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4 Cell Performance: Where Do Inefficiencies Come From?

4 Cell Performance: Where Do Inefficiencies Come From?

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Chemistry of Sustainable Energy

Cell

power



Maximum

power



Cell voltage

Short

circuit



Open

circuit



FIGURE 6.5  Relationship between power and voltage. (Reprinted with permission from

Cracknell, J.A., K.A. Vincent, and F.A. Armstrong. 2008. Enzymes as working or inspirational electrocatalysts for fuel cells & electrolysis. Chem. Rev. 108:2439–2461. Copyright

2008, American Chemical Society.)



match E°cell. In reality, however, VOC is often less than Ecell due to the inefficiencies

we are about to discuss. Clearly, the closer VOC is to Ecell, the greater the integrity

of the cell.

At the other extreme, when the circuit is closed with no load, a maximum current results. This is denoted the short circuit current (ISC). Given the relationship

between the short circuit and the open circuit, the maximum power output for a

given cell will lie somewhere in between, as depicted in Figure 6.5 (Cracknell

et al. 2008).



6.4.2  Polarization

In an ideal reversible cell, the plot of voltage versus current would yield a straight

line. However, as Figure 6.6 illustrates, the cell potential typically decreases nonlinearly at high and low current draw. A close examination of Figure 6.6 reinforces

some important relationships regarding cell potentials:

• The thermoneutral cell potential (Etn) is the absolute, perfect, maximum

potential that can be obtained only in theory.

• The reversible cell potential (Ecell(reversible)) represents that maximum that

can be obtained in theory and taking into account inevitable losses due to

entropy (the second law of thermodynamics).

• The actual cell potential is diminished from Erev by irreversible losses due

to inefficiencies in mass transport, resistance, and kinetics (polarization).

These irreversible losses, generally referred to as polarization (or overpotential or

overvoltage), fall into three basic categories and areas on a voltage/current curve:

activation polarization, ohmic polarization, and concentration polarization. The

sources of these losses are briefly described below.



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Fuel Cells

Thermoneutral voltage (Tn)

Reversible loss



Cell potential (V )



Reversible potential:

The ideal cell potential–current relation

Total losses

Activation polarization

Ohmic polarization



Concentration polarization

Cell current I (A) or cell current density J (A/cm2)



FIGURE 6.6  A typical fuel cell polarization curve. (Reprinted with permission from Li, X.

2006. Principles of Fuel Cells. New York: Taylor & Francis.)



6.4.2.1  Loss Due to Activation

Loss of potential due to activation polarization is a result of the lag time of a slow

electrochemical reaction as the current is drawn. In essence, the reaction cannot

“keep up” with the current demand, so excess energy is required to speed up the

reaction.

6.4.2.2  Ohmic Losses

Ohmic losses are exactly what one would predict: there is invariably resistance in

the cell, whether it is resistance to ionic or electronic conductivity. In either case, the

efficiency of the cell is reduced.

6.4.2.3  Concentration Effects

Losses due to concentration differentials are related to mass transfer. When current

density is high, accumulation of products or depletion of reactants at the relevant

electrode can significantly hinder the transfer of the reacting species, leading to

decreased rate of reaction relative to the current demand.



6.4.3 Exchange Current

One additional source of inefficiency is seen in the fact that even at nearly zero

current, something is going on to reduce the potential significantly below

Ecell(reversible) (see Figure 6.6). This is a result of what is known as the exchange current, a phenomenon that amounts to “leakage” of electrons through the electrolyte.

Electrons are transported to the cathode and ions to the anode, where more electrons



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Chemistry of Sustainable Energy



are released to migrate back through the electrolyte to the cathode, continuing the

exchange. This process can reduce the efficiency of a real fuel cell by as much as

8–16% from the reversible cell efficiency (Li 2006).



6.4.4  Cell Performance and Nernst Equation

Lastly, the actual cell potential obtained from an electrochemical cell must somehow

take into account the actual concentration of the reacting species. For example, in

the hydrogen fuel cell, the oxidant is typically not 100% oxygen but is instead air.

The Nernst equation







°

Ecell = Ecell





RT

lnQ

nF





(6.18)



allows the calculation of Ecell from the standard potential by taking into account

concentration in the form of the reaction quotient, Q (Q = [red]/[ox]):







°

Ecell = Ecell

=



RT (nH2 /VH2 )

ln

nF (nO2 /VO2 )







(6.19)



6.5  FUEL CELL ELECTROCATALYSTS

As noted in the introduction, there are many different types of fuel cells and all

are made up of basically the same components. There are important differences,

however, in terms of the makeup of the catalysts needed to carry out the electricitygenerating reactions. In this section, we will focus on some important aspects of the

electrodes and catalysts that carry out fuel cell electrochemistry.



6.5.1 Electrocatalysis

Certainly among the most crucial components of a fuel cell are the electrodes, and

the sine qua non of the electrode is the electrocatalyst. Electrode construction and

catalyst preparation is a combination of art and science that has a profound impact

on fuel cell performance. All that needs to happen—adsorption of reactants, reaction, conduction of electrons and ions, and diffusion of products—must take place

at the atomic level at the boundary of the catalysts and the reactants. Ultimately,

mass transport of the reacting species to the electrodes is key to maximizing the

improvement in kinetics provided by the catalyst. Reactants must be able to diffuse

to the reaction sites, adsorb onto the electrode surface, react, and the products diffuse off—plus the electrons must be collected and allowed to flow through the circuit

with minimization of the exchange current. Thus the catalyst material, surface areas

and morphology all impact the overall efficiency of the cell, although a detailed

treatment of transport phenomena in fuel cells is beyond the scope of this text.

At this point, we will consider only the H2/O2 polymer electrolyte membrane fuel

cell (PEMFC), although many of the following concepts apply across the spectrum



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Fuel Cells



of fuel cell types. An electrocatalyst is needed for both hydrogen oxidation at the

cathode and oxygen reduction at the anode, and the platinum group metals (PGM)

are currently the most commonly used. The electrodes themselves must be porous

so that the gases can diffuse through the cell to the electrocatalytic surface—hence

the label gas diffusion electrodes. The usual preparation of the catalyst takes place

by adsorption of an aqueous solution of the metal salt (e.g., PtCl2) onto the support material, typically carbon black. This mixture is then treated with a reducing

agent (hydrogen or a metal hydride, for example) to reduce the metal. Other materials are added and the resultant “catalyst ink” is coated onto a gas diffusion layer.

Unfortunately, this method of electrode preparation leaves as much as 30% of the

metal inaccessible—and therefore catalytically inactive—so that developing new

methods for preparing the electrocatalyst is a vigorous area of research (Mitzel

et al. 2012).



6.5.2 Oxygen Reduction Reaction

While efficient transformations at both the cathode and the anode are required for

successful fuel cell performance, it is the oxygen reduction reaction (ORR) that is

the prime target for improving efficiency. The ORR is notoriously slow, an Achille’s

heel that impacts any fuel cell that uses oxygen as the oxidant. Given this (and the

acidic reaction conditions), platinum is largely considered indispensible as the ORR

electrocatalyst, presenting a serious sustainability and cost issue.

The mechanism of the ORR is quite complex and dependent upon the specific

electrode material. As a result, it is not well understood beyond the knowledge that

there are several elementary steps and uncertain intermediates. Two simplified overall pathways were introduced by Wroblowa et al. in 1976 (Wroblowa et al. 1976): the

two-electron peroxide pathway and the four-electron pathway:





Four-electron pathway: O2 + 4e − + 4H +



2H 2 O



(6.20)



and

Two-electron (“series”) pathway: (1) O2 + 2e − + 2H +





(2 ) H 2 O 2 + 2 e − + 2 H +



2H 2 O 2

2H 2 O



(6.21)



The intermediacy of hydrogen peroxide and the specific role of the metal have

yet to be fully understood. The peroxide/two-electron pathway predominates on

most carbon materials, gold, and metal oxides (Erable et al. 2012). The four-electron pathway predominates when platinum is involved; a plausible role for the metal

is shown in Figure 6.7. Molecular oxygen, pi-bound to the metal, accepts a large

amount of electron density into its π* orbital through back-bonding, thus weakening the oxygen–oxygen bond. Sequential protonation and reduction leads to a bishydroxyl species that can undergo reductive elimination to generate water (Okada

and Kaneko 2009).



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Chemistry of Sustainable Energy



Pt + O



O



Pt



O



or



O



Pt



OH

OH



2H+ + 2e–



Pt



O



H+ + e–



O



Pt



O



H



H+ + e–



O



Pt + 2H2O



FIGURE 6.7  Proposed intermediates in the oxygen reduction reaction. (Adapted from

Toda, T. et al. 1999. J. Electrochem. Soc. 1 146 (10):3750–3756.)



While platinum is an effective catalyst for the ORR, it comes at a cost: about onehalf of the expense of a fuel cell stack (even at high production volumes) is a result

of its use (Garland 2008). Given this reality and the fact that platinum is a limited

resource, reducing the amount of Pt (or finding reliable, efficient, sustainable, and

less expensive alternatives) is a high priority. A multitude of approaches exist and

several representative examples follow.

Platinum alloys. In a seminal discovery, Pt3Ni(111) was determined to be much

more active than the state-of-the-art Pt/C catalysts for PEMFC (Stamenkovic et al.

2007). The notation “111” is a crystallographic descriptor that designates the type of

exposure of the surface atoms, as indicated by the shaded area of a unit cell shown

in Figure 6.8. While the reason for this dramatic increase in activity is not yet well

understood (Sha et al. 2012), binary and ternary platinum alloys with 3d transition

metals (Ni, Co, Mn, Fe, Cu) are among the most active ORR electrocatalysts to date,

with Pt3Ni(111) the most active by far (van der Vliet et al. 2012).

Much research has been devoted to understanding the role of the bimetallic surface in catalytic activity. The presence of the metal heteroatoms somehow alters the

electronic and chemical properties of the catalyst as the alloy shows activity different from that of either metal alone. Two factors responsible for this phenomenon are



FIGURE 6.8  Representation of the {111} crystallographic plane.



Fuel Cells



153



postulated: (1) formation of heteroatom bonds at the metal surface, thus changing

the electronic environment and modifying its electronic structure and (2) alteration of the geometry of the structure so that the average metal–metal bond length

is changed. As a result, strain energy is introduced that modifies the electronic

structure through changes in orbital overlap (Kitchin et  al. 2004). These factors

are believed to lessen the adsorption of spectator species on the alloy surface and,

therefore, make additional active sites available to increase activity (Wang et  al.

2012; Yu et al. 2012).

Inroads via nanotechnology. Controlling the synthesis of the electrocatalyst

in terms of particle size, shape, stoichiometric composition, and homogeneity

has greatly elevated the performance of fuel cell catalysts (Wang et al. 2012), and

incorporation of nanoparticles has improved both the performance and durability of these catalysts. “Nano-segregated” PtNi catalysts synthesized at Argonne

National Laboratories (US) have shown activity seven times higher than the Pt/C

comparison for the ORR (Papageorgopoulos 2011) and a synthesis of PtNi nanoparticles using dimethylformamide as both solvent and reductant showed an enhancement almost 15 times higher than a “state-of-the-art” Pt/C catalyst (Carpenter et al.

2012). Nanostructured thin-film (NSTF) ternary electrocatalysts consisting of a

Pt68Co29Mn3 core have been “the workhorse cathode and anode of choice” according to a recent U.S. DOE report (U.S. Department of Energy 2011b) (U.S. DOE

2011). Overall, impressive progress has been made toward the reduction of platinum

use in fuel cells, with PGM content being reduced to 0.05 mg/cm2 of PGM on the

anode and 0.1 mg/cm2 on the cathode (U.S. Department of Energy 2011b) (U.S.

DOE 2011).

Non-PGM electrocatalysts. As platinum remains the primary factor in fuel cell

cost, research toward non-PGM or even metal-free catalysts is especially relevant.

In the realm of non-PGM catalysts, iron, cobalt, and manganese have all shown

promise (Morozan et  al. 2011; Tan et  al. 2012; Zhang and Shen 2012). Nitrogendoped carbon nanotubes, porous carbon, and carbon nitride are among the metalfree alternatives that have been studied with some initial success (Yang et al. 2011;

Yu et al. 2012). Treatment of graphene oxide (see Section 6.10.4) with ammonia in

the presence of boric acid yields a boron- and nitrogen-doped material with better

electrocatalytic activity than a commercially available Pt/C electrode (Wang et al.

2012). Enzymes are, potentially, the ultimate metal-free catalyst alternative for the

ORR. We will examine their role in fuel cell chemistry in the section on microbial fuel cells (MFCs) (Section 6.8). While the performance of these alternatives to

platinum is still quite low compared to the standard Pt/C catalyst, early results are

encouraging.

Although most research has focused on the ORR at the cathode, reducing platinum loading at the anode (where hydrogen oxidation takes place) is also critical.

Furthermore, given that most production methods for hydrogen result in some proportion of the by-product CO, development of an electrocatalyst that can effectively

tolerate this poison in the H2 gas stream has been a high priority. Incorporation of

ruthenium (in the form of a Pt0.5Ru0.5 alloy) increases the tolerance of the anode

to poisoning by CO (daRosa 2009). The presence of ruthenium–platinum bonding

on the catalyst surface removes the CO from the platinum, allowing for the water



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Chemistry of Sustainable Energy



gas shift reaction (Equation 5.5) to take place on the ruthenium surface (Takeguchi

et al. 2012). The rate-determining step for oxidation of hydrogen is the adsorption of

hydrogen onto the metal surface; thus, model studies on the kinetics and transport

phenomena at the catalyst boundaries are helping to elucidate the electrochemical

mechanisms (Zenyuk and Litster 2012).



6.5.3  Characterization of Catalysts

It would be a serious omission to conclude this coverage of electrocatalysts for

fuel cells without at least a brief mention of the experimental techniques used in

their characterization. As should be clear, being able to investigate the surface

properties as well as the electrochemical properties of potential electrocatalysts

is critical to evaluation of their potential. As regards methods to evaluate the

homogeneity, particle size, surface roughness, etc. on catalyst activity, techniques

such as scanning electron microscopy (SEM), transmission electron microscopy

(TEM), scanning transmission electron microscopy (STEM), extended x-ray

absorption fine structure (XAFS), x-ray diffraction (XRD), x-ray photoelectron

spectroscopy (XPS), and energy-dispersive x-ray spectroscopy (EDS) are routinely used to give an atomic-level view of the topography, particle size, and

homogeneity of these materials. Cyclic voltammetry and other electrochemical

studies (rotating disk electrode and rotating ring-disk electrode) provide a kinetic

and mechanistic probe of the redox chemistry. Finally, theoretical methods (particularly density functional theory) have proven to be useful in the calculation

of charge density and modeling the mechanism of the redox reactions on novel

catalyst surfaces.



6.6  POLYMER ELECTROLYTE MEMBRANE FUEL CELL

6.6.1 Introduction

With a basic understanding of the role of the electrocatalysts and the redox chemistry of hydrogen and oxygen, we can now examine some specific fuel cells in depth.

We will look at four: (1) the PEMFC (alternatively described as the proton exchange

membrane fuel cell), (2) the direct methanol fuel cell (DMFC), (3) the SOFC, and

(4) MFCs. While this selection is limited, it covers the most important fuel cells and,

arguably, the most promising in terms of sustainable energy generation.

A detailed schematic of a hydrogen PEMFC is shown in Figure 6.9. (Note: it

should not be assumed that a PEMFC is inevitably using hydrogen as a fuel. As we

will see in Section 6.6.4, the DMFC is a PEMFC that uses methanol as the fuel.

However, unless otherwise noted, in this text, PEMFC does presume that hydrogen

gas is the fuel source.) The PEMFC is a fuel cell that is of particular interest in portable applications, that is, transportation. It is considered a low-temperature (–20°C to

100°C) fuel cell, although significant effort has been devoted to the development of

higher-temperature PEMFCs that can operate above 100°C. Exclusive use of hydrogen as a fuel in vehicles powered by a PEMFC would erase the carbon footprint of

the transportation sector (neglecting, of course, the carbon cost of producing the



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Fuel Cells

4H+ + 4e– + O2 → 2H2O

Cathode flow channel



Air (O2)



Cathode GDL/MPL

Cathode electrocatalyst layer

MEA



H+



H2O



e–



Ionomer membrane

Anode electrocatalyst layer



Load



Anode GDL

Anode flow channel



H2



H2 → 2H+ + 2e–



FIGURE 6.9  Schematic of a PEM fuel cell. GDL, gas diffusion layer; MPL, microporous

layer; MEA, membrane electrode assembly. (Reprinted with permission from Borup, R. et al.

2007. Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chem.

Rev. 107 (10):3904–3951. Copyright 2007, American Chemical Society.)



hydrogen initially and the carbon cost in making the fuel cell materials). However,

not only are technological challenges associated with the generation of the necessarily huge volume of hydrogen fuel, the development of a sufficiently robust PEMFC

remains a substantial challenge.

What are the constraints faced by polymer chemists as they design a polymer for

use as an electrolyte membrane in a fuel cell?

• Perhaps foremost, the polymer membrane must be mechanically stable,

having a reasonably high Tg for operation at relatively high temperatures

(up to 120°C; U.S. Department of Energy 2011a).

• Water is a critical component in the successful operation of a PEMFC so

the polymer must perform well and be durable under conditions of high

humidity.

• In addition to thermal/mechanical stability, the polymer must be robust

with respect to its chemical reactivity.

• As with any electrolyte, it must act as an insulator to electrons.

• It must also be impervious to the fuel, preventing its crossover through the

electrolyte membrane to the cathode which would lower the VOC.

Because PEMs are used as the electrolyte in both the hydrogen/oxygen fuel cell

and the DMFC, much polymer development has been motivated by trying to minimize the methanol crossover problem: diffusion of methanol through the PEM to the

cathode. Crossover amounts of up to 10% can occur leading to a significant decrease



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