In Situ Water Electrolyzer Stack for an Electrobioreactor
Round 1
Reviewer 1 Report
Manuscript Number: energies-498603
The manuscript entitled “In situ water electrolyzer stack for electrobioreactor” by Givirovskiy et al. describes the realistic application of water electrolyzer stacks for electrobioreactors. Overall, the manuscript is well written and interesting to the broader readership in energies journal. However the following concerns should be cleared prior to consider this manuscript for acceptance in Energies Journal.
1. Reviewers understand the authors intention of showing the real time performance of stacks and electrobioreactor, however, there is a lack of materials description and detail experimental procedure is lacking. The main important criteria in the research publication is giving the as much as details to the readers for the reproducibility of the work
2. Authors should be clearly explained what kind of electrode materials used and how did the stack assembled what kind of separators used etc.
3. This looks like a report rather than a scientific article.
4. If the materials are synthesized by authors, then the detail synthesis procedure as well as physico-chemical characterizations should be provided as the supporting information
5. How about the long-time run and durability of these reactors?? Did the authors study the post-mortem analysis of these electrodes after run? This analysis is very important to prove the real time application of these devices
6. Language needs to be polished well. Some part of manuscript cannot be read with pleasure
Author Response
Dear Sir or Madam,
Thank you for the review of our manuscript. The detailed answer to your comments is included in the attached response letter. We are looking forward to hearing from you soon and will be happy to discuss our paper further whenever needed.
Best regards,
Authors of the manuscript
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this paper, a conceptual design for scale up of a bioelectrochemical system (BES) was presented based on water electrolysis and CO2 fixation by hydrogen-oxidizing bacteria. The BES consists of a bioreactor and an electrolyzer stack and the medium solution is circulated between two units via a pump. The performance of the electrolyzer was evaluated based on calculation of the voltage-current curves. The effect of electrode distance on the cell performance was analyzed by using a model. In contrast to the alkaline solution in a conventional alkaline electrolyzer, the culture medium exhibited a low conductivity and hence a high resistance. As a result, a substantial overcharge gave a low hydrogen efficiency (<50%). However, the design provides a possible prototype for BES scale up.
L45:” The highest reported average H2 conversion efficiencies are below 80%”. Does it refer to the conversion efficiency of electrical energy to hydrogen in water electrolysis? Or is it the conversion efficiency of hydrogen to biomass?
Eq. 4 needs a conversion factor of hydrogen volume and mass.
L115: “the simplified construction substantially decreases the internal resistance of the whole system,” This statement needs rewording. In contrast to a PEM electrolyzer, the membraneless structure increases the distance between electrodes, which increases the internal resistance, especially in a dilute electrolyte solution such as a culture medium.
Fig.1. What was the hydrogen evolution rate before inoculation? What was the specific growth rate before the outlet hydrogen became zero?
Fig. 2. If the microbes could not use all hydrogen and oxygen generated in the electrolyzer, would a gas mixture formed in the electrolyzer? It could be a safety issue because of the explosive gas mixture.
Fig. 2. Would electrode fouling be a problem?
L177: “No membrane is needed to separate hydrogen and oxygen gases in opposite to the traditional alkaline electrolyzer stack”. A barrier is used in alkaline electrolyzer, which is different from the membrane in a PEM electrolyzer.
L224: “The main electrolyzer stack efficiency parameters have been calculated based on the collected current–voltage relationships”. Is it possible to measure the hydrogen evolution rate? The culture medium solution may have a current efficiency different from pure water.
Table 1. The efficiency (<50%) is lower than the normal efficiency of commercial electrolyzers. What would the efficiency be if the culture medium were replaced with an alkaline solution?
L249:” a bioreactor with volume of 1000 L with specific production rate of 0.2 g/(L h)”. Is this production rate achieved in a small BES? From Figure 1, the productivity was ca. 0.01 g/(L.h).
Author Response
Dear Sir or Madam,
Thank you for the review of our manuscript. The detailed answer to your comments is included in the attached response letter. We are looking forward to hearing from you soon and will be happy to discuss our paper further whenever needed.
Best regards,
Authors of the manuscript
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Authors improved the manuscript. Manuscript in its present form may consider for publication
