CO2 Reduction: study by chronopotentiometry

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As-deposited and after-test material characterization

Unless otherwise noted on the scientific papers of section III.3. , for most samples, X‐ray diffraction was performed with a PANalyticalX’pert Pro from Anton Paar with Cu– Kα1 radiation (λ 1.54056Å). The diffraction pattern was obtained by scanning between 20 and 80° by steps of 0.02 (2θ°) with a fixed counting time of 2.3 s and sample rotation of 3 rpm. Once the data were obtained, Scherrer analysis calculation was used to obtain the average crystal size of the deposited layer by determining the angular position and FWHM of the peaks between 28 and 60 2theta degrees. Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) analyses were performed with a ZEISS® Ultra 55 microscope to evaluate the surface morphology and to measure layer thicknesses. In some cases, Raman spectra were obtained in a Lab Raman HR‐800 from Horiba Jobin Yvon® spectrometer with a spectral resolution of 2 cm‐1 and Olympus® microscope using a 10X or 100X objective lenses. Spectra were recorded from 100 to 1800 cm‐1 with a He‐Ne laser of wavelength 632.81 nm and grid of 600 or 1800.
Spectrometer calibration was performed with Si (1 1 1). Some samples were also analyzed by X‐ray photoelectron spectroscopy (XPS), the X‐ray photoelectron spectra were recorded using a Thermo Scientific K‐Alpha X‐ray photoelectron spectrometer with a monochromator Al Kα X‐ray (hν 1486.61 eV) for sample excitation. An argon ion beam was used to prevent samples surface charging. The spectrometer energy calibration was obtained using Au 4f 7/2 and Cu 2p 3/2 photoelectron lines. The position of the adventitious carbon C 1s peak at 284.6 eV was used as an internal reference in each sample to determine the binding energies with an accuracy of ± 0.1 eV. The residual pressure in the analysis chamber was maintained below 1×10‐8 Torr, during data acquisition. The survey spectra of each concerned sample were obtained between 0 and 1000 eV. The spectra was collected and analyzed with Advantage 4.0 software.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) studies the response of an electrochemical system to the application of a periodic small amplitude alternating current (ac) signal. The measurements are based in acquiring data of the system response for various ac frequencies. That is why it is called impedance spectroscopy.
From electrical circuit theory, any electrical circuit could be expressed as a complex impedance by following Ohm’s and Kirchhoff laws and by taking the ( ) expressions of a resistance (R), a capacitance (1⁄ ) or an inductance ( ). Thus, for example, the impedance of a resistance connected in parallel with a capacitance will be expressed by: ( ) (II.1).
To extrapolate the circuit response to the phenomena taking place in an electrochemical cell, equivalent electrical circuits can be modeled to approximate the real impedance response of a given electrochemical system. Thus, interpreting the electrochemical behavior through distributed elements in an equivalent circuit is complex, but essential for interpreting impedance spectra. Ideally, a faradaic process will be composed of an electrolyte resistance Re, a double layer capacitance Cd, and a faradaic impedance ZF, the circuit that will represent those contributions is shown in Figure II.3-a). Impedance ZF can be decomposed in two components, a resistance accounting for the charge transfer resistance Rct and in the case of a semi-infinite diffusion process a Warburg element Zw defined as: [ / ( / )].

Modified cathodes and electrolyte

A half‐cell set up was used for testing the modified cathodes and electrolytes. The high‐temperature electrochemical cell was a single‐compartment crucible of dimensions 70 x 50 mm2 contained in an alumina Al2O3 reactor of dimensions 250 x 60 mm2, hermetically sealed by a stainless steel cover with a Viton™ O‐ring. The whole electrochemical set‐up shown in Figure II.7 consists on a cylindrical furnace k) that is used to heat up the carbonate mixture i) contained into an alumina crucible h). An alumina spacer j) is used to center the crucible in the hot zone of the furnace. A water cooled stainless steel cover g) closes the reactor hermetically. The cover has six leak free inlet/outlet ports to introduce alumina tubes of different diameters, through these ports the electrodes, thermocouples and gases are introduced. In this way, a controlled atmosphere is maintained into the reactor [92,93]. Temperature was controlled at a constant temperature of 650°C by means of a calibrated chromel/alumel thermocouple, the electrolyte was a mixture of lithium and potassium carbonates of high grade purity >98% (Sigma‐Aldrich®), in a proportion of 62:38 mol%. The standard cathode atmosphere was a mixture of Air/CO2 (70:30 mol%) of high grades purity (Air Liquide®) at 650°C and a pressure of 1 atm. A carbonate melt was prepared and stabilized 24 h at 650°C for each coated sample. After stabilizing the molten carbonate eutectic under the selected cathode atmosphere, samples were immersed in the melt and electrochemical measurements were performed. In order to determine the electrochemical performance of the coated samples, a three‐electrode system was used. The working electrode being the as‐deposited sample on porous nickel substrate of dimensions 10 x 10 x 0.5 mm, the counter electrode was a gold wire of 1mm diameter and the reference electrode a silver wire dipped into an Ag2SO4 (10‐1mol kg‐1) saturated (Li0.62‐K0.38)2CO3 eutectic in an alumina tube sealed by a porous alumina membrane. Electrochemical experimental data were collected using a potentiostat/galvanostat (AutoLab® PGSTAT‐302N).

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CO2 reduction in carbonate media

The same type of half‐cell set up was used to investigate CO2 reduction by means of chronopotentiometry, the electrode was a gold flag of of dimensions 10 x 10 x 0.5 mm and a graphite plate of dimensions 10 x 10 x 1 mm that were used alternately for each condition tested. The molten carbonates used as electrolytes were (Li0.62‐K0.38)2CO3, (Li0.52‐Na0.48)2CO3 and (Na0.56‐K0.44)2CO3. For each carbonate eutectic three atmosphere compositions were studied each with both gold and graphite electrodes, the compositions were 5, 10, 20, 30, 50 and 100 % CO2, the dilution gas was Ar. Gas control, potentiostat and gas quality used were the same as in previous section. Gold and Gas Composition graphite electrodes were polished to grain 2400 and then on 4000 followed by immersion in an ultrasonic bath (50:50 vol%, ethanol:water) for ten minutes. For the chronopotentiometric study, the imposed current was varied between 0.5 and 30 mA in steps of 0.5 or 1 mA per cycle, each current was imposed for a 30 s period. After each imposed current, a re‐equilibration time of 20 min was set before the next measurement.

Electrochemical performance of protective coatings

The as‐deposited samples were immersed into molten Li‐K eutectic carbonate mixture; for each sample, the crucible, reference and counter electrodes, and Li‐K eutectic were new to avoid crossed contamination. Figure III.4 shows the OCP evolution over 230 h immersion times for all the samples covered by a thin coating of around 50 nm. Thicker samples did not present a good OCP behavior and are not presented herein. Bare porous Ni cathode was used as reference; the black line shows the typical behavior of Ni immersed in eutectic Li‐K molten carbonates. Before Ni reaches its balance potential, around 0 V vs. Ag/Ag , where it is considered ready to work as MCFC cathode, it goes through a complex oxidation/lithiation process.

Table of contents :

I. Introduction
I.1. The molten carbonate fuel cell.
I.2. Molten Carbonate Fuel Cell Optimization
I.2.1. Cathode optimization
I.2.2. Electrolyte optimization
I.3. The aim of this thesis
II. Materials and Methods
II.1. Atomic Layer Deposition (ALD)
II.2. As-deposited and after-test material characterization
II.3. Electrochemical methods
II.3.1. Electrochemical Impedance Spectroscopy (EIS)
II.3.2. Chronopotentiometry and Chronoamperometry
II.3.3. Modified cathodes and electrolyte
II.3.4. CO2 reduction in carbonate media
II.4. Ni solubility measurements
II.5. Single-cell set-up
III. ALD processed coatings for MCFC cathode protection
III.1. Metal oxides deposited by ALD
III.1.1. Deposition parameters
III.1.2. Characterization of as-deposited materials
III.1.3. After tests characterization
III.1.4. Electrochemical performance of protective coatings
III.1.5. Ni solubility
III.2. Conclusions
III.3. Scientific papers
IV. Electrolyte modification with Cs and Rb additives
IV.1. Theoretical approach
IV.1.1. Reaction-rate equations for oxygen reduction
IV.1.2. Nishina’s reaction order analysis.
IV.2. Oxygen and CO2 kinetics in Cs modified electrolyte
IV.2.1. Data analysis by Nishina’s diffusion cases
IV.2.2. The charge transfer resistance dependency on and
IV.3. Porous NiO behavior in Cs and Rb modified electrolytes
IV.4. Conclusions
IV.5. Scientific papers
V. CO2 Reduction: study by chronopotentiometry
V.1. The importance of CO2 reduction in molten carbonates
V.2. Synthesis of previous works by cyclic voltammetry
V.3. Chronopotentiometry on Gold and Graphite electrodes
V.3.1. Chronopotentiometric measurements
V.3.2. Current reversal chronopotentiometry (CRC)
V.3.3. Transition time analysis
V.4. Chronoamperometry
V.5. Conclusions
V.6. Scientific papers
VI. Single-cell tests in fuel cell mode
VI.1. General concepts
VI.1.1. MCFC operated with hydrogen and CO
VI.1.2. MCEC for CO2 reduction
VI.2. Cell assembly and operation
VI.2.1. The gas recovery flange concept and design modification
VI.2.2. Tape-cast of carbonate eutectic
VI.2.3. Calculating the carbonate/matrix ratio
VI.3. Electrochemical measurements on reference cell
VI.3.1. OCP
VI.3.2. I-E curves and fuel cell output power
VI.3.3. EIS
VI.4. Conclusions
General conclusions and prospects
Bibliography

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