Conventional polymer electrolyte water electrolysis (PEWE) requires a large amount of noble metal catalysts such as Pt and/or Ir. Platinum (ranging from 0.5 to 2 mg_Pt cm⁻²) has been used as the cathode catalyst for the hydrogen evolution reaction (HER).^{1, 2} Development of highly active and durable catalysts for the PEWE is essential for the efficient production of hydrogen with reduced amounts of noble metals.³ While some Pt alloys have been examined for the HER in the PEWE, the durability was not sufficient due to dealloying.² Recently, we have found that Pt_xAL–PtM (M=Fe, Co, Ni)/C (with stabilized Pt skin of 1 to 2 atomic layers, Pt_xAL) nanoparticles on carbon supports exhibited superlative activity for the hydrogen oxidation reaction (HOR) in H₂-saturated 0.1 M HClO₄ at 70 and 90ºC.⁴ In this presentation, we demonstrate that the Pt_xAL–PtM and PtM/C catalysts also exhibited higher activity for the HER, compared with that of Pt.

Pt₃Co/C catalyst was prepared by the nanocapsule method,⁵ followed by heat-treatment in 5% H₂ (denoted as Pt₃Co_HT/C). The Pt_xAL–PtM/C catalysts were prepared in the same manner as that described previously.^{6, 7} Two commercial catalysts, c-Pt/C (d_TEM =2.6±0.9 nm) and c-Pt₃Co/C (d_TEM =5.1±1.8 nm), were used for comparison. To measure the HER activities of the catalysts in H₂-saturated 0.1 M HClO₄ solution, the channel flow electrode (CFE) cell⁴ was used to minimize the ohmic loss by H₂ bubble evolution. Each catalyst was uniformly dispersed on an Au substrate as the working electrode in the CFE cell with a constant loading of carbon support of 11 µg cm⁻², which corresponds to approximately two monolayers in height of the carbon particles. Nafion film was coated on the catalyst layer with an average thickness of 0.10 µm.

It was found for the Pt₃Co_HT/C by TEM, XRD and ICP that Pt₃Co nanoparticles with narrow size-distribution (d_TEM =2.9±0.5 nm) were uniformly dispersed on the carbon support. The values of electrochemically active area (ECA) of all catalysts examined accorded well with those estimated from d_TEM with the assumption of spherical particles. Figure 1(a) shows iR-free polarization curves for our Pt₃Co_HT/C and commercial catalysts at 80ºC in H₂-saturated 0.1 M HClO₄ with the mean flow rate U_m = 111 cm s⁻¹. Apparent area-specific activities j_S for the HER at −20 mV vs. reversible hydrogen electrode (RHE) are summarized in Fig. 1(b). The value of j_S on the Pt₃Co_HT/C was about 1.6 times higher than that of c-Pt₃Co/C and 3 times higher than that of c-Pt/C. The value of apparent mass activity MA at −20 mV on the Pt₃Co_HT/C was as high as 4.3 A mg_Pt⁻¹ (see Fig. 1(a)), indicating the possibility of reduction of the Pt cathode loading to a level as low as 0.23 mg_Pt cm⁻² for operation at 1 A cm⁻² with an overpotential of −20 mV. Evaluation of performance and durability of PEWE cells with these cathode catalysts is now in progress.

This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid No. 17H01229 for Scientific Research (A) from the Ministry of Education, Science, Sports and Culture, Japan.

References

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