therefore, this algorithm was used to extract the tagged words in this paper. The keyword clustering visualization mapping of the 3E system is obtained, and the keyword visualization analysis of the research literature can visually and clearly show the research cores and research hotspots in the research field. As shown in the keyword co-occurrence map in figure 6 and the keyword frequency statistics table in table 3, energy, environment, CO₂ emission impact, China, sustainability, energy consumption, model, system, emission, and consumption are the ten keywords that appear most frequently. By analyzing the keyword co-occurrence map and keyword frequency statistics table, the research hotspots can be summarized as follows:

• (1)  
  Energy consumption and economic development with energy as the coreThrough the keyword co-occurrence knowledge map, it can be found that the keywords about energy and economic development are the most. This indicates that the number of studies is relatively large and the scope of research is relatively concentrated, indicating that scholars pay more attention to social economic development issues in 3E system research. In addition to the search term energy, recent research hotspots are mainly derived from the electricity of fossil energy, energy consumption from an economic perspective, and renewable energy, which contrasts with traditional high-emission and highly polluting fossil energy. Bioma and others are all research hotspots, with energy as the core. The energy and economic subsystems focus on economic research. Consumption, cost, optimization, growth, and trade are the main research hotspots. Its research is consistent with traditional economic systems that focus on economic growth and consumption, there is little difference between optimization and trade research.
• (2)  
  Environmental issues centered on sustainable developmentThe 3E system has many research points in the environmental subsystem, and research on the themes of CO₂ emissions, emissions, and climate change presented in the knowledge map is a common concern in current energy economics, environmental economics, and climate change economics. The issue of carbon dioxide-based greenhouse gas research and climate change research has received much attention. For a long period of time in the future, related research on the impact of carbon dioxide emissions on China's economic system and the path of carbon dioxide emission reduction will show a spurt of growth. Water, pollution, environmental impact, and other research hotspots are the research on traditional environmental issues, and the research on life cycle assessment and circular economy is still the development concept of 'Lucid waters and lush mountains are invaluable assets' pursuing the main content will receive further attention in the environmental economic subsystem.
• (3)  
  Systematic problems with the energy economy and environment as a wholeIn the 3E system, the problems with the system and Environment Kuznets Curve as keywords are all research on the 3E system from the comprehensive perspective of economic development, energy consumption and environmental changes, financial development, innovation, and so on. Financial development and technology focus on the impact of the problem itself on the system, and all reflect the systematic characteristics of the relationship between energy, economy, and environment as a whole.
• (4)  
  The carrier of economic development -- regional issuesEnergy, environment and economic studies all have typical regional characteristics, which are reflected in the 3E system keyword knowledge map. The research on China issues represented by China is the focus of both Chinese and English studies. Economic growth runs through the entire system of research, with county and city as the research objects, mainly relying on influencing factors for further research regional differences in economic development. The urbanization problem is closely related to economic development, and at the same time, it corresponds to the population and environmental problems. Therefore, the regionalization problem has become one of the hotspots in the 3E system research.
• (5)  
  Mechanism issues centered on policy and managementAll aspects of social and economic development and energy development and use involve issues related to policy regulations and management control mechanisms. Therefore, the research on policy, performance, management, and other issues in the 3E system is mainly due to the Energy-Environment-Economic system has distinct externalities. Energy itself has economic attributes and people's livelihood attributes. Therefore, performance and management are closely related to the policies and mechanisms.

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The time zone diagram shows the changes in the research direction of the 3E system in different periods, as shown in figure 7.

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From 2001 to 2003, scholars had the largest amount of research on 3E system energy and environmental issues, and there were many types of economic issues. Research has focused on economic growth, energy efficiency, and costs. The earliest keywords in this period were economic growth, economy, efficiency, energy, energy efficiency, and cost. At the same time, the environmental pollution problem of climate change has also begun to appear.

During the period 2004–2007, the term system appeared, indicating that research has expanded from a single system to multiple subsystems. The study of economic subsystems has changed from macro to micro and is closely integrated with the environment. The scope of research on environmental issues has increased. Energy systems pay more attention to renewable energy and biomass energy. In addition to single economic growth, the economic system begins to focus on the circular economy and innovation.

During the period 2008–2012, the high-frequency keywords decreased. Among them, the largest number of studies focused on carbon dioxide emissions in the environmental field and energy consumption in the energy field. In 2007, China became the world's largest carbon dioxide emitter, surpassing the United States. In China, the research focus CO₂ emissions have become the keyword with the highest frequency. The energy field has begun to focus on biofuel research and influencing factors in energy development. The research issues in the economic field are generally scattered. Cointegration and so on belong to the category of econometrics, revealing the economic laws of causality between the research objects. The Environmental Kuznets Curve focuses on the relationship between the economic system and the environmental system. The research shows that the research has begun to shift from single system research to the research on the relationship between them, and pay attention to the evaluation of development results and policy implementation.

During the period from 2013 to 2020, the number of nodes is relatively large, and the area of nodes is relatively reduced, which indicates that the scope of research issues at this stage has been further expanded, the number of single issues has been reduced, the research issues have been further focused, and the overall research relationship between economy, environment, and energy has become closer.

Based on the CNKI source documents, visual clustering was performed with keywords as the analysis basis, and the log-likelihood method was used to extract the labels, and the visual map of keyword clustering of Chinese 3E system research was obtained. As shown in the keyword co-occurrence knowledge map in figure 8 and the keyword statistics table in table 4, energy structure, economic growth, energy consumption, energy consumption, environment, low-carbon economy, renewable energy, industrial structure, carbon emissions, and sustainable development are the top ten keywords in the frequency ranking. Through the keyword co-occurrence knowledge graph and keyword word frequency statistics, the research hotspots can be summarized as follows:

• (1)  
  There are more studies on a single subsystem, but less research on system collaboration.It can be seen from the knowledge graph that the most frequently studied keywords in the research of energy structure, energy, economic growth, energy consumption, and renewable energy are all research on a single subsystem. There is little research on the internal coordination mechanism represented by coupling, coordinated development, and the Environmental Kuznets Curve.
• (2)  
  Pay more attention to the energy and economic subsystems.High-frequency keywords reflect the close relationship between economic development and energy consumption. As a developing country, the industrial economy, urbanization, and other processes are all driven by energy. The scale of high-polluting industries continues to expand, and the rapid growth of car ownership has led to topics such as energy consumption and energy structure favored by researchers.
• (3)  
  Environmental pollution prevention and a low-carbon economy are the core of environmental system research.The key research issues of the environmental subsystem are environmental pollution prevention and control, and a low-carbon economy. Economic development and the ecological environment influence and restrict each other. The ecological environment is the foundation of sustainable economic and social development, and economic development guarantees ecological environmental governance. In the Chinese study, the author's research combined with the current situation in China has confirmed that there is an inverted U-shaped curve relationship between the level of economic development and the environment, namely Environmental Kuznets Curve.

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Through the visual analysis of the keyword time zone map, the evolution characteristics of the Chinese 3E system in the time dimension were analyzed, as shown in figure 9.

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During the period 2001–2003, the high-frequency keywords in the time zone were energy, energy structure, and environment. The beginning of the 21st century was a period when China was vigorously developing its industrial economy, and research on energy and energy consumption was relatively extensive. Environmental issues that follow a substantial increase in energy consumption are environmental issues. In the early stage of development, the awareness of the harm of energy emissions to environmental pollution and climate change was low, the area of keyword nodes was small, and the frequency of research was low.

During 2004–2007, words such as environmental pollution, sustainable development, environmental protection, and ecological environment, as shown in the keyword time zone chart, began to appear. As the country and society pay attention to environmental issues, the green development concept, green water, and green mountains are golden mountains and silver mountains have become important considerations for social and economic development. Internationally, the World Climate Conference in Copenhagen in 2009 agreed on a global emission reduction agreement from 2012 to 2020. In essence, developed countries began to undertake the obligation to reduce carbon emissions in 2005, while developing countries began to undertake the obligation to reduce carbon emissions in 2012.

During the period from 2008 to 2010, research on the low-carbon economy was prominent, and the research on energy saving and emission reduction, energy efficiency, industrial structure, etc centered on the efficient use of energy increased significantly. After the rapid growth stage of energy consumption, people realized that energy promotes economic development and the contradiction between environmental pollution and climate change caused by energy consumption, and began to try to reduce the problem of carbon emissions from energy consumption by improving energy use efficiency. 2001–2010 is also a transitional stage of China's air pollution prevention and control. Due to the acceleration of China's industrialization and urbanization during this period, energy consumption represented by coal has risen rapidly, causing further pressure on the environment. In addition to the strict environmental requirements for the 2008 Beijing Olympics, environmental protection has entered a stage of in-depth advancement. Air pollution has been shown to have both regional and complex characteristics. Key areas such as Beijing-Tianjin-Hebei, the Yangtze River Delta, and the Pearl River Delta have implemented joint prevention and control of air pollution.

From 2011 to 2020, it can be seen from the keywords of CNKI sample data that the research is more focused on the environmental subsystem. In 2011, China's environmental governance entered a critical stage. After large-scale haze weather occurred in central and eastern China in January 2013, dust particles, such as PM2.5, have attracted attention in 3E system research. During this period, China revised the Environmental Protection Law, the Air Pollution Prevention and Control Law, and the Energy Conservation Law, which brought unprecedented attention to environmental governance. In 2016, in response to climate change, China joined the Paris Agreement and promised China's independent emission reduction plan. Facing the high consumption and high emissions of fossil fuels, research on environmental regulations has appeared in large numbers since 2015. In the field of economic subsystems, low-carbon economy and industrial restructuring are frequently studied around 2010, instead of pursuing a single, extensive economic system. Research based on the energy perspective pays more attention to energy saving and emission reduction, energy efficiency, renewable energy, and so on. During this period, China's energy structure began to enter a stage of real adjustment. As energy consumption continued to rise, the proportion of coal, which had remained high for a long time in the energy structure, began to decline, and economic growth and coal consumption began to decouple.