# The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells--A Review
## 2 Tubular Solid Oxide Fuel Cells
The distinguishing characteristics of planar versus micro-tubular SOFCs are seen in Table 1. Micro-tubular fuel cells have inherently facile sealing and gas flow control, given that the dense electrolyte separates gas streams rather than an interconnect. Tubular fuel cell designs exhibit similar power densities to planar devices at up to 1.5 W\(\cdot\)cm\({}^{-2}\)[40; 41]. The \(\upmu\)T-SOFC concept lends itself to high mechanical strength and high thermomechanical  
Figure 4: Tubular (and planar) SOFC cell support configurations and the progression of the state of the art. durability. This strength is inversely proportional to cell diameter. The durability to mechanical shock and thermal cycling of \(\upmu\)T-SOFC allows for rapid start-up times in the order of a few seconds [1]. The aforementioned enables near-instant on-demand power, permitting \(\upmu\)T-SOFCs to be used for both stationary and portable power applications [42]. This widens the market for SOFC technology not typically accessible by planar devices. Several commercial companies have worked on \(\upmu\)T-SOFC development including Adelan, Acumentrics, AMI, Ezzelero, Protonex, Komico, NGK, Toto, Tokyo Gas and others [38, 43]. Tubular SOFC products have reached the commercial market globally, most widely adopted in Japan and Korea [44]. Portable devices have included the tubular Kraftwerk [45], a 5 V (10 W peak power) charging device (which was eventually never produced). The number of journal and conference articles in the academic literature on tubular SOFCs has followed a declining trend over the past five years, as seen in Figure 5a. Instead, research has widened into more fundamental problems associated with \(\upmu\)T-SOFCs (and in most cases planar SOFCs), such as sealing, manifolding and material development. Research in the field has been dominated by authors from China, showing nearly double the number of studies as from the USA which is the next largest contributor, as seen in Figure 5b. Material development and testing, which are common SOFC development areas, are easier to conduct in planar form. Interconnection/current collection in tubular SOFCs is a critical area for development and remains a bottleneck in the commercialisation of tubular cells and stacks. The focus of \(\upmu\)T-SOFC current collection in this review aims to identify what has been achieved to date in order to identify the most viable current collection design(s) for the future and highlight future challenges. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristic** & **Microtubular** & **Planar** \\ \hline Power Density & Medium & High \\ Mechanical Strength & High & Lower \\ Start-up & Fast & Slower \\ Interconnect & Difficult/Cumbersome & Straightforward \\ Sealing & Facile & Complex \\ Manifold & Simple & Simple \\ Stack Power Density & Medium & High \\ System Compactness & High & High \\ Manufacturing Cost & Medium-High & Lower \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of micro-tubular (\(\upmu\)T)-SOFC and planar SOFC. Figure 5: Number of journal and conference articles between 2015 and 2020 mentioning tubular and solid oxide fuel cells (SOFCs) from an Engineering Village search (**a**) by year and (**b**) by location.