E.C. Kumbur

All-MXene (2D titanium carbide) solid-state microsupercapacitors for on-chip energy storage

On-chip energy storage is a rapidly evolving research topic, opening doors for the integration of batteries and supercapacitors at the microscale on rigid and flexible platforms. Recently, a new class of two-dimensional (2D) transition metal carbides and nitrides (so-called MXenes) has shown great promise in electrochemical energy storage applications. Here, we report the fabrication of all-MXene (Ti3C2Tx) solid-state interdigital microsupercapacitors by employing a solution spray-coating method, followed by a photoresist-free direct laser cutting method. Our prototype devices consisted of two layers of Ti3C2Tx with two different flake sizes. The bottom layer was stacked large-size MXene flakes (lateral dimensions of 3–6 μm) serving mainly as current collectors. The top layer was made of small-size MXene flakes (∼1 μm) with a large number of defects and edges as the electroactive layer responsible for energy storage. Compared to Ti3C2Tx micro-supercapacitors with platinum current collectors, the all-MXene devices exhibited a much lower contact resistance, higher capacitances and better rate-capabilities. Areal and volumetric capacitances of ∼27 mF cm−2 and ∼357 F cm−3, respectively, at a scan rate of 20 mV s−1 were achieved. The devices also demonstrated excellent cyclic stability, with 100% capacitance retention after 10 000 cycles at a scan rate of 50 mV s−1. This study opens up a plethora of possible designs for high-performance on-chip devices employing different chemistries, flake sizes and morphologies of MXenes and their heterostructures.

Validated Leverett Approach for Multiphase Flow in PEFC Diffusion Media III. Temperature Effect and Unified Approach

The final paper in this series is devoted to delineating the effects of temperature on the multiphase transport characteristics of thin-film polymer electrolyte fuel cell (PEFC) diffusion media (DM). Direct measurements of capillary pressure-saturation of various commercial DM coated with a wide range of poly(tetrafluoroethylene) (PTFE) loadings (from 5 to 20 wt % PTFE) were performed at different operating temperatures (20, 50, and 80°C). The benchmark data gathered from these experiments (available upon request) were compiled into the existing database generated from the first and second phase of this study, which examined the hydrophobicity and compression effects. The expanded database was then utilized to deduce a unified form of an empirical correlation appropriate for the tested DM. This semiempirical approach can predict capillary pressure of the tested DM as a function of liquid saturation, hydrophobic additive content, uncompressed porosity, compression pressure, and operating temperature within an uncertainty of ±14% of the measured capillary pressure over the entire saturation domain, showing considerable improvement over the traditional Leverett approach.

Characterization of Interfacial Structure in PEFCs: Water Storage and Contact Resistance Model

In this work, an analytical model of the microporous layer MPL and the catalyst layer CL interface under compression is developed to investigate the effects of the MPL|CL interfacial morphology on the ohmic and mass transport losses at the MPL|CL interface in a polymer electrolyte fuel cell PEFC. The model utilizes experimentally measured surface profile data as input. Results indicate that the uncompressed surface morphology of mating materials, the elasticity of PEFC components, and the local compression pressure are the key parameters that influence the characteristics of the MPL and CL contact. The model predicts that a 50% drop in the MPL and CL surface roughness may result in nearly a 40% drop in the MPL|CL contact resistance. The model also shows that the void space along the MPL|CL interface can potentially store a significant amount of liquid water 0.9‐3.1 mg/cm 2 , which could result in performance loss and reduced durability. A 50% drop in the MPL and CL surface roughness is expected to yield nearly a 50% drop in the water storage capacity of the MPL|CL interface. The results of this work provide key insights that will enhance our understanding regarding the complex relation between MPL|CL interfacial structure and cell performance.

Validated Leverett Approach for Multiphase Flow in PEFC Diffusion Media II. Compression Effect

This work is the second part of a series of papers to describe the multiphase transport mechanism in thin-film polymer electrolyte fuel cell (PEFC) diffusion media (DM). The present work is devoted to delineating the effects of compression. Direct measurements of drainage capillary pressure-saturation curves for SGL series carbon paper DM tailored with a range of mixed wettability were performed at room temperature under various compressions (0, 0.6, and 1.4 MPa) typically encountered in a fuel cell assembly. Based on these benchmark data, an appropriate form of the Leverett approach, including a Leverett-type empirical function that incorporates the effect of compression and the mixed wettability characteristics of the tested DM samples, was developed. The presented approach can determine the capillary pressure as a function of hydrophobic additive content, liquid saturation, compression, and uncompressed porosity of the DM. Compression leads to an increase in capillary pressure, effectively caused by the corresponding reduction in effective porosity. Any increase in hydrophobicity amplifies the compression effect, yielding a higher capillary pressure for the same saturation level. Furthermore, the fraction of connected hydrophilic pores is observed to be reduced with an increase in compression, leading to a favorable reduction in water storage capacity of the fuel cell DM.

Composite Manganese Oxide Percolating Networks As a Suspension Electrode for an Asymmetric Flow Capacitor

In this study, we examine the use of a percolating network of metal oxide (MnO2) as the active material in a suspension electrode as a way to increase the capacitance and energy density of an electrochemical flow capacitor. Amorphous manganese oxide was synthesized via a low-temperature hydrothermal approach and combined with carbon black to form composite flowable electrodes of different compositions. All suspension electrodes were tested in static configurations and consisted of an active solid material (MnO2 or activated carbon) immersed in aqueous neutral electrolyte (1 M Na2SO4). Increasing concentrations of carbon black led to better rate performance but at the cost of capacitance and viscosity. Furthermore, it was shown that an expanded voltage window of 1.6 V could be achieved when combining a composite MnO2-carbon black (cathode) and an activated carbon suspension (anode) in a charge balanced asymmetric device. The expansion of the voltage window led to a significant increase in the energy density to ∼11 Wh kg(-1) at a power density of ∼50 W kg(-1). These values are ∼3.5 times and ∼2 times better than a symmetric suspension electrode based on activated carbon.

Towards High‐Energy‐Density Pseudocapacitive Flowable Electrodes by the Incorporation of Hydroquinone

This study reports an investigation of hydroquinone (HQ) as a multielectron organic redox molecule to enhance the performance of flowable electrodes. Two different methods to produce high-performance pseudocapacitive flowable electrodes were investigated for electrochemical flow capacitors. First, HQ molecules were deposited on carbon spheres (CSs) by a self-assembly approach using various HQ loadings. In the second approach, HQ was used as a redox-mediating agent in the electrolyte. Flowable electrodes composed of HQ showed a capacitance of 342 F g(-1), which is >200 % higher than that of flowable electrodes based on nontreated CSs (160 F g(-1)), and outperformed (in gravimetric performance) many reported film electrodes. A similar trend in capacitance was observed if HQ was used as a redox agent in the electrolyte; however, its poor cycle life restricted further consideration. In addition, a twofold increase in capacitance was observed under flow conditions compared to that of previous studies.

Pore-Morphology-Based Simulation of Drainage in Porous Media Featuring a Locally Variable Contact Angle

Since the first publications by Hazlett (Transp Porous Med, 20:21–35, 1995) and Hilpert and Miller (Adv Water Res, 24:243–255, 2001), the pore-morphology-based method has been widely applied to determine the capillary pressure–saturation curves of porous media. The main advantage of the method is the simulation of a primary drainage process for large binary images using moderate computational time and memory compared to other two-phase flow simulations. Until now, the pore morphology model was restricted to totally wetting materials or those with a constant contact angle. Here, we introduce a similarly computationally efficient extension of the model that now enables the calculation of capillary pressure–saturation curves for porous media, where the contact angle varies locally within, due to a composite structure.

Model to Investigate Interfacial Morphology Effects on Polymer Electrolyte Fuel Cell Performance

The purpose of this work is to investigate the impact of the interfacial contact morphology between the catalyst layer (CL) and micro porous layer (MPL) on the polymer electrolyte fuel cell (PEFC) performance. A single-phase anisotropic mathematical model has been developed to evaluate the role of interfacial morphology on ohmic, thermal and gas-phase transport. The novel feature of the model is inclusion of directly measured surface morphological information of the cathode catalyst and the micro porous layers. The preliminary results indicate that thermal disruption due interface morphology has low absolute impact in comparison to ohmic disruption. Ultimately, this model will be used as a tool to understand and minimize the PEFC performance loss, and to develop guidelines for optimal CL and MPL surfaces.

Chapter 1 – Durability of Polymer Electrolyte Fuel Cells: Status and Targets

Polymer electrolyte fuel cells have attracted significant interest from governments and similar organizations due to their promise as an alternative energy system in a wide range of applications. To promote and guide future research and development activities, these regulatory organizations have developed targets for key fuel cell performance parameters, namely; cost, efficiency, and durability. The technical targets developed by the United States, the European Commission, and Japan are presented to provide a context for subsequent chapters, which focus on specific durability concerns and mechanisms. In addition, cost and efficiency targets are presented due to the close coupling between performance and durability parameters.

Microstructure Analysis Tools for Quantification of Key Structural Properties of Fuel Cell Materials

The objective of this work is to develop advanced microstructure analysis tools for direct quantification of the key structural properties of complex fuel cell materials. Computationally efficient algorithms have been developed to extract the key structural parameters from measured microstructure datasets of these materials. In addition to determination of the traditional structural measures (e.g., porosity, surface area, phase connectivity), two novel microstructure analysis techniques are introduced for the quantification of pore size and tortuosity distributions. For initial demonstration purposes, the methods developed are applied to a digitally reconstructed sample of the micro-porous layer (MPL) of a polymer electrolyte fuel cell (PEFC). The results produced from these analyses are compared to previously reported experimental and model-derived values where applicable.