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New Nano-Scale Procedure can Enhance Electrode Material for Improving Hydrogen Production

Researchers developed new nano-scale procedure to enhance electrode material for improving the performance of hydrogen production.

Hydrogen is essential for mitigating climate change. However, the chemical reaction used to manufacture the gas is often sluggish and less efficient which makes the gas production more expensive and dirtier. The new proposed procedure can ultimately make the production of hydrogen efficient and more affordable.

A description of the production of the material and a comparison of its performance against existing commercial competitors appeared in the journal Nano Research on April 7.

Hydrogen will be necessary for a range of clean energy applications as the world attempts to decarbonize its economy to mitigate the effects of climate change. Electrolysis-;the use of electricity to split water into its constituent hydrogen and oxygen atoms-;is the most commonly deployed technique of producing hydrogen without also producing greenhouse gases (so long as the electricity itself is cleanly produced by renewables or nuclear power).

But electrolysis is incredibly energy intensive, and thus costly, rendering the option less commercially competitive than the conventional high-emission way to make hydrogen-;via the splitting of natural gas (which defeats the climate purpose).

This means that efforts to ramp up the efficiency of electrolysis have been keenly sought in recent years. In comparison to conventional alkaline electrolysis technology, proton exchange membrane electrolysis (PEM) has been considered as much more promising in the future due to greater energy efficiency, higher purity of the hydrogen gas produced, and more flexible structure design-;all of which work to slash costs.

In alkaline electrolysis, the reaction takes place between two electrodes (one positive, the cathode, and one negative, the anode) in a liquid solution of water and an electrolyte that conducts ions. PEM electrolysis depends upon a solid rather than a liquid to conduct the ions-;a polymer called a proton exchange membrane. Upon the application of an electric current between the two electrodes, oxygen gas gathers at the anode and hydrogen at the cathode.

However, there remains a sluggishness to the oxygen-producing part of the overall chemical reaction (the sister 'half-reaction' to the hydrogen-producing component), especially in the acidic environments typical of the electrolytic process, meaning that there is still a lot of room for efficiency improvements.

To overcome the oxygen-sluggishness problem, researchers have proposed ruthenium (Ru) and Iridium (Ir)-based oxides as state-of-the-art electrode materials, and a number of schemes have been proposed to improve this still further, including alloying (mixing the oxides with other metals), and better engineering of the interface between the electrodes and the membrane.

This has been demonstrated to improve the amount of activity occurring at the active sites of the oxides (the physical locations on the material where the reaction takes place). Meanwhile clever designs of the structure of the material at the nanoscale ("nanostructures"), including enhancing the porosity (the empty spaces within the material), have been proven to increase the amount of active sites that there are on the surface.

Despite these improvements, the catalytic activity (initiating or speeding up the chemical reaction) and acid resistance of the Ir or Ru-based oxides still need to be substantially improved for practical applications in PEM electrolyzers.

Very recently, the use of "atomic steps" has been proposed as a way to boost intrinsic catalytic activity in a range of other chemical reactions, but until now had not been applied to the challenge of Ir or Ru-based oxides within electrolysis. While one thinks of the surface of a material at the atomic level as basically flat, in reality, the crystalline surface can have defects in which there are a series of atom-scale terraces or 'steps'. Depending on their shape, these atomic steps can produce substantially different chemical properties of the material.

"We thought that if we combined all these strategies, developing an alloyed Ir- and Ru-based oxide that was structured at the nano scale with enhanced porosity and an abundance of these atomic steps, we could produce what you might call a 'step-change' in their efficiency," said Bo Li, an associate professor at the Shenyang National Laboratory for Materials Science and one of the authors of the paper.

The researchers developed ingots of an alloy that combines iridium, ruthenium and aluminum (Ir6Ru6Al88) and spun out super-thin ribbons of the material with a width of around 2000 micrometers and thickness of 30 micrometers. During the formation and growth of the crystals that make up the material, ultra-small crystal clusters exhibited a bounty of the desired atomic steps.

Microscopy also revealed that the ribbons had an array of porous cogwheel-like structures on their surface. An iridium-ruthenium oxide (Ir0.5Ru0.5O2) was then obtained via the electrochemical activation of the alloyed ribbons in sulfuric acid (H2SO4).

The electrocatalytic performance of the oxide was then compared against commercially available iridium dioxide, and the novel nanomaterial easily beat its competitor, while also showing enhanced stability in a strongly acidic environment.

Density function theory calculations (a computational modelling technique used to explore the electronic structure of atoms and molecules) showed that the sharp increase in performance came from the high density of atomic steps and increased number of active sites.

Both the technique and the explanation for its success now provide a foundation for development of high performance iridium and ruthenium-based catalysts for commercial electrolysis applications-;and hopefully giving the emissions-intensive alternative a run for its money.

Source: http://www.tup.tsinghua.edu.cn/en/index.html

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