Commercial Cr2O3 (3 μm, 99.5%, Sinopharm Chemical Reagent Co., Ltd.), GeO2 (500 nm, 99.9%, Sinopharm Chemical Reagent Co., Ltd.), graphite powders (10 nm, 99.8%, Sinopharm Chemical Reagent Co., Ltd.) with different molar ratios (1:1:1, 1:2:1, 1:3:1, and 1:4:1), and 10 wt% polyvinyl butyral (PVB, Sinopharm Chemical Reagent Co., Ltd.) were mixed by ball-milling at 300 r/min for 5 h to prepare the powdered Cr2O3/GeO2/C precursor. About 0.5 g of the obtained mixed powders were pressed under 10 MPa to fabricate a Cr2O3/GeO2/C disc (10 mm in diameter). The Cr2O3/GeO2/C disc was wrapped by nickel foam and fixed on a Mo wire (2 mm in diameter, Shanghai Non-Ferrous Metals (Group) Co., Ltd.) to form the Cr2O3/GeO2/C cathode system. A high-purity graphite rod (5 mm in diameter, 99.999%, Shanghai Carbon Co., Ltd.) fixed with the Mo wire was used as the anode. CaCl2 and NaCl (Shanghai Aladdin Biochemical Technology Co., Ltd.) were commonly baked at 300–400°C for 24–48 h and then used as electrolytes in a 1:1 M ratio. The electrodes and mixed salts were assembled in a corundum crucible to form an electrolytic cell, which was then placed in an electrolysis furnace sealed on one end. High-purity Ar gas was continuously introduced into the electrolytic furnace to create an inert atmosphere. The electrolysis furnace temperature was then ramped up to 700°C with a heating rate of 5°C/min. Pre-electrolysis was then performed between two graphite rods (5 mm in diameter, 99.999%, Shanghai Carbon Co., Ltd.) at 2.0 V for 2–5 h to eliminate residual purities in molten salts. A constant voltage of 3.0 V was applied between the Cr2O3/GeO2/C cathode and the graphite anode for pre-set times. After electrolysis, the obtained electrolytic samples were washed with deionized water to remove solid salts and then, dried at 100°C in a vacuum drying oven for further characterization.
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