1.5.3 Rotating Ring-disk Electrode (RRDE)
In the RRDE method, the O2 reduction reaction occurring on the disk electrode produces intermediates, which can be detected on the ring and are used to deduce the ORR mechanism. An example is using RRDE to study the O2 reduction reaction catalyzed by Pt/C catalysts with different Pt particle sizes. On the disk, 2-electron and 4-electron ORR can occur, and on the ring electrode, H2O2 is further oxidized to H2O.[149, 150]
The 2-electron reduction current (I2e-) is given by I2e-= IR/N
Where I2e- is the 2-electron ORR on the disk electrode and N is the collecting coefficient number. The ORR current (ID) on the disk electrode can be expressed with the equation:
ID= I2e-+ I
4e-where I4e- is the 4-electron ORR current. The following equation is used to obtain the average electron number (ne-) that involves in ORR:
ID ⁄ ne-=I4e- ⁄4+I2e- ⁄ 2
By rearranging this equation, we receive the following equation to calculate ne-: ne-=4ID/(ID+IR/N)
Chapter 1. Introduction
catalyst, the onset potential is normally the first to be measured. However, it is difficult to obtain an exact value of the onset potential since the currents at this point are naturally very low. Therefore, for OER and HER, the value of the potential at 10 mA cm-2 (Ej=10) is considered more reliable and commonly used for comparison.
The overpotential is mainly used in OER and HER and indicates the difference between the potentials observed for a certain current density and 0 V vs. RHE for HER, and 1.23 V vs. RHE for OER. Usually, the overpotential is measured in mV. As example, if an OER catalyst achieves Ej=10 = 1.43 V, an overpotential of 200 mV at 10 mA cm-2, while for a HER catalysts with Ej=10 = -0.15 V, an overpotential of 150 mV at 10 mA cm-2 is derived. Generally, a catalyst with an overpotential in the range of 300-400 mV can be considered already as excellent catalyst for OER and indeed there are just very few catalysts that have overpotentials less than 300 mV. A good HER catalyst usually has an overpotential in the range of 100-200 mV, while very few catalysts can reach the overpotentials within 100 mV.[5]
1.6.2. Half-wave potential and limiting current density
The half-wave potential is another important parameter for evaluating the ORR activity of a catalyst, since ORR is not only determined by the activity of the catalysts but also by the oxygen diffusion efficiency on the surface of the catalysts. The half-wave potential is defined as the point where the reaction is equally controlled by each part, which can mostly reveal the working efficiency of the catalyst. The limiting current density is the current which is fully determined by the diffusion speed of oxygen on the surface of the catalysts and is mainly influenced by the porous structure and surface properties of the catalyst. Generally, the limiting current density of a certain catalyst can be improved by increasing the surface area of the catalyst. A suitable ORR catalyst should usually show a limiting current density between 5-6 mA cm-2.[123]
1.6.3. Tafel slope
The Tafel analysis is usually employed to understand the reaction kinetics and mechanism, which is needed to compare the catalytic activities of different catalysts.
The Tafel slope also helps to define the rate determining step by examining the sensitivity of the current response to the given voltage by using the followed equation.[125]
η = b ·log(j/j0)
where η denotes the overpotential, b represents the Tafel slope, j is the current density, and j0 is the exchange current density. The quality of ORR/OER/HER catalysts can be compared with these values as a desirable performance is expressed in a small Tafel slope and large current density.
1.6.4. Exchange current density
The exchange current density (j0) is an important kinetic parameter representing the electrochemical reaction rate at equilibrium. The exchange current density is the current in absence of net electrolysis and at zero overpotential. It can be thought of as a background current to which the net current observed at various overpotentials is normalized. The magnitude of j0 determines how rapidly the electrochemical reaction can occur. It reflects intrinsic rates of electron transfer between an analyte and the electrode. Such rates provide insights into the structure and bonding in the analyte and the catalysts.
1.6.5. Turnover frequency (TOF)
TOF can be calculated as TOF=(j×A)/(4×F×n), where j (mA cm-2) is the current density at a certain overpotential, A is the area of the working electrode, F is Faraday constant (96,500 C mol-1) and n is the number of moles of the catalyst or active
Chapter 1. Introduction
compounds in the catalyst. However, it is almost impossible to calculate an exact TOF value for a composite catalyst, since the activity of all the involved elements and structures are certainly not constant. Still, TOFs can be relevant and useful parameters for comparing rather similar catalytic materials, for example when comparing porous and non-porous catalysts of otherwise the same composition.
2 Two-Dimensional Microporous Carbons Prepared from Layered Organic-Inorganic Hybrids
This chapter was published in Advanced Materials with the title of “Two-Dimensional Porous Carbons Prepared from Layered Organic-Inorganic Hybrids and their Use as
Oxygen Reduction Electrocatalysts”
Shuang Li, Chong Cheng, Hai-Wei Liang, Xinliang Feng,* and Arne Thomas*
Adv. Mater. 2017, 29, 1700707 https://doi.org/10.1002/adma.201700707
It reprints the abstract and conclusion of the publication and gives a summary of the results. Detailed information can be found in the publication reprint in Chapter 7.1