Normal hydrogen electrode electrochemical cells

For the reaction of hydrogen and oxygen to generate a current in a fuel cell, the anode needs to be polarized more positive than 0 V vs. NHE (Normal Hydrogen Electrode, the reference potential for all electrochemical reactions) for the oxidation of hydrogen, while the cathode needs to be polarized more negative than 1.229 V vs. NHE for the reduction of oxygen. 

Therefore, if a normal hydrogen electrode (NHE) is used as reference electrode, one can describe the reaction in the electrochemical cell as ... 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

The electrode potential of an electrode reaction at equilibrium can be measured as the electromotive force of an electrochemical cell composed of both the reaction electrode and the normed hydrogen electrode. The potential of the reaction electrode thus measured is taken as the equilibrium potential of the electrode reaction relative to the normal hydrogen electrode. 

Fig. 6-6. Electrochemical cell composed of an elecirode of redox electron transfer on the right hand side and a normal hydrogen electrode on the left hand side.

Figure 6-5 shows an electrochemical cell of the redox reaction involving electron transfer coupled with a normal hydrogen electrode reaction. The cell diagram and cell reaction can be written, respectively, in Eqns. 6-13 and 6-14 ... 

Each electrochemical couple exhibits a characteristic electrochemical potential and a characteristic Acell voltage C, defined above, depends on the couples combined in a cell and on their concentrations. In order to quantify the properties of electrochemical couples reference electrodes are used. The reference electrode which is primarily used is the standard hydrogen electrode (SHE), or normal hydrogen electrode (NHE). In this case it is an inert Pt electrode around which hydrogen is flushed (see Fig. 3.4). The reaction involved is given by... 

The Oxygen Evolution Reaction (OER). The OER is the primary electrochemical reaction in water electrolysis, metal electrowinning, and recharging of metal-air cells. The standard electrode potential for this reaction at 25°C is 1.299 V vs the normal hydrogen electrode (NHE) in acid media and 0.401 V in alkaline media. The pertinent... 

The equilibrium potential of an electrochemical reaction is defined as the potential of an electrode (with respect to the potential of a normal hydrogen electrode) when immersed in an electrolytic cell containing the reactive species, but without current flow. When a current is applied, the electrode potential is shifted. In the case of an anodic reaction ... 

These electrodes are electrochemical half cells providing a constant potential they are basically nonpolarizable, which means that their potential is independent from a current flow through the cell, or at least they follow Ohm s law. They contain a redox couple where both partners are present in well-defined concentrations (in fact, activities). Basically it is possible to realize a normal hydrogen electrode (NHE) as a potential reference with 0.000 V (per definition) but the practical efforts are enormous (proton activity 1.000 mol/L, hydrogen gas pressure 1.000 atm using a specially activated platinum electrode) additionally the potential is not very robust because small current flows or temperature changes may cause significant deviations already. 

Reference electrode An electrochemical half-cell with a stable and known potential. The most common is the saturated calomel electrode (SCE,-1-0.242 V vs NHE). The primary standard reference electrode is the normal hydrogen electrode (NHE, 0.0000 V by definition). 

All electrochemical measirrements were carried out in sulphuric acid (1 M) at 25°C. The voltammetric experiments were performed using an Autolab-PGSTAT20 potentiostat with a three-electrode cell and a saturated calomel electrode (SCE) as a reference electrode (+0.245 V vs. normal hydrogen electrode, NHE). The catalyst sample was deposited on a rotating disk electrode (EDT 101, Tacussel), used as working electrode. All details, fiom sample preparation to experimental conditions, are extensively described in reference [9]. 

The potential of this electrode is defined (Section 5.2) as the voltage of the cell Pt H2(l atm) H (a = 1) M M, where the left-hand electrode, = 0, is the normal hydrogen reference electrode (described in Section 5.6). In Chapter 6, we derive the Nemst equation on the basis of the electrochemical kinetics. Here we use a simplified approach and consider that Eq. (5.9) can be used to determine the potential E of the M/M electrode as a function of the activity of the products and reactants in the equilibrium equation (5.10). Since in reaction (5.10) there are two reactants, and e, and only one product of reaction, M, Eq. (5.9) yields... 

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