Supporting information for. The development of cobalt hydroxide as a bifunctional catalyst for oxygen. electrocatalysis in alkaline solution.

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1 Supporting information for The development of cobalt hydroxide as a bifunctional catalyst for oxygen electrocatalysis in alkaline solution Yi Zhan, a Guojun Du, b Shiliu Yang, a Chaohe Xu, a Meihua Lu, a Zhaolin Liu, b Jim Yang Lee a* a Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore , Singapore. Fax: ; Tel: ; cheleejy@nus.edu.sg b Institute of Materials Research and Engineering (IMRE), Agency of Science, Technology, and Research (A*STAR), 3 Research Link, Singapore , Singapore. 1

2 Fig.S1 TEM images of CoO (a) and Co3O4 (b) and their corresponding XRD patterns (c). Fig.S2 BET surface areas of the cobalt catalysts in this study (a) and ORR kinetic mass activities of Co(OH)2, RuO2 and Pt/C at 0.9 V (b). 2

3 Fig.S3 ORR Polarization curves for CoO (a) and Co3O4 (c) in O2-saturated 0.1 M KOH at different rotating speeds and the corresponding Koutecky-Levich plots (J 1 versus ω 1/2) (b, d) in the 0.30 V~0.60 V potential window. 3

4 Table S1. Recent literature on the ORR performance of Co(OH) 2. Catalyst Hexagonal Co(OH) 2 nanoplates n ~3.6 (0.3~0.6 V) Mass activity at 0.6 V (A g -1, based on catalyst) (900 rpm) This work Reference Co(OH) 2 plates 3.1 (0.35~0.5 V) ~105.9 (900 rpm) ACS Appl. Mater. Interfaces, 2014, 6 (13), electrodeposited Co(OH) (0.37 V) N/A Journal of Power Sources, 2010, 195, 3135 Co(OH) 2 sheets 3 (0.3~0.5 V) ~8.57 (1200 rpm) Journal of Power Sources, 2011, 196,

5 Fig.S4 Capacitive and ir corrections of the measured OER polarization curves; using the LSV of Co(OH) 2 in O 2 -saturated 0.1 M KOH at 1600 rpm as an example (a). Values of the Tafel slopes of Co(OH) 2 and RuO 2 (b). Table S2. Recent literature on the OER performance of Co(OH) 2 Catalyst Electrolyte Mass activity at η=0.3 V (Ag -1, based on Reference catalyst) Co(OH) M KOH 15.5 This work exfoliated Co-based layered double hydroxide 1 M KOH ~12.9 Nature Communications 5, doi: /ncomms5 477 Co(OH) 2 1 M KOH 19.4 ACS Appl. Mater. Interfaces, 2014, 6 (13),

6 Fig.S5 XRD (a) and XPS (b) measurements of Co(OH)2 after OER stability test. Fig.S6 ORR (a) and OER (b) polarization curves of Co(OH)2+N-rGO with different NrGO contents. The total solid content was 30 wt% Co(OH)2 70 wt% carbon materials. Within the latter the N-rGO content was varied from 0 wt% to 70 wt%. Table S3. Comparison of the catalysts in this study Catalyst E [V] at J = 3 ma cm 2 for ORR Co(OH)2 CoO Co3O4 Co(OH)2+N-rGO RuO2 Pt/C E [V] at J = 10 ma cm 2 for OER N/A 6 E (OER-ORR) [V] N/A

7 Table S4. Literature survey on non-pgm bifunctional catalysts Catalyst Total solid E (OERloading (mg cm 2 ) ORR) [V] Reference Co(OH) 2 +N-rGO This work Ir/C J. Am. Chem. Soc. 2010, 132, LaCoO 3 /NC Chem. Mater. 2014, 26, 3368 Co 3 O 4 decorated blood powder derived heteroatom doped porous carbon CoFeN/NMC NiCo 2 O Co 3 O 4 /Co 2 MnO 4 nanocomposite NiCo 2 S ~1.00 α-mno N and p co-doped porous carbon 3D mesoporous graphene 0.15 > Co/N-C Adv. Funct. Mater. 2014, 24, 7655 ACS Appl. Mat. Inter. 2015, 7, 1207 Nanoscale 2014, 6, 3173 Nanoscale. 2013,5, 5312 ACS Appl. Mater. Interfaces 2013, 5, 5002 J. Am. Chem. Soc. 2014, 136, Nat. Nanotechnol., DOI: /nnano Chem. Commun., 2015,51, 6773 Nanoscale, 2014, 6,

8 Table S5 Comparison with the state-of-the-art Zn-air batteries using bifunctional catalysts Cathode catalyst Key parameters Reference Co(OH) 2 +N-rGO MCF/N-rGO LaNiO 3 supported on nitrogen-doped carbon nanotubes Co 3 O 4 nanowires grown on stainless steel mesh Co 3 O 4 NPsdecorated CNFs La 2 NiO 4 NiCo 2 O 4 ma cm -2 with 40 min per step for 75 cycles, voltage polarization after cycling: V; roundtrip efficiency: 46% ma cm -2 with 40 min per step for 60 cycles, voltage polarization after cycling: 1.5 V; round-trip efficiency: 38% ma cm -2 with 10 min per step for 75 cycles, voltage polarization after cycling: 1.4 V; round-trip efficiency: 41% ma cm -2 with 10 min per step for 100 cycles, voltage polarization after cycling: 1.1 V; round-trip efficiency: 45% ma cm -2 with 60 min per step for 55 cycles, voltage polarization after cycling: 0.85 V; round-trip efficiency: 58% ma cm -2 with 2.5 min per step for 20 cycles, voltage polarization after cycling: 1.2 V; round-trip efficiency: 50% ma cm -2 with 20 min per step for 50 cycles, voltage polarization after cycling: 0.9 V; round-trip efficiency: 54% This work J. Mater. Chem. A, 2014, 2, Nano Lett. 2012, 12, 1946 Adv. Energy Mater. 2013, doi: /aenm Nanoscale, 2015, 7, 1830 ACS Appl. Mater. Interfaces 2013, 5, 9902 Nanoscale, 2014, 6,