Revealing the Role of the Ionomer at the Triple-Phase Boundary in a Proton-Exchange Membrane Water Electrolyzer.

In the anodic catalyst layer of a proton-exchange membrane (PEM) water electrolyzer, the triple-phase boundary (TPB) is mainly distributed on the surface of ultrafine iridium-based catalysts encapsulated by the ionomer within the catalyst-ionomer agglomerate. It is found that the ionomer at the TPB acts as a barrier to mass transport and a buffer for the bubble coverage during the oxygen evolution reaction (OER). The barrier effect can decrease the OER performance of the catalysts inside the agglomerate by ≤23%, while the buffer effect can separate the bubble evolution sites from the OER sites, turning the instant deactivation caused by the bubble coverage into a gradual performance loss caused by local water starvation. However, this local water starvation still deteriorates the catalyst performance because of the affinity of the ionomer surface for bubbles. Introducing additional transport paths into the agglomerate can reduce the barrier effect and regulate the bubble behavior, reducing the overpotential by 0.308 V at 5 A cm-2.

Insight into Oxygen Transport in Solid and High-Surface-Area Carbon Supports of Proton Exchange Membrane Fuel Cells.

Understanding the oxygen transport mechanism through an ionomer film that covered the catalyst surface is essential for reducing local oxygen transport resistance and improving the low Pt-loading proton exchange membrane fuel cell performance. Besides the ionomer material, the carbon supports, upon which ionomers and catalyst particles are dispersed, also play a crucial role in local oxygen transport. Increasing attention has been paid to the effects of carbon supports on local transport, but the detailed mechanism is still unclear. Herein, the local oxygen transports based on conventional solid carbon (SC) and high-surface-area carbon (HSC) supports are investigated by molecular dynamics simulations. It is found that oxygen diffuses through the ionomer film that covered the SC supports via "effective diffusion" and "ineffective diffusion". The former denotes the process by which oxygen diffuses directly from the ionomer surface to the Pt upper surface through small and concentrated regions. In contrast, ineffective diffusion suffers more restrictions by both carbon- and Pt-dense layers, and thus, the oxygen pathways are long and tortuous. The HSC supports exhibit larger transport resistance relative to SC supports due to the existence of micropores. Also, the major transport resistance originates from the carbon-dense layer as it inhibits oxygen from diffusing downward and migrating toward the pore opening, while the oxygen transport inside the pore is facile along the pore's inner surface, which leads to a specific and short diffusion pathway. This work provides insight into oxygen transport behavior with SC and HSC supports, which is the basis for the development of high-performance electrodes with low local transport resistance.

Ultrafine Core@Shell Cu1Au1@Cu1Pd3 Nanodots Synergized with 3D Porous N-Doped Graphene Nanosheets as a High-Performance Multifunctional Electrocatalyst.

Rationally combining designed supports and metal-based nanomaterials is effective to synergize their respective physicochemical and electrochemical properties for developing highly active and stable/durable electrocatalysts. Accordingly, in this work, sub-5 nm and monodispersed nanodots (NDs) with the special nanostructure of an ultrafine Cu1Au1 core and a 2-3-atomic-layer Cu1Pd3 shell are synthesized by a facile solvothermal method, which are further evenly and firmly anchored onto 3D porous N-doped graphene nanosheets (NGS) via a simple annealing (A) process. The as-obtained Cu1Au1@Cu1Pd3 NDs/NGS-A exhibits exceptional electrocatalytic activity and noble-metal utilization toward the alkaline oxygen reduction, methanol oxidation, and ethanol oxidation reactions, showing dozens-fold enhancements compared with commercial Pd/C and Pt/C. Besides, it also has excellent long-term electrochemical stability and electrocatalytic durability. Advanced and comprehensive experimental and theoretical analyses unveil the synthetic mechanism of the special core@shell nanostructure and further reveal the origins of the significantly enhanced electrocatalytic performance: (1) the prominent structural properties of NGS, (2) the ultrasmall and monodispersed size as well as the highly uniform morphology of the NDs-A, (3) the special Cu-Au-Pd alloy nanostructure with an ultrafine core and a subnanometer shell, and (4) the strong metal-support interaction. This work not only develops a facile method for fabricating the special metal-based ultrafine-core@ultrathin-shell nanostructure but also proposes an effective and practical design paradigm of comprehensively and rationally considering both supports and metal-based nanomaterials for realizing high-performance multifunctional electrocatalysts, which can be further expanded to other supports and metal-based nanomaterials for other energy-conversion or environmental (electro)catalytic applications.

Electronic and Potential Synergistic Effects of Surface-Doped P-O Species on Uniform Pd Nanospheres: Breaking the Linear Scaling Relationship toward Electrochemical Oxygen Reduction.

Developing efficient oxygen reduction reaction (ORR) electrocatalysts is critical to fuel cells and metal-oxygen batteries, but also greatly hindered by the limited Pt resources and the long-standing linear scaling relationship (LSR). In this study, ∼6 nm and highly uniform Pd nanospheres (NSs) having surface-doped (SD) P-O species are synthesized and evenly anchored onto carbon blacks, which are further simply heat-treated (HT). Under alkaline conditions, Pd/SDP-O NSs/C-HT exhibits respective 8.7 (4.3)- and 5.0 (5.5)-fold enhancements in noble-metal-mass- and area-specific activity (NM-MSA and ASA) compared with the commercial Pd/C (Pt/C). It also possesses an improved electrochemical stability. Besides, its acidic ASA and NM-MSA are 2.9 and 5.1 times those of the commercial Pd/C, respectively, and reach 65.4 and 51.5% of those of the commercial Pt/C. Moreover, it also shows nearly ideal 4-electron ORR pathways under both alkaline and acidic conditions. The detailed experimental and theoretical analyses reveal the following: (1) The electronic effect induced by the P-O species can downshift the surface d-band center to weaken the intermediate adsorptions, thus preserving more surface active sites. (2) More importantly, the potential hydrogen bond between the O atom in the P-O species and the H atom in the hydrogen-containing intermediates can in turn stabilize their adsorptions, thus breaking the ORR LSR toward more efficient ORRs and 4-electron pathways. This study develops a low-cost and high-performance ORR electrocatalyst and proposes a promising strategy for breaking the ORR LSR, which may be further applied in other electrocatalysis.

Manipulating the oxygen reduction reaction pathway on Pt-coordinated motifs.

Electrochemical oxygen reduction could proceed via either 4e--pathway toward maximum chemical-to-electric energy conversion or 2e--pathway toward onsite H2O2 production. Bulk Pt catalysts are known as the best monometallic materials catalyzing O2-to-H2O conversion, however, controversies on the reduction product selectivity are noted for atomic dispersed Pt catalysts. Here, we prepare a series of carbon supported Pt single atom catalyst with varied neighboring dopants and Pt site densities to investigate the local coordination environment effect on branching oxygen reduction pathway. Manipulation of 2e- or 4e- reduction pathways is demonstrated through modification of the Pt coordination environment from Pt-C to Pt-N-C and Pt-S-C, giving rise to a controlled H2O2 selectivity from 23.3% to 81.4% and a turnover frequency ratio of H2O2/H2O from 0.30 to 2.67 at 0.4 V versus reversible hydrogen electrode. Energetic analysis suggests both 2e- and 4e- pathways share a common intermediate of *OOH, Pt-C motif favors its dissociative reduction while Pt-S and Pt-N motifs prefer its direct protonation into H2O2. By taking the Pt-N-C catalyst as a stereotype, we further demonstrate that the maximum H2O2 selectivity can be manipulated from 70 to 20% with increasing Pt site density, providing hints for regulating the stepwise oxygen reduction in different application scenarios.

Evaluation of Electrocatalytic Activity of Noble Metal Catalysts Toward Nitrogen Reduction Reaction in Aqueous Solutions under Ambient Conditions.

Electrochemical nitrogen reduction reaction (NRR) is intensively investigated by researchers for its potential to be the next-generation technology to produce ammonia. Many attempts have been made to explore the possibility of electrochemical ammonia production catalyzed by noble metals. However, the produced ammonia in most reported cases is in ppm level or even lower, which is susceptible to potential contaminants in experiments, leading to fluctuating or even contradictory results. Herein, a rigorous procedure was adopted to systematically evaluated the performance of commercial noble metal nanocatalysts toward NRR. No discernible amount of ammonia was detected in either acidic or alkaline solutions. Further, nitrogen-containing contaminants in catalysts that might cause false positive results were detected and characterized. An effective way to remove pre-existing pollutants by consecutive cyclic voltammetry scan was proposed, helping to obtain reliable and reproducible results.

Lithium-mediated electrochemical nitrogen reduction: Mechanistic insights to enhance performance.

Green synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) shows great potential as an alternative to the Haber-Bosch process but is hampered by sluggish production rate and low Faradaic efficiency. Recently, lithium-mediated electrochemical NRR has received renewed attention due to its reproducibility. However, further improvement of the system is restricted by limited recognition of its mechanism. Herein, we demonstrate that lithium-mediated NRR began with electrochemical deposition of lithium, followed by two chemical processes of dinitrogen splitting and protonation to ammonia. Furthermore, we quantified the extent to which the freshly deposited active lithium lost its activity toward NRR due to a parasitic reaction between lithium and electrolyte. A high ammonia yield of 0.410 ± 0.038 µg s-1 cm-2 geo and Faradaic efficiency of 39.5 ± 1.7% were achieved at 20 mA cm-2 geo and 10 mA cm-2 geo, respectively, which can be attributed to fresher lithium obtained at high current density.

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

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.

Insight into the Rapid Degradation Behavior of Nonprecious Metal Fe-N-C Electrocatalyst-Based Proton Exchange Membrane Fuel Cells.

In the past few years, great progress has been made in nonprecious metal catalysts, which hold the potential as alternative materials to replace platinum in proton exchange membrane fuel cells. One type of nonprecious metal catalyst, Fe-N-C, has displayed similar catalytic activity as platinum in rotating disk electrode tests; however, rapid degradation of Fe-N-C catalyst-based fuel cells is always observed, which limits its practical application. Although considerable research has been devoted to study the degradation of the catalyst itself, rather less attention has been paid to the membrane electrode assembly that makes the mechanism of fuel cell degradation remain unclear. In this work, a high-performance Fe-N-C catalyst-based membrane electrolyte assembly is prepared and used to study its degradation mechanism. The fuel cell performs with an initial peak power density as high as 1.1 W cm-2 but suffers a current loss of 52% at 0.4 V over 20 h only. The experimental and DFT calculation results indicate that Fe at active sites of catalysts is attacked by hydroxyl free radicals decomposed from H2O2, which is further leached out, causing an increase in activity loss. The ionomer of the catalyst layer and the membrane is further contaminated by the leached Fe ions, which results in an enlarged membrane resistance and cathode catalyst layer proton conduction resistance, greatly influencing the cell performance. In addition, it has been assumed in previous studies that the quick performance loss of Fe-N-C-based fuel cells is caused by water flooding within the catalyst layer, which is proved to be incorrect in our study through a dry-out experiment.