Promoting Effects of Au Submonolayer Shells on Structure-Designed Cu–Pd/Ir Nanospheres: Greatly Enhanced Activity and Durability for Alkaline Ethanol Electro-Oxidation

Abstract

Rationally engineering the surface physicochemical properties of nanomaterials can improve their activity and durability for various electrocatalytic and energy conversion applications. Cu–Pd/Ir (CPI) nanospheres (NSs) anchored on N-doped porous graphene (NPG) [(CPI NSs/NPG)] have been recently demonstrated as a promising electrocatalyst for the alkaline ethanol oxidation reaction (EOR); to further enhance their electrocatalytic performance, the NPG-supported CPI NSs are coated with Au submonolayer (SML) shells (SMSs), through which their surface physicochemical properties can be tuned. CPI NSs/NPG is prepared by our previously developed method and possesses the special structures of composition-graded Cu1Pd1 and surface-doped Ir0.03. The Au SMSs with designed surface coverages are formed via an electrochemical technology involving incomplete Cu underpotential deposition (UPD) and Au3+ galvanic replacement. A distinctive volcano-type relation between the EOR electrocatalytic activity and the Au-SMS surface coverage for CPI@AuSML NSs/NPG is revealed, and the optimal CPI@Au1/6ML NSs/NPG greatly surpasses commercial Pd/C and CPI NSs/NPG in electrocatalytic activity and noble metal utilization. More importantly, its electrocatalytic durability in 1 h chronoamperometric and 500-cycle potential cycling degradation tests is also significantly improved. According to detailed physicochemical characterizations, electrochemical analyses, and density functional theory calculations, the promoting effects of the Au SMS for enhancing the EOR electrocatalytic activity and durability of CPI NSs/NPG can be mainly attributed to the greatly weakened carbonaceous intermediate bonding and properly increased surface oxidation potential. This work also proposes a versatile and effective strategy to tune the surface physicochemical properties of metal-based nanomaterials via incomplete UPD and metal-cation galvanic replacement for advancing their electrocatalytic and energy conversion performance.

Potential-Dependent Mechanistic Study of Ethanol Electro-oxidation on Palladium

“…where U 0 and E 0 refer to the PZC and the corresponding energy at the PZC, respectively. Since the EOR is a complex dehydrogenation reaction, the Gibbs free energy of the solvated proton and electron p a i r ( H + + e − ) w a s c o n s i d e r e d t o b e e q u a l t o E eU k Tln (10)…”

“…Although the rate of ethanol oxidation reaction is a key factor to determine the performance of DEFCs, the understandings of the detailed ethanol oxidation reaction (EOR) mechanism on electrocatalysts remain unclear and contradictory . A widely recognized “dual-pathway mechanism” theoretically suggests two main possible routes in which the EOR could proceed either through a complete oxidation (C1) route to produce carbon dioxide (CO 2 ) involving 12 electrons or through a partial oxidation (C2) route to produce acetaldehyde (CH 3 CHO) or acidic acid/acetate (CH 3 COOH/CH 3 COO − ) involving 2 or 4 electrons. , In fact, it has been shown experimentally that the partial oxidation route is the dominating one on the state-of-art Pd-based electrocatalysts. − The production of CO 2 through the complete oxidation route only takes up less than 2.5% of the current efficiency. , This favor over the partial oxidation route leads to an insufficient activity of the EOR on the electrocatalysts in DEFCs. To address this issue, great efforts have been devoted to explore ways to enhance the catalytic activity of Pd-based electrocatalysts for the EOR, such as alloying with other metals, − , introducing extra compounds , modifying morphology, − and changing supports. − At present, the state-of-art Pd-alloy electrocatalysts can achieve a catalytic activity at least five or even ten times higher than that of the commercial Pd/C, , while the performance of DEFCs can reach over 180 mW/cm 2 . , A further improvement of DEFC performance requires a detailed mechanistic understanding of the complex EOR pathways on Pd surfaces.…”

Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy Insights into CO Adsorption and Oxidation on Iridium Surface

“…Iridium (Ir) has attracted much attention recently due to its important role in the surface-sensitive electrocatalytic reactions such as alcohol oxidation, − ammonia oxidation, − oxygen evolution reaction, and so forth. , Specifically, ethanol oxidation reaction (EOR), serving as the anodic half-reaction of direct ethanol fuel cells which could be a promising sustainable portable power source, needs to promote its efficiency. Also, it has been demonstrated to benefit the catalysis performance through the addition or decoration of the Ir metallic component. ,, In detail, it was found that Ir could display excellent ability in breaking the C–C bond , and the C–H bond of ethanol molecule to form the so-called C1 intermediates (CO ad and CH x species), which could follow the so-called C1 reaction pathway and eventually result in the considerable conversion of ethanol into CO 2 . ,, Along this line, some Ir-based or Ir-contained EOR catalysts were reported either in acidic or alkaline media, such as PtIr–SnO 2 , IrSn/SnO 2 , and IrPd/C . Note that the pristine Ir itself can hardly exhibit EOR catalytic activity, which could be due to the poisoning effect from the as-generated CO ad and/or CH x species on the Ir surface.…”

“…Also, it has been demonstrated to benefit the catalysis performance through the addition or decoration of the Ir metallic component. 3,10,11 In detail, it was found that Ir could display excellent ability in breaking the C−C bond 3,12 and the C−H bond of ethanol molecule 13 to form the so-called C1 intermediates (CO ad and CH x species), which could follow the so-called C1 reaction pathway and eventually result in the considerable conversion of ethanol into CO 2 . 1,10,13 Along this line, some Ir-based or Ir-contained EOR catalysts were reported either in acidic or alkaline media, such as PtIr− SnO 2 , 3 IrSn/SnO 2 , 14 and IrPd/C.…”

Alloyed Palladium–Lead Nanosheet Assemblies for Electrocatalytic Ethanol Oxidation

“…Wang et al synthesized palladium–lead (Pd–Pb/C) bimetallic alloyed electrocatalysts for the EOR . Thus far, electrocatalysts with various morphologies, like nanocubes, nanochains, , nanowires, − nanospheres, network nanocomposites, and nanosheets, have been reported with improved electrocatalytic activities. Cabot et al synthesized the regular Pd 3 Pb nanocube foroxidation of alcohols .…”