Ultra-Low Platinum Nano-Alloy Encapsulated in Carbon Nanotube Matrix: A Practical PEM Fuel Cell Electrocatalyst
Authors: Zaman Shahid 
Published: December 2020

Hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are the crucial PEM fuel cell reactions. However, the sluggish kinetics of these reactions particularly ORR require efficient catalysts to reduce the larger voltage losses and boost the electrochemical activity. Hence developing efficient and durable platinum (Pt)-based electrocatalysts is of enormous significance for practical PEM fuel cell. The state-of the-art carbon black supported Pt nanoparticles (Pt/C) catalyst has sever activity and stability issues. The tiny Pt nanoparticles often undergo the dissolution and redeposition under electrochemical operation, which would destroy their initial state of highly uniform dispersion and high activity. Meanwhile, the amorphous carbon support suffers from the corrosion and oxidation at the cathodic working conditions, which leads to the initial interaction disruption of Pt and carbon support. It causes Pt nanoparticles detachment and agglomeration with a consequent significant activity degradation. Herein we report an integrated ultra-low Pt nanoalloy encapsulated in nitrogen-doped carbon nanotubes matrix (PtCoNi@NCNTs) for efficient oxygen reduction in practical fuel cell. This hybrid Pt-based catalyst achieves a higher mass activity of 3.46 A mgPt -1 at 0.9 V vs. RHE with a negligible stability decay after 10,000 cycles. More importantly, this excellent half-cell activity can be fully transformed into practical full cell. The fuel cell based on this hybrid electrocatalyst delivers a power density of 11.2 W mgPt
-1
cathode, which is 8 times of commercial Pt/C 20 % based cells (1.4 W mgPt
-1
cathode). Furthermore, PtCoNi@NCNTs also demonstrates an excellent fuel-cell durability by retaining its initial current after 10 h operation at the constant voltage of 0.7 V without any loss. This excellent performance is ascribed to the synergetic response by the integration of encapsulated Pt nano-alloy and hierarchical nitrogen-doped carbon nanotube matrix. It is well acknowledged that the fine tuning of transition metals in the PtM alloy offers extra Pt structural strain that controls the weakening of surface bonds and results in a hike in the electrocatalytic activity. Moreover, the incorporation of metal-nitrogen-carbon (M-N-C) sites also additionally boosts catalytic activity, where the hierarchical architecture is not only the robust substrate to anchor/embed ternary alloy nanoparticles but also ensures sufficient contact of the reactant on the catalysts, which is always ignored in the previous catalysts for fuel cells. Respectable hydrophilic NCNTs matrix restricts comprehensive contact between electrolyte and active sites thereby regulating the oxygen diffusion to the active sites and avoid flooding, hence the enhanced mass transport boosts fuel cell performance. Decent hydrophilic nature of the PtCoNi@NCNTs catalyst facilitates the three-phase interface at the cathode that offers efficient mass transport and excellent conductivity for efficient oxygen reduction reaction. Higher graphitic properties offer advanced durability to the NCNTs support against corrosion and oxidation at cathodic working conditions, while few layers carbon coatings shield the Pt nanoalloy against dissolution in acidic conditions thereby offering pronounced durability for long term fuel cell operation. Therefore, such novel graphitic NCNTs support material endows superior durability to the electrocatalyst in real working conditions of PEM fuel cell. In this work the existing gaps between the RDE activity and fuel cell performance have been investigated and optimized magnificently, which endorses the complete transformation of RDE activity to the MEAs level. This work may provide significant insights in designing the low-Pt integrated highly durable electrocatalysts for practical PEM fuel cells.

Figure 1