Peer review of: Multi-functional anodes boost the transient power and durability of proton exchange membrane (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The authors describe an original way to improve the stability of PEMFC during start/stop experiments 
and fuel starvation by integrating a layer of WO3 between the gas diffusion layer and the catalyst 
layer, at the anode side. The authors well described the synthesis, nanostructural and electrochemical 
characterizations of the WO3 with Carbon Nanotubes. They also give the details of the integration of 
this compound in the membrane electrode assembly. According to the authors, this layer allows to 
make the PEMFC more resilient to transient power demand. The extensive electrochemical 
characterizations clearly and smartly support their conclusions. Considering the details in the 
experimental part, we can be confident that this works gives fruitful information and sufficient to 
reproduce the fabrication, the integration and the evaluation in real operation of this WO3 based layer. 
The approach is novel and of interest for the PEMFC community and also to a broader audience for 
researcher working of electrochemical energy conversion devices with proton conducting electrolyte. 
Thus, I would strongly recommend to publish this article, after some minor revisions. 
In Figure 1, the authors give the electrochemical reaction of WO3 in acidic conditions (equation (iii)). 
Could the author give, somewhere in the manuscript or in the supplementary information, the 
standard redox potential of this reaction and relate it to the Oxidation/Reduction peaks on the cyclic 
voltammograms shown in Figure 2F. 
The fast response of the WO3/CNT layer was evaluated in a PEMFC like configuration but without 
anode catalyst layer. The WO3/CNT is mixed with Nafion to make a porous composite layer on the 
anode Gas Diffusion Layer (GDL) and it is in direct contact with the membrane (details given in the 
supplementary information). In this case, the WO3 is in the presence of acidic water when hydrated 
thanks to the proton coming from Nafion. This is not absolutely clear in the description, but the same 
composition of the layer (WO3/CNT layer+Nafion) seems to be used in real PEMFC. Could the author 
state it more clearly? 
The whole mechanisms rely on the possibility to WO3 to exchange protons with the membrane. This 
explains why the authors mixed this compound with ionomer. When water activity decreases the 
proton resistance of this layer decreases and this makes the whole electrochemical reaction less facile. 
This could make this layer less efficient in drier conditions. Could the authors comment? Experiments 
in fuel cell have been conducted in fully humidified conditions, the impact of the results (already 
important) would have been greatly enhanced if additional experiments in drier conditions, e.g. lower 
RH along with higher temperature (>50°C), had been performed. If the authors have conducted such 
experiments, could they provide the results? Is this lower hydration of the ionomer in the WO3/CNT 
layer could explain the decrease in efficiency of the strategy when the temperature is raised from 30 
to 50°C? Could the authors comment? 
The authors wrote: “This design enables the fabrication of novel PEMFCs with dramatically enhanced 
performance at low cost”. This statement seems to me a bit strong. Indeed, the performance 
demonstrates with the polarization curve on Figure S8 is reasonably good but not really at the state-
of-the-art. The layer does not allow to increase steady state performance as shown on Figure 3C. I 
agree that the stability upon start-up/shut-down and fuel starvation is enhanced. The resilience to 
transient is also improved thanks to this layer but it does not allow to increase the steady-state 
performance (of course, this is not the primarily objective). This is true that the initial objective for 
which this layer has been designed is reached. However, experiments with Air at lower relative 
humidity (e.g. 70%RH or lower) and at higher temperature (e.g. 70°C) would have definitely 
demonstrated the interest of this layer. It seems to me that this observation of the authors should be 
nuanced. Note, that it does not call into question the originality, the relevance and the quality of their 
work. 
Could the authors explain why they did use CNT instead of carbon black? Is this possible to integrate 
WO3 in the PEMFC with carbon black which is more conventionally used in PEMFC than CNT? I maybe 
missed something but it does not seem that the cost of CNT has been integrated to the calculation of 
the cost of the composite layer made of WO3. Could the authors comment? 
A short bibliography study on the use of WO3 in PEMFC as well as on the mitigations strategy to 
reduce the impact of fuel starvation and start-up/shut-down would have been useful in the 
introduction. 
In figure 3, E and F seems to me a bit redundant, as well as H and I. 
The authors may refer to a patent number in case their strategy has been patented. 

Arnaud Morin 
CEA, Grenoble 



Reviewer #2 (Remarks to the Author): 

The manuscript presents a thorough electrodynamic assessment of the transient performance of 
WO<sub>3</sub>-modified PEMFC anodes to address continuity of power delivery following practical 
operational issues such as variations in power demand, fuel starvation and start-up. The rationale for 
the study is based on the variable availability of protons under transient polarisation of conventional 
Pt/C anodes with the proposed solution being the use of an integrated hexagonal tungsten bronze 
layer as a proton reservoir. 

Several studies have reported on the properties of 
WO<sub>3</sub>/H<sub>x</sub>WO<sub>3</sub> as catalysts relevant for fuel cell applications, 
particularly the well-known potential dependent intercalation/de-intercalation of H<sup>+</sup> ions 
and the increased electronic conductivity of the tungsten bronze (H<sub>x</sub>WO<sub>3</sub>) 
[e.g. Liu, et al, Int J Hydrogen Energy, 43, (2018), 8944; Micoud, F., et al, Phys. Chem. Chem. Phys., 
12, (2010), 1182; Wickman, B., et al, Electrochimica Acta 56 (2011) 9496, in addition to refs 21 and 
22 of the current manuscript]. These properties are confirmed by the CV (Fig 2B), the 
H<sup>+</sup> supply /consumption by the tungsten bronze being driven by the polarisation of the 
bronze (anode) under the different fuel cell conditions described. Charge balance in the bronze is 
maintained by the oxidation/reduction of tungsten ions (W<sup>5+</sup>/W<sup>6+</sup>) and 
the accompanying electronic charge transfer between the bronze and the anode current collector. 
Consequently, variation of the anode potential, induced by operational conditions, will influence the 
direction and magnitude of the reaction described in Fig 1 reaction (iii) which accounts for much of the 
reported cell behaviour. 

A H<sub>x</sub>WO<sub>3</sub> supported ORR reaction is suggested by Fig 1 reaction (iv) but 
its viability, or details of any required conditions, e.g. potential, is not discussed or supported by any 
mechanism or cited literature. Hernández-Pichardo, et al, [Int J Hydrogen Energy, 40, (2015), 17371] 
proposes an ORR mechanism requiring the presence of Pt (as is available in the current study at the 
interface between the WO<sub>3</sub> layer and the PtC electrode); note that this requires anode 
polarisation to (0.8V vs NHE). 

H<sub>x</sub>WO<sub>3</sub> = xH<sup>+</sup> + WO<sub>3</sub> + xe<sup>-</sup> 
+ Pt-O<sub>2</sub> = Pt-O<sub>2</sub>H 

The nature of the WO<sub>3</sub>-Pt interaction therefore becomes of interest. The proximity of Pt 
and WO<sub>3</sub> was shown to be important in promoting the beneficial role of 
WO<sub>3</sub> in the CO tolerance of Pt [Micoud, F., et al, Phys. Chem. Chem. Phys., 2010, 12, 
1182; Micoud, F., et al, Electrochem. Commun., 2009, 11, 651–654]. This aspect of the performance 
dependence is not addressed but may be significant. 

The subsequent measurements support the general concepts presented. Although WO<sub>3</sub>-
modified electrodes exhibit similar polarisation curves to the control system (Fig 3C), they are more 
responsive under current switching and the analyses through modelling supports the reduction of 
transient polarisation which could be accounted for by the enhanced availability of protons from the 
WO<sub>3</sub> layer. 
The response of fuel cells to power demand is increasingly important in their development for 
automotive applications and this manuscript directly addresses this through a practical solution. Whilst 
further discussion might be expected on mechanisms and constraints, the general principles illustrated 
could have an important impact in the field.