Self-Standing Pd-Based Nanostructures for Electrocatalytic CO Oxidation: Do Nanocatalyst Shape and Electrolyte pH Matter?

Abstract

Tailoring the shape of Pd nanocrystals is one of the main ways to enhance catalytic activity; however, the effect of shapes and electrolyte pH on carbon monoxide oxidation (COOxid) is not highlighted enough. This article presents the controlled fabrication of Pd nanocrystals in different morphologies, including Pd nanosponge via the ice-cooling reduction of the Pd precursor using NaBH4 solution and Pd nanocube via ascorbic acid reduction at 25 °C. Both Pd nanosponge and Pd nanocube are self-standing and have a high surface area, uniform distribution, and clean surface. The electrocatalytic CO oxidation activity and durability of the Pd nanocube were significantly superior to those of Pd nanosponge and commercial Pd/C in only acidic (H2SO4) medium and the best among the three media, due to the multiple adsorption active sites, uniform distribution, and high surface area of the nanocube structure. However, Pd nanosponge had enhanced COOxid activity and stability in both alkaline (KOH) and neutral (NaHCO3) electrolytes than Pd nanocube and Pd/C, attributable to its low Pd-Pd interatomic distance and cleaner surface. The self-standing Pd nanosponge and Pd nanocube were more active than Pd/C in all electrolytes. Mainly, the COOxid current density of Pd nanocube in H2SO4 (5.92 mA/cm2) was nearly 3.6 times that in KOH (1.63 mA/cm2) and 10.3 times that in NaHCO3 (0.578 mA/cm2), owing to the greater charge mobility and better electrolyte–electrode interaction, as evidenced by electrochemical impedance spectroscopy (EIS) analysis. Notably, this study confirmed that acidic electrolytes and Pd nanocube are highly preferred for promoting COOxid and may open new avenues for precluding CO poisoning in alcohol-based fuel cells.

Assessing the Intrinsic Activity of Pt‐Group Electrocatalysts for Carbon Monoxide Oxidation: Best Practices and Benchmarking Parameters

“…We have studied the effect of electrolytes on the electrocatalytic CO Oxid over self-standing Pdbased nanostructures, but the activity was in the order of H 2 SO 4 > KOH > NaHCO 3 , as shown in the higher I Co and lower E Oxid . [81] The ability of electrolytes to adsorb anions in the supporting electrolytes also led to different voltammogram features and activity. Thereby, the CO Oxd measured on Pt black showed one sharp peak with an I CO of nearly 200 μ/cm 2 in H 2 SO 4 electrolyte and two peaks with broadening and an I CO of nearly 100 μ/cm 2 in the HClO 4 electrolyte (Figure 7f-g).…”

“…[83] Recently, we have synthesized and tailored the morphology of unsupported Pd nanocrystals (i. e., nanocubes and nanosponge) with a clean surface and high surface area using the chemical reduction method with NaBH 4 at 0 °C and ascorbic acid at 25 °C, respectively (Figure 9a-c). [81] The electrocatalytic CO Oxid activity and stability of Pd nanocube nanosponge were systematically investigated relative to commercial Pd/C in different electrolytes (i. e., H 2 SO 4 , NaHCO 3 , and KOH), and the activities depended on the shape of Pd and electrolyte pH. [81] Interestingly, self-standing Pd nanocubes and nanosponges were more active and durable than commercial Pd/C in all electrolytes in terms of higher I CO and lower E Oxid and E Onset (Figure 9d-e, g-h, j-k), originated from the higher ECSA of Pd nanocubes and superior CO ads charge.…”

“…[81] The electrocatalytic CO Oxid activity and stability of Pd nanocube nanosponge were systematically investigated relative to commercial Pd/C in different electrolytes (i. e., H 2 SO 4 , NaHCO 3 , and KOH), and the activities depended on the shape of Pd and electrolyte pH. [81] Interestingly, self-standing Pd nanocubes and nanosponges were more active and durable than commercial Pd/C in all electrolytes in terms of higher I CO and lower E Oxid and E Onset (Figure 9d-e, g-h, j-k), originated from the higher ECSA of Pd nanocubes and superior CO ads charge. Pd nanocubes and nanosponge were close, but nanocubes were the best in H 2 SO 4, and nanosponge was more active in other electrolytes.…”

Controlling the Synthesis Protocols of Palladium‐Based Nanostructures for Enhanced Electrocatalytic Oxidation of Carbon Monoxide: A Mini‐Review

“…Also, Pd‐based catalysts are endowed with impressive properties (i. e., great electrochemical active surface area, low density, quick charge mobility, and ease adsorption/diffusion of reactants) that benefit CO Oxid electrocatalysis [11,12] . To defeat the high‐cost and earth‐rarity of Pd metal, it is usually mixed with other low‐cost and earth‐abundant metals/metal oxides (i. e., Co, TiO 2 , SiO 2 , Ce 2 O 3 , Fe 2 O 3 , Ni, Fe, Cu) [13–17] .…”

“…[6][7][8] Also, Pd-based catalysts are endowed with impressive properties (i. e., great electrochemical active surface area, low density, quick charge mobility, and ease adsorption/diffusion of reactants) that benefit CO Oxid electrocatalysis. [11,12] To defeat the high-cost and earth-rarity of Pd metal, it is usually mixed with other low-cost and earth-abundant metals/metal oxides (i. e., Co, TiO 2 , SiO 2 , Ce 2 O 3 , Fe 2 O 3 , Ni, Fe, Cu). [13][14][15][16][17] This not only reduces the usage of Pd but also modulates the d-band center of Pd (i. e., upshifting or downshifting), resulting in accelerating the CO Oxid kinetics at low potential and facilitating desorption of intermediates/products.…”