(482g) Design of a Novel Electrochemical Membrane Reactor for Hydrogen Production Via the S-NH3 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 during the oxygen evolution step to
produce electricity. Thereby, the electrooxidation of ammonium sulfite is
carried out with the same advantages as described above. Nevertheless, a more
detailed analysis is needed in order to consolidate a high-performance hydrogen
production step. 

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 applying 700 mV (vs
Ag/AgCl). 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. Note that such experimental
conditions enforced hydrogen production via electrooxidation, while
prevented water electrolysis in contrast to those experimental conditions
earlier reported, 125°C and 1,00 V by Taylor et al. (2014). Whereas
hydrogen production was verified by Gas Chromatography with Thermal
Conductivity Detector (GC-TCD), production of ammonium sulfate was proven by
Raman spectroscopy, as shown in the inset in Figure 1. 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 through a Central Composite Design (CCD), to establish optimal
operating conditions. Ammonium sulfite concentration (mM) and applied voltage (mV)
were selected as the studied factors. The response variable was the normalized
half-life time, a parameter linked directly to the reaction rate and calculated
through chronoamperometric data of each ammonium sulfite electrooxidation.
Extensive experimental tests were carried out for each condition and a second
order (quadratic) model for the response variable was obtained. Through this
model, suggested optimal operating conditions are 350 mV (vs Ag/AgCl)
and a concentration of ammonium sulfite at 55 mM. Finally, hydrogen production
rate at optimal operating conditions was verified. Hydrogen gas produced during
reaction was stored inside a collection chamber over the cathode. Continuous
samples were taken and measured by GC-TCD. A calibration curve was used to
obtain the amount of hydrogen produced. To compare hydrogen production levels,
tests at 800 mV / 50 mM were conducted as well. 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.