Scaling up of the hybrid direct carbon fuel cell technology

A hybrid direct carbon fuel cell (HDCFC), combining molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) technologies, is capable of converting solid carbon directly into electrical energy without intermediate reforming. The performance level achieved on small-scale cells (area <4 cm²) suggests that engineering developments should now be undertaken to scale up and demonstrate the feasibility of practical systems. The scaling up of the HDCFC through the design and test of single stack repeat unit with realistic cell sizes was investigated in this study. A single cell of ∼12.56 cm² active area produced a maximum power of ∼1.2 W at 800 °C and a current density of ∼200 mA cm² at 0.6 V, using wood-based pyrolyzed medium density fiberboard (p-MDF) as fuel. In comparison, the HDCFC with activated carbon as fuel produced a maximum power density of 36 and 53 mW cm⁻² at 700 and 800 °C, respectively, and an electric efficiency of ∼40% evaluated under 0.7 V for 17 h at 700 °C. These results demonstrated the applicability of HDCFC to practical systems while stack units were operated in batch mode and an appropriate fuel feeding mechanism has to be designed. Moreover, more engineering advances should be done to enhance power output since a HDCFC stack unit involves multiple challenges that have not been addressed yet, including system configuration and corrosion protection, and durability.     

Microstructure and electrochemical characterization of solid oxide fuel cells fabricated by co-tape casting

A co-tape casting technique was applied to fabricate electrolyte/anode for solid oxide fuel cells. YSZ and NiO-YSZ powders are raw materials for electrolyte and anode, respectively. Through adjusting the Polyvinyl Butyral (PVB) amount in slurry, the co-sintering temperature for electrolyte/anode could be dropped. After being co-sintered at 1400 °C for 5 h, the half-cells with dense electrolytes and large three phase boundaries were obtained. The improved unit cell exhibited a maximum power density of 589 mW cm⁻² at 800 °C. At the voltage of 0.7 V, the current densities of the cell reached 667 mA cm⁻². When the electrolyte and the anode were cast within one step and sintered together at 1250 °C for 5 h and the thickness of electrolyte was controlled exactly at 20 μm, the open-circuit voltage (OCV) of the cell could reach 1.11 V at 800 °C and the maximum power densities were 739, 950 and 1222 mW cm⁻² at 750, 800 and 850 °C, respectively, with H₂ as the fuel under a flow rate of 50 sccm and the cathode exposed to the stationary air. Under the voltage of 0.7 V, the current densities of cell were 875, 1126 and 1501 mA cm⁻², respectively. These are attributed to the large anode three phase boundaries and uniform electrolyte obtained under the lower sintering temperature. The electrochemical characteristics of the cells were investigated and discussed.     

Electrochemical oxidation of graphite in an intermediate temperature direct carbon fuel cell based on two-phases electrolyte

As a promising intermediate temperature fuel cell, Direct Carbon Fuel Cell (DCFC) with composite electrolyte composed of Samarium-Doped Ceria (SDC) and a binary carbonate phase (67 mol% Li₂CO₃/33 mol% Na₂CO₃) has a much higher efficiency compared with conventional power suppliers. In the present work, SDC powder has been synthesized by an oxalate co-precipitation process and used as solid support matrix for the composite electrolyte. Single cell with composite electrolyte layer is fabricated by a dry-pressing technique using LiNiO₂/Li₂Na₂CO₃/SDC as cathode and 1:9 (weight ratio) graphite mixture with 67 mol% Li₂CO₃/33 mol% Na₂CO₃ molten carbonate as anode. The cell is tested at 600–750 °C using electrolytical graphite mixture as fuel and O₂/CO₂ mixture as oxidant. A relatively good performance with high power density of 58 mW cm⁻² at 700 °C is achieved for a DCFC using 0.8 mm thick composite electrolyte layer. The sensibility of the 1 cm² DCFC single cell performance to the anode gas nature is also investigated. At temperatures higher than 700 °C, both carbon (C) and carbon monoxide (CO) can be considered as reacting fuel for the DCFC system.     

A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte

A composite electrolyte containing a Li/Na carbonate eutectic and a doped ceria phase is employed in a direct carbon fuel cell (DCFC). A four-layer pellet cell, viz. cathode current collector (silver powder), cathode (lithiated NiO/electrolyte), electrolyte and anode current collector layers (silver powder), is fabricated by a co-pressing and sintering technique. Activated carbon powder is mixed with the composite electrolyte and is retained in the anode cavity above the anode current collector. The performance of the single cell with variation of cathode gas and temperature is examined. With a suitable CO₂/O₂ ratio of the cathode gas, an operating temperature of 700 °C, a power output of 100 mW cm⁻² at a current density of 200 mA cm⁻² is obtained. A mechanism of O²⁻ and CO₃²⁻ binary ionic conduction and the anode electrochemical process is discussed.     

A high performance composite ionic conducting electrolyte for intermediate temperature fuel cell and evidence for ternary ionic conduction

The performance of a composite electrolyte composed of a samarium doped ceria (SDC) and a ternary eutectic carbonate melt phase was examined. The formation temperature of a continuous carbonate melt phase is crucial to the high conductivity of this material. The electrolyte contains 30 and 50 wt% carbonate exhibited a sharp increase of conductivity at a temperature close to the melting point of the eutectic carbonate, ca 400 °C, which is more than 100 °C lower than those electrolytes using binary carbonate. At around 650 °C, and with CO₂/O₂ used as the cathode gas, the fuel cell gave a power output 720 mW cm⁻² at a current density 1300 mA cm⁻². Water was measured in both the anode and cathode outlet gases and CO₂ was detected in the anode outlet gas. When discharged at 800 mA cm⁻², a stable discharge plateau was obtained. The CO₂ in the cathode gas enhances the power output and the stability of the single cell. Based on these experimental facts, a ternary ionic conducting scheme is proposed and discussed.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

Fabrication and characterization of a cathode-supported tubular solid oxide fuel cell

A cathode-supported tubular solid oxide fuel cell (CTSOFC) with the length of 6.0 cm and outside diameter of 1.0 cm has been successfully fabricated via dip-coating and co-sintering techniques. A crack-free electrolyte film with a thickness of ∼14 μm was obtained by co-firing of cathode/cathode active layer/electrolyte/anode at 1250 °C. The relative low densifying temperature for electrolyte was attributed to the large shrinkage of the green tubular which assisted the densification of electrolyte. The assembled cell was electrochemically characterized with humidified H₂ as fuel and O₂ as oxidant. The open circuit voltages (OCV) were 1.1, 1.08 and 1.06 V at 750, 800 and 850 °C, respectively, with the maximum power densities of 157, 272 and 358 mW cm⁻² at corresponding temperatures.     

Preparation of supported SrCeO3-based membrane by spin coating method

SrCe_0.9Y_0.1O_3−δ (SCY10) powder with a pure perovskite phase is prepared by solid-state reaction method. NiO is dispersed uniformly in SCY10 powder to fabricate NiO-SCY10 anode substrate. The starting powder, the mixture of SrCO₃, CeO₂ and Y₂O₃, is deposited directly on the green substrate instead of SCY10 powder by spin coating. After co-firing at 1300 °C for 3 h, the starting powder reacts to form SCY10 top layer on the substrate. SEM micrographs show that the top layer is defect-free and adheres well with the anode substrate without any delamination. A single fuel cell is assembled with anode-supported SCY10 membrane as electrolyte membrane and Ag as cathode. The electrochemical property of the fuel cell is tested with hydrogen as fuel in the temperature range of 600-800 °C. The open circuit voltage (OCV) reaches 1.05 V at 800 °C, and the maximum power density is 50 mW cm⁻², 155 mW cm⁻², 200 mW cm⁻² at 600 °C, 700 °C, 800 °C, respectively.     

Study of slurry spin coating technique parameters for the fabrication of anode-supported YSZ Films for SOFCs

A slurry spin coating method was developed to fabricate gas-tight anode-supported YSZ films for solid oxide fuel cells (SOFCs). Several technique parameters for slurry spin coating, such as the slurry viscosity, spinning speed, number of coating cycles, film thickness and their effects on YSZ electrolyte film were investigated. SEM results, open-circuit voltage (OCV) values and cell performance indicated that these parameters had crucial and obvious influences on YSZ film quality and fuel cell performance. Based on the optimized parameters, anode-supported YSZ films and several single fuel cells were successfully fabricated and tested. An OCV as high as 1.06 V was obtained at 800 °C and maximum power densities of 900, 1567, 2005 mW cm⁻² were achieved at 700, 750, 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Performance evaluation of tubular fuel cells fuelled by pulverized graphite

A fuel cell fuelled by carbonaceous graphite is proposed. The tubular fuel cell, with the carbon in a fixed-bed form on the anode side, is employed to convert directly the chemical energy of carbon into electricity. Surface platinum electrodes are coated on the cell electrolyte, which is a yttria-stabilized zirconia (YSZ) tube of 1.5 mm thickness. The effect of using different sizes of graphite powder (in the range 0-180 μm) as fuel is analyzed. Power density and actual open-circuit voltage (OCV) values are measured as the temperature is varied from 0 to 950 °C. The cell provides a maximum power density of 16.8 mW cm⁻² and an OCV of 1.115 V at the highest temperature condition (950 °C) tested in this study.     

Performance of direct carbon fuel cell

A direct carbon fuel cell (DCFC) is a variation of the molten carbonate fuel cell (MCFC) which converts the chemical energy of carbon directly into electrical energy. Thus, the energy conversion efficiency is very high and correspondingly CO₂ emission is very low for given power output. DCFC as a high temperature fuel cell performs better at elevated temperatures (>800 °C) but because of the corrosive nature of the molten carbonates at elevated temperatures the degradation of cell components becomes an issue when DCFC is operated for an extended period of time.We explored the DCFC performance at lower temperatures (at 700 °C and less) using different sources of carbon, different compositions of electrolytes and some additives on the cathode surface to increase catalytic activity. Experiments showed that with petroleum coke as a fuel at low temperatures the ternary eutectic (43.4 mol % Li₂CO₃ - 31.2 mol% Na₂CO₃ - 25.4 mol % K₂CO₃) spiked by 20 wt % Cs₂CO₃ performed better than any binary or ternary eutectics described in the published work by other researchers. Maximum power output achieved at 700 °C was 49 mW/cm² at a current density of 78 mA/cm² when modified cathode was fed with O₂/CO₂ gases.     

Improvement of the performances of tubular solid oxide fuel cells by optimizing co-sintering temperature of the NiO/YSZ anode-YSZ electrolyte double layers

The effects of co-sintering temperature on anode microstructure, electrolyte film microstructure, and final cell performance of tubular solid oxide fuel cells (SOFCs) were fully studied. The co-sintering of the NiO/YSZ anode-YSZ electrolyte double layers at temperature ranging from 1350 to 1400 °C for 5 h was carried out. Porosity and electrical conductivity were measured to examine the anodes microstructure, and the electrolyte films microstructure were characterized by scanning electronic microscope (SEM). A higher open current voltage (OCV) value of 0.99 V was achieved by co-sintering the cell at 1400 °C indicating denser electrolyte film, while the maximum power density of the cell co-sintered at 1380 °C was achieved with 322 mW cm⁻² at 800 °C, which was higher than that (241.3 mW cm⁻²) of the cell co-sintered at 1400 °C because of better anode microstructure.     

Performance improvement of direct carbon fuel cell by introducing catalytic gasification process

In this paper, the effects of catalytic gasification on the solid oxide electrolyte DCFC (direct carbon fuel cell) performance are experimentally investigated and analyzed using K, Ca, Ni as catalyst in carbon black and controlling the temperatures of cell and carbon black at 750 °C and 700-1000 °C, respectively. The average power densities are 976, 1473 and 1543 W m⁻² respectively for 900, 950 and 1000 °C pure carbon black gasification. Catalytic gasification improves the DCFC performance significantly. For the same performance of pure carbon black, the gasification temperatures decrease about 200, 130 and 150 °C with K, Ca and Ni additives, respectively. The catalytic effects for carbon black gasification with CO₂ are: K > Ni > Ca. For typical identical temperature DCFC operating at 750 °C, the power densities of 0.7 V discharging are 1477, 1034 and 1123 W m⁻² for the carbon black with K, Ca and Ni additives, respectively. It is possible to reduce the operation temperature of DCFC to the medium temperature range of solid oxide electrolyte (600-800 °C) by introducing catalytic gasification process.     

Behavior of 3 mol% yttria-stabilized tetragonal zirconia polycrystal film prepared by slurry spin coating

A dense and uniform 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3YSZ) electrolyte film of 6 μm in thickness was fabricated by slurry spin coating on a porous NiO/3YSZ anode substrate. Composite cathodes of La_0.7Sr_0.3MnO₃ impregnated with Sm_0.2Ce_0.8O_1.9 were fabricated on the 3YSZ films. A single cell produced in this way was tested at 700, 750 and 800 °C with hydrogen as fuel and stationary air as oxidant. Test results revealed an open-circuit voltage of 1.04 V at 800 °C, and maximum power density of 551, 895 and 1143 mW cm⁻² at 700, 750 and 800 °C, respectively. Impedance spectra results demonstrated that the cell performance was determined by the polarization resistance of the cathode.     

Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells

Highly active and stable carbon composite catalysts for oxygen reduction in PEM fuel cells were developed through the high-temperature pyrolysis of Co–Fe–N chelate complex, followed by the chemical post-treatment. A metal-free carbon catalyst was used as the support. The carbon composite catalyst showed an onset potential for oxygen reduction as high as 0.87 V (NHE) in H₂SO₄ solution, and generated less than 1% H₂O₂. The PEM fuel cell exhibited a current density as high as 0.27 A cm⁻² at 0.6 V and 2.3 A cm⁻² at 0.2 V for a catalyst loading of 6.0 mg cm⁻². No significant performance degradation was observed over 480 h of continuous fuel cell operation with 2 mg cm⁻² catalyst under a load of 200 mA cm⁻² as evidenced by a resulting cell voltage of 0.32 V with a voltage decay rate of 80 μV h⁻¹. Materials characterization studies indicated that the metal–nitrogen chelate complexes decompose at high pyrolysis temperatures above 800 °C, resulting in the formation of the metallic species. During the pyrolysis, the transition metals facilitate the incorporation of pyridinic and graphitic nitrogen groups into the carbon matrix, and the carbon surface doped with nitrogen groups is catalytically active for oxygen reduction.     

High-performance cathode-supported SOFCs prepared by a single-step co-firing process

Cathode-supported solid oxide fuel cells (SOFCs), comprising porous Pr_0.35Nd_0.35Sr_0.3MnO_3−δ (PNSM)/Sm_0.2Ce_0.8O_1.95 (SDC) cathode supports, SDC function layers, YSZ electrolyte membranes and NiO/SDC anode layers, were successfully fabricated via suspensions coating and single-step co-firing process. The microstructures of electrolyte membranes were observed with scanning electron microscope (SEM). The assembled single cell was electrochemically characterized with humidified hydrogen as fuel and ambient air as oxidant. The open circuit voltage (OCV) of the cell was 1.036 V at 650 °C, and the peak power densities were 657, 472, 290 and 166 mW cm⁻² at 800, 750, 700 and 650 °C, respectively. Impedance analysis indicated that the performance of cathode-supported cell was determined essentially by electrode polarization resistance, which suggested that optimizing electrodes was especially important for improving the cell performance.     

Clay as a dispersant in the catalyst layer for zinc–air fuel cells

This study proposes a four-layer membrane electrode assembly (MEA) consisting of air-electrode, proton exchange membrane, Zn-electrode with KOH or NaCl aqueous electrolyte and a steel supporter, for use in Zn–air fuel cells. Montmorillonite clay was used to disperse carbon black (CB) and MnO₂ catalyst to improve the performance of the air-electrode. The microstructures of the air-electrode and cell characteristics were investigated by field emission scanning electron microscopy (FE-SEM), optical microscopy (OM) and an electrochemical analyzer. The experimental results indicate that the four-layer MEA for Zn–air fuel cells reached a power density of 6 mW cm⁻² (at 10 mA cm⁻²) without electrolyte leakage from the cells. The open circuit voltage (OCV) and current density were improved by adding clay to the air-electrode as clay can minimize CB aggregation. In the polarization test, the OCV value (1.40 V) reached approximately 90% of the standard potential (1.65 V) and remained steadily over 48 h. These experimental results demonstrate the four-layer MEA can replace conventional Zn–air fuel cells that utilize aqueous electrolyte.     

Solid oxide fuel cells with bi-layered electrolyte structure

In this work, we have developed solid oxide fuel cells with a bi-layered electrolyte of 2 μm SSZ and 4 μm SDC using tape casting, screen printing, and co-firing processes. The cell reached power densities of 0.54 W cm⁻² at 650 °C and 0.85 W cm⁻² at 700 °C, with open circuit voltage (OCV) values larger than 1.02 V. The electrical leaking between anode and cathode through an SDC electrolyte has been blocked in the bi-layered electrolyte structure. However, both the electrolyte resistance (R_el) and electrode polarization resistance (R_p,a+c) increased in comparison to cells with single-layered SDC electrolytes. The formation of a solid solution of (Ce, Zr)O_2−x during sintering process and the flaws in the bi-layered electrolyte structure seem to be the main causes for the increase in the R_el value (0.32 Ω cm²) at 650 °C, which is almost one order of magnitude higher than the calculated value.     

Electrochemical testing of suspension plasma sprayed solid oxide fuel cell electrolytes

Electrochemical performance of metal-supported plasma sprayed (PS) solid oxide fuel cells (SOFCs) was tested for three nominal electrolyte thicknesses and three electrolyte fabrication conditions to determine the effects of electrolyte thickness and microstructure on open circuit voltage (OCV) and series resistance (R_s). The measured OCV values were approximately 90% of the Nernst voltages, and electrolyte area specific resistances below 0.1 Ω cm² were obtained at 750 °C for electrolyte thicknesses below 20 μm. Least-squares fitting was used to estimate the contributions to R_s of the YSZ bulk material, its microstructure, and the contact resistance between the current collectors and the cells. It was found that the 96% dense electrolyte layers produced from high plasma gas flow rate conditions had the lowest permeation rates, the highest OCV values, and the smallest electrolyte-related voltage losses. Optimal electrolyte thicknesses were determined for each electrolyte microstructure that would result in the lowest combination of OCV loss and voltage loss due to series resistance for operating voltages of 0.8 V and 0.7 V.     

Synthesis and characterization of platinum nanoparticles on in situ grown carbon nanotubes based carbon paper for proton exchange membrane fuel cell cathode

Multi-walled carbon nanotubes (MWCNTs) based micro-porous layer on the carbon paper substrates was prepared by in situ growth in a chemical vapor deposition setup. Platinum nanoparticles were deposited on in situ grown MWCNTs/carbon paper by a wet chemistry route at <100 °C. The in situ MWCNTs/carbon paper was initially surface modified by silane derivative to incorporate sulfonic acid–silicate intermediate groups which act as anchors for metal ions. Platinum nanoparticles deposition on the in situ MWCNTs/carbon paper was carried out by reducing platinum (II) acetylacetonate precursor using glacial acetic acid. High resolution TEM images showed that the platinum particles are homogeneously distributed on the outer surface of MWCNTs with a size range of 1–2 nm. The Pt/MWCNTs/carbon paper electrode with a loading of 0.3 and 0.5 mg Pt cm⁻² was evaluated in proton exchange membrane single cell fuel cell using H₂/O₂. The single cells exhibited a peak power density of 600 and 800 mW cm⁻² with catalyst loadings of 0.3 and 0.5 mg Pt cm⁻², respectively with H₂/O₂ at 80 °C, using Nafion-212 electrolyte. In order to understand the intrinsically higher fuel cell performance, the electrochemically active surface area was estimated by the cyclic voltammetry of the Pt/MWCNTs/carbon paper.