Electrocatalytic upgrading of itaconic acid to methylsuccinic acid using fermentation broth as a substrate solution

Article abstract of DOI:10.1039/c6gc03153f

Biomass presents a promising renewable feedstock allowing access to valuable platform chemicals. In particular, biotechnological processes enable a highly selective product formation but are carried out in aqueous electrolyte-containing solutions. Consequently, the separation of usually polar products poses severe challenges on product separation associated with a high energy demand of product purification. A direct further catalytic transformation within fermentation broth reduces the number of unit operations and the need for an energy intensive separation. We herein study the potential of a chemo- and electrochemical reduction of itaconic acid (IA) to methylsuccinic acid (MS) using acidic media or crude fermentation broth as a case study. Despite an efficient chemo-catalytic hydrogenation of neat IA over Ru/C or RANEY nickel, the presence of various salts as well as glucose prohibits a direct chemo-catalytic valorisation in fermentation broth. In contrast, the electrochemical hydrogenation enabled very benign conditions. The selection of the electrode material proved to be decisive and had, together with the voltage, a strong influence on the conversion and faradaic efficiency of electrolysis facilitating 99% faradaic efficiency. The conversion of IA only slightly declined for an IA fermentation broth instead of neat IA in a diluted sulfuric acid environment reaching 60 versus 64%. Moreover, a full conversion and yield could also be achieved by simple optimizations of the reaction period and the substrate concentration. The electrocatalytic valorisation of a crude biotechnological product stream reduces not only energy demand and unit operations but presents a promising approach to introduce renewable electrical energy in biomass utilization.

Full text of DOI:10.1039/c6gc03153f

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Electrocatalytic upgrading of itaconic acid to
methylsuccinic acid using fermentation broth as a
Cite this: DOI: 10.1039/c6gc03153f
substrate solution†
F. Joschka Holzhäuser,a Jens Artz,a,b Stefan Palkovits,b Dirk Kreyenschulte,c
Jochen Büchsc and Regina Palkovits*a
Biomass presents a promising renewable feedstock allowing access to valuable platform chemicals. In
particular, biotechnological processes enable a highly selective product formation but are carried out in
aqueous electrolyte-containing solutions. Consequently, the separation of usually polar products poses
severe challenges on product separation associated with a high energy demand of product purification.
A direct further catalytic transformation within fermentation broth reduces the number of unit operations
and the need for an energy intensive separation. We herein study the potential of a chemo- and electro-
chemical reduction of itaconic acid (IA) to methylsuccinic acid (MS) using acidic media or crude fermen-
tation broth as a case study. Despite an efficient chemo-catalytic hydrogenation of neat IA over Ru/C or
RANEY® nickel, the presence of various salts as well as glucose prohibits a direct chemo-catalytic valori-
sation in fermentation broth. In contrast, the electrochemical hydrogenation enabled very benign con-
ditions. The selection of the electrode material proved to be decisive and had, together with the voltage,
a strong influence on the conversion and faradaic efficiency of electrolysis facilitating 99% faradaic
efficiency. The conversion of IA only slightly declined for an IA fermentation broth instead of neat IA in a
diluted sulfuric acid environment reaching 60 versus 64%. Moreover, a full conversion and yield could also
be achieved by simple optimizations of the reaction period and the substrate concentration. The electro-
catalytic valorisation of a crude biotechnological product stream reduces not only energy demand and
unit operations but presents a promising approach to introduce renewable electrical energy in biomass
utilization.
Received 15th November 2016,
Accepted 14th December 2016
DOI: 10.1039/c6gc03153f
www.rsc.org/greenchem
major platform molecules derived from biomass feedstocks.5
Since the 1960s the production of IA is in general accomplished
Introduction
Since the beginning of the 21st century, the societal need for by the fermentation of Aspergillus terreus on sugar containing
eco-friendly and renewable energy sources has become increas- media.6,7 Unsaturated di-carboxylic acid is typically used as a
ingly imperative. In this regard, biomass resources have starting material in the manufacturing of polymers,6,7 but it is
attracted plenty of attention over the last decade as a renewable also considered to be a promising substrate for tailor made
carbon source for the production of potentially CO2-neutral fuels, such as 3-methyltetrahydrofuran.8–11 In particular the
chemicals and fuels.1–4 Besides building blocks such as levuli- use of IA as an alternative monomer for the production of poly
nic acid and 5-hydroxymethylfurfural, the US Department of (methyl methacrylate) is of recent interest and will likely
Energy (DOE) named itaconic acid (IA) as one of the 12 future promote the production of IA further.12,13 In addition, studies
on the economic potential of IA estimate the projected market
for IA to nearly 410 000 MT with a value of 570 million USD in
2020.14 This also shows the strong increase of the market size
aChair of Heterogeneous Catalysis and Chemical Technology, Institut für Technische
of IA since the year 2001.6 Methylsuccinic acid (MS) can be
und Makromolekulare Chemie, RWTH Aachen University, Aachen, Germany.
obtained from IA via hydrogenation of the double bond.15 The
saturated product of IA can e.g. find applications as a substrate
E-mail: palkovits@itmc.rwth-aachen.de
bCenter for Molecular Transformations, RWTH Aachen University, Aachen, Germany
cChair of Biochemical Engineering, Aachener Verfahrenstechnik,
for polymerization reactions and as an additive in coating
RWTH Aachen University, Aachen, Germany
agents.16 The diethylester of methylsuccinic acid on the other
†Electronic supplementary information (ESI) available: Calculations, NMR
hand is considered as a suitable solvent in cosmetics.17
spectra and cyclic voltammograms of the different electrode materials. See DOI:
Recently, the Wong Chang group showed that methylsuccinic
10.1039/c6gc03153f
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monoesters have great properties as a substrate for the electro- describes water soluble manganese complexes such as manga-
chemical coupling reaction called “Kolbe electrolysis”. The nese porphyrin as efficient electrocatalysts for the oxidation of
obtained product 2,5-dimethyladipic acid after release of the different unsaturated dicarboxylic acids. However, these
ester groups might possess very promising characteristics in complexes are very pH-dependent and therefore difficult to
the polymer industry.18
One of the key steps in the production of biofuels or value- chemical reactions.38
implement for the often very acidic environments in electro-
added chemicals from biomass presents a suitable connection
Herein, we present the successful direct transformation of
between the highly selective biotechnological transformation biotechnologically produced IA in an electrochemical process
of biomass, e.g. cellulose, into platform chemicals such as running at ambient pressure and temperature over an in-
ethanol and lactic acid or IA with subsequent processing.2,19,20 expensive full metal electrode. Prior to the electrocatalytic con-
Indeed, fermentation processes delivering high-boiling version of crude biotechnological IA solution the electrode
and polar products in aqueous electrolyte-solutions often material, which provided the highest MS yield and best fara-
cause highly demanding and energy intensive separation daic efficiency, was optimized by varying the applied negative
processes.21–24 These challenges make a close integration with potential. This provided valuable insights into achieving high
further transformation attractive avoiding intermediate selectivity of the ECH over the competitive hydrogen evolution
product separation.25 However, despite promising progress in reaction. With the goal of assessing the impact of ECH, we
the design of efficient catalysts for a valorisation of biomass- have also examined the ability of Ru/C and RANEY® nickel
derived platform chemicals, few examples demonstrate a catalysts in a conventional hydrogenation reaction (applying
direct catalytic transformation of crude fermentation broth.
Beside conventional chemo-catalysis, electrocatalysis poses
an efficient methodology for the production of bio-derived
chemicals.26–35 In this regard, in particular the hydrogenation
(or reduction) of levulinic acid into valeric acid and
H2 pressure and heat) for comparison.
Experimental
γ-valerolactone reported by Nilges, Xin, Qiu and Dos Santos Fermentation
et al. revealed that electrochemistry represents a feasible tool
Cultivation of Aspergillus terreus DSM 23081 was performed in
for the effective and sustainable production of promising
chemicals.26,28,31,33 The mentioned reaction was carried out in
aqueous acidic solutions and at room temperature, using Pb
electrodes for high valeric acid selectivities and glassy carbon
electrodes for high γ-valerolactone selectivities.28 Among these
electrochemical approaches also the mild electrosynthesis of
2,5-furandicarboxylic acid from HMF has received attention, as
it does not require high temperatures and oxygen pressure.29,36
The mild process conditions fulfil the major criteria of green
chemistry and open a window of opportunity to integrate
renewable electrical energy into biomass valorization.
In the recent past, literature studies described the electro-
chemical hydrogenation (ECH) of muconic acid to different
diacid monomers34 and the reduction of 5-hydroxymethyl-
furfural to 2,5-bis(hydroxymethyl)furan.32 A recent contri-
bution by Matthiesen et al. also highlights the potential of an
electrochemical transformation of biologically produced
muconic acid.35 Regarding the electrocatalytic transformation
of IA, only a few examples exist. In 1989 Smigielski et al.
reported an electrocatalytic conversion of IA (83%) to methyl-
a minimal medium according to Kuenz et al.39 and Hevekerl
et al.40 The medium contained 180 g L−1 glucose, 0.8 g L−1
KH2PO4, 3 g L−1 NH4NO3, 1 g L−1 MgSO4·7H2O, 5 g L−1
CaCl2·2H2O, 0.00167
g
L−1 FeCl3·6H2O, 0.008
g
L−1
ZnSO4·7H2O and 0.015 g L−1 CuSO4·5H2O. The main substrate
glucose was obtained from Carl Roth GmbH & Co. KG
(Karlsruhe, Germany) as a monohydrate with a purity of
≥99.5%. The medium was completed from stock solutions,
which, except for the CaCl2 and FeCl3 solutions, were adjusted
to pH 3.1 with 1 M H2SO4 and were autoclaved separately.
Experiments were conducted at 33 °C in unbaffled 250 mL
shake flasks with a filling volume of 20 mL and an initial
spore concentration of 5 × 106 spores per mL. The flasks were
shaken at 300 min−1 with a shaking diameter of 50 mm. After
130 hours of cultivation without interruption of shaking, the
culture broth was harvested and fungal biomass was removed
via filtration.
Electrocatalysis
succinic acid (MS) at 60 °C under strongly basic conditions All electrochemical reactions were conducted in a custom
using a high excess of RANEY® Ni as a catalyst and an elec- designed single compartment three-electrode cell under
trode (molar ratio of Ni/IA = 1.4, Fig. 1).37 The second study potentiostatic or galvanostatic control using a potentiostat/
galvanostat (Autolab PGSTAT 302N). The electrode reactions
were undertaken using a three-electrode arrangement. In this
regard, EWE stands for potential applied vs. reference electrode,
for which Ag/AgCl saturated KCl electrodes (ProSense, 0.195 V
vs. standard hydrogen electrode (SHE)) were used throughout
the study. All electrode potentials in this study are reported vs.
Ag/AgCl sat. KCl, but normalized to SHE for energetic calcu-
Fig. 1 The proposed ECH of IA to MS.
lations and for better comparability.
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A pre-treatment procedure was applied to remove contami- D device. The samples were directly injected without any
nants from the electrode and membrane surfaces. All metal additional treatment.
electrodes were treated with sandpaper (400, 1500 and
2500 grits subsequently) and washed with deionized water.
Glassy carbon electrodes were first treated with different
intense sandpapers (400, 1500 and 2500 grits) and washed
Results and discussion
Initial screening studies with different metal electrodes
with ethanol in an ultrasonic bath. After use, the membranes
were stored in a diluted sulfuric acid solution (0.5 mol L−1
and cleaned with deionized water before being used again.
)
The electrochemical production of MS was up to now only
investigated by one group.37 Moreover, since previous studies
required additional heating the conditions need further
optimization. These findings motivated us to investigate the
ECH reaction of IA to MS over different electrodes in more
detail. In this regard, the metals Cu, Ni, Fe, Pb and Cu–Pb
(plastic bronze) have been investigated for electrochemical
activity using a fixed voltage (Fig. 2).
Initial experiments to compare various metal electrodes
and measure the impact of different EWE were performed with
16 mL solution of IA (0.208 g, 1.6 mmol or 1.187 g, 9.1 mmol)
dissolved in 0.5 M sulfuric acid (16 mL) as an electrolyte. The
cell was equipped with a glassy carbon (Hochtemperatur
Werkstoffe GmbH) counter electrode and divided via an ion-
exchange Nafion membrane (NR-324, Alfa Aesar). Testing of
the fermentation broth was conducted with 16 mL IA fermen-
tation broth with IA (1.187 g, 9.1 mmol), glucose (0.375 g,
2.1 mmol) and various minor salt amounts (MgSO4, CaCl2,
FeCl3, ZnSO4, CuSO4, KH2PO4 and NH4NO3). The cell was
equipped with a glassy carbon counter electrode. All reported
current densities are normalized to the geometric surface area
of the working electrode (surface = 8 cm2). All reported results
were conducted at least 3 times in total. All utilized full-metal
working electrodes had technical grade.
The ECH of IA was studied in an aqueous acidic solution
(0.5 mol L−1 H2SO4). For all experiments, a potential of −1.06
V vs. SHE has been applied for a 1 h reaction period. Cu and
Cu–Pb electrodes showed only a very low electrochemical per-
formance for the hydrogenation reaction at this voltage,
whereas for Pb the voltage was not high enough to achieve a
current flow. The highest catalytic activity as well as faradaic
efficiency was observed for Ni, while Fe still revealed a notable
yield and conversion. Interestingly, when switching the
applied potential from −1.06 V to −1.41 V vs. SHE Pb electro-
des meet the required overpotential for hydrogenation and
start to clearly stand out over the other tested electrodes in
terms of substrate conversion (96%), yield of MS (96%) and
faradaic efficiency (16%, calculation described in the ESI†).
The obtained current value at −1.41 V is comparable to the
values observed when using other electrodes at lower poten-
tials (see cyclic voltammograms in the ESI†). As it was already
stated by dos Santos et al.,28 the higher performance of Pb for
ECH reactions can be often explained by the higher overpoten-
tial of water electrolysis.41 Hydrogen evolution at Cu-, Fe- and
Ni-cathodes start already before −1.0 V vs. SHE in acidic
media.41,42
Chemocatalytic batch reactions
Heterogeneous catalysts were tested in a 50 mL batch reactor
(stainless-steel autoclave) with a glass or Teflon inlet, which
contained a stirring bar, temperature controller and pressure
controller. The pressure of H2 gas could be controlled by using
a barometer. The batch autoclave was charged with water
(15 mL or 10 mL) containing IA (1.187 g, 9.1 mmol or 0.791 g,
6.1 mmol) or fermentation broth (15 mL or 10 mL) and Ru/C
0.5 wt% (100 mg) or RANEY® nickel (33.3 mg, 4 wt% of the
substrate amount), 10 bar of hydrogen pressure were applied
and the mixture was heated to 70 °C for 1 h. The stirring speed
of the stirrer was set to 500 rpm. All reported results were aver-
aged over 3 experiments.
Product analysis
1H NMR spectra were recorded at 25 °C on a Bruker Avance
400 MHz spectrometer. The samples were in the case of low
concentrations of IA (0.1 mol L−1) dried at 60 °C under
vacuum, reconstituted in deuterium oxide (Deutero), and sub-
sequently analysed. In the case of high concentrations of IA
and when using fermentation broth, 0.2 mL of the product
mixture was diluted in deuterium oxide. Ethanol was in each
case used as an internal standard during the quantitative
measurements. The spectra of the reactant and product are
depicted in the ESI.†
Inductively coupled plasma atomic emission spectroscopy
(ICP-AES)
Fig. 2 Screening of various metal electrodes in the ECH of IA to MS.
Conditions: r.t., 1 h, counter electrode: glassy carbon, E = −1.06 V vs.
standard-hydrogen-electrode (SHE), water (16 mL), H2SO4 (0.5 mol L−1),
IA (0.1 mol L−1, 1.6 mmol, 0.208 g). *E = −1.41 V vs. SHE (X denotes con-
For the determination of the metal amount in the aqueous
solutions, the samples were analysed on a spectro-flame version, Y means yield).
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In summary, MS can be obtained from IA using acidic transport of charged or uncharged species through the solu-
aqueous solutions and Pb-cathodes with excellent selectivity tion.43 Using a stirrer or a rotational disk electrode can shift
and yields, while the faradaic efficiency needs to be optimized. the limitation to a higher value emphasising the high impact
However, the faradaic efficiency is very dependent on the of reactor design in such a transformation. Another important
initial substrate concentration and therefore, can be increased factor relates to an electrode material with a higher specific
using higher substrate concentrations.
surface area; however, also there mass transfer limitations will
be encountered at a certain potential.43 According to the
obtained results, the highest faradaic efficiency of 62% is
achieved after 10 min when applying lower potentials. The
difference in higher potentials rises when it comes to longer
reaction periods; however, this behaviour is probably intensi-
fied due to low IA amounts in solution after 50% conversion.
This suggestion implies a better faradaic efficiency for all
potentials when using higher substrate solutions and that low
efficiencies are also connected to lower concentrations of IA in
solution. On this account, it is expected that the faradaic
efficiency can be clearly increased when using higher initial
substrate concentrations or when performing the ECH of IA in
a continuous setup. Nevertheless, a very mild process for the
production of MS from IA has been elaborated, which does not
require the addition of thermal heat as it was necessary in a
previous study of Smigielski et al.37 For this reason, and
especially with regard to the rules of green chemistry, the ECH
of IA when using Pb-cathodes and an acidic aqueous solution
presents a very sustainable system and promising setup for
continuing investigations.
Kinetic potential dependent experiments with Pb-cathodes
Considering the results from the section before, another set of
experiments was conducted in order to reveal the impact of
different applied potentials on conversion, and the change of
the faradaic efficiency over a 1 h reaction time in comparison
with the studied electrode potentials (Fig. 3).
Since Pb-cathodes showed the highest reaction rates and
best faradaic efficiencies, they were utilised for further experi-
ments. The effect of the voltage ranging from −1.31 to −1.51 V
vs. SHE shows that the time for reduction decreases for higher
voltages. However, when reaching −1.41 V vs. SHE the effect
becomes negligible, which can likely be attributed to mass
transfer limitations during the process. As mentioned in the
literature applying a potential to a certain electrode, it is found
that, upon increasing the applied voltage the current value at
first increases until it reaches a saturation level, called a
current plateau. It then requires clearly higher potentials to
achieve higher current growth again. When this limiting is
encountered, it is in most cases caused by the slowness of
Fig. 3 The effect of the applied potential on the conversion, yield and faradaic efficiency: (a) IA conversion, (b) yield of MS, and (c) faradaic
efficiency. Conditions: r.t., working electrode: Pb, counter electrode: glassy carbon, water (16 mL), H2SO4 (0.5 mol L−1), IA (0.1 mol L−1, 1.6 mmol,
0.208 g). The dashed lines are added as guide to the eye.
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Comparison study of neat IA solutions and IA fermentation
broth
slightly lower efficiency (93%) for the fermentation broth in
comparison with pure IA solution (99%). The main reason for
this quantity distinction might be explained by the fact that
the broth still contains remarkable amounts of sugar
(0.13 mol L−1) which can undergo side reactions, for example
the already well known reduction of glucose into sorbitol.44
Nevertheless, the discussed differences are very small and
therefore it could be proven that fermentation broth represents
a very promising alternative substrate solution for a direct
electrochemical further transformation.
The ECH of IA to MS was carried out in a custom designed
electrolysis cell consisting of two half-cells, which were separ-
ated through an ion permeable membrane (Nafion). The only
additive was H2SO4 (0.5 mol L−1) when using neat IA solution
as a substrate solution. However, using almost pure IA solution
requires the purification of the previous downstream extract
meaning the fermented sugar containing media with signifi-
cant amounts of several salts and still notable quantities of
glucose. The regeneration of fermentation broth presents an
additional cost factor and of course, it is economically
favoured to reduce the overall amount of unit operations in a
process. In this context, a comparison of the transformation of
a neat IA solution and an IA fermentation broth has been con-
ducted utilising chronopotentiometry in order to keep the
current flow (0.3 A) constant and achieve better comparability.
Fig. 4 presents the conversion, yield and faradaic efficiency
for neat IA solution and fermentation broth after 1 h.
Noteworthily, the IA concentration in the fermentation broth
is almost 6-fold higher compared to the previously shown
results with neat IA solution. Therefore, concentrations for
neat IA solution were adjusted to 0.57 mol L−1 to preserve com-
parability. Despite a 6-fold higher IA concentration, 64% con-
version and 63% yield could be achieved after a 1 h reaction
time. Applying the same conditions to the fermentation broth
as to the neat IA solution, we could confirm that IA conversion
(60%) and MS (60%) yield are only slightly lower, despite the
presence of high glucose amounts and several salts (MgSO4,
CaCl2, FeCl3, ZnSO4, CuSO4, KH2PO4 and NH4NO3) in the sub-
strate solution.
Comparison of thermal batch reactions to ECH
In addition to the electrochemical conversion of IA, we have
focused on the conventional reaction path to the target
product MS, since it is usually produced by applying heat,
hydrogen pressure and using a noble metal catalyst. In this
regard, the mild conditions could be a major challenge consid-
ering the maximum product yield. In this work, electro-
chemistry has proven to be a worthwhile method for the mild
and fast production of MS using only lead cathodes and no
additional heat. Interestingly, it was observed that a chemo-
catalytic hydrogenation of pure IA over a supported ruthenium
catalyst (Ru/C) required at least 70 °C and 10 bar of hydrogen
for a yield of 75%.
In order to determine the Ru/C stability in pure IA solution
and fermentation broth recycling tests were conducted (Fig. 5).
Between the cycles the catalyst was washed with distilled water
and methanol and dried at 60 °C under vacuum. Ru/C shows
excellent recyclability with only slight loss of yield and conver-
sion under these conditions and using neat substrate solution.
However, only 3% conversion and 1% yield were achieved after
the first experiment when applying this catalyst to fermentation
broth. These findings imply that because of high amounts of
glucose, which might plug the Ru/C pores, the catalyst de-
activates fast. In addition, the various salts could inhibit the
performance. In order to ensure that the low activity was not
only related to the properties of the Ru/C catalyst, additional
tests were conducted with a conventional hydrogenation cata-
lyst.45 RANEY® nickel (4 wt% of the substrate amount) showed
Although, the applied potential has to be increased
(approximately −1.4 to −4.4 V vs. SHE) as the resistance of the
mixture is higher when using fermentation broth, this
problem is easily solved when mixing the broth with a diluted
sulfuric acid solution (0.5 mol L−1). According to the obtained
faradaic efficiencies for both substrate mixtures, there is a
Fig. 4 Comparison of neat IA-solution with fermentation broth using
Pb as WE. Conditions: r.t., 1 h, counter electrode: glassy carbon, I = 0.3 A,
Fig. 5 Chemo-catalytic batch hydrogenation of IA over a Ru/C catalyst
including recycling: 100 mg Ru/C in pure IA solution (0.61 mol L−1
,
neat IA-solution: water (16 mL), H2SO4 (0.5 mol L−1), IA (0.57 mol L−1
9.1 mmol, 1.187 g).
,
9.1 mmol, 1.187 g). Reaction conditions: 1 h, 70 °C, 10 bar of H2, 15 mL
water, 500 rpm stirring speed.
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similar activity compared to Ru/C in neat IA solution (43% con-
version, 41% yield) and was even more active in a first run with
Acknowledgements
fermentation broth (30% conversion, 30% yield) compared to We acknowledge financial support by the Cluster of Excellence
Ru/C. However, upon recycling the Ni catalyst became comple- “Tailor-Made Fuels from Biomass” (TMFB) funded by
tely inactive. This can be assigned most likely to notable leach- the Excellence Initiative of the German federal and state
ing of the active species into the solution, which led to a green governments to promote science and research at German
solution after the reaction. ICP-AES confirmed 0.545 mg g−1 Ni universities.
corresponding to almost 39% of the initial amount of the cata-
lyst. It is clear that although a neat IA solution as a substrate
might provide similar results in comparison with the mild
electrochemical conversion it cannot be easily applied to the
downstream mixture from the fermentation. In contrast, the uti-
lized Pb electrode for the ECH was not changed throughout the
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Conclusions
The direct electrocatalytic valorisation of platform chemicals
without prior separation from crude fermentation broth pre-
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unit operations and energy demand. Focusing on the reduction
of itaconic acid (IA) as a case study, an efficient chemo-catalytic
hydrogenation of neat IA over Ru/C or RANEY® nickel required
at least 70 °C and 10 bar of hydrogen pressure facilitating 75%
yield of methylsuccinic acid (MS). However, the presence of
various salts as well as glucose prohibited a chemo-catalytic
valorisation of an IA fermentation broth. In the case of RANEY®
nickel a distinct catalyst leaching occurred. In contrast, we were
able to transform IA into methylsuccinic acid (MS) over various
electrodes at ambient temperature and pressure. Pb and Ni
facilitated the highest activity in a separated single electrolysis
cell. Pb showed under slightly different conditions the highest
activity with a maximum yield of MS of 98% at room tempera-
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applied voltage has a distinct impact on the conversion and
faradaic efficiency with an increasing faradaic efficiency at lower
potentials but increasing conversion for higher potentials
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