Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is essential for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), which are applied to fuel cell vehicles (FCVs) and residential co-generation systems. At the present stage, costly Pt or its alloys have been employed as the cathode catalyst, having appreciable ORR activity and durability in strong acidic electrolyte at low operating temperatures <100 °C.

The mass activity MA of Pt-based catalysts is defined as MA (A g_Pt⁻¹) = j_S (A m⁻²) × ECA (m² g_Pt⁻¹). To increase the area-specific activity j_S (current density per active surface area), Pt-M alloys (Pt-Fe, Pt-Co, Pt-Ni, Pt-Cr, etc.) have been examined. Assuming spherical catalyst particles, the electrochemically active surface area ECA is inversely proportional to the particle diameter. To increase the ECA, it is effective to disperse Pt or Pt-alloy nanoparticles on high-surface-area carbon supports (Pt/C or Pt-M/C). However, for developing such catalysts having both high ORR activity and high durability, there has been long-standing controversy surrounding important issues such as the crystal structures (ordered and disordered) and the chemical composition of Pt-M alloys, as well as their optimum particle size. Furthermore, because the mechanism of enhancement of the ORR activities at Pt-M alloys is still unclear, the strategy for designing new potential catalysts has not yet been established.

This presentation focuses on the following topics for the cathode catalysts from bulk single crystals to practical nanoparticle catalysts.

1) Pt particle-size effect on the durability for an accelerated test, which simulates load-cycles for FCVs

So far, much research has suffered from trade-offs in determining the optimal particle-size of the catalysts, due to the different trends of Pt particle-size effects on the MA and durability. However, by the use of our n-Pt/C catalysts with a very narrow size distribution (σ_d ≤ 10%), the most durable catalyst with the highest MA over the whole test period (65°C, 0.6 ↔ 1.0 V, up to 30,000 cycles) was found to be n-Pt_{2 nm}/C.¹

2) Enhanced ORR activities at Pt-M alloys^{2, 3}

2-1) Enormously enhanced ORR activity at Pt-skin layer formed on Pt₃Co(111) single crystal electrode

Pt-skin/Pt–Co(111) single crystals exhibited extremely high ORR activity, j_S. The j_S value at 0.9 V reached a maximum at Pt₇₃Co₂₇(111), the value of which is ca. 27 times higher than that on a pure Pt(111) electrode.⁴ By the use of in situ STM, in situ SXS, ex situ XPS, and DFT calculation, we have succeeded in correlating such a high j_S with the specific surface structure.

2-2) ORR activity and durability of ordered- and disordered-Pt₃Co/C⁵

We have recently clarified the effect of the crystal structure of Pt₃Co alloy nanoparticles on the ORR activity, H₂O₂ yield, and durability, for the first time, by the use of the ordered- and disordered-Pt₃Co/C catalysts with the nearly identical average particle size, size distribution, and composition.⁵

3) Enhancement in the ORR activity and durability at stabilized Pt-skin–PtCo alloy catalysts^{6, 7}

We have successfully prepared PtCo alloy nanoparticles, having a stabilized Pt skin (one to two atomic layers: xAL), supported on carbon black or graphitized carbon black (Pt_xAL–PtCo/C or Pt_xAL–PtCo/GCB). These new catalysts exhibited high MA for the ORR, together with superlative durability.

This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from the NEDO of Japan.

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