A NiRhS fuel cell catalyst - lessons from hydrogenase

Article abstract of DOI:10.1039/d0cc04789a

We present a novel fuel cell heterogeneous catalyst based on rhodium, nickel and sulfur with power densities 5-28% that of platinum. The NiRhS heterogeneous catalyst was developedviaa homogeneous model complex of the [NiFe]hydrogenases (H₂ases)

Full text of DOI:10.1039/d0cc04789a

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A NiRhS fuel cell catalyst – lessons from
hydrogenase†
Cite this:DOI: 10.1039/d0cc04789a
ab
Seiji Ogo,
*
Tatsuya Ando,ab Le Tu Thi Minh,a Yuki Mori,a
Takahiro Matsumoto,ab Takeshi Yatabe,ab Ki-Seok Yoon,ab Yukio Sato,c
Takashi Hibino
Received 11th July 2020,
Accepted 24th August 2020
bd
bc
and Kenji Kaneko
DOI: 10.1039/d0cc04789a
rsc.li/chemcomm
We present a novel fuel cell heterogeneous catalyst based on rhodium, demonstrate the catalytic properties of these dry-distilled com-
nickel and sulfur with power densities 5–28% that of platinum. The pounds and their application as the first DDCs containing two
NiRhS heterogeneous catalyst was developed via a homogeneous different metals and sulfur that function as both the anode and
model complex of the [NiFe]hydrogenases (H2ases) and can act as cathode of functioning fuel cells (Fig. S1 and Table S1, ESI†).6
both the cathode and anode of a fuel cell.
A NiIIRhIII dichloride complex [NiIICl(X)RhIIICl(Z5-C5Me5)] (1) was
prepared from the reaction of [NiII(X)] with [RhIII(Z5-C5Me5)Cl2]2.
We have previously reported a NiRu model complex, [NiII(X)- This water-soluble complex was then characterized by X-ray analysis
(H2O)(m-H)RuII(Z6-C6Me6)](NO3) {[3](NO3), X = N,N0-dimethyl- (Fig. S2 and Table S2, ESI†), electrospray ionization-mass spectro-
3,7-diazanonane-1,9-dithiolato}, which can mimic the chemical metry (ESI-MS, Fig. S3, ESI†) and 1H NMR (Fig. S4, ESI†) and IR
functions of O2-tolerant [NiFe]H2ases.1–3 This complex is based spectroscopies (Fig. S5, ESI†). Its structure is based around a
on a central NiRu bimetallic core with flexible m-S bridges, Ni(m-S)2Rh butterfly core, which is similar to our previously-
which allow the close approach of the two metal centres. This reported [NiFe]H2ase mimics.1,7
complex can perform both H2-oxidation and O2-reduction, in a
Having synthesized 1, we proceeded to investigate its catalytic
similar manner to O2-tolerant [NiFe]H2ases,1,3,4 and thus acts properties. Complex 1 activated H2 (0.1 MPa) heterolytically in
as both an anode and a cathode catalyst for H2–O2 fuel cells.5 water to afford a NiIIRhIII hydride complex [NiIICl(X)(m-H)RhIII-
However, the successful functioning of the compound did not (Z5-C5Me5)] (2) in a similar manner to H2ases (Fig. 1 and 2a).1,3,7,8
extend as far as producing an efficient fuel cell.5 Since organo- The structure was determined by X-ray analysis (Fig. 1), UV-vis
metallic catalysts are not as robust as their solid-state, heterogeneous spectroscopy (Fig. S6 and Tables S2 and S3, ESI†), ESI-MS (Fig. S7,
counterparts, we investigated the possibility of using dry distillation ESI†) and IR spectroscopy (Fig. S8, ESI†). The X-ray analysis
(pyrolysis) to remove the volatile elements and leave behind the showed that the Ni1ꢀꢀꢀRh1 interatomic distance had been shortened
active m-S bimetallic core. We investigated three bimetallic com- to 2.7454(16) Å, with a bridging hydride ligand between the two
plexes, where nickel was partnered with ruthenium, rhodium and
iridium and the NiRh complex gave the best results.
Herein, we describe the synthesis, characteristics and catalytic
properties of the NiRh organometallic complex before detailing
its conversion into NiRhS dry-distilled catalysts (DDCs). We then
a Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, International Institute for Carbon-Neutral Energy Research
(WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395,
Japan. E-mail: ogo.seiji.872@m.kyushu-u.ac.jp
b Center for Small Molecule Energy, Kyushu University, 744 Moto-oka, Nishi-ku,
Fukuoka 819-0395, Japan
c Department of Materials Science and Engineering, Faculty of Engineering,
Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
d Graduate School of Environmental Studies, Nagoya University, Furo-cho,
Fig. 1 ORTEP drawing of 2 with ellipsoids set at the 50% probability level.
The hydrogen atoms of the ligand X (N,N’-dimethyl-3,7-diazanonane-
1,9-dithiolato) and C5Me5 are omitted for clarity. Selected interatomic
distances [Å] and angles [1]: Ni1ꢀ ꢀ ꢀRh1 2.7454(16), Rh1–H1 1.84(9),
Ni1–H1 1.96(9), Ni1–S1–Rh1 71.15(7) and Ni1–S2–Rh1 70.65(7) for 2.
Chikusa-ku, Nagoya 464-8601, Japan
† Electronic supplementary information (ESI) available: Experimental details,
Tables S1–S9 and Fig. S1–S22. CCDC 1996959 and 1996960. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d0cc04789a
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The hydride complex 2 can also reduce O2 to H2O (Fig. 2b,
Fig. S12 and Table S3, ESI†). The O2-reduction in water was
confirmed by an 18O2-experiment wherein H218O was formed from
the reaction of 2 with 18O2, as observed by gas chromatography-
mass spectrometry (GC-MS). The TON value (TON, [(mol of H218O)/
(mol of 1)]/2) was determined to be 0.48 in the reaction using 1
with H2 as an electron donor (Table S4, ESI†).10–12
Having satisfied ourselves that the NiRh catalyst was a
successful analogue of our NiRu catalyst, we set about rendering
it in a heterogeneous, solid-state form. To achieve this, we used
dry distillation of 1 with carbon black, in pyrolysis experiments
at 100–800 1C under vacuum (ca. 15–20 Pa).13 Following this
process, the volatile elements were removed to mainly leave Ni,
Rh and S atoms. The structural properties of the resulting solids
were analysed by thermogravimetry–differential scanning calori-
metry (TG–DSC), X-ray photoelectron spectroscopy (XPS), elemental
analysis, X-ray powder diffraction (XRD) analysis, (scanning) trans-
mission electron microscopy ((S)TEM) and energy-dispersive X-ray
spectroscopy (EDS) mapping (Table S5, ESI†).
The thermal properties of 1 were investigated by TG–DSC
under an N2 atmosphere (Fig. 3 and Fig. S13, ESI†). Weight loss
began around 250 1C and was mostly completed by about
400–450 1C. Some endothermic peaks in the process suggest
vaporization of the decomposed products that should mainly
contain Cl, H and N atoms, as detailed below.
XPS measurements also show that the structural change of 1
depends on the temperature of dry distillation (Fig. S14, ESI†).
Fig. 2 Proposed catalytic reaction mechanisms for (a) H2-oxidation and
(b) O2-reduction with NiRh complexes in water. DCIP: 2,6-dichlorobenzene-
indophenol. DCIPH2: reduced-form of DCIP.
Above 300 1C, the binding energies at 162.3–163.6 eV for S 2p3/2
,
307.4–307.8 eV for Rh 3d5/2 and 853.0 eV for Ni 2p3/2 are
observed in the XP spectra, which might be derived from metal
sulfides coexisting with Rh0.6c,14,15
metal centres (Fig. 1). This structural change upon the activation of
H2 and complexation of the hydride is similar to that found in our
other [NiFe]H2ase models and the natural [NiFe]H2ases them-
selves.1,7,9 The origin of the bridging hydride was derived from
dihydrogen gas, as confirmed by the formation of a deuteride
complex [NiIICl(X)(m-D)RhIII(Z5-C5Me5)] (D-labelled 2) from the
treatment of 1 with D2 (Fig. S7 and S8, ESI†).
XRD analysis also indicates the transformation of 1 to DDCs
depending on the dry distillation temperatures (Fig. S15, ESI†).
Complex 1 should change to DDCs at 200–300 1C and then the
DDCs could be structurally changed further at 700–800 1C. The
We then investigated the catalytic properties of 1 by using 2,6-di-
chlorobenzeneindophenol (DCIP) as an acceptor of 2eꢁ and 2H+
under stoichiometric conditions with 2 and catalytic conditions with
1 (eqn (1)). Under stoichiometric conditions (2:DCIP = 1:1) in the
absence of H2 in water, 2 is capable of reducing DCIP to DCIPH2
(Fig. S9, S10 and Table S3, ESI†). Under catalytic conditions
(1: DCIP = 1 : 27) in the presence of H2 in water (buffer solution),
H2 reduces DCIP to DCIPH2 with the assistance of 1 (Fig. 2a). The
catalytic reaction is pH-dependent (Fig. S11 and Table S4, ESI†) and
it shows a maximum turnover number (TON) value of 14.3–15.2 at
pH 4.0–5.0 (TON, mol of DCIPH2/mol of 1), similar to our previously
reported system.4
Fig. 3 A TG curve obtained from TG–DSC analysis of a mixture of 1 and
carbon black (1 : 1 weight ratio) (black line), and dry distillation temperature-
dependent ratios of S (red line), N (blue line) and H (green line) atoms in
DDCs determined by elemental analysis. At the point of 25 1C, a mixture of
1 and carbon black (1 : 1 weight ratio) is used, i.e., no dry distillation.
(1)
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XRD patterns of DDCs prepared at 300–700 1C seem to be
similar to those of Rh17S15 and Rh3S4,15 but do not completely
correspond to them. The DDC prepared at 800 1C might be
attributed to the S-doped NiRh alloy.14b
Elemental analyses were performed to determine the ratios
of C : H : N : S, according to the heating temperatures (Fig. 3 and
Table S6, ESI†). The ratio of N decreased from 2.30% to only 0.25%
upon raising the heating temperature from 200 1C to 300 1C, with a
similar trend for H (Table S6, ESI†). These profiles are almost the
same as the TG curve shown in Fig. 3 and strongly indicate that the
weight loss at 200–300 1C is caused by vaporization of N- and
H-containing fragments. S, however, only deceased from 5.36% to
3.88%, even after heating to 800 1C, indicating that the DDCs bear
significant amounts of this element.
Bright-field (BF-)TEM, annular dark field (ADF-)STEM and
STEM-EDS analyses were conducted to analyse the particle form
and elemental distribution in the DDCs (Fig. 4 and Fig. S16,
ESI†). At heating temperatures above 200 1C, nanoparticles with
a particle size of 50 nm or less were formed. Elemental mapping
reveals that Ni, Rh and S atoms are distributed homogeneously
under 280 1C but inhomogeneously above 300 1C.
Based on these results, we conclude that the homogeneous
NiII(m-S)2RhIII catalyst is converted into heterogeneous NiRhS-
based catalysts by loss of N, H and Cl atoms at 300–600 1C, with
a slight loss of S atoms at 800 1C.15
To examine the catalytic activities of DDCs, H2-oxidation and
18O2-reduction are investigated in flask experiments. The DDCs are
able to remember the reactivity of 1 to catalytically oxidize H2 and
reduce 18O2. The composition of NiRhS should remind us of
[NiFe]H2ases. The TON values of H2-oxidation and 18O2-reduction
are increased to 4.3 and 3.5 times higher than those of 1,
respectively, by dry distillation (Tables S4 and S7, ESI†).
Following the successful enhancement of the catalytic activities
by dry distillation, we assembled polymer electrolyte fuel cells
(PEFCs) with 1 or DDCs as anode and/or cathode catalysts
(Fig. S17, ESI†). A membrane electrode assembly (MEA) with Nafion
membrane was made in the same way as molecular fuel cells.5,16
Fig. 5 Maximum power densities of H2–O2 fuel cells depending on the
dry distillation temperature. At the dry distillation temperature of 25 1C, 1 is
used as it is as a fuel cell catalyst. (a) Anode: 1 or DDCs. Cathode: Pt/C.
(b) Anode: Pt/C. Cathode: 1 or DDCs.
The dependence of the maximum power densities on the dry
distillation temperatures was investigated with DDCs as electrode
catalysts for either the anode or the cathode (Fig. 5, Fig. S18, S19
and Tables S4 and S8, ESI†). The maximum power densities and
open circuit voltages (OCVs) obtained, where DDCs were employed
as the anode and Pt/C as the cathode, were significantly enhanced
above 300 1C (Fig. 5a). The highest maximum power density of
4.85 mW cmꢁ2 and OCV of 1.00 V were obtained by using the DDC
prepared at 600 1C (Fig. 5a, Fig. S18 and Table S8, ESI†). This
maximum power density is 164 times higher than that of the
precursor 1 and nearly 5% of that of Pt. Thereafter, the same
catalysts were applied as the cathode to fabricate and evaluate
PEFCs. As before, the performance was drastically enhanced above
the dry distillation temperature of 300 1C, with the DDC prepared
at 600 1C exhibiting the best performance with an OCV of 0.78 V
and a maximum power density of 26.1 mW cmꢁ2, which is 28% of
that of Pt (Fig. 5b, Fig. S19 and Table S8, ESI†). This power density
is 139 times higher than that of 1.17
Fig. 4 (a) BF-TEM image, (b) ADF-STEM image and STEM-EDS elemental
mappings of (c) Rh, (d) Ni and (e) S of the dry-distilled catalyst (DDC)
prepared at 600 1C.
Electrochemical aspects for H2-oxidation and O2-reduction
with 1 and the DDC prepared at 600 1C were investigated by
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In this paper, we have shown that a Rh analogue of our
previous [NiFe]H2ase models successfully functions in a similar
manner. Crucially, however, we have been able to transform it
by dry distillation into heterogeneous catalysts with maximum
power densities 5–28% that of platinum. While these new
catalysts fall short of the performance of platinum, they point the
way towards linking the stepwise development of new, hetero-
geneous, fuel cell catalysts with the development of homogeneous
organometallic catalysts.
This work was supported by JST CREST Grant Number
JPMJCR18R2, Japan, JSPS KAKENHI Grant Numbers JP26000008
(Specially Promoted Research), JP18H02091, and JP19K05503
and the World Premier International Research Center Initiative
(WPI), Japan.
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Conflicts of interest
There are no conflicts to declare.
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