Supplementary Information

Heat-induced magnetic transition accelerates redox couple mediated electrocatalytic water oxidation in alkaline media

Duanduan Liu,1,2 Mengfei Lu,1,2 Depei Liu,1,2 Shicheng Yan,*2 Wei Zhou,*3 Lunyong Zhang,*4 Zhigang Zou1,2

1National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory for Nano Technology, School of Physics, Nanjing University, No. 22 Hankou Road, Nanjing, Jiangsu 210093, P. R. China

2Jiangsu Key Laboratory of Artificial Functional Materials, Collaborative Innovation Center of Advanced Microstructures, Eco-materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, No. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China. Email: yscfei@nju.edu.cn

3Department of Physics, Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, P. R. China. Email: weizhou@tju.edu.cn

4 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R. China. Email: allen.zhang.ly@gmail.com

Supplementary Figure 1 | OER activity of α-NixFe1−x(OH)2 electrocatalysts as a function of Fe content in 1 M KOH at 30 oC without iR compensation.

Supplementary Figure 2 | a, XRD pattern of α-Ni0.5Fe0.5(OH)2. b, High-resolution TEM lattice image for α-Ni0.5Fe0.5(OH)2. c, High-resolution TEM lattice image for α-Ni0.5Fe0.5(OH)2 after LSV scanning at 40 oC. d, High-resolution TEM image for α-Ni0.5Fe0.5(OH)2 after LSV scanning at 90 oC.

Supplementary Figure 3 | Differential scanning calorimetry (DSC) and thermogravimetry (TG) curves for a, Ni(OH)2. b, Ni0.5Fe0.5(OH)2. c, Ni0.5Fe0.5OxHy. d, XRD patterns of Ni0.5Fe0.5(OH)2 before and after magnetic testing.

Differential scanning calorimetry (DSC) and thermogravimetry (TG) were carried out in air atmosphere with a temperature rising step of 10 oC/min to check the thermal stability of Ni(OH)2, Ni0.5Fe0.5(OH)2, and Ni0.5Fe0.5OxHy. As shown in Supplementary Fig.3a-c, only the removal of adsorbed water or crystalline water was observed when these samples were heated at T < 250 oC. The XRD patterns confirmed that the sample was stable during the 200-500K (-73.15oC - 226.85oC) temperature-course magnetic testing (Supplementary Fig.3d). All the samples exhibited about 10 wt. % weight loss from the removal of adsorbed water or crystalline water. Therefore, the moment for Ni0.5Fe0.5(OH)2 and Ni0.5Fe0.5OxHy was calculated by using the quality of sample without the adsorbed water or crystalline water (Fig.2c).

Supplementary Figure 4 | ZFC and FC magnetizations of Ni(OH)2 as a function of temperature with applied magnetic field H = 500 Oe.

Supplementary Figure 5 | XRD patterns for the Ni0.5Fe0.5 (OH)2 with and without

electrochemical oxidation at 1.4 V or 1.55 V for 1h at 30 oC in 1 M KOH electrolyte.

To check the phase transformation of Ni0.5Fe0.5(OH)2 during OER, the electrochemical oxidation of Ni0.5Fe0.5(OH)2 was performed at different potentials at 30 oC for 1h. At 1.4 V of a Ni2+/Ni3+ oxidation potential or 1.55V of an OER potential, the XRD pattern indicated that the crystallinity of Ni0.5Fe0.5(OH)2 decreased and no XRD peaks of Ni0.5Fe0.5OOH were detected (Supplementary Fig. 5). However, the Raman spectra (Fig. 4c) confirmed the formation of Ni0.5Fe0.5OOH after electrochemical oxidation of Ni0.5Fe0.5(OH)2. These facts verified that the amorphous Ni0.5Fe0.5OOH produced during the OER. The mixture (denoted as Ni0.5Fe0.5OxHy) of amorphous Ni0.5Fe0.5OOH and low-crystalline Ni0.5Fe0.5(OH)2 was also indicated by the unsharp or no lattice spacing in the high-resolution TEM lattice image (Supplementary Fig. 2).

Supplementary Figure 6 | a, OER activity of Ni(OH)2 and Ni0.5Fe0.5(OH)2 electrocatalysts on NF at 30-90 oC with 85% iR compensation. b, OER activity of IrO2 and Ni0.5Fe0.5(OH)2 electrocatalysts on NF at 30-90 oC with 85% iR compensation.

Supplementary Figure 7 | a, The temperature-dependent slope of OER current-voltage curves for Ni0.5Fe0.5(OH)2 b, The temperature-dependent Tafel slope of Ni(OH)2.

Supplementary Figure 8 | H2 and O2 evolution of Ni0.5Fe0.5(OH)2 for 5 min measured at different voltage region at 40oC and 90oC.

Supplementary Figure 9 | The temperature-dependent EIS spectra at 1.45V.

Supplementary Figure 10 | a-g, Cyclic voltammograms scanning for the Ni0.5Fe0.5(OH)2 electrocatalyst loaded on NF at 30 oC-90 oC with different scan rates ranging from 20 to 120 mV·s-1. h, Calculations of electrochemical avtive surface area.

Supplementary Figure 11 | The energy of compounds (E) and free energy changes (ΔG) for oxidation of Ni0.5Fe0.5(OH)2 to Ni0.5Fe0.5OOH, with calculated structures.

The Gibbs energy changes for Ni0.5Fe0.5(OH)2/Ni0.5Fe0.5OOH oxidation (ΔGNi2+→Ni3+) were calculated by ΔGNi2+→Ni3+ = GNiOOH - GNi(OH)2 - μH, where GNiOOH is the free energy of NiOOH as calculated from DFT, GNi(OH)2 is the free energy of Ni(OH)2 as calculated from DFT, and μH is the chemical potential of hydrogen. The free energy of materials were obtained from vibration frequency calculations.

Supplementary Figure 12 | The electrochemical performance of Ni0.5Fe0.5(OH)2 at 40 and 90 oC. The samples were electrodeposited on carbon paper to avoid the influence of the foamed nickel substrate on the electrocatalytic material.

Supplementary Figure 13 | a-c, Total DOS and projected DOS for Ni(OH)2 and Ni0.5Fe0.5(OH)2 to show the effects of Fe 3d orbitals on Ni 3d orbitals. d-f, Total DOS and projected DOS for NiOOH and Ni0.5Fe0.5OOH to show the effects of Fe 3d orbitals on Ni 3d orbitals.

Supplementary Figure 14 | High-revolution XPS spectra for Ni0.5Fe0.5(OH)2 after LSV scanning at 40 oC or 90 oC. a, Ni 2p. b, Fe 2p.

Supplementary Figure 15 | a, A photo to show the temperature coefficient correction setup. The two Ag/AgCl electrodes were connected by a salt bridge to build the initial potential equilibrium. The one Ag/AgCl electrode at 30oC was used as a reference to monitor the potential changes of the another electrode with increasing the temperature. b, The relationship between potential changes and temperature changes. The potential difference ∆V is defined as the VT-V30, where the VT is the detected potential at testing temperature and V3o is the potential for electrode at 30oC. The temperature-dependent potential correction relationship was fitted to be ∆V=8.886x10-5T+1.076x10-4.

Supplementary Table 1. Fitting parameters of EIS data obtained by using the equivalent circuit proposed in Fig. 3c.

Temperature (oC)	Rs (Ω)	Rct,M (Ω)	Cct,M (F)	Rct,OER (Ω)	Cct,OER (F)
1.4 V	30	0.968	1.984	0.037	0.706	0.026
40	0.783	1.484	0.041	0.229	0.025
50	0.684	0.996	0.057	0.188	0.026
60	0.625	0.723	0.061	0.132	0.027
70	0.571	0.494	0.063	0.108	0.028
80	0.534	0.409	0.065	0.085	0.027
1.45 V	30	0.744	2.851	0.109	0.108	0.204
40	0.665	1.614	0.118	0.109	0.205
50	0.582	0.727	0.119	0.108	0.205
60	0.540	0.485	0.121	0.109	0.205
70	0.499	0.270	0.134	0.109	0.206
80	0.468	0.166	0.135	0.110	0.207

Supplementary Table 2. Faraday efficiency for water splitting on Ni0.5Fe0.5(OH)2 at 40 oC and 90 oC at different applied potentials.

Applied potentials	Temperature	H2	O2
1.4 V	40oC	-	-
90oC	59.2 %	53.0 %
1.44 V	40oC	55.5 %	1.3 %
90oC	85.3 %	71.9 %
1.47 V	40oC	86.8 %	81.0 %
90oC	91.7 %	86.4 %
1.5 V	40oC	92.6 %	90.3 %
90oC	93.9 %	96.3 %