Catalysts: Efficient hydrogen production via structure
Regeneratively produced hydrogen is considered the ecological raw material of the future. In order to produce it efficiently by electrolysis of water, researchers today also investigate perovskite oxides. The Journal of Physics: Energy invited Dr. Marcel Risch from the Helmholtz-Zentrum Berlin (HZB) to outline the current state of research.
The sustainable production of industrial raw materials and energy sources is one of the greatest social challenges of our time. One promising candidate for this is hydrogen. That is because this lightest of all elements can be not only converted into electrical energy by fuel cells, serve as gasified fuel, and be processed into synthetic fuels that can be transported and stored – it can also reduce metal ores to high-quality metal and is the feed stock for chemicals and fertilisers.
“The only truly sustainable source of hydrogen is water”, says Marcel Risch. This can be split into its constituent parts by applying electric current. To do this, acids or alkalis increase its conductivity, while the two electrodes supply the electrical energy. Hydrogen gas then rises at one of the two electrodes and oxygen at the other. “The manner by which the hydrogen is produced is well understood by scientists and the materials for this have already been optimised”, explains Risch. “The challenge is the other reaction that always has to take place when splitting water: oxygen production. This is less well understood.”
The inner values count
And this is precisely where there is great potential for improvement. For if it were possible to make this half of the electrolysis reaction more efficient and minimise the energy losses at the electrode, the overall efficiency would increase – and with it also the economic efficiency.
One approach to realise this efficiency potential is to introduce specialised catalysts into the electrode material. Besides expensive precious metals such as iridium, perovskite oxides have emerged as a promising class of materials. Sporadic experiments with them were already being carried out in the 1970s and 1980s. It was not until 2011, however, that they attracted the attention of scientists. This class of materials is named after a very common mineral consisting of calcium, titanium, and oxygen. It is the internal structure that makes perovskite oxides interesting for science and technology. “All perovskites have the same characteristic crystal structure or atomic arrangement”, explains Risch, who is also researching perovskite oxides as catalysts for electrolysis at HZB. “They offer us the advantage of exchanging the chemical elements very easily without altering the structure itself.”
But with countless variations. For example, research teams can replace a portion of the titanium in an experimental sample with cobalt, and a portion with iron in another sample. They then can contrast the electrolytic efficiency of both these modified perovskites. “That makes the experiments much more comparable”, says Risch. “This is because we can modify a very specific aspect of the chemistry while everything else often only changes marginally.” A single element can also be replaced by several other elements, even in different proportions and combinations. “The material with the best catalytic properties to date has had its calcium replaced by barium and strontium. As well, cobalt and iron have been incorporated into the perovskite structure in place of the titanium”, the researcher explains.
The right metric for searches
The number of theoretically possible materials is therefore extremely large. The search for the optimal combination of elements could quickly become that proverbial search for a needle in a haystack. This is where Risch‘s survey of highly ordered epitaxial layers of perovskite comes in. He presents not just the current state of research – he also critically evaluates two proposed figures-of-merit of catalyst activity. “In our literature review, we found out that a frequently used figure-of-merit for epitaxial layers of perovskites as catalysts is unfortunately not very suitable”, summarises Risch. “But we were able to confirm the predictive power of another proposed figure.” This insight is not insignificant in the continuing search for the optimal catalyst. With a valid figure-of-merit in hand, it is far easier to conduct a search for an optimum.
A second discovery he made during his research is also important to Risch: “We have to be very careful when interpreting the catalysis results”, he says. “Because some perovskites are semiconductors, and there is a danger that semiconductor effects could be incorrectly attributed to catalytic behaviour.” Simply put, being a poor catalyst does not mean it is due to a lack of catalytic properties. Sometimes semiconductor effects are responsible for the electrons not getting to where they are needed. On the other hand, materials researchers have a few tricks up their sleeves for easily getting these materials back in the race and perhaps making a decisive step forward towards the sustainable production of hydrogen.
The author Dr. Marcel Risch heads a young investigator group at HZB which is financed by the European research grant ME4OER. He and his team study the mechanism of the electrolytic oxygen evolution reaction by water splitting and how the catalysis of this reaction can be optimized.
The review article has been published by The Journal of Physics: Energy in the special volume entitled Focus on Ion-related Properties of Oxides at the Nanoscale: From Fundamentals to Applications.