Impact of Various Acids and Bases on the Voltammetric Response of Platinum Group Metal Oxides

The voltammetric response of platinummetal oxides is discussed with respect to novel pH sensors combining bothminiaturization and stability. For practical applications in solutions of any kind, for example, in tap water and in domestic sewage, various interferences must be considered, such as chloride and reducing agents. This work clarifies the voltammetric behavior of RuO 2 electrodes in solutions of different pH values and ionic strengths.

Platinum metal oxides, in contact with aqueous solutions, are able to adsorption water, whereby a carpet of hydroxyl (OH) groups is formed by proton displacement during anodic charging.Two sorts of hydrogen groups exist: OH with bridging oxygen atoms which acts as Lewis acid or base, and OH that works as a Brønsted base [10].Protons from the solution can be exchanged with the electrode surface by following simplified equation: Using Nernst's equation, the redox potential of the RuO 2 electrode depends on the pH value.By definition in the solid state, the activities of Ru(III) and Ru(IV) approach  = 1, so that (3) is received.(

𝐸 = 𝐸
At 25 ∘ C,  =  0 − 0.059 ⋅ pH. ( The thermodynamically calculated standard potential equals  0 = 0.94 V [11].According to the above equations, RuO 2 electrodes can be utilized for a potentiometric sensor.Usually, buffer solutions are used to investigate the pH sensitivity, whereas the impact of ionic strength has scarcely been considered in the literature so far.At present, the redox behavior of RuO 2 in aqueous solutions is not fully understood.We present new findings on the impact of various acids and bases on the performance of RuO 2 electrodes by the help of cyclic voltammetry (CV), open circuit potential (OCP) measurements, and electrochemical impedance spectroscopy.As well, the hydrogen insertion reaction into the lattice structure of RuO 2 electrodes is considered.

Instruments and Methods
Electrodes were prepared by thermal spray pyrolysis from commercially available ruthenium(III) chloride hydrate (Sigma-Aldrich).The substrate, titanium foil (Ti) of 0.05 mm thickness (Ankuro Int.GmbH), was pretreated with abrasive paper and degreased with acetone to ensure good adhesion of the oxide layer.The RuCl 3 ⋅ x H 2 O was solved in acetone and then decomposed on the substrate in a furnace in air at 500 ∘ C for 2 h.Our previous investigations by thermogravimetric analysis revealed that the transition temperature of oxide formation is at about 360 ∘ C. The active area of the sensor was 1 cm 2 , and the thickness of the oxide layer was about 2 m.The electrochemical measurements were carried out using a three-electrode arrangement, a potentiostat/galvanostat (EC301 Stanford Research System, Inc.), and a frequency response analyzer (Solartron SI 1250).The reference electrode (RE) was a reversible hydrogen electrode (RHE), the counterelectrode (CE) was a platinum-foil, and the working electrode (WE) was the above-mentioned RuO 2 electrode.As electrolytes, 70 mL of 1-molar solutions of sulfuric acid (H 2 SO 4 ), hydrochloric acid (HCl), phosphoric acid (H 3 PO 4 ), nitric acid (HNO 3 ), sodium hydroxide (NaOH), and potassium hydroxide (KOH) were used.The temperature was 25 ± 0.5 ∘ C.

Results and Discussion
3.1.Open Circuit Potential in Different Solutions.The known problems of RuO 2 electrodes in aqueous solutions are (i) potential drift and (ii) hysteresis.With respect to the pH sensitivity, the OCP of a RuO 2 electrode were observed in 1molar acids and bases for a period of several days (see Table 1).
The titanium support plays no role, as far as RuO 2 is coated sufficiently thick.The quality of the electrode was proved by the help of microscopy and impedance spectroscopy.Any weakly conducting layer of TiO 2 on the substrate material, which might be formed by corrosion under an insufficient RuO 2 coating, causes a dramatic increase of the resistance of the electrode within several days.With respect to quality assurance, any undesired peeling off of the RuO 2 coating can easily be checked under a magnifying glass.
In H 2 SO 4 , the RuO 2 electrode provided a stable potential of about 920 mV RHE, which is in good agreement with the thermodynamically calculated standard potential (940 mV [11], (1)).The theoretical pH values in Table 1 were calculated using activity coefficients  from the literature [12], pH = − log(), for  = 1 mol/L.
Unfortunately, the potentials in HCl, HNO 3 , and alkali hydroxides vary dramatically depending on the ionic composition of the solution.According to the p a values, HCl behaves as an extremely strong acid, followed by H 2 SO 4 and HNO 3 , whereas H 3 PO 4 appears moderately strong.In contrast to the expectation that the strongest acid should have the highest potential, Table 1 reveals no clear trend, because the solution is determined not only by the protons, but also by the total ionic strength of all ions present.For example, RuO 2 shows a significant sensitivity to chloride.Therefore, any empirical correlation between acidity (p a value) and the OCP remains vague, as shown in Figure 1.
Chloride ions play a special role, as they interact with the RuO 2 lattice, so that they change the open circuit potential as well as the slope dE/dpH and the cyclic voltammogram (Figure 3).Since chloride and chlorine compounds are found in municipal wastewater, the conventional potentiometric pH measurements using metal oxides are difficult.

Cyclic Voltammetry in Acidic Solutions
. With respect to clarifying the electrochemical behavior of RuO 2 in different acids, cyclic voltammetry was applied.The investigation was to show changes in the voltammograms depending on the pH value.Figure 2 presents the fundamental difference between the cyclic voltammogram of a RuO 2 electrode in H 2 SO 4 in contrast to NaOH.
In H 2 SO 4 (pH 0.1), 14 different peaks can be distinguished besides the hydrogen and oxygen evolution reaction in the potential range between 0 and 1.4 V RHE.
At  ≈ 0V RHE the hydrogen adsorption reaction occurs on the onefold coordinatively unsaturated ruthenium site (in short: 1f-cus Ru) and the bridging oxygen atom.Above 0.4 V RHE the oxidation of the surface takes place.In acidic solutions the redox transitions of Ru(III) → Ru(IV) → Ru(V) → ⋅ ⋅ ⋅ → Ru(VIII) appear to be the predominant oxidation states: Interestingly, different acids produce clearly different voltammograms (Figure 3).
Oxygen Reduction.The shapes of the anodic cycles are quite similar in different acids.However, the cathodic ramp is substantially altered by the presence of chloride in HCl.In HCl, the oxygen evolution reaction occurs at a lower potential than in the other acids.The oxygen evolution at first appears in HCl and then shifts to HNO 3 , H 2 SO 4 , and finally H 3 PO 4 , whereby the electrolyte resistance plays a minor role.These low overpotentials are known for the chlorine evolution reaction in the chloralkali electrolysis with dimensionally stable anodes (TiO 2 /RuO 2 , DSA).According to the Deacon process, traces of chlorine might be formed by reaction with anodically generated oxygen.
The oxygen reduction peak at about 1.1 V RHE was attributed to Ru(VIII), especially to the species RuO 4 in the measured solution, based on in situ reflectance spectroscopy in 0.5 M H 2 SO 4 by Kötz et al. [13].However, RuO 4 is formed at high positive potentials during oxygen evolution and appears to be the main corrosion product in acidic solutions (see (7)).The formation of RuO 4 in HCl is attributed to the shifted potential and therefore earlier oxygen evolution reaction.
Hydrogen Adsorption.By extending the potential window to more negative values, the hydrogen adsorption region appears more clearly and the peak heights of the anodic hydrogen oxidation increase.Starting the voltammogram at −0.1 V RHE, the largest current flows in H 2 SO 4 .Surprisingly, H 3 PO 4 follows, comparable to HCl, whereas HNO 3 reveals the weakest current (due to the smallest hydrogen evolution current).
We conclude that hydrogen oxidation current depends on the extent of the hydrogen evolution, which can be controlled by the potential window.Assuming that the potential window of 1.53 V leads to distinct peaks in 1 M H 2 SO 4 , the required potential difference was not reached in 1 M HNO 3 (about 1.48 V).About 1.4 V in 1 M HCl is enough to show a pronounced voltammogram of ruthenium oxide.
The oxygen reduction reaction seems to be unaffected by the cathodically altered potential window.

Cyclic Voltammetry in Alkaline Solutions.
In alkaline solution, the predominant oxidation states read Ru(III) → Ru(IV) → Ru(VI) → Ru(VII).At high potentials in the region of oxygen evolution, Ru(VIII) is generated and can be reduced in the solution at lower potentials.The redox reactions can be expressed as The cyclic voltammograms of RuO 2 in NaOH and KOH are shown in Figure 4.
Using a potential window of 0 to 1.5 V, the hydrogen oxidation region at  ≈ 0 V is not pronounced.The shape of the voltammograms remains the same.However, there are differences in the peak heights, that is, for the Ru(VI)/Ru(VII) couple at 1.3 V.If the potential window is extended to more negative potentials, the hydrogen oxidation region becomes more intense, and peak height of the Ru(III) → Ru(IV) oxidation at 0.6 V changes too.Note that in acids, this peak was not changed (or only in a negligible extent).

Impedance Spectroscopy in Acidic and Alkaline Solutions.
The RuO 2 electrode was characterized in different acidic and alkaline solutions by impedance spectroscopy in a frequency range from 65 kHz to 10 Hz.The electrochemical activity of the oxide−electrolyte interface is evaluated by the help of capacitance [14].
Figure 5 shows that the RuO 2 electrode exhibits the largest interface activity in H 2 SO 4 (1450 F/cm 2 at 10 Hz), whereas the capacitance in H 3 PO 4 is the smallest (650 F/cm 2 at 10 Hz).HCl, HNO 3 , and NaOH range is around 1250 F/cm 2 .
Interface capacitance allows distinguishing strong and weak acids.Although the p a of HCl is the highest, the capacitance is less pronounced than that of H 2 SO 4 .As seen with the cyclic voltammograms, the chloride ions compete with International Journal of Electrochemistry hydroxide sites in the RuO 2 lattice.Other ions present in the acids do not affect the behavior of the electrode, that is, sulfate, phosphate, and nitrate.The Nernst slope, which generally deviates from 59 mV pH −1 (25 ∘ C), is nearly independent of dissolved anions in the solution (such as SO 4 2− , PO 4 2− , and NO 3 − ).However, commercial RuO 2 resistive pastes, which contain PbO, exhibit a slope which depends on different anions significantly [15].

Proton Insertion in Ruthenium Dioxide.
As shown in Figure 3, the extension of the cathodic potential window leads to a significant increase in hydrogen oxidation current at  ≈ 0 V.This is consistent with the fact that RuO 2 forms a 0 0.2 0.4 0.6 0.8 quasi-hydrogen electrode, once a negative potential has been applied.The formally adsorbed hydrogen (even in the form of hydroxide species) is anodically oxidized according to the reversed equation (1).Cyclic voltammograms in the same solution with different starting potentials are shown in Figure 6.
A more cathodic electrode potential (in the hydrogen evolution direction) leads to (i) larger hydrogen oxidation currents and (ii) a shift of hydrogen oxidation towards more positive potentials.The impact of hydrogen on the behavior of a ruthenium oxide electrode was already mentioned in 1974 by Galizzioli et al. [11].Continuous hydrogen evolution leads to the collapse of the atomic layers under the surface, but there is no lasting effect.
Michell et al. reported that hydrogen can adsorb in the lattice of the ruthenium oxide.This protonation of the oxide lattice leads to the formation of a large porous anodic film, which might break Ru-O bonds [16][17][18].Today, we imagine proton insertion in RuO 2 by the dissociative adsorption of water [9] and bridged OH groups [10].
(1) In acid solution, the electrode potential increases with rising pH, in the same way that is known of the hydrogen oxidation at a hydrogen electrode.Protons are released by the dissociative adsorption of water and superacid OH groups.Simply, by the help of rutile lattice sites [Ru], the potential determining surface process at the more negatively charged RuO 2 electrode reads It is unclear whether Ru(II) exists during the anodic oxidation of hydrogen near 0 V according to (12). Figure 6 shows that the larger the potential is displaced in the negative direction, the higher the reversible oxidation and reduction currents of the Ru(III)/Ru(IV) couple at 0.5 V are.
(2) In alkaline solution, the electrode potential decreases with rising pH, in the same way that is known of the oxygen reduction at an oxygen electrode.By the dissociative adsorption of water, hydroxide sites are formed and bound in ruthenium cluster ions.

[Ru
If the potential is driven far in the positive direction, the oxygen reduction at 1.4 V and the Ru(IV) → Ru(III) reduction at 0.4 V get stronger.On the other hand, further oxidation states such as Ru(IV)/Ru(V) seem to be unaffected by the hydrogen insertion.

Diffusion Control.
The fact that the hydrogen adsorption is a diffusion controlled reaction can be seen in Figure 7.The hydrogen region is clearly affected by stirring the solution, because the exchange of hydrogen ions between RuO 2 surface and solution is disturbed.This affects to a certain extent the oxidation of the electrode at 0.8 V, because H + is involved in the Ru(III)/Ru(IV) redox reaction.

Conclusion
This work compiles the impact of different acids and hydroxide solutions on the ruthenium dioxide electrode.With International Journal of Electrochemistry respect to pH measurements, the determination of the OCP appears to be problematic in many respects, because the electrode potential varies in time and can hardly be reproduced due to hysteresis and aging effects.
Depending on the ionic strength, solutions even having the same pH and the same concentration cause different electrode potentials.Chloride ions affect strongly the behavior of the electrode.The cyclic voltammograms in acid solutions reveal alterations of the oxidation and reduction peaks due to interfering ions.In alkaline solution, the electrode seems to be affected by the potential window only.
We also demonstrated the hydrogen insertion into the bulk of the RuO 2 , forming [Ru]-OH, which plays a role in the pH-dependent Ru(III)/Ru(IV) redox couple, whereas higher oxidation states remain unaffected.
This study shows that further work has to be done to understand in detail the impact of interfering ions on the pH response of a RuO 2 electrode.

Figure 1 :
Figure 1: Fuzzy correlation of acid strength p a and open circuit potential of a RuO 2 electrode in different acids and bases.

Figure 3 :Figure 4 :
Figure 3: (a) Cyclic voltammograms of acids at 25 ∘ C. Impact of an extended potential window: solid lines: 0 to 1.5 V; dashed lines: −0.1 to 1.5 V RHE.Scan rate: 100 mV/s.(b) Full scale to show the oxygen reduction (RuO 4 ) and the impact of chloride at 1.1 V RHE.

Figure 5 :
Figure 5: Frequency response of capacitance of a Ti/RuO 2 electrode (versus RHE) in different solutions at 25 ∘ C.

Figure 6 :
Figure 6: Increasing oxidation of adsorbed hydrogen (by hydrogen insertion into bulk RuO 2 ) in a more and more cathodic potential window (hydrogen evolution) and more positive potentials (oxygen evolution).Picture inside: Ru(IV) → Ru(III) reduction at 0.4 V.

Table 1 :
[12]t of the OCP (E versus RHE) of a RuO 2 electrode in various acids and bases at 25 ∘ C in the course of several days.For comparison: negative decadic logarithm of the dissociation constants of acids (p a ) and bases (p b ) from[12].Theoretical potential of the reversible hydrogen electrode: V RHE = 0.059 pH (25 ∘ C).