Research Hotspots and Evolution Trends of Carbon Neutrality—Visual Analysis of Bibliometrics Based on CiteSpace

Abstract

Climate change is one of the most urgent challenges facing the world. All countries should take joint actions to achieve the goal of carbon neutrality, which include controlling global warming to within a 1.5 °C temperature rise, to mitigate the extreme harm caused by climate change. However, ways in which to achieve economically and environmentally sustainable carbon neutrality are yet to be established. Carbon neutrality appears frequently in international policy and the scientific literature, but there is little detailed literature. It is necessary to conduct an in-depth analysis of the development context of its research. This paper analyzed the literature on carbon neutrality using bibliometric methods. A total of 1383 research papers were collected from the “Web of Science core database” from 1995 to 2021. Descriptive statistical analysis and keyword co-occurrence and literature co-citation network analyses were utilized to sort the research hotspots, and the detected bursts, the top 30 keywords in terms of word frequency, and 12 clusters were selected. It was found that the existing carbon neutrality research literature mainly focuses on carbon neutrality energy transformation, carbon neutrality technology development, carbon neutrality effect evaluation, and carbon neutrality industry examples. The analysis process involved comprehensively reading the key articles and considering the co-citation, burstiness, centrality, and other indicators under clustering; the carbon neutrality research was then divided into three stages, and evolving themes were observed. Based on the burst detection, this paper holds that with the energy structure transformation, energy consumption assessment and carbon neutrality schemes of various industries, carbon dioxide capture technology, and biogas resource utilization, urban carbon neutrality policy will become a research hotspot in the future. This paper helps to provide a reference for scholars’ theoretical research and has important reference value for policymakers to formulate relevant policy measures. It is helpful for enterprises to make strategic decisions and determine the direction of technology, for R&D and investment, and it is of considerable significance to promote the research of carbon neutrality technology.

1. Introduction

According to the “Global Warming of 1.5 °C” issued by the Intergovernmental Panel on Climate Change (IPCC) [1], carbon neutrality refers to the realization of net-zero CO₂ emissions when anthropogenic CO₂ emissions are offset globally for a specified period of time. The report also emphasizes that only by achieving the global net-zero carbon emissions/carbon neutrality target in the middle of the 21st century is it possible to control global warming within 1.5 °C, thereby mitigating the extreme harm caused by climate change. However, the “Emission Gap Report 2019” issued by the United Nations Environment Program (UNEP) [2] points out that there is a large gap between current countries’ emission reduction ambitions and the 1.5 °C target requirement. About two thirds of the available budget for controlling the temperature rise below 2 °C has been discharged. As the most important emission, carbon dioxide emission status shows that global emissions urgently need to start to decline. In this way, the temperature rise can be controlled to far below 2 °C as much as possible. The window most likely to limit the temperature rise below 1.5 °C appears to have been closed. This attitude is expressed in the Paris Agreement, which aims to reach the peak of global greenhouse gas (GHG) emissions as soon as possible and achieve a “balance” between human-made emissions and greenhouse gas removal in the second half of this century [3].

To narrow the carbon emission gap, an increasing number of countries have increased their emission reduction efforts by participating in climate actions such as carbon neutrality. In December 2017, 29 countries signed the “Carbon Neutral Alliance Statement” [4] at the “One Planet Summit”, committing to achieve zero-carbon emissions in the mid-21st century. At the United Nations Climate Action Summit in September 2019, 66 countries committed to carbon neutrality targets and formed the Climate Ambition Alliance. In May 2020, 449 cities participated in the zero-carbon competition proposed by experts in the United Nations climate field. As of 12 June 2020, 125 countries have pledged to achieve the goal of carbon neutrality by the middle of the 21st century. Among them, Bhutan and Suriname have achieved the goal of carbon neutrality. The United Kingdom, Sweden, France, Denmark, New Zealand, and Hungary have written carbon neutrality targets into their laws, and four countries and regions including the European Union, Spain, Chile, and Fiji have proposed relevant draft laws.

In the context of the ever-increasing influence of international carbon neutrality actions, the status quo and development trends of international commitments should be actively studied. This will help achieve the goal of carbon neutrality, seek opportunities for international cooperation to reduce emissions, and promote the process of global climate governance. At present, there have been studies and analyses of international long-term emission reduction actions and strategies [5]. For example, Monica Salvia et al. [6] conducted a comparative analysis of the emission reduction targets announced by 327 European cities in their local climate plans. The study analyzed whether the plan type, city size, climate network members, and their regional location are related to varying degrees of participation.

The main tool used for the bibliometric approach is the CiteSpace software developed by Chaomei Chen’s team. The software is developed by Java, based on co-citation analysis theory and the pathfinding network algorithm, showing the overall situation of a specific field. It visually displays important issues such as iconic work in the field, mainstream themes, field relevance, and research frontier evolution [7]. At present, there have been some bibliometric studies on carbon neutrality and related fields trying to characterize the research to date. Liwen Sun et al. [8] used 806 documents collected from the Web of Science from 2004 to 2019 as the research basis. Based on the 806 documents collected from the Web of Science from 2004 to 2019, Sun [8] and others used the method of literature measurement to find that predicting the time of the carbon peak and carrying out a variety of industrial layouts according to carbon emissions are the hotspots of carbon emission and industrial structure research, and region, industrialization, and environmental efficiency are expected to become emerging trends. Syie Luing Wong et al. [9] demonstrated the scientific pattern of global carbon dioxide utilization research from 1995 to 2019 through a bibliometric analysis of 1875 papers included in the Web of Science. Keyword co-occurrence analysis reveals the evolution process of the CO₂ utilization strategy. From initially relying on carbonate and epoxide fixation, and then using catalyst synthesis to convert carbon dioxide into fuel and chemicals, it has now developed into power-to-gas (PTG) and power-to-liquid (PTL) research. In addition to the use of carbon dioxide through chemical pathways, bio-utilization pathways are also under development. Osaze Omoregbe et al. [10] used the Web of Science database to search papers from 1998 to 2018 to investigate the research trends of three major carbon capture technologies, namely, pre-combustion, post-combustion, and oxygen-containing fuel combustion, and found that the United States has the most research achievements, followed by the United Kingdom and China. Post-combustion capture technology is the most studied carbon capture technology. The existing literature confirms that carbon neutrality research is of great significance, but there are also limitations such as narrow literature sources, small sample sizes, single research perspectives, and a lack of systematic bibliometric analysis.

To further analyze and compare the progress of carbon neutrality research at home and abroad and overcome the above research limitations, this article was based on the research literature of the ISI Web of Science database, using bibliometrics and knowledge graph methods, with the help of CiteSpace software, to analyze the 25 years of international carbon neutrality. This paper addresses research hotspots, the development context, and future trends, in order to provide a reference for future related research and practice.

2. Methodology

2.1. Methods

This article used bibliometric methods to build a knowledge graph using CiteSpace software. Bibliometrics is based on the distribution of authors disclosed by Price’s law and the distribution of specific disciplines described by Bradford’s law in journals as the theoretical basis. In 1963, Price [11] established the following definition in his representative work. Half of the papers on the same topic were written by a group of highly productive authors, and their number was approximately equal to the square root of the total number of all authors. For selected professional documents, the distribution in the journals that produced them follows a common pattern, which is Bradford’s law [12,13]: A large number of documents on related topics are arranged in descending order of the number of authors’ creations; three regions can be marked so that each region contains one third of all related documents. The first area is the core area, containing a small number of high-productivity journals, called n1; the second area contains a larger number of medium-productivity journals, called n2; the third area contains a large number of low-productivity journals, called n3. The dispersion law is expressed as: n1:n2:n3 = 1:α:α³ (α is a constant). Combining the above two important theorems of bibliometrics, we reviewed the field by the most important authors and journals.

2.2. Data Sources

The literature data were sorted and matrixed to explore the information and laws existing in the literature. Through information visualization, the information can be transformed into graphics to mine the hidden content of the information. A knowledge map uses information visualization and has the dual nature of a “graph” and a “spectrum”. On the one hand, it transforms the mathematical expression of scientometrics into a graphical expression, that is, a visualized knowledge graph; on the other hand, it transforms the scientific and technological activities and knowledge of the knowledge map. Geographical distribution is transformed into a map display of the knowledge relationship structure and evolution law, that is, the serialized knowledge pedigree, which shows many complex relationships between knowledge units or knowledge groups, such as network, structure, interaction, intersection, evolution, or derivation [14]. The CiteSpace software developed by Professor Chaomei Chen of Drexel University in the United States integrates bibliometrics and information visualization principles and characterizes the evolution process and structural relationships of scientific knowledge through graphs. It is an important tool for creating knowledge graphs [7]. This article used CiteSpace version 5.7.R5W, selected the plain text format, selected all records and cited references for the record content, saved the selected documents from the Web of Science, and set the time period to 1995~2021 and the time step to 1 year. Author, institution, country, keyword, and other functions calculate and draw graphs to analyze the hotspots and trends in the field of carbon neutrality.

This article used the field of carbon neutrality as the research object. The date of data retrieval was 31 August 2021. The title of “Carbon neutral” OR “Carbon-neutral” OR “Carbon neutrality” in the WOS core collection was set from 1995 to 2021, the “article” option in the results was selected, and a total of 2287 articles were obtained. To improve the validity of bibliometrics, after the manual screening, documents that did not meet the theme, meeting notices, and other information were removed, the document database was merged and deduplicated, and other pre-processing operations were carried out. Finally, 1383 valid documents were obtained.

2.3. Framework of This Study

In this study, we propose an integrated analysis framework to interpret the trend and thematic change of carbon neutrality of a total of 1383 publications from 1995 to 2021, as shown in Figure 1. By repeatedly adjusting the parameters, the results are stable. This study carried out the following work: (1) descriptive statistical analysis, including distribution of publications and high-yield journals, aiming to draw a panoramic view of the field; (2) network analysis, including national cooperation network, institutional cooperation network, and co-authorship network, intended to analyze their cooperation; (3) research hotspots, analyzing the current research hotspot of carbon neutrality through keyword co-occurrence network analysis and keyword clustering; (4) research topic evolution analysis, dividing the research topic into three stages from the time dimension, analyzing the evolution of the research topic, and summarizing the important literature; (5) through burst detection, five possible key research trends in the future were identified.

3. Results

3.1. Descriptive Statistical Analysis

The distribution of publication can be used to analyze the relationship between the number of papers published in a specific field and the change in time, and it is an important method to evaluate the current research status of the field and predict development and research trends [8]. It can be seen in Figure 2 that the number of publications in the carbon neutrality field has shown an exponential upward trend in the past 25 years. According to Price [11], a well-known scholar in the field of bibliometrics, a research field in which the number of publications is increasing exponentially is in a stage of rapid development. New theories, new methods, and new technologies are constantly emerging [15]. The number of papers published in the carbon neutrality field increased slowly from 1995 to 2007. After that, the number of papers stabilized at more than double digits each year. In 2008, the results increased rapidly. In 2017, the number exceeded 100 papers. By the end of August 2021, the number of papers in that year reached 299. From the perspective of annual cumulative percentages, the number of articles in this field grew slowly in the early days, starting in 2010. In recent years, the speed of publishing articles has accelerated, and articles published in the past four years have accounted for more than half of the total number of articles.

High-Yield Journals

By analyzing the distribution of journals that publish research in the field of carbon neutrality, one can understand the distribution of disciplines in the field, and the influence and preferences of core journals, and it is also easy to track follow-up research. To analyze the distribution of journals, the top 9 high-percentage journals are presented in

Table 1. Core area journals.

Number	Journal	TP	Percentage	IF
1	Journal of Cleaner Production	63	4.56%	9.30
2	Sustainability	62	4.48%	2.58
3	Applied Energy	54	3.90%	9.75
4	Energies	41	2.96%	2.70
5	Energy	36	2.60%	6.08
6	Energy Policy	33	2.39%	6.14
7	Fuel	27	1.95%	6.61
8	Global Change Biology Bioenergy	21	1.52%	5.32
9	Nature Communications	21	1.52%	12.12

Note: TP = the number of publications, IF = impact factor.

. Based on the statistics from WOS, there are a total of 474 articles in the literature database of this article, of which 42.01% are published in 10 or more journals. This indicates, to a certain extent, that the publication of articles in international journals in the field of carbon neutrality is relatively scattered, and a stable group of journals has not yet formed in this field and is still in the development stage. According to Bradford’s law, the number of articles in the core areas of a journal can be calculated by r0 = 2ln(eE·Y), where r0 is the number of core areas, E is Euler’s coefficient, E = 0.5772, and Y is the maximum number of publications in the journal. The calculation shows that r0 = 9.19. It can be seen that the top nine journals in the field of carbon neutrality are the core areas.

The core area journals account for 25.88% of this article’s literature database, with a total of 358 articles, and an average impact factor of 6.73. They are mainly authoritative journals in the fields of the environment, energy, and biology. The trend of intersectionality is apparent. Journals such as the “Journal of Cleaner Production”, “Sustainability”, and “Applied Energy” rank at the forefront and are important knowledge dissemination platforms and knowledge carriers for carbon neutrality research.

3.2. Network Methods

3.2.1. National Cooperation Network

According to the country cooperation network diagram in Figure 3, the research cooperation in the field of carbon neutrality between countries is relatively close. There are 93 nodes and 151 connections in the cooperative network, and the network density is 0.035. From the perspective of the number of posts, the size of a node represents the number of posts. The larger the node, the greater the number of posts in the country. The top five countries in terms of the number of papers are the United States (332 articles), China (247 articles), the United Kingdom (101 articles), Germany (98 articles), and Japan (82 articles). Centrality can reveal the important positioning of the research field. The higher the centrality, the more important the country is and the greater the contribution it has in this professional field. The nodes are marked with pink circles, indicating that the centrality of these nodes is more than 0.1, and that they have a large centrality and are called key nodes. In this article, they are Germany, Belgium, Sweden, and Norway, and the centrality is 0.74, 0.61, 0.56, and 0.56, respectively. The connection between the nodes indicates that there is a cooperative relationship between the two countries. The color of the connection represents the cooperation time. The closer the color is to the cold color, the earlier the cooperation time; the closer the color is to the warm color, the closer the cooperation time. From the perspective of time, many countries established cooperation networks in the early years, including Germany, Italy, and Norway; the United States and Belgium; Finland and Belgium; China and Sweden; and France and Switzerland. In recent years, countries have actively participated in research in this field. For example, New Zealand and Portugal have established certain cooperative relations with many countries in recent years.

3.2.2. Institutional Cooperation Network

In terms of the cooperation networks of research institutions, Figure 4 shows a total of 242 nodes and 174 connections; the network density is 0.006, which is low, and the overall network form is relatively scattered. The Chinese Academy of Sciences is the scientific research institution that has published the most articles, with a total of 25 articles, followed by Aalto University and Tsinghua University with 24 and 18 articles, respectively. The University of Helsinki, Southeast University, Technical University of Denmark, Swiss Federal Institute of Technology, and the Delft University of Technology have also achieved more results in the field of carbon neutrality. In terms of cooperation, the Chinese Academy of Sciences, Swiss Federal Institute of Technology, Aalto University, and Tsinghua University are more active. In terms of cooperation time, Aalto University, the Chinese Academy of Sciences, and various research institutions cooperated earlier. The Swiss Federal Institute of Technology and Tsinghua University have established cooperation with many institutions in recent years. From the perspective of centrality, many institutions in China and Switzerland serve as important bridges for cooperation, such as the Chinese Academy of Sciences (0.19), Peking University (0.13), and the Swiss Federal Institute of Technology (0.11).

3.2.3. Co-Authorship Network

In terms of carbon neutrality research scholars, according to Figure 5, a cooperative network of scholars with a density of 0.0029, 548 nodes, and 431 connections has been formed. In general, the density of the cooperation network is low, and most scholars have less cooperation. The largest network is a group of authors with Li Jun at the University of Toronto as the core. In addition, Qiang Cui has published the most articles in the field of carbon neutrality, with a total of 12 articles. He has worked closely with Li Ye of Nanjing University of Finance and Economics to jointly conduct academic cooperation around the Aviation Carbon Neutral Growth Strategy (CNG2020). With Nico Bauer, the Institute for Climate Impact Research in Potsdam, and Germany as the core, a cooperation network with the theme of carbon prices and carbon neutrality has been formed.

3.3. Research Hotspots

3.3.1. Keyword Co-Occurrence Network Analysis

Keywords are highly condensed on the subject of the literature. Research hotspots and important issues in the subject field can be investigated based on high-frequency keywords. This process involves setting the node type to keyword, selecting the critical path algorithm (pathfinding), running CiteSpace, obtaining a keyword visualization map of carbon neutrality research, and merging synonyms, as shown in Figure 6. It can be seen from the large number of keywords that scholars’ research on carbon neutrality is relatively extensive. The close network map reflects the strong correlation between keywords. The node size of the keyword co-occurrence network represents the keyword frequency. It can be seen in Figure 5 that the top five keywords for carbon neutrality research are energy, biomass, carbon neutrality, climate change, and emissions. Through the frequency analysis of relevant keywords, it can be found that in the research on carbon neutrality, the carbon dioxide emitted by energy consumption intensifies climate change, and renewable energy plays an important role in the realization of carbon neutrality, especially biomass energy. In terms of centrality, carbon neutrality research has formed many key nodes, which are represented by the pink outer circle of the nodes. The top five keywords for centrality are bioenergy, carbon dioxide (CO₂), climate, energy, and biomass; they constitute the key path of the carbon neutrality domain knowledge network. From the perspective of time evolution, in the early days, the keyword energy was the core, forming a co-occurrence network with carbon neutrality and biofuel. In recent years, with technology as the core, a keyword co-occurrence network has been formed with carbon capture, hydrogen production, efficiency, and strategy.

To analyze the specific content of keywords, the top 30 high-frequency keywords are presented in

Table 2. Top 30 high-frequency keywords.

Number	Frequency	Centrality	Keyword	Number	Frequency	Centrality	Keyword
1	158	0.23	energy	16	55	0.14	CO2 emission
2	148	0.21	biomass	17	52	0.01	combustion
3	130	0.04	carbon neutrality	18	49	0.03	conversion
4	117	0.16	climate change	19	46	0.06	sustainability
5	106	0.12	emission	20	43	0.04	storage
6	104	0.11	life cycle assessment	21	41	0.13	optimization
7	103	0.41	CO2	22	40	0.04	technology
8	87	0.01	impact	23	40	0.11	carbon
9	84	0.21	biofuel	24	38	0.05	fuel
10	82	0.01	system	25	38	0.07	growth
11	78	0.02	performance	26	37	0.08	biodiesel
12	70	0.11	renewable energy	27	36	0.04	hydrogen
13	65	0.13	model	28	34	0.05	management
14	62	0.43	bioenergy	29	34	0.09	China
15	60	0.12	greenhouse gas emission	30	33	0.07	sequestration

. It can be seen in the table that in terms of research content, among the top 30 high-frequency keywords, the research scope is relatively clear, and the research content is more specific, mostly involving energy, climate change, model CCU technology (carbon capture, utilization, and storage), sustainable development, and management. Among the existing studies, the energy research is relatively concentrated, including the utilization of renewable energy, the exploration of the application of hydrogen energy, and the emission of fossil energy combustion. To deal with the climate change caused by fossil energy combustion, the first thing is to measure greenhouse gas and carbon emissions. Therefore, the life cycle assessment model, as a mature model, has been widely used. It can calculate the carbon footprint of products from manufacturing to use, to promote carbon neutrality in the whole life cycle. Secondly, by capturing carbon dioxide emissions and using them for a series of industrial applications or underground storage and other related technologies, it is an important means to reduce global carbon dioxide emissions and achieve the goal of carbon neutrality. Finally, achieving the carbon emission target is a systematic project, which needs to be completed by multiple countries, departments, and industries. It is also an important issue for the government to guide energy transformation and industrial optimization, to control the growth of carbon emissions through public management, and to achieve sustainable development. In addition, the emergence of China as a high-frequency keyword shows that China, as a country with clear carbon neutrality goals, is an important research area of concern for scholars.

3.3.2. Keyword Clustering

Keyword clustering involves running the clustering function on the CiteSpace visual interface and clustering the keywords into 11 categories using the LLR log-likelihood algorithm. The modularity Q value is 0.78 (>0.3), the weighted mean silhouette value is 0.92 (>0.4), and the clusters are similar. There is a high performance and reasonable structure between clusters.

Table 3. Keyword clustering table.

Number	Cluster Label	Keyword
0	bioenergy	bioenergy; biomass; carbon neutral; carbon neutrality; emission
1	climate change	climate change; carbon sequestration; China; carbon dioxide; model
2	pyrolysis	pyrolysis; combustion; biodiesel; lignin; gasification
3	biofuel	biofuel; ethanol; decarbonization; management; algae
4	hydrogen	hydrogen; CO2 capture; calcium looping; carbon neutrality
5	sustainability	sustainability; biodiversity; harvested wood products; sustainable development; forest carbon
6	eddy covariance	eddy covariance; substitution; fertilization; temperate grasslands; energy balance
7	energy efficiency	energy efficiency; energy policy; nationally determined contributions; water; greenhouse gas emission
8	LCA	LCA; methane; bioethanol; life cycle assessment; greenhouse gases
9	catalyst	catalyst; CO2 reduction; peat; electrocatalysis; adsorption
10	energy transition	energy transition; CO2 emissions; supercritical steam cycle; cost of electricity; biomass power
11	cng2020 strategy	cng2020 strategy; airline; airline efficiency; pollution abatement costs; network environmental production function

shows the 11 cluster names and corresponding core keywords. The smaller the number, the larger the cluster size. The top three themes of the cluster size are bioenergy, climate change, and pyrolysis.

According to the cluster analysis of keywords, the highly cited articles are summarized under the cluster keywords, and the clustering topics are classified; it can be found that the carbon neutrality research hotspots are mainly concentrated in the following four aspects.

(1)

Research on carbon-neutral energy transition. Due to the depletion of resources and the accumulation of greenhouse gases in the environment having exceeded the “dangerous high threshold” of 450 ppm CO₂-e, the use of fossil fuels is now generally considered unsustainable [16,17]. Carlo N. Hamelinck [18] and others used the dynamic Aspen Plus process to evaluate the impact of model parameters or technical means on investment costs, indirect liquefied coal-to-diesel (FT diesel), and power efficiency, and the resulting FT diesel cost. David P.B.T.B. Strik [19] and others developed plant microbial fuel cells. The main idea is that plants produce rhizome materials, mainly in the form of carbohydrates, and bacteria convert these rhizome materials into electricity through fuel cells. Schenk [16] analyzed a second-generation biodiesel production system using microalgae and focused on its advantages and limitations. Jiao et al. [20] used the life cycle assessment (LCA) method to evaluate the energy efficiency and environmental impact of partially or completely using cassava ethanol fuel in 11 provinces in China. Studies have found that the life cycle energy consumption of cassava ethanol is better than that of gasoline, but the environmental performance is not always better than that of gasoline.

(2)

Research on the development of carbon-neutral technology. Carbon neutrality technology routes are relatively diversified, and most of them are carbon capture and carbon storage technologies. Dawid P. Hanak [21] demonstrated the feasibility of introducing a polygeneration CHP-DAC process from the perspective of thermodynamic properties and economic benefits. Its technology can reduce the environmental burden and ensure product competitiveness. I. Martínez [22] demonstrated the feasibility of second-generation calcium cycle technology in combination with large-scale tests and obtained accurate energy efficiency and cost estimates. Zhu et al. [23] developed a typical two-dimensional heterostructure for photocatalytic reduction of CO₂ and realized an efficient photocatalytic CO₂ reduction system, which effectively converts carbon dioxide into fuel. Through experiments, M. Farooq et al. [24] found that the adsorption and desorption of CO₂ through the conductive activated carbon physical adsorption system is a cost-effective, safe, and environmentally friendly new method.

(3)

Evaluation of carbon neutrality effects. In carbon neutrality research, how to effectively evaluate its actual effects has always been the focus of attention. At present, the focus is on biomass reduction and the quantification of CO₂ emissions. Giuliana Zanchi [25] studied woody biomasses as different sources of bioenergy and found that the ability of woody biomass to reduce anthropogenic emissions in the atmosphere mainly depends on the biomass source used and the time frame considered. Francesco Cherubini et al. [26] refined a method to quantify the climate effects of biological CO₂ emissions. Their method used a biological global warming potential (GWPbio) index to estimate its impact on climate and verified that the GWPbio index based on a carbon cycle approach (firf) considering marine and terrestrial soils is the most reliable and accurate choice. E.I. Wiloso [27] added considerations of system boundaries, carbon emission patterns, and biomass estimates to the bio-sourced carbon neutrality assumption and found that these factors will bias the assessment of the impact of global warming. Cui [28] proposed a three-stage strategic operating framework for airline pollution reduction costs and analyzed the pollution reduction costs of 12 European airlines from 2012 to 2014 through the network environment production function. Felix Schreyer [29] and others used the modeling tool REMIND to simulate the corresponding scenarios in representative industrialized regions (the European Union, the United States, Japan, and Australia) that will achieve net-zero carbon emissions by 2050.

(4)

Industry case studies of carbon neutrality. In the future planning of many industries, carbon neutrality is one of the important goals, and better consideration of energy consumption and greenhouse gas emissions will help improve energy efficiency and mitigate climate change. Wang et al. [29] compared the power intensity and related carbon emissions of WWTPs (wastewater treatment plants) in four countries: the United States, Germany, China, and South Africa, and confirmed that it is feasible for sewage treatment plants to achieve net-zero electricity. Ayobami Solomon Oyewo [30] used linear optimization models to explore the model for Nigeria to achieve a fully sustainable energy system by 2050. Additionally, the study simulated the cost optimization path of Nigeria’s transition to a 100% renewable energy power system. Based on the forecast data of 29 international airlines from 2021 to 2023, Li [31] proposed a network-wide adjusted environment DEA model and discussed the CNG2020 strategy and the environmental inefficiency changes under the condition of not implementing the CNG2020 strategy.

4. Research Topic Evolution Analysis

By analyzing the evolution of research topics, the development context and characteristics of the field of carbon neutrality are revealed in Figure 7. The co-citation of documents can reveal the information behind the evolution of knowledge association through the intellectual base and research front [9,26]. Co-citation means that two documents are cited by one or more documents at the same time, indicating that they have common related research topics [27]. Therefore, document co-citation analysis can group related documents according to the similarity of the content, and by analyzing the documents in each group, the core theme of the research field can be determined [28]. With the help of the document co-citation clustering function in CiteSpace, it is possible to sort the knowledge structure of the carbon neutrality field and cluster the knowledge links between documents. The clustering timeline view visually displays the historical span of clustering topics and the relationship between clustering topics in the process of evolution. The horizontal axis shows the publication times of the literature, and the vertical axis shows the cluster numbers, which are arranged vertically by scale. Figure 6 shows the top 15 clustering topics sorted by cluster size, including 2629 nodes, 8237 links, and a co-citation network with a density of 0.0024. The Q value is 0.98 (>0.3), which shows that the clustering structure is reasonable, the boundaries of the research topics are clear, and the field differentiation is significant. The mean silhouette value is 0.97 (>0.4), which shows the strong homogeneity within the cluster. The sizes of the nodes represent the co-citation frequencies of the document. Colors of the cluster themes correspond to the times when co-citation appeared for the first time. Knowledge flows of the clusters are displayed in red, blue, green, yellow, and purple.

The time spans of the clusters vary significantly. Number 3 Energy System Model and number 10 Energy Transition have the longest time span. Lasting for more than 10 years (2009 to present), they are still active topics, and the research contents continue to deepen. Number 0 Triacylglycerides, number 6 Hydroxymethyl Furfural (HMF), and number 7 Socio-ecological System were once popular research topics from 2003 to 2011, but in recent years, no new literature has been published under these three themes. Compared with the above three clusters, number 1 Forest Biomass’s and number 8 Micro-algae’s popularity lasted from 2004 to 2014. Similarly, no new literature has been published under these two topics since 2015. The rise and fall of the research topics of the above five major clusters indicate that the research of these clusters may have reached a clear conclusion, entered the stage of maturity, or switched to a new research path when there were breakthrough discoveries. Number 5 Methanol Economy, number 15 China, number 18 Integrated Assessment Model, and number 19 Climate Change Mitigation have been research hotspots since 2015. At present, the duration of these research foci is still relatively short, but they represent thriving new directions in the field of carbon neutrality research. Finally, number 16 Emissions, number 33 Logging Residues, number 36 Wood Construction, and number 56 Soil Carbon Sequestration are not long in duration, and all have entered the stage of silence.

Vertically, the links between different cluster topics represent inner connections between different clusters. Number 6 HMF, number 8 Micro-algae, and number 33 Logging Residues have multiple links pointing to number 1 Forest Biomass and number 3 Energy System Model, which shows that the first three topics and the last two topics have deep connections. Multiple red lines are generated from number 18 Integrated Assessment Model and number 19 Climate Change Mitigation and point to number 5 Methanol Economy, and this also shows that numbers 18 and 19 have notable connections with number 5. Several documents under cluster number 3 Energy System Model have centralities that exceed 0.07. Among them, the report “Climate Change 2014: Mitigation of Climate Change” published by the IPCC in 2014 is an important connecting node with a centrality of 0.08, and this is the document with the highest centrality among all clusters [29].

To better classify the research topics, we summarized the evolution analysis path of carbon neutrality research hotspots, which is divided into three stages: exploration of alternative fossil energy and theoretical construction (2003~2014); clarification of the concept of carbon neutrality, determining goals, and exploring paths (2015~2018); and technological development and industrial applications of new technologies (2019~2021).

The first phase (2003~2014). The first phase, or the exploration phase, of carbon neutrality research mainly includes 11 clusters: number 0 Triacylglycerides, number 1 Forest Biomass, number 3 Energy System Model, number 6 HMF, number 7 Socio-ecological System, number 8 Micro-algae, number 10 Energy Transition, number 16 Emission, number 33 Logging Residues, number 36 Wood Construction, and number 56 Soil Carbon Sequestration. From 1995 to 2002, the number of cited documents on carbon neutrality research was small because researchers focused on the concept of carbon emission of fossil fuel energy, the connotation of energy efficiency, carbon emission effects, measurement, evaluation, and solutions and had not formed a unified concept of carbon neutrality; thus, this is not reflected in the figure as carbon neutrality was in the incubation period of the research. From 2003 to 2014, the content focused on exploring alternative energy sources of fossil energy, especially bioenergy, biofuels, and biodiesel. This stage mainly studies the determination, measurement, and evaluation of the concept of biomass energy and compares it with fossil fuel energy. It forms the basis for the initial research of carbon neutrality, whereby the co-cited amount and high light intensity are large. In

Table 4. Major articles distributed in clusters (numbers 0, 1, 3, 6, 7, 8, 10, 16, 33, 36, and 5).

First Author	Year	Article	Journal	Co-Citation	Burstiness	Centrality	Cluster
Mckechnie, J.	2011	Forest Bioenergy or Forest Carbon? Assessing Trade-Offs in Greenhouse Gas Mitigation with Wood-Based Fuels [39]	Environmental Science & Technology	26	11.96	0.02	#1
Cherubini, F.	2011	CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming [26]	GCB Bioenergy	21	10.5	0	#1
Searchinger, T.D.	2009	Fixing a Critical Climate Accounting Error [32]	Science	15	7.87	0	#1
Fargione, J.	2008	Land Clearing and the Biofuel Carbon Debt [33]	Science	14	5.77	0.01	#1
Repo, A.	2011	Indirect carbon dioxide emissions from producing bioenergy from forest harvest residues [37]	GCB Bioenergy	14	7.35	0.01	#1
Searchinger, T.	2008	Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land-Use Change [40]	Science	13	5.82	0.01	#1
Haberl, H.	2012	Correcting a fundamental error in greenhouse gas accounting related to bioenergy [35]	Energy Policy	13	6.31	0	#1
Chisti, Y.	2007	Biodiesel from microalgae [36]	Biotechnology Advances	12	5.91	0	#8
Melillo, J.M.	2009	Indirect Emissions from Biofuels: How Important? [37]	Science	11	5.76	0.01	#1
Holtsmark, B.	2012	Harvesting in boreal forests and the biofuel carbon debt [38]	Climatic Change	11	5.87	0	#1

, we list major articles whose total co-citation frequencies are greater than 10, and their main indicators. The co-citation frequency indicates the closeness of the relations between one article and other articles, the burstiness represents the frontier position of the article in the research field, and the centrality represents the strength of the connection between the article and articles from other clusters. In the first phase, articles are linked by green and yellow lines in the map, and there are many links across various clusters, indicating that different clusters are closely connected. In terms of the number of co-citations, there are nine articles in cluster number 1 with a co-citation frequency greater than 10, and the time distribution is concentrated in 2008~2012. Cluster number 8 has one article with a co-citation value of 12. The total co-citation frequencies of articles in other clusters are less than 10. The most frequently cited articles combine life cycle assessment (LCA) and forest carbon analysis to assess the total greenhouse gas emissions of forest bioenergy over time. A case study of applying this method to the production of wood pellets and ethanol from forest biomass has shown that forest carbon is significantly reduced due to bioenergy production [30]. This article has been co-cited 26 times, with a burstiness of 11.96 and a centrality of 0.02, indicating that this article has a strong research correlation with articles inside and outside of its cluster and occupies a frontier position in the field.

Francesco Cherubini et al. [26] conceived a method to estimate the damages caused by CO₂ emissions from biomass by using CO₂ impulse response functions (IRF) from C cycle models. They also proposed an index, GWPbio, which was expressed as a function of the rotation period of the biomass and whose purpose was to quantify the impact from CO₂ emissions on global warming. T.D. Searchinger et al. [32] fixed an accounting currently used for evaluating compliance with carbon reduction goals set in the Kyoto Protocol and took into consideration the previously neglected CO₂ emission from tailpipes and smokestacks when bioenergy was involved, as well as the previously remised changes in emissions from land use when biomass grew. J. Fargione et al. [33] investigated the “biofuel carbon debt” issues in depth and discovered an astonishing truth behind such a carbon reduction plan: they found that converting rainforests, peatlands, and grasslands to crop-based biofuels in many regions in the world was an erroneous decision because the conversion process generated 17~420 times more CO₂ emissions than annual GHG reductions provided by these crop-based biofuels. A. Repo et al. [34] proposed a method to quantify indirect emissions from logging residues during the process of bioenergy production, which focused primarily on calculating land use-related indirect emissions to calculate the reduction in CO₂ emissions from bioenergy. They discovered that carbon emissions from logging residues could be compared to emissions from fossil fuels in that emissions from stumps could last 22 years and emissions from branches could last 4 years, until the level of CO₂ emissions drops below that of natural gas. H. Haberl et al. [35] emphasized a popular accounting flaw: GHG emissions from bioenergy were often neglected, and suggested that such an error could be corrected by applying only the emission reduction from “additional biomass”. Y. Christi [36] explored the potential of microalgal biodiesel in comparison with petro-diesel and reached the conclusion that microalgal biodiesel seemed to be the only option for renewable biodiesel, which could satisfy the significant demand for global transportation fuels. Moreover, microalgae could even generate better oil productivity than oil crops. Y. Christi also examined alternative options for biodiesels from oil crops and waste cooking oil and animal fat but pointed out that these sources of biodiesel failed to meet even a small percentage of the real-world demand for transportation. J.M. Melillo et al. [37] studied direct and indirect effects of GHG emissions over the 21st century from expanding cellulosic bioenergy programs across the globe. They developed a model to predict carbon loss from direct and indirect land use and discovered that indirect land use contributed twice as much carbon loss as direct land use. They also emphasized that nitrous oxide emissions from the growing fertilizer use should also be taken into consideration when designing a GHG emission reduction scheme. B. Holtsmark [38] discussed whether wood harvesting is a carbon-neutral activity and proved that it is not. He estimated that it took 190~340 years to repay biofuel carbon debt generated from an increased harvest of a boreal forest and verified that high levels of harvest indeed lead to low levels of carbon stock.

The duration of this stage was long, and the specific concept of carbon neutrality and temperature rise control objectives were not put forward; thus, emission reduction schemes could not be designed in combination with specific objectives. This phase mainly focused on theoretical accounting, simulation, and prediction of the contribution of different biomass energies to emission reduction effects, and there was no substantive technical progress. Therefore, in this fashion, it is a basic stage of carbon neutrality research.

The second phase (2015~2018). More than half of the clustering studies in the previous phase have entered the stage of silence, and no new literature has been published in the second phase. Number 3 Energy System Model, number 5 Methanol Economy, number 10 Energy Transition, number 18 Integrated Assessment Model, and number 19 Climate Change Mitigation are the research foci of the second phase, among which clusters 3 and 10 span three research phases. From 2015 to 2018, with the deepening of carbon neutrality research and the determination of temperature rise control objectives in the Paris Climate Agreement, climate change, the formation of the carbon neutrality concept, and the determination of carbon neutrality objectives were the main hotspots in this period. To support the research hotspots in this stage, it is necessary to integrate the economic system and ecosystem into a model framework for climate policy evaluation, continuously improve the model conditions, and improve the comprehensive assessment model, so that it can be widely used in the climate field. At this stage, discussions on the development of various alternative energy sources and the planning of gradually replacing fossil energy emerged, and the energy system model was gradually formed. In addition, discussions on the methanol economy and the research on renewable energy-related technologies also gradually emerged, especially in the power-to-gas technology (PTG), which converts electric energy into natural gas or hydrogen, stores the obtained gas in the natural gas pipe network or natural gas storage equipment, converts and stores it during the peak output of renewable energy, and supplies energy in case of power shortage, to improve the consumption capacity of renewable energy in the system. In

Table 5. Major articles distributed in clusters (numbers 3, 5, 10, 18, and 19).

First Author	Year	Article	Journal	Co-Citation	Burstiness	Centrality	Cluster
Gotz, M.	2016	Renewable Power-to-Gas: A technological and economic review [42]	Renewable Energy	17	6.36	0	#3
Griscom, B.W.	2016	Natural Climate Solutions [41]	National Climate Change	14	5.23	0.05	#18
Masson-Delmotte, V.	2018	Global Warming of 1.5 °C. An IPCC Special Report on the impacts of global warming of 1.5 °C [45]	Summary for Policymakers	14	6.39	0	#19
Fuss, S.	2014	Betting on negative emissions [43]	National Climate Change	12	6.65	0	#3
Rogelj, J.	2016	Paris Agreement climate proposals need a boost to keep warming well below 2 °C [3]	Nature	11	4.11	0	#18
Edenhofer, O.	2014	Climate Change 2014 Mitigation of Climate Change Working Group III Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [44]	Climate Change 2014: Mitigation of Climate Change	10	4.38	0.08	#3

, we summarize articles under cluster numbers 3, 5, 10, 18, and 19 whose co-citation frequencies are greater than 10. Griscom B.W. [41] proposed a quantitative index for natural climate solutions to achieve the goal of containing temperature rise within 2 °C in accordance with the Paris Climate Agreement. The centrality of this article is 0.05, indicating that it is closely related to articles from other clusters. Masson-Delmotte V. [1] discussed the impact of global warming of 1.5 °C on science, technology, and the social economy of human society and pointed out that the current global average temperature is 2 °C higher than the pre-industrial level. The burstiness of this article is 6.39, implying that this paper received widespread attention after its publication. Rogelj J. [3] assessed the impact of the current Intended Nationally Determined Contributions (INDC) submitted by various countries on the reduction in total greenhouse gas emissions, as well as each country’s contribution to achieving the temperature targets according to the Paris Climate Agreement and potential for over-realization. Compared with the current policies, INDC have generally reduced greenhouse gas emissions, but they will still increase the temperature by 2.6~3.1 °C around 2100. According to this paper, humankind needs to significantly increase the current INDC by increasing government and non-government actions to achieve the goal of keeping the temperature rise well below 2 °C. M. Gotz et al. [42] discussed the whole PTG process chain by comparing various available electrolysis and methanation technologies and investigated their process conditions and requirements, including low capital expenditure, high efficiency, and high flexibility. Specifically, they examined three water electrolysis technologies: alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis technologies, and discovered that PEM could potentially be the best option for the PTG process chain. S. Fuss et al. [43] discussed carbon capture and storage technologies with a focus on bioenergy and carefully considered various possible negative effects caused by deploying such technologies. They placed great emphasis on the fact that implementing large-scale bioenergy faces social, technical, and biophysical difficulties such as the physical constraints on bioenergy carbon capture and storage (BECCS), the response of natural land and ocean carbon sinks to negative emissions, the financial constraints of these unproven technologies, and public sentiment toward these ground-breaking technologies. O. Edenhofer et al. [44] conducted extensive literature research on the scientific, technological, environmental, economic, and social aspects of climate change mitigation and assessed different approaches to the mitigation of climate change at various levels of governance and in different regions, but they did not propose one specific solution for mitigation. They presented this summary for policymakers based on model results, quantitative analysis of observations, and expert judgement.

At this stage, the concept of carbon neutrality was clarified, the goal was determined, and the path was explored. Discussions on the realization path of climate change and temperature rise control goals emerged, and the mathematical model and theoretical framework of carbon neutrality research gradually formed and improved. Comparing the research topics from 2003 to 2014 and 2015 to 2018, it can be found that the development of carbon neutrality research showed an evolution from microtechnology to macro-system planning. This stage focused on the improvement in theory and the construction of the model and began to try to explore from the perspective of technology, but there was still no specific technology application, which was still in the laboratory stage.

In the past three years (2019–2021), the publication time of the literature has been relatively short, so there are fewer citations, and they cannot be selected by comprehensive consideration indicators. Therefore, we read influential articles in the past three years and conducted a literature review. Compared with the second stage, the researchers’ perspective has gradually narrowed down and begun to explore different types of carbon dioxide emission reduction schemes and the same dilemma of energy demand and carbon footprint responsibility, and a deep study into the practical application of renewable technologies in transportation and the actual contribution of biomass energy to greenhouse gas emission reduction has been carried out. The research focus has shifted to the development of more feasible sustainable fuels. Sustainable fuels, biofuels, and carbon dioxide emission reduction have become hot research topics. The development of new technologies and industry application cases have become the characteristics of research in this period. Compared with previous studies, with the support of industry application cases and increasingly mature new technologies, the research quality has been higher, the research topics have been more subdivided and diversified, and empirical analysis research has been gradually rising.

Haseeb Yaqoob et al. [46] conducted a comparative study on electricity, diesel, gasoline, liquified natural gas, compressed natural gas, liquified petroleum gas, and bioethanol and biodiesel as alternative fuels for transportation in Pakistan and found that electricity, compressed natural gas, and alternative fuels outperformed other types of fuels. They also proposed several strategies and policies for the government that involved using these sustainable fuels in electric vehicles. Shivali Banerjee et al. [47] reviewed strategies adapted for the bioprocessing of urban waste that could be combined with other waste treatment methods to improve the efficiencies of waste management. Mehak Sikander et al. [48] examined a series of sustainable production techniques whose goal was to reduce the environmental footprint in seven tanneries in Pakistan and reached the conclusion that by adopting their strategies, 71,131 m³/year water and 1643.166 m³/year compressed air could be saved, and that overall CO₂ emissions could be reduced by an amount of 300,842 kg/year. H. Gilani et al. [49] proposed a mixed-integer linear programming method, which could design a global network of sugarcane-to-biofuel supply chains. They employed a robust optimization approach to maximize profit, minimize the resulting environmental impacts, and maximize job opportunities and also verified the performance of the model by a case study performed in Iran. N. Verma et al. [50] compared the potential ability of untreated and alkali-treated wheat straw, bagasse, and groundnut shell waste in cellulase production and showed that compared with the untreated materials, alkali-treated raw materials were significantly more effective for cellulase production. S. Vakalis et al. [51] proposed a method which combines co-combusting conventional fuels with biocoals from agro-waste for the purpose of reducing carbon emissions from coal plants and proved that when more than 40% of fuel blends consisted of biocoal, the use of lignite coal led to optimal results. These results answered the question of how to reduce the carbon footprint of coal production facilities for decision makers in European energy sectors. A.M. Mauerhofer et al. [52] employed the dual fluidized bed biomass gasification technology to reduce CO₂ emission by using the produced CO₂ within the process as a gasification agent and showed that the proposed method successfully gasified CO₂. M. Ameen et al. [53] were concerned with the fact that oil palm residues generally cause serious damage to the environment and therefore proposed a method to reduce oil palm residues as solid wastes. They assumed that wastes from oil processing factories had high potential to be converted into renewable energy and conceived a hydrothermal carbonization (HTC) method to produce hydro-char under hot-compressed water using oil palm residues including palm leaves, palm fronds, and palm shells. Huihui Wang et al. [54] studied the characteristics of carbon emissions from commuter travel in Beijing using a bottom-up approach. They focused on the latest changes in the trends of CO₂ emissions by estimating emissions from commuter travel modes in Beijing to determine the main sources of increases in carbon emissions. They concluded that to contain the growing trend of carbon emissions, the Beijing government needed to take actions to facilitate the development of industry, public transport, and residential facilities in suburban areas as well as residential facilities along the Ring Road and the Radix Road. In addition, the researchers also stated that recent development in the sharing economy and digitalization would counteract or enhance the predicted energy efficiency gains, which, in turn, will significantly influence the future energy demand. Heike Brugger et al. [55] presented 12 new social trends which will greatly determine the future energy demand, based on extensive interviews with European experts and literature research. They also evaluated the spatially explicit carbon footprint (CF) at a district and household scale by importing the results of a large-scale household-level consumer survey into a global supply chain database. They assessed 12 new societal trends and their impacts on all economic sectors and eventually simulated 4 case scenarios for energy demand in 2050. They concluded that in the “best case” scenario, the total energy demand in Europe will be reduced by 67% compared to the EU “Baseline”, while in the “worst case” scenario, the total energy demand will increase by 40%. Jemyung Lee et al. [56] studied the variations in the household CF in India by economic, demographic, and cultural factors by investigating micro consumption data from 203,313 households in 623 districts in India and discovered that high-expenditure households contributed seven times more carbon emissions than low-expenditure households. Based on these findings in India, the researchers believed that high-expenditure individuals and households need to be more responsible. Haidi Gao et al. [57] focused on non-CO₂ GHG emissions, built a global multi-regional input–output (MRIO) model for 2004, 2007, and 2011, and conducted a structural decomposition analysis (SDA) to identify major driving forces in increases in consumption-based emissions. They found that among non-CO₂ GHG emissions, CH₄, N₂O, and F gas emissions experienced the fastest growth, although the net exports of non-CO₂ GHG emissions have greatly decreased in recent years. They also found that household consumption was the most critical factor behind the growth in consumption-based non-CO₂ GHG emissions, while investment in the total final consumption demand was the top contributing factor for CO₂ emissions.

The evolution of keywords in the highly cited literature in various periods shows that the global research on carbon neutrality has gradually shifted from the exploration of basic concepts in the first stage, to the determination of macro concepts and the formulation of grand goals in the second stage, to microtechnology development and industrial case exploration in the third stage, forming a macro–micro development path. However, compared with the first stage, the application of quantitative methods, such as econometrics and game theory, goes deeper into the level of industries and enterprises. Moreover, since the second stage defines the global warming control objectives for the next few decades, the microanalysis depth of the third stage combines the macro-objectives established in the second stage, and the research has a clear objective. However, due to the global energy crisis caused by COVID-19 at this stage, the carbon neutrality program has been shelved in many countries and regions around the world, and the global carbon neutrality process has been retrogressive. This has also worried people about the development of this research field and allowed us to reflect on whether the previous carbon neutrality design is feasible enough. However, it may only remain as theory and may lack the design of appropriate emission reduction schemes for industrial applications. Considering the global energy crisis and financial crisis, in the next stage of research, researchers should timely adjust the carbon dioxide emission reduction targets, overturn the unrealistic simulation predictions made previously in combination with the global political and economic situation, accelerate the exploration of feasible new energy technologies, and fully consider the needs of the industry and the public.

5. Burst Detection

The research frontier is an active direction or theme in discipline development, which is derived from the knowledge base. Burst detection can identify the emerging or upcoming research frontier [58]. Through the analysis of burst detection, we can find periods and dynamic changes with a high keyword emergence intensity, in order to reflect cutting-edge situations and development trends in the research field.

Table 6. Top 20 keywords with the strongest citation bursts.

Keywords	Strength	Begin	End	1995–2021
climate	3.7	2001	2010
carbon sequestration	4.64	2003	2012
balance	4.78	2003	2015
growth	3.64	2005	2015
reduction	3.76	2008	2009
biofuel	10.57	2008	2015
biomass	12.22	2008	2016
biodiesel	4.62	2009	2011
ethanol	4.4	2009	2016
microalgae	5.16	2011	2016
bioenergy	5.16	2011	2016
management	4.01	2012	2016
oil	3.77	2013	2017
coal	3.37	2017	2018
aviation	4.1	2017	2019
transition	5.56	2018	2021
CO2 capture	4.5	2018	2021
city	3.35	2018	2021
consumption	4.78	2019	2021
gas	4.38	2019	2021

shows the first 20 keywords in terms of emergence frequency, their emergence intensity, and the start and end years. The keyword with the largest emergence intensity is biomass, with a value of 12.22. The research under this keyword has attracted extensive attention in the academic community. The longest time of emergence belongs to balance, which was an active theme from 2003 to 2015. According to emergence, the frontier development in the field of carbon neutrality can be divided into three stages. (1) Since 2001, a large number of emergent words have appeared, including climate, carbon sequestration, growth, and reduction, showing that the research in the field of carbon neutrality has increased greatly since that year. Scholars have paid more attention to the natural and social problems caused by carbon emissions, as well as the phenomena, causes, and influencing factors of climate change, but the discussion on how to mitigate and solve them has not been fully carried out. (2) Since 2008, renewable energy research has become the focus, and the emerging words include biofuel, biomass, biodiesel, ethanol, microalgae, and bioenergy. (3) Since 2012, the academic community has paid great attention to fossil energy management, including management, oil, coal, aviation, and transition. The use of fossil energy releases a large amount of carbon dioxide into the atmosphere, which is the main cause of climate change. The keywords emerging in the last three years are transition, consumption, CO₂ capture, gas, and city, indicating that the topics represented by these keywords have been more active in the field of carbon neutrality research in recent years. The emerging research has a certain continuity, and the above five keywords continue from the beginning year of emergence to 2021, which represents the frontier progress of academic research at present.

Energy structure transformation. Fossil fuels are used to produce heat and electricity and as transportation fuels, accounting for 80% of global greenhouse gas emissions [59]. The energy structure transformation focuses on the supply side adjustment. Through technical support such as clean energy substitution, fossil energy decarbonization, power system decarbonization, energy efficiency improvement, and carbon reduction technology, supplemented by green finance, carbon emission trading, and policy support, the energy system is clean, low-carbon, and efficient. Sithole h. [60] helped the transformation of the UK power industry by building an energy optimization calculator, focused on the policy goal of 80% carbon emission in 2050, and optimized the sustainable power generation portfolio by considering factors such as the lowest-cost power generation portfolio, emission intensity, and total investment required for power generation. It was estimated that it is feasible for the power sector to achieve carbon neutrality through the large-scale deployment of low-carbon technologies, but this requires a strong policy combination to support the development and deployment of mature and emerging technologies. Farsaei A. [61] estimated the impact of Finland’s ban on coal-fired power generation through modeling and found that abandoning coal and nuclear power increased net exports, thereby increasing carbon dioxide emissions in the surrounding areas. Evangelopoulou S. [62] discussed the alternative role of hydrogen in the future energy system in the process of the EU’s transformation to a carbon-neutral economy in 2050. This paper evaluated the advantages and disadvantages of hydrogen as an end-use fuel, a raw material for the production of carbon-neutral hydrocarbons, and a chemical power storage carrier. The results showed that if hydrogen technology reaches a high level and economies of scale, the energy system will benefit in reducing carbon dioxide emissions and total costs.

Energy consumption assessment and carbon neutrality schemes of various industries. Energy consumption is not only the driving force of economic growth but also the main factor of carbon emission growth. To achieve the goal of carbon neutrality, it is necessary to clarify the emission responsibilities of various industries and evaluate the consumption on the energy demand side, and to formulate industrial emission reduction strategies and schemes. Taking the construction industry as an example, green buildings can not only reduce the carbon footprint of buildings by using carbon-negative, carbon-neutral, and carbon storage building materials but also use low-carbon emission sources to meet the energy demand and produce and store renewable energy to minimize the energy demand of fossil fuels [63]. Valencia A. [64] studied a green building reconstruction plan by creating a symbiotic relationship in the food–energy–water relationships that support building operation through the system dynamics model (SDM) and ecological footprint, avoid carbon dioxide emissions through roof agricultural carbon sinks, and improve energy supply reliability and food security.

Carbon dioxide capture technology. Carbon dioxide emissions can be captured from point sources such as flue gas from traditional power plants or waste gas from non-energy sectors such as cement plants; however, there are also some problems such as factories that are too old to be transformed and have a low capture rate. Direct air capture of carbon dioxide (DAC) is a method of capturing carbon dioxide from the atmosphere, diluted gases, and dispersed carbon sources through industrial processes. This method is still in the early stage of commercialization. Traditional carbon capture and storage have proved to be the lowest-cost option for decarbonization in the power, heat, and industrial sectors. Hanak D.P. [21] evaluated the feasibility of a new polygeneration process that uses solid oxide fuel cells for cogeneration and produces lime for power plants, thereby contributing to decarbonization in the power, thermal, and industrial sectors. Studies have shown that decarbonization in the power, heat, and industrial sectors is not enough to achieve the goal of carbon neutrality. Even in power plants with a carbon dioxide emission system, since the average capture rate is in the range of 50~94%, it is impossible to capture all emissions, meaning it is also necessary to capture carbon dioxide directly from the atmosphere [65,66]. Fasihi M. [67] estimated the capital expenditure, energy demand, and cost of carbon dioxide direct air capture (DAC) technology from 2020 to 2050. The research showed that with the commercialization in 2020 and the large-scale implementation in 2040, the cost of DAC systems may be greatly reduced, making them cost competitive with point source carbon capture systems and other affordable climate change mitigation options. Lucas Bonfim-Rocha et al. [68] conducted a literature review of the major manufacturing processes for the synthesis of NaHCO₃ and associated chemical reactions, for the purpose of evaluating environmental damages caused by CO₂ emissions. They discussed advantages and disadvantages for the separation and purification processes after the chemical reaction, and the total processes for each method were also summarized. S. Pérez et al. [69] constructed an unprecedented milli-channel reactor whose internal diameter is in the scale of millimeters, tested this invention for the Sabatier reaction (a process where the heat produced needs to be ceaselessly eliminated to prevent the catalyst from sintering), and concluded that their novel reactor succeeded in limiting heat produced in the reaction process.

Utilization of biogas resources. Biogas is a renewable energy source derived from the anaerobic digestion of biomass and has the potential to replace natural gas. Research shows that the global biogas supply can replace 20% of the natural gas demand and reduce greenhouse gas emissions [70]. Biomass energy is considered to be one of the main renewable energy sources to replace fossil energy in the future. There are several technologies to convert biomass into energy, the most important of which are gasification, bioethanol, biogas (anaerobic digestion), biodiesel, and combustion. Kheybari S. [71] evaluated biomass-to-biofuel technology based on multi-criteria decision analysis (MCDA) and established a comprehensive framework affecting technology evaluation criteria. The conclusion showed that water consumption is the most important factor in evaluating biomass power generation technology.

Urban carbon neutrality target policy. Urban areas contribute 71~76% of global carbon emissions. Many cities around the world aim to achieve the goal of carbon neutrality by designing schemes, building sustainable development systems, and implementing plans. Linton S. [72] discussed the eight best urban practice cases leading to deep decarbonization. To achieve deep decarbonization at the local level, local governments and stakeholders have developed four types of strategic paths, including non-state actors’ participation in decision making, green economy, policy means, and financial instruments. Dahal K. [73] studied how renewable energy policies act on carbon neutrality, taking the Helsinki metropolitan area as an example. The research showed that the region can take a variety of energy policy measures, including small-scale production of renewable energy at construction sites, the integration of renewable energy (waste heat, heat pump, and solar energy) in buildings and regional heat networks, demand-side solutions for energy utilization, increasing budgets and subsidies for renewable energy production, and improving social acceptance of renewable energy. Hast A. [74] formulated a regional heating scheme to achieve carbon neutrality in 2050 according to the planning and objectives of regional cities and heating companies. In the study area, it is expected to increase biomass and waste utilization, geothermal and waste heat utilization, and carbon capture and storage technologies in the future. However, to limit the increase in heating costs and energy shortages, the diversified use of different technologies should be considered.

6. Conclusions

Based on 1383 documents from 1995 to 31 August 2021, collected from the WOS database, this paper summarized the research status in the field of carbon neutrality and conducted an in-depth analysis on the research of carbon neutrality from four aspects. According to the basic statistical analysis, the amount of literature published in the field of carbon neutrality has been increasing year by year, and more relevant research is expected in the future. In terms of the cooperation network, cooperation in carbon neutrality research happens all over the world in various countries, within institutions, and among scholars. Through the analysis of the keyword co-occurrence network, 11 topics were clustered together, and 4 current key research topics of carbon neutrality were summarized: carbon neutrality energy transformation, carbon neutrality technology development, carbon neutrality effect evaluation, and carbon neutrality industry. By drawing the cited timeline, 15 topics were clustered together, the historical research was divided into three stages, and the theoretical development and knowledge evolution paths of each period were identified. By analyzing the list of emerging words, it was found that a large number of studies showed changes in three themes over time, from climate change to the search for renewable energy, and then to fossil energy management and control. In the future, the research frontier of carbon neutrality will focus on energy structure transformation, industrial energy consumption assessment and its carbon neutrality implementation scheme, carbon dioxide capture technology, biogas resource utilization, and urban carbon neutrality policy. The research field of carbon neutrality is highly interdisciplinary, showing the characteristics of multi-national attention, multi-disciplinary research, and multi-topic exploration. Scholars in different fields have conducted in-depth research from the aspects of market, policy, technology, and behavior and made important contributions to the realization of the goal of carbon neutrality. By analyzing the international research hotspots, development context, and future trends in the field of carbon neutrality in the past 25 years, this paper provides an important reference for relevant research and practice in the future. The innovations of this paper are mainly reflected in the following three aspects: Firstly, while using text mining technology to measure the literature of carbon neutrality research, this paper systematically combed the theme context, knowledge evolution, and emerging hotspots of international carbon neutrality research in the past 25 years. Secondly, the bibliometric method and analysis process used in this paper can be used as a reference for subsequent similar research. This research perspective can also be extended and applied to the research of other disciplines. Finally, the comprehensive use of quantitative methods of literature measurement and qualitative methods of literature research allows the research results to not only conform to subjective experience but also contain objective data, which are more scientific and accurate. They are presented in the visual form of a knowledge map, and the results are clear and readable. In addition, since this study mainly took published academic journals as the data source, in future research, we should integrate different data sources and enrich and expand the research data source channels, in order to discuss and analyze the research theme more comprehensively and completely. This paper excavated effective information from the existing literature related to carbon neutrality, combed the historical context of carbon neutrality research, and discovered the relevant theoretical basis and cutting-edge problems, which will help to provide a reference for scholars’ theoretical research. Mastering the development direction of carbon neutrality technology in the future is helpful for enterprises to take strategic decisions and determine the direction of technology R&D and investment, and it is of great significance to promote the deepening and internationalization of research in the field of carbon neutrality technology. An objective understanding of the problems that need to be faced to achieve the goal of carbon neutrality has important reference value for policymakers to formulate relevant policies and measures.