Cells for Sustainable Energy --- Science meets social responsibility
The Challenge. Sustainable energy has emerged as the most pressing challenge facing humanity in the 21st century. Atmospheric CO2 levels have now reached ~ 382 parts per milliion by volume, the highest observed in 600,000 yrs. Prior to the industrial revolution, CO2 levels were at about 280 ppmv, and hovered around this level for at least 1,000 yrs.
Atmospheric CO2 levels over the past millenium
The increase in atmospheric CO2 over the past 200 years is squarely the result of human activity, with about 75% due to fossil fuel use and about 20% due to deforestation.
On the planet today, humanity uses approximate 13 terrawatts (TW) of power (terra = 1012). Various predictions suggest that if we are to stabilize atmospheric CO2 levels at about 550 ppmv (the safety of which we cannot even guarantee) by the year 2050, the world will require 20 TW of carbon-free power. The most plentiful energy source for meeting this huge global demand is the sun. However, by definition, solar energy is only available when the sun shines, which does not necessarily correlate with the times at which energy is in demand. Other carbon-free sources such as wind and tidal waves suffer from similar intermittant availability. Furthermore, these sources as well as others such as nuclear and geothermal are not available at the place of energy consumption. These considerations introduce the requirement for energy storage and transmission.
Beyond Hydrogen. There has been much national and international attention on hydrogen as the savior to address our energy and climate woes. However, those who are honest recognize that hydrogen is merely an energy carrier, not an energy source. As an energy carrier, or energy storage medium, hydrogen is perhaps the worst choice available. As a low density gas under atmospheric pressures, hydrogen is exceptionally difficult to store, requiring tremendous amounts of additional weight (a hydrogen tank is typically 15-20 or more times the weight of the hydrogen inside) and introducing significant energy penalties. Moreover, because hydrogen can permeate through most materials, transmission losses can be quite substantial. One benefit of hydrogen is the relative ease by which it can be generated through the process of water electrolysis. Here, an electrical voltage is applied to water, which, in turn, causes the hydrogen and oxygen atoms to dissociate from one another. A somewhat more elegant approach is to directly use solar energy to split water via a process called photolysis. Another, and highly touted benefit, of hydrogen is that, when used in conjuction with fuel cells, at the point of use there are absolutely no emssions, with the exception of pure, clean and benign water.
But how important is a carbon-free energy carrier? If one is interested in sustainable energy, an energy production cycle that is carbon-neutral provides exactly the same benefits as one that is carbon-free. What does this mean? This means, if we use CO2 as an input to make an energy carrier, then emitting CO2 at the point of energy consumption does not increase atmospheric CO2. This is precisely the idea behind using biomass for fuel. For example, we can, in principle, react water (H2O) and carbon dioxide (CO2) using our input energy source to make methanol (CH3OH). When we later use the methanol to generate energy, we produce CO2 as the by-product, but this CO2 is recycled in making more methanol. Another way to think of this is simply to consider the methanol as a hydrogen storage medium (if you can't get your mindset beyond hydrogen).
Fuel Cells. One goal of the fuel cell research carried out in the Haile group is to develop fuel cells that do not require hydrogn as the input fuel. Of course our fuel cells function well on hydrogen, but by eliminating the need for hydrogen, the many benefits of fuel cells (high efficiency, zero regulated emissions) can be realized without having to wait for a hydrogen infrastructure to be developed. Thus, we can by-pass the hydrogen distraction, and get on to the serious business of developing a sustainable energy cycle.
The basics of fuel cell operation are shown below. As indicated, the electrolyte component of the fuel cell can either be proton conducting or oxygen ion conducting. For the purposes of illustration, hydrogen is assumed to be the fuel, with more realistic fuels considered below.
Operation of a fuel cell based on a proton conducting electrolyte
Operation of a fuel cell based on an oxide ion conducting electrolyte
In both fuel cell types, the fuel is supplied to the anode and oxygen (typically in the form of air) is supplied to the cathode. The fuel and oxygen would 'like' to react and lower their energy (Gibbs Free Energy, to be exact), but cannot directly do this because of the presence of the electrolyte, which ideally is impermeable to gases. The only species than can migrate across the electrolyte are ions. In the case of the proton conducting electrolyte fuel cell, the hydrogen must drop off its electrons to become protons. The protons then travel across the electrolyte and must pick up electrons at the cathode in order to react with the oxygen and to form neutral water. Thus, the only way for the reactions to proceed is if the proton flux through the electrolyte is balanced by an electron flux through an exterior circuit. This electron flux, or electrical current, is precisely what provides electrical power.
In the case of the fuel cell with an oxide ion conducting electrolyte, the oxygen must pick up electrons at the cathode in order to become mobile oxide ions. After traveling across the electrolyte, the oxide ions must then drop off their electrons in order to react with hydrogen and form neutral water. Again, the oxide ion flux through the electrolyte must be balanced by an electron flux, or electrical current, through an exterior circuit.
For a direct methanol fuel cell using a proton conducting electrolyte, water is supplied to the along with the methanol and the anode reaction becomes
CH3OH + H2O -> 6H+ + 6e- + CO2
whereas the cathode reaction is unchanged. The complete fuel reaction is thus simply the oxidation of methanol
CH3OH + 3/2O2 -> CO2 + 2H2O
For a fuel cell with an oxide ion conducting electrolyte, in principle, any fuel can be directly electrochemically oxidized. That is, all of the carbon is converted to CO2 and all of the hydrogen to H2O by providing an adequate supply of oxygen ions. For this reason, oxide ion fuel cells are better suited to liquid fuels than are proton fuel cells. In practice, in the anode of the oxide ion fuel cell, the H2O and CO2 generated in the initial operation react with the incoming fuel to generate hydrogen and carbon monoxide, which are much more active species electrochemically than carbon containing fuels. Indeed, even CO might react with H2O more readily than undergoing oxidation at the anode. The relevant chemical reactions in the anode in the case where methane is the fuel are:
Steam reforming: CH4 + H2O -> CO + 3H2
Dry reforming: CH4 + CO2 -> 2CO + 2H2
Water-gas-shift: CO + H2O -> H2 + CO2
With background in mind, we aim to develop materials with unprecented properties for ion transport and electrocatalysis that will ultimately lead to exceptionally high power output fuel cells. This work is supported by the Moore Foundation, via the Caltech Center for Sustainable Energy Research (CCSER). Under support from the Global Climate and Energy Project administered by Stanford University, we aim to fabricate fuel cells with optimized architectures and structural features ranging from the nano- to macro-scales, again with the objective of achieving unprecented fuel cell power output.
Moore Foundation; Stanford Global Climate & Energy Project (GCEP)
- A more
in-depth "popular press" article based on a Watson
Lecture delivered by Sossina Haile in January 29, 2003. The article
Rolls and Oreo Cookies" appeared in the Caltech publication
Engineering and Science.