Tag: nickel

Green hydrogen: A cage structured material transforms into a performant catalyst

Green hydrogen: A cage structured material transforms into a performant catalyst

Clathrates are characterised by a complex cage structure that provides space for guest ions too. Now, for the first time, a team has investigated the suitability of clathrates as catalysts for electrolytic hydrogen production with impressive results: the clathrate sample was even more efficient and robust than currently used nickel-based catalysts. They also found a reason for this enhanced performance. Measurements at BESSY II showed that the clathrates undergo structural changes during the catalytic reaction: the three-dimensional cage structure decays into ultra-thin nanosheets that allow maximum contact with active catalytic centres. The study has been published in the journal ‘Angewandte Chemie’.

Hydrogen can be produced by electrolysis of water. If the electrical energy required for this process comes from renewable sources, this hydrogen is even carbon neutral. This ‘green’ hydrogen is seen as an important building block for the energy system of the future and is also needed in large quantities as a raw material for the chemical industry. Two reactions are crucial in electrolysis: hydrogen evolution at the cathode and oxygen evolution at the anode (OER). However, the oxygen evolution reaction in particular slows down the desired process. To speed up hydrogen production, more efficient and robust catalysts for the OER process need to be developed.

Clathrates, a structure build of cages

Currently, nickel-based compounds are considered to be good and inexpensive catalysts for the alkaline oxygen evolution reaction. This is where Dr. Prashanth Menezes and his team come in. ‘The contact between the active nickel centres and the electrolyte plays a crucial role in the efficiency of a catalyst,’ says the chemist. In conventional nickel compounds, this surface area is limited. ‘We therefore wanted to test whether nickel-containing samples from the fascinating class of materials known as clathrates could be used as catalysts’.

The materials are made of Ba8Ni6Ge40 and were produced at the Technical University of Munich. Like all clathrates, they are characterised by a complex crystalline structure of polyhedral cages, in this case, formed by germanium and nickel, enclosing barium. This structure gives clathrates special properties that make them interesting as thermoelectrics, superconductors or battery electrodes. However, until now, no research group had considered of investigating clathrates as electrocatalysts.

Read more on HZB website

Image: The illustration shows schematically how nanothin sheets of nickel compounds are released from the clathrate structure, providing an extremely large surface area for the oxygen evolution reaction. 

Credit: Hongyuan Yang/HZB/TUB

A Goldilocks promoter for a silver catalyst

A Goldilocks promoter for a silver catalyst

Nickel dopants could improve sustainable production of ethylene oxide, a chemical widely used in industrial manufacturing.

Plastics, textiles, detergents, adhesives and antifreeze all have something in common: They were made using ethylene oxide. This colorless gas, a chemical building block in the industrial production of many materials, is itself produced by reacting oxygen with ethylene. However, maximizing the amount of ethylene oxide produced poses unique challenges. 

Adding chlorine increases the efficiency of ethylene oxide production by 25 percent. But chlorine, which is corrosive to metal equipment, has its own drawbacks. Writing in Science, researchers at the University of California, Santa Barbara (UCSB), Tufts University, Brookhaven National Laboratory and Tulane University identified nickel as a promoter that can enhance the selectivity of the silver catalyst by about 25 percent, roughly the same amount as chlorine, but with fewer downsides. The team studied the interaction of nickel with the silver catalyst using X-rays from the Stanford Synchrotron Radiation Lightsource (SSRL) at the Department of Energy’s SLAC National Accelerator Laboratory. 

“From an environmental standpoint, if you remove chlorine, that’s one less toxic and corrosive material out of the process stream,” said Adam Hoffman, a staff scientist at SLAC who contributed to this work. “And if you can improve a catalyst’s activity to a target chemical, it improves the sustainability of the process as a whole.”

Charles Sykes, a chemist at Tufts University who led the effort, said it also makes financial sense. “Every one percent increase in the efficiency of the process saves around $200 million annually,” he said.

If you remove chlorine, that’s one less toxic and corrosive material out of the process stream.

Adam Hoffman SLAC Staff Scientist

A more selective catalyst doesn’t only maximize the amount of product, it is also more efficient overall. Post reaction, ethylene oxide must be separated from the side products and residual reactants, a process that requires additional energy inputs. If the reaction is more selective to ethylene oxide to begin with, it is easier to purify.

Read more on SLAC website

Image: A computer-generated image showing single nickel (Ni) atoms embedded in silver, used to enable efficient production of ethylene oxide. 

Credit: Elizabeth Happel/Tufts University

Sodium-ion batteries: How doping works

Sodium-ion batteries: How doping works

Sodium-ion batteries still have a number of weaknesses that could be remedied by optimising the battery materials. One possibility is to dope the cathode material with foreign elements. A team from HZB and Humboldt-Universität zu Berlin has now investigated the effects of doping with Scandium and Magnesium. The scientists collected data at the X-ray sources BESSY II, PETRA III, and SOLARIS to get a complete picture and uncovered two competing mechanisms that determine the stability of the cathodes.

Lithium-ion batteries (LIB) have the highest possible energy density per kilogramme, but lithium resources are limited. Sodium, on the other hand, has a virtually unlimited supply and is the second-best option in terms of energy density. Sodium-ion batteries (SIBs) would therefore be a good alternative, especially if the weight of the batteries is not a major concern, for example in stationary energy storage systems.

However, experts are convinced that the capacity of these batteries could be significantly increased by a targeted material design of the cathodes. Cathode materials made of layered transition metal oxides with the elements nickel and manganese (NMO cathodes) are particularly promising. They form host structures in which the sodium ions are stored during discharge and released again during charging. However, there is a risk of chemical reactions which may initially improve the capacity, but ultimately degrade the cathode material through local structural changes. This has the consequence of reducing the lifetime of the sodium-ion batteries.

“But we need high capacity with high stability,” says Dr Katherine Mazzio, who is a member of the joint research group Operando Battery Analysis at HZB and the Humboldt-Universität zu Berlin, headed by Prof Philipp Adelhelm. Spearheaded by PhD student Yongchun Li, they have now investigated how doping with foreign elements affects the NMO cathodes. Different elements were selected as dopants that have similar ionic radii to nickel (Ni 2+), but different valence states: magnesium (Mg 2+) ions or scandium ions (Sc 3+). 

Read more on HZB website

Image: The schematic illustration shows a sodium ion battery: The positive electrode or cathode (left) consists of layered transition metal oxides which form a host structure for sodium ions. The transition metal nickel can be replaced either by magnesium or scandium ions. 

Credit: HZB