# Monitoring Electrochemical Reactions in Situ with Low Field NMR: A Mini-Review
## 4 TD NMR for the Study of Magnetoelectrolysis  
In 2014, [PERSON] et al. [8] used a similar system to that used by [PERSON] et al. in 2012 and observed that the reaction rate of the Cu\({}^{2+}\) electrodeposition reaction monitored in situ by TD NMR was faster than the same reaction performed ex situ (Figure 5). The results showed that after 1 h of reaction 40% of the copper ions had been deposited on the electrode surface when the reaction was performed in situ, which is a 48% increase in copper deposition when compared to the 27% that was deposited when the reaction was performed ex situ (Figure 5). The increase in the reaction rate was associated with the magnetoelectrolysis effect, which is known to stir the solution, and increases the quantity of ions that reach the electrode surface. The magnetoelectrolysis effect is proportional to both the magnetic field intensity, **B**, and the current density, **j**, that reaches the electrode surface. The most important force that results from the interaction of these quantities is the magnetic field force density, **F\({}_{\textbf{B}}\)**, which is given by Equation (2). \[\textbf{F}_{\textbf{B}}=\textbf{j}\times\textbf{B} \tag{2}\]  
where **j** has units of A m\({}^{-2}\), **B** is given in T and **F\({}_{\textbf{B}}\)** has units N m\({}^{-3}\). From Equation (2) we can see that **F\({}_{\textbf{B}}\)** will be maximum when **j** and **B** are perpendicular to each other and null when they are parallel. In practice, however, even if at a macroscopic level **j** and **B** are parallel, the electrode surface as well as  
Figure 4: (**A**) Experimental setup using an inductively coupled coil to enhance the NMR signal-to-noise ratio. (**B**) Cupric ion concentration over time. Reactions performed at -1 and -4 V (vs. Ag/AgCl) are represented by black squares and red circles, respectively. Adapted from [26], copyright (2018), with permission from Elsevier. Figure 5: (**A**) Illustration of the electrochemical cell inserted in the NMR relaxometer. (**B**) Variation of Cu\({}^{2+}\) concentration as a function of electrolysis time performed in the electrochemical cell recorded at 23 °C under different configurations: (\(\blacksquare\)) B\(\downarrow\), (\(\bullet\)) **B\(\parallel\)j**, and (\(\blacktriangle\)) **B** = 0. Solution: 0.1 mol:L\({}^{-1}\) Na\({}_{2}\)SO\({}_{4}\) electrolyte containing 0.01 mol:L\({}^{-1}\) CuSO\({}_{4}\). Conditions: E\({}_{applied}=-0.4\) V vs. Ag/AgCl (3 mol:L\({}^{-1}\) KCl). The Cu\({}^{2+}\) concentration data were determined by CPMG pulse sequence. The standard error is represented by the error bars (\(n=3\)). Adapted with permission from [8]. Copyright (2014) American Chemical Society. its edges create local distortions to the electric field which results in there being regions where **j** and **B** are no longer parallel and thus **F\({}_{\textbf{B}}\)** will be minimum, but not zero, as explained [PERSON] et al. in 2015 [13], who call this particular effect micro-magnetohydrodynamic effect. [PERSON] et al. (2014) [8] also discuss the reason for why the reaction rate remains unaltered despite an apparent parallel orientation between **B** and **j** (Figure 5). They explain this result in terms of the effect of the edges of the electrode and gas bubbles that form on its surface during the reaction, which locally alter the electric field, therefore preventing **F\({}_{\textbf{B}}\)** from being zero. [PERSON] et al. in 2001 [12] explained that the increase of an electrochemical reaction's velocity due to **F\({}_{\textbf{B}}\)** is due to the fact that this force agitates the solution, which leads to more molecules reaching the surface of the working electrode and, therefore, reducing the thickness of the electric double-layer (Figure 6). They also explained that this increase only occurs in those reactions which are limited by mass transport, since the magnetic agitation caused by **F\({}_{\textbf{B}}\)** may be compared to a mechanical agitation, which homogenizes the solution. [PERSON] et al.