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Review

Main Trends and Research Directions in Hydrogen Generation Using Low Temperature Electrolysis: A Systematic Literature Review

by
Cristina Hora
,
Florin Ciprian Dan
*,
Nicolae Rancov
,
Gabriela Elena Badea
and
Calin Secui
Faculty of Energy Engineering and Industrial Management (ROMANIA), University of Oradea, Strada Universității 1, 410087 Oradea, Romania
*
Author to whom correspondence should be addressed.
Energies 2022, 15(16), 6076; https://doi.org/10.3390/en15166076
Submission received: 18 July 2022 / Revised: 5 August 2022 / Accepted: 18 August 2022 / Published: 22 August 2022
(This article belongs to the Collection Hydrogen Energy Reviews)
Browse Figures
Versions Notes

Abstract

:
Hydrogen (H2) is the most abundant element in the universe and it is also a neutral energy carrier, meaning the environmental effects of using it are strictly related to the effects of creating the means of producing of that amount of Hydrogen. So far, the H2 generation by water electrolysis research field did not manage to break the efficiency barrier in order to consider H2 production as a technology that sustains financially its self-development. However, given the complexity of this technology and the overall environmental impacts, an up-to-date research and development status review is critical. Thus, this study aims to identify the main trends, achievements and research directions of the H2 generation using pure and alkaline water electrolysis, providing a review of the state of the art in the specific literature. Methods: In order to deliver this, a Systematic Literature Review was carried out, using PRISMA methodology, highlighting the research trends and results in peer review publish articles over more than two years (2020–2022). Findings: This review identifies niches and actual status of the H2 generation by water and alkaline water electrolysis and points out, in numbers, the boundaries of the 2020–2022 timeline research.

1. Introduction

Taking into consideration the environmental impact of fossil fuel usage in regard to the climate change amplified by the international context (e.g., Ukraine 2022 war and its impacts on the energy market) a new star of the energy market emerged under the form of a clean energy carrier, H2. Hydrogen production by its primary energy source is split into fossil fuel and renewable sources. In the context of worldwide agreement on decarbonization the use of renewable energy is the preferred option. Renewable energy sources have the downfall of being unreliable from stability and constant point of view. Hence Hydrogen generation can benefit from the renewable overproduction or renewable output with a low cost. In the same way electricity is produced from other sources, the H2 is produced from different energy sources, but, for now, lacks the infrastructure to be used and deployed. Compared to electricity, the storage of H2 is simpler, cheaper and easy to cache under the form of a tank. Being the most abundant element in the universe, having the biggest energy per weight (out of common fuels, three times higher than gasoline) and the only conversion byproduct being water, makes this energy carrier the target of energy policies by funding and subsequent research. The commonly accepted yield of converting power to H2 to power again has the best round trip conversion efficiency of 46% [1]; therefore, we agree that we must follow closely the steps research takes in this direction. Following reviews conducted in 2018 [2] and 2019 [3], we are showing the results over the 2020–2022 period of published research.
H2 generation methods from fossil fuels are Hydrocarbon Pyrolysis and Hydrocarbon Reforming (Autothermal Reforming; Partial Oxidation, Steam Reforming) [3]. Out of Renewable Sources we see Water Splitting and Biomass Process with Biological (Bio-photolysis; Dark Fermentation; Photo Fermentation) and Thermochemical (Gasification; Pyrolysis; Combustion; Liquefication). Water splitting has three branches Alkaline, Solid Oxide and PEM [3], out of which Solid Oxide operates at higher temperatures (500–800 °C), up to 40% of energy can be supplied by heat and thus a high electric efficiency, Alkaline (60–90 °C) and PEM (25–80 °C) operating at temperatures under 100 °C [2,3]. Alkaline and PEM electrolysis received the focus of our review for their higher compatibility with renewable sources and ability to be deployed at residential level. As stated above, our focus will be on electrolysis, the electro-chemical process that consists of H2O splitting into Hydrogen and oxygen by using electrical energy [2]. In terms of electrolysis, Alkaline electrolysis has a higher degree of maturity, a slight increase in efficiency, 70–80%, and operates at temperatures between 60 °C and 90 °C with an electrolyte that has a pH higher than 7 [2]. On the other hand, PEM electrolysis operates with acidic electrolyte and high-cost Pt catalysts are needed for the reaction to take place at working temperatures between 25 °C and 80 °C, yielding 65% to 80% efficiencies [2]. In order to picture a distribution of Hydrogen generation processes, we show hereunder Figure 1.
The international mainstream trend for large green Hydrogen generation shows a preference for Alkaline electrolyzer with a 260 MW facility in Xinjiang, China, powered by a 300 MW PV field to be completed by 2023, and an already completed, fully operational in December 2021, 150 MW electrolyzer powered by 200 MW PV array in Baofeng, China. This Baofeng project overcame the 20 MW PEM Bécancour project in Canada. This exponential growth shows a political and economic commitment towards green Hydrogen.
The trend of large green Hydrogen generation plants is led by PV powered electrolyzers, as stated above, but we can see a great emphasis on wind powered Hydrogen generation, as well as hybrid PV-wind, PV-solar thermal or multi green energy sources coupled, combined with grid access or with no grid access. These technologies are gathering research focus in identifying the optimum combined green energy power source vs. electrolyzer size.

2. Materials and Methods

Acknowledging the complexity and interdisciplinarity of the area this study is addressing, an up-to-date review is needed at least once a year. Following the trend of [2] (a more general approach to the subject), [3] focusing only on PEMWE, and, more recently, [4]—2020—that focuses on electrode electrodeposited catalysts for PEMWE, we aim to identify the trends in the research and where we stand now with the published research.
Identification of some key characteristics in the cited studies, such as area of research, publishing journal, results, financial criteria, performance of technology deployed, etc., can stand as guidelines or starting point for researchers, investors, manufacturers and other stakeholders that operate in the H2 generation field.
The Systematic Literature Review (SLR) method was used for this review as per Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [5] in order to deliver answers to clearly formulated research questions in a way that is accepted by researchers worldwide. The PRISMA methodology applied to the cited papers points out opportunities and niches, allowing researchers and peers to address gaps and to formulate a clear research agenda.
As per PRISMA checklist, the objectives of this review are to identify the trend and what performances, during 2020 to 2022 timeline, publish research points out, in the H2 generation from H2O and from alkaline H2O using PEM and AEM.
The research questions that were raised are:
  • RQ1: What are the main characteristics and clusters that research focuses on?
  • RQ2: What are the main key performance indicators (KPIs) of the publish studies?
  • RQ3: How the KPIs have evolved over the last years?
We have a large gap between the status of the market available H2 generation technologies and the need to comply with the Kyoto Protocol [6] and Paris Agreement [7]; thus, the main contribution of this review is to highlight the actual research status and give a macro view of this field.
In order to reduce the risk of bias and subjectivity on the field and to allow other researchers to replicate, build over and continue our work we are following the PRISMA guidelines [5]. The PRISMA SLR methodology has a structured process that complies with the dissemination based on the standards stated above.
We structured our process on three large stages [8]: (i) Review the planning, (ii) Conducting the review, (iii) Reporting and dissemination of the results.
(i) In this stage we define the review focus and identify the boundaries of our research. As the next generation fuel, H2 is best suited in regard to the ease of generation and the clean output of its use (H2O). In view of this, we have set our goal on the methods of generating H2 [2]:
  • Have no pollution output;
  • Can integrate renewable energies;
  • Have low CAPEX;
  • Have low OPEX;
  • Integrate technologies with long life;
  • Delivers high purity H2;
  • Have not reached technology maturity.
The above constraints reduce the angle of our research to water electrolysis.
Based on our research area, we defined our protocol, a guide that unites all the steps that are performed in this SLR. First, we define our input database, EBSCO Discovery Service, because it contains an exhaustive list of research papers having a no bias and quality policy.
The best and accurate results were obtained using the following search strategy [5]:
  • Key words:
Hydrogen production                  <AND>
Hydrogen Electrolysis                  <AND>
PEM                  <AND>
Exchange Membrane                  <AND>
Using <AND> as a cumulative key word was a definitory search criteria, using <OR> meant receiving too many results (11,257, 9379, 14,430 articles)—most of them that are not related to the research target area). Using more key words in order to narrow down the search meant fewer results (92, 38, 115 etc.).
  • Disciplines:
    • Applied Sciences
    • Chemistry
    • Engineering
    • Environmental Sciences
    • Information Technology
    • Life Sciences
    • Physics
    • Power & Energy
    • Science
    • Technology
  • Expanders:
    • Also search within the full text of the articles
  • Search modes:
    • Find all my search terms
    • Apply related words
    • Also search within the full text of the article
    • Apply equivalent subjects
  • Results limits:
    • Full text
    • Peer reviewed
  • Published date:
    • January 2020–February 2022
  • Language:
    • English
We agreed, after a skim and scan of the titles received in the database, that the results received, 528, are relevant under the search criteria presented above.
(ii) In the second stage, Conducting the review, the database extracted form EBSCO Discovery Service, 528 titles, in April 2022 was entered in a database. These 528 titles have the following distribution, considering the publishers: MDPI—275 articles, Springer Nature—104 articles, John Wiley & Sons, Inc.—55 articles, Wiley-Blackwell—44 articles, Royal Society of Chemistry—12 articles.
We can observe a decrease over the last 2 years of the total number of articles exported from EBSCO Discovery Service based on the above search strategy as presented in Figure 2. The classical trend of published articles peak in autumn of each year is visible here and we can assume that the large number of articles in the first part of reference period is caused by the COVID-19 lockdown and the increase time availability for research dissemination. On horizontal axis, we have the timeframe and on the vertical one, the number of articles identified.
(iii) The third stage, Reporting and dissemination, is covered in the Results Topic, where we try to synthetize, also graphical, the essence of the target research results [9].
Due to the fact that a large number of research articles were processed in this systematic review a flow of exclusion was identified and presented in Figure 3. This flow chart follows the PRISMA methodology for reporting systematic reviews [5].

3. Results

3.1. RQ1: What Are the Main Characteristics and Clusters That Research Focuses on?

The review of filtered articles showed five large research directions identified, and graphically represented in the Figure 4, hereunder. The research topic clustering was conducted based on the common topic identified in the text of the research articles:
  • Electrodes (anode/cathode) materials;
  • Electrodes (anode/cathode) shape enhancing;
  • Electrolysis cell;
  • Electrolyte;
  • Exchange membrane.
We can see that the main focus is on electrodes (anode/cathode) materials—115 articles distributed as follows:
  • Fifty-four articles on research over Ni alloys and compounds for anode material deposition (OER) [4,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25] and cathode material deposition (HER) [4,11,12,14,15,17,18,20,21,23,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40];
  • Sixteen articles on research over Co alloys and compounds for anode material deposition (OER) [4,41,42,43,44,45,46] and cathode material deposition (HER) [4,41,47,48,49,50,51,52,53,54,55,56];
  • Nine articles on research over Ir alloys and compounds for anode material deposition (OER) [4,57,58,59,60,61,62,63,64] and cathode material deposition (HER) [4,57,58,62,63,65];
  • Six articles on research over Ru alloys and compounds for anode material deposition (OER) [4,66,67] and cathode material deposition (HER) [68,69,70];
  • Five articles on research over Cu alloys and compounds for anode only material deposition (OER) [4,71,72,73];
  • Four articles on research over Mo alloys and compounds for cathode only material deposition (HER) [74,75,76,77];
  • Four articles on research over B alloys and compounds for cathode and anode material deposition (OER and HER) [78], Fe [79], Au [80], Pt [81] alloys and compounds for cathode material deposition (HER);
On the second place the focus is on electrolysis cell—46 articles focused on:
On the third place the focus is on exchange membrane—18 articles focused on:
Eight articles follow the electrolyte area of research:
  • Three articles on research over KOH [130,131,132];
  • One article on research over H2SO4 [132];
  • One article on research over PO3 [133];
On the last research cluster identified we find 4 articles on electrodes (anode/cathode) shape enhancing:
  • Two articles on research over chemical deposition [134,135];
  • Two articles on research over 3D printed [136,137].
Articles that have a topic that doesn’t fit in with the specified subclusters, but have results that were counted in different parts of this article, were cited at that specific location in the text. This remark was made to explain the missing citation count in the total number of clustered research articles.

3.2. RQ2: What Are the Main Key Performance Indicators (KPIs) of the Publish Studies?

The main KPI’s accounted during this review were:
  • Efficiency of the electrolysis cell [%];
  • Faradaic efficiency [%];
  • High frequency resistance HFR [mΩ cm²];
  • Current density [mA/cm2];
  • Electrode deposition decay time [h];
  • Voltage [V];
  • Temperature [°C];
  • Pressure [atm];
  • Electrochemical resistance [mΩ/cm2];
  • Electrolyte pH;
  • Electrolyte flow rate [mL/min];
  • Anode loading [mg/cm2];
  • Cathode loading [mg/cm2];
  • Membrane film thickness [μm];
  • Overpotential (voltage efficiency) HER at 10 mA/cm2 [mV];
  • Overpotential (voltage efficiency) OER at 10 mA/cm2 [mV];
  • H2 production [Nm3/h];
  • Power consumption for H2 production [kWh/Nm3];
  • Electrolysis system size [kW];
  • Ion exchange capacity [meq/g];
  • Ionic conductivity [mS/cm];
  • Electrolyte resistivity [MΩ cm];
  • Hydrogen crossover [mA cm−²];
  • Degradation rate [μV/h];
  • H2 purity [%];
  • Tafel slope [mV dec−1];
  • System lifetime [years];
  • Production cost [EURO/kg].
All KPIs were counted for bit, for the ones that had insufficient information to make a graphical conclusion the number and reference were just mentioned in the text.

3.3. RQ3: How the KPIs have Evolved over the Last Years?

In order to identify the trend of the KPIs, we present a box and whiskers chart and a second temporal chart only of the main KPIs mentioned above: In Figure 5, for example, we can see how the efficiency of the electrolysis cell is reflected in the research papers over the 2020–2022 period.
The Faradaic efficiency was mentioned twice 89% [134] and 96.5 % [56].
High frequency resistance HFR was quantified in three articles varying from 59 [mΩ cm²] in Ref. [89] to 164 [mΩ cm²] in Ref. [90] and 170 [mΩ cm²] in Ref. [115].
The current density as a major indicator of electrolysis cell efficiency was mentioned in 46 research articles and is graphically presented in Figure 6 showing a decreasing trend over 2020–2022 timespan.
The decay time in electrodes or exchange membrane was mentioned in 6 articles starting from 1 [h] in Refs. [70,137], 26 [h] in Ref. [13], 40 [h] in Ref. [143], 100 [h] in Ref. [59] and 200 [h] in Ref. [141]. We can identify an emphasis on larger decay times in recent published articles.
In regard to the voltage applied, we can see that the majority of the research (Q1 to Q3) apply a voltage between 1.5 [V] and 2.2 [V] and that the trend is descending over the focus timespan as presented in Figure 7.
Most articles do not specify the pressure of the electrolyte, so we assume that the large majority of the research is conducted at atmospheric pressure [123,142]: 1.5 [atm] in Ref. [144], 2 [atm] in Ref. [98], 2.47 [atm] in Ref. [89], 4.93 [atm] in Ref. [83], 9.86 [atm] in Ref. [105], 11.84 [atm] in Ref. [113] and 29 [atm] in Refs. [93,101,131]. We did not find any correlation between electrolyte pressure and the efficiency of the cell [%] or the H2 production rate [Nm3/h].
The correlation between voltage and current density in the articles identified is 0.23883.
In Figure 8 we present the evolution of the temperature over the reference period and the correlation with the efficiency and current density.
We can see an ascending trend of electrolyte temperature during the last 2 years studies and also a higher correlation coefficient for the current density and electrolyte temperature. We associate the increase of electrolyte temperature to the developments in the electrodes surface covering materials and, in the PEM, and AEM technologies. New materials used for the exchange membrane are related to the higher values of electrolyte temperature (median to third quartile), like sulfonated poly(phenylene sulfone) [90], phosphoric acid-doped polybenzimidazole [122], aquivion short side chain perfluorosolfonic membrane [123].
The electrochemical resistance was mentioned in four articles: 11.4 [mΩ/cm2] in Ref. [138], 105 [mΩ/cm2] in Ref. [94], 230 [mΩ/cm2] in Ref. [47] and 323 [mΩ/cm2] in Ref. [82].
The electrolyte pH varies from acidic 0 in Ref. [63], 0.6 in Ref. [14] and 1 in Ref. [62] to basic 9.7 in Ref. [78] and 14 in Ref. [10]. The rest of the articles that do not mention the pH of the electrolyte and use water, have the pH 7 in Ref. [47].
The electrolyte flowrate is directly proportional to the size of the electrolysis cell and they vary from 0.00833 [mL/min] in Ref. [143] to 150 [mL/min] in Ref. [98] with 3 research articles mentioning a 40 [mL/min] flowrate, which is also the median of the values, in Refs. [89,94,138].
In Figure 9 we exemplify the anode and cathode loading in [mg/cm2].
We can see a decrease of specific mass deposition in HER electrode for Pt and a trend to focus the research for catalysts with lower cost or higher availability.
In regard to the membrane film thickness [μm], we can identify only three articles that mention this indicator 10 [μm] in Ref. [124], 20 [μm] in Refs. [4,42] and 1270 [μm] in Ref. [98].
In Figure 10, we represent the distribution of overpotential for HER at 10 mA/cm2 in [mV] [4,10,11,12,41,58,59,63,65,69,78,82,94,115,130,134,135,141,145,146], overpotential for OER at 10 mA/cm2 in [mV] [4,11,12,63,130,145] and the Tafel slope [mV dec−1] [4,33,34,35,36,37,38,39,40,41,42,43,44,45,46,49,50,51,57,58,59,60,61,62,63,64,65,66,71,72,73,74,77,78,79,80,83,84,85,86,87,92,93,94]. We can see a slightly better correlation between the overpotential for OER and the Tafel slope with 0.469 value as per the overpotential for HER and the Tafel slope that returns a value of 0.449.
For the H2 production KPI and the electrolysis system size we can see that are also represented by the power consumption for H2 production as presented in Figure 11.
The H2 production indicator varies from 3.09 [Nm3/h] with an electrolysis system size of 20 [kW] in Ref. [120] to a stunning 5000 [Nm3/h] with an electrolysis system size of 25 [MW] in Ref. [101].
Ion exchange capacity is mentioned only in two papers and is specific to the exchange membrane research with 1.12 [meq/g] in Ref. [124] and with 1.88 [meq/g] in Ref. [121].
Ionic conductivity is also specific to the exchange membrane research and was mentioned in three articles with values starting from 4.2 [mS/cm] in Ref. [128], 100 [mS/cm] in Ref. [124] and 230 in Ref. [122].
Electrolyte resistivity is a very important indicator of the cell efficiency but is only mentioned in two articles, 15.6 [MΩ cm] in Ref. [124] and 18.2 [MΩ cm] in Ref. [79].
Hydrogen crossover is also very rarely mentioned, 0.3 [mA/cm2] in Ref. [90] and 1.1 [mA/cm2] in Ref. [89].
Only one article [63] mentions degradation rate of 4 [μV/h].
System lifetime is estimated once 20 [years] and 30 [years] in Ref. [93].
The output of the H2 has a purity varying from 99.1 [%] in Ref. [115], 99.8 [%] and 99.99 [%] in Ref. [93].
The most important indicator for a market release of an electrolysis cell is the operation costs, Figure 12 [91,92,95,96,97,99,100].
Some KPIs were not presented in this review due to the fact that they were not reported in the articles considered or due to the fact that these performance indicators were not clearly defined. We would like to mention some of the KPI’s not covered in this review due to the reasons mentioned above: Turnover frequency (TOF), Mass activity (MA), Specific Activity (SA) and Electrochemically Active Surface Area (ECSA).

4. Discussion

The specific performance indicators covered above, in the topics of H2 production by means of alkaline and pure water electrolysis, are mentioned partially in every article. In this review, we did not encounter an article that covers all the KPIs presented above. Research usually covers niches or aspects of the H2 electrolysis area. We presented the values of the KPIs temporally in order to identify any trends.
We observed only one paper [147] in our search that tackled the hot topic of seawater electrolysis and we did not create a separate analysis for seawater electrolysis, although we acknowledge the development potential of this research area.
We can see from the Operational cost, to the efficiency of the cell, improvements in the indicators have been made during the period 2020–2022. These indicators, however, do not compensate the demand for lower cost and higher sustainability of the materials used in the electrolysis cell. Even more, the demand for electrolysis solutions is reaching a peak so we must see a breakthrough in the near future. Energetic and climatic context pushed also by the war in Ukraine have generated a political movement to push the fossil fuel at the edge of economy that is not counterpartyed by the available yields in H2 generation.
In order to link the dots between scientific research and actual H2 generation equipment market, we are using the statistics of patents filings. The presented trend is in the right direction: 2016 the number of patent filling for water electrolysis technologies exceeded the ones for producing H2 from fossil fuels [148]; 2018 the number of patent fillings for sustainable minerals electrocatalysts became larger than those for traditional but expensive electrocatalysts (e.g., platinum, gold, silver, noble metals and rare elements) [148]. The same trend is observed in the next 2 years period with a high focus on sustainable materials for electrolysis cell components.

5. Conclusions

This work presented a methodology to identify the research focus and research trend in water electrolysis during recent years 2020–2022.
We tried to summarize the KPIs used in recent years to describe the state of the art of current research in H2 generation form alkaline and water electrolysis. Despite the progress that has been made, some challenges still remain, mainly the low roundtrip efficiency of power to H2 to power; the sustainability and durability of materials used in cells; the influence of the electrolyte temperature over the efficiency of the cell etc.
Despite the fact that highlighted advances were made, further research efforts are still needed to build a future based on a green fuel, Hydrogen and to shift the economy toward this new energy carrier.
Green Hydrogen is now considered worldwide as environmental appropriate alternative to traditional fossil fuels due to its high mass energy density and carbon free conversion. AWE is the technology chosen by large PV powered electrolyzer projects, but PEM, on the other hand, is targeted as most desired technology for high purity H2 and compatibility with renewable power generation [149].
  • Expensive catalysts are showing a reduced presence in the research trend for HER (Figure 9). Research made progresses in improving the stability and activity by using sustainable materials, but a gap still remains to be filled for complying with the economical requirements of the market readiness. [149] PGM catalysts need to be replaced in order to be able to deploy large PEM electrolyzers or to achieve economical market readiness. The candidates for this replacement are Ni, Co, Fe, Mo, W and Cu, but not as single metal catalyst. Research is reaching for a compound with high catalytic activity through altering the electronic structures or by increasing the number of active sites [4];
  • OER catalytic processes in PEM mainly focuses on IrOx compounds that deliver high stability but with a high production cost, Ru based catalysts prove high activity with low costs but a low operational stability. [149] It is observed in Ref. [4] that OER activity increases with increasing oxygen—metal bond strength (oxophilicity) but the durability decreases on the same order or materials: Au << Pt < Ir < Ru << Os [4];
  • Ni based catalysts have a distinctive role in our review as they are reflected a large number or articles for HER and OER in AWE. Pure Ni electrodes covering have a low stability and activity during water splitting processes so various alloying are aiming to solving this vulnerability. In HER Ni based materials show a trend to replace the expensive and rare Pt in AWE [4];
  • Cu based catalysts for OER activity in AWE shows some obvious advantages: low toxicity compared with other metal-based materials, low cost and excellent electrical conductivity;
  • Co based catalysts (phosphide-coupled Co) research shows an excellent HER activity in AWE [4].
Many challenges facing the development of water electrolyzer technologies are the increasing the overall efficiency of H2 generation (e.g., AEM for AWE) and the reduction of the CAPEX and OPEX along with increasing the lifespan of the equipment (e.g., higher performance bipolar plate materials for PEM).
Based on the analysis of all the technologies covered in the review, we can answer some research questions:
Q1: What is the current optimized Hydrogen production technology?
A1: From the technology status and the data in the review we can attribute the mature technology label to the AWE and small-scale mature technology for PEM.
Q2: What are the optimal technical parameters for Hydrogen generation?
A2: We summarize our parameter findings in the Table 1 hereunder:
Q3: What are the types of membranes most used in the research?
A3: We have a majority of Nafion membranes (≈55% N115 and ≈25% N117) other experimental membranes (≈20%).

Author Contributions

Conceptualization, F.C.D. and C.H.; methodology, F.C.D. and C.H.; validation, C.H., N.R., C.S. and G.E.B.; formal analysis, C.H. and C.S.; investigation, F.C.D.; resources, C.H.; data curation, F.C.D.; writing—original draft preparation, F.C.D.; visualization, C.H., G.E.B., N.R. and C.S.; supervision, C.H.; funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by University of Oradea, within the Grants Competition “Scientific Research of Excellence Related to Priority Areas with Capitalization through Technology Transfer: INO-TRANSFER-UO”, Project No.315/21/12/2021 and the APC was funded by University of Oradea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
3DThree-dimensional
AEMAnion Exchange Membrane
AuGold
AWEAlkaline Water Electrolysis
BBoron
BWBox and Whiskers chart
CAPEXCapital Expenditures
CoCobalt
CuCopper
DIDeionized
FeIron
H2Hydrogen
H2SO4Sulfuric acid
HERHydrogen Evolution Reaction
IrIridium
KOHPotassium hydroxide
KPIKey Performance Indicator
MoMolybdenum
NAFNot Accounted For
NiNickel
OEROxygen Evolution Reaction
OPEXOperating Expenses
OsOsmium
PEMWEPolymer Electrolyte Membrane Water Electrolysis
PO3Phosphite ion
PRISMAPreferred Reporting Items for Systematic Reviews and Meta-Analyses
PtPlatinum
PTGPlatinum Group Metals
PVPhotovoltaic
RQResearch Questions
RuRuthenium
SLRSystematic Literature Review
WTungsten

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Figure 1. Processes of Hydrogen generation efficiency and costs [2,3].
Figure 1. Processes of Hydrogen generation efficiency and costs [2,3].
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Figure 2. Monthly distribution of published papers as per the defined search strategy.
Figure 2. Monthly distribution of published papers as per the defined search strategy.
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Energies 15 06076 g003a 550Energies 15 06076 g003b 550
Figure 3. Flow chart of the systematic review procedure applied [5,8].
Figure 3. Flow chart of the systematic review procedure applied [5,8].
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Figure 4. Pie chart showing the research area distribution.
Figure 4. Pie chart showing the research area distribution.
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Figure 5. Graphic representation of the efficiency of the electrolysis cell [12,83,84,85,92,93,100,105,108,120,131,138,139,140].
Figure 5. Graphic representation of the efficiency of the electrolysis cell [12,83,84,85,92,93,100,105,108,120,131,138,139,140].
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Figure 6. Graphic representation of the current density of the electrodes [10,11,12,13,41,47,57,58,59,60,62,63,76,77,79,81,89,90,94,98,101,108,115,116,117,119,120,123,126,127,129,130,131,132,134,135,136,137,138,139,140,141,142,143,144].
Figure 6. Graphic representation of the current density of the electrodes [10,11,12,13,41,47,57,58,59,60,62,63,76,77,79,81,89,90,94,98,101,108,115,116,117,119,120,123,126,127,129,130,131,132,134,135,136,137,138,139,140,141,142,143,144].
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Figure 7. Graphic representation of the voltage applied [47,57,60,70,76,77,89,90,94,98,101,113,115,117,123,126,129,130,131,132,126,127,129,130,131,132,135,136,137,138,139,140,141,143,144,145].
Figure 7. Graphic representation of the voltage applied [47,57,60,70,76,77,89,90,94,98,101,113,115,117,123,126,129,130,131,132,126,127,129,130,131,132,135,136,137,138,139,140,141,143,144,145].
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Figure 8. Graphic representation of the electrolyte temperature [12,47,57,59,62,83,85,89,90,93,98,101,103,105,108,113,114,115,117,120,122,123,124,125,129,131,134,137,138,139,140,142,143,144,146].
Figure 8. Graphic representation of the electrolyte temperature [12,47,57,59,62,83,85,89,90,93,98,101,103,105,108,113,114,115,117,120,122,123,124,125,129,131,134,137,138,139,140,142,143,144,146].
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Figure 9. This is a representation of the anode and cathode specific mass catalyst deposition.
Figure 9. This is a representation of the anode and cathode specific mass catalyst deposition.
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Figure 10. BW charts representing overpotential for HER and OER at 10 mA/cm2 in [mV], and the Tafel slope [mV dec−1].
Figure 10. BW charts representing overpotential for HER and OER at 10 mA/cm2 in [mV], and the Tafel slope [mV dec−1].
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Figure 11. Power consumption for producing 1 Nm3 of H2 [86,92,93,102,118,120].
Figure 11. Power consumption for producing 1 Nm3 of H2 [86,92,93,102,118,120].
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Figure 12. Operation cost for H2 production.
Figure 12. Operation cost for H2 production.
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Table
Table 1. Comparison of main electrolysis technologies reviewed.
Table 1. Comparison of main electrolysis technologies reviewed.
ParameterAWEPEM
Cell efficiency [%]≈70 ÷ 89≈53 ÷ 90
Current density [mA/cm2]≈50 ÷ 2500≈100 ÷ 4000
Charge carrierOH−H⁺
Temperature range [°C]≈60 ÷ 90≈25 ÷ 80
Electrolyte≈1.1 ÷ 9.5 M KOH≈H2O/DI H2O
Pressure [bar]≈11.8 ÷ 29≈1 ÷ 29.6
Estimated lifetime [h] [150]≈90,000<20,000
H2 purity [%]≈99.1 ÷ 99.8≈99.99
Energy for H2 gen [kWh/Nm3]≈3.8 ÷ 4.4≈3.54 ÷ 4.5
Production [Nm3/h]≈40 ÷ 12,550≈30 ÷ 5000
Operating cost [EURO/kg]≈3.72 ÷ 4.2≈3 ÷ 8.88
These data are the outcome of the review of the filtered research not the actual state of the market.
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Hora, C.; Dan, F.C.; Rancov, N.; Badea, G.E.; Secui, C. Main Trends and Research Directions in Hydrogen Generation Using Low Temperature Electrolysis: A Systematic Literature Review. Energies 2022, 15, 6076. https://doi.org/10.3390/en15166076

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Hora C, Dan FC, Rancov N, Badea GE, Secui C. Main Trends and Research Directions in Hydrogen Generation Using Low Temperature Electrolysis: A Systematic Literature Review. Energies. 2022; 15(16):6076. https://doi.org/10.3390/en15166076

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Hora, Cristina, Florin Ciprian Dan, Nicolae Rancov, Gabriela Elena Badea, and Calin Secui. 2022. "Main Trends and Research Directions in Hydrogen Generation Using Low Temperature Electrolysis: A Systematic Literature Review" Energies 15, no. 16: 6076. https://doi.org/10.3390/en15166076

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