An ode to electrochemistry

Start with why the field is powerful at all. Spontaneous reactions run downhill and happen by themselves; the interesting ones run uphill and do not. Electrochemistry makes uphill reactions go by applying a voltage. Chiang’s illustration is a lithium-ion cell, which sits at roughly three and a half to four volts. Charging it pushes a lithium ion across about four volts, imparting four electron volts of energy to that ion, which he puts at roughly ten times the heat of vaporization of water and thermally equivalent to about 46,500 degrees kelvin. You do that at room temperature by turning a knob. Set it against mechanical energy, meaning how much you can store elastically in a solid before it snaps, and the electrochemical number is enormous by comparison, which is why he says electrochemistry is powerful enough to break anything. That is meant literally: put an acoustic sensor on a fresh cell and cycle it, and you hear the solid compounds inside cracking. His group later ran that backwards and used electrochemistry as a mechanical actuator, including a DARPA project to twist helicopter rotors in flight.

The limitation follows from the same picture. Electrons have to cross into or out of an electrode, so an electrochemical reaction is a two-dimensional process happening at an interface, while a thermal reaction is a three-dimensional process happening throughout a volume. You can engineer more surface area, which Chiang guesses buys you something like a factor of ten, but the constraint is fundamental and no amount of cleverness removes it. Hence his warning against the hammer looking for a nail. The interesting designs do not electrify every step; they electrify the step where electrochemistry does the most good.

Cement is the worked example. The tempting move, given cheap electricity, is to use it as heat, and that is precisely what Sublime did not do. Instead the electricity goes into an electrolyzer, and the electrolyzer makes reagents. Split water at something above roughly one and a quarter volts and you get hydrogen at one electrode and oxygen at the other; because you are pulling hydrogen off the water at one end and oxygen off at the other, one electrode is left basic and the other acidic. A single device therefore produces acid and base at once, and those reagents do the chemical work downstream in a tank, where the reaction is volumetric rather than interfacial. Water splitting is the familiar illustration; describing his own process he corrects himself in passing to splitting salt. Chiang says the same pattern has since propagated into mining. It also solves a second problem almost for free. Cheap electricity is often intermittent electricity, and acid and base are storable, so they act as chemical storage and let the downstream process run continuously on a front end that runs only when power is cheap. He is careful not to oversell that: if your capacity factor is well below one hundred percent, the capital cost consequence is what it is and cannot be engineered away. What storage buys is decoupling in time, not a free lunch. Kann’s summary, which Chiang accepts, is that you either bolt a battery onto the process or design one into it.

Which gives the hunting-ground heuristic. Look for transformations that are not merely energy-hungry but energy-intense, where the energy has to be concentrated in one place, usually involving small inorganic species, and which today are done by burning fossil fuels for high heat. The contrast is biology, where the total energy involved may be large but the concentration is low, and where bulky organic molecules make it hard to reach high current density, the amount of current you can push through a given area of electrode. The boundary on the other side is solid-to-solid transformations, which electrochemistry handles badly because insulating solid particles do not conduct electrons well enough to react briskly at an electrode. Notably, Chiang names that limitation and then names a solid-to-solid reaction, reducing iron oxide to iron metal in an alkaline electrolyte near room temperature, as one of the things he finds most exciting, on the grounds that his group thinks it has a way around the problem. The boundary is a live research question, not a settled wall.