Fraunhofer-Gesellschaft
Many paths lead to hydrogen! Beside the increased use of renewable energies and electrical energy storage systems, the production of sustainable hydrogen as a precursor for synthetic fuels is the third central building block of the energy transition.
During electrolysis, water is broken down into the gases hydrogen (H2) and oxygen (O2) using an electric current. If the electricity used is generated from renewable sources, the hydrogen is referred to as »green hydrogen«. Each type of electrolysis offers specific advantages, so the choice of technology varies according to the application scenario.
While alkaline electrolysis and proton exchange membrane (PEM) electrolysis are already technologically advanced, there are still unresolved technical issues with alkaline membrane electrolysis and high-temperature electrolysis. For many decades, alkaline electrolyzers have proven to be a robust and reliable technology in power stations and chemical plants, in stationary operation with a constant load. Now, however, a paradigm shift is imminent: load fluctuations due to renewable energies mean that new concepts are needed.
PEM electrolysis is a more recent development than alkaline electrolysis and has various advantages: the current density used can be very high and the design extremely compact. The process is dynamic and the cell can be operated at high pressure. However, due to the acidic medium, PEM electrolysis requires very robust materials, which necessitates for example the use of precious metals in the electrodes. PEM electrolysis is currently in the scale-up and cost reduction phase, and is thus well on the way toward mass production.
More compact electrolyzers and cost-effective catalyst materials can be achieved using alkaline membrane electrolysis cells. Most of the basic processes are understood in sufficient detail, so the focus is currently on application issues.
High-temperature electrolysis takes place at temperatures above 700 °C, and offers the greatest advantages where waste heat is available: no precious metals are required to catalyze the reactions, and very high electrical efficiencies can be achieved. The systems can be used in both electrolysis and fuel cell mode. They also enable co-electrolysis, which involves the cracking of water into hydrogen and oxygen, and the cracking of carbon dioxide into oxygen and carbon monoxide. Together with hydrogen, carbon monoxide is a »synthesis gas«, which serves as a precursor for the manufacture of numerous chemical products. Comparable co-electrolysis is also possible at lower temperatures (< 100 °C), for example using PEM-based electrolyzers. In addition to the production of synthesis gas (hydrogen / carbon monoxide) from carbon dioxide and water, the synthesis of other chemical precursors such as formic acid, ethylene or ethanol is also possible. These processes therefore contribute significantly to the defossilization of chemical products, and to the synthesis of sustainable fuels for aviation or shipping, for example, by closing the carbon cycle. This concept is known as carbon capture and utilization (CCU) technology.
Currently, most so-called »grey hydrogen« is produced by steam reforming of natural gas, but this is associated with considerable carbon dioxide emissions. A sustainable alternative is to replace natural gas with biogas (from biomass gasification). If biogenic residues are used, this process is sustainable. In addition to natural gas, coal can also be used as a fossil fuel. So-called »black hydrogen« is produced via hydrothermal gasification with steam.
»Blue hydrogen« is the term used when the carbon dioxide released during steam reforming is captured and stored in underground geological reservoirs. This process is known as carbon capture and storage (CCS). Although the carbon dioxide is not released into the atmosphere, storing it permanently and safely presents a challenge. »Turquoise hydrogen« is produced from natural gas in a process known as methane pyrolysis, in which methane is split into gaseous hydrogen and solid carbon through the introduction of thermal or electrical energy. The solid can be easily stored, and serves as a precursor for a wide range of applications. Blue and turquoise hydrogen are currently being proposed as a bridging technology until the reduced production costs for electrolyzers and a growing global supply of inexpensive electricity from wind and solar energy ensure that green hydrogen is available in sufficient quantities.
Another process uses sunlight to generate green hydrogen directly, using semiconductors to absorb the light and catalytically split water on their surface. This process takes place in photoelectrochemical cells (PECs), in which charge carriers are generated that then ensure the reduction to hydrogen or oxidation to oxygen. In the future, direct PEC splitting of water by light could offer simple setups with low system complexity. In practice, however, the technology is still at an early stage of technological maturity.
The use of photosynthetic or (photo)fermentative microorganisms is a purely biological method of producing hydrogen. Some microorganisms (e.g. bacteria) and also a few eukaryotic unicellular organisms (e.g. green algae) produce hydrogen as a metabolic product. However, photocatalytic and biological processes are still at an early stage of development and cannot yet be conclusively assessed for their potential marketability.
SkalPro – Scalable production processes for highly efficient electrodes in alkaline electrolysis
In order to mass-produce highly active electrodes for alkaline water electrolysis in the near future, further innovation steps are required. The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM has set the goal of developing and testing a manufacturing process for electrodes. This process will enable the production of highly active electrodes in large quantities and at low cost. Scalable production processes like this are currently not widely available on the market. The overall aim of the project is to develop an efficient, cost-effective and resource-saving production chain on a pilot plant scale for the manufacture of highly efficient and durable electrodes that meet the requirements of the electrolysis industry.
Integrate – Innovative designs for alkaline membrane electrodes for the cost-effective production of green hydrogen on a gigawatt scale
The complete transformation of the energy sector by 2050, from an economy based on fossil fuels to one based on renewable energies: this is the ambitious goal of the industrialized countries. In Germany alone, this will result in an annual hydrogen demand of 78 terawatt hours (TWh) by 2030 and 294 TWh by 2050. The electrolysis capacity required depends heavily on the efficiency of the technology, but will be in the order of 44 gigawatts (GW) of capacity by 2030 and 213 GW by 2050. This model assumes a 12 % increase in the efficiency of electrolysis technology between 2030 and 2050. To achieve this, existing electrolysis technologies (alkaline electrolysis – AEL, polymer electrolyte membrane electrolysis – PEMEL, solid oxide electrolysis cells – SOECs) must be further developed and new, more efficient technologies must find their way to market maturity. A project at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM is focusing on anion exchange membrane electrolysis (AEMEL) as the most promising technology for improving efficiency with cell voltages of 1.8 V at a current density of 1.5 A/cm² by 2030. The main advantage of this technology is that the ohmic internal resistance can be radically reduced by using an anion exchange membrane (AEM) similiar to PEMEL. In contrast to PEMEL, however, the alkaline medium allows the use of transition metals in all components. This ensures low criticality of all the materials involved, and the scalability of the technology. The project includes the design and optimization of a novel anion exchange membrane electrolyzer (AEMEL) designed for the GW market to minimize the cost 1 Project partners: Fraunhofer Institute for Solar Energy Systems ISE, PNE AG (project coordinator), SILICA Verfahrenstechnik GmbH, KONGSTEIN GmbH, Wystrach GmbH of hydrogen production. Innovations include the electrode composition, the chemical structure of the membrane, the optimized catalyst layer structure (achieved by modelling), the use of new porous transport layers and the implementation of the results in an AEMEL stack.
