Fig. 1. The schematic illustration of the experimental setup for applying a magnetic field during electrochemical measurement.

Fig. 2. (a) CV of Cu electrodeposition at polycrystalline Cu electrode. Data was collected in an electrolyte containing 0.5 M H2SO4 + 0.5 M CuSO4 with a scan rate of 10 mV/s for 3 consecutive CV cycles. The 1st and 3rd cycle were at ambient and the 2nd was under a magnetic field of 0.53 T. (b) Chronoamperometry (CA) of Cu electrodeposition at polycrystalline Cu electrode under a magnetic field with different applied potentials. The potential is not iR-corrected. (c) The schematic illustration of the effect of magnetic field (B) on the thickness of diffusion layer under a current flow (j). C* denotes the concentration of electroactive species as a function of the distance from the electrode surface, and a concentration gradient exists in the diffusion layer with a thickness of δd. The magnetic field induces a tangential flow (U), which leads to the formation of a hydrodynamic boundary layer with a thickness of δh. The velocity of U changes from U0 (the value in the bulk stream) to zero within the hydrodynamic boundary layer. The diffusion layer thickness under a magnetic field is δd-B.

Fig. 3. CA of Cu electrodeposition at polycrystalline Cu electrode in 0.5 M H2SO4 + 0.5 M CuSO4 with an applied potential of -0.4 V vs. RHE (not iR-corrected).

Fig. 4. Positive-going polarization curves (a) and CA (b) of HER at commercial Pt/C (40 wt%, Premetek) in electrolytes with different pH values: 0.1 M HClO4, 0.1 M KHCO3 and 0.1 M KOH. The catalyst loading is 2 µgPt/disk (disk area 0.196 cm2). Data was collected at ambient (blue dashed line) and under a magnetic field of 0.53 T (red solid line). The polarization curve was collected with a scan rate of 10 mV/s. During the CA measurement, the 0.53 T magnetic field was applied at the time indicated by the red arrow.

Fig. 5. CV (a) and CA (b) of OER at commercial IrO2 (Sigma-Aldrich, 2.77 m2/g according to Ref. [39]) in electrolytes with different pH values: 0.1 M HClO4, 0.1 M KHCO3 and 0.1 M KOH. The catalyst loading is 48 µgOxi/disk (disk area 0.196 cm2). Data was collected at ambient (blue dashed line) and under a magnetic field of 0.53 T (red solid line). The CV data was collected with a scan rate of 10 mV/s. During the CA measurement, the 0.53 T magnetic field was applied at the time indicated by the red arrow.

Fig. 6. The schematical illustration of Grotthuss mechanism, where proton (a) and hydroxide ion (b) transports along the hydrogen-bonded network in water. The proton (or hydroxide ion) diffuses from left to right. The dash line denotes the hydrogen bond and the arrow denotes the direction of proton diffusion. From top to bottom: it shows the four steps of proton (or hydroxide ion) diffusion along the exemplary four water molecules. Bottom: (a) the hydrogen evolution reaction, which takes the gray hydrogen atom for producing hydrogen gas; (b) the oxygen evolution reaction, which takes the gray oxygen atom for producing oxygen gas.