[Paper Review] Cobalt-based Co$_3$Mo3N/Co$_4$N/Co Metallic Heterostructure as a Highly Active Electrocatalyst for Alkaline Overall Water Splitting

Yuanwu Liu, Lirong Wang+7 more|arXiv (Cornell University)|Apr 17, 2024

Ammonia Synthesis and Nitrogen Reduction19 citations

TL;DR

The paper reports a cobalt-based metallic heterostructure (Co$_3$Mo3N/Co$_4$N/Co/Co) as an efficient, durable catalyst for alkaline water splitting, achieving low cell voltage and long-term stability.

ABSTRACT

Alkaline water electrolysis is considered a commercially viable option for large-scale hydrogen production. However, this process still faces challenges due to the high voltage (>1.65 V at 10 mA cm$^{-2}$) and its limited stability at higher current densities due to the inefficient electron transport kinetics. Herein, a novel cobalt based metallic heterostructure (Co$_3$Mo3N/Co$_4$N/Co/Co) is designed for application for water electrolysis. Operando Raman experiments reveal that the formation of Co$_3$Mo3N/Co$_4$N/Co heterointerface boosts the free water adsorption and dissociation, resulting in a surplus of protons available for subsequent hydrogen production. Furthermore, the altered electronic structure of Co$_3$Mo3N/Co$_4$N/Co heterointerface optimizes the ΔGH of nitrogen atoms at the interface. This synergistic effect between interfacial nitrogen atoms and metal phase cobalt creates highly efficient hydrogen evolution reaction (HER) active sites, thereby enhancing the overall performance. Additionally, the heterostructure exhibits a rapid OH- adsorption rate, coupled with a strong adsorption strength, leading to improved oxygen evolution reaction (OER) performance. Crucially, the metallic heterojunction facilitates fast electron transport, expediting the aforementioned reaction steps and ultimately improving the overall efficiency of water splitting. The water electrolyzer with Co$_3$Mo3N/Co$_4$N/Co/Co as a catalyst exhibits outstanding performance, requiring an impressively low cell voltage of 1.58 V at 10 mA cm$^{-2}$ and maintaining approximately 100% retention over a remarkable 100 h duration at 200 mA cm$^{-2}$. This performance significantly exceeds that of the commercial Pt/C || RuO2 electrolyzer.

Motivation & Objective

• Motivate high-performance, scalable catalysts for alkaline water electrolysis.
• Design and evaluate a cobalt-based metallic heterostructure to enhance HER and OER kinetics.
• Elucidate interfacial effects that boost water adsorption, nitrogen-related electronic structure, and electron transport.
• Demonstrate practical electrocatalytic performance in a water electrolyzer under industrial-relevant conditions.

Proposed method

• Synthesis of a Co$_3$Mo$_3$N/Co$_4$N/Co/Co metallic heterostructure.
• Operando Raman spectroscopy to monitor interfacial formation and water adsorption/dissociation.
• Analysis of electronic structure at the heterointerface and its effect on nitrogen atom ΔGH.
• Assessment of OH$^-$ adsorption dynamics and overall catalytic kinetics for HER and OER.
• Electrical transport studies to connect heterojunctions with rapid electron transfer during water splitting.

Experimental results

Research questions

• RQ1 Can the Co$_3$Mo$_3$N/Co$_4$N/Co/Co heterostructure lower the cell voltage for alkaline overall water splitting?
• RQ2 How does the interfacial Co–N environment influence hydrogen evolution and oxygen evolution activity?
• RQ3 What role does fast electron transport at the metallic heterojunction play in overall electrolysis performance?
• RQ4 What is the stability of the catalyst under prolonged operation at high current densities?

Key findings

• The electrolyzer with the Co$_3$Mo$_3$N/Co$_4$N/Co/Co catalyst requires 1.58 V at 10 mA cm$^{-2}$.
• The catalyst maintains ~100% retention over 100 h at 200 mA cm$^{-2}$.
• Operando Raman shows interfacial Co$_3$Mo$_3$N/Co$_4$N/Co formation boosts water adsorption and dissociation.
• Interfacial nitrogen atom–metal interactions optimize ΔGH and enhance HER activity.
• The heterostructure exhibits rapid OH$^-$ adsorption with strong adsorption strength, improving OER performance.
• Fast electron transport across the metallic junction accelerates the overall water-splitting process.