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Electronic Supporting Information (ESI) for:

Identifying the mechanism and impact of parasitic reactions occurring in

carbonaceous Seawater Battery cathodesWonsuk Leea, Jehee Parka, Jaehyun Parka, Yunseok Choia, Seok Ju Kanga and Youngsik Kim a, b, *

a School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan, 44919, Republic of Korea

b Energy Materials and Devices Lab, 4TOONE Corporation, UNIST-gil 50, Ulsan, 44919, Republic of Korea

* Corresponding author: ykim@unist.ac.kr (Y. Kim)

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020

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Fig. S1 Photographs of (a) components of the experimental setup for seawater battery cell

performance evaluation, (b) pristine ACC, (c) 20mAh charged ACC and (d) 20mAh discharged

ACC.

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Fig. S2 EDS mapping images of (a) charged ACC showing fiber peeling along with oxygen species

on their surface and (b) discharged ACC showing CaCO3 crystallite surface precipitation.

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Fig. S3 Schematic illustration of (a) carbon dioxide circulation in seawater and (b) carbonic acid

equilibria according to pH.

Carbonic acid equilibria were plotted by using the following equations and dissociation constants

=[𝐶𝑂2]

[𝐻 + ]2

[𝐻 + ]2 + 𝐾𝑎2[𝐻 + ] + 𝐾𝑎1𝐾𝑎2

=[𝐻𝐶𝑂3‒ ]

[𝐻 + ]𝐾𝑎2

[𝐻 + ]2 + 𝐾𝑎2[𝐻 + ] + 𝐾𝑎1𝐾𝑎2

= [𝐶𝑂32 ‒ ]

𝐾𝑎1𝐾𝑎2

[𝐻 + ]2 + 𝐾𝑎2[𝐻 + ] + 𝐾𝑎1𝐾𝑎2

Where the fraction of each carbonic species can be expressed in terms of pH and the dissociation

constants given at atmospheric pressure, temperature of 25°C and a salinity of S=351, 2:

CO2 + H2O ↔ H + HCO3- ; Ka1 = 1.085×10-6

HCO3- ↔ H+ + CO3

2- ; Ka2 = 8.12×10-10

Reference

1. S. Bavarella, A. Brookes, A. Moore, P. Vale, G. Di Profio, E. Curcio, P. Hart, M. Pidou and E. J. McAdam, Journal of Membrane Science, 2020, 599, 117682.

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2. C. Mehrbach, C. Culberson, J. Hawley and R. Pytkowicx, Limnology and Oceanography, 1973, 18, 897-907.

Fig. S4 (a) Schematic illustration of the three-electrode cycle experiment setup showing the

configuration of the charge cathode / anode / discharge cathode and (b) its actual setup.

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Fig. S5 (a, b) SEM images and the inset photograph of ACC showing removal of CaCO3

precipitates after charging the pre-discharged cathode. (c) EDS mapping images of (b), showing

traces of CaCO3 after charging.

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Fig. S6 Pre-charging and pre-discharging voltage profiles of the ACC for 20mAh at a current

density of 0.5mA (0.25mAcm-2).