**4.2. The usage of patent**

For answering the question of how patents are involved in e-mobility study and thus providing a reference point for further adoption of patent research, three main categories of the introduction of patents differed in research perspectives, and data formats are identified. **Table 1** outlines the direct use of patent-based indicators (e.g., counts, families, and citations) or patent documents (e.g., abstracts and literature) in view of reasons, including main routes as tracing the technological and industry trend, evaluating policies and innovation, as well as improving patenting activities and patent-based analysis [19, 22, 47–50]. Besides, the importance of patented technologies is highlighted while the rise in patenting activities and their commercialization indicate that a clean technology revolution staged by e-mobility is approaching [51, 52].


**Table 1.** The usage of patent in an e-mobility study extracted from abstracts and methods.

#### **4.3. Limitations and potential topics**

#### *4.3.1. Recent research limitations*

technology analysis [41–43]. Patent citation analysis is the central topic of discussion involved in the fourth cluster with the earliest average publication year of 2000 [44–46]. Namely, the core documents providing a common knowledge base for e-mobility patent study are rela-

By contrast, an analogous network structure could not be found through a bibliographic coupling analysis of 462 citing papers owing to the unclear layout of nodes and their links. It may indicate that the knowledge of these 48 patent studies is expanded to a much broader scientific field with more creative and diverse approaches to exploring e-mobility issues. Moreover, the share of self-citations is comparatively high and implies that there has existed specific groups

For answering the question of how patents are involved in e-mobility study and thus providing a reference point for further adoption of patent research, three main categories of the introduction of patents differed in research perspectives, and data formats are identified. **Table 1** outlines the direct use of patent-based indicators (e.g., counts, families, and citations) or patent documents (e.g., abstracts and literature) in view of reasons, including main routes as tracing the technological and industry trend, evaluating policies and innovation, as well as improving patenting activities and patent-based analysis [19, 22, 47–50]. Besides, the importance of patented technologies is highlighted while the rise in patenting activities and their commercialization indicate that a clean technology revolution staged by e-mobility is

tively new and it is partly an effect of emerging technologies and the changing field.

of authors in this research area and their studies possess certain coherence.

**4.2. The usage of patent**

142 Scientometrics

**Figure 2.** Co-citation analysis of references listed in 48 papers.

approaching [51, 52].

Some of the deficiencies inherent in patent research are synthesized and divided into groups of limitations regarding patents or data sources [21, 53, 54] and patent-based indicators or approaches [47, 55, 56], respectively, thereby pinpointing areas of improvement in the further study on e-mobility. However, the following limitations should not be viewed in isolation, and the specificity of e-mobility field, especially the novelty and complexity of technologies [19, 21], needs to be considered in addition to patent-oriented issues (**Table 2**).

#### *4.3.2. Potential topics*

A series of up-to-date topics captured from citing articles based on recent highly cited ones in **Table 3** could be classified into the extension of the specific technology discussion, patent-based analysis, and research on innovation system or policies in the field of e-mobility [47, 57–62]. The classification of additional research perspectives is inevitably influenced by the usage of patent in highly cited papers (**Table 1**). More specifically, a review on patented technologies has developed the base for further experimental studies, and papers adopting patent-based indicators could arouse growing interests in examining the pattern of technological change [21, 63]. Patents combined with other format of data, such as scientific literatures, surveys, interviews, or press releases, may contribute to a more comprehensive understanding of relevant policy and innovation system research [48, 54].


**Table 2.** Recent research limitations stated in conclusive parts of publications (since 2015).
