Design of a Novel Electrochemical Membrane Reactor for Hydrogen Production Via the Sulfur-Ammonia Water-Splitting Cycle

The solar-driven hybrid
sulfur-ammonia (S-NH₃) water splitting cycle is a novel approach to
produce hydrogen through a large-scale design. Originally proposed by T-Raissi et al.
(2006), this cycle involves the photochemical oxidation of ammonium sulfite as
a solar-photocatalytic hydrogen production step. Later, the resulting ammonium
sulfate is decomposed into ammonia, sulfur dioxide and oxygen through a high-temperature
solar thermochemical oxygen evolution step. Finally, ammonium sulfite is
regenerated by means of chemical absorption of ammonia and sulfur dioxide in
aqueous phase. Thereby, only water is needed to release hydrogen and oxygen in
this cycle. This process has many advantages like high-purity hydrogen
production at ambient temperature; generation of value-added by-products such
as ammonium sulfate; exploitation of the entire solar spectrum and consumption
of greenhouse gases (i.e. sulfur
dioxide). However, even though the hydrogen production step has been studied,
there is a lack of information on aspects which have strong impact on hydrogen
production rates like pH, kinetic parameters, and/or efficient reactor design. Moreover,
new approaches have been proposed to enhance the large-scale viability of the
S-NH₃ cycle, including an electrochemical hydrogen production step
established by Taylor et al. (2014). This
electrochemical approach still makes use of solar energy resources, but via thermodynamic cycles which take
advantage of the high temperature gases to produce electricity. Thereby, the
electrooxidation of ammonium sulfite is carried out with the same advantages as
described above.

In this work, we propose a novel electrochemical
reactor for the hydrogen production step for the S-NH₃ cycle, which
uses an ionic exchange membrane to achieve pH control. The reactor is designed in
such a way that allows a suitable and dynamic analysis of hydrogen production at
laboratory-scale.

In order to support this
proposal, a preliminary pH-control analysis was carried out. Figure 1
illustrates the chronoamperograms resulting from the electrooxidation of
ammonium sulfite at room temperature. The best two conditions of reaction rates
were using a buffer and an Anion Exchange Membrane (AEM), as shown. Thus, the
latter was selected as the most practical pH-control method for this reaction. Moreover,
hydrogen production was verified by Gas Chromatography with Thermal
Conductivity Detector (GC-TCD). Based on these results, a two-chamber
electrochemical membrane reactor separated with an AEM was developed to allow
the hydrogen production analysis, as shown in Figure 2.    

The reactor performance was later
analyzed using a Design of Experiments (DOE) approach to establish an optimal
operating condition. Finally, hydrogen production rate at such condition was verified
by GC-TCD. Currently, kinetic and mass transport studies are being carried out
to discuss a better reaction design for this electrochemical approach.

Figure 1. Electrooxidation kinetics of (NH₄)₂SO₃.

Figure 2. Conceptual principle of operation of the proposed electrochemical membrane
reactor.