Research Trends Analysis on Subject Materials Science for Energy Engineering by Scopus Data

The objective of this article: analysis of research trends on the topic "Materials Science for Energy Engineering" for the period 2012-2021. Materials and methods: bibliometric analysis of data from the Scopus abstract database. Results. It is shown that this topic is developing faster than the general energy topics. The key feature of the research trends is a significant increase in work on the development of materials for renewable energy and a decrease in the number of publications related to nuclear power. The primary goal of many studies is to increase the efficiency of renewable energy production through the use of new materials. The importance of the topic is evidenced by the emergence of new journals that quickly entered the first quartile of the abstract databases Scopus and Web of Science, for example: ACS Applied Energy Materials. There has been a significant increase of interest in perovskite solar cells, layered semiconductors, triboelectric nanogenerators, cobalt compounds, sulfur compounds, and oxygen and hydrogen evolution reaction during Electrolysis. Publications on electric batteries are well represented in all time intervals. The fo-cus is on electrode materials and battery efficiency.

The purpose of this work was: • To analyze the significance of research on materials science for energy engineering according to the metadata of publications in the Scopus abstract database. • To identify the major research trends in scientific publications on the specified topic based on keyword occurrence analysis. • To demonstrate some methods of bibliometric analysis to solve the above tasks.
The relevance of the work is due to the ongoing energy transition to clean energy in all developed countries [1][2][3], supported by both well-funded national programs and international organizations, including the UN. 1 Moreover, the energy transition is an essential component in the ongoing change in the economic paradigm [4][5][6] and forms new, fast-growing markets, the development of which requires a wide range of research works, the significance of which can be assessed using the metadata available in the leading abstract databases.
In the abstract databases of scientific publications, research related to the energy transition is most often classified under the subject categories "Renewable Energy Sustainability and the Environment". 2 By analyzing the bibliometric data, it is possible to understand what scientific and engineering tasks are the most relevant for implementing of the energy transition to clean and affordable energy and what participation Russian researchers can take in their solution.
Such an analysis can be implemented using the metadata of the leading abstract databases Scopus, Web of Science, and the Lens. In this article, we mainly used the materials of the Scopus platform, which provides not only access to metadata but also to a significant number of full-text articles published by Elsevier. To reduce bias in our analysis, we will partially use data from the Lens database, in which the categorization of publication metadata by Fields of Study is close to the categorization on the Scopus platform. It is convenient to use the data of this platform as complementary to the data of the Scopus system.
In the Lens, the subject category "Renewable Energy, Sustainability and the Environment" contains bibliometric data on 299,373 Scholarly Works (as of 03.03.2021). Table 1  The Fields of Study "Materials science" dominates in the above subject area in the publications indexed by the Lens platform, which indicates its importance in solving engineering problems of the energy transition as well. [7][8][9]. This is mostly because of the importance of creating energy storage facilities for renewable energy. California's August 2020 electric "green blackouts" are a reminder that without clean energy storage, utilities are forced to enter spot markets for electricity produced from traditional sources. Without energy storage, new technologies cannot fully replace old ones. [10,11] The relevance of Materials science topics is confirmed even by the names of journals related to the subject "Renewable Energy, Sustainability and the Environment" and included in Q1, as by Scimagojr.com. Examples of such journals are given in Table 2 Here: SJR -The SCImago Journal Rank indicator 3 . The very titles of the journals themselves testify to the importance of materials science in solving the problems of energy engineering.

Materials and methods
Bibliometric materials for a detailed analysis of trends in publication activity on the topic Material Science for Industrial Energy were selected from the Scopus database. In the Scopus there are thematic filters "Subject Areas", allowing to form an actual query to a database, in our case filters Materials Science, Energy, Engineering and restriction on years of publication 2012-2021 were used. An additional filter Engineering was used to narrow the sample, for example, to exclude works closer to environmental or economic problems.
Query: SUBJAREA (ener) AND SUBJAREA (mate) AND SUBJAREA (engi) AND PUBYEAR > 2011 AND (LIMIT-TO (PUBSTAGE, "final") AND (LIMIT-TO (DOCTYPE, "ar")) AND (LIMIT-TO (LANGUAGE, "English")) returned 28,674 document results. The query used additional restrictions to make the sample more homogeneous and suitable for further analysis: language -English, document type -article ("ar"), publication status -"final". The ratio of publications in 2020 vs 2012 is 3.17, which is slightly higher than the total increase in the number of publications in the Scopus database for these years for the entire energy subject -2.6, indicating the relevance of our formulated query.
In addition to the frequency of occurrence of Fields of Study, keywords, and the number of publications in leading journals on the subject in question, this article studied the clustering of authors' and index keywords based on their co-occurrence. The clustering of terms makes it possible to distinguish topics within the collected bibliometric data. This procedure was performed using the VOSViewer software [12]. a software tool for constructing and visualizing bibliometric networks, to cluster terms based on their co-occurrence.
For detailed bibliometric analysis, all data from Scopus for 2021, 2013 and 2012 were exported, for 2014-2020 -2,000 most cited articles (limitation imposed by Scopus on data export) totaling 18,409 articles. For the general analysis, the files "Scopus_exported_refine_values" were used, which contain summary data for the entire sample, including SUBJECT AREA and SOURCE TITLE fields. Table 3 shows the major SUBJECT AREA and SOURCE TITLE for all 28674 publications matching the query SUBJAREA (ener) AND SUBJAREA (mate) AND SUB-JAREA (engi) AND PUBYEAR> 2011 AND (LIMIT-TO (PUBSTAGE, "final") AND (LIMIT-TO (DOCTYPE, "ar")) AND (LIMIT-TO (LANGUAGE, "English")). Energy; Engineering; Materials Science -are database query filters, so these topics apply to all 28674 publications.
A number of terms that occur frequently in publications in 2020 did not occur in the first 160 keywords in 2014 and 2015 and rarely in 2016, so we present them separately, comparing the occurrence in 2016 and 2020. The results for the 22 keywords that either rarely or not at all occur in 2016 but are widely used in 2020 are presented in Table 5.  There is a significant increase in interest in the topic described by the keywords Perovskite, Perovskite Solar Cells. The terms Nanogenerators and Triboelectric Nanogenerator did not appear in the top 160 keywords in 2016, and in 2020 the total number of occurrences of the term Nanogenerator and Triboelectric Nanogenerator reached 302.
The keywords containing the term Compound, which, unlike Lithium Compounds, did not appear in the top 160 keywords in 2016, are noteworthy: Sulfur Compounds (169), Lead Compounds (154) The evolution of the general topic of the publications estimated by the Scopus query considered in the article can also be assessed by the number of publications in different journals. Table 6 shows the data for 10 highly cited journals well reflecting various aspects of research on materials science for energy.  The increasing interest in the topic discussed in this article is confirmed by publications in ACS Applied Energy Materials, a Q1 journal published by the American Chemical Society, indexed by Scopus and WoS since 2018. The total number of journal publications in 2020 according to Scopus is 1364, of which 1330 satisfy our request. Solar RRL is also not yet published in 2016 (previously published as Rapid Research Letters), but is now included in Q1 WoS and Scopus, with 297 of the 411 publications for 2020 included in our sample. Nano Energy -for 5 years the number of publications increased from 579 to 1020, which indicates the growth of interest in nanotechnology in obtaining materials for energy.
In the journals Fusion Engineering and Design, Nuclear Engineering and Design, Physica C Superconductivity and Its Applications, and Progress in Photovoltaics Research and Applications, the publication activity in the subject we analyzed has not increased.
To assess the overall trend in changes in publication topics over the entire study period, a comparison (INNER JOIN) of the 1,000 most frequently used keywords between 2012-2016 and 2017-2021 was conducted; Table 7 shows the 45 terms with the highest growth over the period.  The frequency of keywords nanogenerators, layered semiconductors, perovskite solar cells, oxygen evolution reaction, triboelectricity, perovskite, cobalt compounds, binary alloys, sulfur compounds, triboelectric nanogenerator, sodium-ion batteries have increased by 7 times or more during this period, which corresponds well with the results of Table 5. It should be highlighted that the diversity in the composition of samples for the construction of Tables 4, 5, 7 provides different slices of raw data, which makes the picture of changes in the topics of publications more diverse and sustainable.
The data in Table 7 can be helpful for a more detailed study of the dynamics of publication activity on more specific issues. For example, a precise query to the Scopus database: TITLE-ABS-KEY ("hydrogen evolution reaction") returns 14,608 document results, with only 121 papers published in 2012 and 3,125 in 2020. This means that the topic of "hydrogen evolution reaction" is developing rapidly and in a broader context than that of this article: Materials Science for Energy Engineering and, consequently, the work in this direction may be in high demand. In [13] an excellent overview of this topic is given.
In order to identify possible emerging topics for which keywords are still present in a few of publications, but whose occurrence growth is very high, it is necessary to maximize the list of keywords compared over the two periods under consideration. Table 8 shows the terms that are rarely encountered, but are growing rapidly, according to the results of the analysis of 16,943 keywords. The degree of structural ordering of the material (crystallinity), photocatalytic activity, lead compounds and wearable sensors (a topic for medicine rather than energy, but new materials are needed to convert human activity into energy for such sensors) these may prove to be promising new research challenges.
Trends in research topics can also be assessed by comparing keywords that occur only in the metadata of publications for 2017-2021 but do not occur in 2012-2016, and vice versa. To limit the sample, a keyword occurrence threshold of 10 was used and then the 30 most frequent keywords in publications over the period were selected from these words. Tables 9 and 10 show the main keywords and their occurrence rate for the first and second cases, respectively. Таблица 9. 30 ключевых слов, которые наиболее часто появляются в новых публикациях, но не преодолели порог в 10 повторений в более ранний период The above results on the discovery of trends in scientific publications were mainly based on the analysis of keywords for the query in question. A list of related words can better describe the topic than individual keywords. Words can be clustered using methods based on their co-occurrence. In this paper, keyword clustering was performed using free software VOSviewer [14] (a software tool for constructing and visualizing bibliometric co-occurrence networks). Bibliometric data were grouped for three two-year periods -2015-2016, 2017-2018, and 2019-2020. Each group included 4,000 records, which was sufficient to obtain six stable clusters. Tables 11-13 represent each of the 10 keywords and their occurrence for three of the six clusters with the highest number of co-occurring keywords. Each of the tables reflects data for a two-year interval. The most frequent keyword was used as the field name of the corresponding column of the table. This approach allows us to trace the changes in the position of the keyword cluster over time. It should be noted that the order of occurrence and even the 10 terms under consideration change over time in the cluster.  In this period, solar cells dominate, and the term perovskite is still quite rare. Nuclear power is well represented in the third cluster.  Таким образом, выявлена нарождающаяся тема triboelectric nanogenerator, которая может представлять интерес для дальнейших исследований как для специалистов в области материаловедения и инженерных решений, так и для электропитания носимых датчиков, см. таблицу 8.
The decrease in attention to the topic of materials science for nuclear energy can be explained by the fact that the main funding in the EU, USA and China is aimed at the development of renewable energy. This emphasizes that bibliometric studies are a good reflection of political and economic interests in R&D. It should be noted that in recent years the term renewable energy has been increasingly replaced by clean energy, which includes nuclear energy.

Summary and Conclusions
The topic "Materials Science for Energy Engineering" considered in the article has been steadily developing during the whole period under study, which is reflected in the bibliometric data of scientific publications. The great attention paid to the development of renewable energy is reflected in the fact that there is a significant increase in the number of studies on materials engineering for clean energy tasks, so this topic may have a good prospect for investment in this area of R&D. Emerging areas of research have been identified, an example of this is the triboelectric nanogenerator. The interest in research related to nuclear energy is decreasing -the main funding is going to renewable energy development. The author's private opinion, based not only on the analysis done in this article, is that the production of materials for clean energy is very power-consuming, and in this regard, a sustainable nuclear power industry can be both a reliable energy resource and a base for the development of new materials for clean energy, due to the significant research work done by nuclear industry in previous years, which will allow Russian researchers to take a good position in the global energy world.