Mass Transport Overpotentials Research Articles

A comprehensive multiphysics approach to model solutal buoyancy, thermal convection, and electro-vortex flow phenomena for liquid metal batteries

To alleviate the effects of pollution and global warming, the transition to renewable energy sources is taking place at an accelerating rate. The intermittent nature of renewable energy production can be addressed by an energy storage solution that is low-cost, efficient, and based on earth-abundant materials, and a liquid metal battery (LMB) is coming up as one such solution. Liquid metal battery (LMB) performance, especially its discharge voltage, depends on the concentration profile of cathode which in turn is influenced by fluid flow. In this study, we examine three significant phenomena that impact fluid behavior: solutal buoyancy, internally heated convection, and electro-vortex flow (EVF). Though several attempts have been made to model these processes individually or in a combination of two, a model that simulates all three aspects for resultant fluid flow has not been reported. Therefore, this study develops a comprehensive multiphysics model to simulate the positive electrode that is validated with experimental results for Li||Bi cell operating at 450 °C by coupling all of the aforementioned phenomena. While charging with a current density of 1 A/cm2, our model predicts an average velocity of ∼6 mm/s in the cathode solely due to solutal buoyancy. In the case of discharging, both solutal and internal thermal effects lead to an increase in overpotential to as high as ∼200 mV. Moreover, during discharging, the EVF induced a poloidal fluid flow with a maximum velocity of ∼3–5.5 mm/s at 1 A/cm2, reducing mass transport overpotential by ∼10 mV, but it could not disrupt the lighter intermetallic stratification layer on top of the cathode. Additionally, we investigated the charging overpotential phenomenon due to unstable stratification. This all-inclusive model leveraging solutal, thermal, and EVF effects not only offers insights into the fluid motion, concentration gradient, and voltage profile for a lab-scale LMB but can accurately simulate te conditions prevalent in commercial large LMBs, thereby catalysing the path to commercialization of LMBs.

Improving Water Management and Reactant Distribution in PEM Fuel Cells Via Flow Fields with Biomimetic Auxiliary Channels

The bipolar plate, a component of the polymer electrolyte membrane (PEM) fuel cell is responsible for reactant delivery, product removal, and mechanical stability of PEM fuel cell stacks. Bipolar plates also account for 70-90% of the weight and volume, and 18-28% of the production cost of stacks (1). Improving the function of the flow fields embedded in the bipolar plates can significantly impact the energy density of PEM fuel cells by improving liquid water management and reactant distribution. Previous works show that water preferentially accumulates under the lands of flow fields in the gas diffusion layer (GDL) for various GDL materials, creating a heterogeneous distribution of water and reactants, thereby increasing cathode mass transport overpotential(2–4). Biomimetic channel architectures have been shown to enhance preferential water flow; however, these designs have not been tailored to control water accumulation for improved reactant distribution (5,6). Targeting areas of known water accumulation such as the under-land regions of GDLs, could have a profound impact on reducing mass transport losses and improving reactant homogeneity, thereby improving the power density and efficiency of the PEM fuel cell.In this work, biomimetic auxiliary channels were laser-cut into the lands of a parallel PEM fuel cell flow field to enhance the liquid water removal and reactant distribution in the under-land region of the GDL. Constant-current electrochemical testing and electrochemical impedance spectroscopy revealed a 29% increase in peak power density resulting from a 54% decrease in oxygen mass transport overpotential using the biomimetic flow field (BFF) compared to a baseline trapezoidal flow field (TFF). Operando synchrotron X-ray radiography revealed reduced GDL water accumulation when using the BFF compared to the TFF. Water accumulation under the BFF flow fields was more homogenous especially near the catalyst layer (CL) – GDL interface, indicating enhanced reactant distribution near the CL reaction sites. Therefore, the reduced mass transport overpotential and corresponding power density increase were attributed to enhanced liquid water removal and reactant distribution due to the biomimetic auxiliary channels. These results demonstrate that significant performance enhancements can be realized by embedding alternate pathways for air and water in the lands of flow field components. Ultimately, this work can be used to further optimize the design of bipolar plates for more efficient stacks and accelerate the commercialization of PEM fuel cells. Y. Wang, D. F. Ruiz Diaz, K. S. Chen, Z. Wang, and X. C. Adroher, Materials Today, 32, 178–203 (2020).N. Ge et al., Electrochim Acta, 328, 135001 (2019).D. Muirhead et al., Int J Hydrogen Energy, 42, 29472–29483 (2017).S. Chevalier et al., J Electrochem Soc, 164, F107–F114 (2017)N. Guo, M. C. Leu, and U. O. Koylu, Int J Hydrogen Energy, 39, 21185–21195 (2014).S. Feng et al., Science, 373, 1344–1348 (2021).

Operando Identification of Temperature-Dependent Pore-Scale Gas Saturations in PEM Water Electrolyzers

Polymer electrolyte membrane (PEM) water electrolysis is a promising solution for the chemical storage of intermittent renewable energy in the form of clean hydrogen (1). For the widespread adoption of this technology, the efficiency of PEM electrolyzers must be improved through informed material design and tailored operating conditions to compete with incumbent carbon-emitting technologies (2). A main source of inefficiency is related to mass transport losses, where oxygen gas, a by-product of the anode reaction, accumulates in the anode porous transport layer (PTL) and hinders reactant liquid water delivery to reaction sites (3). Although extensive studies have explored this two-phase flow phenomenon, the consideration of operating temperature on gas accumulation in the PTL adds further complexity. A duality in temperature effects has been reported in the literature where increasing the operating temperature may encourage gas coverage which obstructs liquid water transport, but it may also enhance liquid water delivery due to favorable fluid property changes in viscosity and surface tension (4). Efforts have been made previously to reconcile these competing observations using 2D visualization methods (5), however a closer investigation using 3D visualization techniques will provide valuable pore-scale insights in the comprehensive understanding of the impact of operating temperature on gas transport in the PTL, and subsequently mass transport losses in the PEM electrolyzer.In this work, we employed operando X-ray computed tomography (CT) to visualize three-dimensional (3D) pore-scale gas saturations in the PTL at various industrially relevant operating conditions. Specifically, a custom in-house electrolyzer was imaged with synchrotron CT at five different temperatures (40°C, 50°C, 60°C, 70°C, 80°C) and current densities ranging from 0.5 A/cm² to 4.0 A/cm² with 0.5 A/cm² incremental steps. Electrochemical impedance spectroscopy and Tafel measurements were also acquired to characterize the relationship between gas distributions and corresponding mass transport overpotentials. Considering X-ray transparency, a model PTL made of carbon fibers was used to achieve sufficient contrast to quantify water and gas in the obtained images. Using precise volume registration and subtraction methods, pore-scale gas distributions in the PTL were quantified to reveal changes in bubble accumulation and coverage area at different operating temperatures and over operation time. The novel insights from this work will aid in our understanding of the complex relationship between operating temperature and multiphase flow phenomena in the PTL, which is vital for tailoring operating conditions of PEM electrolyzers to curtail the costs of green hydrogen production.

New Advances in Graded PEMFC Catalyst Layers

Ohmic and mass transport overpotentials remain investigation and optimization areas for Proton Exchange Membrane Fuel Cell (PEMFC) applications. One potential optimization approach involves the preferential distribution of the chemical substituents spatially in the catalyst layer. This would ultimately result in graded catalyst layers (GCLs). The spatial grading directions can thereby be in the in plane and/or through plane direction(s). Chemical constituents often graded in both directions include the binder and the catalyst[1, 2]. The ionomer, an essential constituent for PEMFC applications has also been a key focus area for many research institutes and groups. Novel optimization approaches include finding new ionomers with low equivalent weights (EWs).Typically, in a PEMFC, the highest proton conduction rate is at the membrane interface and decreases towards the GDL[3, 4]. Hence, previous studies with gradients containing high ionomer contents close to the membrane and lower ones at the GDL interface resulted in improved performances. The higher ionomer content at the membrane interface resulted in better protonic conductions, and the bigger void spaces in the GDL interface resulted in improved mass transport properties[1, 5]. Though precise electrochemical analysis is lacking, it is commonly agreed that the graded layers have lower mass transport and ohmic resistances. However, the studies encountered in the literature are almost exclusively based on high EW ionomers, such as Nafion. In contrast, low EW ionomers, which have higher protonic conductivity and water uptake capabilities have not yet been a focus of researchers. Additionally, the few reports encountered in the literature focus solely on GCLs while often disregarding other contributing effects to the overall cell voltage such as the equivalent weight.Low EW ionomers offer a wider operation range for PEMFCs in terms of relative humidities. By combining the low EW ionomers with graded catalyst layers, the operation range becomes even more flexible, and the limits are further stretched. In this study, we push the operating range limits of low EW graded catalyst layers and compare them to non-graded ones. We investigate different manufacturing techniques and use suitable scalable coating methods for the manufacturing of the electrodes. Afterwards, suitable in situ and ex situ analysis techniques are used in detail to trace down the improvements for the gradients. Among the key findings, we notice that potential improvements for low EW ionomer graded catalyst layers become apparent at considerable low relative humidities. The improvements are visible in the kinetic, ohmic, and mass transport regions.

Indispensable Nafion Ionomer for High-Efficiency and Stable Oxygen Evolution Reaction in Alkaline Media.

Addressing the challenge of sluggish kinetics and limited stability in alkaline oxygen evolution reactions, recent exploration of novel electrochemical catalysts offers improved prospects. To expedite the assessment of these catalysts, a half-cell rotating disk electrode is often favored for its simplicity. However, the actual catalyst performance strongly depends on the fabricated catalyst layers, which encounter mass transport overpotentials. We systematically investigate the role and sequence of electrode drop-casting methods onto a glassy carbon electrode regarding the efficiency of the oxygen evolution reaction. The catalyst layer without Nafion experiences nearly 50% activity loss post stability test, while those with Nafion exhibit less than 5% activity loss. Additionally, the sequence of application of the catalyst and Nafion also shows a significant effect on catalyst stability. The catalyst activity increases by roughly 20% after the stability test when the catalyst layer is coated first with an ionomer layer, followed by drop-casting the catalysts. Based on the half-cell results, the Nafion ionomer not only acts as a binder in the catalyst layer but also enhances the interfacial interaction between the catalyst and electrolyte, promoting performance and stability. This study provides new insights into the efficient and accurate evaluation of electrocatalyst performance and stability as well as the role of Nafion ionomer in the catalyst layer.

Microstructural Engineering of Flow Battery Electrodes with Non-Solvent Induced Phase Separation

Rising levels of greenhouse gases urge the decarbonization of human activities. Electrification of strategic sectors such as transport, industry, and heating necessitates affordable, safe, and dispatchable large-scale energy storage systems. Redox flow batteries (RFBs) stand as a promising long-duration and large-scale energy storage technology due to their design features[1]. Ongoing research focuses on engineering key cell components (e.g. membranes, electrodes, active materials) to improve cost competitiveness. This work focuses on the fabrication of novel porous carbon electrodes which strongly influence the performance and cost of the technology. The requirements for porous electrodes liquid-phase electrochemical reactors are contradictory: a large surface area is needed to overall exchange current density, but also minimal pressure losses to decrease pumping costs. Additionally, surface properties of the electrode dictate the reaction kinetics whereas its microstructure controls the mass transport of the reactants within the reactor area[2]. However, the current state-of-the-art porous electrodes for RFBs consists of carbon-fiber materials repurposed from other electrochemical technologies (e.g. polymer electrolyte fuel cells) which do not offer a favorable tradeoff between transport properties and surface area. Additionally, their production involves complex manufacturing processes resulting in high cost and limited morphological versatility. Therefore, there is a need to develop alternative synthetic platforms which offer tailorable microstructures and surface properties to match the requirements of emerging electrochemical applications, such as redox flow batteries. In this work, we draw inspiration from the membrane technology and employ non-solvent induced phase separation (NIPS) as a method to synthesize porous carbon electrodes for RFBs[3,4].Here, we present our latest efforts to develop a manufacturing route enabling the control of microstructural features of carbonaceous porous electrodes for RFBs. The microstructure of the carbon scaffold is determined by the nature and composition of the polymer blend used to cast the electrode precursor. We use polyacrylonitrile as scaffold precursor and polyvinylpyrrolidone as a pore forming agent, dissolved in an organic solvent (i.e., dimethylformamide, dimethylsulfoxide or N-methylpyrrolidone). The polymer blends were cast into a mold and immersed in a water coagulation bath prior to being stabilized (270 °C) and carbonized (1050 °C). Various process parameters (e.g. polymer concentration, bath temperature, solvent type) governing the final electrode properties were leveraged to control structural features, opening new opportunities for the fabrication of diverse microstructures (i.e., porosity gradient, isoporosity and with macrovoids). Electrochemical double layer capacitance and pressure drop measurements show an increased design space compared to carbon-fiber electrodes, confirming the versatility of the synthetic route. Microstructural analysis of the prepared NIPS electrodes reveals two porosity domains: large finger-like macrovoids (throat diameter >50 μm) and smaller interconnected microvoids (pore diameter 2-15 μm). Flow cell polarization and electrochemical impedance spectroscopy measurements in two common flow battery electrolytes, an Fe2+/Fe3+ single electrolyte and vanadium full cell, show improvements in performance compared to traditional electrodes. An increase in the polymer concentration of the polymer blend solution produced NIPS electrodes with smaller pores and resulted in better mass transport within the electrode. When the temperature of the coagulation bath was increased to 40 ºC, the electrode has higher electrochemically active surface area, producing a significant power increase in the sluggish vanadium electrolyte. Overall, the NIPS electrodes operate at significantly lower activation and mass transport overpotentials in all-vanadium RFBs, generating higher electrical power at the expense of a slight pressure drop increase. Although preliminary, we demonstrate the potential of NIPS to manufacture electrodes with tailorable microstructures for convection-enhanced electrochemical technologies.[1] T. M. Gür, Energy Environ. Sci. 2018, 11, 2696.[2] A. Forner-Cuenca, E. E. Penn, A. M. Oliveira, F. R. Brushett, J. Electrochem. Soc. 2019, 166, A2230.[3] R. R. Jacquemond, C. T.-C. Wan, Y.-M. Chiang, Z. Borneman, F. R. Brushett, K. Nijmeijer, A. Forner-Cuenca, Cell Reports Physical Science 2022, 3, 100943.[4] C. T.-C. Wan, R. R. Jacquemond, Y.-M. Chiang, K. Nijmeijer, F. R. Brushett, A. Forner-Cuenca, Adv. Mater. 2021, 10. Figure 1

Effect of perfluorosulfonic acid ionomer in anode catalyst layer on proton exchange membrane water electrolyzer performance

The equivalent weight (EW) and side-chain length of perfluorosulfonic acid (PFSA) ionomers have been found to significantly influence the performance of different proton exchange membrane (PEM)-based electrochemical devices. However, few reports have investigated their influence on PEM water electrolyzers (PEMWE). In this study, we investigate the influence of EW and side-chain length on PEMWE's membrane electrode assemblies (MEAs) which are fabricated by four different PFSA ionomers. It is found that the change in PFSA ionomer EWs barely influences the microstructure of the anode catalyst layer (ACL) but affects its surface wettability and MEA performance. Different from that in PEM fuel cells, MEA in PEMWEs prepared by PFSA ionomers with higher EWs shows better cell performance. The voltage difference comes from the change in the mass transport overpotential which is up to 113 mV with the comparison between PFSA ionomers with the highest EW and the lowest EW, which attributes to the changed wettability and water-oxygen transport of the ACLs. The influence of PFSA ionomer side-chain lengths on cell performance is not observed. This work provides insight into the effect of PFSA ionomers and offers information on how to design high-performance PFSA ionomers for PEMWE applications.

Bubble management in PEM water electrolysis via imprinting patterned grooves on catalyst layer

The demand for cost reduction in proton exchange membrane water electrolyzer (PEMWE) is driving it to increase the operating current density and higher current densities require better bubble management. In this work, we demonstrate that introducing patterned grooves to the catalyst layer surface (CL) is able to enhance the bubble evolution and improve electrolyzer performance. The patterned grooves microstructure on CL provides additional bubble nucleation sites and faster bubble evolution. Smaller average bubble size and higher bubble detachment frequency during water electrolysis are observed through a bubble evolution visualization system and further confirmed by a frequency analysis method. In addition, patterned grooves also improve the hydrophilicity and aerophobicity of CL surfaces. In the PEM water electrolyzer cell, the patterned grooves on CL boost the cell performance and reach high current densities up to 5.0 A cm-2. With better bubble management, patterned grooves significantly reduce the mass transport overpotential at each current density. At 5.0 A cm-2, the mass transport overpotential is reduced by 59%. This work provides a new idea of bubble management in PEMWE and is highly informative for designing CLs for PEMWE and other gas-evolving electrochemical energy conversion applications.

Energy harvesting from liquid cooling systems using thermo-electrochemical flow cells

We report a flow thermocell that is integrated into liquid cooling systems to convert the temperature difference between hot and cold pipes into electrical energy. This flow thermocell uses porous Ni foam electrodes that are inserted into the pipes carrying heated and cooled fluids. An aqueous ferricyanide/ferrocyanide electrolyte is used as a coolant fluid to generate electricity through thermally driven redox reactions on the electrodes. The power generation capability of the fabricated device is evaluated for operating conditions. As the flow rate of electrolytes having a temperature difference of 22 °C increase from 0.3 to 1.5 mL/s, the power density increase from 0.129 to 0.208 mW/cm2 owing to the reduction in the mass transport overpotential for power generation, and a Carnot-relative efficiency of 0.304% is obtained at an optimal flow rate of 1.2 mL/s. The efficiency can be further increased to 0.50% by reducing the heat loss from heated to cooled electrolytes with increasing the inter-electrode spacing. The efforts provide here to optimize the design and performance of flow thermocells would be useful in devising new active cooling systems that incorporate the ability to harvest heat flow in liquid cooling systems into electrical energy while simultaneously performing liquid cooling.

Impact of Porous Microstructure on Performance of Redox Flow Batteries: A Modeling Study

Electrodes are essential components of redox flow batteries (RFBs) that determine performance. Given the advances in 3D printing technologies, bottom-up design and manufacturing electrodes with controllable and reproducible properties can be attainable. In this study, pore network modeling is utilized to understand the coupled transport and reaction processes in Hydrogen-Bromine (H2/Br2) flow batteries. This study highlights the importance of the microstructure while designing a 3D printable microstructure. The 3D pore-scale model is validated against the experimental measurements using an unstructured pore network, extracted from a tomography scan of a commercial carbon paper. A printable ordered cubic lattice with the same permeability as carbon paper increases the maximum power density by 35% due to lower tortuosity and higher mass transport. A parametric study is carried out to understand the role of microstructure, such as pore size, throat size, anisotropy, and lattice unit size, for flow-through and interdigitated flow fields. It is shown that a smaller lattice unit, increases the reactive surface area, lowers the activation and mass transport overpotentials, and consequently enhances the performance of the battery. An improved microstructure design for both flow fields is proposed that intensifies the performance of the battery via engineering the flow path of the electrolyte.

Advanced Porous-Transport-Layer Interface Design for PEM Electrolyzers

Proton-exchange-membrane (PEM) water electrolyzers are promising technologies for generating clean hydrogen for use in a variety of sectors. The anode catalyst layer/porous-transport layer (CL/PTL) interface plays a vital role in determining performance of proton-exchange-membrane (PEM) water electrolyzers. This interface impacts ohmic, kinetic, and mass-transport overpotentials experienced by the cell. PTLs with large surface pores cause undesired catalyst layer deformation into pores of the PTL, which exacerbates degradation, and leads to high ohmic and kinetic overpotentials due to insufficient interfacial contact area.1,2 On the other extreme, low surface porosity at the interface leads to accumulation of oxygen gas that inhibit water transfer, resulting in mass-transport overpotentials.3 Thus, ideal PTLs should have hierarchical structure with high bulk porosity and low tortuosity to mitigate mass-transport overpotentials, while low surface porosity is need to enhance interfacial contact area. In this talk, we explore controlling the interfacial pore structure of these layers through a subtractive method, namely, laser ablation. Laser ablation is a facile method for tailoring the interfacial pore structure of PTLs. Heat from the laser melts titanium, granting a unique morphology advantageous to electrolyzer performance. Laser-ablated PTLs provide improved contact with the CL, while maintaining the bulk pore structure elsewhere, thereby facilitating gas removal and water transport. We also examine how laser-modified PTLs can help enable ultra-low catalyst loadings by minimizing interfacial contact resistance with the thinner catalyst layers. Overall, our work reveals that laser ablation is a facile method to enhance PEM electrolyzer performance. Acknowledgements This work was funded under the H2NEW Consortium by the Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References T. Schuler, R. De Bruycker, T. J. Schmidt, and F. N. Büchi, J. Electrochem. Soc., 166, F270–F281 (2019)T. Schuler, T. J. Schmidt, and F. N. Büchi, J. Electrochem. Soc., 166, F555–F565 (2019).X. Peng et al., Adv. Sci., 8 (2021).

(Invited) Numerical Simulation Approaches for Understanding Transports Behavior inside Polymer Electrolyte Membrane Fuel Cells

Augmentation of polymer electrolyte membrane (PEM) fuel cell (FC) performance at high current densities, particularly under automotive application, is important to improve the overall power density and to reduce the cost of PEMFC systems. Primarily, the fundamental non-linearity of the equations governing PEMFC performance on a three-dimensional model necessitates iterative solutions. As of now, mass transport over-potential is a major barrier to achieving high performance at a high current density. Experimental investigations of oxygen transport in particular are limited by an inability to resolve the water saturation-dependent properties. The alternative approach to understand and overcome transport resistances, predominantly inside the gas diffusion layer, is to use advanced numerical modeling.Numerical simulation techniques using in PEMFC will be presented including conventional computational fluid dynamics and the meshless Lattice Boltzmann method (LBM) which is an alternative advanced modeling technique [1-14]. This approach can be applied to existing models to visualize the transports inside the structure of microscale geometries, such as: a gas diffusion layer (GDL), micro porous layer (MPL), and catalyst layer (CL). The study where the goal was to use these numerical approaches and directly predict the transports behavior across the length scale (macro-, meso-, micro-, and nano- scales) as shown in Figure 1. The understanding of local kinetic activity inside nanoscale CL implemented into the model will also be discussed. The achievement of this work can enhance the potential capability of a model-based investigation of transport physics inside PEMFC to find solution of designs and operational conditions especially for transport applications.

(Invited) Decoupling the Contributions of Different Instability Mechanisms to the Fuel Cell Performance Decay of Fe-Based O2-Reduction Catalysts

The last decade of research on Fe-based O2-reduction single atom catalysts (SACs) has led to the development of SACs with an initial polymer electrolyte fuel cell (PEFC) performance close to that of Pt-based catalyst layers (CLs). However, these inexpensive materials generally suffer from a fast performance decay that has been ascribed to several mechanisms that can be simultaneously at play during device operation. These include (i) the demetallation of the SACs’ active sites; (ii) the potential-induced corrosion of the carbonaceous matrix that hosts these active centers; and/or (iii) the chemical degradation of the CL-ionomer, active sites and/or carbon support by radicals derived from the H2O2 produced as an O2-reduction by-product. Unfortunately, little is known regarding the relative contributions of these mechanisms to the overall PEFC-performance loss and as a function of the operative conditions – a missing understanding of pivotal importance for the design of strategies to mitigate this instability.With this motivation, this contribution will start with a comparison between the device stability in the course of 30 min holds at various currents of two type of SACs featuring similar beginning-of-life PEFC-performance. Particular attention will be paid to the effect of the catalyst loading on the observed degradation, which will be linked to the operando mapping of the distribution of liquid water within these materials’ CLs based on neutron imaging. In a subsequent step, we will present the results derived from a stability protocol in which the combination of different cathode gas feeds (i.e., air vs. N2) and potential hold durations allow decoupling the relative contributions of the above deactivation mechanisms to the overall performance decay. This requires a careful assessment of the kinetic, ohmic and mass transport overpotentials (and changes thereof in the course of the stability measurements) based on Tafel analyses and electrochemical impedance spectroscopy measurements. Moreover, these results are again complemented by an assessment of the protocol-induced changes in catalyst morphology and surface chemistry based on transmission electron microscopy and X-ray photoelectron spectroscopy measurements.In summary, this contribution will showcase our efforts to deconvolute the relative effects of various deactivation mechanisms to the overall PEFC-performance loss of SAC-CLs.

Voltage Loss Analysis of Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzers (PEMWEs) present a promising hydrogen production alternative to common methods like steam methane reforming, particularly for producing green hydrogen from renewable sources. However, scaling up PEM electrolyzers is challenged by the high cost of their platinum group metal catalysts (Ir & Pt) for the oxygen and hydrogen evolution reactions (OER & HER). This study focused on the performance of PEM electrolyzers and its subsequent volage loss breakdown analysis. Current voltage loss breakdown analyses separate voltage losses into three categories: kinetic, ohmic, and mass transport overpotentials. Current models explain the first two but lack proper phenological description of the latter. To better separate and ascribe the mass transport losses due to electrode and PTL dry-out, a new electrolyzer galvanostatic intermittent titration technique (eGITT) was developed in this work. The time-series data from this pulse-based transient diagnostic evaluated through time-scale analysis and basic physical models to extract key transport properties that can be used to quantify the efficacy of different electrode and PTL materials. Results from this method were compared to current model predictions and evaluated against literature values.

Triple-Phase Pore Network for Oxygen and Proton Transport Model in the Cathode Catalyst Layer of Proton Exchange Membrane Fuel Cells

Improvement in the mass transport overpotentials of the oxygen reduction reaction provides a pathway for reducing the high cost associated to platinum group metal loading as well as enhancing the lifetime of electrodes in proton exchange membrane fuel cells (PEMFCs). State-of-the-art PEMFCs typically have Pt loadings of the order of 0.3 mgPtcm-2, which is the main part of the estimated stack cost of $55/kW. Understanding transport phenomena at triple phase boundaries that consist of a (solid) electrocatalyst, ionomer, and (gas-phase) oxygen, where the electrochemical reactions occur has gained importance to improve electrocatalyst accessibility. Mathematical models that address transport of electrons, protons, and oxygen in the cathode is critical for an effective catalyst layer design. Modeling of this multiphysics phenomena is further complicated due to multiscale effects. Pore network model (PNM) is a promising technique to tackle these challenges because of its capability of resolving porous networks locally in greater detail. Moreover, improvements in microscopic imaging techniques (e.g., SEM, TEM, and nano CT) enable the generation of realistic pore networks of the catalyst layers that can be directly integrated into PNM. Most modeling studies to date focus on single networks for oxygen transport in the open pore space, or transport of oxygen and water. A pore network that describes oxygen, proton, and electron transport simultaneously remains uncharted. Here, we construct a triple-phase pore network model using nano CT images of heterogeneous porous catalyst layers. Binary images of solid-open pore space and ionomer-catalyst support are generated, which are then overlapped to create ternary images. Pores and throats are then fit to theseimages to create pore-networks that describe open-pores for oxygen and water transport, ionomer pores for proton transport, and a solid network for electron transport. A schematic of the approach is shown in Figure 1.The study offers a new method that constructs a triple pore network for a porous material in which two levels of open space porosity (micro and macro) exist in connection with solid and ionomer networks. Therefore, arealistic pore network is obtained to be further modeled through the approach presented. Figure 1

Channel Diameter Effect of Porous Carbon Microparticles on PEMFC Performance for Highly Active Ultra-Low Pt Catalysts

Proton exchange membrane fuel cells (PEMFCs) have received much attention as environmentally benign automotive power sources. PEMFCs can offer a large amount of electricity required for autonomous vehicles that battery-powered systems may not be able to provide. However, because PEMFC electrodes are based on expensive and scarce Pt catalysts, Pt minimization is necessary to expand the PEMFC market. To reduce Pt usage, various attempts have been made to increase the intrinsic activity of Pt catalysts, particularly at the cathode where the oxygen reduction reaction (ORR) occurs. Despite decades of progress, high ORR activity was typically reported in half-cell setups and frequently failed to show corresponding performance in single-cell. Catalysts with low Pt content perform poorly in the high-current density region because the thick catalyst layer limits mass transport.[1] In addition, low Pt loading catalysts typically suffer more in long-term operation.[2] As a result, it is critical to develop PEMFC catalysts with low Pt content that can facilitate mass transport while also exhibiting high durability.Herein, we report highly active and durable PtFe@C catalysts with ultra-low amounts of Pt (1 wt%) on channeled mesoporous carbon (CMC) particles. These CMC particles are designed to have continuous channels with open porosity and a large surface area, facilitating the mass transport behavior and maximizing cell performance. Block-copolymer particles (BCPs) with different molecular weights were used to fabricate CMC particles with pore diameters ranging from 13 to 63 nm. Two steps of pre-crosslinking and hyper-crosslinking were conducted prior to the carbonization step to preserve the porous internal structure of the BCP-based carbon support during high-temperature treatment. After depositing Pt onto the support by facile incipient wetness impregnation method followed by reduction, thin layers of carbon shell were observed to encapsulate the PtFe alloy nanoparticles.The channel diameter effect on the mass transport was studied in both half-cell and single-cell. Interestingly, from the cell performance obtained with varying channel diameters and different oxygen concentrations, we concluded that both reactant (proton and oxygen) supply and product (water) removal were greatly enhanced with larger channel size. With the largest channel diameter of 63 nm, initial mass activity in the single-cell was obtained to be 3.5 A mgPt -1, which is the highest value reported to date to the best of our knowledge. Cell performance under H2-air flow, which is the industrially relevant condition, surpassed the commercial 20 wt% Pt/C with only 1/20 of the Pt loading. The origin of enhanced cell performance upon enlarging the channel diameter was investigated by separating kinetic, ohmic (electronic and ionic charge transport), and mass transport overpotentials. Both the proton and oxygen transport resistance were confirmed to be reduced with larger channel size. Moreover, carbon shell protected Fe from getting leached out in an acidic environment, resulting in preserved catalyst structure and high durability. The outstanding performance of 51 kW/gPt in H2/air condition after 30,000 cycles of accelerated degradation tests (ADTs) was observed. This work will open a new paradigm to develop PEMFC catalysts with much higher activity and durability while simultaneously minimizing Pt use.

Effect of Catalyst Aggregate Size on the Mass Transport Properties of Non-Noble Metal Catalyst Layers in PEMFC Cathodes

Non-noble metal catalysts (NNMCs) are regarded as ultimate promising alternative to the high-cost Pt-based catalysts required to catalyze the oxygen reduction reaction (ORR) in the cathode of proton exchange membrane fuel cells (PEMFCs). Indeed, novel syntheses established in recent years have led to NNMCs with initial fuel cell performances comparable to those of Pt-based materials [1]. However, due to their intrinsically lower ORR-activity compared to Pt, NNMC cathodes catalyst layers (CLs) require much higher loadings than those of Pt-based (≈ 4 mgNNMC·cm−2 vs. ≤ 0.3 mgPt·cm−2, respectively) to achieve similar performances. This makes NNMC CLs much thicker (up to ≈ 100 μm) than conventional Pt-based ones, which results in additional mass transport issues [2] that prevent them from reaching the high current densities at high potentials required for automotive applications.In this context, our group recently proposed a novel synthesis [3] that yields NNMCs with a high initial ORR-activity in PEMFC tests (≈ 10 A·g−1 at 0.8 V in H2:O2 at 80 °C, 1.5 barabs, and 100% relative humidity) but poor mass transport properties due to the NNMC’s large particle size. In this study, we have efficiently reduced the aggregate size of this NNMC type from the > 2 μm in Ref. [3] to ≈ 100 nm by substituting the dry ball-milling (BM) originally used to mix the catalyst precursors with wet BM (i.e., in the presence of a solvent). The electrochemical behavior of both kinds of NNMCs was studied through rotating ring disk electrode (RRDE) voltammetry measurements, which revealed significant enhancement in the ORR limiting current of the NNMC with a smaller particle size. Moreover, 3 times higher H2O2 yield (≈ 5% to ≈ 15%) was observed for both catalyst types when lowering their RRDE-loading from 0.5 to 0.1 mgNNMC·cm-2 while keeping the homogenous film quality for both catalyst at low loadings. Subsequently, PEMFC tests were conducted for both kinds of NNMCs with different loadings (1 or 4 mgNNMC·cm-2) using both O2 and air as the cathode reactant feeds. The mass transport overpotential (ηtx) was then assessed by subtracting the polarization curves recorded in O2 vs. air (and correcting for the shift in reversible potential caused by the concomitant change in the O2 partial pressure) [4]. As displayed in Figure 1, increasing the loading of the dry-BM sample resulted in a larger mass transport over-potential, while in the case of wet-BM sample ηtx barely changed as a consequence of the larger loading. Overall, this reduction of the catalyst’s aggregate size led to a ≈ 75 mV improvement in ηtx at low loading, which became more than 2 times larger (≈ 175 mV) in the case of the CLs with a high loading, thus confirming the enhanced mass transport properties of wet-BM NNM-catalyst layer. Acknowledgments: The authors acknowledge the financial assistance of the Swiss National Science Foundation through Sinergia grant CRSII5_18033.

Atomically dispersed antimony on N-doped carbon for highly efficient oxygen reduction reaction

Main-group metal based single-atom catalysts are attracting increasing research attention in electrochemical catalysis owing to their partially occupied valence p-orbitals. Herein, we report a series of atomically dispersed Sb-N-C catalysts (including Sb-N-CNP, Sb-N-CNL and Sb-N-CNT) synthesized by a facile adsorption-pyrolysis strategy for oxygen reduction reaction (ORR). Apart from generating atomically dispersed Sb-N5 sites as active centers, self-evaporation of Sb during pyrolysis process has created abundant micro/meso-pores in Sb-N-C catalysts. Benefitting from these advantageous features, Sb-N-C presents outstanding ORR activity (Sb-N5 sites) and efficient mass transport (micro/meso-pores). By rotating disk electrode (RDE) test in alkaline media, Sb-N-C exhibits a most positive half-wave potential of 0.90 V vs. RHE and a highest kinetic current density up to 43.8 mA cm−2 at 0.85 V vs. RHE. As gas diffusion electrode (GDE), Sb-N-CNP0.2 demonstrates fast O2 diffusion and transport that enables smaller mass transport overpotential at high current densities up to 800 mA cm−2. Finally, Zn-air battery that uses the Sb-N-CNP catalyst as air electrode achieves a maximum power density of 180 mW cm−2 and more than 1000 hs of continuous operation. This work further demonstrates the excellent performance of main-group Sb single-atom catalyst toward ORR and applications in practical energy conversion devices.

Elucidating effects of catalyst loadings and porous transport layer morphologies on operation of proton exchange membrane water electrolyzers

Producing green hydrogen efficiently via proton exchange membrane water electrolysis (PEMWE) is the key for achieving decarbonization targets. Iridium catalyst is expensive, and it is important to minimize its use and to optimize interface between Ir and ionomer or water for higher utilization of catalyst in oxygen evolution reaction. In this paper, x-ray computed tomography along with electrochemical and modeling techniques are used to characterize the interface for two different porous transport layers (PTLs) and catalyst layers at various loadings. We show that low porosity sintered PTLs exhibit higher interfacial contact with the catalyst and the membrane that results in improved kinetics. Radiography and modeling results indicate that oxygen taking multiple transport pathways through the PTL results in slug flow through the channels that reduces mass transport overpotential. Based on the results, we suggest design guidelines for high efficiency and durable PEMWE and their components.

Cell voltage model for Li-Bi liquid metal batteries

Lithium-bismuth bimetallic cells are amongst the best explored liquid metal batteries. A simple and fast quasi-one-dimensional cell voltage model for such devices is presented. The equilibrium cell potential is obtained from a complex two-dimensional fit of data drawn from multiple studies of equilibrium cell potential and rendered congruent with the phase diagram. Likewise, several analytical and fit functions for the ohmic potential drop across the electrolyte are provided for different battery geometries. Mass transport overpotentials originating from the alloying of Li into Bi are modelled by solving a diffusion equation, either analytically or numerically, and accounting for the volume change of the positive electrode. The applicability and limitations of the model are finally illustrated in three distinct experimental settings.