Exploration of significant influences of the operating conditions on the local O2 transport in proton exchange membrane fuel cells (PEMFCs).

A drastic reduction of the Pt loading in the cathode catalyst layers (CCLs) of proton exchange membrane fuel cells (PEMFCs) is much desired. However, a decrease in Pt loading inevitably leads to an unexpected increase of local O2 transport resistance (rLocal) and severely weakens the fuel cell performance, particularly at high current densities. Thus, it is both urgent and meaningful to explore the impacts of the operating conditions on rLocal in CCLs and therefore to clarify the intrinsic mechanism. Herein, we systematically explore the influences of the operating conditions, in terms of the dry O2 mole fraction, the relative humidity, the operating pressure and the temperature on rLocal using limiting current measurements combined with mathematical calculations. The results show that, in contrary to the established rules, rLocal in CCLs of PEMFCs is aggravated when the dry O2 mole fraction or the operating pressure are increased. It is also experimentally found that rLocal in CCLs is alleviated with the increase in the relative humidity or the operating temperature. Moreover, an adsorption controlled solution-diffusion model is proposed to illuminate the local O2 transport behavior in CCLs of PEMFCs, and it accounts for the influence of the dry O2 mole fraction on rLocal in CCLs.

Unraveling the Effects of Carbon Corrosion on Oxygen Transport Resistance in Low Pt Loading Proton Exchange Membrane Fuel Cells.

Cost and durability have become crucial hurdles for the commercialization of proton exchange membrane fuel cells (PEMFCs). Although a continuous reduction of Pt loading within the cathode catalyst layers (CCLs) can lead to cost savings, it also increases the oxygen transport resistance, which is further compounded by key material degradation. Hence, a further understanding of the mechanism of significant performance loss due to oxygen transport limitations at the triple phase boundaries (TPBs) during the degradation process is critical to the development of low Pt loading PEMFCs. The present study systematically investigates the impact of carbon corrosion in CCLs on the performance and oxygen transport process of low Pt loading PEMFCs through accelerated stress tests (ASTs) that simulate start-up/shutdown cycling. A decline in peak power density from 484.3 to 251.6 mW cm-2 after 1500 AST cycles demonstrates an apparent performance loss, especially at high current densities. The bulk and local oxygen transport resistances (rbulk and Rlocal) of the pristine cell and after 200, 600, 1000, and 1500 AST cycles are quantified by combining the limiting current method with a dual-layer CCL design. The results show that rbulk increased from 1527 to 1679 s cm-2, Rlocal increased from 0.38 to 0.99 s cm-1, and the local oxygen transport resistance with the normalized Pt surface area (rlocal) exhibited an increase from 18.5 to 32.0 s cm-1, indicating a crucial impact on the structure collapse and changes in the chemical properties of the carbon supports in the CCLs. Further, the interaction between the ionomer and carbon supports during the carbon corrosion process is deeply studied via electrochemical quartz crystal microbalance and molecular dynamics simulations. It is concluded that the oxygen-containing functional groups on the carbon surface could impede the adsorption of ionomers on carbon supports by creating an excessively water-rich layer, which in turn aggravates the formation of ionomer agglomerations within the CCLs. This process ultimately leads to the destruction of the TPBs and hinders the transport of oxygen through the ionomer.

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.

Progress and prospects of dealloying methods for energy-conversion electrocatalysis.

Developing hydrogen production and utilization technologies is a promising way to achieve large-scale applications of renewable energy. For both water electrolysis and fuel cell electrode reactions, electrocatalysts are critical to their energy conversion efficiencies. Among the various strategies for improving the performance of electrocatalysts, dealloying has been developed as a commonly used effective post-processing method. It originated from anti-corrosion science and can form metal materials with porous or "skin" nanostructures by selectively dissolving the active components in alloys. There are generally two types of dealloying methods: electrochemical dealloying and chemical dealloying. Electrochemical dealloying is more controllable, while chemical dealloying is simpler and less expensive. In this review, the fundamentals, histories, and progress of dealloying methods for energy conversion electrocatalysis are systematically summarized. Furthermore, current problems and prospects are proposed.

Innovative Insight into O2/N2 Permeation Behavior through an Ionomer Film in Cathode Catalyst Layers of Polymer Electrolyte Membrane Fuel Cells.

It is crucial to clarify the permeation behavior of O2 through the ionomer film for enhancing local O2 transport in cathodes of fuel cells. However, all existing studies mainly deal with pure O2 rather than air. Herein, the permeation behavior of the O2/N2 mixture through the ionomer film has been well explored in view of molecular bond length variations by molecular dynamics simulations. The bond lengths for O2 and N2 are shortened under a low hydration level when permeating through a dense layer with small free voids while no obvious change occurs at higher hydration. In the bulk ionomer region, O2 molecules residing in water domains are energetically unstable because the bond lengths deviate far from the equilibrium length; thus, O2 diffuses through the interfacial or hydrophobic regions. N2 molecules show similar properties with O2. This study provides a novel perspective on the permeation behavior of O2 and N2 through the ionomer film, which definitely benefits enhancing local O2 transport.

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.

Dealloyed RuNiOx as a robust electrocatalyst for the oxygen evolution reaction in acidic media.

We report here the dealloying treatment on a RuNiOx catalyst for enhanced acidic oxygen evolution reaction (OER) performance. Specifically, the dealloyed RuNiOx is capable of delivering a current density of 50 mA cm-2 at a low overpotential of 280 mV and demonstrates superior stability after 10 000 potential cycles.

The Experimental Measurement of Local and Bulk Oxygen Transport Resistances in the Catalyst Layer of Proton Exchange Membrane Fuel Cells.

Remarkable progress has been made in reducing the cathodic Pt loading of PEMFCs; however, a huge performance loss appears at high current densities, indicating the existence of a large oxygen transport resistance associated with the ultralow Pt loading catalyst layer. To reduce the Pt loading without sacrificing cell performance, it is essential to illuminate the oxygen transport mechanism in the catalyst layer. Toward this goal, an experimental approach to measure the oxygen transport resistance in catalyst layers is proposed and realized for the first time in this study. The measuring approach involves a dual-layer catalyst layer design, which consists of a dummy catalyst layer and a practical catalyst layer, followed by changing the thickness of dummy layer to respectively quantify the local and bulk resistances via limiting current measurements combined with linear extrapolation. The experimental results clearly reveal that the local resistance dominates the total resistance in the catalyst layer.

Novel Nano-composites SDC-LiNaSO4 as Functional Layer for ITSOFC.

As an ionic conductive functional layer of intermediate temperature solid oxide fuel cells (ITSOFC), samarium-doped ceria (SDC)-LiNaSO4 nano-composites were synthesized by a sol-gel method and their properties were investigated. It was found that the content of LiNaSO4 strongly affected the crystal phase, defect concentration, and conductivity of the composites. When the content of LiNaSO4 was 20 wt%, the highest conductivity of the composite was found to be, respectively, 0.22, 0.26, and 0.35 S cm-1 at temperatures of 550, 600, and 700 °C, which are much higher than those of SDC. The peak power density of the single cell using this composite as an interlayer was improved to, respectively, 0.23, 0.39, and 0.88 W cm-2 at 500, 600, and 700 °C comparing with that of the SDC-based cell. Further, the SDC-LiNaSO4(20 wt%)-based cell also displayed better thermal stability according to the performance measurements at 560 °C for 50 h. These results reveal that SDC-LiNaSO4 composite may be a potential good candidate as interlayer for ITSOFC due to its high ionic conductivity and thermal stability.

Self-assembly of palladium nanoparticles on periodic mesoporous organosilica using an in situ reduction approach: catalysts for ullmann reactions in water.

A Si--H functionalized phenyl-bridged periodic mesoporous organosilica [H-PMO(Ph)] is synthesized via a surfactant-directed assembly approach. Pd nanoparticles are then immobilized onto the PMO catalyst [Pd/H-PMO(Ph)] by a Si--H in situ reduction method. The Ullmann reaction, in water as medium, is used to investigate the catalytic performance of Pd/H-PMO(Ph). The results show that the Pd/H-PMO(Ph) catalyst has excellent catalytic activity and selectivity, which can be attributed to synergetic effects derived from the highly dispersed catalytic species and the hydrophobic microenvironments. Furthermore, the catalyst could be conveniently recovered and recycled five times without significant loss of activity and selectivity.