Mo-doped Ni₂P nanorings boost seawater electrolysis for hydrogen production

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Torus-shaped Mo0.1Ni1.9P nanoparticles with hollow nanorings are synthesized via a one-pot colloidal method. These nanorings serve as efficient electrocatalysts for alkaline seawater splitting to produce green hydrogen, operating at low cell voltage with high turnover frequency, and demonstrating strong stability. Credit: Dr. Sasanka Deka

Burning fossil fuels has led to a global energy crisis, worsening pollution and climate change. To tackle this problem, we must explore cleaner energy alternatives. One promising solution is the use of water electrolysis technology (electrolyzer) powered by renewable electricity to produce high-purity hydrogen (H₂) fuel.

Currently, most systems for producing hydrogen rely on pure water. However, the limited availability of pure water makes it difficult to scale up this technology. On the other hand, seawater, which covers about 97% of the Earth’s surface, offers an endless supply of hydrogen.

Despite its abundance, using seawater for hydrogen production presents challenges due to its complex composition, which includes high amounts of salts like chloride, sodium, and magnesium.

Two key problems arise when using seawater for electrolysis:

  1. Unwanted chemical reactions: The chlorine in seawater tends to react faster than the oxygen we aim to produce, creating competition and reducing efficiency.
  2. Electrode damage: High chloride levels cause corrosion, which lowers the efficiency and lifespan of the system.

To overcome these challenges, researchers are working on developing advanced catalysts that can efficiently handle untreated seawater. These catalysts would help prioritize oxygen production while resisting chlorine-related damage, making large-scale hydrogen production from seawater more practical.

Story of doped-Ni2P electrocatalysts

There is growing interest in developing affordable alternatives to precious metal catalysts for water splitting, a process used to produce clean hydrogen fuel. Among these, transition metal phosphides (TMPs) have gained significant attention because of their versatile compositions, excellent conductivity, and favorable electronic properties.

One TMP of particular interest is Ni2P (nickel phosphide). Despite their potential, TMPs face challenges such as a limited number of active sites, instability during reactions, and interference from chlorine-related reactions (CER poisoning).

To address these issues, doping Ni2P with molybdenum (Mo) has shown promising results. The combination of Mo and Ni creates strong electronic interactions, better adsorption of active species, and structural improvements due to lattice distortion. However, the limited surface exposure of Mo-doped Ni2P remains a barrier to fully optimizing its catalytic performance for water splitting.

Present development

Considering these challenges, our team, led by Sasanka Deka at the Department of Chemistry, University of Delhi, Delhi, developed a novel nanocomposite-based electrocatalyst that is both highly efficient and cost-effective for seawater splitting. We developed a novel torus-shaped (donut or ring-shaped) Mo-doped Ni2P nanoparticle (NP) through shape engineering.

To achieve this, we utilized a new high-temperature method designed to expose more {0001} facets. By increasing the amount of capping agent and phosphorus precursor at 350°C, we induced an aggressive Kirkendall effect, facilitating outward diffusion and ultimately leading to the formation of torus-shaped particles as the central part of spherical Ni2P particles vanished.

This unique shape offers a larger surface area and a higher density of unsaturated surface atoms compared to conventional spherical particles, as supported by surface area-to-volume ratio equations. This marks the first-time monodispersed torus-shaped nanoparticles have been successfully produced and utilized as electrocatalysts for direct seawater electrolysis (SWE).

Our optimized design not only introduced these novel donut-shaped Mo-doped Ni2P nanoparticles but also achieved one of the lowest cell voltages for overall water splitting (OWS) in direct SWE applications, along with excellent stability. Our findings are published in the journal Small.

When the Mo0.1Ni1.9P||Mo0.1Ni1.9P pair is used as bifunctional electrocatalysts for overall water splitting in an alkaline environment, it requires only 1.45 V in 1.0 M KOH and 1.47 V in alkaline seawater at a current density of 10 mA/cm2. Furthermore, the pair demonstrates excellent stability, maintaining performance for more than 80 hours at a high current density of 400 mA/cm2 in alkaline seawater.

As a proof of concept, a video of direct solar to hydrogen energy is also provided in this work using the present electrolyzer and Mo0.1Ni1.9P||Mo0.1Ni1.9P couple electrodes. Here, a do-it-yourself solar panel kit was directly connected to the anode and cathode, where a layer of Mo0.1Ni1.9P paste was deposited.







A commercial solar cell module (Electronic Spices Solar Panel for DIY, 70 mm × 70 mm) was purchased from Amazon and used it as a power generator to drive the two-electrode electrolyzer in the video. Credit: The authors

Compared to undoped Ni2P and other shapes, the Mo-doped Ni2P nanorings show significantly enhanced electrochemical performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolytes. The optimized Mo0.1Ni1.9P catalyst shows low overpotentials and Tafel slopes in addition to high turnover frequencies, mass activities, and exchange current densities.

Industrially relevant current densities of 500 and 1000 mA/cm2 were achieved at record-low voltages of 1.81 and 1.86 V at 25 °C and 1.77 and 1.82 V at 75 °C for overall alkaline seawater splitting. The catalyst is also highly stable under industrial 30 wt% KOH condition.

In summary, all these improvements are attributed to the dual active metal centers and the unique ring-shaped morphology. The latest synthesis protocol introduces a uniquely shaped Mo-doped Ni2P electrocatalyst with significant potential for large-scale industrial production. It also provides valuable insights into the surface chemistry mechanisms involved.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

More information:
Abhinav Yadav et al, Fine Tuning of Torus‐Shaped Mo‐Doped Ni2P Nanorings for Enhanced Seawater Electrolysis, Small (2024). DOI: 10.1002/smll.202408036

Bio:

Dr. Sasanka Deka is Professor of Chemistry, University of Delhi. He received his Ph.D. degree from National Chemical Laboratory (NCL-Pune). He did his postdoctoral research from National Nanotechnology Laboratory, CNR-INFM, Lecce, Italy and Italian Institute of Technology (IIT), Genova, Italy. He has been awarded the TMS Foundation 2008 SHRI RAM ARORA AWARD, by the Minerals, Metals & Materials Society (TMS), Warrendale, USA; DAE-BRNS Young scientist research award 2011, RSC best oral talk–2015, Institute of Physics (IOP), UK best cited paper-India 2019, RSC best cited paper in 2020, and IoE-DU publication award. Dr. Deka has published more than 80 research papers in different international high impact journals, holds three patents, and also wrote two books and three book chapters published by an international publisher. He has successfully handled several extramural national and international research projects. His current research interest deals with synthetic nanochemistry, and advanced nanomaterials for energy conversion and storage.

Journal information:
Small


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Mo-doped Ni₂P nanorings boost seawater electrolysis for hydrogen production (2025, February 3)
retrieved 3 February 2025
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