ADVANCED CU-PEPTIDE DESIGN FOR THE EFFICIENT ELECTROCATALYTIC PRODUCTION OF O2

J. S. Pap1,* Ł. Szyrwiel2,3 D. Lukács1,4 D. F. Srankó1 J. Brasun5 B. Setner6 R. Horvath7

1MTA Centre for Energy Research - Surface Chemistry and Catalysis Dept., H-1121 Budapest, Konkoly Thege 29-33, Hungary 2RIKEN, SPring-8 Center, Hyogo, Japan 3Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany 4University of Pannonia, Doctoral School of Chemistry and Environmental Sciences, H-8200 Veszprém, Hungary 5Department of Inorganic Chemistry, Wroclaw Medical University, 50-552 Wrocław, Poland 6Faculty of Chemistry, University of Wrocław, 50-383 Wrocław, Poland 7Nanobiosensorics Group, MTA Centre for Energy Research, Budapest, Hungary

*pap.jozsef@energia.mta.hu Synopsis CuII complexes with branched peptides undergo proton-coupled electron transfer (PCET) steps and promote the OER at basic pH, in phosphate buffer. C-terminal extensions, N-terminal modifications and multiple branching affect the TOF (kcat, s-1) of the OER very differently.

Self-assembling of complexes with polyelectrolytes has been demonstrated on indium-tin-oxide (ITO) working electrodes by simple layer-by-layer (LbL) surface deposition. The applied optical method (OWLS) revealed surface mass density changes under operando conditions of the OER.

J.S. Pap - János Bolyai Research Scholarship of the Hungarian Academy of Sciences (BO/00424/16)

Ł. Szyrwiel - Polish Foundation of Science within the BRIDGE program (BRIDGE/2012-5/9)

Financed by the VEKOP-2.3.2-16-2016-00011 grant supported by the European Structural

and Investment Funds

equatorial donor groups chelate ring size axial access ligand substituents hydrophilic/hydrophobic steric/electronic effects secondary interactions

1000 2000 3000 4000 5000 6000

2.8 equivs. CuII 1.54

1.64

1.00

1.09rel. spin conc.

1 eqiv. CuII

2 equivs. CuII

dX

"/d

B

Gauss

CuIICl2

X-band EPR CVs, 5 mV/s electrocatalysis at pH = 11 in phosphate

0 600 1200 1800 2400 3000 36000

1

2

3

4

5

6

~75% Faraday

efficiency

3rd2nd

rinsed ITO electrode

1.8:1 - Cu:3DapG

n(O

2)

(mm

ol)

time (s)

2.7:1 - Cu:3DapG 1st

n (Cu) = 0.78 mmol

~90% Faraday

efficiency

2nd

1st

Electrolysis, 1 V [Cu]:[L] effect

Mononuclear Cu

The OER serves as electron source for the production of renewable chemical energy carriers (H2). The presence of a molecular catalyst in the electrolyte promotes the OER at a polarized working electrode surface. First row transition metals (including Cu) are attractive alternatives for the traditional Ir and Ru. The electrochemical oxidation of a catalyst triggers the most challenging chemical step, e.g., the O-O bond formation. How can a molecular catalyst promote the O-O bond formation?

Llobet&Maseras, et al., ACS Catal. 2017, 7, 1712

Meyer, et al., JACS, 2013, 135, 2048

Kieber-Emmons, et al., JACS, 2017, 139, 8586

Pap&Szyrwiel, et al., Chem. Commun. 2015, 51, 6322

Szyrwiel&Pap, et al., RSC Adv. 2017, 7, 24657

Pap&Szyrwiel, Comm. Inorg. Chem. 2017, 37, 59

On the heterogenization options of Cu-peptides

0 30 60 90 120 150 180 210 240 2700

50

100

150

200

250

300

350

400

450

5002. electrolysis

with LbL-ITO

1. polyelectrolyte/rinse

Cu-peptide/rinse

cycles

0.0

0.5

1.0

1.5

2.0

i (m

A)

****

rinse

M (

ng/c

m2)

time (min)

*1.1 V

LbL-ITO working

electrode

Szyrwiel&Horvath&Pap, et al., Chem. Sci. 2016, 7, 5249

O2

solution: pH & complex

WE: XPS (Cu?)

CE: charge RE:

potential

Water splitting by molecular electrocatalysis: the oxygen evolving reaction (OER)

Peptide ligand design for Cu – potential effects

Dinuclear Cu

Cu-branched peptide variations and the OER

Multiple branching leading to advanced catalysis

The redox reactivity of small metallopeptides strongly depends on the amino acid sequence. The equatorial coordination of deprotonated amidic donor groups supports the CuIII/II redox event, moreover, OER was performed with a linear Cu-peptide. It is relevant to investigate how may structural variations of peptides affect the basic descriptors of the OER process?

Branching of tetrapeptides by L-2,3-diaminopropionic acid (Dap) allows access to quasi-tripodal ligands with ”3D” modularity. While C-terminal functionalization leads to palpable differences in kcat (s-1), additional, N-terminal substituents cease catalysis as evaluated under similar conditions by electrochemical methods.

EC-OWLS operando studies

We envisioned LbL deposition of polyelectrolytes and Cu-peptides on ITO that called for adequate analytical methods. The transport and adsorbtion of molecules at a coated surface can be investigated by means of optical waveguide lightmode spectroscopy (OWLS) and its combination with electrochemistry (EC-OWLS).

OWLS LbL formation kinetics

OWLS allows real-time monitoring of processes accompanied by refractive index changes on a waveguide sensor chip. The sensing principle relies on the perturbation of the evanescent optical waves of the guided lightmodes. Refractive index variations near the sensing surface alter the discrete incoupling angles, thus shift the positions of the resonance peaks. CV and CPE experiments on LbL-ITO showed that OER can be sustained for longer periods, if pH and potential are kept at values below those resulting in high initial rates, but conflicting with layer stability. Operando EC-OWLS analysis revealed mass transport events that ceased after initial loss in surface density.

A multiply branched peptide, 3DapG, was designed to furnish multiple metal centers. Each Dap branching unit (3 units) serves as a binding pocket for a cupric ion. UV-vis, CD and EPR spectroscopy suggest {2NH2,2N-}eq for the first two Cu2+ ions, whereas a vastly different mode for the third ion that in turn changes the overall complex structure and results a drop in spin concentration (EPR). According to electrochemical investigations this third Cu2+ triggers OER at a detectably lower overpotential/pH and higher oxygen evolution rate.

3DapG and Cu2+ binding hierarchy

0.2 0.4 0.6 0.8 1.0 1.20

50µ

100µ

150µ

200µ

250µ

300µ

350µ

400µ

-i (

A)

E vs. Ag/AgCl (V)

0.54 mM CuII

0.6 mM 3DapG

0.54 mM CuII

0.3 mM 3DapG

0.54 mM CuII

0.2 mM 3DapG

-0.6 -0.3 0.0 0.3 0.6 0.9

5 mA

3rd Cu