Carbon-Neutrality Research in China—Trends and Emerging Themes

: Carbon neutrality is a key human endeavor to deal with global climate while China is the country producing the most publications on carbon neutrality. However, what are the focuses of carbon-neutrality research in China? To answer such an important question, this study adopts a bibliometric approach to analyze carbon-neutrality journal publications from China-based researchers during the period of 2008–2022 using CNKI and Scopus. Results showed that carbon-neutrality publications in Chinese and English journals by Chinese-based researchers increased from 4 in 2008 to 2879 in 2022. In Chinese journals, X.D. Hao was the most productive author with 10 publications. In English journals, Y.K. Zhou was the most productive author with 14 publications. As a whole, the Chinese Academy of Sciences was the most productive institution with 376 publications. Co-occurrence of keywords analysis revealed seven themes in Chinese journal articles, namely, “carbon neutrality and climate change”, “energy transformation”, “peak carbon emission”, “carbon emission and low carbon economy”, “realization path in China”, “hydrogen energy and clean energy”, and “green ﬁnance and sustainable development”. In English journals, three major clusters were formed— “carbon, carbon neutralities, emission control, and energy utilization”, “carbon dioxide, carbon neutrals, biomass, and electrocatalysts”, and “China, carbon neutrality, sustainable development, and economic development.” Most extant publications focused on policy and technological development. Emphasis shall be paid to social change and changes in people’s behavior, sectoral carbon emissions, and carbon leakage in future research.

Urbanization, on the one hand, enhances efficiency, effectiveness, and productivity of a country and provides job opportunities to all city-dwellers, including migrants from rural areas and immigrants.On the other hand, urban areas consume unproportioned amounts of energy, food, water, and other resources at an ever-increasing rate due to increases in population, affluence, and technology use [4][5][6].According to UN-Habitat's Urban Energy [7], the world's urban areas consume about 80% of global primary energy directly and indirectly, and produce over 60% of the total greenhouse gases (GHG).The International Energy Agency [8] reported that the world's total energy consumption increased almost continuously from 261 Exajoule (EJ) in 1990 to 418 EJ in 2019, then decreased to 401 EJ in 2020 because of the COVID-19 pandemic.In 2020, China was the top country consuming 91 EJ, followed by the US consuming 61 EJ.About thirty years ago, the US was the top country consuming 54.2 EJ while China only consumed 27.5 EJ in 1990.More specifically, China's industrial sector increased its energy consumption by almost five times (from 9.8 EJ in 1990 to 44.9 EJ in 2020) and China's transport sector increased its energy consumption by more than 10 times (from 1.3 EJ in 1990 to 13.5 EJ in 2020) during the thirty-year period.Nevertheless, China's population was 1.41 billion while US's population was 0.33 billion in 2020.Thus, total energy consumption per capita in 2020 was about 65 GJ per capita in China and 184 GJ per capita in the US, respectively.Additionally, many of the products manufactured in China were exported and consumed abroad, including the US.Thus, the actual energy consumption per capita needs to be adjusted due to the complex import, manufacture, and export relationship.
The International Energy Agency [8] indicated that fossil fuels including coal, oil, and gas accounted for 81% of energy production.Thus, when the world's total energy consumption increases, GHG and other air pollutant emissions increase.Chung and To [9] reviewed carbon-neutrality studies that were published as journal articles and reviews, conference papers, books and book chapters in English during the period of 2001-2022 (up to 8 October 2022).Specifically, carbon neutrality is characterized by net-zero carbon emission in which the discharge of carbon compounds into and the removal of carbon compounds from the atmosphere are the same [9].To and Chung [9] indicated that the cumulative net carbon dioxide (CO 2 ) emissions from 1850 to 2019 amounted to 2400 gigatonnes (Gt) [10].However, about 1000 Gt of CO 2 (i.e., 42% of the cumulative total) were emitted to the atmosphere between 1990 and 2019 [10].Concurrently, global temperature had an upward trend from +0.43 • C in 1990 to +0.95 • C in 2020 compared to the 1951-1980 global temperature average [11].Although the Paris Agreement signed by 196 parties in December 2015 aims at limiting global warming to no more than 2 • C, preferably to 1.5 • C, when compared to pre-industrial levels, the current situation is not optimistic because members of the international community are divided [12] and many of them are way behind in limiting their real carbon emissions.Fortunately, China announced in the UN General Assembly on 22 September 2020 that it aims at reaching its peak carbon emission on or before 2030 and achieving carbon neutrality by 2060-known as the "30-60" goals.Since then, many Chinese academics have reprioritized their research focuses to carbon neutrality-related topics and have published their findings in Chinese and English academic journals [13][14][15].In order to understand carbon-neutrality research across the world, Chung and To [9] performed a bibliometric study of carbon neutrality using Scopus-one of the largest abstracting and indexing databases in English as the source of data.Their study focused on carbon-neutrality publications in English academic journals, conference proceedings, books, and book chapters.Chung and To [9] found that the first academic journal article on carbon neutrality was written by Ahamer [16] in 1994 about the influence of biomass fuels on atmospheric CO 2 .The second and third academic journal articles on carbon neutrality were published in 1995 about hydrogen as a fuel [17] and using logging residues as bioenergy [18].In fact, the first English journal article on carbon neutrality by China-based researchers was published in 2006 on energy use in rural households near Xian city in which biomass (as a more carbon-neutral energy source) was widely used [19].It was followed by another two articles about carbon fluxes in the Inner Mongolia of China [20] and production of biomass fuels in China [21].Chung and To [9] then analyzed 8071 documents published during the period of 2001-2022 (up to 8 October 2022).They reported that China-based researchers alone co-authored 44.6 percent of carbon-neutrality publications written in English.A quick search of "carbon neutrality"/"carbon neutral" as a keyword using Chinese National Knowledge Infrastructure, i.e., CNKI, identified the first Chinese journal article on carbon neutrality was published in 2008.It was written by Zeng and Cen [22] with a title "Carbon neutrality and Beijing Green Olympics".The second and third Chinese journal articles on carbon neutrality focused on the relationship between economic development and carbon emissions in different regions [23] and low-temperature municipal wastewater treatment and its efficiency [24].Yet, there is no bibliometric study of Chinese carbon-neutrality publications that have helped the Chinese government to formulate policy, promote technology development, and set regulations.Additionally, carbon-neutrality research published by China-based researchers in Chinese and English journals have yet to be thoroughly reviewed to understand the trends, highly cited topics, and emerging themes.Thus, this study adopts a bibliometric approach to analyze carbonneutrality publications by China-based researchers using CNKI and Scopus as data sources during the period of 2008-2022.Specifically, the study aims to answer the following research questions (RQs).RQ1: Who were the most productive China-based researchers in carbon neutrality in Chinese/English journals?RQ2: Which affiliations were the ones producing most carbon-neutrality publications in Chinese/English journals?RQ3: Which Chinese/English journals were the ones having the most carbon-neutrality publications?RQ4: Which funding bodies were the most supportive ones for carbon neutrality research published in Chinese/English journals?RQ5: Which were the highly cited publications in carbon-neutrality by China-based researchers in Chinese/English journals?RQ6: What were the emerging themes/clusters of carbon neutrality?Our findings can shed light on the past and present of carbon-neutrality research published by China-based researchers.They also reveal the core themes of carbon-neutrality research so that future research efforts can be appropriately developed.Specifically, the contributions of this study are four fold.First, the study identifies the most productive authors and affiliations of carbon-neutrality research in China.Second, it reveals the highly cited carbon-neutrality publications in Chinese/English journals and their focuses, where these focuses may have significant implications for policy development and deployment.Third, it identifies emerging themes of carbon-neutrality research that may help scholars formulate their future research plan.Fourth and finally, it also sheds light in some areas that warrant future investigation.

Materials and Methods
Bibliometric analysis can be defined as the use of quantitative tools to (i) identify the statistical properties of large-scale scholarly publication metadata on a research topic and (ii) reveal the trend and underlying structure of the research topic [25][26][27][28].In the past two decades, there have been significant developments in academic indexing databases such as Scopus, Web of Science and Dimensions in English, and CNKI in Chinese in terms of the coverage and details of bibliographic information.Some bibliometric visualization tools such as VOSviewer and CiteSpace have been developed and made available free of charge [29,30].These tools enable researchers to explore the underlying properties of a selected group of publications such as co-authorship and co-occurrence of keywords.Thus, the number of bibliometric studies has increased rapidly.Donthu et al. [26] found that in the social sciences, business, and management, the number of bibliometric studies increased by over 11 times in a 15-year period (from 170 in 2005 to 1950 in 2020).When a search using "bibliometric" in "Article Title, Abstract, Keywords" then filtering by "Environmental Science" in "Subject Area" was performed on 15 April 2023, 3586 publications were identified.The search results showed that the number of bibliometric studies in envi-ronmental science increased from 7 in 2005 to 1027 in 2022.Among all these bibliometric studies in environmental science, nine of them focused on carbon neutrality in the building sector [31], in the construction industry [32], from the global health perspective [33], from the forest management perspective [34], research changes due to the COVID-19 [35], and in a general sense [36][37][38][39].Additionally, there was another bibliometric study on climate change that has implications on carbon neutrality [40].In fact, carbon emission reduction can lower the risk of climate change and the occurrence of extreme weather [41,42].Most of these bibliometric studies about carbon neutrality used Web of Science as the source of data and one study [40] used CNKI-CSSCI (Chinese Social Science Citation Index) as the main source of data, all focusing publications in English-except Zhong et al.'s [40] study covering both English and Chinese publications.

Sources of the Data
Data were obtained from two academic indexing databases-CNKI and Scopus.CNKI is the largest Chinese academic indexing database launched in 1996 by Tsinghua University and its subsidiaries [43].It has become a privately owned Chinese publishing company since 2014.According to the CNKI website, its Academic Journals Database (AJD) covers articles published in both Chinese and non-Chinese academic journals.Currently, CNKI-AJD covers 8490 Chinese academic journals with 59 million full-text articles and over 75,000 non-Chinese academic journals with 123 million articles' bibliographic records.Bibliographic information of over 90% of journal articles covered by Web of Science and Scopus are available through CNKI-AJD.In the present study, we collected bibliographic data from Chinese journal articles on carbon neutrality using CNKI-AJD that are covered by the Chinese Social Sciences Citation Index (CSSCI), Chinese Science Citation Database (CSCD), and A Guide to the Core National Journals by Peking University.A search was performed using CNKI on 15 April 2023 based on "carbon neutrality" or "carbon neutral" in "Subject" published up to "2022" in Chinese academic journals.The search results showed that 891 journal articles were identified.
Scopus is one of the largest academic abstracting and indexing databases in English.It covers over 87 million documents, 1.8 billion cited references, 17 million author profiles, over 94 thousand affiliation profiles, and over 40,000 journals from seven thousand publishers [44].Scopus is one of the most popular databases used in systematic literature reviews and bibliometric studies because it works closely with bibliometricians to curate its data structure and presentation [45].On 15 April 2023, a keyword search using ["carbon neutrality" OR "carbon neutral"] in "Article title, Abstract, Keywords" was performed in Scopus with the date range from 2008 to 2022.A total of 9804 documents were identified.Among these 9804 documents, 6420 were English journal articles and reviews including 6250 published and 170 articles in press.More specifically, 3195 journal publications in English were authored/co-authored by researchers from China after excluding 103 articles in press.In the Scopus term, the search was [TITLE-ABS-KEY ("carbon neutrality" OR "carbon neutral") AND PUBYEAR > 2007 AND PUBYEAR < 2023 AND (LIMIT-TO (PUBSTAGE, "final")) AND (LIMIT-TO (AFFILCOUNTRY, "China")) AND (LIMIT-TO (SRCTYPE, "j")) AND (LIMIT-TO (LANGUAGE, "English"))].Figure 1 presents a schematic diagram of the data searches.

Performance Analysis and Science Mapping
Generally speaking, a bibliometric study covers performance analysis and science mapping of the selected research topic [28,47,48].Performance analysis reveals the most productive authors, affiliations, and countries.It also highlights the top source titles and the highly cited publications among the selected articles.In the present study, performance analysis was conducted using CNKI and Scopus built-in bibliometric functions.
Science mapping identifies the relational aspects of the selected articles.In the present study, bibliographic details including authors and keywords were extracted from CNKI.Bibliographic data were manually screened for missing information and relevance to the study.No missing information was found and data were then analyzed by VOSviewer (version 1.6.11).Similarly, the Scopus CSV file was manually screened and no missing information was observed.It was then analyzed by VOSviewer (version 1.6.11).The analysis included the identification of co-authorship networks and co-occurrences of keywords [29].

Results
Figure 2 shows the number of carbon-neutrality publications in Chinese academic journals from 2008 to 2022.Before 2021, there were no more than 11 articles per year.The number of carbon-neutrality publications increased sharply to 367 in 2021, then to 476 in 2022.Figure 2

Performance Analysis and Science Mapping
Generally speaking, a bibliometric study covers performance analysis and science mapping of the selected research topic [28,47,48].Performance analysis reveals the most productive authors, affiliations, and countries.It also highlights the top source titles and the highly cited publications among the selected articles.In the present study, performance analysis was conducted using CNKI and Scopus built-in bibliometric functions.
Science mapping identifies the relational aspects of the selected articles.In the present study, bibliographic details including authors and keywords were extracted from CNKI.Bibliographic data were manually screened for missing information and relevance to the study.No missing information was found and data were then analyzed by VOSviewer (version 1.6.11).Similarly, the Scopus CSV file was manually screened and no missing information was observed.It was then analyzed by VOSviewer (version 1.6.11).The analysis included the identification of co-authorship networks and co-occurrences of keywords [29].

Results
Figure 2 shows the number of carbon-neutrality publications in Chinese academic journals from 2008 to 2022.Before 2021, there were no more than 11 articles per year.The number of carbon-neutrality publications increased sharply to 367 in 2021, then to 476 in 2022.Figure 2

Performance Analysis Using CNKI and Scopus Bibliometric Tools
When the selected 891 Chinese journal publications were analyzed by authors, X.D. Hao of Beijing University of Civil Engineering and Architecture was identified to be the most productive author with 10 articles, followed by C. Wang of Tsinghua University with 9 articles.Table 1 shows nine of the top authors with five or more carbon-neutrality Chinese journal publications.Additionally, Table 1 shows   1 The same author.
Among the selected 891 Chinese journal publications, 64 were published by authors affiliated with Tsinghua University, 36 by authors affiliated with the University of Chinese

Performance Analysis Using CNKI and Scopus Bibliometric Tools
When the selected 891 Chinese journal publications were analyzed by authors, X.D. Hao of Beijing University of Civil Engineering and Architecture was identified to be the most productive author with 10 articles, followed by C. Wang of Tsinghua University with 9 articles.Table 1 shows nine of the top authors with five or more carbon-neutrality Chinese journal publications.Additionally, Table 1 shows   1 The same author.
Among the selected 891 Chinese journal publications, 64 were published by authors affiliated with Tsinghua University, 36 by authors affiliated with the University of Chinese Academy of Sciences, and 24 by authors affiliated with Renmin University of China.Table 2 shows the top universities/institutions that produced 13 or more Chinese journal articles from their faculty members.In terms of publications on carbon neutrality in English journals, the Chinese Academy of Sciences ranked first with 345 articles, followed by Tsinghua University with 190 articles and the University of Chinese Academy of Sciences with 148 articles.In terms of the number of carbon-neutrality publications by source titles, Natural Gas Industry was ranked first with 14 articles, followed by Journal of China Coal Society and Proceedings of the CSEE with 6 articles each.Table 3   The Chinese government actively promotes research on carbon neutrality from different perspectives.Table 4 shows the number of carbon-neutrality publications in Chinese supported by various funding agencies.The National Natural Science Foundation was the most supportive one for carbon-neutrality research and publications, followed by the National Social Science Fund and the National Key Research and Development Project.When the number of carbon-neutrality publications in English journals was considered, the National Natural Science Foundation ranked first and was acknowledged by researchers in 1652 publications, followed by the National Key Research and Development Program acknowledged by researchers in 412 publications.
Table 5 shows the top five highly cited Chinese carbon-neutrality publications.Hu's article "China's goal of achieving carbon peak by 2030 and its main approaches" ranked first both in terms of the total citations (520) and citations per year (260 per year) [49].Hu [49] indicated that the 30-60 goals are ambitious in China's plan.In order to achieve peak emissions in 2030, China must establish a reverse mechanism to gradually promote green reform and green innovation, covering a wide range of aspects such as limiting total energy consumption, setting targets, increasing the use of non-fossil-fuel energy, reducing coal production and consumption significantly, etc. [49].The second highly cited article was written by Tan and Huang [23].This article was one of three earliest Chinese academic articles with an implication on carbon neutrality and attracted 437 citations.Tan and Huang [23] reported that due to China's rapid social and economic development, energy consumption increased substantially and the total amount of GHG emissions kept increasing.They estimated the total carbon emissions in the Eastern, Central and Western part of China and showed that Eastern China had the largest carbon emissions, followed by Central China, and then Western China.Tan and Huang [23] also put forward suggestions on policy measures to restrain China's carbon emissions.Wang and Zhang's article "Implementation pathway and policy system of carbon neutrality vision" ranked third with 320 citations [50].Wang and Zhang [50] stressed the importance of the 30-60 goals that set the vision for China to adopt low-carbon high-quality economic development.As carbon neutrality involves all industrial and economic activities, innovative technologies and new ways of doing that could enhance energy efficiency, recycle energy, emit zero carbon and sequester carbon must be developed.Government, enterprises, and individuals play important roles in achieving carbon neutrality.Effective incentive mechanisms shall be established to promote capital investment and talents that can create innovations and their applications to carbon neutrality [50].The fourth and fifth highly cited articles were written by Zou and his associates in the PetroChina Research Institute of Petroleum Exploration and Development [51,52].Each of these two articles attracted 180 citations or more.Zou et al.'s article "The role of new energy in carbon neutral" highlighted that new energy including solar energy, wind energy, nuclear energy, and hydrogen energy plays an important role in lowering carbon emissions from the power sector [51].Green hydrogen is a new energy that can help the industry and transportation sectors to achieve low carbon emissions.Additionally, artificial carbon conversion technology can bridge fossil-fuel energy to new energy as it can reduce carbon emissions from the burning of fossil fuels by using chemical or biological measures to convert carbon dioxide into useful chemical products [51].Zou et al.'s article "Connotation and pathway of world energy transition and its significance for carbon neutral" argued that the world's energy transition has four connotations: political, technological, management, and business.Among them, the political connotation refers to the political synergy centered on extensive consultation and global coordination mechanism [52].The path of energy transition demands the international community to realize low-carbon emissions, low-cost new energy, and intelligent energy management through political alignment, technology innovation and promotion, proper business and management incentives [52].Table 6 shows the top five highly cited English carbon-neutrality publications by Chinabased researchers.The top highly cited publication was a review written by Tu et al. [53] of Nanjing University.Its title was "Photocatalytic conversion of CO 2 into renewable hydrocarbon fuels: State-of-the-art accomplishment, challenges, and prospects" and it attracted over 1100 citations in the past 9 years.Tu et al. [53] provided a review of different mechanisms and procedures for the photocatalytic reduction of CO 2 into hydrocarbon fuels and listed out the challenges for technological and practical development of such methodologies.The second highly cited publication was a review about photoelectrochemical devices for solar water splitting [54].It was written by a group of researchers in the Chinese Academy of Sciences and the University College London.It attracted close to 900 citations in the past 6 years.Jiang et al. [54] reviewed fundamental properties of photoelectrochemical devices and how different configurations of these devices would influence the overall water splitting efficiency.The third highly cited publication was also a review entitled "Cocatalysts in semiconductor-based photocatalytic CO 2 reduction: Achievements, challenges, and opportunities" [55].It was co-authored by researchers with affiliations including Tianjin University, the University of Adelaide, and Kent State University.Ran et al. [55] reviewed mechanisms of photocatalytic CO 2 reduction and highlighted the role of cocatalysts in enhancing the overall efficiency of the reduction and oxidation process.The fourth and fifth highly cited publications were also reviews [56,57]  World 2023, 4 499

Science Mapping Using VOSviewer
Co-authorship in Chinese journal publications was explored using VOSviewer.When the minimum number of publications was set to four, 38 researchers met the threshold, forming 24 clusters without links.There were four clusters with three members, six clusters with two members, and eight clusters with one member.The largest cluster, i. Figure 3 shows a map of co-authorship in English journal publications.When the minimum number of publications was set to seven, 36 researchers met the threshold.The largest cluster (red in color) included 13 members in which the core members were Y.K.  Co-occurrence of keyword analysis was conducted on the selected 891 Chinese journal publications.VOSviewer identified 2095 keywords from all the selected documents.When the minimum number of keyword co-occurrences was set to eight, 33 met the threshold forming seven clusters as shown in Figure 4. Table 7 lists keywords for each of the clusters.The largest group (red in color) included eight items such as carbon neutrality, climate change, carbon sink, carbon dioxide, wastewater treatment, greenhouse gases, carbon sources, and carbon footprint.The other groups had three to six items per group.nal publications.VOSviewer identified 2095 keywords from all the selected documents.When the minimum number of keyword co-occurrences was set to eight, 33 met the threshold forming seven clusters as shown in Figure 4. Table 7 lists keywords for each of the clusters.The largest group (red in color) included eight items such as carbon neutrality, climate change, carbon sink, carbon dioxide, wastewater treatment, greenhouse gases, carbon sources, and carbon footprint.The other groups had three to six items per group.The co-occurrence of keywords analysis was also performed using the selected English journal publications.VOSviewer identified 20,503 keywords from all the selected documents.When the minimum number of keyword co-occurrences was set to 50, 110 keywords met the threshold forming three clusters, as shown in Figure 5.The largest cluster (red in color; Cluster 1) consisted of 40 keywords.Cluster 1 focused on "carbon", "carbon neutralities", and "emission control" in which they were related to "energy utilization", "energy efficiency", "renewable energy", and "alternative energy".The second largest cluster (green in color; Cluster 2) contained 37 keywords.Cluster 2 focused on "carbon dioxide" and "carbon neutrals", while "carbon neutrals" was found to link with "biomass", "electrocatalysts", "electrolytic reduction", "wastewater treatment", and "hydrogen production".The third cluster (blue in color; Cluster 3) contained 33 keywords.Cluster 3 focused on "China", "carbon neutrality", "carbon emission", and "climate change".Additionally, "sustainable development" and "economic development" were also important keywords in cluster 3. Table 8 shows the top 10 keywords of each cluster.

Discussion
Carbon neutrality is a must because global climate change caused by anthropogenic carbon emissions can have disastrous consequences.As such, humans should unite to find ways to cap carbon emissions and achieve carbon neutrality as soon as possible [58].With China announcing the 30-60 goals in the UN General Assembly in September 2020, researchers across the world, particularly China-based researchers, reprioritized their research focuses to carbon-neutrality-related topics.Specifically, the number of Chinese carbon-neutrality journal publications increased sharply from 7 in 2020 to 367 in 2021, then to 476 in 2022 (see Figure 2a).At the same time, the number of English carbon-neutrality journal publications from China-based researchers increased from 59 in 2020 to 535 in 2021, then to 2403 in 2022 (see Figure 2b).This implies that Chinese researchers did not only disseminate their research findings locally, but also internationally to advocate the importance of carbon-neutrality to humankind and scientific advancements.According to Scopus, the percentage share of carbon-neutrality research with contributions from China grew from 18% (=59/328) in 2020 to 49.6% (=535/1079) in 2021, then to 71.5% (=2403/3363) in 2020, consistent with suggestions from To and Yu [59] who studied the rise of academic publications due to China's effort.
Performance analysis showed X.D. Hao to be the most productive author in Chinese journals with 10 publications.According to CNKI, many of Hao's carbon-neutrality articles were published in China Water & Wastewater and Hao has great interest in exploring energy recovery from various processes in wastewater treatment facilities (for example, see Hao et al. [60]).The second most productive author was C. Wang who has a broad interest in discussing technologies and the roadmap of carbon neutrality in China [50,61].Among China-based researchers publishing in English journals, Y.K. Zhou was the most productive author with 14 publications and he has great interest in investigating zero-carbon district energy systems and the use of artificial intelligence in energy utilization and savings [62].The second most productive authors included Y. Geng whose research interests cover environmental economics and environmental engineering, B. Lin whose research interests cover energy economics and energy policy, X.P. Sun whose research interests include electrocatalytic CO 2 reduction using nanosheets, and D.C.W. Tsang whose research interests cover biochar, waste treatment, and carbon neutrality.These findings answered RQ1.When carbon-neutrality publications by affiliations were counted, Tsinghua University was the most productive affiliation in Chinese journals with 64 articles while the Chinese Academy of Sciences was the most productive affiliation in English journals with 345 publications (see Table 2).Overall, the Chinese Academy of Sciences was the most productive affiliation with 376 publications including 31 Chinese journal articles and 345 English journal publications, answering RQ2.Interestingly, all top nine universities/institutions that produced most carbon-neutrality articles in Chinese journals are situated in Beijing, China (see Table 2).According to Scopus, the top three universities/institutions that produced the most carbonneutrality publications in English journals are situated in Beijing, followed by Tianjin University (in Tianjin), Shanghai Jiao Tong University (in Shanghai), and North China Electric Power University (also in Beijing).These findings show that Beijing is the academic center of carbon-neutrality research in China.
In China, Natural Gas Industry published the largest number ( 14) of carbon-neutrality articles in Chinese, reflecting that the oil and gas industry in China put in a lot of effort to explore different ways of including renewable energy technologies to achieve carbon neutrality.In English academic publishing, Sustainability (Switzerland) as a multidisciplinary journal was the one publishing the largest number (156) of carbon-neutrality publications, as shown in Table 3.These findings addressed RQ3.Table 4 reveals that the National Natural Science Foundation was the most supportive funding body for carbon-neutrality research published in both Chinese and English journals, answering RQ4.This finding was consistent with the finding of Chung and To [9] who focused on carbon-neutrality publications in English journals, conference proceedings, and books.With respect to the five highly cited Chinese journal articles, two of them focused on the overall picture of the 30-60 goals and the ways to achieve these goals [49,50] (see Table 5).Another two highly cited articles were published by researchers of the PetroChina Research Institute of Petroleum Exploration and Development that emphasized the importance of the transition from fossil-fuel energy to new and smart energy technologies for achieving carbon neutrality [51,52].With respect to the five highly cited English journal publications, all of them were reviews and three of them focused on photocatalytic conversion/reduction of CO 2 into biofuels and other products [53][54][55].These findings addressed RQ5.Nevertheless, these findings were different from the finding of Chung and To [9] who reported that the five highly cited English journal publications (each with over 3000 citations) focused on turning biomass to biofuels and other carbon-based products but none of the authors were affiliated with Chinese institutions.
Co-occurrence of keywords analysis using the selected 891 Chinese journal articles identified seven clusters.Based on the characteristics and similarity of keywords in each cluster, these seven clusters can be labeled as carbon neutrality and climate change (cluster 1-red in color), energy transformation (cluster 2-green in color), peak carbon emission (cluster 3-blue in color), carbon emission and low carbon economy (cluster 4-yellow in color), realization path in China (cluster 5-purple in color), hydrogen energy and clean energy (cluster 6-orange in color), and green finance and sustainable development (cluster 7-light blue in color).When the number of occurrences was considered, carbon neutrality in cluster 1 (red in color) ranked first with 661 occurrences, followed by peak carbon emission in cluster 3 (blue in color) with 207 occurrences, carbon emission in cluster 4 (yellow in color) with 83 occurrences, and energy transformation in cluster 2 (green in color) with 38 occurrences, as shown in Table 7 and Figure 4. Cluster 1 (red in color) illustrates that carbon neutrality is normally associated with climate change and carbon sink.Cluster 3 (blue in color) shows that peak carbon emission is associated with carbon emission reduction and carbon tax.Cluster 4 (yellow in color) shows that carbon emission is associated with a low-carbon economy and high-quality development.Finally, cluster 2 (green in color) reveals that energy transformation is associated with renewable energy and low-carbon transformation.Topics covered in four clusters (red, blue, yellow, and green) attracted much more research attention than the other three clusters (purple, orange, and light blue) identified in the study, as illustrated in Table 7. Co-occurrence of keywords analysis using the selected 3195 English journal publications identified three clusters.They were "carbon, carbon neutralities, emission control, and energy utilization" (in the red cluster), "carbon dioxide, carbon neutrals, biomass, and electrocatalysts" (in the green cluster), and "China, carbon neutrality, sustainable development, and economic development" (in the blue cluster), as shown in Figure 5.When the number of occurrences was considered, carbon (in the red cluster) ranked first with 1169 occurrences, followed by carbon dioxide (in the green cluster) with 895 occurrences, and China (in the blue cluster) with 770 occurrences, as shown in Table 8.These findings addressed RQ6.Additionally, these findings extended the findings of Chung and To [9] who reported global carbonneutrality research published in English journals to be characterized by four clusters, namely, "carbon neutrals, biofuels, and hydrogen", "carbon neutralities, energy efficiency, energy utilization, and renewable energies" (similar to the red cluster reported in this study), "carbon neutrality, sustainable development, and China" (similar to the blue cluster reported in this study", and "greenhouse gases, gas emissions, coal, and global warming".Thus, it is likely that China will take a pragmatic and balanced approach to achieve the 30-60 goals.

Implications
Carbon neutrality requires a concerted effort to materialize.At the moment, many important Chinese carbon-neutrality journal articles focused on policy analysis, technology innovations, and regulation formulations, reflecting that many articles were written from the government's and corporations' perspectives (see Table 5-the highly cited articles in Chinese journals, and Table 7).When carbon-neutrality publications by China-based researchers in English journals were analyzed, results showed that many of them focused on technological developments such as emission control, energy utilization, photocatalytic conversion, and electrolytic reduction of CO 2 (clusters 1 and 2 in Figure 5, Tables 6 and 8).Additionally, China-based researchers also published frequently about China's strategy on carbon neutrality aiming to fight climate change and sustain economic development (cluster 3 in Figure 5 and Table 8).Indeed, insufficient attention was given to the social, community and individual contributions such as pro-environmental behaviors and green consumption [63] to carbon neutrality in both Chinese and English journal publications on carbon neutrality.Moreover, sectoral analysis of carbon emissions and carbon leakage shall be carried out and reported in Chinese and English journals in order to highlight the current carbon emissions and possible future carbon emission reduction for different sectors [64].For example, the transport sector relies heavily on fossil fuels and accounts for around 37% of CO 2 emissions from end-use sectors [65].Therefore, sustainability analysis of different transport modes based on real operational data shall be conducted using China's data in order to help the Chinese government plan future transport-related infrastructure developments without further accelerating transport-related carbon emissions.Furthermore, regional contexts and cultures shall be considered in order to establish a realistic, sustainable transportation hierarchy [66].Additionally, the building construction and building sectors combined consume about 30% of global final energy consumption and emit 27% of total energy sector CO 2 emissions [67].Thus, it is necessary to carry out building-related carbon emissions using a life-cycle approach at the provincial, regional, and country levels in China.Rules and regulations must be established to prohibit too much energy (including embodied energy) wastage due to empty commercial and residential buildings.The Chinese government should have mechanisms to take over long-term empty buildings and units for proper use.Finally, it is possible to achieve low-carbon or carbon-neutral emissions if an effective multi-level government network with strong political willing and leadership is present [68].
Carbon neutrality can be addressed from different fronts.The adoption of renewable energies, significant improvement of energy efficiency, and, more importantly, energy savings can help China meet the 30-60 goals [69] because they help reduce CO 2 emissions.Forestation including reforestation, proforestation and afforestation, and carbon-capture and storage technologies can help mitigating the ever-increasing CO 2 concentration in the atmosphere [70][71][72].Financial incentives such as carbon credits, carbon trading and carbon taxes can encourage commercial and industrial firms to reduce their carbon footprint [73][74][75] as well as to invest in renewable energy technologies and carbon sequestration projects [72,76].Additionally, it is also feasible to assign a personal carbon quota to individuals in order to accelerate carbon neutrality [77].However, carbon offsetting such as a combination of carbon trading and forestation shall be carried out with great caution [9].It is because carbon offsetting may blind people to believe that new CO 2 emissions due to the burning of fossil fuels, i.e., old biomass can be captured by plants in order to become new biomass.Yet, the truth is that the nature takes millions of years for turning biomass to fossil fuels under complex geothermal-chemical processes [9,78] and the continual burning of fossil fuels has disrupted the atmospheric O 2 /CO 2 ratio and has caused significant climate change [78,79].Finally, the governments can fund cutting-edge research that aims to develop ingenious ways to turn atmospheric CO 2 to carbon-based fuels and products [80][81][82], achieving a long-run, net-zero world.

Conclusions
This paper explored carbon-neutrality research in China during the period of 2008-2022.On 15 April 2023, a search was performed in CNKI using "carbon neutrality" or "carbon neutral" in "Subject" published up to "2022" in Chinese academic journals identifying 891 journal articles.A search was also conducted in Scopus using ["carbon neutrality" OR "carbon neutral"] in "Article title, Abstract, Keywords" with the date range from 2008 to 2022, filtering by English journal publications (i.e., articles and reviews) and author affiliation as "China", identifying 3195 documents.Results showed that the number of carbon-neutrality articles in Chinese journals increased rapidly to 476 in 2022 from 7 in 2020 and the number of carbon-neutrality publications in English journals surged to 2403 in 2022 from 59 in 2020 after China's announcement about the 30-60 goals in the UN General Assembly in September 2020.X.D. Hao was found to be the most productive author with 10 publications in Chinese journals while Y.K. Zhou was the most productive author with 14 English journal publications.As a whole, the Chinese Academy of Sciences ranked first with 376 (31 Chinese and 345 English) publications.Two of the top three highly cited articles in Chinese journals revealed that researchers focused more on policy analysis of carbon neutrality and its implementation plan, while the top five highly cited carbon-neutrality publications by China-based researchers in English journals were all reviews.Co-occurrence of keywords analysis identified seven clusters and the largest cluster centered on carbon neutrality and climate change based on keywords from Chinese journal articles.Co-occurrence of keywords analysis was also performed on the 3915 English journal publications.Three clusters were identified covering different technological aspects and economic implications of carbon neutrality.Nevertheless, insufficient attention has been paid to the social, community and individual contributions to carbon neutrality.Knowing each individual, organization, and community can make a profound difference on energy utilization and pollutant emissions such as CO 2 ; the central and provincial governments of China need to establish policies that highlight the importance and awareness of carbon neutrality and different paths to achieve the 30-60 goals through public participation.In addition, sectoral carbon emissions and carbon leakage shall be thoroughly investigated.Finally, this study has two limitations.First, it was a cross-sectional study and we performed bibliometric analysis on data from CNKI and Scopus covering the period of 2008-2022.As the number of publications on carbon neutrality has increased rapidly since 2020, new hotspots and research themes may emerge in the near future.Future research using our approach can be carried out to reveal such new hotspots and emerging research themes.Second, a bibliometric analysis is useful to provide an overview of a broad research topic.Nevertheless, it may lack sufficient details on some specific topics or technologies.Future research such as systematic literature reviews can be carried out on some important carbon-neutrality technologies such as biomass fuels, and electrocatalysts.
Figure 2 shows the number of carbon-neutrality publications in Chinese academic journals from 2008 to 2022.Before 2021, there were no more than 11 articles per year.The number of carbon-neutrality publications increased sharply to 367 in 2021, then to 476 in 2022.Figure 2 also shows that the number of English journal publications by China-based researchers increased dramatically from 59 in 2020 to 535 in 2020, then to 2403 in 2022.
Figure 2 shows the number of carbon-neutrality publications in Chinese academic journals from 2008 to 2022.Before 2021, there were no more than 11 articles per year.The number of carbon-neutrality publications increased sharply to 367 in 2021, then to 476 in 2022.Figure 2 also shows that the number of English journal publications by China-based researchers increased dramatically from 59 in 2020 to 535 in 2020, then to 2403 in 2022.

Figure 2 .
Figure 2. The number of publications on carbon neutrality 2008 to 2022 in Chinese academic journals from 2008 to 2022 and in English journals by China-based researchers.
that Y.K. Zhou of the Hong Kong University of Science and Technology was the most productive author with 14 English journal publications.Y. Geng of Shanghai Jiao Tong University, B. Lin of Xiamen University, X.P. Sun of the University of Electronic Science and Technology of China, D.C.W. Tsang of the Hong Kong Polytechnic University ranked second, each with 12 English journal publications.It should be noted that C. Wang (the second most productive author in Chinese journals) was also one of the sixth most productive authors on carbon neutrality with 10 English journal publications.

Figure 2 .
Figure 2. The number of publications on carbon neutrality 2008 to 2022 in Chinese academic journals from 2008 to 2022 and in English journals by China-based researchers.
that Y.K. Zhou of the Hong Kong University of Science and Technology was the most productive author with 14 English journal publications.Y. Geng of Shanghai Jiao Tong University, B. Lin of Xiamen University, X.P. Sun of the University of Electronic Science and Technology of China, D.C.W. Tsang of the Hong Kong Polytechnic University ranked second, each with 12 English journal publications.It should be noted that C. Wang (the second most productive author in Chinese journals) was also one of the sixth most productive authors on carbon neutrality with 10 English journal publications.
e., with the most active co-authorship relationship was formed by six members including five carbon-neutrality researchers of the PetroChina Research Institute of Petroleum Exploration and Development.They are S.Q.Pan (six articles), C.N. Zou (five articles), G.S. Zhang (four articles), B. Xion (four articles), and Y. Wang (four articles).
Zhou (zhou y.) of the Hong Kong University of Science & Technology and D.C.W. Tsang (tsang d.c.w.) of the Hong Kong Polytechnic University.They co-authored quite a large number of publications with researchers in South China such as Y.Sun (sun y.) of Sun Yat-Sen University.The second largest cluster (green in color) included eight members in which the core members were C. Wang (wang c.) of Tsinghua University, X. Zhang (zhang x.) of the Ministry of Science and Technology in Beijing, and Y Geng (geng y.) of Shanghai Jiao Tong University.The third largest cluster (blue in color) included seven members in which the core members were W. Liu (liu w.) of Harbin Institute of Technology and C.C. Tang (tang c.-c.) of Xi'an University of Architectural and Technology.This group of researchers focused on wastewater-treatment technologies that help achieve the carbon neutral goal.The fourth cluster (yellow in color) included six members in which W. Cai (cai w.) of Chongqing University, Z. Ma (ma z.) of Chongqing University and M. Ma (ma m.) of Tsinghua University.The fifth cluster (purple in color) had only one member who was B. Lin (lin b.) of Xiamen University.B. Lin has published widely about the economic implications of carbon neutrality.World 2023, 4, FOR PEER REVIEW 11

Figure 3 .
Figure 3. Co-authorship networks among China-based researchers who published in English journals.

Figure 3 .
Figure 3. Co-authorship networks among China-based researchers who published in English journals.Co-occurrence of keyword analysis was conducted on the selected 891 Chinese journal publications.VOSviewer identified 2095 keywords from all the selected documents.When the minimum number of keyword co-occurrences was set to eight, 33 met the threshold forming seven clusters as shown in Figure4.Table7lists keywords for each of the clusters.The largest group (red in color) included eight items such as carbon neutrality, climate change, carbon sink, carbon dioxide, wastewater treatment, greenhouse gases, carbon sources, and carbon footprint.The other groups had three to six items per group.

Figure 4 .
Figure 4. Co-occurrence of keywords analysis using CKNI data (the minimum number of occurrences was set to eight).

Figure 4 .
Figure 4. Co-occurrence of keywords analysis using CKNI data (the minimum number of occurrences was set to eight).

Figure 5 .
Figure 5. Co-occurrence of keywords analysis using Scopus data (the minimum number of o rences was set to 50; 110 out of the 20,503 identified keywords met the threshold).

Figure 5 .
Figure 5. Co-occurrence of keywords analysis using Scopus data (the minimum number of occurrences was set to 50; 110 out of the 20,503 identified keywords met the threshold).
shows the top Chinese source titles that published three or more Chinese carbon-neutrality articles.It also shows that China-based researchers published 156, 135, and 102 articles and reviews in Sustainability (Switzerland), Journal of Cleaner Production, and International Journal of Environmental Research and Public Health, respectively.

Table 5 .
Top five highly cited Chinese carbon-neutrality publications.
. Song et al. [56]-a group of researchers in the Institute of Coal Chemistry of the Chinese Academy of Sciences-reviewed the advancement of organic synthesis using CO 2 during the period of 2012-2016 while Niu et al. [57]-a group of researchers in Xi'an Jiaotong University-reviewed the use of biomass as a renewable energy source but brought up concerns that ash-related issues must be properly addressed through different technological means.Song et al.'s [56] and Niu et al.'s [57] publications attracted 703 and 662 citations, respectively, in the past few years.In terms of citations per year, Ran et al.'s [55] publication ranked first with 157.4 citations per year, followed by Jiang et al.'s [54] publication with 149.3 citations per year.

Table 6 .
Top five highly cited English carbon-neutrality publications by China-based researchers.

Table 7 .
Clusters of keywords in Chinese journals.

Table 8 .
Top 10 keywords of each cluster identified from English journal publications.