Unique (100) Surface Configuration Enables Promising Oxygen Reduction Performance for Pt3Co Nanodendrite Catalysts.

Selective exposure of active surfaces of Pt-based electrocatalysts has been demonstrated as an effective strategy to improve Pt utilization and promote oxygen reduction reaction (ORR) activity in fuel cell application. However, challenges remain in stabilizing those active surface structures, which often suffer undesirable degradation and poor durability along with surface passivation, metal dissolution, and agglomeration of Pt-based electrocatalysts. To overcome the aforementioned obstacles, we here demonstrate the unique (100) surface configuration enabling active and stable ORR performance for bimetallic Pt3Co nanodendrite structures. Using elaborate microscopy and spectroscopy characterization, it is revealed that the Co atoms are preferentially segregated and oxidized at the Pt3Co(100) surface. In situ X-ray absorption spectroscopy (XAS) shows that such (100) surface configuration prevents the oxygen chemisorption and oxide formation on active Pt during the ORR process. Thus, the Pt3Co nanodendrite catalyst shows not only a high ORR mass activity of 730 mA/mg at 0.9 V vs RHE, which is 6.6-fold higher than that of the Pt/C, but also impressively high stability with 98% current retention after the acceleration degradation test in acid media for 5000 cycles, far exceeding the Pt or Pt3Co nanoparticles. Density functional theory (DFT) calculation also confirms the lateral and structural effects from the segregated Co and oxides on the Pt3Co(100) surface in reducing the catalyst oxophilicity and the free energy for the formation of an OH intermediate in the ORR.

Ultrastable Fe-N-C Fuel Cell Electrocatalysts by Eliminating Non-Coordinating Nitrogen and Regulating Coordination Structures at High Temperatures.

Pyrolyzed Fe-N-C materials have attracted considerable interest as one of the most active noble-metal-free electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Despite significant progress is made in improving their catalytic activity during past decades, the Fe-N-C catalysts still suffer from fairly poor electrochemical and storage stability, which greatly hurdles their practical application. Here, an effective strategy is developed to greatly improve their catalytic stability in PEMFCs and storage stability by virtue of previously unexplored high-temperature synthetic chemistry between 1100 and 1200 °C. Pyrolysis at this rarely adopted temperature range not only enables the elimination of less active nitrogen-doped carbon sites that generate detrimental peroxide byproduct but also regulates the coordination structure of Fe-N-C from less stable D1 (O-FeN4 C12 ) to a more stable D2 structure (FeN4 C10 ). The optimized Fe-N-C catalyst exhibits excellent stability in PEMFCs (>80% performance retention after 30 h under H2 /O2 condition) and no activity loss after 35 day storage while maintaining a competitive ORR activity and PEMFC performance.

Stable Pd-Cu Hydride Catalyst for Efficient Hydrogen Evolution.

Pd has been regarded as one of the alternatives to Pt as a promising hydrogen evolution reaction (HER) catalyst. Strategies including Pd-metal alloys (Pd-M) and Pd hydrides (PdHx) have been proposed to boost HER performances. However, the stability issues, e.g., the dissolution in Pd-M and the hydrogen releasing in PdHx, restrict the industrial application of Pd-based HER catalysts. We here design and synthesize a stable Pd-Cu hydride (PdCu0.2H0.43) catalyst, combining the advantages of both Pd-M and PdHx structures and improving the HER durability simultaneously. The hydrogen intercalation is realized under atmospheric pressure (1.0 atm) following our synthetic approach that imparts high stability to the Pd-Cu hydride structure. The obtained PdCu0.2H0.43 catalyst exhibits a small overpotential of 28 mV at 10 mA/cm2, a low Tafel slope of 23 mV/dec, and excellent HER durability due to its appropriate hydrogen adsorption free energy and alleviated metal dissolution rate.

Enhanced CO2 Electrochemical Reduction Performance over Cu@AuCu Catalysts at High Noble Metal Utilization Efficiency.

The electrochemical CO2 reduction reaction (CO2RR) represents a viable alternative to help close the anthropogenic carbon cycle and convert intermittent electricity from renewable energy sources to chemical energy in the form of value-added chemicals. The development of economic catalysts possessing high faradaic efficiency (FE) and mass activity (MA) toward CO2RR is critical in accelerating CO2 utilization technology. Herein, an elaborate Au-Cu catalyst where an alloyed AuCu shell caps on a Cu core (Cu@AuCu) is developed and evaluated for CO2-to-CO electrochemical conversion. Specific roles of Cu and Au for CO2RR are revealed in the alloyed core-shell structure, respectively, and a compositional-dependent volcano-plot is disclosed for the Cu@AuCu catalysts toward selective CO production. As a result, the Au2-Cu8 alloyed core-shell catalyst (only 17% Au content) achieves an FECO value as high as 94% and an MACO of 439 mA/mgAu at -0.8 V (vs RHE), superior to the values for pure Au, reflecting its high noble metal utilization efficiency.

CO-Reductive and O2-Oxidative Annealing Assisted Surface Restructure and Corresponding Formic Acid Oxidation Performance of PdPt and PdRuPt Nanocatalysts.

Formic acid oxidation reaction (FAOR) at anode counterpart incurs at substantial high overpotential, limiting the power output efficiency of direct formic acid fuel cells (DFAFCs). Despite intense research, the lack of high-performance nanocatalysts (NCs) for FAOR remains a challenge in realizing DFAFC technologies. To surmount the overpotential losses, it is desirable to have NCs to trigger the FAOR as close to the reversible conditions (i.e. with over-potential loss as close to zero as possible). Herein, Pd-based binary and ternary NCs consisting of PdPt and PdRuPt have been synthesized via the polyol reduction method on the carbon support. As prepared PdPt and PdRuPt NCs were further subjected to heat treatment (annealed) in CO (namely PdPt-CO and PdRuPt-CO) and O2 (namely PdPt-O2 and PdRuPt-O2) atmosphere at 473 K temperature. By cross-referencing results of electron microscopy and X-ray spectroscopy together with electrochemical analysis, the effects of heat treatment under CO-reductive and O2-oxidative conditions towards FAOR were schematically elucidated. Of special relevance, the mass activity (MA) of PdPt-CO, PdPt-O2, PdRuPt-CO, and PdRuPt-O2 NCs is 1.7/2.0, 1.3/2.2, 1.1/5.5, and 0.9/4.7 Amg-1 in the anodic/cathodic scan, respectively, which is 2~4-folds improved comparative to of as-prepared PdPt (1.0/1.9 Amg-1 in anodic/cathodic scan, respectively) and PdRuPt (0.9/1.4 Amg-1 in anodic/cathodic scan, respectively) NCs. Meanwhile, after chronoamperometric (CA) stability test up to 2000 s, PdPt-CO (72 mAmg-1) and PdRuPt-CO (213 mAmg-1) NCs exhibit higher MA compared to as-prepared PdPt (54 mAmg-1) and PdRuPt (62 mAmg-1) NCs, which is attributed to the increase of surface Pt composition, especially for PdRuPt-CO NC. Besides, the stability of PdPt-O2 (15 mAmg-1) and PdRuPt-O2 (22 mAmg-1) NCs is deteriorated as compared to that of as-prepared NCs due to severe oxidation in O2 atmosphere. Of utmost importance, we developed a ternary PdRuPt catalyst with ultra-low Pt content (~2 wt.%) and significantly improved FAOR performance than pure Pt catalysts. Moreover, we demonstrated that the FAOR performance can be further enhanced by more than 30% via a unique CO annealing treatment.

Optimization of Pt-Oxygen-Containing Species Anodes for Ethanol Oxidation Reaction: High Performance of Pt-AuSnOx Electrocatalyst.

Pt-oxygen-containing species (Pt-OCS) catalysts, in which OCS (e.g., metal-oxides) are decorated on a Pt surface, possess enhanced ethanol oxidation reaction (EOR) activity and stability compared with pure Pt and are promising in practical applications of direct ethanol fuel cells. We investigate the promotion roles of Pt-OCS electrocatalysts toward the EOR via a combination of density functional theory (DFT) calculations and experiments, providing a rational design strategy for Pt-OCS catalysts. It is revealed that Pt-AuO and Pt-SnO excel in EOR activity and stability, respectively, among the DFT screening of various Pt-OCS systems, and this is confirmed by the following experiments. Moreover, an optimized Pt-AuSnO catalyst is proposed by DFT calculations, taking advantage of both Pt-AuO and Pt-SnO. The as-prepared Pt-AuSnO catalyst delivers an EOR activity that is 9.7 times higher than that of Pt and shows desired stability. These findings are expected to elucidate the mechanistic insights into Pt-OCS materials and lead to advanced EOR electrocatalysts.

Promoting formic acid oxidation performance of Pd nanoparticles via Pt and Ru atom mediated surface engineering.

The alteration of surface functional properties via incorporation of foreign atoms is supposed to be a key strategy for the enhanced catalytic performance of noble-metal based nanocatalysts (NCs). In the present study, carbon-supported palladium (Pd)-based NCs including Pd, PdPt and PdRuPt have been prepared via a polyol reduction method under the same reduction conditions as for formic acid oxidation reaction (FAOR) applications. By cross-referencing the results of the microscopic, spectroscopic and electrochemical analysis we demonstrated that adding a small amount of platinum (Pt) into Pd NCs (i.e. PdPt NCs) significantly promotes the FAOR performance as compared to that of Pd NCs via weakening the COads bond strength at a lower voltage (0.875 V vs. NHE) than Pd (0.891 V vs. NHE). Of special relevance, the PdPt NC shows a mass activity (MA) of 1.0 A mg-1 and 1.9 A mg-1, respectively, in the anodic and cathodic scan. These values are ∼1.7-fold (0.6 A mg-1) and ∼4.8-fold (0.4 A mg-1) higher than those of Pd NC. Moreover, PdPt NC retains a higher MA (54 mA mg-1) than that of Pd NC (9 mA mg-1) after chronoamperometric (CA) stability tests over 2000 s. Meanwhile, further addition of ruthenium (Ru) (i.e. PdRuPt NCs) outstandingly enhances the CO tolerance during the CA test via removal of adsorbed COads and thus shows the highest MA (62 mA mg-1) after CA testing, which is higher than that of PdPt (54 mA mg-1) and Pd (9 mA mg-1) NCs. The intriguing results obtained in this study have great significance to provide further strategic opportunities for tuning the surface electronic properties of Pd-based NCs to design Pd-based NCs with improved electrochemical performance.