Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry

Abstract

Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long-term functionality of electrodes. In-depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO₂ into CO₃ ^2- . We report the adaptation of a DEMS system for online CO₂ detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO₃ ^2- as CO₂ in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon-supported nickel boride (Ni_x B/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution.

Keywords: DEMS; carbon corrosion; cell design; differential electrochemical mass spectrometry; electrocatalysis.

Scheme 1

Schematic of carbon oxidation during OER in alkaline electrolytes. The current is the sum of i _OER and i _C,Ox and it is supposed that a highly active OER catalyst is protecting the carbon support against corrosion (a). Concept of CO₂ detection as a marker for electrochemical carbon corrosion in alkaline electrolytes, for example during the OER. The formed CO₂ is converted into CO₃ ²⁻ which is again liberated as CO₂ by injecting an acid and in turn collected through a Teflon membrane at the inlet of the MS (b).

Figure 1

Current density response during potentiostatic polarization of a graphite rod electrode at increasing potentials (first row) in 0.1 m KOH, and the corresponding ion current for O₂ and CO₂, without acidification of the electrolyte (second and third row), and after acidification of the electrolyte upon injection of 0.15 m H₂SO₄ (fourth row). The i _ion signal for O₂ was recorded without acidification.

Figure 2

a) Chronopotentiometric measurements on graphite electrodes at an applied current density of 5.5 mA cm⁻² in electrolytes with pH 1 and 13 (flow rate 270 μL min⁻¹). b) The Electrolytes were acidified by 0.15 m H₂SO₄ (flow rate 270 μL min⁻¹) in front of the DEMS membrane inlet in order to release the primarily formed CO₂ present as carbonate (pH 13), and corresponding detected CO₂ ion charge.

Figure 3

Chronopotentiometric measurements (thick lines) and ion currents (normalized by the mass of Vulcan on the electrode; lines) for CO₂ of electrodes modified with Vulcan and Ni_xB/C‐10 at applied current densities of 4.4 mA cm⁻² (a), 8.8 mA cm⁻² (b), 13.3 mA cm⁻² (c), and 17.7 mA cm⁻² (d) measured in 0.1 m KOH (flow rate 270 μL min⁻¹). The electrolyte was acidified by 0.15 m H₂SO₄ (flow rate 270 μL min⁻¹) in front of the DEMS membrane inlet.