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Observation of Kinetic Isotope Effect in Electrocatalysis with Fully Deuterated Ultrapure Electrolytes

Kinetic isotope effect (KIE) is a common physicochemical effect to elucidate complicated microscopic reaction mechanism in biological, chemical and physical systems. Especially, the exchange of hydrogen to deuterium is a standard approach to investigate kinetics and pathways of a wide spectrum of key reactions involving proton transfer. However, KIE in electrocatalysis is still challenge. One main reason is owing to the high sensitivity to impurities in electrochemical systems. Aiming to establish an appropriate approach to observe KIE in electrocatalysis, we investigated KIE in electrocatalysis by using fully deuterated ultrapure electrolytes. With these electrolytes, we studied oxygen reduction reaction with platinum catalyst, which is well-known to be sensitive to impurity, as the model systems. In conclusion, the electrode processes in these systems can be strongly influenced by a purity of a selected deuterated electrolyte, especially in case of alkaline conditions. Therefore a highly pure deuterated electrolyte is indispensable to study microscopic electrode processes of electrocatalysis by analyzing KIE. This work shows a key criterion and methods to observe a reliable KIE in electrocatalytic systems, and therefore, provides a general approach to investigate complicated multielectron- and multiproton-transfer processes using not only standard electrochemical technique but also surface sensitive spectrometry.

observe KIE in electrocatalysis, we investigated KIE in electrocatalysis by using fully deuterated ultrapure electrolytes. With these electrolytes, we studied oxygen reduction reaction with platinum catalyst, which is well-known to be sensitive to impurity, as the model systems. In conclusion, the electrode processes in these systems can be strongly influenced by a purity of a selected deuterated electrolyte, especially in case of alkaline conditions. Therefore a highly pure deuterated electrolyte is indispensable to study microscopic electrode processes of electrocatalysis by analyzing KIE. This work shows a key criterion and methods to observe a reliable KIE in electrocatalytic systems, and therefore, provides a general approach to investigate complicated multielectron-and multiproton-transfer processes using not only standard electrochemical technique but also surface sensitive spectrometry.

INTRODUCTION
Mutilelectron-, mutilproton-transfer process is the core principle in a wide spectrum of energy transforming systems, such as metabolism in life or fuel-cells in green vehicles. [1][2][3][4][5][6][7][8] Therefore acquiring the microscopic picture of a variety of highly complicated multistep electrochemical processes can advance both fundamental scientific knowledge and modern technology. One of the simplest model systems to study the microscopic mechanism of aforementioned processes is the electrocatalysis at electrode/electrolyte interfaces using noble metals, such as platinum or gold. 5,[9][10][11] However, understanding microscopic electrocatalytic process at these well-known model systems is still a one of the today's challenges, even though there is a wide spectrum of advanced experimental and/or computational approaches to investigate surface reactions so far. [12][13][14][15][16][17][18][19][20][21][22][23] From this point, kinetic isotope effect (KIE) can be a powerful tool to clarify the basic reaction mechanism of electrocatalysis. [24][25][26] A use of heavy water in order to substitute proton in the system to deuteron (H-D exchange) is a classical topic in electrochemistry, 27-34 and this method was applied to analyze complicated electrode processes, such as electrocatalytic reactions on carbon-based electrocatalysts. [35][36][37][38][39] These non-noble metal based catalysts show a promising properties, [40][41][42][43][44][45] therefore is expected that KIE can be used for a part of approaches to find out the alternatives to present Pt-based electrocatalysts. However, a general method to observe/analyze KIE in electrochemical systems based on deuterated water was established recently. 46,47 There are several reasons why it took such long time to reach to proper method to observe KIE in electrocatalysis.
One reason is that thermodynamics in heavy water system is quite different compared to ordinary systems, 32, 46, 47 therefore we need modification to analytical equations for electrochemical reactions. Another reaction is that electrochemical systems are quite 3 sensitive to impurity, [48][49][50][51] and this is one of main reasons for hindering to establish a procedure to ensure a reliable KIE measurement so far. Therefore a procedure shown here to provide the correct KIE observation in electrochemical system can give a huge impact.
In present study, we investigated an influence of impurities towards the KIE in oxygen reduction reaction (ORR) on a polycrystalline platinum (polyPt) electrode surface in acidic (0.05M H2SO4 and 0.05M D2SO4) and alkaline (0.1M KOH and 0.1M KOD) electrolytes. The particular focus lies in the identification of a method to observe correct KIE in electrochemical systems. Aiming to this goal, we prepared ultrapure deuterated water (denoted as Ultrapure) in order to perform the electrochemical experiments and compared with the results with the highly pure deuterated water, but not ultrapure grade, (denoted as Control). As the result, it was found that several key electrochemical features, such as electrochemically active surface area (ECSA), deuteron desorption isotherm, and Tafel slope, obtained in the Ultrapure systems are identical to the values obtained in the well-established ultrapure ordinary water systems. On the other hand, the different values and features are obtained in the Control deuterated electrolytes. This indicates that the Ultrapure-based systems can provide a sufficient clean condition to proceed a contamination-sensitive experiment. The kinetics of ORR electrode process were checked in order to confirm the impact of impurities towards the system by using a rotating ringdisk electrode technique (RRDE), and we found that the impurity can lead to kinetic values with a two or three order of magnitude difference. These different kinetic values can mislead to an unappropriated interpretation of KIE, and to incorrect microscopic picture of electrode processes. Therefore, a use of ultrapure heavy water is indispensable to obtain a reliable microscopic view of complicated multistep electrode processes.

Standard Electrochemical Methods
The chemicals and equipment are the same with the previous reports, 46 Resistance of electrochemical systems was measured prior to each experiment by using impedance measurements, and this value was used to correct iR-drop.  The RF of the electrode was measured in the ultrapure ordinary electrolytes, and applied this same electrode and RF to both in the deuterated Ultrapure and Control systems.
We are aware of that it is possible to obtain VRDeE scale from other reference electrode potentials, such as V vs Ag/AgCl or vs Hg/HgSO4. 55 However, a RDeE is recommended to use as a reference electrode instead of these typical rones. This is because the method to use a RHE for heavy water systems can be applied in a relatively limited acidic concentration range, and also the use of a RDeE is free from suffering from unknown effects, such as liquid junction potential, therefore it is more accurate approach than a use of other reference electrodes.

Analytical Methods for Kinetics of Electrode Processes
The electron transfer number (n) and D2O2 yield were determined by the following equations: where Id is disc current, Ir is ring current and N is a collection efficiency which was determined to be 0.39 in the systems.
where j0, k0, C0, η, and F are, an exchange current density, a rate constant for j0, oxygen concentration, overpotential, and Faraday constant, respectively. The C0 D /C0 H is known to be 1.101. 35 Due to the following equation, we can obtain j0 from a corresponding log jk-η diagram: The ORR kinetic currents on Pt electrode can be separated from diffusion limiting current by using a following equation since we can observe clear diffusion-limited currents: where j and jlim are experimentally obtained current with RRDE technique and diffusion limiting current, respectively.

Ion Adsorption/Desorption onto Pt Surface in Deuterated Acidic Solution
The V than that of Ultrapure, and also the decrease of QD-UPD in Control system. In addition to this, the observation of current at EDL region in Control is also a strong evidence of impurity ( Figure 1a).

ORR Electrode Processes of Pt Surface in Fully Deuterated Acidic Solution
The ORR kinetics in the acidic condition were investigated by using the polyPt electrode to check the effect of the different electrolytes toward the ORR activities. All electrochemical properties in the acidic condition are summarized in the Table 1. First of all, linear sweep voltammetry (LSV) with RRDE technique was applied to check the ORR kinetics, and surprisingly, we found that there is only a small effect of impurities ( Figure  2a). In addition to this result, the two systems show n = 3.98 for a potential range from 0.2 to 0.9 VRDeE (Figure 2b), suggesting the main product is heavy water. The fourelectron ORR pathway of polyPt in acidic condition is well agreed with previous reports using ordinary water, therefore, we conclude that the ORR electrode process undergoes property in the two deuterated electrolytes. 10 In order to acquire further insights of the electrode process, the log jk-η diagrams were analyzed ( Figure 3a). The Tafel relation is applied to analyze the diagram. This means that we hypothesized the existence of at least one linear relation between log jk and η in a certain range of potential. In this study, two Tafel  The TS D 1 in the Ultrapure was 68.6 mV/dec, which is almost identical to the TS H 1 in the ultrapure ordinary electrolyte obtained as 69.1 mV/dec ( As it is indicated by the above data, the KIE on ORR in the acidic condition is less influenced by impurities even though the clear differences were observed in CV and LSV ( Figure 1 and Figure 2a).

Ion Adsorption/Desorption onto Pt Surface in Deuterated Alkaline Solution
In Figure  Ultrapure, which is well-established to prepare a highly clean electrochemical system ( Figure S2).
Furthermore, we compared the isotherm corresponding to QD-UPD for the Ultrapure with the Control in order to check the surface conditions (Figure 4b). The isotherms differs in the two systems. In the Ultrapure system, QD-UPD is 220 μC cm -2 ECSA but 157 μC cm -2 ECSA in the Control system, even with the same polyPt electrode. This huge difference of QD-UPD between the Ultrapure and Control suggest that impurity gives a stronger effect to alkaline conditions that to acidic conditions, and this can be a reason for a larger anodic current of Control from 0.6 to 1.5 V than that of Ultrapure, and also the decrease of QD-UPD in Control system, which observations are same for the Control in the acidic condition.

ORR Electrode Processes on Pt Surface in Fully Deuterated Alkaline Solution
The ORR electrode processes in the alkaline condition were investigated to check the effect of the different electrolytes toward the ORR activities. All electrochemical properties in the alkaline conditions are summarized in the Table 2. First of all, LSV curve was shown to check the ORR kinetics. As the results, on the contrary to the acidic condition, we found that there are significant effects owing to different electrolytes. The LSV curve of the Control much negatively shifted compared to the Ultrapure (Figure 5a).
This shows that the catalytic activity of Pt was obviously altered because of the different electrolytes. In addition to these results, beside it was constantly shows n = 3.98 in the Ultrapure in a potential range from 0.2 to 0.9 VRDeE, in the Control, n shows three plateaus, i.e. n = below 3.95, 3.96, and 3.98 in the potential ranges from 0.7 to 0.9 VRDeE, 0.4 to 0.6 VRDeE, and < 0.4 VRDeE respectively (Figure 5b). The n value in the potential ranges from 0.4 to 0.9 VRDeE is lower than typical values reported in previous studies. 57 Furthermore, it is unusual to observe several plateaus in n value. These results indicate 13 that although the main product is heavy water, however a higher amount of deuteron peroxide was produced in case of the Control compared to the Ultrapure case. Therefore, the two-electron pathway as the side reaction was enhanced in the Control. In addition to this, the reaction path could be altered on changing electrode potential as evident by a change of n value on applied overpotential (Figure 5b).
The log jk-η diagrams were studied in order to acquire further insights of the electrode process ( Figure 6). In the alkaline conditions, only one Tafel line was observed in each condition. The TS D in the Ultrapure was 47.6 mV/dec, which is almost identical to the TS H in the ultrapure ordinary electrolyte obtained as 47.7 mV/dec ( However, this high K H/D value in the Control is not related to these non-trivial effects but because of impurity as indicated by the CV and isotherms ( Figure 5). As such, these results show that an ultrapure condition is indispensable for reliable measurements of KIE, especially in alkaline conditions.

CONCLUSION AND OUTLOOK
Aiming to establish a reliable approach to measure KIE in electrocatalysis, we have studied the ORR kinetics and the corresponding KIE by using different purity of fully deuterated acidic and alkaline electrolytes. The main conclusions are following: ( value is as same as the previous report. (2) The impurity mainly comes from the as-received D2O. We can solve this problem with a commercially available equipment to obtain ultrapure quality D2O. This is exactly same previous efforts to obtain high quality ordinary water for an ultraclean therefore a reliable (3) In order to prepare an equivalent level of purity in deuterated systems as clean as protonated systems with Milli-Q water, ultrapure deuterated electrolytes should be used.
With this ultrapure electrolyte, we can obtain reliable KIE in electrocatalysis combined with appropriate analytical equations and well-established approaches to prepare a highly clean electrochemical system.
All-embracing, this study shows that ultrapure deuterated electrolyte is a key to measure reliable KIE in multi-electron/-proton transfer electrode processes. The approach shown here can be applied to a wide spectrum of research on electrode/electrolyte interfaces combining with surface spectrometry or first-principle calculations in order to unveil complicated reactions at electrode/electrolyte interfaces.