A review of hydrogen production methods and power electronics converter topologies for green hydrogen applications

Citation: Rego, G.; Rocha, J.; Faria, J.A.; Afonso, J.L.; Monteiro, V. A Review of Hydrogen Production Methods and Power Electronics Converter Topologies for Green Hydrogen Applications. Energies 2024, 17, 5579. https://doi.org/10.3390/ en17225579 Academic Editor: Riccardo Berta Received: 20 September 2024 Revised: 25 October 2024 Accepted: 31 October 2024 Published: 8 November 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Review A Review of Hydrogen Production Methods and Power Electronics Converter Topologies for Green Hydrogen Applications Goncalo Rego , Joao Rocha, Jose A. Faria, Joao L. Afonso and Vitor Monteiro * ALGORITMI Research Centre, LASI, University of Minho, 4800-058 Guimarães, Portugal; [email protected] (G.R.); [email protected] (J.R.); [email protected] (J.A.F.); [email protected] (J.L.A.) *Correspondence: vmonteir[email protected] Abstract: Hydrogen has been receiving a lot of attention in the last few years since it is seen as a viable, yet not thoroughly dissected alternative for addressing climate change issues, namely in terms of energy storage, and therefore, great investments have been made towards research and development in this area. In this context, a study about the main options for hydrogen production, along with the analysis of a variety of the main power electronics converter topologies for such applications, is presented as the purpose of this paper. Much of the analyzed available literature only discusses a few types of hydrogen production methods, so it becomes crucial to include an analysis of all known types of methods for producing hydrogen, according to their production type, along with the color code associated with each type, and highlighting the respective contextualization, as well as advantages and disadvantages. Regarding the topologies of power electronics converters most suitable for hydrogen production, and more specifically, for green hydrogen production, a list of them was analyzed through the available literature, and a discussion of their advantages and disadvantages is presented. These topologies present the advantage of having a low ripple current output, which is a requirement for the production of hydrogen. Keywords: hydrogen; green hydrogen; hydrogen colors; alternative fuels; energy; power electronics converters 1. Introduction Hydrogen (H 2 ) is known today as the most abundant element in the universe [ 1 ], and it has many uses that range from ammonia production for fertilizers, to cyclohexane utilized in the plastic industry, or methanol, applied in pharmaceuticals. It can also be part of the hydrogenation process that occurs in oils, in order to form fats, such as margarine. For glassmakers, H 2 creates a protective atmosphere, and it can even be used for manufacturing silicon chips. There is also the classic example of the airships, which took advantage of hydrogen’s low density to propel zeppelins. It is known, however, that this no longer happens due to the Hindenburg disaster [ 2 ]. Automobile companies are investing their efforts into developing vehicles fueled by H 2 , such as the Mirai model, by Toyota, or the Clarity Fuel Cell model, by Honda [ 3 , 4 ]. The H 2 used in these vehicles can be used in fuel cells (FCs), which take advantage of H 2 ’s chemical energy, and through an electrode, an electric current is generated, allowing the motor on these vehicles to rotate, making the car move [ 5 ]. Even though H 2 is used in the automotive industry, via FC stacks, it can also be used in internal combustion engines [ 6 ]. The operational principle of H 2 in this type of engine is similar to the engines fed by fossil fuels [ 3 ], such as gasoline or diesel, and there are companies like Keyou that specialize in converting diesel engines into H 2 internal combustion engines in a cheap manner, smoothing the transition from a polluting way of transportation into a cleaner one [7]. Energies 2024,17, 5579. https://doi.org/10.3390/en17225579 https://www.mdpi.com/journal/energies Energies 2024,17, 5579 2 of 22 H 2 , however, is still not as reliable as other fuels, like diesel or gasoline, for example [ 8 ], and its adoption still requires major efforts for such a transition to happen, in order to lower costs and to boost the investigation in this area [ 9 , 10 ]. The whole infrastructure needs adjustments since H 2 does not present the same characteristics as these other energy carriers [11]. Such changes refer to two specific cases. The first case refers to alterations in terms of gas storage. Previous fueling stations can be repurposed, but with changes to the conditioning of the H 2 in the underground tanks and in the pump supply system, since gas pressure, temperature, and stability are not the same when compared to other fuels [ 12 ]. The second case refers to repurposing and minor adaptations to the preexisting natural gas infrastructure. In Europe, there has been an effort to understand the viability and feasibility of reusing the well-developed and widespread pipeline systems for natural gas. This method presents advantages, besides the preexistence of the transporting infrastructure, such as the easily achievable very high energy transportation, thanks to the lower density of H 2 compared to methane [ 13 ]. When comparing the energy flow of both gases through a pipeline, H 2 ’s outputted volume can be nearly three-fold that of methane, under a given period, at the same pressure, achieving only a slightly reduced energy density, which means that the transition from natural gas to H 2 has a low impact on the capacity of a pipeline to transport energy [7,9,10,14–16]. In [ 16 ], an analysis of the differences between vehicles with distinct types of engines under several test conditions is presented. Their driving range, fuel consumption, and the amount of carbon dioxide (CO 2 ) emitted per vehicle type were compared, and the conclusions are summarized in Figure 1. Energies 2024, 17, x FOR PEER REVIEW 3 of 25 Figure 1. (a) Comparison of fuel consumption and driving range between the analyzed vehicles; (b) differences in lifecycle CO 2 emissions associated with each vehicle type. To this extent, there is a strong need for the development of solutions that allow such technological advancements, namely in the power electronics (PE) field. In order to interface the many components of a power grid, whether it is part of a major grid or part of an islanded grid, there is always the need for PE, and in the case discussed in this paper, the transfer of energy from its source to the electrolyzers that produce H 2 requires several PE converters that work symbiotically as a single converter. Figure 2 displays the relationship established between the energy sources, which for G-H 2 must be renewables, the PE converter, and finally the electrolyzer. This presents an example application of the G-H 2 production and utilization, where PE is at the core of the operation. Another utilization for the produced G-H 2 is also depicted by fuel cells. Figure 1. (a) Comparison of fuel consumption and driving range between the analyzed vehicles; (b) differences in lifecycle CO2emissions associated with each vehicle type. Energies 2024,17, 5579 3 of 22 Besides the transportation sector, H 2 has been receiving special attention in energyheavy industries, such as the steelmaking industry, and it accounts for the list of biggest pollutants, through all types of industry [ 17 , 18 ]. Thus, there has been evidence that an energy transition in this sector is crucial for climate targets to be achieved [ 18 ], which has led to a growing interest in H 2 , since it can be implemented as an eco-friendlier fuel, without major changes to existing hardware, such as furnaces and ovens [ 19 ]. It should be noted, however, that the required H 2 must be produced through processes that use clean energy, such as that produced from renewable energy sources (RESs), which means that for the whole process to be sustainable and to have minimal environmental impact, green hydrogen (G-H2) must be considered. RESs are known to have a deep “flaw”, which is their variance through time, and one of the solutions proposed was the use of energy storage systems [ 20 , 21 ]. These include batteries, supercapacitors, and gases, for example. H 2 is a type of gas that can be used for this end; therefore, it is crucial to study its capabilities as a solution for the energy storage problem. Green hydrogen stands out as a solution for this problem since there is a major utilization of what could otherwise be lost energy. Therefore, G-H 2 is seen as a key driver of the energy transition [20]. To this extent, there is a strong need for the development of solutions that allow such technological advancements, namely in the power electronics (PE) field. In order to interface the many components of a power grid, whether it is part of a major grid or part of an islanded grid, there is always the need for PE, and in the case discussed in this paper, the transfer of energy from its source to the electrolyzers that produce H 2 requires several PE converters that work symbiotically as a single converter. Figure 2displays the relationship established between the energy sources, which for G-H2must be renewables, the PE converter, and finally the electrolyzer. This presents an example application of the G-H 2 production and utilization, where PE is at the core of the operation. Another utilization for the produced G-H2is also depicted by fuel cells. Energies 2024, 17, x FOR PEER REVIEW 4 of 25 Figure 2. Diagram of a G-H 2 use case, from its energy source to the production and storage, or for usage of the gas itself. This paper presents various contributions, namely the following: (i) outlining the various types of H 2 according to their production type, along with the color code associated with each type; (ii) regarding G-H 2 , the specific and main PE converter topologies that allow for its production are presented and described, along with the characteristics that make them suitable for G-H 2 production; (iii) the principle of operation of the PE converters, including a comparison based on advantages and disadvantages, is presented. After this introduction, the second section is dedicated to an introduction to H 2 and its possible applications, as well as an enumeration of the various H 2 production methods, along with the color code associated with each type. Section 3 describes some PE converter topologies associated with the production of H 2 , as well as the characteristics that make them suitable for G-H 2 production. A comparison of all the converters is presented in Section 4, according to their advantages and disadvantages. Lastly, in Section 5, conclusions about the discussed methods for H 2 production and the PE converter topologies are drawn. 2. Hydrogen: Classification and Associated Color Code Before showcasing the different existing methods for H 2 production, a note of warning must be made about the different types of hydrogen since there are three variants; that is, hydrogen can be categorized in its isotopes, namely Protium ( 1 H), Deuterium ( 2 H), and Tritium ( 3 H), and they all refer to the chemical element hydrogen, with the same number of protons, but with 0, 1, or 2 neutrons, respectively. With this being said, in this paper, the term hydrogen or H 2 will always represent the most abundant isotope, which is Protium, and the molecule composed of two Protium atoms [22]. In recent years, H 2 has been appraised as a sustainable alternative for the decarbonization of a multitude of processes, from transportation to energy production, or energydense industries, such as the steel industry [23,24]; it has a variety of applications, and when it is burned or used in fuel cells, the only byproduct is pure drinkable water. Therefore, in this regard, H 2 has an advantage, when compared to fossil fuels [25]. The latter produce hazardous and pollutant byproducts when they are burned to produce energy and are considered key drivers of global warming, which in turn leads to negative impacts on the environment, its inhabitants, and the ecosystem as a whole [26,27]. There are, Figure 2. Diagram of a G-H 2 use case, from its energy source to the production and storage, or for usage of the gas itself. This paper presents various contributions, namely the following: (i) outlining the various types of H 2 according to their production type, along with the color code associated with each type; (ii) regarding G-H 2 , the specific and main PE converter topologies that allow for its production are presented and described, along with the characteristics that make Energies 2024,17, 5579 4 of 22 them suitable for G-H 2 production; (iii) the principle of operation of the PE converters, including a comparison based on advantages and disadvantages, is presented. After this introduction, the second section is dedicated to an introduction to H 2 and its possible applications, as well as an enumeration of the various H 2 production methods, along with the color code associated with each type. Section 3describes some PE converter topologies associated with the production of H 2 , as well as the characteristics that make them suitable for G-H 2 production. A comparison of all the converters is presented in Section 4, according to their advantages and disadvantages. Lastly, in Section 5, conclusions about the discussed methods for H2production and the PE converter topologies are drawn. 2. Hydrogen: Classification and Associated Color Code Before showcasing the different existing methods for H 2 production, a note of warning must be made about the different types of hydrogen since there are three variants; that is, hydrogen can be categorized in its isotopes, namely Protium ( 1 H), Deuterium ( 2 H), and Tritium ( 3 H), and they all refer to the chemical element hydrogen, with the same number of protons, but with 0, 1, or 2 neutrons, respectively. With this being said, in this paper, the term hydrogen or H 2 will always represent the most abundant isotope, which is Protium, and the molecule composed of two Protium atoms [22]. In recent years, H 2 has been appraised as a sustainable alternative for the decarbonization of a multitude of processes, from transportation to energy production, or energy-dense industries, such as the steel industry [ 23 , 24 ]; it has a variety of applications, and when it is burned or used in fuel cells, the only byproduct is pure drinkable water. Therefore, in this regard, H 2 has an advantage, when compared to fossil fuels [ 25 ]. The latter produce hazardous and pollutant byproducts when they are burned to produce energy and are considered key drivers of global warming, which in turn leads to negative impacts on the environment, its inhabitants, and the ecosystem as a whole [ 26 , 27 ]. There are, however, some constraints related to the production methods of H2, and to assess this issue, a color code for different types of H2was developed [28,29]. H 2 itself is a colorless gas, but there are around ten color codes for identifying H 2 , and these refer to the source or the process used to produce it, more specifically green, blue, grey, brown or black, turquoise, purple, pink, red, and white, and they refer to a way of categorizing H 2 concerning its origin, method of production, monetary and energy costs for its production, and life cycle emissions [ 26 , 30 ]. Table 1showcases the different categories created for H 2 according to several specifications. These are the most common H 2 production methods since they have already been used to produce H 2 on an industry level [31]; therefore, not all colors are addressed in this table. Table 1. Main pathways for H2production. Type Source Process Min. Energy Inputs (kWh/kg) Conversion Efficiency (%) Average Cost (€/kg) Life Cycle CO2 Emissions (kgCO2/kg) Black/Brown [23,25,26,28]Coal Gasification ~40 60 3.4 >18 Grey [17,23,24,27,28]Methane SMR 110.8 65–80 1.7 >12 Blue [17,32,33]Methane SMR + CCS 2~14 47–62 2–2.5 3.5–5 Pink [34–36]Water Nuclear Electrolysis ~47.8–51.3 85–90 ~3.3–6 0.559 Turquoise [17,37–39]Methane Plasm Pyrolysis 9–16 14–56 ~2 3–4 Green [17,20,30,33,40,41]Water Electrolysis 55 60–70 5–7 4–5 1SMR—Steam Methane Reforming. 2CCS—Carbon Capture and Storage. Energies 2024,17, 5579 5 of 22 In [ 26 ], the characteristics of each of these colors of H 2 are discussed and presented, along with a benignity assessment of each of them. In [ 33 ], a differentiation of the various colors of H 2 is made, reinforcing the weight that most conventional practices for H2 production have in the transitional phase of H 2 , namely coal and gas burning. In [ 32 ], the authors discuss the production of blue H 2 as a key for the growth of the H 2 market. They emphasize the evolution and improvement of techniques that aim for the production of H 2 in recent years, as well as a comparison between H 2 and natural gas, in terms of their advantages and disadvantages as fuels and energy carriers. Lastly, they refer to CCS as being essential for the implementation of blue H2. 2.1. Black/Brown Hydrogen Black and brown H 2 are similar since they are both produced from coal. The only difference is the type of coal that is used; for the first case, black bituminous coal is used to produce energy from its burning, whereas for brown coal (thereby the name), lignite is used instead [ 23 , 25 ]. This method of production is, on one hand, the cheapest, since coal prices are low, but on the other hand, it has the biggest environmental impact when compared to other methods of H 2 production, since burning coal is responsible for the emission of large amounts of greenhouse gases, aggravating global warming through air pollution [26,28]. 2.2. Grey Hydrogen The most common method of producing H 2 is through the SMR process, and more than 80% of H 2 is produced through it [ 23 ]. At the beginning of the process, natural gas and other hydrocarbons that contain methane are injected into the steam enclosure, where heating and sulfur removal processes happen. When heated, the mixture produces the desired H 2 . Some extra procedures are used to produce more pure H 2 , such as the water shift gas reaction [ 27 ]. The advantages of this method are the already existing infrastructure and its lower cost. However, SMR has an enormous presence among the biggest pollutant industrial processes, accounting for almost 3% of the global industrial sector CO 2 emissions; therefore, it is not an alternative to be looking at in the future, since it does not meet the net-zero carbon targets [42,43]. 2.3. Blue Hydrogen To produce blue H 2 , it is necessary to capture and store the carbon emitted upon the burning of natural gas. This method allows for this H 2 to be labeled as low-carbon, since most of the produced CO 2 is then captured and stored in underground caves [ 32 ]. This method is hailed as more expensive than the black/brown H 2 type; however, with the CCS strategy, the carbon offset is lower, making it an environmentally friendly option [ 17 ]. It is not, however, the greenest, because the captured carbon cannot be kept forever, and there will always be leaks, which leads to an increase in the overall carbon footprint of this alternative [33]. 2.4. Pink Hydrogen There are numerous ways to produce energy, and besides the ones mentioned above, or RESs, there is one energy production method that stays in between, and it is nuclear energy (NE). The use of nuclear reactions to produce energy is no novelty, and this energy can also be used to produce H 2 [ 34 ]. Pink H 2 is produced through the electrolysis of water, and the energy is sourced from nuclear power plants. There are trade-offs when using NE to produce H 2 . In terms of pollution caused, none of it directly affects the environment; i.e., the byproduct of producing this energy is water vapor, which is innocuous to the atmosphere but can contribute to the heating of nearby rivers and water reservoirs, leading to a less positive influence on the local ecosystem. Another great issue related to the use of nuclear power refers to the residues since these are practically impossible to decompose in nature and present a biohazard because of the radiation they produce, as well as their Energies 2024,17, 5579 6 of 22 toxicity, when in contact with living beings. Thus, the production of pink H 2 must be thoroughly calculated so that no greater risks arise [35,36]. 2.5. Turquoise Hydrogen For the production of turquoise H 2 , the process of methane (CH 4 ) pyrolysis via thermal plasma is used. That is, through the burning of CH 4 , using hot plasma, it is possible to produce H2with a high degree of purity [37]. This method has many advantages, such as being a lower-energy-density process, when compared to other processes, such as SMR, or water electrolysis. Furthermore, its usage and production can be accelerated, considering the already existing natural gas infrastructure [ 17 , 37 ]. Another advantage presented by turquoise H 2 is its byproduct: high-value black carbon. Upon burning one molecule of methane, it is possible to produce two molecules of H 2 and one molecule of carbon, which can later be used in a multitude of processes and/or products, including as a pigment, as a filler for rubber-making, for pencil-making, as artificial diamonds for industrial tools, and even for gunpowder, among others [38,39,44,45]. 2.6. Green Hydrogen Lastly, concerning the major types of H 2 , there is G-H 2 . This is the most environmentally friendly type of H 2 since it is produced from RESs through the electrolysis process [ 41 ]. This is an energy-dense process, which may represent a downside; however, since this is a cleaner process, it beats all the other H 2 production methods, because no harmful gases or chemicals are released into the atmosphere and surrounding ecosystems [ 30 , 33 ]. G-H 2 is deeply associated with the decentralization of energy production since for it to be more sustainable and reliable and for the whole production and usage process to be more efficient, G-H 2 must be produced near its final application equipment, namely electric vehicle (EV) charging stations and ammonia and fertilizer production, or simply to produce H 2 to be stored as an energy carrier, for example [ 17 , 40 ]. Many projects, namely in Europe, intend to accelerate the transition to G-H 2 through a goal set for 2030 that relies upon importing 10 million tons of G-H 2 for use in traditionally fossil-fuel-reliant sectors, such as heavy industry and transportation [46]. 2.7. Purple Hydrogen The production of purple H 2 foresees the usage of nuclear power, heat, and H 2 O through electrolysis and thermolysis processes. It has similarities to the production of pink H2; however, they differ, since pink H2does not rely on heat to be manufactured [40]. The vast majority of purple H 2 projects are located in the USA and in the UK. Even though China only has one purple H 2 project, its announced capacity surpasses all the other preexistent infrastructure, with a theoretical maximum capacity of 39.37 kilotons per annum (ktpa), and it is expected to begin its activity in 2025. This is a huge difference from the average purple H 2 project, which outputs around 0.6 ktpa. It is also estimated that a 1 GW reactor could provide 150 ktpa of H 2 production, whereas a 3 GW one could generate enough H2to heat 1 million homes or to feed 40,000 H2buses [47]. 2.8. Red Hydrogen Just like pink and purple H 2 , red H 2 is produced using nuclear energy. This particular type of H 2 is produced solely from the resulting heat of nuclear energy production, through the thermolysis of water [ 40 ]. There is, however, another difference, which relates to the type of nuclear reactor that is used for the production of red H 2 . To be able to produce red H2, a new generation of reactors must be used, and they differ from previous reactors in terms of size and power; i.e., they are smaller and less powerful. This allows higher temperatures than those in classic reactors to be attained. In terms of perspective, the newer reactor power plants will output from 30 MW to 100 MW of thermal power, whereas bigger and older nuclear power plants output approximately 100 times more thermal power. The Energies 2024,17, 5579 7 of 22 problem is that older nuclear reactors are less efficient; therefore, the implementation of these newer reactors could be transformative to this H2production method [48]. 2.9. White Hydrogen H 2 can also be produced naturally. White H 2 refers to the gas that is produced from natural processes and kept in underground deposits. This hydrogen is seen as bliss since it is produced naturally without any kind of interference from humans. However, there are downsides to it, such as its extraction, which accounts for possible gas leaks that are dangerous, since H 2 is flammable, and the environmental impact associated with it, considering that these natural H 2 -occurring areas need to be exploited, and therefore, heavyduty machinery will induce both controlled and uncontrolled damage to the structures that keep the gas stored; a further downside is its bioavailability, since naturally formed H 2 cannot be found in all parts of the world, and natural H 2 reserves are not equally accessible and do not grant the same extraction conditions [49,50]. 2.10. Yellow Hydrogen Yellow H 2 is defined as a derivate of G-H 2 since it is produced from RESs; however, in this case, the only source that can be accounted for is solar energy. Through the use of solar panels, electricity is produced, and when it is fed to the electrolyzer, H 2 is produced, without the release of CO 2 or other harmful gases to the atmosphere [ 51 ]. There is, however, not much consensus on the definition of the origin of yellow H 2 , and some authors claim that it may also be produced from electrolysis, but with its energy directly sourced from the power grid [26]. 3. Power Electronics Converter Topologies for Green Hydrogen Production G-H 2 stands out as an environmentally friendly alternative to fossil fuels since through its use, no pollutant components are emitted to its surroundings, either through its use via fuel cells or by simply burning it, as is done with other fuels, such as diesel or gasoline. The use of H 2 requires, nonetheless, a high degree of purity, i.e., a purity level superior to or equal to 98%; therefore, the equipment responsible for such production must be capable of outputting top-grade H2[52,53]. For this to be possible, every step of the production process must be performed in such a way that the H 2 is as pure as possible. The electrolysis process leads to the fissure of the H 2 O molecules into their components, namely oxygen (O 2 ) and H 2 , by means of large amounts of electric current [ 54 ]. Considering this, it is necessary to think of ways to minimize the losses that occur amidst the energy conversion process and to grant the required conditions for the electrolyzers to work, namely in terms of adequate current and voltage. PE is evident from the beginning to the end of the production process, and thus an alternative to fight energy conversion losses is proposed by using PE converters. These systems are key to having high-quality H 2 , and there are many PE converters that allow the production of G-H 2 . PE converters are an essential part of an electrolysis system since their purpose is to be able to provide the electrolyzer with all its electricity needs, despite the power grid it may be inserted on, either as part of a bigger power grid or as an islanded grid [55]. These converters will be further analyzed. Considering the aforementioned, some authors have already addressed the current ripple influence on the electrolyzers. In [ 56 ], an analysis of how important and relevant the current ripple is to the performance of an electrolyzer, namely a proton exchange membrane (PEM) electrolyzer, is presented. With the results presented in their work, the authors concluded that efficiency is reduced proportionately to the increase in the current ripple since it increases the power consumption of the electrolyzer. In terms of the amount of H 2 that is produced, they stated that it stays constant since this value is solely affected by the average of consumed current, which stays constant no matter the amount of ripple that the current may have. Thus, the lower amount of ripple leads to a reduction in the power consumption from the electrolyzer’s perspective. In [ 57 ], the authors established that there Energies 2024,17, 5579 8 of 22 is a proportional relationship between the watt-hour efficiency of the H 2 production process and the quotient IAVG/PAVG . Another aspect could be considered for the improvement of the power conversion efficiency, namely through the usage of soft switching; however, in the context of this paper, this feature is not an object of analysis [58]. G-H 2 can be compared to pink H 2 , given the fact that a major similarity between both production methods can be identified, namely the fact that both methods rely on the electrolysis of water. However, they differ in terms of power, since a nuclear power plant with commercially available reactors may produce as much as 1500 MW [ 41 ], whereas for G-H 2 production, a standardized electrolyzer stack coupled to RESs could output only about 10 MW [ 59 ]. This leads to the conclusion that PE converter topologies may be similar, independently of the application; however, considering the power required for each method, there must be an adjustment of the type of components that ensure the correct operation of both electrolyzers. Thus, this section analyzes converters that can be employed for an electrolyzer and, more specifically, for electrolyzers whose energy is produced from RESs, in order to produce G-H2. 3.1. Power Electronics Converter Topologies For a certain converter to be deemed suitable for G-H 2 production, some technical aspects must be considered, since H 2 production requires lower voltages and higher current values; i.e., for PEM electrolyzers that present an electrode area of around 1500 cm 2 , current density may vary between 900 A and 3000 A, whereas the voltage range is limited to 1.4 V to 2.5 V [ 54 , 60 , 61 ]. Other aspects must be taken into account when considering PE converter topologies for their integration with electrolyzers, such as, and foremost, a low output current ripple, diminished electromagnetic interferences, higher power density, reduced costs, high conversion ratio, reliability, and the efficiency of the electrolysis process, according to the technique applied [ 62 ]. Electrolyzer efficiencies may vary from 50% to 83% in PEM electrolyzers and may vary for other technologies, such as alkaline water electrolysis (50% to 78%), anion exchange membranes (57% to 59%), and solid oxide electrolysis cells (about 89%), which means that if the H 2 -producing element has such values of efficiency, it is crucial to increase the efficiency of the PE converters as much as possible, in order to feed the electrolyzer units, and thus, this presents a prerequisite for appointing certain PE converter topologies, such as the ones described in this paper, as adequate for H 2 production [60,61,63]. To produce hydrogen using electrolysis, only dc current can be used, and since the intended application cases are RESs, to produce G-H 2 , their coupling is made to a dc bus. The usage of dc has benefits when compared to ac, namely in terms of efficiency, reliability, the integration of battery systems and RESs, and ultimately, the elimination of frequency issues, which lead to transmission losses [63]. There are many power electronics converters that can be applied in H 2 production, and thanks to continuously growing investment and investigation in this area, this number is continuously growing. Thus, a selection of PE converter topologies is dissected in this work for the sake of understanding their position among the remainder by the analysis of their benefits and demerits. 3.1.1. Dual Active Bridge Converter In [ 64 ], the authors discuss a dual active bridge (DAB) converter as a way to interface a high-voltage DC bus with an electrolyzer. This PE converter is pictured in Figure 3. However, to handle the high currents typical in such systems, an alternative approach is proposed, namely by using these converters as modules, that is, to allow the high currents to flow through nconverters. This approach is used to minimize losses as well. The authors analyzed three different configurations for these modules, namely by changing their configuration on the high-voltage (HV) and low-voltage (LV) sides. Energies 2024,17, 5579 9 of 22 Energies 2024, 17, x FOR PEER REVIEW 9 of 25 current density may vary between 900 A and 3000 A, whereas the voltage range is limited to 1.4 V to 2.5 V [54,60,61]. Other aspects must be taken into account when considering PE converter topologies for their integration with electrolyzers, such as, and foremost, a low output current ripple, diminished electromagnetic interferences, higher power density, reduced costs, high conversion ratio, reliability, and the efficiency of the electrolysis process, according to the technique applied [62]. Electrolyzer efficiencies may vary from 50% to 83% in PEM electrolyzers and may vary for other technologies, such as alkaline water electrolysis (50% to 78%), anion exchange membranes (57% to 59%), and solid oxide electrolysis cells (about 89%), which means that if the H 2 -producing element has such values of efficiency, it is crucial to increase the efficiency of the PE converters as much as possible, in order to feed the electrolyzer units, and thus, this presents a prerequisite for appointing certain PE converter topologies, such as the ones described in this paper, as adequate for H 2 production [60,61,63]. To produce hydrogen using electrolysis, only dc current can be used, and since the intended application cases are RESs, to produce G-H 2 , their coupling is made to a dc bus. The usage of dc has benefits when compared to ac, namely in terms of efficiency, reliability, the integration of battery systems and RESs, and ultimately, the elimination of frequency issues, which lead to transmission losses [63]. There are many power electronics converters that can be applied in H 2 production, and thanks to continuously growing investment and investigation in this area, this number is continuously growing. Thus, a selection of PE converter topologies is dissected in this work for the sake of understanding their position among the remainder by the analysis of their benefits and demerits. 3.1.1. Dual Active Bridge Converter In [64], the authors discuss a dual active bridge (DAB) converter as a way to interface a high-voltage DC bus with an electrolyzer. This PE converter is pictured in Figure 3. Figure 3. Dual active bridge converter. However, to handle the high currents typical in such systems, an alternative approach is proposed, namely by using these converters as modules, that is, to allow the high currents to flow through n converters. This approach is used to minimize losses as well. The authors analyzed three different configurations for these modules, namely by changing their configuration on the high-voltage (HV) and low-voltage (LV) sides. Series HV Side and Parallel LV Side (SHV-PLV) As the number of modules (n) increases in this configuration, the amount of current and voltage on each is decreased by a factor of 1/𝑛, which is beneficial, since the workload is divided, allowing for an extended lifetime of the switching elements. Such a configuration can be seen in Figure 4. Figure 3. Dual active bridge converter. Series HV Side and Parallel LV Side (SHV-PLV) As the number of modules (n) increases in this configuration, the amount of current and voltage on each is decreased by a factor of 1 /n , which is beneficial, since the workload is divided, allowing for an extended lifetime of the switching elements. Such a configuration can be seen in Figure 4. Energies 2024, 17, x FOR PEER REVIEW 10 of 25 Figure 4. Series HV and parallel LV configuration. Parallel HV and LV Sides (PHV-PLV) The configuration seen in Figure 5 presents the same advantage in terms of current distribution through the modules as the previous configuration. However, the voltage rating of each bridge switch must match the DC grid voltage. It is even possible to activate or deactivate each of the modules individually when in operation in order to increase the system’s efficiency. Figure 5. Parallel HV and LV configuration. Dedicated DAB Module for Each Electrolyzer Stack In such a configuration, each DAB module concentrates its effort on a single electrolyzer stack, as per Figure 6. The power rating for each system is given by (1/𝑛) of the given system power. Since each module is independent, the overall system efficiency can be altered for the required specifications. This configuration has the downside of being a more expensive approach, since the more stacks there are, the more DAB modules are required, and these modules must be able to handle higher currents on their own since no current is divided through the other modules, as they are separated (on the LV side). Figure 4. Series HV and parallel LV configuration. Parallel HV and LV Sides (PHV-PLV) The configuration seen in Figure 5presents the same advantage in terms of current distribution through the modules as the previous configuration. However, the voltage rating of each bridge switch must match the DC grid voltage. It is even possible to activate or deactivate each of the modules individually when in operation in order to increase the system’s efficiency. Dedicated DAB Module for Each Electrolyzer Stack In such a configuration, each DAB module concentrates its effort on a single electrolyzer stack, as per Figure 6. The power rating for each system is given by (1/n)th of the given system power. Since each module is independent, the overall system efficiency can be altered for the required specifications. This configuration has the downside of being a more expensive approach, since the more stacks there are, the more DAB modules are required, and these modules must be able to handle higher currents on their own since no current is divided through the other modules, as they are separated (on the LV side). Energies 2024,17, 5579 16 of 22 (although it is used in some studies available in the literature). Through the analysis and comparison of several other papers [ 79 – 83 ], the mentioned values were found, and they are displayed in Figure 12. Energies 2024, 17, x FOR PEER REVIEW 17 of 25 simulations, which is displayed in Figure 12, along with the values of the components that compose this model. Therefore, it was possible to analyze the behavior of the distinct power converters with a more realistic electrical model of the electrolyzer, as opposed to the use of a resistor, which is the simplest model but does not guarantee as much accuracy (although it is used in some studies available in the literature). Through the analysis and comparison of several other papers [79–83], the mentioned values were found, and they are displayed in Figure 12. Figure 12. Electrolyzer model and its characteristics. According to the aforementioned, some simulation results are presented in order to demonstrate the advantages/disadvantages of the analyzed converters, along with a simplified assessment of each converter’s efficiency under the circumstances specified in each converter diagram, shown in Figure 13. (a) Figure 12. Electrolyzer model and its characteristics. According to the aforementioned, some simulation results are presented in order to demonstrate the advantages/disadvantages of the analyzed converters, along with a simplified assessment of each converter’s efficiency under the circumstances specified in each converter diagram, shown in Figure 13. Table 4presents the values used in the simulations of the analyzed PE converter topologies. Table 4. Passive element values used in the simulation process. Converter Topology Resistors Capacitors Inductors (a) Dual Active Bridge R1.1 = 10 mΩ R1.2 = 35 Ω R1.3 = 10 mΩ C1 = 12 µF L1.1 = 1.5 µH L1.2 = 10 µH L1.3 = 1.5 µH (b) Standard Buck R2 = 10 mΩC2 = 500 µFL2.1 = 1.5 µH L2.2 = 5 µH (c) Synchronous Buck R3 = 10 mΩC3 = 10 µFL3.1 = 1.5 µH L3.2 = 250 µH (d) Stacked Buck R4.1 = 10 mΩ R4.2 = 10 mΩ C4.1 = 10 µF C4.2 = 200 µF L4.1 = 1 µH L4.2 = 1 µH Ls = 100 µH Lp = 100 µH M=5µH (e) Quadratic Buck R5 = 10 mΩC5.1 = 330 µF C5.2 = 110 µF L5.1 = 6 µH L5.2 = 1.5 µH L5.3 = 50 µH (f) Superimposed Quadratic Buck R6 = 10 mΩ C6.1 = 0.33 µF C6.2 = 1 µF C6.3 = 680 µF L6.1 = 1.5 µH L6.2 = 200 µH L6.3 = 200 µH Energies 2024,17, 5579 17 of 22 Energies 2024, 17, x FOR PEER REVIEW 17 of 25 simulations, which is displayed in Figure 12, along with the values of the components that compose this model. Therefore, it was possible to analyze the behavior of the distinct power converters with a more realistic electrical model of the electrolyzer, as opposed to the use of a resistor, which is the simplest model but does not guarantee as much accuracy (although it is used in some studies available in the literature). Through the analysis and comparison of several other papers [79–83], the mentioned values were found, and they are displayed in Figure 12. Figure 12. Electrolyzer model and its characteristics. According to the aforementioned, some simulation results are presented in order to demonstrate the advantages/disadvantages of the analyzed converters, along with a simplified assessment of each converter’s efficiency under the circumstances specified in each converter diagram, shown in Figure 13. (a) Energies 2024, 17, x FOR PEER REVIEW 18 of 25 (b) (c) Figure 13. Cont. Energies 2024,17, 5579 18 of 22 Energies 2024, 17, x FOR PEER REVIEW 19 of 25 (d) (e) Energies 2024, 17, x FOR PEER REVIEW 20 of 25 (f) Figure 13. Simulation results: (a) dual active bridge converter; (b) standard buck converter; (c) synchronous buck converter; (d) stacked buck converter; (e) quadratic buck converter; (f) superimposed quadratic buck converter. Table 4 presents the values used in the simulations of the analyzed PE converter topologies. Table 4. Passive element values used in the simulation process. Converter Topology Resistors Capacitors Inductors (a) Dual Active Bridge R1.1 = 10 mΩ R1.2 = 35 Ω R1.3 = 10 mΩ C1 = 12 µF L1.1 = 1.5 µH L1.2 = 10 µH L1.3 = 1.5 µH (b) Standard Buck R2 = 10 mΩ C2 = 500 µF L2.1 = 1.5 µH L2.2 = 5 µH (c) Synchronous Buck R3 = 10 mΩ C3 = 10 µF L3.1 = 1.5 µH L3.2 = 250 µH (d) Stacked Buck R4.1 = 10 mΩ R4.2 = 10 mΩ C4.1 = 10 µF C4.2 = 200 µF L4.1 = 1 µH L4.2 = 1 µH Ls = 100 µH Lp = 100 µH M = 5 µH (e) Quadratic Buck R5 = 10 mΩ C5.1 = 330 µF C5.2 = 110 µF L5.1 = 6 µH L5.2 = 1.5 µH L5.3 = 50 µH (f) Superimposed Quadratic Buck R6 = 10 mΩ C6.1 = 0.33 µF C6.2 = 1 µF C6.3 = 680 µF L6.1 = 1.5 µH L6.2 = 200 µH L6.3 = 200 µH 5. Conclusions Hydrogen alone will not solve the problems that the world faces regarding greenhouse gas emissions, but it can be part of the solution, through the investigation, development, and deployment of more sustainable solutions for hydrogen production, storage, and usage. The use of renewable energy sources is highly dependent on the evolution of energy storage techniques, and H 2 is presented as a promising alternative since it is the Figure 13. Simulation results: (a) dual active bridge converter; (b) standard buck converter; (c) synchronous buck converter; (d) stacked buck converter; (e) quadratic buck converter; (f) superimposed quadratic buck converter. Energies 2024,17, 5579 19 of 22 5. Conclusions Hydrogen alone will not solve the problems that the world faces regarding greenhouse gas emissions, but it can be part of the solution, through the investigation, development, and deployment of more sustainable solutions for hydrogen production, storage, and usage. The use of renewable energy sources is highly dependent on the evolution of energy storage techniques, and H 2 is presented as a promising alternative since it is the most abundant element in the universe. Considering this statement, it becomes imperative that engineers and researchers worldwide determine the best ways to make the most out of this gas, without harming the planet that we live on and at the same time ensuring benefits similar to those that we have been enjoying, such as energy and resources, that are traditionally produced/obtained from fossil fuels or in ways that are negative for the environment. There are several ways to produce H 2 , and the main ones are analyzed and presented in detail, with a focus on their advantages and disadvantages, which is an important output to highlight since they are summarized and compared in this review paper. Some of these technologies for producing H 2 are still in a preliminary stage, which means that there is much more room to either strengthen our knowledge of these newer technologies or to expand the other techniques that have already been industrialized and find better ways to use them in order to have more efficient and effective methods of producing this promising gas. As concerns G-H 2 , it is expected that through more research, development, and investment, its costs will decrease to prices that are the same as, if not lower than, the price of H 2 produced from fossil fuels. This would then lead to the worldwide adoption of H 2 in various industries and processes, as aforementioned, with the objective of reaching the 55% minimum reduction in CO 2 emissions in 2030 and the net-zero carbon goals set for 2050. Power electronics technologies for G-H 2 production are indispensable, and their weaknesses and strengths have been analyzed. This work intended to emphasize the role of power electronics, and the main focus of this review was related to the production of G-H 2 . Thus, no topology was referred to as the best or the worst, because the suitability of a topology depends on the final application, the available means, and the manufacturing costs, among other factors. One distinguished and important factor to be noted is that all the aforementioned power electronics converter topologies ensure low output current ripple, either intrinsically or through the application of external filters to limit it. Regarding the power electronics converter topologies, the standard buck, the dual active bridge (including different configurations according to different contexts), the synchronous buck, the stacked buck, the quadratic buck, and the superimposed quadratic buck were presented in detail. A comprehensive comparison between all of them was established. The standard buck converter is also featured in this review paper because, even though it may be a less effective converter due to the fact that it still introduces ripple in the output current, through the implementation of filters, it can still have a decent behavior, and thereby, when its reduced cost is considered, it still may be seen as a viable option for simpler applications. Funding: This paper is supported by the Alliance for the Energy Transition (56) co-financed by the Recovery and Resilience Plan (PRR) through the European Union. This work has been supported by FCT – Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020. Gonçalo Rego has a scholarship with the reference ATE_BI_2024_01_CALG (1). Conflicts of Interest: The authors declare no conflict of interest. References 1. U.S. Department of Energy Los Alamos National Laboratory. Periodic Table of Elements: Los Alamos National Laboratory. 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