Can Carbon be Used as an Anode for Water Splitting?

Abstract

Carbon materials, whose structural and electronic properties can be fine-tuned, are promising material solutions for many energy-related systems. However, due to the lack of fundamental understanding of the carbon surface chemistry, especially when they are used in electrolytes, the rapid development of carbon as electrodes has led to many widely accepted misunderstandings. Focusing on the case of carbon-based electrode for water splitting, this Viewpoint tries to highlight the main problems of the area and demonstrates/presents the dynamic carbon surface chemistry in the application. The role of carbon as an anode for water splitting is revealed and if it can be practically used in water splitting is discussed.

Keywords: Anode; Carbon materials; Surface chemistry; Water splitting.

Figure 1

The possible oxidation process of the selected model molecules (blue color). The thermodynamic energy is calculated for each step with DFT method. The steps are the most energy‐favorable ones. As the model cannot represent real carbon structures, the obtained energy only shows a trend of the energy requirement. Different oxidants like H₂O, H₃O⁺ and OH⁻ are considered for the steps and only OH⁻ show the ability to continuously oxidize those sites. Here, the carbon atoms denoted by gray ball, the oxygen atoms denoted by red ball and the hydrogen atoms denoted by white ball.

Figure 2

The plausible catalytic process of water oxidation when the oxidation sites of the carbon are blocked by iron species.

Figure 3

a) Charge collection of the carbon pellet in, acidic (H₂SO₄, pH 1), alkaline (KOH, pH 13) and neutral (K₂SO₄, pH 7) electrolytes. The dash lines show the initial steps. b) C 1s of the carbon before and after reaction. c) Thermal analysis of different samples. The non‐linear thermal procedure is used for removing different oxygen functionalities (details in d). d) The decomposition species and steps of different functionalities: carboxyl (252 °C, 10 min), lactone (zig‐zag) (310 °C, 25 min), anhydride (515 °C, 42 min), lactone (armchair) (610 °C, 58 min), phenolic (800 °C, 75 min) and carbonyl (1000 °C, 93 min). e) The density of the related functionalities confirmed by the Mass spectra. f) Mass spectra of the water signal and g) ATR spectra for different samples.