A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
Round 1
Reviewer 1 Report
The manuscript titled "A review of hydrogen sensors for ECLSS: fundamentals, recent advances, and challenges" by Chenghao Jia et al. provided a comprehensive overview of different types of hydrogen sensors, their structures, sensing mechanisms, and performance. The manuscript also discusses the latest research advancements and challenges associated with the development of hydrogen sensors for the environmental control and life support subsystem of the space station. Overall, I found the manuscript to be well-written, organized, and informative e.g. covered various types of sensors, including catalyst combustion, heat-conduction, semiconductor, fiber optic, etc., and provided detailed information about their structures, sensing mechanisms, and performance. The manuscript is highly relevant to the field of space station environmental control and life support subsystems (ECLSS), where the development of hydrogen sensors with high accuracy, fast response time, long calibration period, and good stability is critical. However, However, there are a few areas that could be improved as listed below.
1.In the section on semiconductor hydrogen sensors, various semiconducting materials have been actually investigated, but only metal oxides were mentioned. The authors should consider including information on other semiconducting materials, such as nitrides (https://link.springer.com/article/10.1007/s12613-020-2143-8) (https://www.sciencedirect.com/science/article/pii/S0925400518314497), sulfides (https://www.sciencedirect.com/science/article/pii/S0169433219337985) , and others, to provide a more comprehensive overview of the current research status and to allow the reader to better understand the potential advantages and disadvantages of different materials.
2. The authors should consider breaking the section into smaller paragraphs to improve its readability and flow. Additionally, some of the sentences can be rephrased to improve clarity. For example, the sentence "Most of the hydrogen sensors have this problem, as both temperature and humidity affect the response signal of the sensor" could be rephrased as "The response signal of most hydrogen sensors is affected by temperature and humidity, resulting in cross-sensitivity."
3. In terms of reducing cross-sensitivity, the authors could provide more specific suggestions on how to implement temperature and humidity compensation and surface modification of the hydrogen-sensitive material. For example, they could suggest specific materials that have been shown to be effective at reducing cross-sensitivity, or techniques for implementing temperature and humidity compensation. Additionally, they could discuss the trade-offs between different methods for reducing cross-sensitivity, and identify areas where further research is needed.
4. Regarding the development of new hydrogen-sensitive materials, the authors could provide more specific recommendations on which types of materials are most promising for improving sensor performance, and how these materials could be incorporated into existing sensor designs. They could also discuss the challenges associated with introducing new materials, such as the need to optimize the synthesis and fabrication processes to ensure consistent performance across different sensors.
Author Response
Point #1
COMMENT: In the section on semiconductor hydrogen sensors, various semiconducting materials have been actually investigated, but only metal oxides were mentioned. The authors should consider including information on other semiconducting materials, such as nitrides (https://link.springer.com/article/10.1007/s12613-020-2143-8) (https://www.sciencedirect.com/science/article/pii/S0925400518314497), sulfides (https://www.sciencedirect.com/science/article/pii/S0169433219337985), and others, to provide a more comprehensive overview of the current research status and to allow the reader to better understand the potential advantages and disadvantages of different materials.
RESPONSE: Thanks for your valuable comment. Metal oxides are often the semiconductor material of choice for semiconductor hydrogen sensors. We highly agree with the reviewer's suggestion that more information on semiconductor materials is needed. Therefore, in this section, we have added descriptions of other semiconductor materials such as nitrides, sulfides, etc.
The revised part of the article is as follows (on Page 9, Lines 317-329):
In addition to metal oxides, semiconductor materials include sulfides, nitrides, etc. Gottam et al. [54] presented a highly sensitive hydrogen sensor composed of MoS2-Pt nanoparticle films as the active layer. For 100 ppm H2, the sensor achieved ultra-fast response and recovery rates of 4 s and 19 s, respectively. When exposed to 100 ppm H2, the MoS2-Pt composite film exhibited a high sensor response , which was superior to existing metal sulfide based sensors. Hermawan et al. [55] studied a GaN hydrogen sensor. The GaN hydrogen sensor exhibited high stability and sensitivity under high temperature environment. Hermawan et al. [56] studied a wide band gap aluminum nitride (AlN) sensor material, which can withstand high temperature environments. Three unique AlN morphologies (rod, nested, and hexagonal plate) were synthesized by direct nitridation at 1400 °C using -AlOOH as a precursor. The gas sensing performance showed that the hexagonal plate form exhibited good reproducibility and the highest response to 750 ppm H2 leakage at high temperature (500 °C) compared to the rod and nested forms.
Point #2
COMMENT: The authors should consider breaking the section into smaller paragraphs to improve its readability and flow. Additionally, some of the sentences can be rephrased to improve clarity. For example, the sentence "Most of the hydrogen sensors have this problem, as both temperature and humidity affect the response signal of the sensor" could be rephrased as "The response signal of most hydrogen sensors is affected by temperature and humidity, resulting in cross-sensitivity."
RESPONSE: Thank you for taking the time to review my work and providing valuable feedback. We have broken up the longer paragraphs into smaller ones to enhance the readability and flow of the article.
The revised part of the article is as follows (on Pages 20-22, Lines 653-718):
At present, the technology of hydrogen sensors at room temperature is more mature, and the research on sensitivity, response time, safety, lifetime, and other performance is also sufficient to meet the needs of the industry. With the continuous development of hydrogen energy in fuel cells, distributed power generation, and human spaceflight, the research of hydrogen sensors, as the core components of the future hydrogen energy Internet of Things (IoT), will gradually move towards miniaturization, integration and distributed sensing, and the need to achieve stable measurement under harsh environments such as high temperature and high humidity. Future research will focus on the following four points.
(1) Reduce the cross-sensitivity of temperature, humidity, and other interference factors, and achieve accurate measurement of hydrogen in different environments. The response signal of most hydrogen sensors is affected by temperature and humidity, resulting in cross-sensitivity. Some reducing gases are also absorbed by the hydrogen-sensitive material, which affects hydrogen detection. The cross-sensitivity of the sensor can be reduced by temperature and humidity compensation and surface modification of the hydrogen-sensitive material. The temperature and humidity compensation of the sensor mainly uses hardware compensation and software compensation methods. Hardware compensation includes bridge arm parallel resistance method, bridge arm thermistor compensation method, series parallel thermistor compensation method outside the bridge, etc. Hardware compensation is usually based on the test data of the sensor in high and low temperature environments, and the corresponding temperature compensation components are selected to compensate for the initial zero point and output sensitivity drift of the sensor. Common software compensation methods include interpolation, least squares fitting, BP neural network algorithm, etc. Software compensation is based on the real-time ambient temperature collected by the temperature measurement element, the sensor's zero point, sensitivity and temperature change algorithm is integrated into the high-speed computing microcontroller to achieve real-time temperature compensation of the sensor. The use of hardware correction can not be complex algorithm compensation and compensation circuit is also susceptible to the influence of electromagnetic environment, in contrast, the software compensation method has high accuracy, wide range of applications, simple debugging, and therefore become a common method of sensor temperature compensation.
(2) To develop new hydrogen-sensitive materials, the defects of common hydrogen-sensitive materials such as Pd and WO3 can be improved by introducing nanotechnology and adding new materials such as graphene. The research on hydrogen-sensitive materials has never stopped, and the selection of hydrogen-sensitive materials plays a decisive role in the performance of sensors in terms of sensitivity, measurement range, operating temperature, and lifetime. With the development of material science, more new materials will be applied to hydrogen-sensitive materials to improve sensor performance. New hydrogen-sensitive materials, including nanomaterials, hybrid materials, new carbon materials, two-dimensional materials, and metal-organic framework materials, are studied to improve their selectivity and shorten the response time; their sensing materials are gradually developed from traditional metal oxides such as SnO2, ZnO, and Fe2O3 in the early stage to new metal oxides such as WO3, MoO3, and In2O3. Among them, WO3, as an n-type semiconductor, has a wide range of applications in hydrogen sensing. However, it is difficult for WO3 monolithic materials to have the characteristics of fast response, low detection limit and wide detection domain at the same time, and it is necessary to prepare composite materials by physical mixing, loading, and doping to enhance their hydrogen-sensing performance. The future goal can be to develop hydrogen-sensitive materials with fast response, long-term stability, and good selectivity. Currently, Pd alloys have been used to reduce the response time and to increase the stability of hydrogen sensors. In addition, magnesium-titanium (Mg-Ti) and magnesium-titanium-nickel (Mg-Ti-Ni) alloy composite films have been made into sensors with good repeatability and stability. However, little research has been done on the selectivity of hydrogen sensors. Moreover, regardless of the measurement method used, the response time is still quite long (tens of seconds and in some cases even several minutes or hours), therefore, further research on new sensitive materials is needed.
(3) Research on the applicability of hydrogen sensors at high temperatures. The current commercialization of several hydrogen sensors have a narrow operating temperature range, most of them can only be measured at room temperature, high-temperature environments require higher accuracy and life of the sensor compared to room temperature, so the optimization of signal processing and packaging technology should be one of the directions of future research. For example, the performance of fiber-optic hydrogen sensors at high temperatures can be improved by adjusting the internal structure and external packaging of the fiber.
(4) Study the signal detection and demodulation technology of fiber-optic sensors. The excellent performance of fiber optic hydrogen sensors facilitates temperature compensation and distributed multi-parameter measurement, which is conducive to the research and development of intelligent sensing systems in the context of the Internet of Things, but it also poses new challenges to the signal detection and demodulation technology of fiber optic sensors. Currently, the price of a single-channel grating demodulator is over 10,000, and the price of a multi-channel grating demodulator and spectrum analyzer is even higher, which limits the commercialization of fiber-optic hydrogen sensors. Therefore, the development and optimization of software and hardware systems for signal detection and demodulation will be the focus of future research.
Point #3
COMMENT: In terms of reducing cross-sensitivity, the authors could provide more specific suggestions on how to implement temperature and humidity compensation and surface modification of the hydrogen-sensitive material. For example, they could suggest specific materials that have been shown to be effective at reducing cross-sensitivity, or techniques for implementing temperature and humidity compensation. Additionally, they could discuss the trade-offs between different methods for reducing cross-sensitivity, and identify areas where further research is needed.
RESPONSE: We very much agree with this valuable comment you made. We have added in this paragraph how to improve the effect of temperature and humidity on the sensor. Several special materials are described that reduce cross-sensitivity.
The revised part of the article is as follows (on Page 21, Lines 661-680):
Reduce the cross-sensitivity of temperature, humidity, and other interference factors, and achieve accurate measurement of hydrogen in different environments. The response signal of most hydrogen sensors is affected by temperature and humidity, resulting in cross-sensitivity. Some reducing gases are also absorbed by the hydrogen-sensitive material, which affects hydrogen detection. The cross-sensitivity of the sensor can be reduced by temperature and humidity compensation and surface modification of the hydrogen-sensitive material. The temperature and humidity compensation of the sensor mainly uses hardware compensation and software compensation methods. Hardware compensation includes bridge arm parallel resistance method, bridge arm thermistor compensation method, series parallel thermistor compensation method outside the bridge, etc. Hardware compensation is usually based on the test data of the sensor in high and low temperature environments, and the corresponding temperature compensation components are selected to compensate for the initial zero point and output sensitivity drift of the sensor. Common software compensation methods include interpolation, least squares fitting, BP neural network algorithm, etc. Software compensation is based on the real-time ambient temperature collected by the temperature measurement element, the sensor's zero point, sensitivity and temperature change algorithm is integrated into the high-speed computing microcontroller to achieve real-time temperature compensation of the sensor. The use of hardware correction can not be complex algorithm compensation and compensation circuit is also susceptible to the influence of electromagnetic environment, in contrast, the software compensation method has high accuracy, wide range of applications, simple debugging, and therefore become a common method of sensor temperature compensation.
Point #4
COMMENT: Regarding the development of new hydrogen-sensitive materials, the authors could provide more specific recommendations on which types of materials are most promising for improving sensor performance, and how these materials could be incorporated into existing sensor designs. They could also discuss the challenges associated with introducing new materials, such as the need to optimize the synthesis and fabrication processes to ensure consistent performance across different sensors.
RESPONSE: Your suggestions would be very helpful in improving the quality of the paper. We have added in this paragraph a narrative about the development of new hydrogen-sensitive materials to improve the performance of sensors and how these materials can be incorporated into existing sensor designs. We can also discuss the challenges associated with the introduction of new materials.
The revised part of the article is as follows (on Page 21, Lines 681-702):
To develop new hydrogen-sensitive materials, the defects of common hydrogen-sensitive materials such as Pd and WO3 can be improved by introducing nanotechnology and adding new materials such as graphene. The research on hydrogen-sensitive materials has never stopped, and the selection of hydrogen-sensitive materials plays a decisive role in the performance of sensors in terms of sensitivity, measurement range, operating temperature, and lifetime. With the development of material science, more new materials will be applied to hydrogen-sensitive materials to improve sensor performance. New hydrogen-sensitive materials, including nanomaterials, hybrid materials, new carbon materials, two-dimensional materials, and metal-organic framework materials, are studied to improve their selectivity and shorten the response time; their sensing materials are gradually developed from traditional metal oxides such as SnO2, ZnO, and Fe2O3 in the early stage to new metal oxides such as WO3, MoO3, and In2O3. Among them, WO3, as an n-type semiconductor, has a wide range of applications in hydrogen sensing. However, it is difficult for WO3 monolithic materials to have the characteristics of fast response, low detection limit and wide detection domain at the same time, and it is necessary to prepare composite materials by physical mixing, loading, and doping to enhance their hydrogen-sensing performance. The future goal can be to develop hydrogen-sensitive materials with fast response, long-term stability, and good selectivity. Currently, Pd alloys have been used to reduce the response time and to increase the stability of hydrogen sensors. In addition, magnesium-titanium (Mg-Ti) and magnesium-titanium-nickel (Mg-Ti-Ni) alloy composite films have been made into sensors with good repeatability and stability. However, little research has been done on the selectivity of hydrogen sensors. Moreover, regardless of the measurement method used, the response time is still quite long (tens of seconds and in some cases even several minutes or hours), therefore, further research on new sensitive materials is needed.
Special Thanks for your advice and best regards.
Reviewer 2 Report
The authors investigated the operational principles and characteristics of catalyst combustion and heat conduction in hydrogen sensors. This was done considering the practical application requirements of hydrogen sensors in space stations and physical and chemical regeneration systems. The paper briefly described the employment of semiconductor and fibre-optic innovations. The manuscript's topic is interesting. However, the manuscript may be suitable for publishing after a major revision.
The following suggestions have been made to improve the manuscript:
1- The manuscript should be carefully checked and corrected for grammatical, typographical, and punctuation errors.
2- Please consider revising the manuscript title, avoiding the use of abbreviations.
3- The abstract section looks like part of an introduction, so rephrasing it to highlight the most critical obtained results is better.
4- The abbreviation in the manuscript's text should be defined first, and then you can use the abbreviation only. In addition, some repetitive sentences should be deleted.
5- The rationale behind the authors' emphasis on palladium remains unclear despite the absence of explicit reference in the article's title. In addition, sensors that only detect hydrogen gas were the subject of the review. The statement of the author's choice of hydrogen gas only did not meet the criteria required. The idea of author/s choice of hydrogen gas only did not meet the standards required.
6- It is recommended that the author includes citations for recent work at the end of sentences that have not been cited.
7- Many cited works appear outdated, so an update to the list of references is advised.
Good luck.
The manuscript should be carefully checked and corrected for grammatical, typographical, and punctuation errors.
Author Response
Point #1
COMMENT: The manuscript should be carefully checked and corrected for grammatical,typographical, and punctuation errors.
RESPONSE: We are very grateful for the suggestion. Accordingly, We carefully checked and corrected grammatical, typographical and punctuation errors in the manuscript.
Point #2
COMMENT: Please consider revising the manuscript title, avoiding the use of abbreviations.
RESPONSE: Thanks for your valuable comment. "ECLSS" is an abbreviation for "Environmental Control and Life Support System". The article focuses on a review of the fundamentals, recent advances and challenges of hydrogen sensors for ECLSS, which is recognized as a popular acronym in the field of manned spaceflight. The title of the paper is "A review of hydrogen sensors for ECLSS: fundamentals, recent advances, and challenges" in order to highlight the characteristics of the field and object of study. To make it easy for the reader to understand ECLSS, we introduce its full name in the introduction section.
Point #3
COMMENT: The abstract section looks like part of an introduction, so rephrasing it to highlight the most critical obtained results is better.
RESPONSE: We are very grateful for the suggestion. We rewrote the Abstract section to highlight the most critical findings.
The revised part of the article is as follows (on Page 1, Lines 9-21):
The development of hydrogen sensors with high detection accuracy, fast response time, long calibration period, and good stability has become the focus of the space station environmental control and life support subsystem. We analyze the current research status of different types of hydrogen sensors, including catalyst combustion type, heat-conduction type, semiconductor type, fiber optic type, etc. The response signals of most hydrogen sensors are affected by temperature and humidity, resulting in cross-sensitivity. Reducing the cross-sensitivity of temperature, humidity, and other interfering factors to achieve accurate hydrogen measurement in different environments is a challenge that limits the development of hydrogen sensors. Several hydrogen sensors currently commercially available have a narrow operating temperature range, most of them can only measure at room temperature, and the high-temperature environment requires higher accuracy and a lifetime of the sensor than room temperature. Many new hydrogen-sensitive materials have been developed to improve the performance of the sensors. The excellent performance of fiber optic hydrogen sensors is beneficial to temperature compensation and distributed multiparameter measurement and to the research and development of intelligent sensing systems in the context of the Internet of Things. The signal detection and demodulation techniques of fiber optic sensors are the focus of future hydrogen sensor research.
Point #4
COMMENT: The abbreviation in the manuscript's text should be defined first, and then you can use the abbreviation only. In addition, some repetitive sentences should be deleted.
RESPONSE: We are very grateful for the suggestion. Accordingly, w We defined the abbreviations in the body of the manuscript first and, in addition, removed some repetitive sentences.
The revised part of the article is as follows (on Page 2, Lines 57-58; Lines 73-7):
- The catalyst used in this type of sensor is metallic Palladium (Pd), which has two disadvantages.
- Using a sensitive element, a compensating element, and a fixed resistor to form a bridge, the heat generated by the catalytic combustion of the combustible gas is conducted to the wrapped Platinum (Pt) coil, causing the resistance of the coil to rise and thus causing a change in voltage in the bridge path of the sensing signal.
Point #5
COMMENT: The rationale behind the authors' emphasis on palladium remains unclear despite the absence of explicit reference in the article's title. In addition, sensors that only detect hydrogen gas were the subject of the review. The statement of the author's choice of hydrogen gas only did not meet the criteria required. The idea of author/s choice of hydrogen gas only did not meet the standards required.
RESPONSE: We deeply appreciate the reviewer’s suggestion. Pd is widely used in the development of hydrogen sensors due to its excellent ability to absorb large amounts of hydrogen and its high selectivity for hydrogen. At the same time, Pd-based hydrogen sensors have also attracted the attention of researchers because of their simple device structure and preparation process, which has led to their rapid development. The article focuses on the characteristics of hydrogen sensors in environmental control and life support systems in manned spaceflight, and the current status of recent research is reviewed. In addition to Pd, the article also provides a review of other novel hydrogen-sensitive material.
Point #6
COMMENT: It is recommended that the author includes citations for recent work at the end of sentences that have not been cited.
RESPONSE: Thank you for taking the time to review my work and providing valuable feedback. We have added references to recent work at the end of the unquoted sentences.
Point #7
COMMENT: Many cited works appear outdated, so an update to the list of references is advised.
RESPONSE: Thank you for taking the time to review my work and providing valuable feedback. We have updated the latest references.
The revised part of the article is as follows (on Pages 22-26, Lines 720-922):
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Special Thanks for your advice and best regards.
Round 2
Reviewer 2 Report
After the authors have achieved all of the reviewers' notes, I recommend that the manuscript be accepted for publication.
