Influence of Microporous Layers on Interfacial Properties, Oxygen Flow Distribution, and Durability of Proton Exchange Membrane Water Electrolyzers.

The efficient and cost-effective production of green hydrogen is essential to decarbonize heavily polluting sectors such as transportation and heavy manufacturing industries such as metal refining. Polymer electrolyte membrane water electrolysis (PEMWE) is the most promising and rapidly maturing technology for producing green hydrogen at a scale and on demand. However, substantial cost reduction by lowering precious metal catalyst loadings and efficiency improvement is necessary to lower the cost of the produced hydrogen. Porous transport layers (PTLs) play a major role in influencing the PEMWE efficiency and catalyst utilization. Several studies have projected that the use of microporous layers (MPLs) on PTLs can improve the efficiency of PEMWEs, but very limited literature exists on how MPLs affect anodic interfacial properties and oxygen transport in PTLs. In this study, for the first time, we use X-ray microtomography and innovative image processing techniques to elucidate the oxygen flow patterns in PTLs with varying MPL thicknesses. We used stained water to improve contrast of oxygen in PTLs and demonstrate visualization of time averaged oxygen flow patterns. The results show that PTLs with MPLs significantly improve interfacial contact by almost 20% as compared to single layer sintered PTL. For the single layer PTL without MPL, the pore volume utilization for oxygen flow is low and the oxygen follows a viscous fingering flow regime. With MPLs, the pore volume utilization is higher, and the number of oxygen transport pathways is increased significantly. MPLs were also shown to suppress capillary fingering and transition oxygen flow to the viscous fingering regime, which has been proven to decrease site masking effects. Finally, durability tests showed the least voltage degradation for thin MPL and thicker MPLs run into mass transport limitations. Based on these findings, PTL/MPL design optimization strategies are proposed for enabling low catalyst loadings and improving durability.

X-ray Tomography Applied to Electrochemical Devices and Electrocatalysis.

X-ray computed tomography (CT) is a nondestructive three-dimensional (3D) imaging technique used for studying morphological properties of porous and nonporous materials. In the field of electrocatalysis, X-ray CT is mainly used to quantify the morphology of electrodes and extract information such as porosity, tortuosity, pore-size distribution, and other relevant properties. For electrochemical systems such as fuel cells, electrolyzers, and redox flow batteries, X-ray CT gives the ability to study evolution of critical features of interest in ex situ, in situ, and operando environments. These include catalyst degradation, interface evolution under real conditions, formation of new phases (water and oxygen), and dynamics of transport processes. These studies enable more efficient device and electrode designs that will ultimately contribute to widespread decarbonization efforts.

Observation of Preferential Pathways for Oxygen Removal through Porous Transport Layers of Polymer Electrolyte Water Electrolyzers.

Understanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the polymer electrolyte water electrolyzer (PEWE) performance. Operando X-ray computed tomography and machine learning were performed on a model electrolyzer at different water flow rates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. We report a direct observation of oxygen taking preferential pathways through the PTL, regardless of the water flow rate or current density (1-4 A/cm2). Oxygen distribution in the PTL had a periodic behavior with period of 400 µm. A computational fluid dynamics model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer.

Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers.

We present ultralow Ir-loaded (ULL) proton exchange membrane water electrolyzer (PEMWE) cells that can produce enough hydrogen to largely decarbonize the global natural gas, transportation, and electrical storage sectors by 2050, using only half of the annual global Ir production for PEMWE deployment. This represents a significant improvement in PEMWE's global potential, enabled by careful control of the anode catalyst layer (CL), including its mesostructure and catalyst dispersion. Using commercially relevant membranes (Nafion 117), cell materials, electrocatalysts, and fabrication techniques, we achieve at peak a 250× improvement in Ir mass activity over commercial PEMWEs. An optimal Ir loading of 0.011 mgIr cm-2 operated at an Ir-specific power of ∼100 MW kgIr-1 at a cell potential of ∼1.66 V versus RHE (85% higher heating value efficiency). We further evaluate the performance limitations within the ULL regime and offer new insights and guidance in CL design relevant to the broader energy conversion field.

Using operando techniques to understand and design high performance and stable alkaline membrane fuel cells.

There is a need to understand the water dynamics of alkaline membrane fuel cells under various operating conditions to create electrodes that enable high performance and stable, long-term operation. Here we show, via operando neutron imaging and operando micro X-ray computed tomography, visualizations of the spatial and temporal distribution of liquid water in operating cells. We provide direct evidence for liquid water accumulation at the anode, which causes severe ionomer swelling and performance loss, as well as cell dryout from undesirably low water content in the cathode. We observe that the operating conditions leading to the highest power density during polarization are not generally the conditions that allow for long-term stable operation. This observation leads to new catalyst layer designs and gas diffusion layers. This study reports alkaline membrane fuel cells that can be operated continuously for over 1000 h at 600 mA cm-2 with voltage decay rate of only 32-µV h-1 - the best-reported durability to date.