The topic is interested and the experiments seem to be well executed. However, I am afraid about the following points:
(1) Author forgot to mention the recent publication on DFAFC: Electrochimica Acta 235 (2017) 135–142. And compared to this publication it seems that
a. It exist a lack of originality: Fig 1a is similar to the fig 2 inside above mentioned reference, and Fig 2 is similar to fig 4 inside above mentioned reference.
b. The authors observed, in fig 1b, a time series under current control in a DFAFC. These oscillations have regular amplitude, i.e. 0.55V and large dispersion on the values of period, i.e. between 250 and 650 Seg. In above mentioned reference, same amplitude and dispersion on the value of period (except by the absolute values, which in mentioned reference is, dispersed between 40 and 120 Sec, whose difference may be attributed to the larger temperature, the lower period) were already observed in a pseudo-half fuel cell operating with formic acid (cathode operated with H2). There is strong evidence that they are equivalent.
(2) In general, the results do not support the Author`s conclusions. The negative values of potential (potentio inversion) in a DFAFC during oscillations, observed by the authors in fig 1b, solely reflect the enlargement on the anodic overvoltage due to the oscillatory kinetics as demonstrated in Electrochimica Acta 235 (2017) 135–142, which, ultimately, reinforces the arguments defended by Lopes et. al. cited inside the manuscript as ref.7.