Development of Transition Metal-Based Electrocatalysts for Energy-Efficient Hydrogen Production through Hybrid Water Splitting Reactions

Mishra, Viplove (2025) Development of Transition Metal-Based Electrocatalysts for Energy-Efficient Hydrogen Production through Hybrid Water Splitting Reactions. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Viplove Mishra (20RS073)) 20RS073.pdf - Submitted Version Restricted to Repository staff only Download (12MB)

Abstract

Hydrogen (H₂) is a promising fuel for the future owing to its high specific energy and zero carbon emissions, making it an ideal candidate for energy storage applications and a suitable substitute for fossil fuels. Electrocatalytic water splitting is one of the best and most environmentally benign methods to produce ultrapure H₂. Two half-reactions, the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode are involved in the energetically uphill water splitting reaction. Compared to HER, the reaction dynamics of the OER are constrained due to high activation energy barriers, which is the bottleneck for the practical application of water electrolysis. Many TM-based OER catalysts display good performance at lab-scale conditions (1 M KOH, 25°C, and current density of 10 mAcm⁻²geo). However, only a few could efficiently perform and remain stable under industrial conditions (6 M KOH and 55-85 °C) and at a high current density of 200-1000 mAcm⁻²geo due to harsher reaction conditions. Despite the advancement, even noble metal-based electrolyzers need a much more voltage ~ 1.6 to 2.0 V than the theoretical value of 1.23 V. Recently, a promising strategy has been introduced to reduce the overall cell voltage for H2 production, in which kinetically sluggish OER is replaced with a more thermodynamically favorable reaction such as hydrazine, methanol, urea, sodium borohydride, formaldehyde, benzyl alcohol, hydroxymethyl furfural oxidation reactions at the anode. The most prevalent choices are precious metals based on Pt, Ru, and Ir; however, their high cost and limited availability limit their broad use. This has motivated us to develop electrocatalysts for water electrolysis and hybrid water electrolysis using less expensive transition metal-based electrocatalysts. Furthermore, the inevitable requirement of high cell voltage, particularly for reaching high current density (200 - 1000 mA/cm²), and the use of high-temperature synthetic conditions to prepare catalysts motivated us to improve the performance of electrocatalysts by suitably altering the synthesis conditions and applying innovative strategies like heterogenic interfacial coupling, microstructural tuning, etc. The whole presentation is separated into five chapters. Chapter 1 presents a comprehensive overview of the research work in the field of electrocatalytic water splitting for hydrogen production. For instance, the history of energy resources, the current energy crisis, and alternative energy resources, the electrochemical setup, and performance evaluation parameters. Chapter 2 provides a new synthesis protocol to develop crystalline Co3O4 using the microstructural tuning ability of the iodate (IO₃₋) moiety. The microstructural tuning abilities of the iodate (IO₃₋) moiety led to an increase in defects, oxygen vacancies, surface area, and pore volume of Co₃O₄-I. The resulting iodate-derived Co₃O₄-I demonstrates excellent high-current density OER performance and durability under both ambient and industrial conditions. The Co₃O₄-I showed an excellent lab-scale ambient OER performance (249 ± 4 mV@ 10 mAcm⁻²geo) using a nickel foam substrate. Furthermore, the Co₃O₄-I demonstrated splendid OER performance (1.383 ± 0.003, 1.425 ± 0.002, and 1.501 ± 0.004 V @ 10, 100, and 500 mAcm-2geo, respectively) at industrial conditions (6 M KOH, 85 °C). Additionally, the Co₃O₄-I/NF demonstrated prolonged excellent durability for industrial conditions OER (72 h @ 500 mAcm⁻²geo). Chapter 3 discusses the fabrication of Co₃O₄/CoS₂ heterostructure using a facile and low-temperature hydrothermal technique for overall hydrazine-assisted water splitting (OHzWS). The Co₃O₄/CoS₂ heterostructure was synthesized using Co₃O₄ as a template, followed by controlled sulfurization. Such control over sulfurization is expected to provide the optimum sulfurization, attributed to the heterogenic interface formation between Co₃O₄ and CoS₂, leading to excellent hydrazine oxidation reaction (HzOR) performance in the Co₃O₄/CoS₂ heterostructure. Additionally, a developed heterostructure can efficiently perform electro-oxidation of HzOR even at a high current density at a low potential (300 mA/cm² @ 480 mV) using an electrochemically neutral carbon paper-based substrate. The Co₃O₄/CoS₂ heterostructure bifunctional catalyst required a cell potential of 0.49 V to attain a benchmark current density of 10 mA/cm² for OHzWS, which is 1.35 V less than the overall conventional water splitting (10 mA/cm² @ 1.84 V). Chapter 4 depicts the development of the n-n type Co₉S₈/CoTe₂ heterostructure by optimizing the tellurium and sulfur concentration in the system to maximize the impact of synergistic heterogenic interfacial coupling for overall hydrazine-assisted water splitting (OHzWS). Detailed investigation revealed that the n-n heterojunction formation between Co₉S₈ and CoTe₂ phases triggers an oriented built-in electric field, facilitating faster electronic charge transportation at the interface, reducing charge transfer resistance, and improving electrochemical performance for HER and HzOR. The OHzWS cell performance of 10 mA/cm² @ 0.22 V and 100 mA/cm² @ 0.71 V were obtained compared to the values observed for overall water splitting (OWS) cell (10 mA/cm² @ 1.82 V) constructed using Co₉S₈/CoTe₂ heterostructure as a bifunctional (OER, HER) catalyst for both anode and cathode. Chapter 5 provides an idea for the design of efficient electrocatalysts for energy-saving hydrogen production coupled with formaldehyde oxidation. This is achieved by using self-supported thermally reconstructed copper foam for a formaldehyde oxidation reaction (FOR) electrocatalyst in formaldehyde oxidation-assisted energy-efficient hydrogen production. The synthesis of self-supported thermally reconstructed copper foam was performed via a single-step calcination method. The developed self-supported thermally reconstructed copper foam can efficiently perform electro-oxidation of formaldehyde even at a high current density at a low potential (500 mA/cm² @ 169 mV). The TRCF/PtC cell can deliver a current density of 10 mA/cm² @ - 0.01 V for OFWS, which is 1.69 V less than OWS (1.68). formaldehyde oxidation-assisted energy-efficient hydrogen production.

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