Electrocatalysis for Membrane Fuel Cells E-Book
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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Electrocatalysis for Membrane Fuel Cells
Comprehensive resource covering hydrogen oxidation reaction, oxygen reduction reaction, classes of electrocatalytic materials, and characterization methods
Electrocatalysis for Membrane Fuel Cells focuses on all aspects of electrocatalysis for energy applications, covering perspectives as well as the low-temperature fuel systems principles, with main emphasis on hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR).
Following an introduction to basic principles of electrochemistry for electrocatalysis with attention to the methods to obtain the parameters crucial to characterize these systems, Electrocatalysis for Membrane Fuel Cells covers sample topics such as:
- Electrocatalytic materials and electrode configurations, including precious versus non-precious metal centers, stability and the role of supports for catalytic nano-objects;
- Fundamentals on characterization techniques of materials and the various classes of electrocatalytic materials;
- Theoretical explanations of materials and systems using both Density Functional Theory (DFT) and molecular modelling;
- Principles and methods in the analysis of fuel cells systems, fuel cells integration and subsystem design.
Electrocatalysis for Membrane Fuel Cells quickly and efficiently introduces the field of electrochemistry, along with synthesis and testing in prototypes of materials, to researchers and professionals interested in renewable energy and electrocatalysis for chemical energy conversion.
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Seitenzahl: 1079
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Part I: Overview of Systems
1 System-level Constraints on Fuel Cell Materials and Electrocatalysts
1.1 Overview of Fuel Cell Applications and System Designs
1.2 Application-derived Requirements and Constraints
1.3 Material Pathways to Improved Fuel Cells
1.4 Note
References
2 PEM Fuel Cell Design from the Atom to the Automobile
2.1 Introduction
2.2 The PEMFC Catalyst
2.3 The Electrode
2.4 Membrane
2.5 The GDL
2.6 CCM and MEA
2.7 Flowfield and Single Fuel Cell
2.8 Stack and System
Acronyms
References
Part II: Basics – Fundamentals
3 Electrochemical Fundamentals
3.1 Principles of Electrochemistry
3.2 The Role of the First Faraday Law
3.3 Electric Double Layer and the Formation of a Potential Difference at the Interface
3.4 The Cell
3.5 The Spontaneous Processes and the Nernst Equation
3.6 Representation of an Electrochemical Cell and the Nernst Equation
3.7 The Electrochemical Series
3.8 Dependence of the
E
cell
on Temperature and Pressure
3.9 Thermodynamic Efficiencies
3.10 Case Study – The Impact of Thermodynamics on the Corrosion of Low-T FC Electrodes
3.11 Reaction Kinetics and Fuel Cells
3.12 Charge Transfer Theory Based on Distribution of Energy States
3.13 Conclusions
Acronyms
Symbols
References
4 Quantifying the Kinetic Parameters of Fuel Cell Reactions
4.1 Introduction
4.2 Electrochemical Active Surface Area (ECSA) Determination
4.3 H
2
-Oxidation and Electrochemical Setups for the Quantification of Kinetic Parameters
4.4 ORR Kinetics
4.5 Concluding Remarks
Acronyms
Symbols
References
5 Adverse and Beneficial Functions of Surface Layers Formed on Fuel Cell Electrocatalysts
5.1 Introduction
5.2 Catalyst Capping in Heterogeneous Catalysis and in Electrocatalysis
5.3 Passivation of PGM/TM and Non-PGM HOR Catalysts and Its Possible Prevention
5.4 Literature Reports on Fuel Cell Catalyst Protection by Capping
5.5 Other Means for Improving the Performance Stability of Supported Electrocatalysts
5.6 Conclusions
Abbreviations
References
Part III: State of the Art
6 Design of PGM-free ORR Catalysts: From Molecular to the State of the Art
6.1 Introduction
6.2 The Influence of Molecular Changes Within the Complex
6.3 Cooperative Effects Between Neighboring MCs
6.4 The Physical and/or Chemical Interactions Between the Catalyst and Its Support Material
6.5 Effect of Pyrolysis
Acronyms
References
7 Recent Advances in Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Electrolytes
7.1 Introduction
7.2 Mechanism of the HOR in Alkaline Media
7.3 Electrocatalysts for Alkaline HOR
7.4 Conclusions
Acronyms
References
8 Membranes for Fuel Cells
8.1 Introduction
8.2 Properties of the PE Separators
8.3 Classification of Ion-exchange Membranes
8.4 Mechanism of Ion Conduction
8.5 Summary and Perspectives
Acronyms
Symbols
References
9 Supports for Oxygen Reduction Catalysts: Understanding and Improving Structure, Stability, and Activity
9.1 Introduction
9.2 Carbon Black Supports
9.3 Decoration and Modification with Metal Oxide Nanostructures
9.4 Carbon Nanotube as Carriers
9.5 Doping, Modification, and Other Carbon Supports
9.6 Graphene as Catalytic Component
9.7 Metal Oxide-containing ORR Catalysts
9.8 Photodeposition of Pt on Various Oxide–Carbon Composites
9.9 Other Supports
9.10 Alkaline Medium
9.11 Toward More Complex Hybrid Systems
9.12 Stabilization Approaches
9.13 Conclusions and Perspectives
Acknowledgment
Acronyms
References
Part IV: Physical–Chemical Characterization
10 Understanding the Electrocatalytic Reaction in the Fuel Cell by Tracking the Dynamics of the Catalyst by X-ray Absorption Spectroscopy
10.1 Introduction
10.2 A Short Introduction to XAS
10.3 Application of XAS in Electrocatalysis
10.4 Δ
μ
XANES Analysis to Track Adsorbate
10.5 Time-resolved
Operando
XAS Measurements in Fuel Cells
10.6 Fourth-generation Synchrotron Facilities and Advanced Characterization Techniques
10.7 Conclusions
Acronyms
References
Part V: Modeling
11 Unraveling Local Electrocatalytic Conditions with Theory and Computation
11.1 Local Reaction Conditions: Why Bother?
11.2 From Electrochemical Cells to Interfaces: Basic Concepts
11.3 Characteristics of Electrocatalytic Interfaces
11.4 Multifaceted Effects of Surface Charging on the Local Reaction Conditions
11.5 The Challenges in Modeling Electrified Interfaces using First-principles Methods
11.6 A Concerted Theoretical–Computational Framework
11.7 Case Study: Oxygen Reduction at Pt(111)
11.8 Outlook
Acronyms
Symbols
References
Part VI: Protocols
12 Quantifying the Activity of Electrocatalysts
12.1 Introduction: Toward a Systematic Protocol for Activity Measurements
12.2 Materials Consideration
12.3 Electrochemical Cell Considerations
12.4 Parameters Diagnostic of Electrochemical Performance
12.5 Stability Tests
12.6 Data Evaluation (Auxiliary Techniques)
12.7 Conclusions
Acknowledgments
Acronyms
Symbols
References
13 Durability of Fuel Cell Electrocatalysts and Methods for Performance Assessment
13.1 Introduction
13.2 Fuel Cell PGM-free Electrocatalysts for Low-temperature Applications
13.3 PGM-free Electrocatalyst Degradation Pathways
13.4 PGM-free Electrocatalyst Durability and Metrics
13.5 Low-PGM Catalyst Degradation
13.6 Conclusion
Acronyms
References
Part VII: Systems
14 Modeling of Polymer Electrolyte Membrane Fuel Cells
14.1 Introduction
14.2 General Equations for PEMFC Models
14.3 Multiphase Water Transport Model for PEMFCs
14.4 Fluid Mechanics in PEMFC Porous Media: From 3D Simulations to 1D Models
14.5 Physical-based Modeling for Electrochemical Impedance Spectroscopy
14.6 Conclusions and Perspectives
Acronyms
Symbols
References
Note
15 Physics-based Modeling of Polymer Electrolyte Membrane Fuel Cells: From Cell to Automotive Systems
15.1 Polymer Fuel Cell Model for Stack Simulation
15.2 Auxiliary Subsystems Modeling
15.3 Electronic Power Converters for Fuel Cell-powered Vehicles
15.4 Fuel Cell Powertrains for Mobility Use
Acronyms
Symbols
References
Notes
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Impact of cell temperature and relative humidity on metal dissolut...
Chapter 3
Table 3.1 Standard potentials of reduction in aqueous solution at 25...
Table 3.2 Thermodynamic data of fuel cell reactions under standard condition...
Table 3.3 Exchange current densities for Hydrogen Oxidation Reaction (HOR) o...
Table 3.4 Exchange current densities for oxygen reduction reaction (ORR) on ...
Chapter 6
Table 6.1 Bond distance (
d
), first ionization energy (IE), and magnetic mome...
Chapter 7
Table 7.1 Summary of PGM-based electrocatalysts for alkaline HOR.
Table 7.2 Summary of non-PGM-based electrocatalysts for HOR.
Chapter 8
Table 8.1 Targets for ion-exchange membranes and membrane electrode assembli...
Table 8.2 Selected examples of proton- and anion-exchange ionomers.
Table 8.3 Comparative conductivity of selected IEMs.
Chapter 10
Table 10.1 Crystallite size calculated from XRD and EXAFS fitting results ob...
Chapter 12
Table 12.1 Chemical treatments reported for the cleaning of electrochemical ...
Table 12.2 Potentials of commonly employed reference electrodes at 25...
Table 12.3 Activity descriptors for electrocatalytic activity.
Table 12.4 Koutecký–Levich oxygen parameters in common electrolyte media.
Table 12.5 Comparison between
E
(
j
Pt
(5%)...
Chapter 13
Table 13.1 PGM-free MEA durability testing in air protocol.
Table 13.2 PGM-free MEA carbon corrosion in N
2
protocol.
Table 13.3 PGM-free MEA durability testing in N
2
protocol.
Table 13.4 Break-in protocol for PGM-based catalysts conducted in H
2
/air, 80...
Table 13.5 Voltage recovery protocol for PGM-based catalysts for catalyst co...
Table 13.6 Accelerated stress test protocol to assess PGM-based catalyst dur...
Table 13.7 MEA recovery protocol
a)
.
Chapter 14
Table 14.1 Physical parameters of the 1-D analytical model for PEMFC and SOF...
Chapter 15
Table 15.1 Resume of the main design outcomes yielded on output by the propo...
List of Illustrations
Chapter 1
Figure 1.1 Diagrams summarizing the current status of automotive fuel cell s...
Figure 1.2 Schematics of representative fuel cell system designs for automot...
Figure 1.3 Illustrations of (a) the voltage loss mechanisms that contribute ...
Figure 1.4 Plot of the relationship imposed by the heat rejection constraint...
Figure 1.5 Breakdown of fuel cell system cost by major stack and balance of ...
Chapter 2
Figure 2.1 (a) Thermodynamic potential of the HOR and ORR reactions, (b) ORR...
Figure 2.2 Historic performance curves in H
2
/air at various temperatures and...
Figure 2.3 Design of a pressurized, water-cooled PEMFC system [20, 21] conta...
Figure 2.4 PEMFC stack (a) composed of many individual cells (b) of which th...
Figure 2.5 (a) Vulcan XC72 carbon, (b) Graphitized Vulcan XC72 carbon, (c) A...
Figure 2.6 Platinum surface area as measured by CO adsorption (a) for HSC ca...
Figure 2.7 Cathode catalyst specific activity (a) and mass activity (b) as a...
Figure 2.8 Metal ECSA loss rate vs. catalyst particle size: 0.25 mg
Pt
cm
−2
...
Figure 2.9 NSTF pyrelene red whiskers on Kapton substrate (left), NSTF coate...
Figure 2.10 PEMFC electrode (a) and composite M/C representation of possible...
Figure 2.11 Optical images of electrodes coated on a GDL substrate showing s...
Figure 2.12 Performance impact (left, center) and catalyst surface area loss...
Figure 2.13 Thin standard NSTF electrode (a) and dispersed NSTF + ionomer el...
Figure 2.14 Chemical structures of the three main PFSA ionomers used in PEM ...
Figure 2.15 Examples of perfluoro bis(sulfonyl)imide ionomer (a), 3M's perfl...
Figure 2.16 Conductivity vs. equivalent weight for 3M ionomers of various ty...
Figure 2.17 Material components of a manufactured PEM before operation (a). ...
Figure 2.18 Unreinforced membrane (a). 3M membrane reinforced with electrosp...
Figure 2.19 The PEMFC GDL is situated between the flowfield and electrode la...
Figure 2.20 Carbon paper (top) and microporous layer (bottom) for (a) Freude...
Figure 2.21 Performance vs. various cathode GDL types: 70 °C, 50...
Figure 2.22 CCM and MEA layers (a). MEAs of different sizes and configuratio...
Figure 2.23 Impact of temperature on roll-to-roll lamination temperature on ...
Figure 2.24 Different MEA subgasket designs. The CCM subgasket may not overl...
Figure 2.25 (a) A quad-serpentine lab-scale cell (A), a bi-serpentine (B), s...
Figure 2.26 Hypothetical operating conditions for an alternate design of the...
Figure 2.27 Performance after SUSD cycles carried out at 70/70/70 °...
Figure 2.28 Rendering highlighting internal air and hydrogen manifolds exten...
Chapter 3
Figure 3.1 Example of a basic electrochemical cell, displaying the interconn...
Figure 3.2 Structure of the electric double layer under different conditions...
Figure 3.3 Example of a cell: scheme of the Daniell cell.
Figure 3.4 Impact of pH on the equilibrium potentials of the main electroche...
Figure 3.5 Energy map of electronic states on a metal electrode and on elect...
Figure 3.6
G
0
on
q
for an electron transfer reaction. For a heterogeneous re...
Figure 3.7 Effect of potential modulation on the standard free energy of act...
Figure 3.8 Influence of
k°
on the shape of the current–potential curves...
Figure 3.9 Tafel plot: log
i
vs
.
η
for a general case.
η = E − Eeq
...
Figure 3.10
η
vs
. ln
|J|
of a hypothetical electrochemical reaction. At ...
Chapter 4
Figure 4.1 Relations between the overpotential and the kinetically controlle...
Figure 4.2 Cyclic voltammogram (dashed line) and CO-stripping potential scan...
Figure 4.3 Cyclic voltammograms recorded at a scan rate of 10 mV s
−1
o...
Figure 4.4 Nitrite adsorption and stripping steps on Fe–N
x
sites.
Figure 4.5 Pt-mass-normalized cyclic voltammograms recorded on Pt-based elec...
Figure 4.6 Schemes of RDE (a) and RRDE (b) setups.
Figure 4.7 Drawing of the cell (left) and detailed magnification of the plat...
Figure 4.8 Scheme of an SECM operated in TG/SC mode, with a single vs. multi...
Figure 4.9 Scheme of the microstructure of the floating electrode (a) with e...
Figure 4.10 Arrhenius plots of the HOR/HER exchange current densities (
i
0
) o...
Figure 4.11 Scheme depicting the possible pathways of the oxygen reduction r...
Figure 4.12 Oxygen reduction current density recorded on Pt/C catalyst in O
2
Chapter 5
Figure 5.1 Top: The synthesis of composite Pt@mSiO
2
nanoparticles; Bottom: T...
Figure 5.2 The activity in CO thermal oxidation of Pt nanoparticles capped b...
Figure 5.3 Common mechanisms of electrocatalyst nanoparticle degradation inc...
Figure 5.4 HOR polarization curves of Ru/C, Ni/C, Pt/C, PtRu/C, and Ru
7
Ni
3
/C...
Figure 5.5 HEMFC performance with Ru
7
Ni
3
/C as anode catalyst: (a) voltage–cu...
Figure 5.6 Anodic H
2
oxidation in 0.1 M KOH using ternary alloys of Co, Mo, ...
Figure 5.7 (a) The surface Pourbaix diagram for Nickel.(b) The phase Pou...
Figure 5.8 Comparison of the changes in mass activity measured at 0.9 V for ...
Figure 5.9 (a) Substantial change in the morphology of uncapped Pd (top) fol...
Figure 5.10 Performance of a carbon-capped hydrogen evolution (HER) Ni catal...
Figure 5.11 (a) TEM image of a TiCN-supported Pt catalyst. (b) ORR current–p...
Figure 5.12 Polarization curves recorded for H
2
/air fuel cells, with cathode...
Figure 5.13 A schematic presentation of different possible distributions of ...
Chapter 6
Scheme 6.1 Suggested ORR mechanisms of MCs in (A) acidic solution and (B) al...
Figure 6.1 A general structure of MCs where X can be either a carbon or a ni...
Figure 6.2 The molecular structure of porphyrin (a), phthalocyanine (b), and...
Figure 6.3 Cyclic voltammetry of Co-corrole deposited on a glassy carbon ele...
Figure 6.4 Cyclic voltammetry of Fe-phthalocyanine on an acetylene black sur...
Figure 6.5 The SOHMO isovalue surfaces for (a) Co-corrole with (which is sim...
Figure 6.6 Linear sweep voltammograms of metallocorroles on BP2000 carbon wi...
Figure 6.7 Spin-density isosurfaces for O
2
adsorbed on the porphyrin metal c...
Figure 6.8 (a) RRDE measurements of B-halogenated Co-corroles in 0.1 M KOH a...
Figure 6.9 Chemical structure of heme A and heme B molecules.
Figure 6.10 CV of the Fe-TPP and Fe-PP-diester complexes in MeOH solvent (co...
Figure 6.11 Structure of the two phthalocyanins (FePc and 16(Cl)FePc), pyrid...
Figure 6.12 Electroreduction of O
2
on a clean Au(111) surface and modified w...
Figure 6.13 Cyclic voltammetry for ORR on Au(111) modified with different co...
Figure 6.14 Description of the synthesis of porphyrin aerogels (a) and photo...
Figure 6.15 RDE measurements of β-halogenated Co-corroles deposited on...
Figure 6.16 ORR activity of various porphyrins and phthalocyanines before an...
Chapter 7
Figure 7.1 Projected density of states for the d-band of metal atoms around ...
Figure 7.2 Phase diagram showing the free energy change for water in contact...
Figure 7.3 Free energy diagram of Tafel–Volmer mechanism involved in ...
Figure 7.4 (a) Calculated adsorption free energy of H, Δ
G
H...
Chapter 8
Figure 8.1 Structural regions in wet Nafion: (a) hydrophobic region (bundle ...
Figure 8.2 Examples of anion-exchange groups and their degradation products ...
Figure 8.3 Perfluorinated membranes: (a) main conformational structures (15
7
Figure 8.4 Classification of hybrid inorganic–organic polymer electrolytes (...
Figure 8.5 (a) Water uptake of N/(M
x
O
y
)
z
membranes, with
z
= 0 and 5 wt% and...
Figure 8.6 Interactions among ZrO
2
nanoparticles, PBI chains, and H
3
PO
4
mole...
Figure 8.7 “
Core–shell
” nanoparticles: (a) a few examples of [(M1
m
O
n
)(...
Figure 8.8 N/[(M1
m
O
n
)(M2
x
O
y
)
z
] inorganic–organic hybrid membranes: (a) tempe...
Figure 8.9 3D spectra of the imaginary permittivity (
ɛ
″) of dr...
Figure 8.10 Dielectric relaxations detected in perfluorinated IEMs: α-mode i...
Figure 8.11 Conductivity mechanisms in perfluorinated membranes as function ...
Figure 8.12 Schematic depiction of a delocalization body, i.e. a wet polar d...
Figure 8.13 SPEEK-based membranes: average migration distance (<
r
...
Figure 8.14 Conductivity mechanism and dielectric relaxations for [PVBTMA][X...
Figure 8.15 (a) 3D plot of real conductivity for [PVBTMA][OH]-
b
-PMB diblock ...
Figure 8.16 Conductivity mechanisms in hybrid membranes (N/(M
x
...
Chapter 9
Figure 9.1 Cartoon illustrating immobilization of platinum nanoparticles in ...
Figure 9.2 Schematic diagram illustrating (a) importance of the strength of ...
Figure 9.3 Hybrid (functionalized) electrocatalytic system composed of ceria...
Figure 9.4 Diminishing of the formation of platinum oxo-species on catalytic...
Figure 9.5 Nanostructured metal-oxide-containing electrocatalytic systems: (...
Figure 9.6 Cartoon illustrating interactions of hydroxyl groups on ceria wit...
Chapter 10
Figure 10.1 Extent of transmittance or attenuation of X-ray photons by diffe...
Figure 10.2 Scheme depicting various X-ray absorption events, occurring befo...
Figure 10.3 Ru K-edge spectra obtained from pristine Pt/Ru anode catalyst, r...
Figure 10.4 Pt L
3
-edge spectra obtained for commercial Pt/C, pristine and ag...
Figure 10.5 EXAFS FT obtained from Pt L
3
-edge spectra for commercial Pt/C, p...
Figure 10.6 Correlation between particle size and coordination number obtain...
Figure 10.7 Pt L
3
-edge EXAFS FT obtained for pure Pt nanoparticles as well a...
Figure 10.8 Photograph of fuel cell
operando
setup at Spring 8. The scheme o...
Figure 10.9 Amount of RuO
2
·
x
H
2
O formed/remained ...
Figure 10.10 (a) XANES Ru K-edge obtained during fuel starvation operation; ...
Figure 10.11 Janin clusters showing different coordination sites, top left: ...
Figure 10.12 Theoretical Δ
μ
signature obtained for atop CO and at...
Figure 10.13 Left: Δ
μ
signature obtained during methanol operatio...
Figure 10.14 XAS analysis results from
operando
time-resolved measurements ...
Chapter 11
Figure 11.1 An electrochemical cell with two charged electrode/electrolyte i...
Figure 11.2 Surface charging relation of an ideally polarizable interface. ...
Figure 11.3 How chemisorption modifies the electrode–electrolyte interface....
Figure 11.4 Multifaceted double-layer effects on the local reaction conditio...
Figure 11.5 A concerted theory–computation framework for modeling electrocat...
Figure 11.6 (a) Comparison between experimental data and model simulation in...
Chapter 12
Figure 12.1 Milestones that have influenced the protocols of H
2
–O
2
fuel cell...
Figure 12.2 (a) Volcano plot of the hydrogen-exchange current density as a f...
Figure 12.3 (a) ORR RDE curves for Pt alloys at 1500 rpm obtained in 0.1 M H...
Figure 12.4 (a) Electrochemical probe of the SMSI effect using the CO stripp...
Figure 12.5 (a) Cyclic voltammograms of 20 wt% Pt/C (JM) performed on a Tefl...
Figure 12.6 Possible contaminants in electrochemical measurements and their ...
Figure 12.7 Pourbaix diagram of water.
Figure 12.8 (a) Double-layer structure of the HOR process at Pt catalyst in ...
Figure 12.9 (a) Voltammograms of a Pt(111) electrode in N
2
sat. acid electro...
Figure 12.10 (a) Representation of the zero potential of the RHE with respec...
Figure 12.11 (a) Comparison between a well-dispersed ink vs. a badly dispers...
Figure 12.12 (a) Representative cyclic voltammetry curves of clean cubic and...
Figure 12.13 (a) Differences of the electrode surface at different scales, r...
Figure 12.14 Guide for choosing a method for determining RSA of electrocatal...
Figure 12.15 Cyclic voltammogram of a 20 wt% Pt/C (JM) catalyst in 0.1 M HCl...
Figure 12.16 Curves obtained from a Pd sheet in 0.5 M H
2
SO
4
with different u...
Figure 12.17 CO stripping curves of a 20 wt% Pt/C (JM) catalyst in 0.1 M HCl...
Figure 12.18 Cyclic voltammetry of polycrystalline Pt performed in 0.5 M H
2
S...
Figure 12.19 Curves obtained for the double-layer capacitance method for det...
Figure 12.20 (a) Determination of
j
Pt
(5%)
() and
E
(
j
Pt
(5%))
values...
Chapter 13
Figure 13.1 Schematic of a polymer electrolyte fuel cell (PEFC) where H
2
oxi...
Figure 13.2 Stability diagram describing proposed Fe dissolution as a functi...
Figure 13.3 Fe demetallation results from Choi et al. [44] showing operando ...
Figure 13.4 DFT study by Yang et al. [32] depicting the (a)
density function
...
Figure 13.5 Study of the effect of different H
2
O
2
concentrations (inset) on ...
Figure 13.6 Illustration depicting the proposed degradation mechanisms for c...
Figure 13.7 Potential-pH Pourbaix diagram describing Pt dissolution at 298 K...
Figure 13.8 Illustration of potential wave forms from high potential (
E
H
) to...
Figure 13.9 Cyclic voltammogram of polycrystalline Pt electrode in 0.5 M H
2
S...
Figure 13.10 Various corrosion steps on three different types of carbon used...
Chapter 14
Figure 14.1 Scheme of PEMFC with anode and cathode current collector (ACC, C...
Figure 14.2 Typical polarization curve of a PEMFC according to (14.5):
E
is ...
Figure 14.3 Polarization curves for (a) PEMFC and (b) SOFC computed with the...
Figure 14.4 Simulations performed with a 1D model implementing multiphase wa...
Figure 14.5 Simulations performed with a 1D model implementing multiphase wa...
Figure 14.6 An example of lattice Boltzmann simulation of water infiltration...
Figure 14.7 (a) Darcy's model for mass transport in porous media, which link...
Figure 14.8 An example of two-phase flow configuration within a hydrophilic ...
Figure 14.9 Macroscopic effective quantities extracted from X-ray reconstruc...
Figure 14.10 (a) Diffusion layer (DL) placed between membrane and catalyst l...
Figure 14.11 Typical impedance response of a PEM fuel cell simulated at 8...
Figure 14.12 Comparison between simulated (line) and experimental (markers) ...
Figure 14.13 2D effects on impedance spectra not considered with the 1D mode...
Chapter 15
Figure 15.1 Schematic of a hydrogen PEMFC power system for automotive applic...
Figure 15.2 Effect of channel/rib geometry on PEMFC performance: (a) schemat...
Figure 15.3 Calibration of the 1D model on the 3D model simulations to accou...
Figure 15.4 Effect of air and hydrogen stoichiometry on the polarization cur...
Figure 15.5 Transient simulations of a PEMFC stack after a current step was ...
Figure 15.6 Air loop components: (a) and (b) the compressor static map that ...
Figure 15.7 Stack gross and net power generated by the PEMFC stack in a NEDC...
Figure 15.8 Stack voltage and current simulated during the simulation of the...
Figure 15.9 Local membrane water content simulated during the simulation of ...
Figure 15.10 Coolant local temperature at several positions in the stack sim...
Figure 15.11 Relative humidity in the air stream at the stack inlet (after t...
Figure 15.12 Stack and system efficiency computed by the NEDC simulation at ...
Figure 15.13 Peak shaving concept: the PEMFC is only delivering the long-ter...
Figure 15.14 The three-port converter architecture. The unidirectional energ...
Figure 15.15 The synoptic diagram illustrating the entire FCHV codesign proc...
Figure 15.16 The longitudinal dynamic model used in the FCHV codesign proced...
Figure 15.17 Model library utilized to estimate the fuel consumption and bat...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Electrocatalysis for Membrane Fuel Cells
Methods, Modeling, and Applications
Edited by Nicolas Alonso-Vante and Vito Di Noto
Editors
Prof. Nicolas Alonso-VanteUniversity of PoitiersIC2MP-UMR-CNRS 72854 rue Michel BrunetF-86073 Poitiers Cedex 9France
Prof. Vito Di NotoUniversity of PadovaDepartment of Industrial EngineeringVia Marzolo 9I-35131 PadovaItaly
Cover Image: Courtesy of Vito Di Noto, Nicolas Alonso Vante and Keti Vezzù
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Print ISBN: 978-3-527-34837-4ePDF ISBN: 978-3-527-83055-8ePub ISBN: 978-3-527-83056-5oBook ISBN: 978-3-527-83057-2
Preface
In electrochemical energy converters such as low-temperature membrane fuel cells, the slow kinetics of oxygen reduction (ORR) represents one of the main reasons for a high overpotential in a fuel cell, e.g. polymer electrolyte membrane (PEM) type. Despite this inherent phenomenon, in these systems, fuel cells constitute (i) a cornerstone in the energy technologies of the present twenty-first century for transportation and stationary applications and (ii) one of the two pillars, together with electrolyzers, of the future Green Hydrogen Economy of the world. These scenarios are comforted by the rapid advances in the development of materials based on noble and non-noble electrocatalytic materials that encompass a bunch of applications operating in a wide pH range (acidic and alkaline). In this context, in order to find a utility, to the knowledge obtained to date, for current and future researchers in this field of activity, the repository of such an avalanche of information is thus a central resource to be transmitted with a global perspective. It is for this reason that the present book aims to consolidate and transmit this knowledge while providing the necessary forum to complement what is published daily in specialized journals. Thus, the contributions of experts working in both academic and industrial research and development will serve as a reference source for the fundamentals and applications of fuel cells, establishing the state-of-the-art and disseminating research advances within a scope corresponding to textbooks for undergraduate and graduate students.
This book, devoted to fuel cell electrocatalysis, will, we hope, further the development and application of this exciting technology on the road to the successful establishment of a clean and sustainable energy economy in the twenty-first century. For the reader's convenience, this book, with a total of 15 chapters, is organized in seven sections, namely Overview, Fundamentals, State of the Art, Physical–Chemical Characterization, Modeling, Protocols, and Systems.
The first chapter discusses how application requirements and system-level considerations create constraints on fuel cell materials and electrocatalysts, with the goal of informing more strategic and impactful research and development efforts. In the second chapter, the discussion is centered on how an atomically designed catalyst surface efficiently produces protons and electrons from hydrogen on the anode and water from oxygen, protons, and electrons on the cathode. In the third chapter, insights are provided on how fundamental electrochemistry can be exploited to guide fuel cell research, whereas the fourth chapter discloses the quantification of the kinetic descriptors that determine the activity and stability of the anode and cathode electrocatalysts, providing analytical methods and electrochemical set-ups as supports. Moreover, in Chapter 5, the author discusses some means for protecting catalytic sites in order to maintain high performance in the light of recent data from the literature. Chapter 6, furthermore, puts into relevance the state-of-the-art of platinum group metal (PGM)-free ORR catalysts. Herein, the authors provide an overview of important parameters that influence the catalysis of ORR with well-defined ORR catalysts. In Chapter 7, recent development in electrocatalysts for the hydrogen oxidation reaction (HOR) is put on the floor, emphasizing the state-of-the-art PGM- and non-PGM-based electrocatalysts for the HOR in alkaline conditions. An important ingredient in the proton exchange membrane fuel cell (PEMFC) system is the polymeric electrolyte. In this context, Chapter 8 describes the features that a membrane must exhibit to be implemented in a fuel cell. This chapter ends with a comprehensive overview of the mechanisms of ion conduction proposed for fuel cell membranes, followed by a brief summary outlining the perspectives of the research in this field. The characteristics of ORR electrocatalyst support (carbon-based and oxide-based) have been analyzed in Chapter 9. Of importance, in all interface research, is the in operando technique, and/or probing under real fuel cell operating conditions is offered in Chapter 10 with the use of X-ray absorption spectroscopy (XAS). Theoretical modeling and computation to unravel the local reaction environment are given in Chapter 11. This chapter addresses this complex issue by introducing some basic concepts of electrochemical interfaces, especially the surface charging relation. The authors highlight the electrocatalytic interfaces pertaining to the role of chemisorption-induced surface dipoles that could cause nonmonotonicity in the surface charging behavior. The electrocatalytic materials research protocols for investigating fuel cell reactions are deployed in Chapters 12 and 13. In sum, the correct evaluation of fuel cell reactions, selection of reference electrodes, durability tests of PGM-free materials, and fuel cell testing procedures are put forward in the light of the most advanced literature data research. The last section of the book presents Chapters 14 and 15. These chapters analyze the fundamentals of fuel cell simulation by means of a mono-dimensional analytical model considering multiphase water transport affecting the electrical conductivity properties of the cell membrane, whereas Chapter 15 analyzes the optimization of the operative conditions and the prediction of the system durability that back the design of the PEMFC stack and components of the balance of the plant.
The editors appreciate the contributing authors of this book, who maintained high scientific standards.
N. Alonso-Vante acknowledges financial support from the European Union (ERDF) and “Région Nouvelle Aquitaine.”
V. Di Noto thanks the financial support of EIT Raw Materials, project Alpe, and Graphene Flagship, Core 3, of the European Union.
Nicolas Alonso-Vante
University of Poitiers, IC2MP-UMR CNRS 7285
Poitiers, France
Vito Di Noto
University of Padova, Department of Industrial Engineering
Padova, Italy
Part IOverview of Systems
1System-level Constraints on Fuel Cell Materials and Electrocatalysts
Elliot Padgett and Dimitrios Papageorgopoulos
Hydrogen and Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, 1000 Independence Ave., SW, Washington, D.C., 20585, USA
1.1 Overview of Fuel Cell Applications and System Designs
Fuel cells are anticipated to play an important role in the future clean energy economy as versatile energy conversion devices across many applications and sectors. Fuel cells have important current and potential applications in three broad areas: (i) transportation powertrains, in vehicles such as cars, buses, trucks, rail locomotives, ships, and aircraft; (ii) stationary power systems, such as distributed power generation, backup power, and combined heat and power (CHP) systems; and (iii) specialty applications such as material handling equipment as well as portable systems for auxiliary power or devices such as personal electronics or mobile communications equipment. While fuel cells for these diverse applications have some common foundations, the systems for each application have different requirements and priorities, which call for different system designs and technologies to meet them. The development of advanced, application-relevant materials and electrocatalysts is essential to overcoming the technical challenges that remain to bring fuel cells into widespread adoption and realization of their potential. This chapter discusses how application requirements and system-level considerations create constraints on fuel cell materials and electrocatalysts, with the goal of informing more strategic and impactful research and development efforts. The primary focus will be on transportation applications and polymer electrolyte membrane (PEM) fuel cells, but other applications and fuel cell types will also be included for context and comparison.
1.1.1 System-level Fuel Cell Metrics
It is useful to begin by covering the typical high-level metrics for fuel cell systems, which provide a basis for comparing different fuel cell types, application requirements, and alternative technologies as well as for benchmarking technological progress. These metrics are commonly used as specifications for fuel cell products and targets for fuel cell research, development, and demonstration (RD&D) programs [1–3]. For instance, system and stack-level targets for automotive fuel cells set by the U.S. Department of Energy (DOE), along with respective status estimates, are illustrated in Figure 1.1[4]. The most used metric categories include cost, durability, efficiency, system size, and flexibility. Each of these, as well as specific metrics, will be described below.
Figure 1.1 Diagrams summarizing the current status of automotive fuel cell systems (a) and stacks (b) relative to DOE targets.
Source: Reproduced from U.S. Department of energy [4] / https://www.hydrogen.energy.gov/pdfs/20005-automotive-fuel-cell-targets-status.pdf / Public domain.
There are several different metrics in common use that describe the size of fuel cell systems, combining the power output and physical mass or volume of the system. Power output may be given as gross power – the total electrical power output of the fuel cell stack – or as net power – the power output of the stack minus the power consumption of the supporting balance of plant (BOP). This distinction must be specified to avoid confusion and may be included in the power units (as kWgross or kWnet, for example). To address application-driven system size and weight restrictions, the power output can be given as an absolute total, per unit weight of the system (this is known as the specific power, with units such as kW kg−1), or per unit volume of the system (this is known as the power density, with units such as kW l−1).
The energy conversion efficiency of a fuel cell system can be specified in either the electrical power output per fuel input (e.g. ) or as a percentage of the fuel's lower heating value. Fuel cells generally are more efficient at low power than at high power, and the efficiency is closely tied to the fuel cell performance, as the same mechanisms of voltage loss decrease both. There are therefore different definitions for system efficiency specified at different performance levels, most commonly at the peak efficiency (at low power), peak or rated power, or an average efficiency over a particular duty cycle.
Fuel cell durability or lifetime is commonly specified as the number of hours of operation before a certain level of degradation is reached. While, in practice, the tolerable level of degradation will vary depending on the user's needs, it is also useful to use standardized end of life definitions such as 10% voltage degradation at rated power for benchmarking purposes. It is also important to recognize that degradation rates and lifetimes for fuel cell systems will depend on the duty cycle and stressors of each application.
The cost of a fuel cell system is an important metric but is more challenging to determine than other metrics that are rooted in the physical or engineering parameters of the system. The actual cost of deployed fuel cell systems is of interest in business transactions and to assess current market competitiveness. The projected cost of fuel cell systems using earlier stage, lab-demonstrated technologies, and larger manufacturing scales is also useful for tracking advances in technology and informing research and development (R&D) needs. The cost of fuel cell systems is commonly specified per power output (e.g. $ kWnet−1) although this metric depends on both the system size and the definition of the system boundaries (fuel storage, power electronics, and other components are commonly excluded from the fuel cell system, although system definitions vary).
Flexibility and robustness are umbrella concepts that encompass many different potential metrics for the ability of the fuel cell system to adjust to provide power as it is required. These include the time required to start the fuel cell system, its capability to start and sustain power under cold or hot conditions, its ability to quickly adjust to varying power demands, and the reliability of the system.
It is important to recognize that the various aspects of fuel cell systems that are described by these metrics are interrelated. For instance, an alteration to a fuel cell system that lowers its cost may also impact its power output, efficiency, and durability. Composite metrics that constrain related metrics in a particular way can therefore also be useful. For example, DOE has introduced a “durability-adjusted cost” metric for automotive fuel cells, which describes the cost of an 80-kWnet fuel cell system that also meets the requirements for 8000 hour on-road durability [5].
1.1.2 Fuel Cell Subsystems and Balance of Plant (BOP) Components
Fuel cells require supporting BOP equipment to provide high performance and durability, including the supply of air and fuel, cooling, and system monitoring and control. It is important to understand the common subsystems and components used for these purposes. State-of-the-art fuel cell system designs are generally proprietary, but representative model systems have been developed to provide public information. For instance, the DOE has funded the development of a model automotive fuel cell system in a collaboration between Strategic Analysis, Inc. and Argonne National Laboratory and with feedback from the U.S. DRIVE (Driving Research and Innovation in Vehicle efficiency and Energy sustainability) Partnership [6, 7]. This model system is a useful resource for understanding the subsystems and components in transportation fuel cell systems. Similar model systems are being developed for medium- and heavy-duty vehicles [8–10] and have also been developed for stationary and other fuel cell types [11–14].
Example diagrams of fuel cell systems for automotive and heavy-duty vehicle applications are shown in Figure 1.2. These diagrams provide a representative illustration of typical BOP components and subsystems in transportation fuel cell systems.
Figure 1.2 Schematics of representative fuel cell system designs for automotive (top) and heavy-duty vehicle (HDV) applications (bottom) illustrating major balance of plant components and subsystems.
Source: Reproduced from Ref. [6].
As the power-producing component, the stack is the heart of the fuel cell system. The stack is a collection of individual galvanic cells, each of which provides <1 V when operating, connected in series to create a power device that provides a higher, more useful voltage. In some applications, multiple stacks may be used together for modular, higher power systems. Each cell contains a membrane electrode assembly (MEA), which is the electrochemically active stack component, with diffusion media (gas diffusion layers and microporous layers) on each electrode encouraging uniform distribution of reactants over the MEA and removal of water. MEAs are connected in the stack by electrically conductive bipolar plates to collect the electric current produced, which include flow channels facing the MEAs to deliver reactants to the electrodes. Bipolar plate assemblies also include coolant channels running between (and separated from) the MEAs to remove waste heat from the stack. Gas manifolds distribute gases between the cells in the stack, and gaskets are included to seal gas within the desired electrodes. The stack also includes structural components, including tie rods that hold the cells together and housing that encloses the stack.
Fuel must be supplied and prepared for the fuel cell system, requiring different BOP components depending on the type of fuel. If hydrogen is the fuel, the preparation required is minimal: the pressure and flow rate of hydrogen to the stack must be controlled, and in some cases the hydrogen may be humidified. Unused hydrogen may also be recirculated. More complex molecules, such as alcohols or hydrocarbons, may also be used as fuel for different types of fuel cell systems. High-temperature fuel cells (e.g. solid oxide) can use complex fuels directly, reforming them internally in the stack. Low-temperature fuel cells may include an external reformer, which produces hydrogen from the fuel and must also remove by-products, such as carbon monoxide, that are harmful to the fuel cell.
Fuel cell systems also require air to be supplied to the fuel cell stack. To enable high-power performance, the air is typically pressurized by a compressor, which may be a simple compressor or a compressor-expander module, which recoups some energy from the pressurized outlet stream to improve overall efficiency. Compression heats the air supply, so precooling may be necessary before the air enters the stack. The air supply may also be humidified to ensure optimal performance of membrane and electrodes, and air must be filtered to remove potentially harmful contaminants.
The thermal management subsystem removes waste heat from the fuel cell system, using coolant to transfer heat from the stack (and other BOP components, such as the compressor, as needed) to the radiator. Thermal management subsystems typically consist of pumps, coolant lines, and radiators, although the radiator is sometimes considered to be external to the fuel cell system. Multiple thermal management subsystems may be used, such as a high-temperature loop for the fuel cell stack and a low-temperature loop for the air processing subsystem.
The fuel cell system also includes components used to monitor and control the system. Numerous sensors are used in fuel cell systems, including stack voltage and current monitors, pressure and temperature sensors at different points in the system, and hydrogen sensors to detect leaks. These sensors provide information to the system controller, which directs the system to deliver requested power, while maintaining safe operation and avoiding conditions that may degrade the fuel cell.
Several other systems and components commonly accompany the fuel cell system, but are not considered a part of it, such as the fuel storage system, power electronics, and hybrid batteries. The boundaries between the fuel cell system and these other systems necessary for applications are often not defined consistently. However, these external systems have minimal impact on the choice of fuel cell materials and so will not be covered in this chapter.
1.1.3 Comparison of Fuel Cell Systems for Different Applications
The design of fuel cell systems and the technologies used vary significantly between different applications. For transportation fuel cell systems, flexibility and fast startup are critical, making PEM fuel cells the preferred technology. For automotive applications, the fuel cell system is typically sized to provide around 100 kW rated power and is usually accompanied by a hybrid energy storage battery to support transient and peak power demands. Larger fuel cell systems (hundreds of kW to several MW) with multiple stacks are used for heavy-duty vehicles, while smaller (up to tens of kW) but similar systems are used for material handling vehicles such as forklifts. Transportation fuel cell systems are typically direct hydrogen fueled, making the fuel supply subsystem relatively simple. The compressed air supply and heat rejection are both very important to enable high power density and specific power. A low cost is important for the fuel cell system to compete with incumbent combustion engine technologies. Durability is also a key concern for transportation fuel cells, as powertrains are required to endure thousands of hours of operation for automobiles and tens of thousands of hours for heavy-duty vehicles. The relative importance of different system metrics varies significantly between different transportation applications as well. For example, automotive fuel cell developers prioritize lowering capital cost and improving high-power performance to enable system size and cost reductions. In contrast, full lifecycle costs are important for commercial, heavy-duty vehicles, making durability and efficiency important priorities. Furthermore, for heavy-duty vehicle applications that carry heavy loads, the fuel cell system needs to be designed to deliver high power for more sustained periods, which can create more harsh conditions for fuel cell materials.
Stationary fuel cell systems vary widely in scale from <1 kW “micro-CHP” residential systems to large multimegawatt systems. For backup power systems, flexibility and responsiveness are critical, so PEM fuel cells are typically used. Because backup power systems operate only a small fraction of the time, capital cost dominates their overall cost. For distributed power and CHP applications, systems are typically operated continuously for very long periods, making durability and efficiency very important. The fuel processing system is also important for stationary fuel cell systems fueled by methane (natural gas or biogas). Stationary fuel cells have minimal constraints on the system size or weight.
For specialty applications such as material handling, fuel cells must provide at least equivalent performance without significant changes in functionality, size, and counterbalance weight compared to the incumbent technology. They must provide short bursts (15–20 seconds) of high power for lifting a heavy load, plus sustained power to drive the equipment. On the other hand, portable fuel cell systems typically have low power requirements. However, they are subject to extreme system size and weight limitations, and often are designed to minimize the required BOP, for example by operating at or near ambient pressure. Because these constraints impact the fuel storage system as well, liquid fuels are of interest for these applications. Cost may or may not be a serious constraint depending on the application; for consumer electronics, low costs are required to compete with Li-ion batteries, which have seen rapidly falling prices in recent years. However, for military or other specialty applications high costs may be acceptable.
1.2 Application-derived Requirements and Constraints
This section covers constraints on fuel cell operation and material choices that are imposed by the system and application requirements. Fuel cell materials must meet all system-level requirements simultaneously, which makes some otherwise promising materials infeasible. The most fundamental requirement of a fuel cell system is to provide the power demanded by the application. This requirement includes two broad categories: (i) maximum power performance, either instantaneous or sustained, and (ii) flexibility to deliver power under a variety of conditions and in response to changing demand. The fuel cell system, components, and materials must also be durable to provide the required performance not only initially but also after extensive use and exposure to potentially damaging conditions. Finally, fuel cell systems must be available at low cost to be competitive with alternative power systems, considering both initial capital and operating costs. It is important to note that performance, durability, and cost are interrelated, which allows for trade-offs between the three, depending on the lifecycle requirements of the application.
1.2.1 Fuel Cell Performance and the Heat Rejection Constraint
Cell-level performance is a fundamental issue underlying the system-level power density, specific power, cost, and efficiency. The fuel cell system must be sized to deliver the power required depending on the nature of the application and the system architecture. For a fuel-cell-dominant hybridization scheme, the fuel cell must deliver the required sustained maximum power, as the relatively small battery can add to the peak power for a limited period of time. For a battery-dominant hybridization scheme, the fuel cell instead must deliver the average power required, with the battery supplying power for peak demand. For example, fuel-cell-dominant automotive fuel cells operate most of the time at low-power conditions, where the system is most efficient, but occasionally require a high rated power (such as for highway merging). This makes rated power important because it drives system size requirements and is directly related to cost.
The voltage loss mechanisms that determine fuel cell performance, illustrated in Figure 1.3a, have been thoroughly described in many other texts on fuel cells and electrochemistry [15, 16], so we will only briefly recap them here. The ideal potential (for a perfectly reversible process) is determined by the overall thermodynamics of the fuel cell reaction, corresponding to 1.23 V for a hydrogen-oxygen fuel cell at standard ambient temperature and pressure [15]. Nonstandard thermodynamic conditions modify this potential as described by the Nernst equation, but the correction is generally small (on the order of 10 mV) for PEM fuel cells. The actual voltage of an operating fuel cell depends on the current density and is determined by voltage losses from reaction kinetics, Ohmic resistance, and mass transport.
Figure 1.3 Illustrations of (a) the voltage loss mechanisms that contribute to the fuel cell polarization curve and (b) the relationship among the fuel cell voltage, electrical power production, and waste heat that must be rejected.
The largest loss for low-temperature fuel cells under typical conditions is due to the slow kinetics of the oxygen reduction reaction (ORR) on the cathode. ORR kinetics for PEM fuel cells are well described by the Tafel approximation [17] and have a roughly logarithmic dependence on the current density, growing rapidly at low current density and then varying slowly at moderate to high current densities. ORR kinetic losses are impacted by the intrinsic activity of a catalyst material, the active surface area in the fuel cell electrode, and interactions with the polymer electrolyte in the electrode (the ionomer), which may coat the active surface. By contrast, the kinetics of the hydrogen oxidation reaction (HOR) at the fuel cell anode are extremely rapid, and HOR kinetic losses for PEM fuel cells are typically negligible, even with extremely low catalyst loadings. For high-temperature fuel cells kinetic challenges are minimal.
Ohmic losses increase linearly with current density in proportion to the overall resistance of the cell. In PEM fuel cells the membrane is typically the primary source of ohmic resistance, with the electrode ionomer also contributing significantly under some conditions. The carbon-based materials commonly used for gas diffusion media and catalyst supports contribute minimal resistance, although contact resistances and less-conductive, corrosion-resistant alternative materials may contribute significant ohmic losses. For high-temperature, solid oxide fuel cells, the ohmic resistance of the ceramic electrolyte typically dominates overall losses.
Mass transport-related losses are negligible at low current density but grow rapidly at high current density. The primary source of transport losses for hydrogen-air fuel cells is oxygen diffusion in the cathode. This includes both bulk oxygen transport through the electrode and local oxygen transport resistance associated with oxygen diffusion to a limited number of catalytically active sites, which is a particularly important and challenging problem for low-platinum group metal (PGM)-loaded electrodes [18]. Inadequate removal of product water can also lead to condensation or “flooding,” leading to significant mass transport losses. The effectiveness of mass transport is determined by the porous structure of the diffusion media and electrodes, including the catalyst support structure and ionomer dispersion.
Beyond simple single-cell performance, heat rejection puts an important constraint on performance. As illustrated in Figure 1.3b, as the voltage losses increase at higher current densities, the efficiency of energy conversion in the fuel cell declines. Consequently, the increase in power output slows, eventually reaching a peak at high current density and high voltage loss. This also leads to an accelerating growth in the amount of waste heat produced by the fuel cell.
During steady-state operation, the fuel cell system must remove all waste heat produced by the fuel cells stack. For brief periods, the stack can be allowed to generate excessive heat if it is at a relatively low temperature, so higher power is possible in transient operation than the continuous power rating. The heat Q rejected from radiator can be simply described by Newton's law of cooling:
where h is the heat transfer coefficient, A is surface area, and ΔT = Tc − Ta is the difference between the coolant temperature Tc and the ambient temperature Ta. For a given radiator, h and A are fixed, so Q/ΔT must stay below a certain value. This makes a particular value of Q/ΔT a metric to describe radiator capacity, which is limited for vehicle applications. To meet this heat rejection constraint, it is possible to either lower the amount of waste heat produced or raise the operating temperature. This sets a practical limit on the feasible fuel cell operating conditions [19].
A simple formula relates the Q/ΔT heat rejection metric to rated (continuous) power operating conditions, particularly the cell voltage Vr, which determines the fraction of energy converted to waste heat, and the stack coolant outlet temperature Tc, which determines how effectively that heat can be rejected through the radiator:
where Pg is the gross power rating of the stack, and Vi is the ideal cell voltage. This relationship is illustrated in Figure 1.4, which shows the dependence of the cell voltage at rated power Vr on Tc and Q/ΔT, using standard assumptions [20] for an automotive fuel cell system (90 kWgross rated power, 40 °C ambient temperature, and 1.23 V ideal cell voltage). In general, lower voltages (and therefore higher current and power densities) can be used with either a higher coolant temperature or higher Q/ΔT (i.e. radiator size). The DOE has set a target for fuel cell heat rejection of Q/ΔT ≤ 1.45 kW per °C to enable use of practically sized automotive radiators for fuel cell vehicles.
Meeting this automotive heat rejection constraint creates a strong motivation for using higher temperature (e.g. 94 °C) and higher pressure (e.g. 2.5 bar) operating conditions at rated power [19]. Raising the temperature allows higher current (and power) density by relaxing the voltage limitation imposed by heat rejection, for example from 0.76 V at 80 °C to 0.67 V at 94 °C. Raising the operating pressure also enables higher power density by improving kinetics and mass transport, although this performance boost must be balanced against higher parasitic power losses from air compression.
Figure 1.4 Plot of the relationship imposed by the heat rejection constraint among cell voltage at rated power, coolant outlet temperature, and the Q/ΔT metric.
The heat rejection constraint also translates into constraints on the set of viable fuel cell materials. For heat-rejection-constrained fuel cell applications such as transportation, high-power performance beyond the heat rejection limit is not useful, and the cell cannot be operated at the “maximum” point of the power curve shown in Figure 1.3
