lecture6.md

class: title, no-number

Lecture 6 - Fuel cells

.footer[- Return to course contents ]

---

class: roomy

Lecture Summary

$\require{mediawiki-texvc}$

• Fuel cell introduction
• Types of fuel cells
  • Polymer cells
  • Solid oxide fuel cells (SOFCs)
• Materials requirements for SOFCs
  • example materials
• Defect ordering

---

class: compact

Fuel Cells

Fuel cells are similar to batteries; they have a cathode, electrolyte and anode.


Electricity can be generated as long as fuel is supplied (they don't need to be recharged)

---

class: compact


---

Fuel cell fundamentals

$$ \ce{ {Fuel} + O2 -> H2O + nCO2} $$

• Fuel cells grouped into low-temperature (LT, < 200 °C) and high-temperature (HT, > 450 °C).

• H₂ is the preferred fuel

  • Particularly for LT devices.
  • Doesn't produce CO₂ --

• Other fuels (e.g. CH₃OH, CH₄, NH₃) also possible

  • Steam reforming reaction converts fuels to $\ce{H2}$: $$\ce{CH4 + H2O -&gt;[&gt;700 ^{\circ}C] CO + 3H2}$$
    • can be achieved in-situ for HT cells, but must be separate for LT.

---

class: compact

Fuel cell efficiency

Fuel cells are very efficient

• Convert fuel → electricity directly, rather than
  fuel → heat → electricity (as in combustion)

$$ \text{Thermodynamic efficiency} = \frac{\Delta G}{\Delta H} $$

e.g. for $ \ce{2H2 + O2 -> 2H2O} \ (\Delta H = -571.6\ \text{kJ mol}^{-1})$:

$$ \begin{align} \text{Cathode:} & & \ce{4H+ + O2 + 4e- &-> 2H2O & & E} = +1.229 \mathrm{\ V} \\ \text{Anode:} & & \ce{4H+ + 4e- &<- 2H2} & & E = 0.00 \mathrm{\ V} \\ \end{align} $$

$$ \begin{align} \Delta G &= -nFE \\ &= -4 \times F \times 1.229 \\ &= -474.3 \mathrm{\ kJ\ mol^{-1}} \qquad (\mathrm{per\ mole\ O_2}) \end{align} $$

Efficiency = η = -474.3 / 571.6 = 83%

---

class: compact

Efficiency with temperature

$$ \Delta G = \Delta H - T\Delta S, \quad \therefore \quad \frac{\Delta G}{\Delta H} = \eta = 1 - \frac{T \Delta S}{\Delta H} $$

--

For 'ideal' combustion engine (heat engine) the maximum efficiency is the Carnot limit:

• $ \eta = \frac{T_{\mathrm{hot}} - T_{\mathrm{cold}}}{T_{\mathrm{hot}}} $

--


???

A heat engine is one that converts heat into work, for instance by repeatedly heating and cooling a substance while extracting (usually mechanical) motion. The term covers a huge range of technologies including combustion engines, thermal power stations and fridges (a heat pump, which is a heat engine operating in reverse).

A really nice example (in which the heating/cooling is done on a sealed chamber of air) is the Stirling engine:

<iframe width="560" height="315" src="https://www.youtube.com/embed/taDHMw38aE0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

---

class: compact

Types of fuel cell

Type	Mobile ion	Temperature (°C)	Applications
Alkaline	OH-	50-100	Stationary power, space missions
Polymer	H+ or OH-	50-100	Portable devices, transport
Phosphoric acid (PAFC)	H+	220	Medium to large scale combined heat and power (CHP) systems
Molten Carbonate (MCFC)	CO32-	650	⋮
Solid Oxide (SOFC)	O2-	500 - 1000	⋮

.footer[ - [L. Carrette, Fuel Cells, 2001, 5.](https://doi.org/10.1002/1615-6854(200105%291:1%3C5::AID-FUCE5%3E3.0.CO;2-G) ]

---

class: no-number

Main low temperature device:
Proton exchange membrane fuel cell (PEMFC)

• Carbon electrodes with precious metal catalysts (Pt, Pd, Ru)
• Requires acidic proton-conducting polymer
  • e.g. Nafion
• Use H₂ as fuel, but can work with MeOH (less efficiently)

 

.footer[

• Nafion
• Gemini spacecraft ]

???

Sometimes also known as polymer electrolyte membrane fuel cell

---

class: compact

PEMFC + H₂

$$ \begin{align} \text{Anode:} & & \ce{2H+ + 2e- &<- H2} & & E = 0\ \mathrm{V}\\ \text{Cathode:} & & \ce{O2 + 2H+ + 2e- &-> H2O2} & & E = 0.695\ \mathrm{V} \\ & & \ce{H2O2 + 2H+ + 2e- &-> 2H2O} & & E = 1.776\ \mathrm{V}\\ \text{Cathode (Overall):} & & \ce{O2 + 4H+ + 4e- &-> 2H2O} & & E = 1.229\ \mathrm{V} \\ \end{align} $$

• Good Low-temperature (< 100 °C) operation .green[✔]
  • Quick to start/stop
  • Suitable for portable applications
• H₂O₂ forms when acidic .red[✘]
  • Corrodes carbon-containing electrodes
  • Lowers cell voltage
  • Requires expensive Pt or Pd catalysts to decompose H₂O₂
• Need careful hydration to ensure H⁺ conduction .red[✘]

---

exclude: true class: compact

PEMFC + Methanol

Methanol easier to store/transport than H₂

• Readily oxidised, does not require C-C bond breaking

$$ \begin{align} \text{An.:} & & \ce{CO2 + 6H+ + 6e- &<- CH3OH + H2O} & & E = 0.046 \mathrm{\ V} \\ \text{Cat.:} & & \ce{\frac{3}{2}O2 + 6H+ + 6e- &-> 3H2O} & & E = 1.229 \mathrm{\ V} \\ \text{Overall:} & & \ce{ CH3OH + \frac{3}{2}O2 &-> CO2 + 2H2O} & & E= 1.183\mathrm{\ V} \end{align} $$ 

Problems

• MeOH crosses from anode to cathode .red[✘]
  • Reduces cell voltage to ~0.5 V
• CO formed in side-reaction, blocking reaction sites .red[✘]
  • requires more Pt catalyst!

---

exclude: true

Alkaline polymers?

• OH^- as mobile ion prevents H₂O₂ formation .green[✔]
• pH change alters redox energies, allowing Ni catalysts to replace Pt .green[✔]
• Attaching counter-cation to the polymer reduces electrode poisoning .green[✔]


• Current OH^- polymers have low ionic conductivity! .red[✘]

.footer[- Y-J. Wang et al., Chem. Soc. Rev. 2013, 42, 5768-5787. ]

---

class: compact

Main high temperature chemistry:
Solid Oxide (SOFC)

• All-solid-state system (i.e. solid electrolyte)
• Most work at 800 - 1000 °C

--

• Based around redox and conduction of O^2-:

$$ \begin{align} &\text{Anode:} & &\begin{cases} \ce{2H2O + 4e- &<- 2H2 + 2O^{2-}} \\\ \ce{2CO2 + 4e- &<- 2CO + 2O^{2-}} \\\ \ce{H2O + \frac{1}{2}CO2 + 4e- &<- \frac{1}{2}CH4 + 2O^{2-}} \\\ \end{cases} \\\ &\text{Cathode:} & &\quad\ce{O2 + 4e- -> 2O^{2-}} \\\ \end{align} $$

--

• High temperature allows internal steam reforming; many fuels
• No precious metal catalysts
• Excess heat can be used to increase efficiency (to ~90%)
  • drive an electricity turbine or combined heat and power (CHP)

---

class: compact

SOFC Limitations

High temperatures:

• prevent rapid start/stop
• cause reactivity between electrolyte and electrodes
• make thermal expansion important

--

Delicate balance between:

• optimum temperature for redox and/or ionic conductivity
• thermal expansion, reactivity and device construction
• Intermediate-temperature (IT) SOFCs are the current optimum.


---

Requirements for SOFC materials

Property	Anode	Electrolyte	Cathode
Electronic conductivity	High	Low	High
Ionic Conductivity	High	High	High
Chemical stability	reducing conditions	oxidising and reducing conditions	oxidising conditions
Catalytic activity	Fuel oxidation	$\ce{O2}$ reduction	$\ce{O2}$ reduction

Also: chemical compatibility between materials, similar thermal expansion, low cost, ...

---

class: compact

'Perfect' electrodes

Ideally, electrodes should be good electronic and ionic conductors!

• fuel/oxygen reactions would occur at the electrode surface


--

In reality, use a mixture of good ionic and electronic conductors.

• reactions occur at the triple phase boundary


---

Typical anode materials

Usually a cermet (i.e. mixture) of Ni and electrolyte

• Ni → high e^- conductivity and catalytic activity
  • but susceptible to poisoning by S (forming stable $\ce{NiS}$)
• High ionic conductivity from electrolyte


---

class: compact

Typical cathode materials

Composite of $\ce{La_{1-x}Sr_{x}MnO_{3}}$ perovskite (LSMO) and electrolyte

• LSMO gives e^- conduction and high catalytic activity
  • $\ce{Sr^{2+}}$ subtitution generates holes in valence band
• poor performance below 700 °C .red[✘]

--

.pull-right[ ]

Interest in mixed-conductors:

• $\ce{La_{1-x}Sr_{x}CoO_{3-y}}$
  (perovskite with $\ce{V_{O}}$)
  • good ionic/electronic conduction
  • high thermal expansion
• $\ce{La2NiO_{4+x}}$
  • 'layered' $\ce{O_{i}}$ conductor
  • $\ce{2Ni_{Ni} + \frac{1}{2}O2 &lt;=&gt; O_{i}^{''} + 2Ni_{Ni}^{\bullet}}$

---

class: compact

Electrolyte materials

Most studied electrolyte is $\ce{Y_{0.15}Zr_{0.85}O_{1.925} }$ (yttrium-stabilised zirconia, YSZ)

• defective fluorite structure
• $\ce{Y2O3 + 2Zr_{Zr} + O_{O} &lt;=&gt; 2 Y_{Zr}^{'} + V_{O}^{\bullet\bullet} }$
• Sc-doping also effective (but expensive)

.pull-right[ ]

Another commercial material is $\mathrm{Gd_{0.1}Ce_{0.9}O_{1.95}}$ (CGO)

• Better for lower temperature
  • e^- conductor above 600 °C

Many other materials, but issues with cost, stability, manufacturing...

---

class: compact, no-number

Improving Ionic conduction

As $\sigma = nq\mu$, so as [defects] ↑, $\sigma$ ↑

--

However, at high defect concentrations we can get defect clusters

• Local ordering of defects reduces mobility

--

e.g. in YSZ: $(\ce{(1-x)ZrO2 + \frac{x}{2}Y2O3 -&gt; Y_{x}Zr_{1-x}O_{2-\frac{x}{2}} })$

.pull-left[] .pull-right[ ]

.footer[

• Conductivity vs $x$ in YSZ
• Ideal arrangement for $x=0.5, \ce{Y2Zr2O7}$ ]

???

In YSZ, introducing two Yttrium atoms (x=2) results in the formation of one oxygen vacancy. Locally, these are found to order; the optimum arrangement is for each OM₄ tetrahedron to contain 2 Zr and 2 Y, with oxygen vacancies arranging in a 'grid-like'p pattern.

The figure (a) shows the the equilibrium arrangement for Y₂Zr₂O₇ (x = 0.5). Compare this with the ideal fluorite structure consisting of edge-sharing OM₄ tetrahedra, shown below:

---

class: compact

Long-range defect ordering

In some cases defects can form long-range order

• Many show order-disorder phase transition with T

Example: Ba₂In₂O₅

• Brownmillerite structure ($\ce{ABO_{2.5}}$ perovskite with ordered $\ce{V_{O}^{\bullet\bullet}}$)
• Large increase in $\sigma$ at phase transition

.pull-left[ ] .pull-right[  ]

---

time: 48:56 class: compact

Lecture recap

• fuel cells operate like a battery with continous 'charge' supply
  • Many similar materials properties required
• different technologies work at different temperatures
  • advantages and disadvantages for both
• properties of electrolyte, cathode and anode must be optimised
• ideal electrodes would be ionically and electronically conducting
  • more commonly a mixture of materials is used
• Ionic conduction reaches a maximum with defect concentration
  • defect ordering occurs
• Defect ordering can give rise to new structure types

---

Feedback

.footer[- Return to course contents ]