Hydrogen is an energy carrier, not an energy source, which makes it similar to electricity. Both of them can be produced by various energy sources and technologies; both can be used in many different applications; no greenhouse gases, particulates, sulfur oxides, or ground-level ozone are produced by the use of hydrogen or electricity, but they have a high CO2 intensity upstream when produced from fossil fuels or natural gases. This disadvantage can only be overcome by using renewables or nuclear as the initial energy input, or equipping fossil fuel plants with CCUS. The difference between hydrogen and electricity is that hydrogen is a chemical energy carrier, composed of molecules and not only electrons. Chemical energy can be stored and transported in a stable way, like is done with oil, coal, biomass, and natural gas. Molecules can be stored for a long period; it permits transporting it across the sea, burning it to produce high temperature, and using it in existing plants and business models designed for fossil fuels. The hydrogen molecular nature allows it to combine with other elements, like carbon and nitrogen, to make hydrogen-based fuels that are easier to handle, and that can be used as feedstock in industry, helping to reduce emissions. A decarbonized energy system based on electricity would be flow-based, that must match demand and supplies in real time, across distances, and can be vulnerable to disruptions of supply. Chemical energy can add a stock-based element to an energy economy, and it helps the energy system resilience. When there are energy carriers, every time they are produced, converted, or used, there is an efficiency loss that can accumulate across steps in the value chain. Electrocatalytic water-splitting is one of the most ideal and effective ways to produce hydrogen with high purity. The water-splitting reaction is known with two half-reactions: the water oxidation reaction (or oxygen evolution reaction, OER) and the water reduction reaction (or hydrogen evolution reaction, HER). Namely, the HER catalysts produce mass hydrogen but poor OER efficiency in acidic media; the OER catalysts present ideal OER performance but less hydrogen production in alkaline media. How to integrate outstanding HER and OER performance of a catalyst in one cell has been an important problem for determining whether it can be a qualified bifunctional catalyst and overcome the kinetics barrier. The processes in the removal of impurities from crude hydrogen to obtain a pure product can be roughly divided into three steps. The first step is the pretreatment of crude hydrogen for the removal of specific contaminants that are detrimental to subsequent separation processes and for their conversion to easily separable species. Three methods of conventional adsorption, physical absorption, and chemical reactions are effective for these purposes. The second step is the removal of both major and minor impurities to yield an acceptably pure hydrogen level. To be able to use the energy efficiently and as required, large and flexible storage options are required that can compensate for these fluctuations. Electricity cannot provide the necessary large industrial capacities (especially via grid buffers and battery storage) for the foreseeable future at economically viable terms. Alternatively, hydrogen is well-suited as an energy source due to its compressibility and storage capacity in storage facilities, and it can supplement the electricity grid based on the gas storage facilities. Hydrogen contains more energy per unit of mass than natural gas or gasoline, making it attractive as a transport fuel. However, hydrogen is the lightest element and so has a low energy density per unit of volume. This means that larger volumes of hydrogen must be moved to meet identical energy demands as compared with other fuels. If hydrogen is to play a meaningful role in clean, flexible energy systems, it will largely be because it can be used to store energy in large quantities for long periods and to move it over very long distances. Delivery infrastructure choices and costs are thus critically important. Today, hydrogen is usually stored and delivered in compressed gas or liquid form. The majority is either produced and consumed on-site (around 85%) or transported via trucks or pipelines (around 15%). In the future, the balance between these options could change, and new alternatives could emerge. Hydrogen offers ways to decarbonize a range of sectors—including long-haul transport, chemicals, and iron and steel—where it is proving difficult to meaningfully reduce emissions. It can also help improve air quality and strengthen energy security. Despite very ambitious international climate goals, global energy-related CO2 emissions reached an all-time high in 2018. Outdoor air pollution also remains a pressing problem, with around 3 million people dying prematurely each year. Hydrogen is versatile. Technologies already available today enable hydrogen to produce, store, move, and use energy in different ways. A wide variety of fuels are able to produce hydrogen, including renewables, nuclear, natural gas, coal, and oil.

Hydrogen: Energy Vector and Fuel

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发表日期：

2025年

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Pasquale Cavaliere

摘要

Hydrogen is an energy carrier, not an energy source, which makes it similar to electricity. Both of them can be produced by various energy sources and technologies; both can be used in many different applications; no greenhouse gases, particulates, sulfur oxides, or ground-level ozone are produced by the use of hydrogen or electricity, but they have a high CO2 intensity upstream when produced from fossil fuels or natural gases. This disadvantage can only be overcome by using renewables or nuclear as the initial energy input, or equipping fossil fuel plants with CCUS. The difference between hydrogen and electricity is that hydrogen is a chemical energy carrier, composed of molecules and not only electrons. Chemical energy can be stored and transported in a stable way, like is done with oil, coal, biomass, and natural gas. Molecules can be stored for a long period; it permits transporting it across the sea, burning it to produce high temperature, and using it in existing plants and business models designed for fossil fuels. The hydrogen molecular nature allows it to combine with other elements, like carbon and nitrogen, to make hydrogen-based fuels that are easier to handle, and that can be used as feedstock in industry, helping to reduce emissions. A decarbonized energy system based on electricity would be flow-based, that must match demand and supplies in real time, across distances, and can be vulnerable to disruptions of supply. Chemical energy can add a stock-based element to an energy economy, and it helps the energy system resilience. When there are energy carriers, every time they are produced, converted, or used, there is an efficiency loss that can accumulate across steps in the value chain. Electrocatalytic water-splitting is one of the most ideal and effective ways to produce hydrogen with high purity. The water-splitting reaction is known with two half-reactions: the water oxidation reaction (or oxygen evolution reaction, OER) and the water reduction reaction (or hydrogen evolution reaction, HER). Namely, the HER catalysts produce mass hydrogen but poor OER efficiency in acidic media; the OER catalysts present ideal OER performance but less hydrogen production in alkaline media. How to integrate outstanding HER and OER performance of a catalyst in one cell has been an important problem for determining whether it can be a qualified bifunctional catalyst and overcome the kinetics barrier. The processes in the removal of impurities from crude hydrogen to obtain a pure product can be roughly divided into three steps. The first step is the pretreatment of crude hydrogen for the removal of specific contaminants that are detrimental to subsequent separation processes and for their conversion to easily separable species. Three methods of conventional adsorption, physical absorption, and chemical reactions are effective for these purposes. The second step is the removal of both major and minor impurities to yield an acceptably pure hydrogen level. To be able to use the energy efficiently and as required, large and flexible storage options are required that can compensate for these fluctuations. Electricity cannot provide the necessary large industrial capacities (especially via grid buffers and battery storage) for the foreseeable future at economically viable terms. Alternatively, hydrogen is well-suited as an energy source due to its compressibility and storage capacity in storage facilities, and it can supplement the electricity grid based on the gas storage facilities. Hydrogen contains more energy per unit of mass than natural gas or gasoline, making it attractive as a transport fuel. However, hydrogen is the lightest element and so has a low energy density per unit of volume. This means that larger volumes of hydrogen must be moved to meet identical energy demands as compared with other fuels. If hydrogen is to play a meaningful role in clean, flexible energy systems, it will largely be because it can be used to store energy in large quantities for long periods and to move it over very long distances. Delivery infrastructure choices and costs are thus critically important. Today, hydrogen is usually stored and delivered in compressed gas or liquid form. The majority is either produced and consumed on-site (around 85%) or transported via trucks or pipelines (around 15%). In the future, the balance between these options could change, and new alternatives could emerge. Hydrogen offers ways to decarbonize a range of sectors—including long-haul transport, chemicals, and iron and steel—where it is proving difficult to meaningfully reduce emissions. It can also help improve air quality and strengthen energy security. Despite very ambitious international climate goals, global energy-related CO2 emissions reached an all-time high in 2018. Outdoor air pollution also remains a pressing problem, with around 3 million people dying prematurely each year. Hydrogen is versatile. Technologies already available today enable hydrogen to produce, store, move, and use energy in different ways. A wide variety of fuels are able to produce hydrogen, including renewables, nuclear, natural gas, coal, and oil.