Interstitial B-Doping in Pt Lattice to Upgrade Oxygen Electroreduction Performance

Abstract

The dissolution of M in currently popular Pt–M alloy catalysts (M = Co, Ni, and Fe) during the oxygen reduction reaction (ORR) may deter their wide application in proton exchange membrane fuel cells (PEMFCs). In this work, interstitial B-doping in the Pt lattice is instead used to design a durable and active ORR catalyst, by taking advantage of its unique regulation of the electronic structure of surface Pt sites. 3 nm Pt–B nanoparticles on carbon black (Pt–B/C) are obtained using dimethylamine borane (DMAB) as a reductant and the B source in a mixed H2O–ethylene glycol precursor solution. The formation of the B-doped Pt catalyst is verified by inductively coupled plasma-atomic emission spectrometry, X-ray diffractometry, and spherical aberration-corrected scanning transmission electron microscopy. Both half-cell and single-cell tests indicate that the as-synthesized Pt–B/C catalyst outperforms the commercial Pt/C­(com) in terms of activity and durability. In particular, the Pt–B/C-based PEMFC exhibits an initial maximum power density 1.24 times as high as the Pt/C­(com)-based one under otherwise same conditions, with a 15% decay for the former versus a 45% decay for the latter after 30 000 cycles of the accelerated degradation test (ADT). Comparative DFT calculations on B-doped and undoped Pt(111) surfaces reveal that the lowered Pt d-band center and the strong interaction of Pt–B bonding weaken the binding of OH and O species to surface Pt sites and lessen oxidative disruption of surface Pt atoms. This interstitial metalloid doping in conjunction with the simple and scalable synthesis protocol enables the Pt–B/C to be a competitive ORR catalyst for the PEMFCs.

Boron-Doped Platinum-Group Metals in Electrocatalysis: A Perspective

“…The retaining of pristine crystalline structures is applicable to other B-doped PGMs for low Bdoping levels. 32,39,42 Moreover, XRD characterization and model calculations confirm that, for the fcc crystalline structure, the larger interstitial volume in Pt induces less lattice expansion or structural undulation than that in Pd. 42 It is worth noting that the lattice structure regulation, especially the undulation of the outermost layers accompanied by B-doping, may contribute to the overall adsorption energy tuning for reactive intermediates (vide infra).…”

“…33,62 For the B-dopant position, density functional theory (DFT) calculations reveal that the interstitial octahedral occupancy is the most energetically favorable regardless of the B-doping level. 42,44,63 For highly B-doped PGMs, an interstitial tetrahedral occupancy is a second choice. 48 In the early stage, the B-doping content is mainly determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES); 37 the enlarged lattice parameters upon interstitial B-doping are characterized by high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction (XRD), 37,46,64 in which the obtained Pd−B catalysts with Bdoping levels of less than ∼20 atom % maintain the fcc structure, although partial lattice disruption can be inferred by the widened full width at half maximum.…”

“…35 A similar electron transfer discussion was also reported recently on B-doped Pt by means of a Bader charge analysis. 42 Figure 5c compares the valence band spectra on Pd and Pd−B, in which the latter shows a broadened bandwidth (4.11 eV for Pd−B and 3.82 eV for Pd) and a "net" downshift of ε d from −2.51 eV on Pd/C to −2.81 eV on Pd−B with respect to the Fermi level. 55 The downshift of ε d also applied to the hcp structured Pd 2 B, as supported by a decreased magnetic susceptibility or a reduced density of states (DOS) close to the Fermi level as compared to the counterparts for Pd.…”

“…A Pt−B/C catalyst was recently developed by our group with improved activity and excellent stability for ORR in both halfcell and single-cell measurements (Figure 9a). 42 An H 2 −O 2 single fuel cell using Pt−B/C as the cathode catalyst exhibits a 24% higher maximum power density as compared to that with the state-of-the-art Pt/C under otherwise the same conditions. Moreover, the maximum power density with Pt−B/C only decays by 15% after 30000 cycles of accelerated degradation tests (ADTs), while that with Pt/C cathode suffers a loss of 45%.…”

“…On the other hand, the electron transfer and/or orbital hybridization between the PGM and the interstitial dopant manifest a delicate control on the surface electronic structure of the resulting catalysts. , Among the light-element-doped PGM catalysts, B-doped PGMs were historically once studied in the thermocatalytic alkyne hydrogenation reactions. − In 2009, our group came up with the first report on electrocatalysis of the formic acid oxidation reaction (FAOR) on home-synthesized Pd–B/C catalysts with improved electrocatalytic activity and anti-CO-poisoning stability. That preliminary work triggered the ongoing development of B-doped PGM electrocatalysts, through the extension of B-doping to other PGMs and the increase of interstitial B-doping concentration for a transition from solid solution alloys to ordered intermetallics, toward multitude of electrocatalytic reactions (Figure ), such as Os–B, Pd 2 B, RuB, and RhB catalysts for the hydrogen evolution reaction (HER), ,,, Pd–B and Pt–B catalysts for the oxygen reduction reaction (ORR), − and Pd–B for formic acid dehydrogenation (FAD), FAOR, and CO 2 reduction reactions (CO 2 RR). ,− …”