Re: [Phys-L] Electrolysis under Pressure

[John Denker <jsd@av8n.com>]

On 06/16/2014 06:45 AM, Diego Saravia asked:

> > Now, during the electrolysis, applying exactly 1.229 V at 298 K will
> > take the system to deltaG° = 0 kJ
> > This corresponds to an equilibrium system with K = 1: pure water and
> > 1 bar of each gas. Applying double the voltage will drive the reaction
> > to beyond equilibrium state again.
> is posible to do that in practice?

Yes, to a good approximation.

Proof by construction:  Electrochemical /fuel cells/
have been around since the early 1900s.  The reaction
is the reverse of the electrolysis reaction.

A laboratory-grade fuel cell, operating under low-load 
conditions, should come pretty close to thermodynamic 
reversibility.

    Industrial fuel cells are somewhat more lossy.
    This is because they trade off efficiency in
    favor of greater power density.

The thermodynamics of electrolysis is straightforward.
It is spelled out here:
  http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html

The thermodynamics of fuel cells is essentially the 
same, and is spelled out here:
  http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html#c2

Tangential remark:  The hyperphysics site is quite
good overall.  It has wide coverage of physics topics.
It is reasonably clear.  It is incomparably more 
trustworthy than wikipedia.

=============

Exciting story:  Many NASA spacecraft rely on fuel
cells to generate electricity.  On Apollo 13, when 
the oxygen tank blew up, not only did they lose their
supply of breathable oxygen, they lost their source
of electricity.  The crew could easily have died from 
lack of power to operate the command-module during 
reentry.  It took ingenious engineering to solve the 
problem.  The guys in Houston found Rube Goldberg
methods for (a) using the LM to charge the CM batteries, 
and then (b) minimizing the load.  Even so, there was 
just barely enough juice to do the job.

The guys on board were not the only heroes in this
story.