Annual publications and growth trend
The search carried out in the Web of Science obtained a total of 3188 publications on the electro-oxidation of organic pollutants with an average of 31.8 citations per document. Among these, the majority were research articles (2946, 92.41%) followed by event proceedings (214, 6.71%), reviews (147, 4.61%), early access (15, 0.471%), meeting summaries (5, 0.157%), book Chaps. (2, 0.063%), editorials (1, 0.031%) and retraction publication (1, 0.031%). As can be seen in Fig. 1, during the period evaluated there was a growing interest in research related to the electrochemical degradation of organic pollutants, initially with 55 publications in 2001 up to a significant number of 308 scientific studies published on the subject in 2020, which corresponds to an average growth rate of 9.30%. In general, few exceptions to this growth trend were observed, specifically in the years 2006, 2010 and 2016 the number of publications was lower compared to previous years. In the year 2021, up to the time of data research (August/09/2021), 156 studies on the subject had already been published. To assess the correlation between the number of documents and the year (data from 2021 were not included) the polynomial model was applied. A good fit of the polynomial curve to the increasing trend of publications was observed and a high coefficient of determination was obtained (R2 = 0.9681). This result suggests that in the coming years, annual publications on this topic will continue to grow. The continued interest in the electrochemical degradation of organic contaminants, indicated by the increase in the number of publications, can be attributed to different factors, such as: population growth and high demand; commitment to reduce environmental contamination; and importance of electrochemical methods as effective tools for removing organic pollutants.
Analysis of publications by subject area and source
The electrochemical degradation of organic pollutants included 51 thematic areas of the Web of Science. As shown in Fig. 2, the analysis of the results showed that the recovered documents mainly belonged to the chemistry area (1133, 35.54%). In the recovered literature, the areas of engineering (1045, 32.78%), electrochemistry (840, 26.35%), environmental sciences and ecology (793, 24.88%), and materials science (320, 10.04%) also stand out. Due to the assignment of journals to different subject categories, the total percentage of research areas is greater than 100%.
In total, 595 journals participated in research publications on electrochemical degradation of organic pollutants. Table 1 shows the 15 sources with the highest number of publications, representing 42.91% of all publications. The journal with the most publications was Electrochimica Acta (230, 7.22%), followed by Chemosphere (141, 4.42%), Journal of Hazardous Materials (141, 4.42%), Journal of Electroanalytical Chemistry (122, 3.83%) and Chemical Engineering Journal (107, 3.36%). The list of the top 15 sources includes six journals in the field of electrochemistry, while the remaining journals are in the areas of environmental chemistry, environmental science, catalysis, chemical engineering and hydrology. Six of the leading journals are from the Netherlands, four from the UK, two from the USA and Germany, respectively, and one from Serbia. Furthermore, it is important to mention that among the journals on the list, 8 have an impact factor greater than 6.000, of which Applied Catalysis B Environmental has the highest impact factor (19.503).
Analysis of publications by country
A total of 93 different countries participated in research publications on the electrochemical degradation of organic pollutants. The geographic distribution of the retrieved documents is shown in Fig. 3, and the fifteen countries with the highest research production and the highest number of citations are shown in Table 2. In the period evaluated, China is notably the country with the highest number of publications (1004, 31.49%), followed by Spain (282, 8.85%), Brazil (255, 8.00%), India (192, 6.02%) and USA (189, 5.93%), which complete the top five. When evaluating the number of citations by country, some changes in ranking are observed. Among the top five, China (n = 26398) and Spain (n = 18497) still occupy the top two positions, however, Italy (n = 14288) and France (n = 10276) appear, respectively, as third and fourth place, followed by Brazil (n = 7901). Switzerland, which occupies the seventh position in relation to the total number of citations (n = 4978), has the highest average of citations per document (146.41).
Academic cooperation between countries and institutions is a very important practice for the production and dissemination of scientific knowledge. Figure 4 shows the academic collaboration network among the 30 most productive countries. In this figure the nodes represent the different countries, the lines connecting the nodes indicate cooperation between countries, and the line thickness is proportional to the strength of the cooperation. China is the country with the highest number of publications in collaboration with other countries (n = 149), followed by Spain (n = 143), France (n = 107), USA (n = 105) and Brazil (n = 87). Despite occupying the first position, only 14.84% of all publications in China were in collaboration with other countries, which corresponds to the lowest rate among highly productive countries. For example, France cooperated with other countries in 65.24% of its publications, for the USA this percentage was 55.56%, for Italy 50.96% and for Spain 50.70%. Therefore, it is important that China, which is the country with the most research in the area of electrochemical degradation of organic pollutants, intensify its academic cooperation with other countries. The analysis of academic cooperation also revealed that the USA has the most diverse collaboration network, having collaborated on publications with 41 different countries. In this aspect, also stand out Spain (37), Italy (31), France (30) and China (29). Spain and Brazil are the countries that most cooperate with each other (46 publications), besides them an intense academic cooperation was verified between China and the USA (41 publications).
Analysis of publications by institutions and authors
Figure 5 shows the 20 institutions with the most publications between 2001 and 2021. A total of 1947 institutions contributed to the 3188 publications retrieved. The most productive institution was the Universidad de Castilla-La Mancha, which published 91 documents, followed by the Chinese Academy of Sciences (89), the Center National de la Recherche Scientifique CNRS (77), the University of São Paulo (70) and the Federal University of Rio Grande do Norte (69). In addition, 5 of the 15 most productive institutions were from China, the other 10 institutions are spread across 6 countries.
The authors with the most publications on electrochemical degradation of organic pollutants are shown in Table 3. A total of 8017 researchers participated in publications on the subject between 2001 and 2021. Researcher Rodrigo MA appears with the highest number of publications (92, 2.886%), followed by Martinez-Huitle CA (78, 2.447%), Canizares P (74, 2.321%), Oturan MA (69, 2.164%), Saez C (64, 2.008%). Another important parameter to be considered is the number of citations, in this case Panizza M leads the list with a total number of 7860 citations, Rodrigo MA was the second with the most citations (7139), followed by Oturan MA, Brillas E and Cerisola G who had a total of 7041, 6184 and 5834 citations, respectively.
Most cited publications
Table 4 shows the most cited publications on electrochemical degradation of organic pollutants. Among the 10 most cited publications there are 9 review articles and only 1 research article. Spain and Italy are the countries with the highest number of publications among the most cited, each of these countries collaborated in 5 publications belonging to this list. The journals with the most publications among the 10 most cited were Chemical Reviews (n = 2) and Critical Reviews in Environmental Science and Technology (n = 2). The study by Panizza and Cerisola (2009) was the most cited publication, with 1434 citations. In this review, the authors discuss important aspects of the mechanisms and materials used in direct and mediated anode processes for the oxidation of organic pollutants, as well as reviewing performance indicators used to assess the progress and efficiency of electrochemical treatments. The study by Marselli et al. (2003), the only research article among the 10 most cited, presented evidence of the electrogeneration of hydroxyl radicals during electrolysis in the boron-doped diamond (BDD) electrode. The high number of citations generated by this publication can be attributed to the great interest in the use of BDD electrodes for the electro-oxidation of organic pollutants, due to the excellent characteristics of this material, such as high stability, high starting potential for the evolution of O2 and properties of weak adsorption.
The keywords of a scientific article reflect the general themes of the study. Thus, by analyzing the frequency and co-occurrence of keywords, it is possible to identify important topics and trends in a particular field of research (Li et al. 2009; Zhang et al. 2010). In this study, the VOSviewer 1.6.17 software was used to assess the frequency and co-occurrence of keywords. Terms with similar meaning and different spelling were combined, such as “electrooxidation” and “electro-oxidation”, “hydroxyl radical” and “hydroxyl radicals”. In all, 5694 keywords were found in publications related to electrochemical degradation of organic pollutants. The vast majority of these keywords had only one (4323, 75.9%) or two (611, 10.7%) occurrences, while 283 (5.0%) keywords had more than 5 occurrences. Table 5 shows the keywords with the highest number of occurrences in different periods. Between 2001 and 2021, the most frequent keywords were “electrochemical oxidation” (482), “phenol(s)” (261), “boron-doped diamond” (183), “electrooxidation” (180) and “hydroxyl radical(s)” (172). By analyzing the ranking of the most frequent keywords in different periods, search trends can be noticed. For example, the keyword “electro-Fenton” between the years 2001–2007 has the 15th highest frequency, between 2008–2014 it appears in the 7th position and between 2015–2021 it appears in the 4th position, which indicates a growing interest in the process electro-Fenton for degradation of organic pollutants. An important increase in occurrences was also observed for the term “water treatment”, which occupied the 22nd position in 2001–2007 and rose to the 11th position in 2015–2021. “Advanced oxidation processes” also appears as an emerging topic moving from 30th position in 2001–2007 to 14th position in 2015–2021.
Figure 6 shows the co-occurrence network map of the most frequent keywords in publications related to the topic. Each keyword is represented by a node and the lines connecting the nodes indicate co-occurrence between the keywords. Node size is proportional to the number of links keywords have, and line thickness is proportional to the number of co-occurrences between two keywords. For the construction of the map, keywords with at least 20 occurrences were selected, which resulted in 57 keywords. “Electrochemical oxidation” has the highest number of links with other keywords as it is the most frequent word, which reflects its central position in this research field. Thicker lines are observed between the words "electrochemical oxidation" and "boron-doped diamond" (53 co-occurrences), "electrochemical oxidation" and "phenol(s)" (50 co-occurrences), "electrochemical oxidation" and "wastewater treatment" (49 co-occurrences), which indicates that these themes are frequently addressed in the studies together. There is also a relevant co-occurrence between the keywords "hydroxyl radical" and "electro-Fenton", "anodic oxidation" and "electro-Fenton", "electrochemical oxidation" and "hydroxyl radical", therefore, these combinations are focuses in research on the degradation of organic pollutants.
Author keywords with strong correlation were grouped into five clusters indicated by different colors. Among these clusters, three main groups were observed, defined by the number of keywords contained. Cluster 1 (red color) is the largest group and has 25 keywords, focusing on the electrochemical oxidation applied to the degradation of phenolic compounds, adding the terms "electrochemical oxidation", "electrochemical degradation", "oxidation", " phenol”, “phenolic compounds”, “chlorophenol”, “boron-doped diamond electrode”, “PbO2”, etc. Cluster 2 (green color) has 14 keywords, mainly related to the application of combined techniques and advanced oxidative processes for degradation of organic pollutants, this group includes the keywords “anodic oxidation”, “mineralization”, “electro-Fenton”, “photoelectro-fenton”, “photoelectrocatalysis”, “advanced oxidation process”, etc. Cluster 3 (blue color) has 12 keywords, and involves the electro-oxidation of organic pollutants focusing mainly on the use of boron-doped diamond anodes, among the keywords belonging to this group are “electro-oxidation”, “electrolysis”, “organic pollutant”, “wastewater”, “pesticide”, “boron-doped diamond”, among others.