Frontiers in Chemistry CHEM1008 Lecture 1 PDF

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices 1 Fuel Cells Prof. Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 1 Learning Outcomes 1. U...

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices 1 Fuel Cells Prof. Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 1 Learning Outcomes 1. Understand the need for new devices that can help solve problems of climate change 2. Consider some of the main options for generating power sustainably 3. Know why burning a fuel may not be the best way to extract its energy 4. Understand how fuel cells work and appreciate some of the challenges we face in making them economically and technologically viable  Climate Change o “Climate change is the greatest environmental challenge facing the world today” – Dept for Environment, Food and Rural Affairs (DEFRA) o CO2, N2O and CH4 accumulating in the atmosphere prevent radiative escape of heat from the Earth and cause increased temperatures at the surface and the lower atmosphere  The global land-ocean temperature index Source: NASA/GISS o Nineteen of the 20 warmest years all have occurred since 2001 o 2023 on track to be warmest on record The global land-ocean temperature index time series For full animation see https://climate.nasa.gov/vital-signs/global-temperature/  The effects of climate change o 1.5 billion people live in water-stressed regions – 7 billion by 2050 o Some areas could become more fertile – others more barren o Expect regional food shortages, mass migration, more poverty and malnutrition in developing countries – Source: IPCC o More frequent and intense heatwaves, floods, storms, wildfires and droughts o Deaths from cold-related diseases will reduce o Patterns of disease will change  Replacing conventional fossil-fuel economy o A 500 MWe plant may consume coal at 250 tonnes per hour under full load o Our local plant at Ratcliffe-on-Soar is one of the last coal-fired power stations in the country, and it’s a big one – 2.1 GW capacity  Replacing conventional fossil-fuel economy with alternative power sources o Water, wind, geothermal energy o Clean but may involve high capital costs and depend on geography o Solar energy o Enough energy hits the Earth in about 1.5 hours to meet demand for 1 year o Must be used as generated or stored somehow for when Sun goes down o Batteries o Energy from solar plants or wind farms could be stored in large batteries for use when Sun goes down or wind stops blowing o Hydrogen and fuel cells o Use solar power to make hydrogen from water for later use as a fuel?  Hydrogen power and fuel cells o The BMW Hydrogen 7 above was a limited production (100 made) hydrogen-powered internal-combustion engine vehicle built from 2005-2007 o Is a combustion engine the best way to use H2?  Nature’s limit on the efficiency of combustion engines o Consider a simple heat engine (steam and gas turbines) o They turn heat (random motion of atoms) into work (net motion of atoms in a direction) o The Second Law of Thermodynamics tells us that some of the heat must be discarded to a cold sink  Maximum efficiency of heat engines o Carnot worked out the thermodynamic limit for conversion of heat into work in an engine: (T1 – T2)/T1 o All temperatures are in Kelvin, where room temperature is about 290 K o Example: What is the efficiency of steam turbine operating at 400 ℃ (673 K) with the water exhausted through a condenser at 50 ℃ (323 K)?  Fuel cells versus combustion engines o Much higher efficiencies can be achieved using fuel cells (up to about 90% in some cases) o Fuel cells do produce some heat, but we have some clever ways to re-use that heat increasing the efficiency as much as possible  The net chemistry occurring in a hydrogen fuel cell o The net reaction is the same as when we burn H2 in air (or in a H2-powered combustion engine): 2H2(g) + O2(g) → 2H2O(g) ∆rG o The usable energy released is the Gibbs energy, which we will meet in CHEM1011 The detailed chemistry occurring in a hydrogen fuel cell o The H2 and O2 react in 2 conceptual half-reactions o The anode reaction (where oxidation happens) is 2H2 → 4H+ + 4e– o The cathode reaction (where reduction happens)is O2 + 4H+ + 4e– → 2H2O  The key parameter is power! o (Pcell = Icell × Ecell) o Ecell is a measure of the electrical energy pushing electrons from the anode to the cathode – the driving force o It is related to the Gibbs energy of the net reaction, ∆rG, in a simple way: ∆rG = –zFEcell So what is the expected cell voltage of a H2 fuel cell, considering that ∆rG = –228.6 kJ mol–1?  What about the cell performance? o We can change the current – to make it (and the power) as big as possible, we make the electrodes as big as possible o We coat the electrodes with catalysts (usually Pt) to speed up reactions o We put the electrodes as close together as possible What are the electrical properties of a fuel cell? theoretical In real devices we use stacks of multiple cells to boost the power  What can chemists do to make fuel cells viable? o Make new, cheaper, and better catalysts, especially for the oxygen reaction o Make new, cheaper, and better ionically-conducting membranes o Work with engineers to work out better ways to quickly deliver reactants to fuel cells and combat mass-transport losses o Work with social scientists and industry to tell the public about the opportunities offered by hydrogen and fuel cells Maybe one day we will all drive one of these…