Transport Properties Of Electrodes Research Articles

(Invited) Linking Ionomer/Electrocatalyst Interactions to Membrane Electrode Assembly Performance and Durability in Proton Exchange Membrane Fuel Cells

The proper integration of materials to enable high performing low temperature fuel cell operation has never been more crucial to realizing the isolated (e.g. half-cell or ex-situ) improvements of advanced electrocatalysts, supports and ionomers in an effort to maximize efficiency. The integration pathway for oxygen reduction electrocatalysts, from initial testing in a rotating disk electrode (RDE) to their apex form as porous electrodes in a membrane electrode assembly (MEA) is replete with potential pitfalls. Along the route to improving MEA level electrocatalyst kinetics and electrode transport properties are numerous choices relating to ink formulation (i.e. solvents and solvent ratios), ink processing and deposition methodology, ionomer content and chemistry and electrochemical MEA conditioning. Combined with carbon morphology/porosity, such variables can alter the final electrode morphology, influencing active site accessibility and ORR activity as well as oxygen transport resistance and electrode proton conductivity.[1–5]Here, the development and utilization of in situ diagnostics designed to probe specific phenomena (e.g. catalyst/ionomer interactions, molecular and Knudsen diffusion, electrochemical active site accessibility and catalyst layer proton resistance) will be presented as tools to contrast the ink, processing and fabrication methods necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) and PGM-free (e.g. FeNx/C) electrocatalysts based electrodes. For PGM based MEAs, the dynamic effects of electrochemical conditioning will be examined in an effort to both baseline MEA level performance and decouple the contributions of simultaneous electrocatalyst particle growth and the removal of system impurities.[6] The prerequisite understanding gleaned from the application of electrochemical diagnostics and MEA conditioning procedures will be combined with the development of a novel sulfonated ionic liquid block copolymer ionomer and tailored integration methodology to illustrate a pathway towards improved MEA level performance.[7] Lastly, alterations in ionomer interactions as a function of accelerated stress tests will be discussed, providing insight into the evolving electrode structure as the community targets more robust systems capable of efficient operation for 25,000 hrs.[8]References[1] A. Orfanidi, P.J. Rheinlander, N. Schulte, H.A. Gasteiger, Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC, Journal of The Electrochemical Society. (2018) 11.[2] S. Khandavalli, J.H. Park, N.N. Kariuki, D.J. Myers, J.J. Stickel, K. Hurst, K.C. Neyerlin, M. Ulsh, S.A. Mauger, Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks, ACS Appl. Mater. Interfaces. (2018) 13.[3] D.A. Cullen, R. Koestner, R.S. Kukreja, Z.Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H.M. Meyer, C.M. Parish, K.L. More, Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy, J. Electrochem. Soc. 161 (2014) F1111–F1117. https://doi.org/10.1149/2.1091410jes.[4] T. Van Cleve, S. Khandavalli, A. Chowdhury, S. Medina, S. Pylypenko, M. Wang, K.L. More, N. Kariuki, D.J. Myers, A.Z. Weber, S.A. Mauger, M. Ulsh, K.C. Neyerlin, Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application, ACS Appl. Mater. Interfaces. 11 (2019) 46953–46964. https://doi.org/10.1021/acsami.9b17614.[5] G. Wang, L. Osmieri, A.G. Star, J. Pfeilsticker, K.C. Neyerlin, Elucidating the Role of Ionomer in the Performance of Platinum Group Metal-free Catalyst Layer via in situ Electrochemical Diagnostics, J. Electrochem. Soc. 167 (2020) 044519. https://doi.org/10.1149/1945-7111/ab7aa1.[6] S. Kabir, D.J. Myers, N. Kariuki, J. Park, G. Wang, A. Baker, N. Macauley, R. Mukundan, K.L. More, K.C. Neyerlin, Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study, ACS Appl. Mater. Interfaces. 11 (2019) 45016–45030. https://doi.org/10.1021/acsami.9b11365.[7] Y. Li, T.V. Cleve, R. Sun, R. Gawas, G. Wang, M. Tang, Y.A. Elabd, J. Snyder, K.C. Neyerlin, Modifying the Electrocatalyst−Ionomer Interface via Sulfonated Poly(ionic liquid) Block Copolymers to Enable High- Performance Polymer Electrolyte Fuel Cells, ACS Energy Letters. (2020) 6.[8] D.A. Cullen, K.C. Neyerlin, R.K. Ahluwalia, R. Mukundan, K.L. More, R.L. Borup, A.Z. Weber, D.J. Myers, A. Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation, Nature Energy. (2021) 1–13. https://doi.org/10.1038/s41560-021-00775-z.

Investigation of Mass Transport Properties of Fibrous Electrodes in Vanadium Redox Flow Batteries By Lattice Boltzmann Simulation

Redox flow batteries (RFBs) are gathering much attention as one of the promising candidates for a large-scale electrical energy storage device that is vital to utilize intermittent renewable energy in the grid [1]. In RFBs, fibrous electrodes have been widely applied presumably due to their large specific surface area and better electric conductance. It has been well recognized that the electrode structure profoundly affects cell performance that is attributed to the reaction and transport properties of the electrode [2,3].In this study, we developed a numerical simulation method to analyze pore-scale inhomogeneous behaviors of fluid and electrochemical reaction in fibrous electrodes using Lattice Boltzmann method (LBM). We focused our attention on porous structure affecting mass transport properties and overpotential in the electrodes for the vanadium redox flow battery.We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1(a). As the calculation procedure first, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. All calculation was carried out by the Lattice Boltzmann code we developed. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 50mA/cm2 was performed with different fiber diameter and porosity. As the pressure loss was fixed, the change in total overvoltage was evaluated as the change in energy loss.Figure 1 shows the calculation results of the electrochemical reaction and transport in the fiber electrode with a diameter of 3.75 μm and a porosity of 0.70. A path through which the electrolyte selectively flows is formed in the sparse area of the fibrous electrode and the inhomogeneous vanadium ion concentration and local reaction current density are identical in the electrode. This indicates that the electrolyte flow is locally biased due to the non-uniformity of the electrode. The reactive species concentration decreases due to the electrochemical reaction in the dense fiber region and causes local reaction current density.Based on a series of simulation with different diameter and porosity of the fibrous electrode, we obtained an equation for the electrolyte velocity and pressure drop in the electrode. We also evaluated averaged mass transport coefficients. We confirmed that the mass transfer characteristics are improved when the fiber diameter is large and the porosity is high. This is attributed to the larger the voids, the higher the permeability and the transport by advection was promoted. However, it is noteworthy that the larger diameter and the higher porosity leads to less electrode surface area. In our previous study [3], an optimized electrode architecture with a diameter of 2.4 μm and a porosity of 0.89 was presented by macro-scale simulations.In this fiber-scale simulations, we evaluated overpotential depending on the fiber diameter and the electrode porosity, and demonstrated the LBM simulation applied for optimization of fiber diameter and the electrode porosity in the RFB applications. Acknowledgements This research was supported by a Precursory Research for Embryonic Science and Technology (PRESTO) grant (grant number JPMJPR12C6) from the Japan Science and Technology Agency(JST).

Cation Intercalation Materials and Cell Design for Low-Energy Electrochemical Desalination

Due to the growing population and human-made climate change spurred by the use of fossil fuels, the urgent need for energy-efficient desalination has motivated research into electrochemical desalination as a technology to remove salt ions from water. Prussian Blue analogues (PBAs) are a promising class of redox-active intercalation compounds with high capacity and cycle life used in sodium-ion batteries. While their functionality was previously shown to enable desalination in symmetric cation intercalation desalination (CID) cell architectures [S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel, and K. C. Smith, Electrochim. Acta 255, 369 (2017)], they have low electronic conductivity which limits their charge and discharge rates while causing high energy consumption due to ohmic losses [A. Shrivastava and K. C. Smith, J. Electrochem. Soc. 165, A1777 (2018)]. A common technique to increase electronic conductivity in battery electrodes is the inclusion of additives such as carbon black, however increasing conductive particle content reduces active particle loading and overall salt absorption capacity. To overcome these limitations, the effects of volume fraction of either C45 or Ketjen Black conductive additive on electrode transport properties were studied, and we demonstrated that orders of magnitude improvement in electronic conductivity could be achieved using the smaller Ketjen Black particles [E. R. Reale, A. Shrivastava, and K. C. Smith, Water Res. 165, 114995 (2019)]. Through this discovery, the low electronic conductivity of PBA electrodes was overcome, and new higher-conductivity electrodes were successfully fabricated and used in a flow-through cation intercalation desalination cell built with a custom-designed 3D printed flow field. The new CID cell functioned using a PBA anode and cathode separated by an anion exchange membrane to transport salt into one half of the cell while desalinating the other, and displayed energy consumption an order of magnitude lower than that of other electrochemical desalination technologies while removing similar quantities of salt from an influent feed. For a salt concentration of 100 mM, a salinity comparable to brackish water found in areas such as estuaries and aquifers, approximately 25% of influent salt was removed from one feed and transferred into the other. Further improvements can be made through the use of intercalation materials with higher volumetric storage capacity and increased mobility for sodium, which were both limiting factors when using PBAs which lowered utilization and prevented higher salt removal. Early tests with such electrodes have shown their electronic conductivity to be several times higher than PBA electrodes, further reducing energy consumption. Another modification to the cell is the use of an in-house designed recirculation system to increase salt removal over multiple passes rather than single-pass flow. The goal of our research is to develop CID technology into a form of energy-efficient desalination capable of surpassing reverse osmosis in salt removed for a given energy input.

(Invited) Towards Obtaining Rotating Disk Electrode Activity in a Membrane Electrode Assembly; Implications of the Electrocatalyst-Ionomer Interface for the Oxygen Reduction Reaction

The proper integration of materials to enable high performing low temperature fuel cell operation has never been more crucial to realizing the isolated (e.g. half-cell or ex-situ) improvements of advanced electrocatalysts, supports and ionomers in an effort to maximize efficiency and fuel. This integration pathway for oxygen reduction electrocatalysts, from initial testing in a rotating disk electrode (RDE) to their apex form as porous electrodes in a membrane electrode assembly (MEA), is replete with potential pitfalls. Along the route to improving MEA level electrocatalyst kinetics and electrode transport properties are numerous choices relating to ink formulation (i.e. solvents and solvent ratios), ink processing and deposition methodology, ionomer content and chemistry and electrochemical MEA conditioning. Combined with carbon morphology/porosity, such variables can alter the final electrode morphology, influencing active site accessibility and ORR activity as well as oxygen transport resistance and electrode proton conductivity.[1–5]Here, the development and utilization of in situ diagnostics designed to probe specific phenomena (e.g. catalyst/ionomer interactions, molecular and Knudsen diffusion, electrochemical active site accessibility and catalyst layer proton resistance) will be presented as tools to contrast the ink, processing and fabrication methods necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) and PGM-free (e.g. FeNx/C) electrocatalysts based electrodes. For PGM based MEAs, the dynamic effects of electrochemical conditioning will be examined in an effort to both baseline MEA level performance and decouple the contributions of simultaneous electrocatalyst particle growth and the removal of system impurities.[6] The prerequisite understanding gleaned from the application of electrochemical diagnostics and MEA conditioning procedures will be combined with the development of a novel sulfonated ionic liquid block copolymer ionomer and tailored integration methodology to illustrate a pathway towards improved MEA level performance.[7]References[1] A. Orfanidi, P.J. Rheinlander, N. Schulte, H.A. Gasteiger, Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC, Journal of The Electrochemical Society. (2018) 11.[2] S. Khandavalli, J.H. Park, N.N. Kariuki, D.J. Myers, J.J. Stickel, K. Hurst, K.C. Neyerlin, M. Ulsh, S.A. Mauger, Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks, ACS Appl. Mater. Interfaces. (2018) 13.[3] D.A. Cullen, R. Koestner, R.S. Kukreja, Z.Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H.M. Meyer, C.M. Parish, K.L. More, Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy, J. Electrochem. Soc. 161 (2014) F1111–F1117. https://doi.org/10.1149/2.1091410jes.[4] T. Van Cleve, S. Khandavalli, A. Chowdhury, S. Medina, S. Pylypenko, M. Wang, K.L. More, N. Kariuki, D.J. Myers, A.Z. Weber, S.A. Mauger, M. Ulsh, K.C. Neyerlin, Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application, ACS Appl. Mater. Interfaces. 11 (2019) 46953–46964. https://doi.org/10.1021/acsami.9b17614.[5] G. Wang, L. Osmieri, A.G. Star, J. Pfeilsticker, K.C. Neyerlin, Elucidating the Role of Ionomer in the Performance of Platinum Group Metal-free Catalyst Layer via in situ Electrochemical Diagnostics, J. Electrochem. Soc. 167 (2020) 044519. https://doi.org/10.1149/1945-7111/ab7aa1.[6] S. Kabir, D.J. Myers, N. Kariuki, J. Park, G. Wang, A. Baker, N. Macauley, R. Mukundan, K.L. More, K.C. Neyerlin, Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study, ACS Appl. Mater. Interfaces. 11 (2019) 45016–45030. https://doi.org/10.1021/acsami.9b11365.[7] Y. Li, T.V. Cleve, R. Sun, R. Gawas, G. Wang, M. Tang, Y.A. Elabd, J. Snyder, K.C. Neyerlin, Modifying the Electrocatalyst−Ionomer Interface via Sulfonated Poly(ionic liquid) Block Copolymers to Enable High- Performance Polymer Electrolyte Fuel Cells, ACS Energy Letters 5 (2020) 1726-1731.

Effect of conductive additives on the transport properties of porous flow-through electrodes with insulative particles and their optimization for Faradaic deionization

Deionization devices that use intercalation reactions to reversibly store and release cations from solution show promise for energy-efficient desalination of alternative water resources. Intercalation materials often display low electronic conductivity that results in increased energy consumption during desalination. Accordingly, we performed experiments to quantify the impact of the size and mass fraction of conductive additives and insulative active particles on the effective electronic conductivity, ionic conductivity, and hydraulic permeability of porous electrodes. We find that Ketjen black conductive additives with nodules <50 nm in diameter produce superior electronic conductivity at lower mass fractions than the larger carbon blacks commonly used in capacitive deionization. Hydraulic permeability and effective ionic conductivity depend weakly on carbon black content and size, though smaller active particles decrease hydraulic permeability. Based on these results we analyzed the energy consumption and salt removal rate of different electrode formulations by constructing an electrochemical Ashby plot predicting the variation of desalination performance with electrode transport properties. Optimized electrodes containing insulative Prussian blue analogue (PBA) particles were then fabricated and used in an experimental cation intercalation desalination (CID) cell with symmetric electrodes. For 100 mM NaCl influent energy consumption varied from 7 to 33 kJ/mol when current density increased from 1 to 8 mA/cm2, approaching ten-fold increased salt removal rate at similar energy consumption levels to past CID demonstrations. Complementary numerical and analytical modeling indicates that further improvements in energy consumption and salt removal rate are attainable by enhancing transport in solution and within PBA agglomerates.