Designing Co-N/C Cathode Catalysts with Dense Atomic Cobalt Sites for Enhanced PEMFC Performance

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Non-platinum group catalysts are crucial for acidic oxygen reduction reaction (ORR) in fuel cells.
• Metal-nitrogen/carbon (M-N/C) catalysts, especially Fe, Co, or Mn-based, show promise.
• High catalyst loading can improve performance but often leads to aggregation.

Purpose of the Study:

• To synthesize and characterize a high-loading Co-N/C catalyst with exclusively atomic cobalt sites.
• To evaluate the catalyst's performance in oxygen reduction reaction (ORR) and proton-exchange membrane fuel cells (PEMFCs).
• To investigate the relationship between active site density and overall fuel cell performance.

Main Methods:

• Synthesis of a Co-N/C catalyst with 3 wt% cobalt.
• Extended X-ray absorption fine structure (EXAFS) analysis to determine coordination environment.
• Nitrite stripping method to quantify electrochemically active sites.
• Rotating ring-disk electrode (RRDE) measurements for ORR activity.
• Single-cell PEMFC testing.

Main Results:

• The optimized catalyst, (3.0)Co-N/CΔ, features a tetrapyridinic Co-N4 coordination and exclusively atomic cobalt sites.
• A high density of active sites (3.58 × 10^19 sites g⁻¹) was achieved.
• Excellent ORR activity was observed (E1/2 = 0.76 V at 0.2 mg cm⁻² loading) with a mass activity of 3.5 A g⁻¹ at 0.80 V.
• Peak power density exceeding 1.3 W cm⁻² in H2-O2 PEMFCs.
• Comparable fuel cell performance to Fe-N/C catalysts was achieved, despite lower intrinsic turnover frequency of Co sites.

Conclusions:

• High cobalt loading in M-N/C catalysts can significantly increase active site density.
• Atomic cobalt sites in a Co-N4 configuration are highly effective for ORR.
• Achieving high site density is critical for maximizing catalyst performance in fuel cells, comparable to catalysts with higher intrinsic activity but lower site density.