The Oxygen Evolution Reaction at Manganese oxide Films in Base: Kinetics & Mechanism

Various manganese oxide type Dimensionally Stable Anode (DSA) electrodes were prepared though a simple thermal decomposition technique at different annealing temperatures for their effect on the oxygen evolution reaction (OER). Along with the change in different annealing temperatures, the effects of different metal substrates were also explored. Cyclic Voltammetry was used to analyze the redox transitions of all annealing temperatures on the nickel and titanium substrates. Steady state polarization curves (also known as Tafel Plots) were used to determine tafel slopes, Open Circuit Potential decay analysis was also used to determine tafel slopes. Reaction orders of the 350°C nickel MnxOy electrode were determined and calculated as a half reaction order. A mechanism for this electrode is also proposed. SEM-EDX and FTIR analysis are also reported on some MnxOy films and powders. Introduction Currently the interest in developing an alternative to fossil fuels has increased dramatically in recent years due to the imminent threat of the depletion of the earth’s coal, oil and gas reserves. As a consequence, the costs of these fossil fuels have also increased. Another disadvantage associated with fossil fuels is the issue of carbon based emissions from the fossil fuels when burnt into the earth’s atmosphere which leads to global warming. Many renewable alternatives have already been developed as fossil fuel alternatives e.g. wind power, solar power and hydropower. However all these alternatives may not suit people in different parts of the planet due varied geological/weather conditions being unreliable. Alkaline water electrolysis has been proposed as one of the solutions to this problem; the equations for this process are shown below; Anode: (OER) 40H 2H2O + O2(g) + 4e E° = + 0.40 V Cathode: (HER) 4H20 + 4e 2H2(g) + 4OH E° = 0.83 V Overall: 2H2O 2H2(g) + O2(g) E°= -1.23 V [1] [2] [3] 10.1149/05329.0059ecst ©The Electrochemical Society ECS Transactions, 53 (29) 59-77 (2013) 59 ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 134.226.254.162 Downloaded on 2013-10-29 to IP The cathodic process produces large amounts of hydrogen gas which can then be used in vital applications such as energy conversion and energy storage devices. In contrast to the use of fossil fuels, the production of hydrogen, as an energy source, from alkaline water electrolysis offers an environmentally inoffensive and reliable route to the production of the large volumes of hydrogen gas required by a possible hydrogen economy (1-3). Of course during water electrolysis oxygen is also produced at the anode, according to equation 1 above, the production of oxygen is the main energy consuming step in the reaction. In practice, the efficiency of water electrolysis is limited by the large anodic over-potential of the oxygen evolution reaction (OER) (3). Thus, a vast amount of research is being dedicated into fully understanding the kinetics and mechanism of the OER reaction. This knowledge should lead to the development of an OER anode material that produces oxygen at the lowest overpotential. Currently there are various electrodes being studied for the oxygen evolution reaction, these include thermally prepared DSA type oxide electrodes, electrochemically prepared hydrous oxide electrodes, and electrodeposited bulk oxide/hydroxide electrodes using various metal oxides (4). In this work DSA type electrodes are examined. DSA electrodes were first used by the chlori-alkai industry. These electrodes consist of a titanium support which was then coated with a mixed metal oxide (5). The optimum oxide materials for the preparation of these electrodes are RuO2 and IrO2 due to their ability to produce oxygen at the lowest overpotential compared to other metal oxides. However these materials are highly expensive and do not exhibit long term stability in alkaline water electrolysis conditions (5). Much interest into the use of the first row transition metals (Mn, Fe, Ni and Co) have been reported for OER anode material due to their good electrochemical performance and their long tern stability in alkaline media (4). In recent years manganese oxide has been studied as an alternative anodic material for alkaline water electrolysis due to its relative low cost, eco-friendly properties and long term corrosion resistance in alkaline solution compared to RuO2 and IrO2 (6). Manganese oxides electrodes can be produced/synthesized in various ways such as Electrodeposition (7), Chemical precipitation (8), Sol-gel (9) and by Thermal Decomposition (10). In this communication, the behaviour of thermally decomposition prepared DSA type manganese oxide electrode as an OER anode material will be reported. Experimental Details Preparation of electrodes The working electrodes in this work were prepared via a thermal decomposition method outlined by Trasatti et al. (9) To prepare the electrodes titanium (as supplied by Alfa AesarJohnson Matthey company, purity 99.99% (metals basis), with a diameter of 1.0mm) or nickel wire (as supplied by Alfa AesarJohnson Matthey company, purity 99.99% (metals basis)) were sealed in glass. The electrodes were then dipped in 5M H2SO4, polished with 1200 grit carbimet paper and washed with deionised water and set aside to dry. A 0.01M MnxOy precursor solution was made from dissolving Mn(NO3)2.H20 in a 10ml conical flask, which was weighed using a weighing balance (Sartorius, model 1872). The resulting mixture was evaporated on a hot plate (Jenway ECS Transactions, 53 (29) 59-77 (2013)