Correlation between Preparation Condition and Performance of Pt/SnO2/KB for Cathode Catalyst of PEMFC

Proton exchange membrane fuel cells (PEMFCs) are promised as the power sources for stationary and automobile application.  However, further improvement of the durability of cathode catalyst is required for full-scale commercialization.

It is recognized that Pt/C catalyst is degraded by electrochemical Ostwald ripening and migration of Pt particles at the cathode [1].  And also, carbon corrosion is serious under increasing the cathode potential.  Considering the degradation mechanisms, we reported that surface modification of carbon support with high durable material as tin dioxide is promising technique [2].  

In this study, correlation between preparation condition and performance of Pt/SnO₂/KB is investigated. 

 For preparation of Pt/SnO₂/KB, Ketjen black modified with tin oxide particles (SnO₂/KB) is firstly prepared and then Pt was loaded on SnO₂/KB.

SnO₂/KB was prepared by our recipe as reported elsewhere [2, 3]. Ketjen Black EC-300j (abbr. as KB, Ketjen Black International Co. Ltd) was dispersed in Tin fluoride solution for 24 h.  Then, it was dispersed in boric acid aqueous solution and finally was heated at various temperatures under Ar atmosphere.

Loading of Pt particles was carried out by the impregnation method [2].  Then, they were heated at various temperatures under N₂atmosphere.  The prepared catalysts were characterized by TG, SEM, ICP and CO absorption pulse test.  In particular, Pt particle size was estimated by CO absorption pulse test.

Pt/SnO2/KB, Nafion^® solution (Aldrich) and i-propanol was mixed ultrasonically to prepare the catalyst ink.  The ink was dropped on a glassy carbon disk and was used for the working electrode to measure the catalytic activity for ORR and the durability. An electrochemical cell filled with 0.1 mol dm^-3 HClO₄ aqueous solution was used. ORR activity was measured under O₂atmosphere by a rotating disk electrode technique.

The potential pulse durability tests were conducted between 1.0 V and 0.6 V to evaluate Pt durability.  Electrochemical active surface area (ECA) of Pt catalysts was estimated after an arbitrary period by cyclic voltammetry (CV) during the durability test.

 Figure 1 shows SEM image of Pt/SnO₂/KB heated at 120^oC and 300^oC.  SnO₂ particles of ca. 10 nm in the diameter were observed on KB surface. 

The Pt particle size of Pt/SnO₂/KB heated at 120^oC is estimated as ca. 2.3 nm; on the other hand, that of Pt/SnO₂/KB was ca. 1.6 nm after heating at 300^oC.

Table 1 shows specific and mass activity for ORR of Pt/SnO₂/KB heated at 120^oC and 300^oC.  ORR activity of both catalysts is approximately same.  We reported that ORR activity is increased by SnO₂/KB when SnO₂ weight ratio is over 35wt% [3].  Unfortunately, ORR activity is almost same for the catalyst without SnO₂ in this study, which may be caused by aggregation of SnO₂particles.

Figure 2 shows Pt durability test results.  The ECA remaining rates for both of these catalysts are approximately same.

 The detail results including the effect of weight ratio of SnO₂, heat treatment temperature before and after Pt loading on the activity as well as the durability will be also report.

References

[1] Y. Shao-Horn, et al., Top. Catal., 46, 305 (2007).

[2] T. Kinumoto, et al., Electrochemistry, 79, 334 (2011).

[3] T. Kinumoto, et al., ECS Trans., 58(1), 1259 (2013).