Electrochemical Oxidation of Carbon‐Containing Fuels and Their Dynamics in Low‐Temperature Fuel Cells

Abstract

Fuel cells can convert the energy that is chemically stored in a compound into electrical energy with high efficiency. Hydrogen could be the first choice for chemical energy storage, but its utilization is limited due to storage and transport difficulties. Carbon-containing fuels store chemical energy with significantly higher energy density, which makes them excellent energy carriers. The electro-oxidation of carbon-containing fuels without prior reforming is a more challenging and complex process than anodic hydrogen oxidation. The current understanding of the direct electro-oxidation of carbon-containing fuels in low-temperature fuel cells is reviewed. Furthermore, this review covers various aspects of electro-oxidation for carbon-containing fuels in non-steady-state reaction conditions. Such dynamic investigations open possibilities to elucidate detailed reaction kinetics, to sense fuel concentration, or to diagnose the fuel-cell state during operation. Motivated by the challenge to decrease the consumption of fossil fuel, the production routes of the fuels from renewable resources also are reviewed.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…For disturbing adsorbates a performance increase may also be achieved by periodic regeneration of the catalyst via dynamic changes of the reactant concentration or the feed load. This has been shown recently for electrochemical cells 106. The resulting reducing or oxidizing conditions reactivate the catalyst surface either by oxidation or reduction of the catalyst or by desorption of adsorbates.…”

“…Macroscopic models are already widely and successfully applied to steady state—and in electrochemistry to dynamic operation106—though without taking into account changes of the state of the catalyst. As chemical reactors are usually operated in steady state, most kinetic models contain lumped kinetics ignoring elementary steps and dynamic behavior, but reproducing temperature and concentration dependence.…”

“…Furthermore, owing to the still limited options for monitoring the electrode state, except by current and voltage, electrochemists apply a sophisticated set of dynamic electrochemical analysis methods for kinetic studies, such as impedance spectroscopy and nonlinear frequency response analysis, to gain more information. These measurements are increasingly used for kinetic modeling, comprising model and parameter identification and analysis 106. Coupling them to simultaneous dynamic concentration monitoring by operando characterization techniques will further aid micro‐ and macrokinetic modeling, especially if the technique allows for quantifying concentration dynamically.…”

Voltage Inversion Caused by Self-organized Oscillations in a Direct Formic Acid Fuel Cell

“…Oscillations in electrocatalytic reactions have a long history in electrochemistry [1] [2], and example include many oxidation reactions which are relevant in the interconversion between chemical and electrical energies. Beyond half-cell, laboratory experiments, kinetic instabilities have been also found in polymer electrolyte membrane fuel cells (PEMFC) fed with CO contaminated H 2 [3] [4] [5] and, more recently, methanol [6] and formic acid [7].…”

“…Beyond half-cell, laboratory experiments, kinetic instabilities have been also found in polymer electrolyte membrane fuel cells (PEMFC) fed with CO contaminated H 2 [3] [4] [5] and, more recently, methanol [6] and formic acid [7]. Under oscillatory regime, most systems are expected to result in higher efficiency [2] [3] [4] [8] [9]. Motivated by the recent reports of oscillatory kinetics in fuel cell fed directly with carbon-containing fuels [6] [7], we have investigated the occurrence of voltage oscillations in a Direct Formic Acid Fuel Cell (DFAFC).…”