Engineering Multimetallic Aerogels for pH‐Universal HER and ORR Electrocatalysis

Abstract

The advent of noble metal aerogels (NMAs), that feature the high catalytic activity of noble metals and unique structural attributes of aerogels, has stimulated research on a new class of outstanding electrocatalysts. However, limited by the available compositions, the explored electrocatalytic reactions on NMAs are highly restricted and certain important electrochemical processes have not been investigated. Here, an effective gelation approach is demonstrated by using a strong salting‐out agent (i.e., NH4F), thereby expanding the composition of NMAs to various multimetallic systems and providing a platform to investigate composition‐dependent electrocatalytic performance of NMAs. Combining structural features of aerogels and optimized chemical compositions, the Au–Pt and Au–Rh aerogel catalysts manifest remarkable pH‐universal (pH = 0–14) performance surpassing commercial Pt/C and many other nanoparticle (NP)‐based catalysts in the electrocatalytic oxygen reduction reaction, hydrogen evolution reaction, and water splitting, displaying enormous potential for the electrochemical hydrogen production, fuel cells, etc.

Unveiling reductant chemistry in fabricating noble metal aerogels for superior oxygen evolution and ethanol oxidation

“…Compared with a number of strategies where the gelation resembles a precipitation process 13,14,16,21,23 , here, visible gel pieces occurred throughout the bulk solution upon initiation ( Supplementary Fig. 1).…”

“…8), PVP can strongly stabilize NPs by steric effects due to its large size 28,38 . Therefore, PVP-stabilized gold NPs were unaffected by almost all previously reported gelation methods (e.g., by using elevated temperature, H 2 O 2 , NH 4 F, CaCl 2 , dopamine, or ethanol 5,13,16,20,21,37 ). The remarkable destabilization power delivered by the current method can be interpreted from the as-discussed mechanism, which allows effectively wiping out PVP via the combined BH 4 − exchange/transformation and H 2 bubbling.…”

“…18), attributing to its strong stabilization ability and the shape-directing function 28 . This allows for a waterbased mass production of nanostructured gold gels from a much higher HAuCl 4 concentration (up to 5 mM) compared to previous reports (typically 0.2-0.6 mM) 5,[11][12][13]16,20,21 . Regulating the ligament sizes can further render the specific surface areas, pore size distributions, and pore volumes ( Supplementary Fig.…”

“…Intriguingly, BO 2 − ions feature a very low bonding strength to gold (E b = 0.397 kcal mol −1 ), thus will easily detaching from NPs and promoting destabilization; meanwhile, H 2 bubbles, in situ generated exactly on the NP surfaces, can strongly promote collisions between NPs. In this way, excessive NaBH 4 can serve as a unique and powerful gelation initiator, substantially accelerating the gelation process compared with previously reported methods (e.g., by using common salts, a small amount of NaBH 4 , or other additives) 5,12,13,16,20,21,37 . Regarding the one-step gelation of the HAuCl 4 solution with an excessive amount of NaBH 4 , gold NPs will be produced before and during the aforementioned process, while the general story would be similar.…”

“…Imparting the unique physicochemical properties of nanostructured noble metals to aerogels, NMAs manifest abundant catalytically/optically active sites, rapid mass/electron transfer channels, and excellent structural stability, confirming their broad application prospects such as catalysis and surface-enhanced Raman scattering [8][9][10][11] . Particularly in electrocatalysis, NMAs exhibit extraordinary activities surpassing commercial noble metal catalysts [11][12][13][14][15][16] . However, the underlying structure-performance correlations are not well understood, presumably because an insufficient understanding of the sol-gel process retards manipulating versatile parameters (e.g., ligament sizes, compositions, and spatial element distributions) 6,7 .…”