Supporting Information

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1 Supporting Information Enhancing Full Water Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO2- TMO-Ni Nano-interfaces Xia Long, a,b He Lin, a Dan Zhou, a Yiming An, a Shihe Yang a,b* a. Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b. Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University, Shenzhen, China Correspondence and requests for materials should be addressed to S. Y. ( chsyang@ust.hk) Experimental details Materials. Chemical reagents including Ce(NO 3 ) 3, NaCl, FeCl 3, Mn(Ac) 2, NiCl 2, H 2 SO 4, HNO 3, C 2 H 5 OH, and KOH were purchased from Sigma-Aldrich. All the chemical reagents were analytical grade (AR) and were used without any additional purification. Deionized water was used in all experimental processes. Carbon cloth were pre-treated by complying following steps before each experiment: firstly, they were immersed in a H 2 SO 4 /HNO 3 (v/v=1/1) mixed aqueous solution (30 wt%)and sonicated for 2 hours, then washed with deionized water (DI water) under sonication for 3 time, 30 min/time. Preparation of the catalysts. (1). CeO 2 film was fabricated by electrodeposition on carbon cloth (CC) in a mixed solution containing 2 mm Ce(NO 3) 3 and 10 mm NaCl at 0.25 ma/cm 2 for 10 min at 70. (2). Ceira/TM-OH were synthesized by electrodeposition NiMnFe hydroxide nanosheets on ceria coated carbon cloth. The electrolyte for electrodeposition were 10 mmol, 10 mmol, 10 mmol of Fe, Ni, Mn, respectively. The electrodeposition was carried out at -0.9 V (vs RHE) for 200 s at room temperature if not otherwise indicated. (3). Ceria/Ni-TMO and Ni-TMO were fabricated by post annealing treatment of corresponding ceria/tm-oh and NiMnFe hydroxides at 400 for 5 hours in H 2/N 2 (2.5%) atmosphere, respectively. The reduction process of metallic Ni nanoparticles was described by the following formula: TM(OH) 2 + H 2 TMO + 3H 2O +TM Herein, TM-OH indicates NiMnFe hydroxides (Fe and Mn co-doped layered Ni(OH) 2), which has been described as TM(OH) 2 in the above reaction formula. TM (transition metal) represent Ni, Fe and Mn (with atomic ratio of Ni/Fe/Mn in TM-OH to be around 7/2/1). All samples were washed several times by DI water and then blow-dried by N 2 gas to remove the physical deposited nanomaterials before use and characterizations. Electrochemical testing. Electrochemical measurements were conducted in a standard threeelectrode glass cell on a CHI 760 E workstation at room temperature in a standard three-electrode system using as-electrodeposited catalysts on carbon cloth as working electrodes, Pt wire (for

2 OER)/graphite rod (for HER) as the counter electrodes, and Ag/AgCl (3M) as the reference electrode respectively, if not otherwise indicated. The OER, HER and full water splitting were tested in 1 M KOH (ph=13.74) and all potentials measured were calibrated to RHE using the following equation: E (RHE) = E (Ag/AgCl) ph. Cyclic voltammetry (CV) testes were carried out at a scan rate of 10 mv/s for more than 50 cycles until a stable CV curve was achieved before measuring the water splitting performances of the catalysts. Polarization curves were obtained using linear sweep voltammetry (LSV) with a scan rate of 5 mv/s with 95% ircorrection to avoid the influence of electrolyte resistance. Tafel plots were collected at a scan rate of 1 mv/s. The long-term durability tests were carried out using the chronopotentiometry measurements under current density of 10 ma/cm 2 for both OER in a three-electrode system and the whole water splitting in a two-electrode system (the as-synthesized ceria/ni-tmo on carbon cloth were applied as both the anode and cathode). Electrochemical impedance spectroscopy (EIS) tests were carried out in a frequency range from 100 KHz to 0.1 Hz. The electrochemical active surface area (ECSA) was measured by measuring the double layer capacitance (C dl ) via a simple cyclic voltammetry (CV) method in a non-faradaic region at different scan rates. Morphology and structure characterizations. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were characterized by JEOL JSM Transmission electron microscopy (TEM) samples were prepared by sonicating the catalysts from carbon cloth and then the catalysts suspensions were drop dried onto copper grids and were measured by JEOL JEM Gas chromatography measurements. Water splitting were performed in a gas-tight electrochemical cell with 1 M KOH electrolyte. Both the working and counter electrode were assynthesized ceria/ni-tmo on carbon cloth. The wires were stamped with the carbon cloth and glued by epoxy to make sure the good connection. Chronopotentiometry at the current density of 5 ma/cm 2 to maintain constant gas production. The reaction cell was connected to the gas-sampling loop of the gas chromatograph (GC-7900, Techcomp Co.) with argon as the carrier gas. A thermal conductivity detector (TCD) was used to detect and quantify the hydrogen and oxygen generation. The Faraday efficiency (FE) was estimated by comparing the amount of experimentally measured hydrogen and oxygen, as well as that from the theoretical calculations by the equation of It/nF, where I is the current (A), t is the duration (s), n is the number of electrons transferred, and F is the Faraday constant ( C/mol). Other characterizations. X-ray diffraction (XRD) were measured by a multi-purpose XRD diffractometer of PANanalytical, model Empyrean. X-ray photoelectron spectroscopy (XPS) spectra were collected on Physical Electronics, model PHI 5600.

3 Scheme S1. Scheme for the formation of ceria/ni-tmo on carbon cloth. Figure S1. SEM images of (A, B) bare carbon cloth, (C, D) CeO 2 film and (E, F) NiMnFe hydroxides nanosheets formed on carbon cloth by electrodeposition.

4 Figure S2. SEM (A) and corresponding EDX mapping images (B-F) of ceria/tm-oh. (B) Ce, (C) O, (D) Ni, (E) Fe, and (F) Mn mapping.

5 j (ma/cm 2 ) Ceria/TM-OH Ceria/TM-OH (n) Ceria/Ni-TMO Ceria/Ni-TMO (n) E (V vs RHE) Figure S3. Polarization curves of ceria/tm-oh and ceria/ni-tmo. Ceria/TM-OH (black curve) and ceria/ni-tmo (blue curve) were collected from low potential to high potential, while ceria/tm-oh (n) and ceria/ni-tmo (n) indicate LSV curves were collected from high potential to low potential. j (ma/cm 2 ) ceria/ni-tmo ceria/tm-oh Ni-TMO ceria ceria/nimn-oh E (V vs RHE) Figure S4. Polarization curves of ceria/ni-tmo (black cueve), ceria/tm-oh (red curve), Ni-TMO (blue curve), bare ceria film (magenta curve) and ceria/nimn-oh (olive curve).

6 j (ma/cm 2 ) j (ma/cm 2 ) ph 14 ph 13.5 ph 13 ph V (vs RHE) ph E (V vs RHE) Figure S5. Polarizations curves of ceria/ni-tmo on OER collected in different electrolytes with different ph values: ph 12.5 (0.03 M KOH), ph 13 (0.1 M KOH), ph 13.5 (0.56 M KOH), and ph 14 (1 M KOH). Inset was the current densities tested in electrolytes with different ph values at 1.5 V (vs RHE). Inset was the current densities tested in electrolytes with different ph values at 1.5 V (vs RHE).

7 3d 3/2 Ce 3d Intensity (a.u.) 3d 5/2 Ce 4+ Ce 3+ Ce 4+ Ce 3+ shake-up 3d 3/2 shake-up Ce 4+ shake -up Binding Energy (ev) Figure S6. High resolution XPS spectra of ceria/ni-tmo in Ce 3d region. A Ni-TMO ceria/ni-tmo Mn 2p B Ni-TMO ceria/ni-tmo Ni 2p C Ni-TMO ceria/ni-tmo Fe 2p Intensity(cps) Intensity(cps) Intensity(cps) Binding Energy(eV) Binding Energy(eV) Binding Energy(eV) Figure S7. XPS spectra of Ni-TMO (black curves) and ceria/ni-tmo (red curves) at the spinorbital of (A) Mn 2p, (B) Ni 2p and (C) Fe 2p. The peaks of Fe, Ni and Mn 2p spin-orbital in ceria/ni-tmo (red curves) were positively shifted compared with that of Ni-TMO (black curves), indicating the existence of ceria film offered opportunities to generate strong electron interactions with the formed Ni-TMO, which further benefit the water splitting performance of the catalysts.

8 A Ni fitting: ceria/ni-tmo Ni 2p 3/2 B Ni fitting: ceria/tm-oh Ni 2p 3/2 Intensity (a.u.) Ni 2p 1/2 Satellite ev ev ev Intensity (a.u.) Ni 2p 1/2 Satellite ev Binding Energy (ev) Binding Energy (ev) Figure S8. Deconvoluted high resolution XPS spectra of (A) ceria/ni-tmo, and (B) ceria/tm- OH. Metallic Ni was fitted from XPS spectra of ceria/ni-tmo. Figure S9. Deconvoluted high resolution XPS spectra of (A) ceria/tm-oh, (B) Ni-TMO, and (C) ceria/ni-tmo in Mn 3d spin-orbital region.

9 ceria/tm-oh Z''/ohm -40 Ni-TMO ceria/ni-tmo Z'/ohm Figure S10. EIS spectra of ceria/ni-tmo (black curve), Ni-TMO (red curve) and ceria/tm-oh (blue curve) tested in the overpotential of 100 mv for HER. A j (ma/cm 2 ) Ceria/Ni-TMO 100 mv/s 5 mv/s B j (ma/cm 2 ) Ceria/TM-OH 100 mv/s 5 mv/s C j (ma/cm 2 ) Ni-TMO 100 mv/s 5 mv/s E (V vs RHE) E (V vs RHE) E (V vs RHE) Figure S11. CV curves of catalyzed OER collected at different scan rates of 5 mv/s, 10 mv/s, 20 mv/s, 30 mv/s, 40 mv/s, 50 mv/s, 70 mv/s, and 100 mv/s. (A) ceria/ni-tmo, (B) ceria/tm-oh, and (C) Ni-TMO.

10 Figure S12. Gas chromatography curve of the gaseous products from the water splitting by using ceria/ni-tmo on carbon cloth as both the cathode and anode.

11 Table S1. Comparison of OER performances for ceria/ni-tmo with other reported non-noble metal based electrocatalysts. represent onset overpotential (if not otherwise indicated) or overpotential at the current density of 1 ma/cm 2, 2 ma/cm 2, and b represent overpotential, and Tafel slope, respectively. (onset) indicates the onset overpotential and the overpotential at 10 ma/cm 2, and the OER performance was collected in 1M KOH if not otherwise indicated. refs catalysts /mv b/(mv/dec) Working electrode This work ceria/ni-tmo Carbon Cloth FeNi-O S1 FeNi LDH NiO GC Ni(OH) S2 RuO GC Exfoliated CoCo LDH S3 Exfoliated NiCo LDH Exfoliated NiFe LDH GC IrO S4 NiFeO x NiCoO x Carbon cloth S5 NiFe LDH/CNT CFP (0.1 M KOH) S6 Ni 0.9Fe 0.1O x 297 (1mA/cm 2 ) 30 Au/Ti QCM Ni 0.5Co 0.5O x 320 (1mA/cm 2 ) 35 crystal; Ni 0.25 Co 0.75 O x 340 (1mA/cm 2 ) 36 ITO Ni 0.75 Co 0.25 O x 312 (1mA/cm 2 ) 33 S7 NiCoO x Cr/Au coated glass FeNi 10 LDH 220 (2 ma/cm 2 ) 55 S8 FeNi 9Co LDH 210 (2 ma/cm 2 ) 52 Ni foam FeNi 8Co 2 LDH 200 (2 ma/cm 2 ) 42 S9 Porous FeNi oxide GC (0.1 M KOH)

12 Table S2. Comparison of HER performance for ceria/ni-tmo with other reported non-noble metal based electrocatalysts. ref Electrocatalysts Overpotential (mv) Tafel slope (mv/dec) Electrolyte This ceria/ni-tmo 93 (j=10 ma/cm 2 ) 69 work Ni-TMO 198 (j=10 ma/cm 2 ) M KOH S10 Ni/NiO 46 (j=20 ma/cm 2 ) 65 1 M KOH S11 Ni/NiO-CNT 80 (j=10 ma/cm 2 ) 82 1 M KOH S12 Ni-MoS 2 98 (j=10 ma/cm 2 ) 60 1 M KOH S13 NiMo/Ni foam 30 (j=10 ma/cm 2 ) 86 1 M KOH S14 Ni 4 Mo 34 (j=20 ma/cm 2 ) M KOH S15 Ni-Mo-N 53 (j=20 ma/cm 2 ) M H 2SO 4 43 (j=20 ma/cm 2 ) 40 1 M KOH S16 NiS 2 /CC 243 (j=10 ma/cm 2 ) 69 1 M KOH S17 NiCo 2 S 4 /NF 210 (j=10 ma/cm 2 ) M KOH S18 Ni/NiO 145 (j=10 ma/cm 2 ) 43 1 M PBS Table S3. Comparison of full water splitting performance for ceria/ni-tmo in two-electrode setup with other reported non-noble metal based bifunctional electrocatalysts. ref Electrocatalysts E (V, j=10 ma/cm 2 ) Electrolyte This work ceria/ni-tmo/cc M KOH S19 NiSe/NF 1.63 Bare NF M KOH Pt/C on NF 1.67 S20 Ni 5P 4-NiOOH < 1.70 Pt >1.8 1 M KOH Pristine Ni >1.9 S21 NiFe LDH/NF M KOH S22 NiCo 2S 4 NA/CC 1.68 NiCo 2O 4 NA/CC M KOH S23 a-cose/ti M KOH S24 NiFe/NiCo 2O 4/NF M KOH S25 Co(S xse 1-x) 2/NF M KOH S26 CoP/CNT(-) Co 0.7Fe 0.3P/CNT(+) M KOH S27 CoP/GO M KOH S28 NiC//NiD-PCC M KOH S29 Co/CoP M PSB

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