Walter Mérida


Relevant Degree Programs


Postdoctoral Fellows

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Cost optimization of hydrogen fuel supply chain with environmental policy integration: the case for British Columbia (2020)

By powering fuel cell electric vehicles hydrogen can contribute to greenhouse gas emissions reduction in British Columbia (B.C.) The province is well positioned to capitalize on its natural resources and policies towards the development of a hydrogen fueling supply chain (HFSC). However, such development requires significant investment with high risks of negative cash flow for years to decades.A spatially explicit multi-period optimization model was developed to design a minimum-cost HFSC based on a mixed integer linear programming formulation. The model was applied to the light duty passenger vehicle sector in B.C. under three hydrogen demand scenarios. The model considered different capacities for all components of the supply chain, covered the on-site production and capacity expansion options as well as minimum storage requirement for fueling stations. Different combinations of the current and potential environmental mandates and the government economic instruments were integrated in the model explicitly. The model measured the effectiveness of the policies on reducing the cost and greenhouse gas (GHG) emissions of the HFSC for each demand scenario. To this end, the GHG emissions were monetized using the social cost of carbon. The results suggested that hydrogen can be cost competitive with gasoline. However, the cost optimal hydrogen infrastructure relied heavily on steam methane reforming (SMR), with small GHG emissions reduction benefits. Nonetheless, the monetary benefits of well to wheels (WTW) GHG emissions reduction justified the switch from gasoline to SMR-based hydrogen. It was found that central electrolysis can be financially justified by addition of production tax credits or electricity incentives to the current provincial carbon control policies (i.e., carbon tax and low carbon fuel standard).This study assessed the effectiveness of current policies in emissions mitigation from the road freight transport. Moreover, the WTW energy requirement and GHG emissions reduction potential of the all-electric trucking were measured to meet the provincial emissions reduction targets. The results suggested that the B.C. hydroelectricity will fall short of generating sufficient energy to support all-electric trucking. Thus, B.C. has to undertake policies to incentivize electricity generation from diversified renewable energy resources.

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Electrospun nanofibrous membranes for water vapour transport applications (2017)

This study discusses the development of selective water vapour permeable membranes for the separation of water vapour from air. These membranes are useful as separation media for membrane-based energy recovery ventilation devices. Current generation composite membranes for these devices consist of a polymer film layer which is permeable to water vapour but selective for water vapour over gases. In these composite membranes, this film layer is attached to one surface of a microporous polymeric support substrate. However, as it is demonstrated in this work, the microporous support contributes 30 to 50% of the resistance to water vapour transport in the composite membranes.In an attempt to eliminate this microporous substrate and its associated resistance, a membrane was developed using an electrospun nanofibrous layer as a support structure for the selective coating layer. In these membranes, the electrospun nanofibers are deposited on a non-woven micro-fibrous carrier and then the nanofibers are impregnated with a selectively permeable polymer to make impregnated electrospun nanofibrous membranes (IENM). The nanofibers are found to contribute a resistance to vapour transport in these membranes and their effect on water vapour permeability is quantified through a fiber-filled-film model. An optimization study of the IENMs demonstrated that fiber diameter and fiber volume in the film layer effect the water vapour permeance of these membranes and reducing these variables improved water vapour permeance. IEMNs were demonstrated to have water vapour permeance (>10000 GPU) while still having sufficient selectivity for the application, exceeding the performance of current generation composite membranes.The membranes were demonstrated to be particularly useful in the fabrication of ‘formable membranes’ which could be thermally-formed into exchanger plates for energy recovery ventilator exchangers. Thermal and mechanical properties of the membrane components were reported individually and as complete membranes. A membrane composition was demonstrated to fabricate exchanger plates from formable IENMs.This work contributes to the development of membranes for air-to-air exchangers specifically for energy recovery ventilation. A new class of membrane based on electrospun nanofibers for these devices was successfully demonstrated to have improved performance properties and a novel formable membrane was developed.

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New Design Tools and Characterization Methods to Study Polymer Electrolyte Membrane Fuel Cell Degradation (2016)

An increasing energy demand and the need to reduce greenhouse gas emissions promotes a need for sustainable energy systems. Polymer-electrolyte-membrane fuel cells, comprising a carbon-platinum catalyst layer (CL), have reached (pre-)commercial viability. However, performance, durability, and cost targets are not entirely met yet. In a larger perspective, this thesis aims to contribute to the development of novel catalyst quality monitoring methods, gradual catalyst loading strategies, and the link between water management and catalyst degradation. The objectives of this thesis are to design tools and develop methods of investigation for catalyst degradation, characterize different accelerated stress test (AST) protocols, indicate stressors of the occurring degradation, deconvolute the performance decay, and to analyze the locally occurring degradation. Carbon corrosion and platinum dissolution are the major degradation mechanisms during the operation. Start-up and shut-down (SU/SD) events where air purges through the hydrogen containing anode, accelerates the progression of these mechanisms, and amplified performance decay results. This study develops an ex-situ tool and a method to link the water content within the CL to the degradation behavior. Higher water content may accelerate CL failure. The comparison of different SU/SD ASTs revealed the degradation behavior and the influence of various mitigation strategies to reduce the occurring voltage decay. The application of an external resistance resulting in the consumption of electrons reduces the degradation. If the voltage is suppressed by applying an external load, the degradation in high current densities is marginal. The deconvolution of the voltage decay at high current densities reveals the ratio of carbon to non-carbon related losses. The non-carbon related losses remain dominant throughout the initial stages of the AST; later the carbon corrosion related losses become significant. The novel multi-channel-characterization-system revealed larger losses towards the cell’s exit during SU/SD and platinum dissolution ASTs. Chemical platinum degradation caused by higher water content towards the exit dominates. Longer residence times of oxygen within the anode promote degradation towards the exit. In conclusion, this study developed novel diagnostics, assessed the ratio of performance decay related to its mechanisms, estimated a dominance of the non-carbon related losses throughout the initial phase, while localizing the degradation.

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Novel Transport Layer Characterization and Synthesis for Proton Exchange Membrane Fuel Cells (2016)

Fuel cells are a promising energy conversion technology compatible with developing renewably sourced primary energy distribution. Proton exchange membrane (PEM) fuel cells are particularly suitable for automotive and portable applications. The present thesis advances novel PEM fuel cell porous transport layer (PTL) characterization and materials research. These layers link macro and nano scales by mediating energy and mass transport between reactant distribution channels and catalyst layers. Contemporary commercial PTLs are limited in selection. Moreover, typical characterization methods ignore essential material anisotropy. Herein, a novel transport layer synthesis concept is introduced. By adapting electrospinning technology, structures with engineered morphology are created. PTLs are produced with fibre diameters from 0.2 to 1.6 µm, and are characterized experimentally ex-situ and in-situ. Electrospun PTLs are shown to deliver 85% of equivalent commercial PTL current densities. Furthermore, the state-of-the-art for electronic resistance measurement of PTLs is improved, with rigorous attention given to the anisotropy of the fibre-based media. Novel method and apparatus provide this information as a function of mechanical strain. PTL in-plane resistivities are a unique contribution, where for commercial materials 4.5x10-⁴ to 1.5x10-⁴ Ω∙m are observed for strains from 0.0 to -0.5 m∙m-¹. Finally, electrospun PTLs are developed to investigate the effect of within-plane anisotropy upon fuel cell performance. Electrospun layers are produced with progressively greater fibre alignment to effect anisotropy. This anisotropy is visualized via microscopy, and quantified using the aforementioned electronic resistivity methods. In-situ results with electrospun PTLs, of anisotropy ratios from 1 to 6, suggest greater performance with average fibre alignment perpendicular to gas distribution channels. The present thesis’ contributions strengthen development of a PTL structure-property-performance relationship. With integration into a cell-level relationship, this can empower rational PEM fuel cell design.

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High Frequency Data Analysis for Wind Energy Applications (2015)

High frequency data (HFD) of three site studies in different geographic locations were analyzed to reproduce the power spectrum illustrated by Van der Hoven in 1957. His work represents the basis of wind energy standards such as averaging and variability in the frequency domain. The results presented in this thesis unveil discrepancies with Van der Hoven’s approach. A major eddy-energy peak is illustrated at a period of 2 days and a smaller eddy-energy peak contribution at frequencies higher than the region known as the spectrum gap. The variance in the microscale region was calculated by integrating the Power Spectral Density (PSD) over the corresponding range of frequencies. The economic value of this energy variance based on the turbulence kinetic energy of the wind data set is calculated. It is also concluded that, given the results of the study, HFD analysis in the frequency domain uncovers eddy-energy peaks that determine energy fluctuations in the short and long terms. An algorithm was developed to detect delay times in the turbulence kinetic energy (TKE) and the energy dissipation rate ε on a continuous basis (thereby identifying the highest cross-correlation coefficients between them). The Kolmogorov turbulence order is applied to calculate the energy dissipation rate ε through the identification of the inertial subrange. The time scale in the variations of both parameters was successfully calculated and it is close to the time the air takes to circulate between the surface and the top of the atmosphere’s mixed layer. High correlation coefficients are found in the three site studies from 4am to 8am, and from 8pm to 12pm. The cross-correlation function also determines delay time scales in the range of 10-20 minutes and approximately 2 hours. The energy dissipation rate can be calculated to characterize wind variability in a particular site that might affect the performance of a wind turbine. With these results, more information is generated that can be used in the wind turbine’s control system routines to improve its response under wind turbulence variations.

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New Methodologies to Characterize PEMFC Performance Losses (2015)

As fuel cells reach the early stages of commercialization, the need for effective research methods and efficient characterization tools is increasing. Industrial demands allowing for serial production require methods for standardized testing and efficient quality control while developing new materials and production methods. This thesis work presents two methodologies: Design of Experiments (DoE) applied to Polymer Electrolyte Membrane Fuel Cells (PEMFCs) to analyze for sparsity of effects and performance mapping; and a systematical Voltage Loss Breakdown (VLB) method based on experimental polarization featuring anodic contributions. Both concepts were validates and the effects of commercial porous transport layer (PTL) material on the fuel cell’s voltage under galvanostatic conditions were investigated. The results of this work demonstrate the use of DoE to assess the differences and parameter dependencies of different materials in the PTL of PEMFC. Split-plot and general blocked design models were used to analyze the voltage and pressure drop of PEMFC at different current densities of 1.0 A cm^-², 1.4 A cm^-² and 1.6 A cm^-². The empirical models show good fit and prove that these methodologies based on experimental designs can be useful to predict and analyze fuel cell performance within this design space. The use of designed experiments allows a scientifically objective analysis of the data compared to one-factor-at-a-time (OFAT) testing while reducing the overall required test runs. Our results show, that this analysis can capture and model the effects of PTL materials and operating conditions as statistically and physically significant. The VLB method developed in this work systematically analyzes the different dominant loss contributions and shows the relevance of the anode under varying operating conditions. A reference electrode system was designed and validated in order to measure the anodic and cathodic contributions to the cell’s polarization separately. A mathematical approach was developed to break down the polarization curve into the individual contributing losses, distinguishing between anode and cathode and the individual kinetic, ohmic and mass-transport overpotentials. Based on this study it can be concluded, that a micro-porous layer (MPL) leads to reduced mass-transport losses inside the cathode electrode and decreases the ohmic losses.

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New approaches to fuel cell diagnostics (2011)

The durability and reliability of fuel cell products need to be improved. The lack of early diagnosis and failure-prevention techniques is one of the limiting factors. This thesis presents a non-invasive method for the early diagnosis of flooding, dehydration and low fuel stoichiometry (three common failure modes). The method is based on micro sensing electrodes (SE) that are placed at appropriate locations in a single cell. These electrodes have a characteristic potential response to each of the failure modes, which enables detection prior to overall fuel cell failure. The specific features in the measured responses (or combinations thereof) can be used to discern between different failure modes, and initiate corrective actions.This thesis also reports on the separation of anodic and cathodic potentials in a working fuel cell via reference electrodes maintained at constant conditions. The reference electrodes consisted of four platinized platinum electrode wires and two patches of the same catalyst layer used in the anode. All the reference electrodes were unaffected by the operating conditions of the fuel cell and those with patches provided the most stable potentials. Individual anodic and cathodic overpotentials (activation, ohmic, concentration and mass transport) were obtained in a segmented and un-segmented fuel cell for the first time. An array of reference electrodes and gases with different diffusion coefficients were used to discern the different overpotentials. The results show that the anodic overpotentials cannot be ignored, even if the conditions are changed at the cathode only. The oxygen concentration has an effect on the anode and in particular hydrogen oxidation and proton flux. Under dry conditions the current in-plane gradients are very large and the heat generation profiles are affected, creating an uneven temperature distribution in the catalyst layer due to concurrent effects of the half-cell reactions and the water vaporization.The combination of reference electrodes and multi-component gas analysis, allows the measurement and calculation of kinetic and diffusion parameters that can be used for modeling and to understand the behavior of different layers of a fuel cell.

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Water transport in proton exchange membranes (2008)

Water transport across Nafion membranes was investigated under activitygradients at atmospheric pressure. The activity gradients across the membrane werecontrolled by exposing one side of the membrane to dry gas under laminar flow, whilemaintaining liquid or vapour equilibrium with water on the other side of the membrane.The measurements were made under steady state and transient conditions.The main objective was to identify the rate limiting mechanism among the threemajor water transport processes in Nafion: diffusion in the bulk, and sorption across bothinterfaces. The proposed hypothesis was to represent the overall water transport acrossthe membrane as the sum of resistances across the membrane bulk and interfaces.The experimental implementation required new hardware and techniques. A dualchamber cell with temperature, humidity, and pressure control was designed with a gatedvalve to control the initial starting time for transient measurements. Unlike previouslyreported work, this design enabled the individual control of membrane thickness,temperature, pressure, relative humidity, and dry gas flow rate in each chamber.The ability to control these variables made the experimental results amenable totheoretical simulations. Two phenomenological models were proposed to separate thecontributions of bulk transport and sorption processes. The Varying Diffusion Coefficientmodel (VDC) was a preliminary effort to generate mass transport coefficients. TheVaporization-Exchange Model (VEM) provided an approximation for the steady stateand transient water transport data through the definition of a novel boundary conditionthat describes the kinetics at the interface, while diffusion is described by Fick’ s law.The VEM yielded interfacial water transport rates:kv= 0.75 cms⁻¹ for liquid-, andkv= 0.63 cms⁻¹ for vapour-equilibrated membranes. Such results contribute to fill the gapfor the membrane interfacial kinetics in the fuel cell literature. The analysis with theVEM revealed that interfacial water transport became rate limiting at membranethickness below Ca. 100 μm.Analysis of transient data with VEM generated bulk diffusivity coefficients: D2-7x 10⁻¹° m²s⁻¹, for liquid equilibrated membranes at 30-70°C, which agreed with literaturedata.A case study is presented for Nafion-Si0₂ composite membranes to study theeffect of the membrane water content, by addition of silicon dioxide to Nafionmembranes. The status of water in the membrane was characterized with long-establishedtechniques, such as vapour sorption, scanning calorimetry, and water uptakemeasurements. Information from these measurements was coupled with measurementsfrom the dual chamber cell. Experimental results indicated a threshold at 16 wt% ofsilicon dioxide above which the water transport properties of the composite differedsignificantly from additive-free Nafion. Analysis with the VEM suggested that theaddition of the composite produced structural changes to the polymer matrix.

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Master's Student Supervision (2010 - 2018)
Electrochemical evaluation and modulation of surface wettability (2018)

Smart surfaces with dynamic and reversible wettability offer much versatility to meet various application needs. However, application of the smart materials remains challenging, mostly due to complex preparation procedure, a small change in the contact angle, and slow wettability switching. A thorough understanding of wetting phenomena is needed to achieve a breakthrough in the design of superwettability systems.This thesis describes an electrochemical technique to evaluate the surface wettability of rough surfaces based on the wetted area under the droplet. The proportionality between the double layer capacitance and the ion accessible solid-liquid interfacial area is exploited to determine the actual wetted area. While the contact angle fails to describe the wetting mode, the electrochemical approach is capable of discerning the effective wetted area. The electrochemical wettability metric is also used to understand an anomalous wetting behaviour at which the contact angle and interfacial area of an intrinsically hydrophilic substrate show a concurrent increase with roughness. These observations contradict predictions from the Wenzel and Cassie- Baxter models. Based on a coupled optical and electrochemical analysis, the limitations of contact angle are highlighted.An in-situ control over the wettability of electrodeposited copper structure through electrochemical modulation of oxidation state is demonstrated for the first time. Precise control over the rate and extent of the wetting transition is achieved by tuning the magnitude and duration of the applied voltage. Moreover, air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min restores the initial superhydrophobicity. Microstructural and electrochemical analysis show that the active wetting control is based on the Faradaic phase transformation of the surface-bound CuxO phase shielding the Cu core. Based on the wetting switching functionality of the Cu-CuxO core-shell structures, a smart oil-water membrane is designed. The as-deposited or air-dried Cu mesh exhibits superhydrophobicity and superoleophilicity, thus is effective for heavy oil-water separation. On the other hand, the application of a small reduction voltage (
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Tunability of photogenerated charge carrier density on semiconductors by in-situ electrochemical treatments (2018)

With fossil fuels increasingly being exhausted and environment pollutions getting worse, it is necessary for our generation to look for sustainable, renewable and environmentally safe alternative energy sources. Solar energy is so far the most available resource, with around 120,000 TW of solar energy striking the surface of the earth. However, sunlight is just available in the daytime, can change within hours or seasons, and it is spread over low-density collection areas. An efficient way of energy storage is required for the utilization of solar energy. So far, one of the most practical ways to store a significant amount of energy is through a chemical energy carrier. Hydrogen fuel is one of the prime candidates as a future energy carrier which is environmentally friendly during its production, delivery, and consumption. Hydrogen production by photoelectrochemical water splitting using a semiconductor catalyst could be one of the most promising ways to harvest solar energy. In this thesis, an in-situ potentiodynamic approach (cyclic voltammetry) was used to modify the photoelectrocatalytic properties of nanostructured electrodes in different media. The effect of the morphology was studied by comparing TiO₂ nanotube and nanorod. Also, the influence of the modification on WO₃, ZnO materials was evaluated. The photogenerated charge carrier separation was studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and Mott-Schottky plots. The morphologies of the samples were tested by scanning electron microscope (SEM). X-ray diffraction (XRD) was used to analyze crystallinity. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemical composition. The experimental data proved that electronic properties could be changed by the self-doping process, hence, improving the optical absorption properties and increase charge transfer rates. In this way, the photoelectrocatalytic activity of semiconductors was enhanced. The observed behaviors from electrochemical measurements suggested that morphology has a vital role in the capacitive properties. A semiconductor tailored via band structure modification indicates that the electrochemical treatment can be a systematic and straightforward technique for developing novel photoelectrocatalysts with enhanced performances under visible light.

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Improved Characterization and Movement of the Platinum Band in a Proton Exchange Membrane Fuel Cell (2016)

Proton exchange membrane (PEM) fuel cells are devices that produce zero emission electricity, offering one pathway towards a sustainable energy future. They are in the early stages of commercialization in automotive, backup power and mobile power applications, with cost and lifetime of the cells still representing major barriers. Platinum (Pt) degradation in a PEM fuel cell leads to reduced performance and lifetime, while raw material costs represent a large portion of the overall cost. One of the Pt degradation modes leads to Pt dissolving from the cathode and precipitating in the membrane, forming a “Pt band”. The Pt band is disconnected and unused Pt. If this Pt could be moved, it might be able to be returned to the cathode and be made useful again. This Pt can also have effects on membrane degradation.Pt bands were created in two unique locations using accelerated stress tests (AST) of 10,000 square wave potential cycles from 0.6-1.0V. The locations of the bands were accurately predicted using an existing model and are dependent on the concentrations of oxygen and hydrogen in the membrane. It was hypothesized that increasing the oxygen concentration around the Pt particles in the membrane, would lead to dissolution and movement of the Pt. A new more quantified analysis of the Pt band using SEM imaging is implemented to measure the Pt movement and more fully characterize the Pt band. After two experiments, one trying to move the Pt for 28h and another for 100h, no Pt movement was observed. Another experiment created 2 Pt bands in the same membrane and characterized them for the first time. The second band formed in this experiment did not influence the first band, nor was it influenced by the first band during its formation. The new characterization techniques demonstrated that the distributions of Pt mass and particles were different for the bands formed in the two different locations, while the dual band displayed a superposition of these distributions. The differences distributions have never been quantified before and could have different effects on membrane degradation, which should be the focus of future work.

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Interfacial morphology and contact resistance between the catalyst and micro porous layers in PEM fuel cells (2016)

The interface between the catalyst layer (CL) and the micro porous layer (MPL) in proton exchange membrane fuel cells (PEMFCs) has been studied in ex-situ experiments. The interfacial morphology, specifically the area, origin and dimensions of interfacial gaps in between compressed CLs and MPLs were investigated with high-resolution X-ray micro computed tomography. In a separate experiment, the electric contact resistance (CR) was evaluated using a custom four-point-probe setup for CLs with different compositions as a function of compression pressure and relative humidity (RH). The interfacial gap area (fraction of the interface separated by gaps) was higher for gas diffusion layers (GDL, with MPL) – catalyst coated membrane (CCM) assemblies with large differences in the surface roughness of the CL and MPL. The interfacial gap area decreased with increasing compression and with increased similarity in roughness. Relatively large continuous gaps were found in proximity of specific cracks in the MPL. These are hypothesized to form due to the presence of large pores on the surface of the GDL, in which the MPL sags and cracks. Relatively small gaps form by means of the regular surface roughness features throughout the CL-MPL interface. Smaller pores on the GDL surface serving as substrate for the MPL could reduce the number of MPL crack-induced gaps. Moreover, adjusting the CL and MPL surface roughness parameters to achieve similar orders of roughness can result in fewer enclosed gaps, and therefore, enhance the mating characteristics. The electric CR followed a similar trend for all the CL compositions, featuring a non-linear decrease in resistance with the increase in the compression pressure. Moreover, the CR was also found to increase with the ionomer content in the CL and with the increase in RH. Physical characterization of the CL surfaces revealed that this increase in the ionomer content enhances the surface roughness features and the surface coverage by the ionomer, both of which affecting the electrical CR towards the MPL. With increasing RH, the CR values doubled for all CL compositions as a result of humidity induced ionomer swelling with the uptake of water.

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