Spontaneous Hydrogen and Electricity Production in a Nested Carbon Fuel Cell

Abstract

Introduction Hydrogen is a desirable fuel because it is an effective energy carrier and because it has minimal impact on the environment. However, there are no natural reservoirs of hydrogen, and the majority of hydrogen currently used needs to be produced either from steam reforming of hydrocarbons or electrolysis of water. Both of these methods are energy intensive and have significant drawbacks. Hydrogen from steam reforming of hydrocarbons contains trace amounts of CO that acts as a poison for the Pt catalyst in PEM fuel cells, and hence limits the usability of the hydrogen produced. Similarly, electrolysis of water is an electrochemically uphill process, which requires significant energy input. The steam-carbon-air fuel cell (SCAFC) concept is a viable alternative that is capable of producing carbon-free hydrogen and electricity spontaneously and at high primary efficiency. The SCAFC couples an air-carbon fuel cell [1] with a steam-carbon fuel cell [2] through a shared carbon bed at the anode, as shown in Figure 1. Both cells employ an ionically conducting and electrically insulating ceramic electrolyte membrane for selective transport of oxide ions from the cathode to the anode. While both the air-carbon and the steam-carbon cells are spontaneous (i.e., downhill in electrochemical potential), the steam-carbon cell is endothermic, while the air-carbon is highly exothermic. By coupling the air-carbon with the steam-carbon cell, the SCAFC achieves a novel device that produces both hydrogen and electricity spontaneously without the need for external heat or power input. This presentation builds upon modeling and experimental studies of both types of carbon fuel cells conducted in our laboratory [1-3] and describes how this novel idea can achieve efficient and clean production of hydrogen from solid carbonaceous fuels. Modeling of the Steam-Carbon-Air Fuel Cell A model of a SCAFC has been developed that takes into account reaction chemistry in the carbon bed, electrochemistry at the electrodes, mass transport of gases, and heat transfer. In the anode chamber, the carbonaceous fuel undergoes dry gasification via the reverse Boudouard reaction, forming CO that gets oxidized at the anode surfaces. The CO2 formed on the anode surfaces can then once again gasify the adjacent carbon, thus forming a self-sustained “CO shuttle” mechanism [4]. Practical carbonaceous fuels not only contain carbon, but also small amounts of hydrogen, which when released, will lead to an analogous “H2 shuttle” mechanism. Hydrogen gets oxidized at the anode surface forming steam that will then diffuse back to the carbon bed and react to form more CO and H2. How hydrogen in the fuel stock affects the fuel cell operation and performance will be discussed. A detailed mechanism for dry and wet gasification of the carbon bed has previously been developed in the lab and is used in the model [5]. The kinetic parameters needed to describe the half-cell reactions are derived experimentally. Electrochemical impedance spectroscopy (EIS) measurements are taken as a function of temperature, gas composition and cell voltage. The EIS data is used to extract polarization resistances for each half-cell reaction and then fitted to a detailed electrochemical reaction mechanism. Mass transport is important in determining the local gas composition, which affect carbon bed and electrode kinetics, as well as to determine fuel utilization. Any CO or H2 in the exhaust is fuel that is not fully oxidized, and is still capable of undergoing the oxidation reaction to produce electricity in the cell. Thus, fuel utilization improves with an increase in the ratio of (CO2 + H2O)/(CO+H2) in the exit gas, and this influences the overall efficiency of the cell. Since both the Boudouard gasification and the half-cell reactions are temperature dependent, it is important to understand the coupled relationship between reaction rates, heat release, and local temperature. Thus, consideration of heat transfer effects is necessary to develop a better understanding of the cell operation. The model is used to map out the operating space for hydrogen production, power output and cell efficiency. Furthermore, geometrical parameters are tuned to minimize the mole fraction of CO and H2in the exhaust and maximize the overall efficiency. Optimal operating conditions are identified for maintaining high efficiencies while also achieving realistic hydrogen production rates.