Bimodal nanoporous platinum on sacrificial nanoporous copper for catalysis of the oxygen-reduction reaction

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Research Letter

Bimodal nanoporous platinum on sacrificial nanoporous copper for catalysis of the oxygen-reduction reaction Masataka Hakamada, Yuto Sato, and Mamoru Mabuchi, Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo, 606-8501 Kyoto, Japan Address all correspondence to Masataka Hakamada at [email protected] (Received 21 August 2018; accepted 7 November 2018)

Abstract Bimodal nanoporous platinum (BNP-Pt) is synthesized by using a sacrificial nanoporous copper (NP-Cu) support for oxygen-reduction-reaction (ORR) catalysts in fuel cells. The specific ORR catalytic activity of BNP-Pt increases by the dissolution and removal of supporting NP-Cu, suggesting that the BNP structure improves the intrinsic catalytic properties of platinum. The lattice contraction of BNP-Pt containing residual copper even after NP-Cu removal is milder than predicted by Vegard’s law. The BNP structure governs the intrinsic catalytic activity of the platinum by relaxing the lattice contraction and by alloying with copper and/or misfit strain at the Pt/Cu interface.

Introduction Platinum is used as a catalytic electrode in fuel cells. Fuel-cell performance is often governed by the oxygen-reduction reaction (ORR) at an air electrode made of platinum.[1] Platinum is a rare metal and is found disproportionately in the earth’s crust. Hence, the platinum price is high and fluctuates, which prevents fuel cells from being used widely. Many efforts have been made to improve the catalytic activity of platinum in the ORR. One strategy is nanostructuring of platinum; that is, a reduction in the typical length scale of platinum is advantageous in terms of an effective surface area. Platinum nanoparticles[2,3] and hollow platinum nanoparticles are good candidates for efficient ORR catalytic electrodes.[4,5] On the other hand, the synthesis of nanoporous metals by dealloying or selective dissolution of one element from a binary alloy is another strategy for nanostructuring. Such a threedimensional nanoporous structure is obtained in various metals.[6,7] Nanoporous copper (NP-Cu) can be also produced by dealloying ductile Cu–Mn alloys,[8–10] and similarly, porous Raney copper is synthesized by dealloying Cu–Al alloys.[11] Platinum can be deposited in the form of a dense film or fine particles on copper by electroless plating or galvanic replacement.[5,12] Thus, NP-Cu can be converted to nanoporous platinum with a skeletal structure replicated from NP-Cu and deposits of fine platinum particles. The supporting NP-Cu can be removed readily by HNO3 which dissolves copper selectively out through the porous particulate platinum layer, and leaves a bimodal nanoporous platinum (BNP-Pt) [Fig. 1(a)]. Here we show the synthesis of BNP-Pt for ORR by electroless plating of platinum on the sacrificial NP-Cu and subsequent dissolution of NP-Cu. We focus on the contraction of the

crystal lattice of platinum in BNP-Pt as a source of high ORR catalytic activity per surface area.

Material and