In Situ Determination of pH at Nanostructured Carbon Electrodes Using IR Spectroscopy

Lolade Bamgbelu,Katherine B Holt

doi:10.3390/ma12244044

Abstract

Changes in pH at electrode surfaces can occur when redox reactions involving the production or consumption of protons take place. Many redox reactions of biological or analytical importance are proton-coupled, resulting in localized interfacial pH changes as the reaction proceeds. Other important electrochemical reactions, such as hydrogen and oxygen evolution reactions, can likewise result in pH changes near the electrode. However, it is very difficult to measure pH changes located within around 100 µm of the electrode surface. This paper describes the use of in situ attenuated total reflectance (ATR) infrared (IR) spectroscopy to determine the pH of different solutions directly at the electrode interface, while a potential is applied. Changes in the distinctive IR bands of solution phosphate species are used as an indicator of pH change, given that the protonation state of the phosphate ions is pH-dependent. We found that the pH at the surface of an electrode modified with carbon nanotubes can increase from 4.5 to 11 during the hydrogen evolution reaction, even in buffered solutions. The local pH change accompanying the hydroquinone–quinone redox reaction is also determined.

Highlights

Carbon nanostructured electrodes are ubiquitous in electrochemistry, used as electrocatalyst supports [1], as energy storage electrodes with a large surface area, found in supercapacitors [2], as redox flow batteries [3], and in sensing and electroanalysis [4,5,6]
Using a more versatile attenuated total reflectance (ATR) IR spectroelectrochemical cell arrangement, with a working electrode located directly above a non-conducting ATR prism, we have previously shown pH changes at the surface of an electrode modified with iron sulfide under conditions of hydrogen evolution and reduction of carbon dioxide [22]
The aim of this work was to demonstrate the applicability of in situ IR spectroscopy to measure changes in pH in electrode surfaces modified with carbon nanomaterial while a potential is applied

Summary

Introduction

Carbon nanostructured electrodes are ubiquitous in electrochemistry, used as electrocatalyst supports [1], as energy storage electrodes with a large surface area, found in supercapacitors [2], as redox flow batteries [3], and in sensing and electroanalysis [4,5,6]. The biocompatible nature of some nanocarbons, such as carbon nanotubes, has led to their use in the modification of electrodes for biological sensing [7], the detection of neurotransmitters [8], the stimulation of nervous tissues [9], pathogen detection [10], and interfacing with enzymes [11]. Relatively high potentials are applied to carbon electrodes during in vivo neural stimulation, and these may be sufficient to oxidize or reduce water—reactions that liberate or consume protons and change the local pH. Such pH changes can perturb redox equilibria or even damage tissue or enzymes in the case of embedded electrodes or biosensors. This paper describes how in situ infrared (IR) spectroscopy can reveal significant pH changes at carbon electrode interfaces during operation

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