Mass Spectrometry for Electrochemists
Jul 12 2021
Author: Peter Hatton on behalf of Hiden Analytical
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Electrochemistry is concerned with the range of oxidation-reduction (redox) reaction that cause electrons to transfer between the molecules of reactants. It is the study of the relationship between electricity and an identifiable chemical change, which was first observed in 1780. The chemical aspect of this relationship was largely ignored until Michael Faraday established the fundamental laws of electrolysis, laying the groundwork for modern voltaic technology. Yet the chemical aspect of many redox reactions remains ambiguous, particularly with respect to the specific interfacial behaviour between electrodes and electrolytes.
Mass spectrometry was first highlighted as an essential tool for electrochemical gas analysis by Bruckenstein and Gadde in 1971. They explored the use of a porous electrode for real-time determination of volatile electrode reaction products using an electron impact ioniser. This electrode contacted the electrolytic solution on one side and the high-vacuum chamber of a mass spectrometer on the other, facilitating the rapid identification of products generated in a typical electrochemical reaction. This in-situ dissolved gas and off-gas analysis at the electrode surface provided a much better understanding of the reactions that occur in a galvanic cell or half-cell structure. Differential electrochemical mass spectrometry (DEMS) is a sophisticated tool that was engineered to improve upon this pioneering method. This technique has been improved and now offer quantitative insights into cell chemistry by integrating a mass spectrometer with a nanoporous gas diffusion electrode, enabling the acquisition of mass ion currents proportional to the electrode’s faradaic current. Hiden Analytical manufacture several different Mass Spectrometry systems specifically for measurement and investigation of a wide variety of these Electrochemical applications. Thereby being a powerful tool for the researchers to gain insight and expand knowledge.
This article includes a brief overview of how mass spectrometry works for electrochemical studies, and we describe the main applications where mass spectrometry is being used by electrochemists.
Differential Electrochemical Mass Spectrometry - How it works
Mass spectrometers separate ionised species as a function of their mass to charge ratio. The key to success for most mass spectrometry techniques is to present the species of interest as a gas or vapour at the optimum pressure for ionisation.
Operation depends on the conversion of gas molecules into charged particles, typically positive ions. This is achieved by electron bombardment from an electron beam generated from a hot filament wire. This process is known as electron impact ionisation. Ions are passed through the quadrupole mass filter and separated by mass /charge ratio before being detected by the detector, typically a Faraday cup and secondary electron multiplier dual detector.
The ion current detected is proportional to the partial pressure of the species measured.
The species of interest to electrochemists are dissolved within the electrolyte. The interface between the liquid electrolyte and the mass spectrometer in a ‘DEMS’ mass spectrometer is a nano porous membrane. The nano porous membrane allows the dissolved gas and vapour species generated in the electrolyte to permeate into the mass spectrometer ion source region, whilst maintaining a barrier to prevent the liquid entering.
High performance vacuum pumping maintains the high vacuum necessary for mass spectrometer operation.
OEMS - on-line electrochemical mass spectrometry:
In applications where a nano-porous membrane interface is not practical, off-gas from the electrolyte can be analysed by sampling through a capillary sampling tube. In off-gas experiments its critical to match the sample flow required by the capillary to the off-gas volume produced by the experiment. In some cases, this can be as low as 10 micro-litres per minute. Specialist sampling solutions for sampling at very low flow rates with good response time are offered as ultra-low flow capillary inlets that provide viscous flow sampling with fast response time.
Applications of mass spectrometry for electrochemists
The technique of carbon dioxide, CO2, reduction is used to produce fuels from CO2. This is carbon capture from CO2 that might otherwise go into the atmosphere and contribute to global warming. This technique of investigation derives from Redox reaction typical of an electrochemical half-cell. DEMS is an ideal technique for quantitative gas measurement due to the fast response, low detection limits and linearity, all in real time. Hiden instruments are noted for their stability and sensitivity. Scientists are looking to new materials such as graphene and carbon nanotubes to catalyse this reaction, leading to reduced energy requirements for the reaction and improved efficiency.
Water Splitting - Electrolysis
The technique of electrolysis uses electric current to drive a chemical reaction. A common method uses water electrolysis to produce hydrogen and oxygen. This is being investigated as a way of delivering small amounts of hydrogen for Fuel cells, either static or vehicular.
The graph below shows data for hydrogen (with other gases not shown for clarity). The changes in hydrogen evolution relate to the efficiency of the electrolysis. Hydrogen response and Potential is plotted versus Current and Time below.
Hydrogen is typically generated by atmospheric condensation which is carried out at a large plant for better efficiency. However, this transfers the production cost to distribution. Transporting by road vehicle is inefficient and costly as the containers need to be highly pressurised and therefore thick walled and heavy. Pumping hydrogen is also less efficient as it is an extremely light gas, reducing the effectiveness of pumps to move along pipelines. Distribution of sufficient quantities for practical use becomes costly. Using hydrogen as a fuel has strong environmental benefits, as the waste product is water vapour. Other fuels such as methanol (also considered for fuel cell use) produce a mixture of gas products, water vapour and carbon dioxide (a greenhouse gas). An electrolysis station can produce hydrogen on-site for manufacturing plant use, or deliver directly using a refuelling station to a fuel cell vehicle. This localised distribution of hydrogen fuel thereby reduces safety concerns and transportation costs of hydrogen. Research into electrolysis aims to improve energy efficiency and so that this hydrogen production is economically viable, offers a key advantage over the current inefficient hydrogen transportation.
Electro Catalyst Studies
DEMS can be used to determine the kinetics of various oxidation and reduction reactions. Understanding these factors and reaction is of great importance to design and improve electrocatalysts. Also, to measure their efficiency and response to various different feedstock sources.
Mass spectrometer systems can simplify the analysis of otherwise complex cracking patterns from multi-component gas and vapour mixtures via the method of soft ionisation. This method provides user-defined ionisation to facilitate mass analysis and species discrimination with identification
Potential and Hysteresis Behaviours
By following reaction intermediates and products, such as oxygen, O2, hydrogen, H2, and peroxides, H2O2, during cyclic voltammetry experiments, potential and hysteresis behaviours can be obtained.
Lithium-Ion Battery Development
Lithium-ion batteries are being widely used due to their lightweight, rechargeable and power density. These technological advantages were recognised when three researchers shared a Nobel Prise for Chemistry, for key developments in Lithium-Ion Battery Technology. Active research and development continue to further improve power density, reduce recharge degradation and control temperature (which can degrade performance).
A significant proportional of the lithium-ion batteries manufactured goes to the automotive sector. Electric and hybrid vehicles are helping to reduce global emissions, whilst still maintaining the transport infrastructure.
Fuel Cell Studies
Solid Oxide Fuel Cells operate at high temperature. To improve efficiency, catalytic compounds can be used to reduce operating temperature and improve safety aspects. The Hiden mass spectrometer can be used to follow gas compositions for reaction dynamics and can read in thermocouple temperatures to incorporate into the data display. This research was carried out by Georgia Tech, using the HPR-20 to look at the effect of temperature on reforming of iso-octane as a fuel source, as there is already an infrastructure for distribution.
The materials used for electrodes have a key effect on the performance of a battery. During battery usage, electrons should flow from the negative electrode (the anode) to the positive one (cathode) during discharge. For a rechargeable battery, they also need to flow in the reverse direction. Therefore, both the anode and cathode material properties influence the battery. The anode needs to readily lose electrons and cathode readily accept electrons. It was realised that Lithium being an element which readily loses electrons could be used for both electrodes to make a rechargeable battery.
Work done by the researchers Goodnough, Whittingham and Yoshino on electrode materials earned them the Nobel Prize for Chemistry in 2019. Their work on the development of Lithium Ions batteries showed that the effect of the electrode materials was critical to achieve safe and powerful batteries, enabling the many lightweight portable electronic devices in today’s market. Initially Lithium metal was used, but it is reactive to air and could be explosive in the device. The Lithium element was replaced by Lithium ions, supported within a metal sulphide structure, so that the batteries were no longer explosive. By changing the sulphide material to a metal oxide, the capacity was doubled. The anode material was changed to carbon-based, which stored the lithium ions within the structure (intercalated), avoiding reactions with the electrode materials. This gives the battery a longer lifetime, due to the stability during the discharge and recharge cycle. These developments in materials greatly improved efficiency of a product that is so ubiquitous, especially for portable electronics. However, further improvements in materials chemistry are possible and many researchers are looking at materials for the electrodes and electrolyte which transport the ions and electrons, which can deliver higher capacity, more stable hysteresis during discharging and recharging cycles.
Mass Spectrometry provides an insight into electrochemical reactions not available by other techniques. The specificity, dynamic range and speed of response provided by mass spectrometry opens a window of discovery into the current/voltage dynamics of the electro-chemical world.
As the world moves to use more battery and hydrogen power in the future, the research from the applications discussed above will help provide efficient, reliable and affordable solutions.
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