The comparison between hydrogen and synthetic hydrocarbons such as e-methane and e-kerosene in energy systems requires a detailed examination of their life cycle impacts, particularly in terms of carbon intensity and resource efficiency. Both pathways aim to decarbonize energy systems but differ significantly in production methods, infrastructure requirements, and overall environmental footprints.

Carbon intensity is a critical metric for evaluating the climate impact of energy carriers. Hydrogen produced via electrolysis using renewable electricity, known as green hydrogen, has near-zero carbon emissions during operation. However, the life cycle emissions depend on the electricity source, electrolyzer efficiency, and upstream processes like material production for electrolyzers. For example, if renewable electricity is used, the carbon footprint is minimal, but reliance on grid electricity with fossil contributions increases emissions significantly.

Synthetic hydrocarbons, such as e-methane and e-kerosene, are produced by combining green hydrogen with carbon dioxide captured from the air or industrial sources. The carbon intensity of these fuels depends on the hydrogen production method and the carbon source. If renewable energy powers both hydrogen production and direct air capture (DAC), synthetic hydrocarbons can be carbon-neutral over their life cycle. However, the additional energy required for DAC and synthesis processes (like Fischer-Tropsch or methanation) increases overall emissions compared to standalone hydrogen.

A key difference lies in the carbon cycle. Synthetic hydrocarbons re-release CO2 when combusted, but since the carbon is sourced from the atmosphere, the net effect can be neutral if the entire process is powered by renewables. In contrast, hydrogen combustion emits only water vapor, but any leakage of hydrogen contributes indirectly to global warming due to its role as an indirect greenhouse gas.

Resource efficiency is another major consideration. Hydrogen has a high energy content by weight but low energy density by volume unless compressed or liquefied, requiring significant energy for storage and transport. Electrolysis typically achieves 60-80% efficiency, with further losses in compression or liquefaction.

Synthetic hydrocarbons, while easier to store and transport due to their compatibility with existing fossil fuel infrastructure, suffer from lower overall efficiency. The conversion of hydrogen into e-methane or e-kerosene involves additional steps, each with energy losses. Methanation, for example, has an efficiency of around 50-60%, and Fischer-Tropsch synthesis is similarly energy-intensive. When considering the full pathway from electricity to final fuel, synthetic hydrocarbons often require two to three times more renewable energy input than direct hydrogen use for the same energy output.

Material requirements also differ. Hydrogen production via electrolysis demands catalysts like iridium or platinum for PEM electrolyzers, raising concerns about material scarcity. Alkaline electrolyzers use nickel-based electrodes, which are more abundant but less efficient at high current densities. Synthetic hydrocarbon production requires catalysts for CO2 hydrogenation, often based on nickel, cobalt, or iron, with varying availability and environmental impacts from mining.

Land and water use are additional factors. Green hydrogen production via electrolysis requires large amounts of water—approximately 9 liters per kilogram of hydrogen—though this is lower than fossil-based methods. Solar or wind farms for powering electrolysis also need significant land area. Synthetic hydrocarbons amplify these demands due to their lower efficiency, requiring more renewable energy generation and thus more land. DAC systems for CO2 capture further increase land and energy requirements.

Infrastructure compatibility plays a role in life cycle impacts. Hydrogen requires new pipelines, storage tanks, and refueling stations, which incur embedded emissions from material production and construction. Retrofitting natural gas pipelines for hydrogen is possible but limited by material compatibility issues like hydrogen embrittlement. Synthetic hydrocarbons can use existing fossil fuel infrastructure, avoiding new construction emissions but perpetuating reliance on combustion technologies.

A comparative assessment of energy losses illustrates the efficiency gap. For hydrogen, the pathway from electricity to end-use may lose 30-40% of the original energy due to electrolysis, compression, and conversion losses in fuel cells. For synthetic hydrocarbons, losses can exceed 60-70% when accounting for DAC, synthesis, and combustion inefficiencies.

The choice between hydrogen and synthetic hydrocarbons depends on application priorities. Hydrogen excels where direct use is feasible, such as in fuel cells or industrial processes requiring high-purity hydrogen. Synthetic hydrocarbons are better suited for sectors where energy density and compatibility with legacy systems are critical, such as aviation or long-haul shipping.

In summary, hydrogen offers lower carbon intensity and higher resource efficiency when produced renewably and used directly, while synthetic hydrocarbons provide infrastructural advantages at the cost of higher energy losses and more complex production chains. The optimal pathway varies by use case, with trade-offs between emissions, efficiency, and practicality shaping their roles in future energy systems.