Introduction

Coal-based resources are currently used as the main source of power generation all over the world [1]. These resources are non-renewable and are a danger to the environment, due to the fact that they are a source of greenhouse gasses [2]. In 2019, greenhouse emissions originating from fossil fuels were reported to have reached their highest level [3]. The emission of greenhouse gases results in more alarming issues such as global warming and climate change [4]. The increase of population has resulted in the increase of energy that is needed for survival; this energy is currently generated by using fossil fuels. Over the years, fossil fuel combustion has resulted in carbon dioxide (CO2) concentrations in the atmosphere to rise; this leads to negative climate change and rise to sea level that is destructive in sea life and humans [5]. In 2016, the world energy resource reported that by the year 2050, CO2 emissions will have reached 565–886 billion tons [6].

Fuel cells have the potential to serve as an alternative power source; this is because their use can help reduce the formation of CO2 [7]. A device that generates electricity from the transformation of chemical energy without any combustion is known as a fuel cell [6]. Fuel cells are reliable, compact, and noise-free devices that exhibit lower electrochemical impedance spectroscopy (EIS) compared to other power-producing devices during operation [2]. Fuel cells are much more convenient than traditional batteries because of their durability, simple recharge, increased operation time, and compact load [8]. Batteries and fuel cells both produce electrical energy from chemical energy. The major difference between the two is that batteries can at any point stop producing electricity when its chemical components are finished, while fuel cells are externally provided with fuel and oxidants giving them the advantage to continuously provide electrical energy [9]. In comparison to lithium batteries, direct methanol fuel cells (DMFCs) have a much higher theoretical energy density of 6.13 kWh kg−1 [10]. In contrast to traditional diesel engines, fuel cells have much greater efficiency that ranges between 50 and 60% [11].

Fuel cells are classified into six main kinds; these include the proton exchange membrane fuel cell (PEMFC) alkaline fuel cell (AFC), solid oxide fuel cells (SOFCs), direct methanol fuel cell (DMFC), phosphoric acid fuel cells (PAFCs), and molten carbonate fuel cells (MOFCs) [12]. The grouping of these fuel cells is based on their electrolyte types, the fuel they use, how they are used, and their allocated temperatures [13]. Methanol is a cheap and environmentally friendly fuel that is used to power up direct methanol fuel cells (DMFCs) [14]. DMFCs are of great interest due to how easy it is to transport and store methanol, their environmentally friendly by-products, and high volumetric energy density [15]. Polymer exchange membrane fuel cells (PEMFCs) use H2 gas as fuel. H2 gas is expensive when compared to methanol, that is why DMFCs are a better option [16]. Methanol is also known to have volumetric energy density that is around 17.28 MJ L−1, while H2 has a volumetric energy density of 1.9 MJ L−1 at 20 MPa and liquid H2 stands at 4.7 MJ L−1 [17]. DMFCs need electro-catalysts for methanol oxidation reaction (MOR) in order for them to fully function. Electro-catalysts help to speed up the rate at which the electrochemical reaction takes place at the electrode–electrolyte interface of fuel cells [18]. The breakthrough of DMFCs has been held back by the slow operations of the methanol oxidation reaction (MOR) of the anode reaction [19]. There are two materials that are considered as the main active electro-catalysts for MOR; these are platinum (Pt)-based and palladium (Pd)-based materials [20]. Researchers have proposed the use of catalysts that do not feature metals for MOR as they have shown potential by being active towards MOR, but the high MOR onset eliminates these non-noble metal in DMFC application [21, 22]. The poor kinetics of the DMFC at the anode and the methanol crossover that takes the anode to cathode route through the proton exchange membrane are the major challenges faced by the development of the DMFC [23,24,25]. Selecting the correct electro-catalysts to use for MOR in DMFCs has been a huge stumbling block in the fuel cells’ success [26, 27]. Methanol oxidation proceeds through a wide variety of reactions, but the highly accepted reactions are the adsorption of methanol and the oxidation of CO as follows [28]:

$${\text{CH}}_{3}\text{OH }\to {({\text{CH}}_{3}\text{OH})}_{\text{ads}}$$
(1)
$${({\text{CH}}_{3}\text{OH})}_{\text{ads}} \to {(\text{CO})}_{\text{ads}} + {4\text{H}}^{+} + {4\text{e}}^{-}$$
(2)
$${(\text{CO})}_{\text{ads}} + {\text{H}}_{2}\text{O }\to {\text{CO}}_{2} + {2\text{H}}^{+} + {2\text{e}}^{-}$$
(3)

Hammnet explained the DMFC is made up of an anode where methanol is electro-oxidized to Co2; this is further explained by the reaction (Fig. 1):

Fig. 1
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Scheme of a direct methanol fuel cell that uses liquid acid as an electrolyte [29]

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CH3OH + H2O → CO2 + 6H+  + 6e−, and a cathode where oxygen is reduced to steam explained by this reaction: \(\frac{3}{2}\) O2 + 6e−  + 6H+  → 3H2O. It has been proven by various researchers that the DMFC operates at low temperatures, uses a renewable source of energy as fuel (liquid methanol), and its refuelling system is fast and efficient [30,31,32]. The CO2 produced at the anode can be managed. This happens when the CO2 released proceeds from the membrane electrode assembly (MEA) to the reservoir; this is followed by the liquid methanol and water flowing from the reservoir to the MEA. This process is also helpful in refining and handling the methanol and water in DMFCs [33]. Even though CO2 is produced at the anode, the DMFC is a fuel cell that has very low emission of CO2 unlike the conventional ways of producing energy [34,35,36].

This study reports on bibliometric model to formulate the qualitative and quantitative analysis of the global citation network of electro-catalysts for DMFC application. The search engine that was used to obtain the publications analysed in this study is Web of Science (WoS). This study is very informative, because it is not only limited to a continent, and it is applicable in all parts of the world. This study also has the potential to give incisive information to policy makers on the development and use of DMFC technology. It will also provide researchers with more knowledge that can be useful in bridging the knowledge gaps that exist within this field of study.

Methodology

Data extraction

Web of Science (https://www.webofknowledge.com) was used as the search engine to obtain data for this study. (“Electro-catalysts” OR “Catalysts”) AND (“Direct methanol fuel cells” OR “Methanol fuel cells”) was used as the search keywords with the “Topic” search mode. The selection of Web of Science is based on how easy it is to access, ease of use, its dependability when formulating bibliometric analysis, and also the fact that it covers a broad spectrum of literature in non-identical fields of study [37, 38]. The search was carried out on the 5th of March 2024, to retrieve data from 1992 to 2023. The types of documents that were considered are book chapters, articles, reviews, and proceedings papers. Documents that include abstracts, letter to the editor, corrigendum, publications that are not written in English, news, note retraction, and editorial publication were not included in the data extraction. A total of 1912 documents was extracted (article = 1630, book chapter = 15, proceedings paper = 189, review = 78). These documents were downloaded in Bibtex file format and were imported into RStudio for bibliometric analysis.

Data analysis

In this study, the RStudio software (VERSION 3.4.1) was used to develop tables, visuals, and systematic makers from the retrieved data from bibliometric analysis. https://www.blibliometric.org was used to obtain research code of analysis including annual scientific production, most productive authors, top manuscripts per citations, corresponding author’s countries, total citations per country, most relevant sources, and most relevant keywords.

Results

Between 1992 and 2023, a total number of 1912 documents were extracted from WoS. The documents are on electro-catalysts for direct methanol fuel cells (Table 1). The documents have an average citations per article of 49.98.

Table 1 Publication variables of studies based on electro-catalysts for DMFCs between 1992 and 2023 [37, 39]
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This study focuses on laying out the tendency of publication production on research that is based on electro-catalysts for DMFC application. The search that was conducted on WoS produced a total number of 1912 documents. All the retrieved documents are written in English. A total of 345 sources (books, journals, etc.) were obtained from the search. These documents contain keywords (ID) and author’s keywords (DE) totalling up to 2625 and 2975, respectively. The authors that appeared were a total of 4690; these authors were further categorized into authors of single-authored documents and authors of multi-authored documents, which resulted in 24 and 4666 documents, in that order. This led to a ratio of 0.408 documents per author and 2.45 authors per document. The collaboration index was 2.48. This clearly reveals that this field of study consists of very high contribution from multiple authors, an indication that collaborative work is high in this field.

The annual percentage growth in this study is 7.35%. A few drops are observed; for instance, in 2021, 38 articles were published; in 2022, 20 articles were published; and in 2023, 27 articles were published (Fig. 2). These few years show a huge drop from the 100 articles that were published in 2020. The decrease in the number of articles reported each year could be an indication of reduced interest in this particular field [39]. Researchers are also considering other fuel cells and finding ways of making them efficient in order to implement growth in fuel cell technology. These numbers are still higher than the number of articles that were published between 1992 and 2003, which range from 1 to 24 articles per year. The lowest being 1 in 1993. The highest number of articles were published in 2016, with a total number of 157 publications. The growth in publications on research based on electro-catalysts for DMFC application can be associated with the fact that the world is currently dependent on fossil fuels which are non-renewable and are the main source of greenhouse gases. As the population grows, more energy is needed, and if countries of world continue to excessively use fossil fuels, more damage will be done to the environment. People are also realizing the importance of commercializing DMFCs and selecting the most suitable electro-catalysts for their application.

Fig. 2
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Annual scientific production of research articles based of electro-catalysts for DMFCs

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Electro-catalysts with high number of publications between 1992 and 2023 for DMFCs

Pt on its own as an electro-catalyst for DMFCs has the highest number of publications between 1992 and 2023. This high number is also influenced by the fact that researchers have alloyed Pt with other metals and support materials; this increases the number of times in which Pt appears in publications produced between 1992 and 2023. Pt is mostly used because it is a pure metal that possess great activity as an anode electro-catalyst [40, 41]. When Pt is used on its own as an anode electro-catalyst for DMFCs, CO poisoning is promoted and it decreases the rate at which the cell produces electrical energy [42]. This is considered to be the reason why Pt cannot be an efficient electro-catalyst on its own for MOR within the functional temperatures of DMFCs [43,44,45,46]. According to this data, ruthenium (Ru) is the transition metal that has been alloyed with Pt the most between the years 1992 and 2023. The reason why researchers have used Pt-Ru alloys as electro-catalysts for DMFCs over the years is because they are able to withstand the CO; this is of great benefit to MOR [47, 48]. Advantages associated with the alloying of Pt with Ru are that it leads to a less intense process involved in the removal of CO [49, 50]; it gives Pt catalysts the ability to withstand the CO, reduces costs, improves the stability of the overall cell, and improves the cell’s activity [41]. In Table 2, Pt-Ru is ranked fourth place with a total of 254 publications; this is in support of the positive attributes of this anode electro-catalyst. Even though Ruthenium is able to successfully encourage the oxidation of CO to CO2 at the anode, it is necessary to additionally decrease the poisoning of Pt and Pt-Ru electro-catalyst so that the efficiency of DMFCs can improve [51,52,53]. Electro-catalysts displayed in Table 2 are mostly binary electro-catalysts. Binary electro-catalysts are known to be costly and having low catalytic activity [54]. This has revealed the necessity of transforming these binary electro-catalysts to ternary electro-catalysts; this is done by introducing a third transition metal to binary electro-catalysts. Ternary electro-catalysts that are found in Table 2 are Pt-Ru-Ni with 10 publications, Pt-Ru-Os with eight publications, Pt-Ru-Sn with six publications, and Pt-Ru-Ir with four publications. Figure 3 shows the gap between publications based on Pt-based electro-catalysts and non-Pt electro-catalysts. Pt electro-catalysts are the commonly used because Pt is attributed to outstanding electro-catalytic activity [55,56,57]. Among the listed electro-catalysts, Pd is ranked third with a total of 553 publications. This is associated with the growing interest in Pd-based electro-catalysts for DMFCs. The development of Pd-based electro-catalysts is implemented so as to bridge the gap caused by the negative characteristics of Pt as an anode electro-catalyst. Pd and Pt are quite similar which makes Pd a suitable non-Pt electro-catalyst for DMFCs, it is also available in abundance unlike the Pt, and this has led to Pd being cheaper [58,59,60,61]. The increase of electronic structure of Pd-based electro-catalysts leads to more electro-catalytic activity; therefore, Pd alloyed with other transition metals can perform the oxygen reduction reaction efficiently [62].

Table 2 Top 15 electro-catalysts used in DMFCs between 1992 and 2023
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Fig. 3
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Graphical representation of publications containing Pt and non-Pt electro-catalysts for DMFCs between 1992 and 2023

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Most published support materials for electro-catalysts used in direct methanol fuel cells between 1992 and 2023

Figure 4 represents the electro-catalyst supports used in DMFCs that appear in most publications that were produced between the years 1992 and 2023. Graphene is the leading electro-catalyst support in the graph with a total of 462 publications. Graphene gained the attention of researchers after it was discovered that it has the potential to replace other carbon support materials that are prone to decay [63]. Advantages associated with graphene are high conductivity (chemical and electronic), enhanced chemical steadiness, increased mechanical strength, how easy it is to alter, and graphene has distinctive properties (physical and chemical properties) [64]. Pt/graphene was observed to consist of smaller Pt nanoparticles on the graphene surface than the Pt found in Pt/C, with sizes 2.83 nm and 4.21 nm respectively. This is because 2D graphene sheets have a bigger surface area available to load metals [65, 66]. Over the past decades, it can be seen that carbon-based supports for DMFCs have been extensively studied. These include the carbon black (CB) with 147 publications, carbon nanofibers (CNFs) with 52 publications, mesoporous carbon with 86 publications, and carbon nanotubes (CNTs) with 313 publications. CNFs and CNTs have been reported to be among the highly stable supports; this is attributed to their high graphitic character [67]. These support materials are among the top 5 support materials represented in Fig. 4. Carbon materials are dominating as support materials for DMFCs because they are available abundantly, they are stable in acidic and basic environments, they are not harsh to the environment, and they are said to have exceptional physical properties [68, 69]. CNTs are ranked second in Fig. 4; they have made great progress in attracting researchers as they were only discovered in 1991 [70]. CNTs are further divided into other types such as single-walled CNTs, double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs), and hallow-structured MWCNTs/CNFs, bamboo-structured MWCNTs/CNFs, and many CNTs [71, 72]. Among these supports, it is seen that non-carbon materials are also making their way into research based on electro-catalysts for DMFCs. These non-carbon support materials include polyaniline (PANI) with 52 articles, tin oxide with 29 articles, polypyrrole with 23 articles, and titanium nitride with 12 articles to mention a few. The “others” in the graph represents the articles that did not include support materials and the support materials that were not part of the top 15.

Fig. 4
figure 4

Graphical representation of the electro-calalysts support materials for DMFCs with the most publications between 1992 and 2023

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Methods used to synthesize electro-catalysts for DMFCs that have the highest number of publications between 1992 and 2023

The hydrothermal synthesis has been used the most to synthesize electro-catalysts for DMFCs between 1992 and 2023 according to Table 3. This method appears in a total of 133 publications during this period. This method has been used in the synthesis of nanocomposites and semiconductor nanocrystals [73]. The hydrothermal method can be used in the doping of CNTs with nitrogen [74]. This method is favoured for this process as it prevents the destruction of the graphitic structure of carbon materials unlike the peroxidation of these materials [75]. This method involves the CNTs being treated with very high temperatures with heteroatom precursors [76]. The polyol process is in second place with 72 articles. The polyol process involves the reduction of metal precursors in a medium such as ethylene glycol. This medium then serves as a solvent and reducing agent [77, 78]. This method has been successful in synthesizing a number of electro-catalysts; these electro-catalysts display high performance, durability, and high tolerance to CO poisoning [79]. Electrodeposition is ranked third on the top 15 methods used in the synthesis of electro-catalysts for DMFCs. This method is mostly used in electroplating, synthesis of thin and thick films metals, synthesis of supercapacitor metals, and synthesis of nanomagnetic materials to mention a few [80, 81]. There are two ways to this method; there is either a two electrode system or a three electrode system. These electrodes are immersed in electrolyte containing ions [82]. The chemical reduction and impregnation method both have 60 publications on research based on electro-catalysts for DMFCs. The sol–gel method follows with a total of 29 publications. The methods with the least publications are the microwave-assisted synthesis, photochemical synthesis, and sonochemical synthesis, all with seven publications each.

Table 3 Top 15 methods used to synthesize electro-catalysts for DMFCs
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Countries with high number of publications on electro-catalysts for DMFCs

Figure 5 shows the top 20 countries out of 50 that were a part of producing articles that are based on electro-catalysts for DMFCs. The top 3 countries being China, USA, and Korea as observed in Fig. 5 as well as in Tables 4 and 5. China published the most articles, with a total number of 838 articles contributing 43.8% of the overall countries articles published. The USA at number 2 accounts for 10.1% which is a total of 193 publications. Korea is ranked at number 3 with a total of 167 articles which is 8.7% of the overall countries articles published. The last three countries in the top 20 are France with 13 articles which is 0.68% of the overall countries articles published, Russia with 12 articles which is a 0.63% contribution on the countries overall articles published, and ranked number 20 is Australia with 11 articles which accounts for 0.58% of the overall countries published articles. In Table 5, the top 20 for the total number of citations per country differs a bit, because Greece is part of the top 20 while it did not appear in Table 4. Greece is ranked number 18 with a total of 410 citations and average article citation of 82.00. The top 3 countries remain the same for the total citations. China has 38,121 total citations and average article citations of 45.49. USA follows with 12,852 total citations with average article citations of 66.59. At number 3, Korea has a total number of 6264 citations and average article citations of 37.51. Even though France is ranked number 10 with a total citations of 2351, it has the highest average article citations of 180.85. This is a difference of 38.62 from UK with the second highest average article citations of 142.23. Egypt is the only African country that made it to both the top 20 countries with most productive research and top 20 countries with high number of overall citations on electro-catalysts for DMFCs. Egypt is ranked 17 on Table 4, with a total of 16 articles. In Table 5, it takes the twentieth position, with 32 overall citations and average article citations of 20.75. South Africa is the only other African country that made it to the top 50 on both lists. In Table 4, it is at number 34, with a total of four articles. In Table 5, it sits at number 39 with 52 overall citations and average article citations of 13.00. This is an indication of the fact that more African countries need to be involved in research on electro-catalysts for DMFCs. The application of DMFCS has the potential of providing Africa with a much more clean energy as compared to the coal that is mainly used for electricity production. Countries like South Africa that experience loadshedding need to implement the application and commercialization of the DMFC. Loadshedding goes as far as reaching stage 6; this is very draining to the South African economy as most businesses depend on electricity for their operation [83].

Fig. 5
figure 5

20 countries with highest productivity in research on electro-catalysts for DMFCs

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Table 4 20 countries with highest productivity in research on electro-catalysts for DMFCs
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Table 5 Overall number of citations for 20 most productive research on electro-catalysts for DMFCs
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Most relevant sources

The wide variety of journals that have produced publications on research based on electro-catalysts for DMFCs is much needed, as it represents growth and more attention being given to this field of study. Table 6 analyses the quantity of articles produced by the top 50 of most relevant journals. The journal that produced the most articles is Journal of Power Sources with a total of 169 articles which is 12.41% of the total articles produced by the most relevant journals. At number 2, International Journal of Hydrogen Energy has a total of 159 articles which is accountable for 11.67% of the overall articles published by the top 50 most relevant sources. The Electrochemical Acta is at number 3 with 137 articles published. This is 10.06% of the overall articles published. Following these top 3 journals are Journal of the Electrochemical Society (n = 66, 4.85%), Applied catalysts B-Environmental (n = 49, 3.60%), Journal of Material Chemistry A (n = 49, 3.60%), RSC Advances (n = 47, 3.45%), Journal of Electroanalytical Chemistry (n = 36, 2.64%), and ACS Applied Materials and Interfaces (n = 34, 2.50%); these above mentioned journals make up the top 10 of the most relevant sources on research that is on electro-catalysts for DMFCs. The bottom ten journals of these top 50 most relevant sources are ACS Applied Energy Materials with ten articles which is 0.73% of the overall articles produced by the most relevant sources. The journals that follow are Chemistry of Materials (n = 10, 0.73%), Energies (n = 10, 0.73%), Iconics (n = 10, 0.73%), Journal of Fuel Cell Science and Technology (n = 10, 0.73%), Materials Chemistry and Physics (n = 10, 0.73%), Journal of Industrial and Engineering Chemistry (n = 9, 0.66%), Journal of Nanoparticle Research (n = 9, 0.66%), Journal of New Materials for Electrochemical Systems (n = 9, 0.66%), ACS Sustainable Chemistry and Engineering (n = 8, 0.59%), and Dalton Transactions (n = 8, 0.59%).

Table 6 50 most relevant sources on research based on electro-catalysts for DMFCs
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Collaboration

When researchers collaborate, they work together to achieve a goal that has been set. Co-authoring is one of the many forms of collaboration. Researchers achieve the goal they set through gathering knowledge, different skills, and resources that the other lack [84]. Collaboration is known to have the ability to restore and enrich relations among scientists, but can also bring about problems and arguments [85]. The mere fact that these institutions, countries, and authors were able to come into agreement, collaborate, and publish these documents shows their level of determination to work together. The growing need to collaborate can be associated with the need to incorporate different interdisciplinary research, more skills, and critical thinking [86, 87]. Collaboration can also be viewed as an integral consequence that occurs in nature, where elite researchers groom promising young scientists [88]. Scientific research involves big projects that have complexities, technology that is updated all the time, new set of acquired knowledge, and different fields of specializations [89]. The above mentioned requires collaboration in order for it to be successful. When analysing the top 20 most productive countries on research based on electro-catalysts for DMFCs, China sits at number 1 in both single country publications and multiple country publications, with a total of 681 single-country articles which is 81.26% of its overall publications, and 157 multiple-country publications (18.73%). Following China, at number 2 is USA with 160 single-country publications and 33 multiple-country publications, which is 82.90% and 17.10%, respectively. The third placed is Korea with 140 single-country publications and 27 multiple-country publications accounting for 83.83% and 16.17%, respectively. The collaborative network in Fig. 6 is a representation of the 20 most performing countries in research based on electro-catalysts for DMFCs. The junction lines/intersection lines traveling across the network show the amount of collaborations these countries have with each other. The size of the circles around each country shows their individual collaborative power. Figure 7 is a visual network of the top 10 institutional collaborations on research based on electro-catalysts for DMFCs. Eight out of the ten institutions that are present on the top 10 collaborating countries are in China. The two exceptions are University of Sao Paulo which is in Brazil and Seoul Nalt University which is in South Korea. This can be explained by the fact that China been a developed country for the longest time, so it is expected of it to be advanced in such research.

Fig. 6
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Visualization network of top 20 country collaborations

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Fig. 7
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Visualisation network of top 10 institutional collaborations

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Some of the funding agencies that have supported research on electro-catalysts for DMFCs along with their countries between 1992 and 2023.

Table 7 shows some of the funding agencies that supported research based on electro-catalyst supports for DMFCs between 1992 and 2023. The National Natural Science Foundation of China (NSFC) is ranked first place with a total of 1761 publications. This is in agreement with the fact that China has been excelling in publishing articles on research based on electro-catalysts for DMFCs between 1992 and 2023. The second placed funding agency is Department of Energy (DOE) which is based in the USA. This also aligns with the place at which USA was ranked in other categories such as “countries with the most publication.” Another funding agency that originates from the USA is National Science Foundation (NSF) appearing on 169 publications. The National Research Foundation (NRF), a South African funding agency, takes on fourth position with a total of 123 publications. Other countries that were ranked high in other categories seem to have a decreased appearance in funding agencies which supported research based on electro-catalysts for DMFCs. For instance, Engineering and Physical Sciences Research Council (EPSRC) from the UK is in ninth place with only 19 publications. Korea was highly ranked in other categories but the South Korean funding agency Korea Institute of Energy Research (KIER) is in eight place with a total of 22 articles. This shows that there is a need for funding agencies and countries to invest more on research based on electro-catalysts for DMFCs.

Table 7 Some of the funding agencies that have supported the development of DMFCs
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Most relevant keywords

Table 8 and Fig. 8 represent frequently used keywords on research based on electro-catalysts for DMFCs. The table analyses the author key words (DE) and keywords-plus (ID) along with the number of articles published for each keyword. Fifty keywords participated in this study, and in Table 4, only the top 20 is represented. The first ten author keywords (ID) are “direct methanol fuel cells” with 218 articles, “methanol oxidation” with 208 articles, “direct methanol fuel cell” (singular of the first author keyword) with 177 articles, “methanol” with 142 articles, “electro-catalyst” with 126 articles, “platinum” with 122 articles, “oxygen reduction reaction” with 114 articles, fuel cells with 112 articles, “electro-catalysts” (plural of the fifth author key word) with 104 articles, and at tenth place “fuel cell” (singular of author keyword at eight place) with 94 articles. The top 10 for the keywords-plus (ID) include “nanoparticles” with 585 articles, “oxidation” with 585 articles, “electrooxidation” with 552 articles, “catalysts” with 493 articles, “performance” with 475 articles, “electro-catalysts” with 398 articles, “methanol fuel-cells” with 387 articles, “platinum” with 362 articles, “fuel-cells,” and “oxygen reduction” with 218 articles. Most of these author keywords (DE) and keywords-plus (ID) are a repetition of each other but they are either written differently or plurals or each other. The keywords-plus have quite higher publications than the author keywords (DE). Figure 9 is a visualisation network of keywords that co-occur on research based on electro-catalysts for DMFCs. The thickness of the lines between the keywords catalysts, oxidation, platinum, performance, electrooxidation, oxidation, nanoparticles, methanol, fuel cells, and electro-catalysts indicates their frequency of occurrence on research based on electro-catalysts for DMFCs.

Table 8 Top 20 frequently used keywords in research based on electro-catalysts for DMFCs
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Fig. 8
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Top 20 visualisation network of the most relevant keywords

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Fig. 9
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Keyword co-occurrence visualization network

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Authors with the most publications

Table 9 and Fig. 10 show the 20 authors who have published the most articles on research based on electro-catalysts for DMFCs between 1992 and 2023. Zhang Y has a total of 52 articles published which puts the author at number 1. Following Zhang Y is Wang H, with a total of 51 publications. In third place is Wang Y with a total of 49 publications. Even though Arico AS sits at number 11 with a total of 35 articles, the author has the most overall citations of 2856. The author with the second most citations is Wang X with a total of 51 articles and 2556 overall citations. Following Wang X on overall citations is Wang H with a total of 2394 and 51 articles. In this study, the number of articles published is shown to not be a determining factor of how much the author will be cited. An author that is in position 11 on the total number of articles published over the years was able to take place on total citations. Figure 11 shows how productive the authors have been over the selected years. The blue circles show the productivity of authors each year, and the lines crossing the blue circles show the consistency of the research outputs between 1992 and 2023 on research based on electro-catalysts for DMFCs. This study shows that the most consistent author in research based on electro-catalysts for DMCFs is Baglio V. Baglio V had more publications between the years 2013 and 2018. The second author who shows consistency is Wang Y who started publishing articles on this research in 2003; Wang Y has the most articles between the years 2014 and 2020. Zhang Y started being consistent in 2008 and has the most articles published between 2013 and 2020. Liu Y started publishing articles based on electro-catalysts for DMFCs in 2006, with the most articles published in 2019. Li Y started publishing articles on this research in 2010, after that the publications kept on increasing between then and 2020; this authors publications have since decreased between 2020 and 2023. Wang L started publishing articles on research based on electro-catalysts for DMFCs in 2010, with the most publications in 2019. Zhang J started publishing articles on this research in 2007 and has been constantly publishing from then until 2022, with the most publications in 2017. Zhang X started publishing articles on this research in 2008; the author has been publishing from then until 2022, with the most publications in 2019. While in 2011 and 2012, the author did not publish articles. Most of these authors are Chinese authors, which corresponds with it being the most productive country on data produced in this study.

Table 9 Top 20 authors with the most publications and citations
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Fig. 10
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Graphical representation of the 20 most productive authors

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Fig. 11
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Productivity of most performing authors over time

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Most cited articles

Citations are in high demand among scientists because they are how they are rewarded on the system of science [90]. Getting citations for a published paper is not easy as many published papers remain unread and uncited; others are only cited by the authors [91]. Citations keep certain research findings from fading away over time and develop its academic characteristics [92]. In this study, the top 20 articles with the highest number of citations on research based on electro-catalysts for DMFCs is recorded in Table 10. The publication with the highest citations is by Guo YG which was published in the journal “advanced materials” in 2008 with a total of 1967 citations and 115.7 average citations per year. In second place is an article by Serp P published in the journal “applied catalysis A: general.” This article has a total of 1711 citations and average citations of 77.8 per year. Taking third place is an article by Arico AS published in the journal “Fuel Cells,” with a total of 1431 citations and 5.9 average citations per year. The article that is number 1 on the list by Guo YG touches on the importance of developing the discoveries of nanostructured Pt-based electro-catalysts for DMFCs [93]. A total of 16 journals participated on the 20 top cited articles relating to research that is based on electro-catalysts for DMFCs. The sources that appeared twice in the list include “Advanced Materials” with articles taking up the first and tenth positions. The second source is “Fuel Cells” with articles ranked in third and fourth positions. The third journal “Journal Power Sources” with articles in seventh and twelfth places. The last journal that appears twice is “Chemical Society Reviews” with articles placed thirteenth and seventeenth in Table 10. The other 12 journals were only sourced once in the 20 frequently cited articles on research based on electro-catalysts for DMFCs.

Table 10 Top 20 research with most citations
Full size table

Discussion

This study has shown that the attention given to DMFCs has increased between 1992 and 2023. China has been the most performing country in the development of research based on electro-catalysts for DMFC technology. This can be driven by the fact that China is a developed country and is more exposed to fuel cell technology. Other countries that have shown a great contribution in the development of DMFCs are USA and Korea. All the above-mentioned countries are developed, and it is well within their rights to be performing well in DMFC technology. China is the leading country in both single-country and multiple-country publications, with a MCP ratio of 0.187. Most of the institutions that were assessed in institutional collaborations are Chinese institutions. Their collaboration is quite frequent; this shows that China is guaranteed progress in this type of research if it aims to continue exploring and developing electro-catalytic technology for DMFCs. It was quite expected that the results show that the most productive authors are Chinese; this is in line with all the other analysed data, where China was the leading country. Zhang Y started publishing articles relating to research based on electro-catalysts for DMFCs in 2008, with H index of 22, G index of 46, M index of 1.2941176, 2146 total citations, and 52 publications. The second author is Wang H who started publishing in 2006, with H index of 28, G index of 46, M index of 1.4736842, 2394 total citations, and 51 publications. The author in third place is Wang Y, whose first publication year is 2003, with H index of 25, G index of 48, M index of 1.1363636, 2353 total citations, and 49 publications on research based on electro-catalysts for DMFCs. A big challenge that is faced by DMFC application is its high cost that is caused by the Pt usually used as an electro-catalyst for oxidation of methanol [30]. Also, carbon monoxide poisoning that is produced by MOR degrades the activity and durability of electro-catalysts [94]. Pt-based electro-catalysts are usually alloyed with other metals to improve their catalytic properties [95]. This alloying helps to reduce the PT loadings in electro-catalysts that is directly associated with high cost and CO poisoning thus improving the MOR performance in DMFCs [26, 96]. Over the years, researchers have shown the importance of including support materials in the synthesis of electro-catalysts for DMFCs such as carbon black (CB), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanohorns (CNHs), and graphene. These support materials play a huge role in reducing Pt load of electro-catalysts, increasing their surface area, and improving their catalytic activity [18]. The above mentioned carbon support materials also have the ability to greatly enhance the efficiency of electro-catalysts, decrease poisoning thus increasing their stability, and improve the catalytic dispersion while creating a structure that supports electron conductivity and gas diffusion [97]. Platinum has been observed to be the most active metal for dissociative adsorption of methanol [39] but the negative side to it is that at room temperature, Pt can be poisoned by CO; this is a by-product of the MOR [28]. What has been used as a solution to this is using binary and ternary Pt electro-catalysts while incorporating ruthenium to promote the activity of electro-catalysts [98, 99]. When preparing electro-catalysts, the impregnation method is the most preferred because of how simple it is [100]. There are three steps involved in the impregnation method; these are (1) contacting a selected support material with an impregnation solution for a certain time and (2) drying of the support material to extract the diffused liquid, and the last step (3) is to activate the catalysts using calcination, reduction, or any other suitable treatment [101]. The catalysts prepared using the impregnation method often consist of a large average size of the metal particles, the particles are of a broad particle distribution, and relative crystallinity is often high, and it accommodates low oxygen reduction of electro-catalytic activity [102].

Conclusion and recommendation

In this study, the growth and decline of publications on research based on electro-catalysts for DMFCs between 1992 and 2023 were shown. Pt-based electro-catalysts have been used the most for DMFCs; this is due to the high catalytic activity Pt holds. Even though Pt has been successful in this field, Pd has to be intensively studied so as to bridge the gaps associated with using Pt-based electro-catalysts for DMFCS. Pd is available abundantly, and it is much cheaper than Pt. Carbon support materials have dominated the field of electro-catalysts for DMFCs. Graphene is the top ranked and is followed by CNTs, CNFs, CB, and mesoporous carbon. This is because these supports have graphitic characteristics. Graphene has positive attributes such as chemical and electronic conductivity, mechanical strength, and chemical steadiness to mention a few. The top 3 methods used in research based on electro-catalysts for DMFCs are the hydrothermal synthesis, polyol process, and electrodeposition. The least used methods in the top 15 are the microwave-assisted synthesis, photochemical synthesis, and sonochemical synthesis. It can be observed that over the years, the interest in such research has grown, with a slight decline of the number of articles published from 2021 to 2023. The most dominating journals in this research were found to be Journal of Power Sources, International Journal of Hydrogen Energy, and Electrochimica Acta; these are energy or renewable energy journals. China, USA, and Korea have been found to be the most successful countries in producing articles on research based on electro-catalysts for DMFCs. These countries are all developed countries, and it is expected of them to be leading in such research. Only two African countries appeared on the top 50 countries, Egypt and South Africa. Egypt was able to take the seventeenth position on the table representing the total number of articles published by each country and the twentieth position on the table representing the total number of citations. South Africa sits at the thirty-fourth position in the table representing total articles and thirty-ninth position on the table representing total citations. This is great achievement for these African countries, especially South Africa as it is one of the countries that are currently experiencing loadshedding which is straining its economy. More African countries need to be involved in this research so as to help to improve the state of the environment and eliminate the dependence on fossil fuels for power generation and consider this more environmentally friendly approach. The National Natural Science Foundation of China (NSFC) has been supporting a lot of studies based on research based on electro-catalyst for DMFCs between 1992 and 2023. This is in line with the fact that this study revealed how China has been excelling in this field. It would be of great benefit to see other top ranked countries and their funding agencies investing more in this field. There are still a lot of challenges that need to be worked on, like the selection of suitable electro-catalysts for DMFCs, the issue of methanol crossover, and the weakness of the kinetics of the DMFC [74, 103,104,105] to mention a few. It is therefore recommended that research in this field should be tailored towards addressing the aforementioned challenges in order to ensure the overall efficiency of this fuel cell.