Water makes up more than 70 percent of our planet and up to 60 percent of our bodies. Water is so common that we take it for granted. Yet water also has very strange properties compared to most other liquids.

Its solid form is less dense than its liquid form, which is why ice floats; its peculiar heat capacity profile has a profound impact on ocean currents and climate; and it can remain liquid at extremely cold temperatures.

In addition to ordinary water and water vapor, or steam, there are at least 17 forms of water ice, and two proposed forms of super-cooled liquid water.

New work from Carnegie high-pressure geophysicists Chuanlong Lin, Jesse Smith, Stanislav Sinogeikin, and Guoyin Shen found evidence of the long-theorized, difficult-to-see low-density liquid phase of water. Their work is published by Proceedings of the National Academy of Sciences.

The normal density of water is one gram of water molecules per each cubic centimeter. Studies of anomalies in water's behavior have indicated the existence of liquid water with both lower and higher densities than this standard. But observing these phenomena experimentally has been difficult.

Each molecule has what's called a phase diagram--a sort of chart indicating how its bulk molecular structure changes form under different temperature and pressure conditions. The parts of the phase diagram where low-density water is thought to occur are notoriously difficult to explore, the so-called "water's no-man's land," because they require a path through a series of very specific, very difficult conditions.

But the Carnegie team was able to observe low-density water as an intermediate phase using a newly developed rapid-decompression technique to turn the high-pressure crystalline phase ice-VIII to the diamond-like ice Ic at temperatures between about -207 and -163 degrees Fahrenheit (140 and 165 kelvin).

Sophisticated x-ray analysis confirmed the observation of the low-density liquid water phase, which only lasted for about half a second at -163 degrees Fahrenheit (160 kelvin).

When ice-VIII was decompressed at moderate speeds, it formed other phases of ice, indicating that the speed of decompression is key to observing the low-density liquid water phase.

"Our newly developed, very fast decompression method was the key to this exciting observation of low-density liquid water as an intermediate between two crystalline phases," Shen explained.

related report High-efficiency and low-cost catalyst for water electrolysis
Seoul, South Korea (SPX) Feb 15 - A research team led by Professor Sangaraju Shanmugam of Energy Science and Engineering at DGIST has developed highly efficient and ultra-durable core-shell nanostructured electrocatalyst and successfully replaced the precious anode in water electrolysis, through the collaboration with the research group of Pacific Northwest National Laboratory (PNNL), USA.

The replacement of conventional fuels with renewable energy resources is a suitable approach to achieving an eco-friendly environment and decreasing future energy demands. To these ends, electrochemical energy generation or conversion in renewable energy devices, which depends on anode and cathode reactions, has received much attention.

In electrocatalytic water splitting, oxygen gas generates in the anode due to the oxygen evolution reaction (OER), and it is a slow electrochemical reaction as compared with the hydrogen evolution reaction (HER). Thus, a suitable electrocatalyst is needed for promising and stable electrocatalytic water splitting.

Development of efficient, durable, low-cost OER electrocatalysts is a great challenge and paid more attention to the renewable energy devices of the water electrolyzer. Until now, the Ruthenium and Iridium oxides are considered as state-of-the-art electrocatalysts in OER, but the lack of stability limit their use in large-scale water splitting and hinder widespread commercialization.

Hence, Professor Shanmugam's and PNNL teams has focused on developing an alternative, low-cost, non-noble metal electrocatalyst to replace the noble metal anode electrode in efficient water splitting. Carbon-supported metal has been considered as an efficient electrocatalytic material for the enhanced OER in water splitting. So far, most of the developed electrocatalysts have featured higher carbon content and less metal active specious content. The higher carbon amount mired the real metal active sites and thus resulted in a faster carbon corrosion conditions. This further led to lower electrocatalytic activity, stability and large-scale water splitting processes (scalability).

In the study, the researchers found that a large amount of inorganic cobalt metal ions bridged by organic ligands in the Prussian blue analog were shown to be a suitable precursor for developing efficient and ultra-stable, metal-rich, nitrogen-doped graphitic nanocarbon-encapsulated core-shell electrocatalysts for the sluggish OER (anode) in water splitting.

When heated (600-900C) in an inert atmosphere, the cobalt metal ions and organic ligands in the salt are transformed into cobalt metal and nitrogen-doped graphitic thin carbon layers, respectively, which form the thin carbon layer, encapsulated metallic, cobalt core-shell nanostructures (Core-Shell Co@NC). The thin carbon layers have a strong interaction with cobalt metal, which can promote less carbon corrosion, excellent electron movement, and more cobalt metal exposure to the reaction medium, including the formation of nanosized morphology without particle aggregation.

Accordingly, the combined effect of carbon and cobalt metal in the electrodes achieves the more efficient electrocatalytic activity of the OER than that of the precious metal electrodes to allow efficient water splitting. Therefore, the non-noble metal-rich electrode is an alternative, active, stable, and less expensive OER anode for cost-effective H2 gas production in commercial-scale water electrolysis.

"Anticipate this to be a unique approach to developing metal-rich, reduced-carbon composite nanostructures that have enhanced metal active sites, which feature thin carbon layer protection and ultra-fast electron movement in the catalyst surface, that will enhance the electrochemical activity and stability of electrocatalysts," says Professor Shanmugam. "We will carry out the follow-up studies that can be used to understand the real OER mechanism on the active species in the presence of nanocarbon coating".

This research outcome was published in the online edition of Advanced Energy Materials on 11th of January 2018, a reputed international journal in the field emerging materials.

The following is an interview with Professor Sangaraju Shanmugam (Department of Energy Science and Engineering):

Q. What are the differences compare to the previous researches?

A. In the previous research, the researchers have prepared the carbon coated metals from various precursors, including metal-organic frameworks (MOFs). The obtained catalysts exhibit more carbon with less graphitic nature, and the carbon covered the active metal sites. Thus most of the active metal sites are not utilized properly by the electrochemical reactions.

Also, due to the substantial carbon corrosion, those catalysts are not suitable enough for the sluggish OER in water splitting at the higher positive potential with lack of instability in harsh electrolyte conditions. Accordingly, in this work, we prepared the metal-rich, thin nanocarbon (NC) layers encapsulated electrocatalyst of core-shell Co@NC nanostructures from a single precursor Prussian blue (PB) analogue. The Co@NC showed enhanced oxygen evolution activity and ultrastability on the current collector of nickel foam.

Overall, the thin and uniform carbon layers provide the fast electron movements, more metal active sites utilization with easy electrolyte penetration. Most importantly, it can protect the active metal sites from the corrosion with minimal exposing and also the strong interaction between metal and carbon layers exhibits the synergistic effect towards the excellent activity and ultra-stability (over 350 h) of core-shell Co@NC nanostructures with less possibility of carbon oxidation.

Q. How can it be utilized?

A. Based on the remarkable OER performance, kinetics and long-term stability of core-shell Co@NC nanostructures as compared to the state-of-the-art Noble metal based electrocatalysts, such as IrO2 and RuO2, it can be the most suitable candidate to replace the precious OER electrode for reducing the overall cost of the water electrolyzer system. Thus the development of efficient and durable non-noble metal electrocatalyst in water electrolyzer is the main obstacle for successful commercialization of water electrolyzers.

Q. How long will it be required for commercialization?

A. The know-how process is readily available for the fabrication of cost-effective catalyst. But, still we have to evaluate the integration of this catalytic system in a polymer electrolyte membrane electrolyzer, and moreover, studies are underway to understand the OER mechanism on this electrocatalyst and which can help us to realize their activity lose and some other issues in the large-scale water electrolyzer. So, for the commercialization, it may require a year with complete understanding towards activity and stability.

Q. What are the challenges for commercialization?

A. We have to make the uniform coating of this catalyst on the larger size current collectors without any peeling off. So, we need to find the more suitable coating methodology. Also, as like precious OER electrocatalysts we have to understand the precise OER mechanism on this electrocatalysts to maintain/avoid activity losses due to the unwanted side reactions, etc.

Q. Please let us know the motivation on you research.

A. The primary motivation of this research work is that the development of non-precious, metal active sites rich, core-shell electrode material to replace the precious anode in the water electrolyzer system with excellent activity and stability. So, to improve the activity and stability, we tried to introduce the very thin carbon coating on the metal active sites. Overall, the development metal-rich and carbon less OER electrocatalysts with proper utilization of metal-active species and metal-carbon synergistic effect to overcome the sluggish anode reaction in water electrolysis.

Q. What is the final goal you would like to achieve through this research? A. Based on this research, we understand that the metal-rich electrocatalysts are one of the most suitable materials for the excellent OER activity. So, we want to prepare the cheapest anode electrocatalysts by using the same methodology and want to eliminate the use of precious electrodes in the water electrolyzer system for the production of green and sustainable hydrogen in large scale.

DGIST (Daegu Gyeongbuk Institute of Science and Technology)

Research paper

Related Links
Carnegie Institution for Science
Water News - Science, Technology and Politics

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