Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Based on the current trends in hydrogen production from biomass by fermentation, few questions could arise:

1.     How does the collaboration between institutions impact the progress of research in this field?

2.     How has the publication landscape evolved over time in terms of the number of articles, journals, and citation impact?

3.     How are different substrates such as lignocellulosic biomass, glycerol, sludge, and wastewater being utilized for hydrogen production, and what are their respective advantages and challenges?

4.     What are the key microbial species involved in hydrogen production through fermentation, and how do they interact within the fermentation environment?

5.     What are the potential environmental benefits and challenges associated with scaling up hydrogen production via fermentation on an industrial level?

6.     What are the upcoming opportunities for innovation and collaboration in this field, and how can they be leveraged to accelerate progress towards a low-carbon economy?

7.       How does hydrogen production from biomass fermentation compare to other renewable energy sources in terms of sustainability and environmental impact?

Comments on the Quality of English Language

Minor editing required

Author Response

Reviewer #1:

Thank you for your comments and contributions. The article has significantly improved after the modifications. The responses to all inquiries are provided below and have been marked in yellow in the manuscript.

Comments: Based on the current trends in hydrogen production from biomass by fermentation, few questions could arise:

1. How does the collaboration between institutions impact the progress of research in this field?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

“Collaboration between institutions can significantly impact the progress of research in fermentation for hydrogen production, as shown by the bibliometric analysis that highlights, among other aspects, collaboration agreements between different institutions and significant partnerships [28,52,54,58]. These collaborative networks are valuable for accumulating knowledge and resources, bringing a diversity of approaches to the field, where different institutions can contribute with specific focuses, from microbiology to energy policies, enriching research by addressing hydrogen production from various perspectives [215–217]. A bibliometric review, evaluating many articles, suggests that contributions from multiple institutions can broaden the database available for analysis, providing a more comprehensive view of the current state and trends in research [52,54,55]. Thus, identifying prominent countries, institutions, and authors makes it easier to visualize international collaboration, mapping the closest relationships that allow interaction between researchers from different parts of the world to accelerate the dissemination of knowledge and the implementation of discoveries globally [28,218–220]. Furthermore, collaboration also facilitates the transfer of technology between institutions, accelerating the practical application of scientific discoveries, especially when there is effective interaction between academic and industrial researchers, and can lead to an improvement in the quality of studies by allowing different institutions to approach gaps in knowledge [42,55,218,221,222]. Such interaction promotes technological innovation, with the combination of skills and resources leading to the development of new techniques, methodologies or technologies that drive the advancement of hydrogen production [182,223,224].”

2. How has the publication landscape evolved over time in terms of the number of articles, journals, and citation impact?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

"The scenario of publications in hydrogen production through biological fermentation has shown significant evolution. This is reflected by the significant increase in the number of articles published, the expansion of journals that cover the topic, and the growing impact of these publications, expressed by the volume of citations. This field of study began with a modest number of publications in the initial years of research but has seen impressive growth, with a peak of 371 articles in 2022 and 235 articles by the last database update in 2023. These results draw attention to the increasing importance attributed to research in the area of fermentative hydrogen [156,171,210]. Furthermore, the number of journals publishing work related to this topic reached 541, accumulating an impressive total of 181,769 citations. This diversification in publications and the collaboration between different research groups highlight the multidisciplinary nature of the field and a continuous effort for innovation, strategies, and advances in knowledge about hydrogen production via fermentative processes [156,210,211]. This quantitative increase also signals an increase in interest and study of the topic, attracting the attention of the scientific and industrial community to anaerobic fermentative processes [23,212,213].”

3. How are different substrates such as lignocellulosic biomass, glycerol, sludge, and wastewater being utilized for hydrogen production, and what are their respective advantages and challenges?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

" In summary, we see that different substrates, such as lignocellulosic biomass, glycerol, sludge, and wastewater, can be used in the production of fermentative hydrogen, and this reflects efforts towards more sustainable and economically attractive raw materials, taking advantage of industrial waste and by-products. [189,225–227]. Lignocellulosic biomass is valued for its abundance and sustainability, transforming waste into energy and minimizing dependence on fossil fuels [66,102,212]. However, it faces challenges such as the need for pre-treatments to obtain sugars and the complexity of conversion processes [212,228]. Glycerol, derived from biodiesel production, stands out for its availability and reduced cost, offering an opportunity to value an industrial byproduct. However, the glycerol's purity and the catalysts' efficiency represent challenges [61,226,228]. In turn, using sludge and wastewater, treated by processes such as anaerobic digestion, not only solves waste management problems but also generates renewable energy, facing challenges in optimizing biogas production and infrastructure costs [46,70,79,229].”

4. What are the key microbial species involved in hydrogen production through fermentation, and how do they interact within the fermentation environment?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

“Furthermore, several microbial species are involved in transforming organic substrates in fermentative processes to produce hydrogen [65,76,187,230]. Knowing that such processes can occur in the absence or presence of light, known as dark fermentation and photo-fermentative fermentation, respectively [44,225,231]. In dark fermentation, anaerobic microorganisms, such as species of the genus Clostridium (eg Clostridium butyricum, Clostridium cellolosi, and Clostridium acetobutylicum), play a fundamental role, in converting sugars and other organic compounds into hydrogen, in addition to generating organic acids (eg lactic acid, acetic acid, and butyric acid) and alcohols [185,232]. Another important genus is Enterobacter, with species such as Enterobacter aerogenes also contributing to hydrogen production under anaerobic conditions [63,232,233]. Photofermentative fermentation depends on solar energy to generate hydrogen, with phototrophic bacteria such as Rhodobacter sphaeroides and Rhodopseudomonas palustris leading this process [234–236]. These microorganisms work by capturing solar energy, allowing the transformation of organic substrates into hydrogen [232,235]. Within the fermentation environment, microbial interactions are complex and directly influence hydrogen production efficiency [44,153,185,231]. Competition for substrates between different microorganisms can affect the distribution of resources. At the same time, syntrophic associations, in which the metabolic products of one species serve as substrates for another, can optimize the hydrogen production process [65,83,119,182,205].”

5. What are the potential environmental benefits and challenges associated with scaling up hydrogen production via fermentation on an industrial level?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

“Fermentative hydrogen production is seen as a sustainable alternative, with advantages such as organic waste and low greenhouse gas emissions, contributing to waste management and the transition to a low-carbon economy [44,199,237]. However, it faces significant challenges, including the need for large amounts of biomass to compete with food production, impact ecosystems through land conversion, and increase water consumption [119,153,185,231]. Furthermore, the efficiency and competitiveness of the process, the management of by-products, and the impact on biodiversity are relevant concerns [156,185,187,205]. An integrated approach and sustainable practices at all stages are essential to maximize environmental benefits and effectively contribute to more sustainable energy sources and climate change mitigation [44,193,203,237].”

6. What are the upcoming opportunities for innovation and collaboration in this field, and how can they be leveraged to accelerate progress towards a low-carbon economy?

Answer: Thank you for your comments and contributions. We created the "6. Overview" section and added the information to the manuscript.

“To achieve a low-carbon economy, innovations and collaborations are required on several fronts to accelerate development in this field of research. Improvements in engineering technologies are essential to increase efficiency and reduce costs, while integration with renewable energies such as solar and wind promotes more sustainable systems [63,235]. The efficient use of renewable raw materials (eg agricultural and forestry residues), and interdisciplinary collaboration between research, industry, and government are fundamental to overcoming challenges [44,185,238]. Incentive policies, investments in infrastructure, and specialized education are essential to support the energy transition [239,240]. Public-private collaboration and the internationalization of research are vital to promoting advances [63,191,241,242]. Thus, exploring these opportunities can accelerate innovation in hydrogen production through fermentation and contribute to a more sustainable economy [23,242,243].”

 

 

7. How does hydrogen production from biomass fermentation compare to other renewable energy sources in terms of sustainability and environmental impact?

Answer: Thank you for your comments and contributions. We corrected and added the information in section “4. Overview of hydrogen produced by fermentation”.

“In contrast to conventional methods, which often involve energy-intensive processes and significant carbon emissions, hydrogen fermentation is a promising alternative with lower environmental impact [99] and geographic flexibility. Factors such as geographic location or availability of specific resources do not become limiting factors compared to other energy sources (e.g., solar and wind), as fermentation offers greater adaptability and can be implemented in different regions [101]. This contributes to the decentralization of energy production and provides a circular approach, reusing organic waste and transforming it into a valuable, clean, renewable energy source.”

 

We thank you again for the important observations and believe that your revisions have significantly enhanced the scientific potential of the article.

Sincerely,

Reviewer 2 Report

Comments and Suggestions for Authors

This is an interesting bibliometric analysis of recent publications related to biological hydrogen production from biomass, accompanied by a brief review indicating the most important aspects of current research. Although I am not directly involved in such research at the moment, I found it very interesting and informative, and I think other researchers would also find it useful. As such, if the editors consider bibliometrics analysis to be in scope for this journal, I believe that this work is suitable for publication.

There are a few minor issues that should be addressed. There is a slight error in lines 59-60, where it is stated that 'combustion of fossil hydrocarbons produces only water and energy'; I presume this should read 'combustion of hydrogen'. Text in line 135 states that there were 5 relevant publications in 2003, but Figure 2 shows only 4. Also, the meaning of Figure 3 was not clear to me (perhaps just my own unfamiliarity with this type of analysis); what do the colours in 3A indicate, and what do the lines connecting journal represent? Are these citations of one journal by another? I think a little more explanation might be useful, as other readers may also find it confusing. In line 219 (caption to Figure 3) I presume that 'newspapers' should read 'journals'.

In line 282, I was not sure what 'citations per equivalent' means; should this read 'citations per year'? In Table 2, commas are used to indicate decimal points (eg '66,91') whereas in the main text full stops are used (eg '66.91'). I presume full stops should be used. In Table 3, 'TLs' in the column headings should read 'TLS' as the caption indicates that it stands for Total Link Strength. In Table 4, it might be useful to add divisions between the word clusters, to make the separation clearer.

Lines 412-413 are slightly misleading, in that lignin can not be broken down into sugars (as far as I am aware). In lines 591-592, I was not sure what is meant by 'high energy yield and low calorific value'; isn't the calorific value essentially the same as the energy yield? Apologies if I misunderstand. 

Section 4 is essentially a literature review, and some parts of it repeat material which was covered in Section 1 and elsewhere; for example, lines 381-388, 474-480, 575-588, 632-637. These parts could perhaps be deleted or shortened to reduce length and improve flow, though there is some new information and relevant reference citations.

There are some minor inconsistencies in formatting which can be addressed during editing; for example, headings 4 (line 380) and 5 (line 764) should presumably be bold, and heading 3.3 (line 307) should be italic. Also there are two sections numbered 4.1, so the second and all subsequent parts of section 4 need to be renumbered.

Comments on the Quality of English Language

The quality of English is generally excellent, except that there are a few incomplete sentences: eg lines 298-299, 337-338, 343-345.

Also a few statements are rather vague or almost meaningless; for example in line 278 'fermentation is unique among biological processes.' But generally the language is very clear and easy to follow.

Author Response

Reviewer #2:

Comments: This is an interesting bibliometric analysis of recent publications related to biological hydrogen production from biomass, accompanied by a brief review indicating the most important aspects of current research. Although I am not directly involved in such research at the moment, I found it very interesting and informative, and I think other researchers would also find it useful. As such, if the editors consider bibliometrics analysis to be in scope for this journal, I believe that this work is suitable for publication.

Answer: Thank you for your comments and contributions. We want to express our gratitude for recognizing the scientific potential and considering the article for publication in the Journal Biomass. The article has significantly improved after the modifications. The responses to all inquiries are provided below and marked in green in the manuscript.

 

Comments: There are a few minor issues that should be addressed. There is a slight error in lines 59-60, where it is stated that 'combustion of fossil hydrocarbons produces only water and energy'; I presume this should read 'combustion of hydrogen'.

Answer: Thank you for your comments and contributions. We corrected the sentence.

“Furthermore, hydrogen combustion produces only water and energy [24].”

Comments: Text in line 135 states that there were 5 relevant publications in 2003, but Figure 2 shows only 4.

Answer: Thank you for your comments and contributions. We corrected the sentence.

“As can be seen in Figure 2, the number of publications related to fermentative hydrogen production has shown remarkable growth, as in 2001 and 2003 only three and four publications were recorded, respectively, representing the lowest values within the period selected for investigation.”

Comments: Also, the meaning of Figure 3 was not clear to me (perhaps just my own unfamiliarity with this type of analysis); what do the colours in 3A indicate, and what do the lines connecting journal represent? Are these citations of one journal by another? I think a little more explanation might be useful, as other readers may also find it confusing.

Answer: Thank you for your comments and contributions. We made the corrections and added the information related to Figure 3.

“Figure 3A illustrates the interconnection between journals with at least five publications, using lines that indicate the Total Link Strength (TLS) and Clusters to demonstrate this relationship. TLS indicates the degree of connection between two or more distinct journals, while clusters identify each journal by a specific color node. INTERNATIONAL JOURNAL OF HYDROGEN ENERGY stands out for being the journal with the largest number of publications on the topic and for its significant collaboration with other journals, thus positioning itself in the center of the figure with a considerably thicker line. This indicates a high Total Link Strength, highlighting the INTERNATIONAL JOURNAL OF HYDROGEN ENERGY extensive network of collaborations, mainly with journals such as BIORESOURCE TECHNOLOGY, RENEWABLE ENERGY, FUEL, RENEWABLE SUSTAINABLE ENERGY REVIEWS, JOURNAL OF CLEANER PRODUCTION and BIOMASS BIOENERGY. Figure 3B illustrates how these relationships became closer over time.”

Comments: In line 219 (caption to Figure 3) I presume that 'newspapers' should read 'journals'.

Answer: Thank you for your comments and contributions. We corrected the legend of Figure 3.

 “(B) a Temporal map of the number of journals that published the most in the area.”

Comments: In line 282, I was not sure what 'citations per equivalent' means; should this read 'citations per year'?

Answer: Thank you for your comments and contributions. We corrected the sentence.

“Secondly, the article “Hydrogen production from renewable and sustainable energy resources: A promising green energy carrier for clean development”, published in 2016, had an average of 157.5 citations per year.”

Comments: In Table 2, commas are used to indicate decimal points (e.g. '66,91') whereas in the main text full stops are used (e.g. '66.91'). I presume full stops should be used.

Answer: Thank you for your comments and contributions. We corrected Table 2, in the manuscript.

Comments: In Table 3, 'TLs' in the column headings should read 'TLS' as the caption indicates that it stands for Total Link Strength.

Answer: Thank you for your comments and contributions. We corrected Table 3, in the manuscript.

Comments: In Table 4, it might be useful to add divisions between the word clusters, to make the separation clearer.

Answer: Thank you for your comments and contributions. We corrected Table 4. We added spaces between groups of words to make the separation more straightforward.

Comments: Lines 412-413 are slightly misleading, in that lignin can not be broken down into sugars (as far as I am aware).

Answer: Thank you for your comments and contributions. We corrected the sentence.

“They have an advantage over competitors because they contain a large amount of cellulose, which microorganisms can break down to release sugars that can be fermented [103].”

Comments: In lines 591-592, I was not sure what is meant by 'high energy yield and low calorific value'; isn't the calorific value essentially the same as the energy yield? Apologies if I misunderstand.

Answer: Thank you for your comments and contributions. We corrected the sentence.

“The main advantages perceived by these industries are its low calorific value (120 MJ/Kg), making green hydrogen a more efficient fuel than fossil fuels based on non-renewable hydrocarbons [156,158].”

Comments: Section 4 is essentially a literature review, and some parts of it repeat material which was covered in Section 1 and elsewhere; for example, lines 381-388, 474-480, 575-588, 632-637. These parts could perhaps be deleted or shortened to reduce length and improve flow, though there is some new information and relevant reference citations.

Answer: Thank you for your comments and contributions. We corrected the lines provided.

1) Lines 381-388 were corrected, and their parts were shortened.

“Hydrogen production is carried out by anaerobic bacteria [90] that decompose organic matter [91,92], generating molecular hydrogen (H₂) as one of the byproducts [93]. This process, which does not require oxygen, allows the collection of hydrogen, which is then compressed, purified, and stored for future use [94–96].”

2) Lines 474-480 were corrected, and their parts were shortened.

“Generating hydrogen through biological means is seen as one of the most promising approaches for the future. Despite the absence of industrial facilities specialized in biological hydrogen production to date, it is important to highlight the significant growth in research in this field [138].”

3) Lines 575-588 were corrected, and their parts were shortened.

“Due to the scarcity of energy resources and the rise in prices of foods  [135,156,158–160] such as corn and sugar, research is looking for new renewable sources of energy  [156,159]. One approach is to convert agricultural waste into hydrogen through sustainable methods such as fermentation. This has led to the adoption of green hydrogen in industries such as steel and agriculture as an energy alternative [157,161,162,162–164].”

4) Lines 632-637 were corrected, and their parts were shortened.

“The technological evolution in the production of renewable energy for the sustainable production process of biofuels can be one of the most interesting alternatives to satisfy the planet's energy needs as much as possible [165–169].”

Comments: There are some minor inconsistencies in formatting which can be addressed during editing; for example, headings 4 (line 380) and 5 (line 764) should presumably be bold, and heading 3.3 (line 307) should be italic. Also there are two sections numbered 4.1, so the second and all subsequent parts of section 4 need to be renumbered.

Answer: Thank you for your comments and contributions. We made corrections in titles 4 (line 380) and 5 (line 764) and in title 3.3 (line 307). We also made all the corrections in section 4, which was divided into two subsections “4.1.1 Main biomasses used” and “4.1.2 Fermentative processes”.

Comments on the Quality of English Language

Comments: The quality of English is generally excellent, except that there are a few incomplete sentences: eg lines 298-299, 337-338, 343-345.

Answer: Thank you for your comments and contributions. We made corrections to the manuscript.

Comments: Also a few statements are rather vague or almost meaningless; for example in line 278 'fermentation is unique among biological processes.' But generally the language is very clear and easy to follow.

Answer: Thank you for your comments and contributions. We corrected the sentence.

“As a result, they argue that fermentation is among the most widely applied biological processes for hydrogen production [71].”