Peer review of: Protonic solid-state electrochemical synapse for physical neural networks (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Emond et al: This interesting paper deals with a WO3-based device, the conductivity of which can 
be modified by electrochemically pumping hydrogen in and out. I.e., the conductivity of the 
channel can persistently be changed back and forth, not only on/off but with multiple steps in 
between. Such devices termed "electrochemical synapses" could be very beneficial for analog 
implementations of neuronal networks. The principle has been realized before with other materials 
(organic semiconductors, or WO3 with lithium insertion) but the present combination of WO3 with 
hydrogen pumping is expected to be advantageous with respect to long-term stability and energy 
consumption. 
The physical mechanism of changing WO3 conductivity by hydrogen insertion=reduction is 
straightforward (and well supported by XPS and DFT results), also the appearance of the different 
regimes in fig. 2a. 
It is important to explore several options how to realize such devices (which channel material, 
modified by which ion insertion, which operation mode...), and thus overall I recommend 
publication. 
 
Some points related to the practical applicability should be discussed in more detail: 
* regarding the multi-state capability, authors should be more specific which magnitude of 
conductance change is regarded necessary for reliable and reproducible access of one state, and 
how many states can realistically be implemented in one cell 
* low hysteresis is decisive for multi-state operation. I'm a bit surprised that even under current 
control quite big hysteresis occurs under some conditions, e.g. fig. 2c,d. Any idea why, and how to 
improve that? 
* According to the reported results, current gating is preferred as operation mode over voltage 
gating. Please comment if this would lead to any significant complications for construction of real 
devices comprising many such synapse cells (current is more difficult to control than voltage), or 
how big the chance is to get good performance also in voltage gating 
* it would be good to comment on the scatter in conductance between a larger number of 
nominally identical cells (i.e. how reliably can the number of cells be scaled up) 
* the cells need humidity in the Nafion layer, and oxygen in- and/or hydrogen out-diffusion must 
be avoided. Authors report some preliminary encapsulation tests, but longer times than in fig S14 
should be reported 
* a related issue: Nafion is an acidic polymer. Any long-term degradation in contact with palladium 
hydride (the hydrogen reservoir) by releasing hydrogen from proton+hydride ions, and 
incorporation of Pd2+ into Nafion for charge compensation? 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
In this manuscript the authors present a very complete work on the development of three-terminal 
synapses based on proton intercalation in inorganic materials. The work is very complete, as it 
provides clear information about the fabrication of the devices, complete morphological and 
electrical characterization, and atomistic modelling. The electrical characteristics measured are 
impressive, specially the linearity achieved and the controllability of the potentiation and 
depression processes. The conductance modulation mechanism is clearly identified using “in 
operando” chemical measurements. Overall, the work provides important insights in its field of 
research, and the reliability of the findings is higher than in most reports in this field due to the 
multiple and high quality techniques used. In my opinion, this work is above the level of most 
others in the field, and for that reason I would like to recommend its publications in Nature 
Communications. 
 The only weakness that I see, which the authors should definitely further discuss, is the 
integration capability of this work: 
 
1- First of all, they use devices with channels of 100 um (is that the length or the width?). These 
devices are very big. Did the authors try to do smaller devices? This is highly recommended in 
order to observe if the findings also apply to miniaturized devices. 
 
2- Moreover, as the authors wrote the word “computing” in the title of their abstract and because 
the introduction of their paper is discussing about “in memory computing”, the authors should 
discuss what is the potential of their device for computation, which in its current form seems to be 
unclear to me. The authors should clarify which kind of computation may be done with their device 
and the path towards such implementation. I do not think it is necessary they demonstrate such 
computation, but they may reference some works that did such things. 
 
3 – The authors should also indicate which is the advantage and disadvantage of their devices 
compared to two-terminal resistive switching devices. I think two-terminal resistive switching 
devices are the most commonly used in most companies, and therefore it seems to me that they 
have most potential to end up being the hardware for such “in memory computing”. Therefore, 
from a technological point of view, the authors should clarify the potential of their work. 
 
The work is really good, I really enjoyed reading it, but the authors should provide more practical 
information about the technological potential of their work. I also recommend to remove the word 
“computing” from the title; some readers may get too excited when seeing the title and later 
disappointed when reading the manuscript, as there is not computing in this manuscript. In the 
future, the authors may try to associate different devices to solve a complex problem. They may 
even indicate such thing in the conclusions to encourage the community to work in such 
application. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
Authors in this work demonstrate a three terminal device based on proton migration. The device 
shows a great reversibility for analog signal processing under current pulses. They have also 
performed impressive and careful materials studies both experimentally and theoretically to 
conclusively reveal the switching mechanism. It is a very interesting and timely work. I strongly 
recommend it for publication. 
I do have some minor comments for the authors to consider. 
1. The authors obtained good XAS and XPS results to make comparisons between WO3 and 
HxWO3, but the exact ratio of O to W of the as-deposited film was not revealed. It would be great 
if the stoichiometric ratio of O to W is also given in the manuscript. Did author fabricate the WOx 
film with a non-stoichiometric ratio? How would that affect the performance of the device? How to 
eliminate the effect of oxygen vacancies migration in WOx channel? 
2. The WO3 film was annealed in air at 450 ℃ for 1 h. Please comment on the concentration and 
effects of nitrogen in the annealed film. In addition, 450C is higher than 400C, which means this 
process is not 100% fab compatible yet. Would 400C annealing for a longer time achieve the same 
effect? 
3. Please comment on how to control concentration of hydrogen in Pd, nafion and WO3 during 
fabrication process? How do we know that H has been incorporated into the Pd films? Can 
phase/structure of PdHx or Hydrogen forward scattering (HFS) help? 
4. The relative humidity was 100% during the testing. Does it mean that the devices still somehow 
rely on water electrolysis? Would any change of the environment humidity affect the proton 
conductivity of the device? Would the device work well when the test chamber contains hydrogen 
only? What is the chamber pressure when the devices were tested? 
5. In Figure 2b-d, the conductance updating property of the device at low conductance regime 
shows a better symmetry than that at middle and high conductance regime. Why does the 
symmetry of the conductance updating become worse when the concentration of hydrogen 
increases? Why there is still an asymmetrical updating in low conductance regime? Since the 
symmetry of conductance updating with 200 pulses looks better than that with 40 pulses and 100 
pulses in low conductance regime, would the symmetry be affected by pulse number? 
6. In Figure 3b, is there a plausible explanation for that the high resistive state of the device 
becomes more insulative and low resistive state of the device becomes more conductive after large 
amounts of cycling? 
7. on Page 7, there is a typo, “As demonstrated in Figure 1a, over seven orders …” should be 
Figure 2a. 
8. on Page 9 about Figure 2a “…after 100 positive gating pulses, the conductance of the channel 
increased from 1 μS to 22 μS, ...” it's a bit confusing here, what is the pulsing sequence, from 100 
down counting to 1 in Figure 2b? please use arrows to label them in the figure. 
9. The specific pulse height-width that was used to achieve each conductance regime (low, 
medium and high) should be clearly stated in the caption of Figure 2 or the main text. 
10. Endurance to the level of million has not been demonstrated (and not really expected for this 
paper as well). Is that due to testing time or failure? Please comment on possible failure mode? 
11. As the author mentioned, “When the external circuit is cut off (gate is open circuit), the 
electronic insulating nature of the electrolyte prevents the back flow of electrons, and 
consequently that of protons too.” Therefore, a two-terminal threshold volatile switch was used to 
provide both selector and threshold functions for programming (e.g. NatMat 16, 396, 2017), 
without which an array of such devices may only be programmed column-wise or row-wise (not 
randomly and individually programming a device in the array during learning). Will the device 
studied here also need such as threshold switch on the gate? 
12. A constant current source (compared to a voltage source) is usually not preferred in real 
circuits due to various reasons.