MOF-Derived Trifunctional High Entropy Sulfides as an Electrocatalyst for Alkaline Unitized Regenerative Fuel Cell Application
Authors: Kalahasti V R Siddhartha Sairam , Sarkar Mithun , Jha Anshu , Kharwar Yashwant Pratap , Ghosh Prakash Chandra , Dutta Arnab 
Published: February 2024

Electrochemical devices such as electrolyzers (EL), fuel cells (FC), metal-air batteries (MAB), and unitized regenerative fuel cells (URFC) are key components in our drive toward a renewable energy-based power infrastructure. The potential scale of these devices requires the usage of catalytic materials that exhibit not only excellent catalytic activity but also commendable durability. Conventional precious metal catalysts such as Pt, Ir, Rh, Ru, and Pd are utilized for variable deployments during electrocatalysis.
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Recent trends in alloying, tuning the band structure, nanosizing of particles, and application of earth-abundant transition metals, have immensely increased the economic feasibility and practical applications of a new genre of electrochemical devices.
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Here, strategically designed materials, such as metal-organic frameworks (MOF), offers a new route for synthesizing the alloys and their derivatives with high porosities and excellent catalytic activity.
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High entropy alloys (HEA) containing multiple (four or more) components of metallic oxides, sulfides, and phosphides have emerged as a new class of materials. Effects such as lattice distortion, entropy stabilization, cocktail effect, and formation of multiple oxidation states in such HEA enable access to multiple catalytic centers for catalyzing more than one small molecule activation reaction.
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The current work focuses on the synthesis of FeCuCoMnNi-based high entropy sulfide (HES) via a unique MOF route. This HES material successfully demonstrates three distinct catalytic reactions: oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The surface morphology and phase of the HES can be critically curated via sulfurizing the multi-metal MOF precursor, which was monitored with a battery of surface characterization techniques. This HES displayed an impressive bifunctionality index of 0.80 V and 1.87 V for application in alkaline URFC (ORR/OER) and EL (HER/OER), where moderate to low overpotentials were observed for OER (η OER = 380 mV), ORR (η ORR = 420 mV), and HER (η HER = 260 mV). The superior electrochemical performance of this HES is attributed to the surface reconstruction mechanism upon the inclusion of transition metals such as Fe, Co, and Mn to Ni leading to the formation of the key Ni-OOH layer. Along with excellent performance, the HES exhibited good durability at +1.5 V vs. RHE up to 12 hours while retaining ~75% of the initial current density, indicating the presence of a high configurational entropy in the material.

References

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