Production of a monolithic fuel cell stack with high power density

The transportation sector is undergoing a technology shift from internal combustion engines to electric motors powered by secondary Li-based batteries. However, the limited range and long charging times of Li-ion batteries still hinder widespread adoption. This aspect is particularly true in the case of heavy freight and long-range transportation, where solid oxide fuel cells (SOFCs) offer an attractive alternative as they can provide high-efficiency and flexible fuel choices. However, the SOFC technology is mainly used for stationary applications owing to the high operating temperature, low volumetric power density and specific power, and poor robustness towards thermal cycling and mechanical vibrations of conventional ceramic-based cells. Here, we present a metal-based monolithic fuel cell design to overcome these issues. Cost-effective and scalable manufacturing processes are employed for fabrication, and only a single heat treatment is required, as opposed to multiple thermal treatments in conventional SOFC production. The design is optimised through three-dimensional multiphysics modelling, nanoparticle infiltration, and corrosion-mitigating treatments. The monolithic fuel cell stack shows a power density of 5.6 kW/L, thus, demonstrating the potential of SOFC technology for transport applications.


Reviewer #2 (Remarks to the Author):
The reviewer's comments and requests have been addressed adequately.

Reviewer #3 (Remarks to the Author):
The manuscript was first submitted to Nature Energy and has been reviewed by three reviewers. The authors were encouraged to transfer the manuscript to Nature Communications after considering the three reviews.
General comment: Metal supported fuel cells are highly attractive for the transportation sector due to their large potential with respect to thermal management (high heating rates) and robustness. The manuscript describes a method to produce a new type of metal supported SOFC stack, which is characterized by its assembling as a monolith. Based on electrochemical tests done on a single repeat unit (SRU, size of 70 x 40 mm2), the authors state a power density of 5.6 kW/L for a monolithic stack made of these SRUs. This power density clearly exceeds the requirements of SOFC stacks in the transportation sector, e.g. as range extender for battery electric vehicles. Furthermore, the authors present a production route for this monolithic stack, which is optimized with respect to cost effectiveness amongst others by reducing the thermal treatments during cell processing. The results are promising and of interest for the SOC community.
The authors accurately considered the recommendations of the reviewers and changed the manuscript accordingly. Also further supplementary information is added for clarification.

Point-by-point response letter to the reviewers
The authors would like to thank the reviewers for their time and their useful comments, helping to improve this paper. All comments from the reviewers were answered and the manuscript was modified accordingly. Below the detailed reply to each issue addressed by the reviewers can be found.
The comments from the reviewers are written in red, the answers and comments from the authors are marked in blue. The modifications made in the manuscript are highlighted in yellow.

REVIEWER COMMENTS
Reviewer #2 (Remarks to the Author): The reviewer's comments and requests have been addressed adequately.
Reviewer #3 (Remarks to the Author): The manuscript was first submitted to Nature Energy and has been reviewed by three reviewers. The authors were encouraged to transfer the manuscript to Nature Communications after considering the three reviews.
General comment: Metal supported fuel cells are highly attractive for the transportation sector due to their large potential with respect to thermal management (high heating rates) and robustness. The manuscript describes a method to produce a new type of metal supported SOFC stack, which is characterized by its assembling as a monolith. Based on electrochemical tests done on a single repeat unit (SRU, size of 70 x 40 mm2), the authors state a power density of 5.6 kW/L for a monolithic stack made of these SRUs. This power density clearly exceeds the requirements of SOFC stacks in the transportation sector, e.g. as range extender for battery electric vehicles. Furthermore, the authors present a production route for this monolithic stack, which is optimized with respect to cost effectiveness amongst others by reducing the thermal treatments during cell processing. The results are promising and of interest for the SOC community.
The authors accurately considered the recommendations of the reviewers and changed the manuscript accordingly. Also further supplementary information is added for clarification. There are pressure peaks related to PMMA in all cases because PMMA is also contained in the electrode tapes (as pore-forming agent). The experiment was performed on SRU monoliths therefore even the monolith containing only graphite as sacrificial material to form gas distribution channels (figure 2.a) contained some PMMA.
To clarify, we have modified a sentence in the manuscript (page 9, lines 10-12): " Figure 2 shows SRU monoliths manufactured with different gas channel forming materials (a. 100 % graphite,and c. 100 % PMMA) in addition to the organics use in the tapes."  A sentence and two references were added to the method section to specify how the ASC-SRUs benchmark was estimated:  41 ,42 SRU height in the range from 1.4 to 4 mm is reasonable to assume for SoA SRU and these numbers were applied to compare the electrochemical performances of the monolith with SoA SOFCs in terms of power density per volume, as presented in Figure 4." -Reference 21: Not clear if journal publication or patent, please revise.
Reference 21 is a patent. The reference has been revised and the patent number has been specified in the manuscript (page 22).