Hydrogen Technology Development and Policy Status by Value Chain in South Korea

: Global transitions from carbon-to hydrogen-based economies are an essential component of curbing greenhouse gas emissions and climate change. This study provides an investigative review of the technological development trends within the overall hydrogen value chain in terms of production, storage, transportation, and application, with the aim of identifying patterns in the announcement and execution of hydrogen-based policies, both domestically within Korea, as well as internationally. The current status of technological trends was analyzed across the three areas of natural hydrogen, carbon dioxide capture, utilization, and storage technology linked to blue hydrogen, and green hydrogen production linked to renewable energy (e.g., water electrolysis). In Korea, the establishment of underground hydrogen storage facilities is potentially highly advantageous for the storage of domestically produced and imported hydrogen, providing the foundations for large-scale application, as economic feasibility is the most important national factor for the provision of fuel cells. To realize a hydrogen economy, pacing policy and technological development is essential, in addition to establishing a roadmap for efﬁcient policy support. In terms of technological development, it is important to prioritize that which can connect the value chain, all of which will ultimately play a major role in the transformation of human energy consumption.


Introduction
A global transformation from a carbon to hydrogen economy is underway to help alleviate the effects of climate change.Such an economy consumes hydrogen as its main energy source, where hydrogen brings about fundamental changes in the national economy, society, and lifestyle of citizens through an environmentally friendly energy source [1].Hydrogen produced from coal, natural gas, nuclear energy, and renewable energy sources is utilized as a primary energy source through its storage and application.Recently, there has been increasing interest in hydrogen technology to realize carbon neutrality, as no greenhouse gas emissions are produced by green hydrogen production technologies, or their application to transportation and power generation [2].Currently, 50 million tons of hydrogen is consumed annually worldwide, and is used not only as an energy source, but also an industrial raw material across various areas, such as semiconductor and display manufacturing, ammonia production, chemical raw materials, and petroleum refinement [3].Hydrogen generates electricity and heat through a simple chemical reaction with atmospheric oxygen, and water is the only byproduct.Although hydrogen is an abundant resource that accounts for 75% of all materials in outer space, it exists solely in the forms of water or organic compounds; thus, the direct application of hydrogen requires advanced technical expertise.Regardless, hydrogen production has the advantage of enabling long-term and large-capacity storage for supply or utilization, there is a significant expectation of commercialization; accordingly, various policies regarding the development of hydrogen-related core technologies are being established in participatory countries to promote a hydrogen economy [1].Hydrogen production technology is thus being developed to minimize the associated greenhouse gas emissions, thereby reducing the impacts on climate change in large-volume hydrogen manufacturing methods based on natural gas and coal.The importation of hydrogen is also being considered [4].To use hydrogen as the main energy source, the domestic supply and charging infrastructure require expansion, as should receiving terminals and storage facilities when importing from abroad; thus, technologies for the production, storage, transportation, and application of hydrogen are being developed simultaneously.In this study, the technological development trends within the overall hydrogen value chain were investigated in terms of hydrogen production, storage, transportation, and application, with the aim of identifying patterns in the announcement and execution of hydrogen-based policies both domestically in Korea and internationally.

Country-Specific Policies
In connection with the New Deal and carbon neutrality policies, the technological development of the renewable energy sector is progressing rapidly, and policy proposals are being continuously announced accordingly [5].In particular, the global demand for hydrogen has increased, while the hydrogen industry is expected to create 30 million new jobs and generate USD 2.5 trillion annually by 2050 [6].Accordingly, individual countries have announced various policies, visions, and technological roadmaps for promoting the hydrogen economy (Table 1).

Europe
The European Union (EU) announced "A Hydrogen Strategy for a Climate-Neutral Europe" in July 2020 [7].With almost no pollutant emissions, including carbon dioxide, hydrogen was considered an effective and suitable technology for carbon reduction, and was presented as an essential technology for achieving carbon neutrality by 2050 [8].The EU officially established the European Clean Hydrogen Alliance, which provides a strategic roadmap for the hydrogen field, and promotes cooperation between public institutions, industry, and civil society.
The "2050 Hydrogen Roadmap" of the EU focuses on renewable hydrogen derived from solar energy, but presents the need for low-carbon hydrogen as a medium-term transitional stage [9].Notably, the utilization of low-carbon hydrogen requires price competitiveness in carbon dioxide capture.
The plan for 2050 was divided into three stages for the completion of the European hydrogen ecosystem.The first stage, 2020-2024, targeted the decarbonization of industrial plants and large-scale commercialization by installing renewable hydrogen electrolytic cells with a minimum of 6 GW, and a maximum renewable hydrogen production of 1 million tons.Regarding policies, measures to introduce a hydrogen market regulatory framework and revitalize the hydrogen market were included, as well as methods to support the establishment of a robust investment plan.In the second stage (2025-2030), the goal was to install a minimum of 40 GW electrolytic cells and produce a minimum of 10 million tons of renewable hydrogen, while measures for the establishment of hydrogen are to play a key role in the integrated energy system.For policies, measures were presented to promote investments in the strengthening of EU resources, and the establishment of a full-fledged hydrogen ecosystem within 5 years.The final goals in the third stage (2030-2050) surround the decarbonization of all fields, where additionally carbon reduction is difficult through renewable hydrogen technology that has entered maturity.Additionally, a plan for the amendment of related legislation was presented for sequential progression, in accordance with the infrastructure establishment for each stage [10].

United States
The United States (US) has announced the "Road Map to a US Hydrogen Economy" through the Fuel Cell and Hydrogen Energy Association in March 2020 [11].This plan revealed the long-term vision for the future hydrogen and fuel cell industry, as well as policy proposals to achieve it considering the technological development stages and market maturity, with respect to 2030 and 2050 policy timelines.Notably, the US is pursuing a hydrogen policy led by the state of California, where the state and federal government have formed a "public-private partnership" to promote hydrogen energy policies.
The US Department of Energy (DOE) has also shown a significant interest in hydrogen development to reduce the costs of various energy sources, while improving the production and application of domestic energy resources [12].In particular, hydrogen produced from fossil fuels will be an important element in transitioning to a low-carbon economy; corresponding technological developments are underway [13].Relevant departments within the DOE, including the Office of Fossil Energy, Office of Renewable Energy, and Office of Nuclear Energy, are involved in various technologies, such as gas reforming, blue hydrogen connected with carbon dioxide capture, utilization, and storage (CCUS), and green hydrogen (using water electrolysis) [11].
The proportion of blue hydrogen production in the US is relatively high, as the maturity of national CCUS technologies are relatively advanced with a price competitiveness of approximately USD 2 per kg, even when compared with fossil fuels.Within the US, the majority of the demand is accounted for by petroleum refinement and ammonia production, and it is expected that hydrogen demand will increase over a large scale, even across the fields of power generation, buildings, and transportation until 2050 [14].To increase hydrogen production, related technology projects and programs, such as the Hydrogen Program Plan, are being implemented [12].

Japan
Japan issued its Basic Hydrogen Strategy in December 2017, and presented a roadmap for 2050 [15].Nationally, the hydrogen economy was intensively cultivated to support a self-sufficient energy supply following the 2011 Fukushima accident; Japan has since played a leading role in the development of natural gas reforming hydrogen production and liquefaction technology.The main components of the roadmap include the establishment of an international hydrogen supply chain utilizing unused energy sources from overseas, such as Australian lignite, as well as the presentation of a hydrogen production method that actively employs renewable energy and unused regional resources [16].In terms of application, goals have focused on stable and large-scale consumption through hydrogen power generation, including the expanded use of hydrogen in transportation (hydrogen-based automobiles, ships, trains, and forklifts), and energy reduction through the application of fuel cells for home use [17].Furthermore, Japan is focused on the development of a hydrogen supply chain, including low-cost hydrogen consumption, liquid hydrogen, P2G, and overseas production.

China
In March 2021, China had announced the "3060 Target" that aims to achieve carbon neutrality by 2060, after reaching peak national carbon emissions before 2030 [18].China is in the process of transitioning its national energy system into a clean and low-carbon system.Accordingly, hydrogen was selected as the main energy source for decarbonization and realizing carbon neutrality; thus, major advancements in the hydrogen energy industry are being carried out by major companies [19].The hydrogen market size and growth rate in China have been rapidly increasing since 2020.Through the "Eco-friendly Automobile Industry Advancement Plan (2020-2025)", China announced its intention to have 20% of all automobiles sold in 2025 be environmentally friendly [20].Moreover, plans for largescale investment facilities, including public charging stations and smart cities have been presented.Notably, China has the third highest number of hydrogen refueling stations in the world (69 stations by the end of 2020), following Japan (154) and Germany (91).As such, China is conducting large-scale research and development investments to become a leader in hydrogen energy-related technologies, and is expanding its corresponding market shares in fuel cells and transportation [21].

South Korea
In January 2019, Korea had announced its vision to become a world-class leading nation in the hydrogen economy by adopting the "Hydrogen Economy Activation Roadmap" (Table 2) [22].In particular, the establishment of an economical, stable hydrogen production and supply system was presented based on the creation of an industrial ecosystem that could lead to hydrogen economies across the transportation and power generation sectors [23].Strategies for systemic establishment included the production of a crossministerial hydrogen technology development roadmap, ecosystem cultivation, safety management standards, and the enactment of related legislative policies to serve as the foundations for the hydrogen economy [24].In October 2019, a cross-ministerial hydrogen technology development roadmap was established to support the implementation of the hydrogen economy by improving the domestic technological competitiveness in hydrogen energy.
On 7 October 2020, the "Vision for Becoming a Leading Hydrogen Economy Nation" was announced, representing the selection of the hydrogen industry to achieve carbon neutrality by 2050 [25].Through this announcement, the government presented its plans to establish a domestic 1-million-ton clean hydrogen production system; cultivate 30 global hydrogen companies; create 500,000 hydrogen-related jobs by 2030; and invest USD 36 billion by 2030.
On 26 November 2021, the first hydrogen economy blueprint was announced, representing the first plan after the enactment of the Hydrogen Economy Promotion and Hydrogen Safety Management Act.This plan contained 15 implementation tasks based on four major strategies of leading domestic and overseas clean hydrogen production, building a tight-knit infrastructure, utilizing hydrogen in everyday life, and strengthening ecosystem foundations.The government announced plans to transition domestic and overseas hydrogen production into clean hydrogen supply systems, and among the green, blue, and gray hydrogen supplies, only the former two will account for the 27.9 million annual tons of hydrogen by 2050 [26].In addition to production, the commercialization of hydrogen power generation will be initiated to expand the supply of fuel cell power generation facilities, and apply them to coal-ammonia co-firing power generation (mixed combustion) and liquefied natural gas (LNG) hydrogen co-firing techniques.For the transport sector, the production of hydrogen cars is to be expanded to 5.26 million vehicles annually by 2050 through the improvement of hydrogen car performance to levels on par with that of internal combustion engine vehicles by 2030, and the expansion of hydrogen application to various transportation modes, including ships, drones, and trams [23].Moreover, industrial processes that produce high amounts of greenhouse gases, such as steel, petrochemical, and cement production, will be transitioned to hydrogen-based processes; whereas fuels and raw materials will be replaced with hydrogen.Cross-ministerial hydrogen R&D, and professional manpower cultivation are also planned, where its corresponding implementation will lead to hydrogen accounting for 33% of the total national energy consumption, and 23.8% of power generation by 2050 [26], making hydrogen the largest single energy source.Hydrogen thus has the most powerful means of achieving carbon neutrality by 2050, and various hydrogen economy supporting policies to be are expected to be proposed continuously.

Technology Development Status for Each Hydrogen Value Chain
The hydrogen technology value chain can be divided into four parts, namely production, storage, transportation, and application [28], whereas the basic materials for hydrogen production can be further divided into fossil fuels, renewable energy, and overseas imports, and the actual hydrogen produced can be divided into byproducts and hydrogen.Hydrogen storage itself can be split into physical storage methods including tanks and trailers, as well as compound storage methods.For transportation, tube trailers and long-term supply using natural-gas pipelines are available options.In the early stages of hydrogen adoption, the tube trailer is expected to be widely used in the mobility field, which generally has a smaller capacity, while future expansions of applications to homes and industry are planned [29] (Table 3).

Hydrogen Production
Hydrogen production technologies start with hydrogen-containing compounds and are categorized according to the raw materials and manufacturing methods employed [30].Global energy infrastructure (GEI) attempted to set the "color debate" and reported clear definitions of various hydrogen production technologies.In total, hydrogen is designated by nine colors according to the production method (Figure 1) [31].
heat ing, among others securing a power generation system using carbon-free fuel

Hydrogen Production
Hydrogen production technologies start with hydrogen-containing compounds and are categorized according to the raw materials and manufacturing methods employed [30].Global energy infrastructure (GEI) attempted to set the "color debate" and reported clear definitions of various hydrogen production technologies.In total, hydrogen is designated by nine colors according to the production method (Figure 1) [31].
In this study, the current status of technology trends was analyzed across the three areas of natural hydrogen, CCUS technology linked to blue hydrogen, and green hydrogen production powered via renewable energy (e.g., water electrolysis).In this study, the current status of technology trends was analyzed across the three areas of natural hydrogen, CCUS technology linked to blue hydrogen, and green hydrogen production powered via renewable energy (e.g., water electrolysis).

Natural Hydrogen
Natural hydrogen refers to hydrogen in its natural form from within the Earth's system.Hydrogen is virtually nonexistent in the crust or mantle, as it geochemically reacts with oxygen during the formation of the primitive crust to release energy, and form water or hydrated minerals [32].Furthermore, despite the argument that crustal hydrogen will not readily exist due to its extremely light weight, hydrogen can be generated through geological processes within the crust, such as hydrothermal alteration [33].Accordingly, natural hydrogen is defined as "hydrogen produced inorganically through solid-solid, solid-liquid, or liquid-liquid reactions that occur due to the internal energy within the crust or mantle", and its primary sources are as follows [32]: Hydrogen ejected from magma on continents and oceans.

2.
Produced in large-scale fragmentation processes of rocks.
Produced by the reduction of water during the reaction process of hydrothermal water and minerals containing Fe 2+ .
Most natural hydrogen found on land exists in the gaseous form within pure gas or fluid inclusions near volcanic rock, metamorphic rock, and iron ore.It has been estimated that the amount of natural hydrogen released is 23 million tons per year [32].
Natural hydrogen is extremely likely to exist in the terrestrial continental crust because of the oxidation-reduction reactions of Fe and S formed during the Paleozoic era.Notably, such hydrogen will be a critical component of the future global energy industry, considering its environmentally sustainable development conditions, and low economical production cost (<1 USD per kg).

Production via Fossil Fuels
(1) Grey Hydrogen; natural Gas Reforming Decomposing the natural gas methane using steam methane reforming (SMR) at high temperatures and pressures results in the extraction of gray hydrogen [34].Hydrogen production via SMR has already been commercialized, and is advantageous for its largescale production potential at low production costs; however, carbon dioxide (a notable greenhouse gas) is produced during the hydrogen manufacturing process [35].
(2) Blue Hydrogen with CCUS Since any produced CO 2 must be processed to use hydrogen as the main carbon neutral energy source, the application of CCUS technology is being explored to produce blue hydrogen [36].Although the unit cost of gray hydrogen is competitive (USD 1.3 per kg), the application of CCUS technology to produce blue hydrogen doubles the cost [37]; thus, to commercialize blue hydrogen and supply it on a large scale, attaining competitiveness through the advancement of CCUS technology is critical.Specifically, CCUS technology can be categorized into carbon dioxide capture and utilization (CCU), as well as and carbon dioxide capture and storage (CCS) technologies.Currently, the application of CCS technology is more economically viable for the supply of blue hydrogen due to its relatively higher technological maturity, and the capacity to store large amounts of CO 2 in underground spaces (e.g., gas reservoirs) [38].
Recently, the first blue hydrogen product project in Korea was carried out in connection with the CCS integrated demonstration project using the East Sea gas reservoir [39].This project is the first domestic commercial scale R&D project and blue hydrogen production project, and is an outstanding demonstration project that is stable and economical.Additionally, the world's first blue hydrogen plant has been planned for construction in Boryung, Chungnam, to supply hydrogen power plants and refueling stations by producing 250,000 tons of hydrogen starting in 2025 [39,40].
To fulfill the carbon neutrality policy, active research on blue hydrogen is being conducted to supply commercial supplies that are economically feasible, safe, and business viable.

Green Hydrogen
Water electrolysis produces green hydrogen with zero carbon emissions.Here, hydrogen is produced by electrolyzing water using electricity generated via renewable energy sources (e.g., solar and wind) [38]; thus, the two most important aspects of green hydrogen production are: (1) securing renewable energy power, and (2) water electrolysis technology.Hence, power-to-gas (P2G) technology must be developed in conjunction with hydrogen production technology by utilizing renewable energy power [41].
Accordingly, renewable energy facilities are a prerequisite for the production of green hydrogen; however, some countries, such as Korea, cannot create large-scale renewable energy facilities owing to geographical or environmental limitations.In such cases, it is necessary to install facilities overseas, or import green hydrogen produced overseas.
Notably, approximately 80% of the cost of producing green hydrogen is related to electricity generation from renewable energy; the lack of competitiveness in the price of electricity generation will consequently make domestic green hydrogen commercialization difficult.To overcome this limitation, imported hydrogen can be produced overseas as part of the supply.The greatest advantage of overseas hydrogen production is that it contributes to the stable supply of hydrogen and cost stability and reduces the costs of domestic hydrogen production and greenhouse gas emissions.To import hydrogen, hydrogen transport ships with storage containers and other related technologies must also be developed; thus, there is a positive effect of fostering other related industries.For the stable import of overseas hydrogen, the infrastructure of hydrogen-receiving terminals and storage facilities, such as LNG storage, must be expanded and related technologies developed.
In Korea, green hydrogen commercialization is being developed on Jeju Island by carrying out the demonstration of large-scale green hydrogen production, storage, and application in connection with renewable energy for the first time in Korea [42].In addition, through the hydrogen economy activation roadmap of the government, the goal is to move toward becoming a green hydrogen-producing nation using water electrolysis and hydrogen produced overseas.In connection with this, the plan is to expand hydrogen production from 130,000 tons in 2018 to 5.26 million tons in 2040 and reduce the cost of hydrogen cost through stable mass production [27].Therefore, the production technology of green hydrogen is important in terms of hydrogen supply and cost stability [27].

Hydrogen Production from Biomass
The hydrogen production method using biomass is a technology that produces hydrogen through anaerobic fermentation of microorganism from biomass and biomass-derived organic compounds.This method is also one of the technologies that is attracting attention as an extension of waste resource utilization technology.There are three methods for producing hydrogen using biomass, namely thermochemical, biological, and electrochemical [43].
(1) Aqueous phase reforming (APR) APR method is one of the thermochemical approaches that converts an oxygenated compound into hydrogen [44].The APR process must have a catalyst, with a temperature range of 220-270 • C and pressure of 30-60 bars [44].The solution maintains a liquid state to prevent vaporization, which has an advantage in terms of energy [45].An important factor in maximizing the efficiency of hydrogen production in the APR process is the design of catalysts, which is as follows: (1) dehydrogenation, C-C bond breaking, H 2 O activation, and water gas shift reaction should be favored; (2) C-O bond breaking, methanation/Fischer-Tropsch and dehydration should be avoided [44].The APR mechanism is shown in Figure 2.  2

) Microbial systems
Hydrogen can be produced through electrolysis using microorganisms as catalysts.The simplest carbon source is acetate as a method of activating the fermentation of soluble organic substances found in wastewater.The system consists of an anode and a cathode, a cation exchange membrane between the electrodes, and external electricity [46].In this (2) Microbial systems Hydrogen can be produced through electrolysis using microorganisms as catalysts.The simplest carbon source is acetate as a method of activating the fermentation of soluble organic substances found in wastewater.The system consists of an anode and a cathode, a cation exchange membrane between the electrodes, and external electricity [46].In this system, bacteria are located on the anode surface, and through redox reactions, they oxidize the carbon source of wastewater to carbon dioxide, electrons [47], protons, and on the anode surface, they react.Electrons are transferred to the positive electrode via an external power source, which is a system in which hydrogen is generated by combining with free protons in the electrolyte [47,48].External power can be of a different source including solar power, microbial fuel cells, and wind energy.
The energy required during this process is 0.11 V lower than water electrolysis, hence the low energy loss [49].This technology is more efficient when combined with a dark fermentation process, and therefore promising [50].
The cost values and properties of each hydrogen production technology were compared in Table 4. Natural hydrogen is an unfamiliar field compared to other technologies, but it can be produced 23 million tons per year and reproduced.Although it is an attractive technology because of its low production cost, it is very difficult to capture hydrogen from underground.Grey hydrogen from the SMR process has a low production cost and it is a mature technology for commercialization.However, there is a disadvantage in that it emits a large amount of CO 2 .Blue hydrogen technology, which removes CO 2 from the SMR process, complements these shortcomings and has the advantage of only adding the CO 2 utilization/storage processes and costs in the existing system.Green hydrogen produced by wind power has a high production cost, even if it is a clean and sustainable technology.Hydrogen production systems using renewable energy have limitations in geological characteristics, and for example, it is difficult to adopt large renewable energy systems in Korea due to the small land area.The common characteristic of biomass-derived hydrogen production technology is a high hydrogen production rate.A relatively simple system is also an advantage, but there is a disadvantage in that catalysts are required and a separation process is required for hydrogen capture.In terms of the economy, it is between blue and green hydrogen.

Hydrogen Storage
Hydrogen storage has both physical and chemical components [58].The volume of hydrogen is large at room temperature; thus, a reduction in transportation costs requires large-scale storage and transport technologies; however, methods other than compressing hydrogen gas for storage and transporting remain in the technological development stages.Therefore, increasing the transportation capacity of hydrogen through the advancement of gas storage and transport technology, as well as liquid hydrogen and liquid hydride storage technologies are being pursued.

Physical Storage (1) Liquid Hydrogen Storage [59]
Liquid hydrogen can be stored by applying pressure after cooling.When the temperature is lowered, the volume of hydrogen decreases to approximately 1/800th; hence, it is advantageous in that the volume decreases with less pressure than at room temperature.Unlike the high-pressure storage hydrogen tanks currently used, this method allows storage at atmospheric pressure, enabling a safer and lower-cost transport.Notably, the US, Japan, and Europe sell and operate commercial hydrogen liquefaction plants.
Research on large-capacity liquefied hydrogen storage facilities is underway in Europe, though it will take time to store and use large-capacity hydrogen depending on the demand for hydrogen.In addition, the cost for technology for storing large amounts of liquefied hydrogen should also be considered.The storage and transportation of liquefied hydrogen relays on the LNG field.The LNG acquisition and the system of the large-scale storage tanks can help facilitate research on liquefied terminals and the diversification of the size of storage tanks [60].
(2) Geological Storage [61] When the hydrogen supply infrastructure is established and demand increases, the tube trailers may not then be sufficient, hence medium-to-large scale storage facilities will be necessary.Currently, leading nations such as the US and European countries are developing underground storage technologies to meet hydrogen energy needs.Furthermore, underground storage technologies must be considered for countries with high population densities and a low land area, as it is advantageous in terms of safety, site selection, and the alleviation of storage capacity limitations.
When storing liquid hydrogen aboveground at extremely low temperatures (−235 • C), the vaporization loss rate is significant, owing to the temperature variation from seasonal changes and climate change, while the overall stability decreases due to exposure to natural disasters and accidents, such as earthquakes, typhoons, heat waves, and acts of terror; thus, aboveground hydrogen energy storage facilities are not only accompanied by safety risks, but also difficulties related to site selection and costs for large-scale storage as well.Such facilities are mostly located in remote locations, thereby reducing the economic feasibility owing to the required transportation along with issues related to the residents' opposition [62].
Currently, all domestic and international underground hydrogen energy storage cases employ gaseous hydrogen storage technology using existing natural and manmade environments, such as salt domes or unused oil and gas fields.When cryogenic temperature conditions have to be maintained, the bedrock can act as insulation from aboveground temperature changes, thereby minimizing loss from vaporization.Underground communal storage technology is optimal for cryogenic liquid hydrogen storage, offering high site utilization, and is advantageous for the establishment of power generation and production facilities, as well as common infrastructure as there is little exposure to aboveground [63].
In particular, the Global Technology Network Program Hydrogen Technology Collaboration Programme (TCP) under the International Energy Agency (IEA) reported that large-scale underground storage facilities, and consequently transportation technology will be necessary by 2030, when the global supply of hydrogen is expected to significantly increase.In Korea, the establishment of hydrogen underground storage facilities is ex-pected to be highly advantageous for the storage of domestically produced and imported hydrogen, providing the foundations for the application of large-scale hydrogen, as well as mobility along related with power generation and industry.

Chemical Storage
Chemical storage of hydrogen relays on compounds, such as ammonia or a liquid organic hydrogen carrier (LOHC), to transform hydrogen into different substances for liquid storage.Transformation using a material containing a carbon compound results in an organic hydride, whereas using carbon-free materials results in inorganic hydrides, the most representative of which is ammonia.
(1) Ammonia [64] Ammonia storage of hydrogen works by conversion to ammonia, and subsequent transformation back to hydrogen for use [65].Ammonia can be transported and stored in liquid form at room temperature and pressure, and has doubled the storage density of liquefied hydrogen at the same volume [66].Furthermore, ammonia maintains a liquid form even at −33 • C, and requires less energy for liquefaction than liquid hydrogen [67].When returning to hydrogen, less voltage is required in the extraction of ammonia compared to that via water electrolysis, and the reaction rate is also faster; however, there is risk of toxicity and explosion, and a conversion facility is required to extract hydrogen from ammonia [65].
(2) Liquid Organic Hydrogen Carrier (LOHC) [68,69] LOHC is a hydrogen storage method in which hydrogenated and dehydrogenated materials exchange hydrogen via a catalytic reaction.The safe storage and transportation of large quantities of hydrogen in liquid form at room temperature and pressure are possible with LOHC, which reduces the costs associated with hydrogen infrastructure establishment, storage, and transportation.In addition, LOHC technology has a great advantage that it can be applied directly to the current industrial site.This can be used as an opportunity to promote active R&D in terms of accelerating the hydrogen economy the fastest [70].
(3) Metal Hydride [71][72][73] Compound storage uses the chemical bond between hydrogen and chemical substances; specifically, metal hydrides can be used to trap hydrogen within a chemical substance to be stored in a solid form.This method stores hydrogen in an alloy between inorganic materials containing light metals, such as palladium, magnesium, lanthanum, and aluminum, or metalloids such as boron.Such solid storage methods remain in their early stages of development; thus, the storage capacity is small, and the problem of long-term technology development demands further attention.

Transportation
The hydrogen transport method is almost similar to LNG.Most countries require hydrogen to be supplied to pipelines of small capacity transported via tube trailers.Since it is very important to understand the basic properties of hydrogen in hydrogen transportation, basic information related to hydrogen power density and ignition are summarized in Table 5.The most important thing in hydrogen transport is to move it safely, since the power density of hydrogen is approximately 33.3 kWh/kg, 2.5 times larger than that of LNG (12.5-14.0kWh/kg) [66].In addition, if more than a certain percentage of oxygen is encountered in an enclosed space, it is likely to explode [69].
Hydrogen cars, taxis, buses, and trucks were included in the roadmap of transportation hydrogen economy activation [74].Specifically, the plan is to expand hydrogen refueling stations and clean transportation infrastructure, along with the construction of production lines for all types of hydrogen vehicles.In total, 12.4 million hydrogen vehicles of all types (5.8 million for Korea domestic consumption) will be manufactured with 1200 hydrogen refueling stations by 2040.The costs for a hydrogen taxi pilot study are to be supported, and 10 vehicles will be operated on real road environments until reaching their endurance limits (200,000 km) to verify, analyze, and improve the core component performance.Moreover, there are plans to establish infrastructure for hydrogen buses, increase their provision, and verify the phased replacement of police buses with hydrogen buses.The establishment of the infrastructure mentioned above is thus important for the supply of hydrogen cars, taxis, and buses.In addition, the economic and institutional support for taxi and bus companies should be strengthened for the phased increase in supply.Regarding public transport, there needs to be significant improvements in fuel efficiency and durability, as many consumers use these services, in addition to the unified establishment of safety standards [75].
In response to recent environmental regulations, technology development and verification are underway, regarding fuel cell-powered transportation modes such as hydrogen ships, trains, and drones.For the latter, long endurance flights are possible in comparison to lithium battery drones; thus, hydrogen drones might be able to enter the commercial drone market for those requiring longer flight times, such as during agricultural use, in the fields of logistics and delivery, and in particular, drones used for initial response, including searching for survivors following a disaster [76].Boiling point (K) 20 Diffusion coefficient in air (cm 2 /s) 0.610

Power Generation
The goal of power generation is to use hydrogen as an energy source for eco-friendly distributed power.The supply target of 2.1 GW in 2040 was derived from the current 7 MW for fuel cells in homes and buildings, whereas fuel cells for power generation are to be greatly expanded from the current 307.6 MW to 15 GW by 2040 [42].Through increased fuel cell installations, correlated installation and power generation unit costs will be substantially reduced to the levels of small-and medium-sized gas turbines.Through a connection with installation expansion, an industrial ecosystem will be created through 100% localization of parts [25].The removal of investment uncertainty and securement of economic feasibility are to be supported through the introduction of a fuel cell-exclusive gas rate system according to the supply of fuel cells and the preferential treatment of green hydrogen renewable energy certificates (RECs).Furthermore, there are mid-to long-term plans for commercialization after 2030 through the development of hydrogen gas turbine power generation technology [23].
Specifically, in the case of fuel cells for power generation, unit cost reduction is possible through scaled economies when supplying 1 GW cumulatively in 2022; whereas there are plans to attain 35% of the installation cost, and 50% of the power generation cost by 2040 compared to present day.
Economic feasibility is the most important factor for the provision of fuel cells.The Korean government has presented a target of introducing a fuel-cell-exclusive LNG rate system and maintaining fuel-cell RECs for a specified period to remove investment uncertainty, and secure economic feasibility.Moreover, in the mid-to-long terms, RECs are planned to receive preferential treatment when green hydrogen is used.Considering the current status of domestic technology, research on cell electrodes and catalysts, along with fuel converter catalysts, remains lacking.The localization of all fuel cell core components, except for imported materials (e.g., catalysts) is to be completed by 2022, and competitiveness will be strengthened through the long-term technology development for imported materials (catalysts, electrodes, and separator, among others) [79].
Hydrogen gas turbines, an important technology for power generation, are necessary to overcome the disadvantages of renewable energy (e.g., high price for system construction and environmental pollution) [80], and corresponding technological development will be promoted by categorization into hydrogen co-firing (large scale), and hydrogen single-firing (small scale), simultaneously establishing demonstration infrastructure.
In the case of hydrogen co-firing, the goal of expanding the application of large-scale gas turbines by 2026 in addition to commercial applications after 2030 was presented based on technological development and empirical research.In addition, in the case of hydrogen single firing, commercialization in 2028 is planned through the joint development and demonstration of 1 MW hydrogen gas turbines in 2026.When developed, hydrogen gas turbine technology can be applied to power plants and expanded to apply hydrogen co-firing technology through demonstration.
The Hydrogen value chain and specific technologies of each technology were summarized in Figure 3.
Economic feasibility is the most important factor for the provision of fuel cells.The Korean government has presented a target of introducing a fuel-cell-exclusive LNG rate system and maintaining fuel-cell RECs for a specified period to remove investment uncertainty, and secure economic feasibility.Moreover, in the mid-to-long terms, RECs are planned to receive preferential treatment when green hydrogen is used.Considering the current status of domestic technology, research on cell electrodes and catalysts, along with fuel converter catalysts, remains lacking.The localization of all fuel cell core components, except for imported materials (e.g., catalysts) is to be completed by 2022, and competitiveness will be strengthened through the long-term technology development for imported materials (catalysts, electrodes, and separator, among others) [79].
Hydrogen gas turbines, an important technology for power generation, are necessary to overcome the disadvantages of renewable energy (e.g., high price for system construction and environmental pollution) [80], and corresponding technological development will be promoted by categorization into hydrogen co-firing (large scale), and hydrogen single-firing (small scale), simultaneously establishing demonstration infrastructure.
In the case of hydrogen co-firing, the goal of expanding the application of large-scale gas turbines by 2026 in addition to commercial applications after 2030 was presented based on technological development and empirical research.In addition, in the case of hydrogen single firing, commercialization in 2028 is planned through the joint development and demonstration of 1 MW hydrogen gas turbines in 2026.When developed, hydrogen gas turbine technology can be applied to power plants and expanded to apply hydrogen co-firing technology through demonstration.
The Hydrogen value chain and specific technologies of each technology were summarized in Figure 3.

South Korea's Status and Strategy
In this chapter, we discussed the trends and standards in Korea in terms of technology through the value chain mentioned above.The roadmap for hydrogen technology

South Korea's Status and Strategy
In this chapter, we discussed the trends and standards in Korea in terms of technology through the value chain mentioned above.The roadmap for hydrogen technology development recently announced in 2019 is being reorganized in accordance with the current situation and global trends.We will organize new goals and key indicators together (Table 6).
In the field of hydrogen production, South Korea is currently developing technologies to produce large-capacity blue hydrogen in cooperation with natural gas modified hydrogen production technologies.In South Korea, demand for hydrogen is increasing, but due to geographical requirements, there remain limitations that hinder the construction of a large-scale renewable energy system, and hence blue hydrogen plays an important role in the transition to green hydrogen.In the case of green hydrogen, some domestic renewable energy systems are being used for production, but most of the products are imported from overseas.We are also conducting pilot-scale research on technologies that utilize waste resources/biomass to produce hydrogen.In the case of hydrogen production, a strategy has been announced to increase price competitiveness and efficiency by receiving electricity.The goal is to reduce hydrogen prices to approximately USD 2 per kg in 2050, with water electrolysis efficiency set at 43 kWh/kgH 2 .
For hydrogen storage, South Korea is developing the domestic production of smallcapacity hydrogen liquefaction plants, LOHC, as well as ammonia storage and extraction systems.To limit dependency on overseas technology, we aimed to develop our own technology and have a strategy to build a large-scale hydrogen supply infrastructure to meet domestic hydrogen demand.We are considering storage materials such as green hydrogen that can transport large volumes of overseas hydrogen stably, underwriting bases and hydrogen/methane mixed combustion technology to use existing LNG pipelines.In the case of hydrogen liquefaction plants, the goal is to build plants and core equipment domestically by 2030 at 5 tons per day, which will increase to 300 tons per day by 2050.
For hydrogen utilization, we are developing hydrogen reduction steel technology along with research to reduce the price and increase efficiency of fuel cell materials/parts/systems.Currently, the city's gas-based fuel cell power generation system is the main focus; the core materials and components of the fuel cell are imported.Therefore, the main goal in the field of fuel cells is to develop domestic production technology with the strategy to develop a power generation system utilizing carbon-free fuels (hydrogen and ammonia).Securing hydrogenreducing steelmaking technology is an important strategy as greenhouse gases are emitted via the use of fossil fuels in coal power generation and steel/cement industries.By 2030, the power generation sector has established a core indicator of over 90% efficiency of fuel cell systems, 50% hydrogen-mixed combustion gas turbine power generation, and 20% ammonia mixed combustion boiler power generation.
Due to the rapid increase in demand for hydrogen, South Korea aims to realize the hydrogen economy through technology development for all value chains.Therefore, the low price and high efficiency performance of hydrogen production are important.Securing storage and transportation infrastructure and establishing a technology system that utilizes domestic production will be significantly beneficial strategies.

Discussion
There is increasing interest in hydrogen technology in relation to the New Deal and carbon-neutral policies.In each country, hydrogen economy implementation guidelines and a hydrogen technological development roadmap at the government level were announced, including research, development, policy, and institutional support.In R&D, a roadmap including the entire value chain from hydrogen production to storage, transportation, and application was presented with the goal of expanding support for the entire process, from basic fundamental technologies to demonstration and commercialization, in conjunction with R&D in the fields of production, storage, and transportation.Regarding applications, support has been planned for the localization of core technologies to main-tain global competitiveness in hydrogen vehicles and fuel cells, and the establishment of large-scale demonstration platforms has been announced for the commercialization and establishment of infrastructure, including hydrogen refueling stations.Moreover, the plan is to stably implement a hydrogen economy by establishing the safety of all hydrogen technology fields across the value chain.For this, professional manpower in safety management must be cultivated, as well as significant support for fostering human resources regarding the development of core technologies.The establishment of standard cooperation systems is promoted through leadership in international standardization activities, along with the localization of technological development in an effort to become a leader in the hydrogen economy.A hydrogen industry cluster and hydrogen city are planned, and institutional foundations will be prepared through the enactment of legislation for hydrogen economic support.
To realize a hydrogen economy, it is important to keep pace with policy and technological development, in addition to establishing a roadmap for policy support so that development can proceed efficiently and effectively.Such efforts also require support for customized technological development and policies within each country, which are notably not in accordance with the global roadmap.
In terms of technological development, it is important to prioritize those that can connect the value chain, as opposed to a single area of chain.If the pace of technological development in all fields is aligned, the hydrogen economy can be implemented more efficiently and successfully, which will ultimately play a major role in the transformation of human energy.

Energies 2022 ,
15,  x FOR PEER REVIEW 10 of 20 sign of catalysts, which is as follows: (1) dehydrogenation, C-C bond breaking, H2O activation, and water gas shift reaction should be favored; (2) C-O bond breaking, methanation/Fischer-Tropsch and dehydration should be avoided[44].The APR mechanism is shown in Figure2.

Figure 2 .
Figure 2. Mechanism underlying APR conversion of a diol component into hydrogen (reprinted with permission from ref. [43] Copyright 2022 Elsevier).

Figure 2 .
Figure 2. Mechanism underlying APR conversion of a diol component into hydrogen (reprinted with permission from ref. [43] Copyright 2022 Elsevier).

Figure 3 .
Figure 3. Specific technologies by hydrogen vale chain.

Figure 3 .
Figure 3. Specific technologies by hydrogen vale chain.

Table 1 .
Hydrogen policy strategies and plans for key countries.

Table 3 .
Definition and future technologies according to hydrogen value chain.

Table 6 .
[81]a's hydrogen technology and economy strategies by value chain from Korea's hydrogen technology development roadmap 2.0[81].