Peer review of: Elastic conducting polymer composites in thermoelectric modules (2020)

RR2

REVIEWERS’ COMMENTS:
Reviewer #1 (Remarks to the Author):
The authors report the fabrication of a novel PEDOT-ionic liquid composite that exhibits high conductivity, elasticity, stretchability, and a low young’s modulus. The authors attribute the high performance of their material to favorable nano- and micro- scale percolation networks, and plasticizing effect of the ionic liquid, and then use this material to fabricate an intrinsically stretchable organic thermoelectric module. To support their hypothesis of the importance of percolation networks, the authors investigated the properties of their composite over a wide range of composition, and measured the size distribution of aggregates in the aqueous dispersions of the composite that they used to fabricate films of their materials, as well as performed atomic force microscopy measurements to visualize the surface morphology of their films.
Although intrinsically stretchable thermoelectric materials have been previously demonstrated, the fabrication of a thermoelectric module with tens of micron thick legs from such a material, along with the increased electrical conductivity and supported hypothesis for this increase, represents a significant advance for the understanding and utility of these materials. This manuscript is both sufficiently novel, and relevant to a broad audience to justify publication in Nature Communications after addressing the following comments:
1)
Although the authors report electrical properties as a function of strain and reference cycling tests for the physical properties of their materials, it is unclear how the electrical properties evolve with strain cycling. If the conducting polymer is formed of a percolated network of nanofibrils, it seems reasonable that cycling would induce changes in this morphology that would affect the electrical conductivity or Seebeck coefficient. Additionally, although electrical conductivity can be inferred from the change in resistance and geometric changes for strain up to 100%, the Seebeck coefficient is shown only up to strains of 40%. If the authors have additional data on electrical properties as a function of cycle, and Seebeck coefficients for larger strain values, these data would be useful for a reader to understand the importance of the demonstrated thermoelectric module.
2)
Similar to above, although the resistance has increased and therefore current output of the thermoelectric module decreased by a factor of approximately two for tensile strains of 40%, these changes are explained by geometric changes in the material only if it behaves as a material whose resistivity is an intensive material property. Given that percolation through a network of fibers is a more complex model of conduction, the authors should comment on the validity of their assumptions when using an effective medium model to determine the conductivity of the embedded conducting polymer phase, or uniform geometric model that invokes intensive material properties, such as for 2(h) and S3.
3)

The DC conductivities were measured on spun-cast films that were significantly thinner than the legs of the drop-cast thermoelectric modules. The authors should comment on potential differences in morphologies of materials deposited in these ways, and specifically how those differences might affect a percolation model of conduction.
Reviewer #2 (Remarks to the Author):
This is an interesting paper on the development of flexible thermoelectric based on PEDOT. Inclusion of ionic liquids (ILs) into the material make possible the development of stretchable thermoelectrics. The work has been carried out very well and the results should be of interest to the thermoelectric community. Unfortunately, while I think the work is good, I don't believe it has the level of impact and novelty that one would expect for the journal. I don't believe it will impact on thinking in the field to a sufficient level.
On a scientific level, I note that the authors haven't really addressed loss or leaching of the plasticiser from the materials. In the development of composite materials based on ILs often must determine the stability, dispersion and loss of the material in/from the host material. I think this is an important issue that must be addressed.
Reviewer #3 (Remarks to the Author):
The authors report on the stretchable PEDOT:PSS-IL-WPU composite for the stretchable thermoelectric (TE) module. In this manuscript, the solution processable TE ink is prepared using the PEDOT:PSS-IL-WPU composite and this composite exhibits quite good electrical and mechanical properties. However, materials based on PEDOT:PSS with elastomer have widely been studied even with their use in stretchable electronics. Furthermore, the addition of IL for the TE application of PEDOT:PSS is a routine process. The fabricated TE module in parallel direction without n-type materials also shows quite low output power despite the good mechanical stability.
Although the paper is well written and presented in a scholarly manner with few exceptions, I believe
this manuscript, while interesting, is insufficient to provide readers with useful information in the field of thermoelectronics. Therefore, I strongly recommend rejection for publication in Nature Communications.
1) What is the change in the Seebeck coefficients depending on the composition ratio of three substances, PEDOT:PSS, IL, and WPU?
2) On line 206, the author referred to a paper that is not yet published.

3) On line 209, the author claims that the conductivity of composite decreased by dedoping is recovered via chemical equilibrium with the oxygen in air. However, no evidence or reference can be found to support the author's claim in the manuscript. Provide the appropriate evidences or refer relevant papers.
4) On line 318, are the Au-TiO2 NWs stretchable? (If they are stretchable enough, please provide the proper evidence.) Resistance of the Au-TiO2 NWs is significantly increased with an increase of tensile strain in Figure S15. I wonder the drop in the output power of the TE module in Figure 5f is closely related to increased resistance of the Au-TiO2 NWs.
5) In page 30, the authors evaluated the TE properties of the stretchable TE module in only one direction. Since the active part of the TE module shown in Figure 5 has a long rectangular shape with anisotropy (not a square), they should be characterized in both perpendicular and parallel direction to elongation.
6) On line 650, the elasticity of TE module with the composite is different from that of the pristine active material because the TE module includes both substrate and electrode. Is there no effect of the substrate and the electrode on the performance of the active layer? Also, direction of stretching should be indicated with an arrow in Figure 5 for the reader's understanding.
7) On line 305, for wearable and stretchable TE modules, vertical configuration is more suitable than parallel one due to its easy build-up of the temperature gradient and low electrical resistance. If the PEDOT:PSS-IL-WPU composite forms the enough thickness, vertical TE module can be prepared by using only p-type TE legs. Is the thickness of the PEDOT:PSS-IL-WPU films (~40 μm) sufficient for the TE legs of vertical configuration? If it is impossible, how can the thickness be increased or controlled? How can the electrical conductivity and mechanical properties be optimized depending on thickness?