Nickel Complexes with Non-innocent Ligands as Highly Active Electrocatalysts for Hydrogen Evolution

Article abstract of DOI:10.1002/cjoc.201800359

Rational design of a molecular catalyst for a hydrogen evolution reaction (HER) with both high rates and low overpotential remains a challenge. Here we report a series of Ni-based catalysts incorporating non-innocent ligands and aimed at reduction of the overpotential by reserving electrons in the ligand, which could decouple the correlation between metal center reduction and metal hydride formation. The third-order rate constant of our catalysts can be increased by 2 orders of magnitude without increase of overpotential by altering the substituents in the imide since the electronic effect of substituents can change the kinetics of metal hydride formation dramatically. Furthermore, constructing a proton shuttle in the ligand further improves the activity. The catalyst with both of these features can catalyze HER at a turnover frequency of 2572 s^–1 with low concentrations of acid and an extremely high third-order rate constant of 2.7×10⁶ M^–2·s^–1 at a low overpotential.

Full text of DOI:10.1002/cjoc.201800359

Nickel Complexes with Non-innocent Ligands as Highly Active
Electrocatalysts for Hydrogen Evolution
Zhixin Chen,† Tao Wang,† Tingting Sun, Zhiyong Chen, Tian Sheng, Yu-Hao Hong, Zi-Ang Nan, Jun Zhu,* Zhi-You Zhou,*
Haiping Xia,* Shi-Gang Sun
ABSTRACT Rational design of a molecular catalyst for a hydrogen evolution reaction (HER) with both high rates and low overpotential remains a
challenge. Here we report a series of Ni-based catalysts incorporating non-innocent ligands and aimed at reduction of the overpotential by reserving
electrons in the ligand, which could decouple the correlation between metal center reduction and metal hydride formation. The third-order rate constant
of our catalysts can be increased by 2 orders of magnitude without increased of overpotential by altering the substituents in the imide since the electronic
effect of substituents can change the kinetics of metal hydride formation dramatically. Furthermore, constructing a proton shuttle in the ligand further
improves the activity. The catalyst with both these features can catalyze HER at a turnover frequency of 2572 s-1 with low concentrations of acid and an
extremely high third-order rate constant of 2.7×106 M-2 s-1 at a low overpotential.
KEYWORDS hydrogen evolution, molecular electrocatalysis, non-innocent ligand
natural enzymes, and this can stabilize the intermediate
containing a low valence metal center in order to reduce the
overpotential for reduction reaction (Scheme 1).
Introduction
Producing hydrogen by electrolysis of water is an
environmentally-friendly and sustainable method because it
requires only water and electricity.1 Hydrogen is a promising
Scheme
1
Similarities between hydrogenase and Ni complex with
candidate as a clean fuel. The oxidation of hydrogen through fuel
cells can regenerate energy without CO2 emission and within the
limit of the Carnot cycle. Hydrogen produced by steam reforming
contains CO and causes poisoning of Pt catalysts. In contrast,
hydrogen produced by an electrocatalytic hydrogen evolution
reaction (HER) is very clean, and can be used in fuel cells directly.
As a side benefit, intermittent electricity from solar cells and wind
turbines can be utilized effectively in such a process.
non-innocent diiminopyridine ligands.
For molecular catalysts,2 the overpotential and the reaction
rate of HER are related to metal center reduction and metal
hydride formation but in general, these two element procedures
are deleteriously coupled. On the one hand, the modification of
the catalysts with electron-withdrawing substituents would
reduce the electron density on the metal center, lowering the
overpotential and making it easily reducible. On the other hand,
the low electron density on the metal center hinders its
protonation, which lowers the rate of metal hydride formation, or
lowering the reaction rate of HER.2f-g Consequently, rational design
of a molecular catalyst for HER with both low overpotential and a
high reaction rate remains a challenge.3,4
Hydrogenases are highly active catalysts for hydrogen
evolution.5 Previous work showed that an amine base positioned
near the metal center can function as a proton shuttle which can
speed up the proton transfer and facilitate the formation of the
H-H bond.2 Furthermore, [4Fe-4S] clusters in prokaryotic [FeFe]
hydrogenase have multiple redox centers, allowing storage of
electrons, which helps to stabilize electron-rich low valence metal
centers,6 suggesting that a building block for electron storage may
play a vital role in the catalytic redox process of the enzyme.
Non-innocent ligand is a ligand in a metal complex where the
oxidation state is not clear.7a Metal complexes with non-innocent
ligands can operate as an electron reservoir and usually exhibit
high catalytic activity and controllability in catalytic reactions.3d-i,7
We assume non-innocent ligands such as diiminopyridine may be
able to store electrons for redox reactions like [4Fe-4S] clusters in
Herein, we demonstrate that the design of Ni complexes with
suitable non-innocent diiminopyridine ligands can lead to efficient
catalysts for HER. In particular, we found that replacing the
π-acceptor aryl substituent of a ligand with a -donor cyclohexyl
group would tune the basicity of a one electron reduced metal
center and enhance the kinetics of metal hydride formation
without increasing the overpotential, indicating that the
substituent of the non-innocent ligand could significantly affect
the performance of
a catalyst. Decoupling the correlation
between metal center reduction and the kinetics of metal hydride
Z. X. Chen,† T. Wang,† T. T. Sun, Z. Y. Chen, Dr. T. Sheng, Y. H. Hong, Z. A. Nan,
Prof. Dr. J. Zhu, Prof. Dr. Z. Y. Zhou, Prof. Dr. H. P. Xia, Prof. Dr. S. G. Sun,
Collaborative Innovation Center of Chemistry for Energy Materials, State Key
Laboratory for Physical Chemistry of Solid Surfaces, College of Chemistry and
Chemical Engineering
E-mail: jun.zhu@xmu.edu.cn
zhouzy@ xmu.edu.cn
hpxia@x mu.edu.cn
† These authors contributed equally to this work
Xiamen University, Xiamen, 361005 (China)
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may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800359
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formation is a new approach.
Results and Discussion
The catalysts 1, 2 and 3 were synthesized via the one-pot
reaction of NiCl2 with a slight excess of 2,6-diacetylpyridine and
excess of an amine in THF at 85 °C in a sealed tube. The product
precipitated in the reaction was purified by recryst- allization from
CH2Cl2/Et2O. The solid-state structures of 1, 2 and 3 verified by
single-crystal X-ray diffraction as well as high-resolution ESI(+)-MS
and elemental analysis are shown in Figure 1.
Scheme 2 Synthesis of complexes 1, 2, and 3.
Figure 2 (a) Comparison of HER behaviors of 1, 2, and 3 (0.15 mM in
CH2Cl2) with 10 mM TfOH (b) Linear plot of icat/ip versus acid
concentrations with 0.15 mM of 1 (blue), 2 (green) and 3 (red). Complexes
2 and 3 decompose when the acid concentration is beyond 15 mM.
Figure 1 X-ray crystal structure of 1, 2, and 3 at 30% ellipsoid probability.
Hydrogen atoms have been omitted for clarity.
Table 1 Half wave potential, icat/ip and third-order rate constant for 1, 2
and 3 catalysing HER. Experimental conditions: 0.2 M Bu4NBF4; scan rate:
100 mV s-1.
The HER catalytic activity was tested in CH2Cl2 solution with
Bu4NBF4 as supporting electrolyte, trifluoromethanesulfonic acid
(TfOH) as proton source, and a glassy carbon disk ( = 3 mm) as
the working electrode. Figure 2a shows linear sweep
voltammograms of complex 1, 2, and 3 catalyzing HER in 10 mM
TfOH. The comparison of kinetic parameters among 1, 2, and 3 is
shown in Figure 2b and Table 1 and Figure S8-S9. In order to
quantitatively evaluate the HER performance of this catalyst, we
made a linear plot of icat/ip vs acid concentrations, as shown in
Figure 2b. The icat is the catalytic current of HER, and ip is the peak
current of a one-electron reduction wave before the catalytic
wave (Figure S10).3b,3p The icat/ip represents the catalytic turnover
number of each molecular catalyst molecule making contact with
the electrode surface. The linear plot indicates a second order
catalytic rate dependent on acid according to Equation 1.
Onset potential / V vs.
Cp2Fe+/0
(ic/ip)max
k / M-2 s-1
(10 mM TfOH)
1
2
3
-0.70
-0.76
-1.01
123
36
7
2.7 × 106
1.5 × 106
3.7 × 104
In this work, we prefer using a third-order rate constant and
overpotential to assess the activity of catalysts, since TOF is
relative to the concentration of protons. The moderate HER
catalytic activity of complex
3 is consistent with previous
reports.3e Surprisingly, we found that the catalytic activity is
increased significantly when the aryl substituent in 3 is replaced
by a cyclohexyl group, as in 2: the third-order rate constant is
increased by 2 orders of magnitude, and the HER onset potential
is decreased from -1.01 to -0.76 V. On the other hand, the
increase in the activity from 2 to 1 is relatively lower than from 3
to 2, and consequently, our discussion begins with the catalysts 2
and 3. Through theoretical calculations, we found metal hydride
formation of 3 occurs only after two-electron reduction whereas
in 2, it occurs just after one-electron reduction, indicating the
significant change achieved in the kinetics of metal hydride
formation by altering the imide substituents. Possible catalytic
In Equation 1, n is the number of electrons transferred in the
catalytic process (n = 2 for HER); ν is potential scan rate; R is the
universal gas constant; T is the temperature, and F is Faraday’s
constant.
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cycles are presented in Scheme 3. The only difference between
these two complexes are the substituents on the nitrogen atom in
the imine groups (the cyclohexyl group in 2, and the phenyl group
in 3), and we carried out the density function theory (DFT)
calculations (for details, see the SI) on these two catalysts to
reveal the origin of the substituent effect. The calculated one
electron reduced potentials of 2 and 3 are -0.54 and -0.63 V,
respectively, in good agreement with experimental values (-0.68
and -0.76 V), supporting the reliability of our calculations.
Furthermore, there is no major difference in the one electron
reduced potentials between 2 and 3, so we consider the
characters of the geometries after the catalysts accept one
electron and lose a chlorine ligand, namely, 2’ and 3’.
HSOMO energy in CH2Cl2 of 3’ is lower than that of 2’ by 0.12 eV,
suggesting that it is more difficult for the proton to attack 3’ than
2’. This is understandable because as a π-acceptor, the phenyl
group is able to lower the molecular orbitals whereas the
cyclohexyl group is a -donor only. Second, in the HSOMO of 2’,
the metal contribution is 44% but becomes 40% in that of 3,
which could be another factor for the lower reactivity of 3’
because an electron-rich metal center should have higher
reactivity towards protons. Third, the lower spin density a metal
center has, the lower reactivity it has towards protons. The spin
densities on the metal center in complexes 2’ and 3’ are 0.892 and
0.859, respectively, in the doublet state (Figure 3), indicating that
the Ni atom in 2’ is more favorable for the proton attack than the
Ni atom in 3’. Furthermore, the spin density on the phenyl group
in 3’ is higher than that on the cyclohexyl group in 2’ by 0.106,
which could again be attributed to the π-acceptor character of the
phenyl ring. Thus the interplay among HSOMO energy, orbital
contribution and spin density results in a lower reactivity of
catalyst 2’ than that of 3’. Consequently, the protonation did not
occur on 3 after the one electron reduction. The inverse kinetic
isotope effect of 3 is kH/kD=0.52, which is different from that of 2
(Table S5), indicated a proton-coupled electron transfer (PCET)
process during the hydrogen evolution reaction.4
Scheme 3 Possible electrocatalytic hydrogen evolution cycles of catalysts
2 and 3. Protonation occurs on 2 after a one electron reduction while the
protonation of 3 occurs only after a two electron reduction.
Our approach to construct a proton shuttle in the ligand was
adopted to improve the activity of catalysts. Figure 4 shows the
cyclic voltammograms of complex
1 for electrochemically
catalyzing hydrogen evolution with various concentrations of TfOH.
The onset potential of the catalytic peak is at about -0.67 V (vs.
Cp2Fe+/0, or -0.04 V vs. NHE) with 31.1 mM TfOH. Without the
complex 1, the background current on the glassy carbon electrode
is very low (<0.2 mA cm-2 at -1.5 V), confirming the catalytic
function of the complex (Figure S11). A second feature is that
prior to the main peak, there is a weak reduction peak at a
potential from -0.69 to -0.52 V, which shifts positively as the acid
concentration increases (Figure 4, inset). This result indicates a
combination of protonation and electron transfer processes
during the hydrogen evolution reaction, which was also supported
by KIE (Table S5).4 To evaluate the Faradaic efficiency of HER,
constant potential electrolysis at -1.1 V was performed for 4 h to
generate H2 gas in the head space of electrolytic cell. The
concentration of H2 was analyzed by gas chromatography, and the
Faradaic efficiency for H2 was determined to be >99% (Table S6).
Figure 3 Orbital contribution, HSOMO energy (eV) and spin density of 2’,
3’ in the doublet state and 3’- in triplet state.
The much lower overpotential of 2 compared to 3 for HER
suggests that complex 3’ does not react with H+ in CH2Cl2 but 2’
does, leading to the formation of a complex in which the H atom is
bonded to the Ni center. So we first compared the geometries of
2’ and 3’. The highest singly occupied molecular orbital (HSOMO)
and spin densities of the optimized geometries for complexes 2’
and 3’ in the doublet state are shown in Figure 3. First, the
Figure 4 Cyclic voltammograms of 1 (0.15 mM) in CH2Cl2 with varying
concentrations of TfOH.
The half wave potential of 2 is about 0.2 V higher than that
observed for 1, indicating that the pendant bases into the second
coordination sphere promote the HER. As described previously by
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Dubois et al.2a, pendant bases in ligand can couple the electron
and proton transfer, and avoid the large energy barrier of Ni(III)-H
for the hydrogen oxidation reaction. In our case, the proton relay
may also promote the proton coupled electron transfer process,
and provide a new pathway for hydrogen evolution reaction
without a high energy intermediate Ni(0)-H being involved, thus
lowering the overpotential significantly. Besides, the pendant base
1326. (b) Z. Wang, L. Wang, Sci. China Mater. 2018, 61, 806; (c) F.
Xiao, W. Zhou, B. Sun, H. Li, P. Qiao, L. Ren, X. Zhao, H. Fu, Sci. China
Mater. 2018, 61, 822; (d) Z. Zhao, Y. Xing, H. Li, P. Feng, Z. Sun, Sci.
China Mater. 2018, 61, 851; (e) Z. Qin, M. Wang, R. Li, Y. Chen, Sci.
China Mater. 2018, 61, 861; (f) K.-H. Ye, Z. Wang, H. Li, Y. Yuan, Y.
Huang, W. Mai, Sci. China Mater. 2018, 61, 887; (g) X. Sun, J. Huo, Y.
Yang, L. Xu, S. Wang, J. Energy Chem. 2017, 26, 1136; (h) S. Wang,
S. P. Jiang, Natl. Sci. Rev. 2017, 4, 163; (i) Y. Chen, Y. Zhang, Y. Ma, T.
Tang, Z. Dai, J. Hu, L. Wan, Chin. J. Chem. 2017, 35, 911; (j) X. Dou, W.
Liu, Q. Liu, Z. Niu, Chin. J. Chem. 2017, 35, 405; (k) L. Wang, W. Wang,
Chin. J. Chem. 2017, 35, 148.
in the ligand can be protonated preferentially to form
a
quaternary ammonium cation in acidic conditions, which helps to
prevent the complex decomposition, because the cation will
hinder further protonation of the diiminopyridine ligand. The
complex 1 can be stable in 50 mM TfOH but as a control, the
complex 2 and 3 without a pendant base decompose when the
acid concentration is in excess of 15 mM.
The icat/ip of complex 1 is as high as 123 when the acid
concentration is 31.1 mM. Through the slope of the linear plot, a
third-order rate constant was calculated to be 2.7×106 M-2 s-1,
indicating a remarkable turnover frequency of 2572 s-1 in 31.1 mM
TfOH. This third-order rate constant is 10-100 times larger than
that of Ni complexes with non-redox active cyclic diphosphine, but
also with pendant amines in the second coordination sphere
reported previously by DuBois (Table S4).2 Electron paramagnetic
resonance (EPR) of complex 1 after one-electron reduction by 1
eq of 1.0 M Li(BEt3H) shows the existence of the ligand radical
(Ni2+  Ni+) after one-electron reduction (Figure S12), which
supports our assumption that is presented in Scheme 1.
[2] (a) M. L. Helm, M. P. Stewart, R. M. Bullock, M. R. DuBois, D. L.
DuBois, Science 2011, 333, 863; (b) B. Ginovska-Pangovska, A. Dutta,
M. L. Reback, J. C. Linehan, W. J. Shaw, Acc. Chem. Res. 2014, 47,
2621; (c) T. Liu, M. R. DuBois, D. L. DuBois, R. M. Bullock, Energy
Environ. Sci. 2014, 7, 3630; (d) A. D. Wilson, R. H. Newell, M. J.
McNevin, J. T. Muckerman, M. Rakowski DuBois, D. L. DuBois, J. Am.
Chem. Soc. 2006, 128, 358; (e) O. A. Ulloa, M. T. Huynh, C. P. Richers,
J. A. Bertke, M. J. Nilges, S. Hammes-Schiffer, T. B. Rauchfuss, J. Am.
Chem. Soc. 2016, 138, 9234; (f) P. Huo, C. Uyeda, J. D. Goodpaster, J.
C. Peters, T. F. Miller, ACS Catal. 2016, 6, 6114; (g) B. H. Solis, S. J.
Hammes-Schiffer, J. Am. Chem. Soc. 2011, 133, 19036.
[3] (a) P. Du and R. Eisenberg, Energy Environ. Sci. 2012, 5, 6012; (b) R. S.
Nicholson, I. Shain, Anal. Chem. 1964, 36, 706 (c) J. R. McKone, S. C.
Marinescu, B. S. Brunschwig, J. R. Winkler and H. B. Gray, Chem. Sci.
2014, 5, 865; (d) B. D. Stubbert, J. C. Peters and H. B. Gray, J. Am.
Chem. Soc. 2011, 133, 18070; (e) O. R. Luca, S. J. Konezny, J. D.
Blakemore, D. M. Colosi, S. Saha, G. W. Brudvig, V. S. Batista and R. H.
Crabtree, New J. Chem. 2012, 36, 1149; (f) M. T. M. Koper and E.
Bouwman, Angew. Chem., Int. Ed. 2010, 49, 3723; (g) X. Hu, S.
Brunschwig and J. C. Peters, J. Am. Chem. Soc. 2007, 129, 8988; (h) E.
J. Thompson and L. A. Grapperhaus, Angew. Chem., Int. Ed. 2015,
54,11642; (i) X. Chen, H. Ren, W. Peng, H. Zhang, J. Lu and L. Zhuang,
J. Phys. Chem. C 2014, 118, 20791; (j) A. Z. Haddad, B. D. Garabato, P.
M. Kozlowski, R. M. Buchanan and C. A. Grapperhaus, J. Am. Chem.
Soc. 2016, 138, 7844; (k) O. R. Luca, J. D. Blakemore, S. J. Konezny, J.
M. Praetorius, T. J. Schmeier, G. B. Hunsinger, V. S. Batista, G. W.
Brudvig, N. Hazari and R. H. Crabtree, Inorg. Chem. 2012, 51, 8704; (l)
V. Lyaskovskyy and B. de Bruin, ACS Catal. 2012, 2, 270. (m) D.
Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S.
Demeshko, F. Meyer, M. Orio, V. Artero and C. Duboc, Nat. Chem.
2016, 8, 1054. (n) A. Z. Haddad, B. D. Garabato, P. M. Kozlowski, R. M.
Buchanan and C. A. Grapperhaus, J. Am. Chem. Soc. 2015, 137, 9238.
(o) E. J. Thompson and L. A. Berben, Angew. Chem., Int. Ed. 2015, 54,
11642. (p) J. M. Saveant, E. Vianello, Electrochimica Acta 1965, 10,
905.
Conclusions
We have designed a Ni complex with high electrocatalytic
activity for hydrogen evolution by incorporation of a non-innocent
diiminopyridine ligand with a pendant base in the second
coordination sphere for nickel. We further showed that the
electronic structure of a non-innocent ligand plays a critical role in
regulation of the activity of molecular catalysts. The electronic
withdrawing/donating effect of substituents could alter the
kinetics of metal hydride formation without making any obvious
change in the potential for molecular reduction, and this finally
leads to a large difference in catalytic activity. Thus, a practical
approach was found to eliminate the deleterious coupling
between metal center reduction and metal hydride formation.
Our findings are critical to a further understanding of the
properties of non-innocent ligands and the hydrogen evolution
reaction, which opens an avenue to the design of new catalysts
with non-innocent ligands for redox reactions.
[4] (a) S. C. Marinescu, J. R. Winkler, H. B. Gray, Proc. Natl. Acad. Sci. USA
2012, 109, 15127; (b) L. M. A. Quintana, S. I. Johnson, S. L. Corona, W.
Villatoro, W. A. Goddard, M. K. Takase, D. G. VanderVelde, J. R.
Winkler, H. B. Gray, J. D. Blakemore, Proc. Natl. Acad. Sci. USA 2016,
113, 6409. (c) H. Kotani, R. Hanazaki, K. Ohkubo, Y. Yamada and S.
Fukuzumi, Chem. Eur. J., 2011, 17, 2777.
[5] S. Shima, O. Pilak, S. Vogt, M. Schick, M. S. Stagni, W. Meyer-Klaucke,
E. Warkentin, R. K. Thauer, U. Ermler, Science 2008, 321, 572.
[6] A. Adamska, A. Silakov, C. Lambertz, O. Rüdiger, T. Happe, E. Reijerse,
W. Lubitz, Angew. Chem. Int. Ed. 2012, 51, 11458.
[7] (a) P. J. Chirik, Inorg. Chem. 2011, 50, 9737; (b) C. H. Hsieh, S. Ding, O.
F. Erdem, D. J. Crouthers, T. Liu, C. C. McCrory, W. Lubitz, C. V.
Popescu, J. H. Reibenspies, M. B. Hall, M. Y. Darensbourg, Nat.
Commun. 2014, 5, 3684; (c) O. R. Luca, R. H. Crabtree, Chem. Soc.
Rev. 2013, 42, 1440; (d) V. Lyaskovskyy, B. de Bruin, ACS Catal. 2012,
2, 270; (e) T. D. Manuel, J. U. Rohde, J. Am. Chem. Soc. 2009, 131,
15582; (f) J. V. Obligacion, S. P. Semproni, P. J. Chirik, J. Am. Chem.
Soc. 2014, 136, 4133; (g) A. C. Bowman, C. Milsmann, E. Bill, E.
Lobkovsky, T. Weyhermꢀller, K. Wieghardt, P. J. Chirik, Inorg. Chem.
2010, 49, 6110; (h) J. L. Dempsey, J. R. Winkler, H. B. Gray, J. Am.
Chem. Soc. 2010, 132, 16774; (i) W. H. Harman, Z. P. Lin, J. C. Peters,
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
Dedicated to Academician Xiyan Lu on the occasion of his 90th
birthday.
This study was supported by grants from Major State Basic
Research Development Program of China (2015CB932303),
Top-Notch Young Talents Program of China, and National Natural
Science Foundation of China (21373175, 2151101071, and
21321062). We thank Huayan Yang and Dongyu Liu for helpful
discussions and sample test.
References
[1] (a) A. Le Goff, V. Artero, B. Jousselme, P. D. Tran, N. Guillet, R. Metaye,
A. Fihri, S. Palacin, M. Fontecave, Science 2009, 326, 1384; (b) B.
Rausch, M. D. Symes, G. Chisholm, L. Cronin, Science 2014, 345,
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Angew. Chem. Int. Ed. 2014, 53, 1081.
Version of record online: XXXX, 2017
Manuscript received: XXXX, 2017
Revised manuscript received: XXXX, 2017
Accepted manuscript online: XXXX, 2017
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Entry for the Table of Contents
Page No.
Nickel Complexes with Non-innocent
Ligands as Highly Active Electrocatalysts
for Hydrogen Evolution
Zhixin Chen,† Tao Wang,† Tingting Sun,
Zhiyong Chen, Tian Sheng, Yu-Hao Hong,
Zi-Ang Nan, Jun Zhu,* Zhi-You Zhou,*
Haiping Xia,* Shi-Gang Sun
Regulation of metal center reduction and metal hydride formation for HER - lower overpotentials
and faster reaction rates.
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