Supplying Energy Sustainably (VCE SSCE Chemistry): Revision Notes

Supplying Energy Sustainably

The challenge of fossil fuels

Fossil fuel combustion represents the primary source of greenhouse gas emissions and atmospheric pollutants that contribute to human-caused climate change. There is widespread recognition of the need to improve energy conversion efficiency and reduce both fuel consumption and pollutant emissions. In recent decades, scientists and engineers have been searching for alternative energy sources to protect our planet from further environmental damage.

Why we need alternatives to fossil fuels

Traditional power generation and transportation systems rely heavily on burning fossil fuels, which releases large quantities of carbon dioxide and other pollutants into the atmosphere. These emissions are driving global climate change and causing serious environmental harm. To address this crisis, we need to find more sustainable ways to meet our energy needs.

Biofuels as partial alternatives

Biofuels are fuels derived from plants or organic waste materials that can serve as alternatives to fossil fuels. Different types of biofuels offer varying levels of environmental benefits.

Types of biofuels and their benefits

• Bioethanol: Produces approximately $12\%$ less greenhouse gas emissions compared to equivalent fossil fuels
• Biodiesel: Generates about $10-20\%$ fewer greenhouse gas emissions than traditional diesel
• Solid biofuels: Include materials like wood and dried animal waste
• Biogas: Produced from organic decomposition, though with lower methane content than natural gas

Limitations of biofuels

Despite their environmental advantages, biofuels face significant constraints that limit their potential as complete replacements for fossil fuels.

Energy quality issues: Solid biofuels, whilst not competing with food production, typically have low energy content and don't produce much energy when burned. Similarly, biogas releases less energy per gram than natural gas because of its lower methane content.

Partial solution only: Because of these limitations, biofuels are likely to provide only a partial replacement for fossil fuels rather than a complete solution.

Fuel cells: a more efficient approach

For energy supplies to become truly sustainable and minimise carbon dioxide emissions, developed nations need to look beyond both fossil fuels and biofuels. Fuel cell technology is predicted to play a crucial role in our energy future.

Why fuel cells are more efficient

Fuel cells offer significant advantages over traditional power generation methods because they avoid the energy losses that occur in coal-fired power stations and combustion engines. This results in substantially reduced greenhouse gas emissions.

Fuel cell versatility

Whilst fuel cells can be designed to use various fuels including natural gas, methanol, ethanol and ammonia, most are designed to use hydrogen. Hydrogen-oxygen fuel cells produce only water as their product, making them particularly attractive for reducing pollution.

Hydrogen as a fuel for the future

Some scientists envision a future hydrogen economy where hydrogen replaces hydrocarbons as the principal energy source for transport and other applications. In this vision, fuel cells would replace internal combustion engines.

The hydrogen economy concept

The hydrogen economy offers the potential for dramatically reduced greenhouse gas emissions and pollutant production without compromising quality of life. However, this depends on producing hydrogen from renewable sources rather than fossil fuels.

Challenges for hydrogen adoption

Widespread use of hydrogen as an energy source presents several challenges:

• Production methods
• Distribution infrastructure
• Storage solutions
• Safety considerations

Hydrogen production methods

Current production: steam reforming

At present, approximately $95\%$ of hydrogen is produced from fossil fuels such as natural gas, oil and coal through a process called steam reforming. In this process, steam reacts with fossil fuel at high temperature in the presence of a nickel catalyst.

For methane:

$\text{CH}_4(g) + \text{H}_2\text{O}(g) \xrightarrow{\text{Ni}} \text{CO}(g) + 3\text{H}_2(g)$

The carbon monoxide produced can generate additional hydrogen using a copper or iron catalyst:

$\text{CO}(g) + \text{H}_2\text{O}(g) \xrightarrow{\text{Cu or Fe}} \text{CO}_2(g) + \text{H}_2(g)$

Limitations of steam reforming

Whilst a fuel cell using hydrogen can be described as a 'zero-emission' device (producing only water, electricity and heat), the production of hydrogen through steam reforming creates significant problems.

Potential bridging solution

Some advocates suggest that carbon dioxide from steam reforming could be captured at source and stored, preventing its release to the atmosphere. Hydrogen from this source has been proposed as a temporary solution to help meet climate objectives whilst the cost of producing hydrogen from renewable sources decreases.

Sustainable hydrogen production

Currently, only two practical methods exist for generating hydrogen sustainably:

1. Electrolysis using renewable energy: Using electricity from solar farms, wind farms or other renewable sources to convert water into hydrogen (covered in later chemistry studies)
2. Biogas conversion: Collecting methane from landfill sites and converting it to hydrogen through steam reforming

The cost challenge

Hydrogen sourced from renewable sources (often called 'green' hydrogen) is the most desirable option because the energy it produces would be truly sustainable. However, it currently costs approximately twice as much as hydrogen produced from coal and natural gas.

Hydrogen supply, storage and safety

Infrastructure requirements

Widespread adoption of hydrogen as the primary fuel for vehicles and electricity generation would require massive investment in infrastructure, including:

• Modified or new pipelines
• Hydrogen filling stations
• Improved storage facilities

Storage challenges

Although hydrogen has very high energy content by mass ($143 \text{ kJ g}^{-1}$ compared to petrol's $44 \text{ kJ g}^{-1}$), storage presents significant challenges, particularly for vehicles.

Materials-based storage solutions

Scientists are investigating alternative storage methods known as materials-based storage, which show considerable promise but require further development. This technique uses materials—either solids or liquids—that can absorb or react with hydrogen and then release it when needed.

Advantages: These methods may allow hydrogen storage in:

• Smaller volumes
• Low pressure
• Temperatures close to room temperature

Metal organic frameworks

Metal organic frameworks (MOFs) represent a promising new class of materials for capturing, storing and delivering gases including hydrogen and carbon dioxide.

Applications: Beyond hydrogen storage, MOFs have potential uses in:

• Gas separation
• Catalysis
• Drug delivery
• Conducting solids

Safety considerations

Hydrogen use poses unique safety challenges because hydrogen is highly flammable and potentially explosive. Implementation would require:

• Strict codes and standards
• Hydrogen sensors to detect leaks
• Appropriate safety protocols

Designing better fuel cells

Fuel cells are likely to play a critical role in developing technologies for:

• Renewable energy generation
• Energy storage
• Energy management
• Greenhouse gas reduction

Scientists continue to optimise electrode materials, electrolytes and cell designs to improve characteristics such as cost, energy storage capacity, safety, lifetime and performance.

Environmental and humanitarian considerations

One important consideration is avoiding materials that pose environmental and humanitarian risks, particularly heavy metals like cobalt and nickel.

Better alternatives: The platinum group metals (platinum, rhodium, palladium) widely used in modern fuel cells are considered to have fewer associated risks than some other metals.

Green chemistry principles

The development of new battery and fuel cell technologies exemplifies the move from a linear economy (products discarded after use) to a circular economy (optimal use and reuse of resources). These developments also demonstrate the importance of creativity and innovation in applying green chemistry principles to design safer and more sustainable products.

Emerging technologies

Society increasingly expects products designed for reuse rather than disposal. Two examples of emerging fuel cell technologies that embody these principles are:

• Microbial fuel cells
• Redox flow batteries

Microbial fuel cells

A microbial fuel cell (MFC) is a device that converts organic material into electrical energy through the action of microorganisms.

How microbial fuel cells work

Construction: An MFC contains two compartments separated by a membrane:

Chemical reactions in MFCs

At the anode:

$\text{organic material (s or aq)} \rightarrow \text{CO}_2(g) + \text{H}^+(aq) + e^-$

At the cathode:

$\text{O}_2(g) + 4\text{H}^+(aq) + 4e^- \rightarrow 2\text{H}_2\text{O}(l)$

Advantages of microbial fuel cells

Operating conditions: MFCs operate at or near room temperature, making them practical for many applications.

Fuel sources: They can use low-grade waste materials including:

• Soils and sediments
• Wastewater
• Agricultural waste

Applications:

• Sustainable wastewater treatment
• Contaminant removal
• Low-power electricity generation

Energy efficiency: MFCs achieve energy efficiencies in the $21-35\%$ range.

Redox flow batteries

Redox flow batteries represent an interesting type of fuel cell under development, offering potential advantages over traditional battery technologies.

Construction and operation

A redox flow battery has several key features:

• Two separate compartments containing solutions of reactants
• Storage tanks from which solutions are pumped into the cell
• Membrane between compartments that allows ion exchange whilst preventing contact between solutions
• Reversibility: Can be recharged by applying opposite voltage, causing the reverse reaction

Advantages

• Lower cost: Potentially cheaper energy storage than devices like lead-acid batteries
• Safer materials: Don't contain potentially toxic metals like lead, cadmium, zinc or nickel
• Easy transportability: Can be moved relatively easily
• High efficiency: Some designs achieve up to $85\%$ energy efficiency
• Scalability: Can be joined together for large-scale deployment

Disadvantages

• Complexity: Storage tanks, pumps and flow control units add weight and operational complexity
• Size: Not ideal for small-scale applications

Current applications

The main development focus is on:

• Remote area power systems
• Grid energy storage
• Compensating for fluctuations in renewable energy supply

Types of redox flow batteries

Vanadium redox flow batteries: First developed in Australia nearly 40 years ago, these batteries:

• Offer immediate energy release
• Have long lifespans
• Use vanadium, which is much less toxic than lead or cadmium
• Achieve energy efficiencies up to $85\%$

Zinc-bromine redox flow batteries: Another type being commercialised that:

• Offers sustained energy output
• Has a lifetime exceeding 10 years
• Has been described as a type of electroplating machine (zinc metal plates onto an electrode during operation)