Environmental profile of so called green hydrogen
Published on December 29, 2022
Hydrogen is a promising solution to store and deliver energy in the context of global outreach to decarbonate the economy and mitigate climate change. When it is produced from the electrolysis of water using electricity from renewable sources (non-competing sources), the so-called “green hydrogen” may provide relatively clean energy for various sectors, including transportation. Yet less than 5% of global hydrogen production is produced through water electrolysis, only a small fraction of which is “green”. The rest is produced by reforming natural gas, from coal gasification, or as a by-product from chemical production.
Although it may seem like a new field of research, the main technology to produce hydrogen through alkaline water electrolysis is over a hundred years old. Upcoming technologies include polymer exchange membrane water electrolysis (PEMWE) and solid oxide electrolyser cells (SOEC). Each technology comprises cells within which electric current passes through electrodes into a circulating flow of water (with the addition of KOH in the case of alkaline electrolysis) under given pressure and heat conditions, generating hydrogen and oxygen.
Life cycle assessment (LCA) studies of the alkaline and PEM electrolytes reveal that electricity consumption is responsible for most hydrogen production impacts. WeLOOP conducted a cradle-to-gate LCA study of those two hydrogen production systems, leading to the same conclusions when using impact assessment methods for the PEF 3.0 method. Even though using renewable energy sources reduces the impact by approximately 80% compared to a typical European electricity mix, it remains true when considering offshore wind energy as the only source of electricity for the process. Nevertheless, raw materials and water consumption are also significant contributors to the environmental impact of the process.
Aside from the origin of electricity, the three most important parameters to consider for optimising the environmental impact of the process are:
- the efficiency of the process in terms of energy conversion (electricity to hydrogen)
- the lifetime of the cells
- the material consumption of the cells
Indeed, the alkaline cells comprise hefty amounts of nickel in their anodes and cathodes, a metal with important impacts on resource use, ecotoxicity of freshwater, and climate change indicators. On the other hand, the membranes used for PEM electrolysis use considerable amounts of platinum group metals, whose production also implies important impacts on the same impact categories as nickel. Furthermore, both systems rely on a fair amount of the increasingly rare copper metal, impacting the resource use category.
There are different ways to reduce the share of impacts due to metal consumed for the electrolysis systems: reducing the amount of metal, extending the longevity of the cells, and recycling. For instance, we found that reducing the nickel content of alkaline electrolysis cells by half would reduce the impact of hydrogen production by approximately 10% if all other parameters remain unchanged. The challenge for improving the environmental profile of green hydrogen in the future thus resides in finding a proper balance between the efficiency of the process , the lifetime of cells, and the consumption and recycling of metals used within cells.
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