Nonprecious metals in the spotlight as catalysts for hydrogen production

Green hydrogen is considered a cornerstone of the energy transition, but producing hydrogen by electrolysis is an energy-intensive process. A team at the University of Oldenburg, Germany, is working on ways to boost the efficiency of the chemical reaction using powerful catalysts made from inexpensive materials.

Many of us are likely to have done experiments with the electrolysis of water in chemistry lessons at school. Jasmin Schmeling, a doctoral candidate in the Technical Chemistry research group at the University of Oldenburg, Germany, performs the procedure with a practiced hand: she takes a small beaker containing a solution of dissolved salts, immerses two electrodes in the liquid, applies electric voltage and, lo and behold, tiny bubbles of gas stream upwards from both electrodes – hydrogen at the negative cathode and oxygen at the positive anode. All that's missing is the famous oxyhydrogen test for identifying the hydrogen.

There is, however, one small difference: if you look closely, you can see a fine-grained, brownish-green layer on the anode. "This is the catalyst, which reduces the amount of energy required for electrolysis," Schmeling explains. Catalysts made of rare precious metals such as ruthenium or iridium are usually used to split water. Here, however, a compound that consists of only nonprecious metals, mainly nickel, is at work.

Schmeling and Laura Gronewold, another doctoral candidate in Prof. Dr Michael Wark's group at the Institute of Chemistry, are working on a promising new class of catalysts known as metal-organic frameworks (MOFs), which consist of ions of common metals such as iron, nickel or cobalt that are connected by organic molecules (known as linkers) and together form an ordered, crystalline structure. The advantage of MOF catalysts is that they can be produced from inexpensive, readily available materials and have the potential to achieve a good energy balance. This is comparable to precious metal catalysts, which are expensive and problematic due to their low availability and large CO₂ footprint in mining. "Another advantage is that their properties, such as the size distribution, can be varied and adjusted for different purposes," Professor Wark explains.

Hydrogen produced in remote regions could be converted into ammonia

The chemist and his team began their work with the catalysts in the "Water Electrolysis Innovation Lab" (InnoEly), a project funded by the state of Lower Saxony which recently came to an end. Schmeling and Gronewold are now continuing this research in the state post-graduate programme "Hydrogen and Hydrogen Derivative Ammonia", which is part of the "Transformation of the Energy System of Lower Saxony" (TEN.EFZN) research programme launched in October. The researchers are experimenting with ammonia, which has the potential to play an important role in the energy system as a cost-effective, CO₂-free hydrogen carrier – particularly given that it is easier to transport than hydrogen. In the future, hydrogen produced in remote regions could be converted into ammonia using atmospheric nitrogen, transported by ship and then converted back into hydrogen.

The background to both projects is the German government's National Hydrogen Strategy, which envisages the production of large quantities of hydrogen, primarily to store surplus wind-generated energy. One of the challenges here, as Wark explains, is that "every conversion from one form of energy to another entails losses." Currently, around half of the energy is lost when electricity is used to generate hydrogen and then converted back into electricity later on. The situation is similar with the production of ammonia and the subsequent recovery of the hydrogen. The aim of the projects in Lower Saxony is to keep these energy losses to an absolute minimum. Wark's team is tackling two challenges: firstly, to produce the MOF catalysts themselves, using as little energy as possible. Secondly, to optimise the properties of the oxide electrocatalysts obtained from MOF precursors, for example by adding other metals.

In the InnoEly project, Dr Danni Balkenhohl achieved a breakthrough regarding the first task: he developed a method by which the catalysts grow directly on the electrodes at room temperature. "The standard procedure for synthesising such mixed metal oxide electrocatalysts is conducted in pressurised containers known as autoclaves at temperatures of at least 120 degrees Celsius. It takes up to 48 hours," says Wark. Being able to synthesise the MOFs and the finally resulting mixed metal oxides without applying heat is almost "revolutionary" and significantly reduces the amount of energy required, he explains.

A simple process is used to synthesise the catalysts from their starting materials

Balkenhohl used a simple precipitation process to synthesise the MOF catalysts from their starting materials – metal salts and organic substances. Thin, uniform MOF layers form directly from the solution on suitable metal substrates. The team is also experimenting with microwave synthesis, in which the mixture of metal salts, organic linkers, solvent and the metal substrate is heated in a microwave. This reduces the reaction time to about 15 minutes, much shorter than the time needed with autoclaves. Wark and Balkenhohl have been granted a patent for this method.

In their current research in the state post-graduate programme, doctoral candidates Jasmin Schmeling and Laura Gronewold are focused on finding catalysts that split ammonia into its components and produce hydrogen in the process. "In principle, the reaction is the same as with the splitting of water," Wark emphasises.

For both the electrolysis of ammonia and the electrolysis of water, the researchers are investigating which synthesis strategy is most effective for controlling key parameters such as porosity or the stability of the compound. This is no easy task. Although the MOFs are crystalline, the oxide structures formed from precursors are often less crystalline and sometimes even amorphous, meaning that they have a disordered structure similar to glass, Wark explains. "This makes the characterisation of the resulting mixed metal oxide electrocatalysts difficult. It is also a challenge to reliably reproduce the structure and composition."

Nevertheless, the team has already discovered that adding ions of the metal manganese can have a stabilising effect on the catalysts. "In catalysis, you always struggle with the fact that the most active states are also the most unstable," Wark observes. The crucial challenge now is to establish further stable catalysts and to transfer the successes achieved on a small scale in the lab to large-scale industrial use, he adds. Wark is convinced: "Metal-organic catalysts are an exciting field in which there is still plenty of work to be done."