Cu foil

For this study, 100 μm-thick Cu foil (99.8% purity) was obtained from Sinopharm Chemical Reagent Co., Ltd., and 100 μm-thick W foil (99.95% purity) was purchased from Alfa Aesar. Before the graphene growth process, the Cu and W foils were successively cleaned by ultrasonication in deionized water, ethanol and acetone several times, and they were then immersed in dilute hydrochloric acid aqueous solution to remove oxide impurities. For the GNR growth, first, one piece of Cu foil (0.8 × 0.8 cm²) on W substrate (0.5 × 0.5 cm²) was placed in the center of a quartz tube. Afterwards, the whole system was rinsed by Ar gas at an ultrahigh flow rate (5000 standard cubic centimeters per minute, sccm) for about 30 s to remove the trapped air, followed by purging with a carrier gas of 200 sccm Ar and 10 sccm H₂ for 10 min. Second, the furnace was heated up to a temperature of 1120°C within 1 h under the carrier gas flow (200 sccm Ar and 10 sccm H₂). When the set temperature was reached, the substrates were annealed using pure Ar gas for a period to minimize the residual hydrogen induced in the heat-up stage. The GNR was grown at a temperature of 1120°C, with CH₄ gas as the carbon source and a mixture of H₂/Ar as the carrier gas. Finally, the CH₄ flow was turned off, and the furnace was rapidly cooled to room temperature.

Cu foil (0.3 mm, 3 N) was purchased from Sinopharm Chemical Reagent. Sodium hydroxide (NaOH), ammonium persulfate (APS), nitric acid (HNO₃), and sodium sulfide nonahydrate (Na₂S·9H₂O) were purchased from Aladdin Reagents. All these chemicals were analytical reagents and used without further purification.

Zinc acetate dihydrate, lead acetate dihydrate, potassium hydroxide; KOH, Zn foil, and Cu foil were purchased from Sinopharm Chemical Reagent Co., Ltd. without treatments, potassium bicarbonate; KHCO₃ (99.7% Sigma‐Aldrich), carbon papers (TGP‐H‐060), Nafion membrane (Nafion 211, Dupont), Nafion membrane solution (20%, Dupont), deionized water (purified by a Milli‐Q system) was used to prepare all solutions and to rinse samples and glassware.