Alonso Gamero-Quijano

Grégoire Herzog

Micheál D Scanlon
email: micheal.scanlon@ul.ie

Aqueous Surface Chemistry of Gold Mesh Electrodes in a Closed Bipolar Electrochemical Cell

Keywords: Bipolar electrochemistry, bipolar electrode, redox active surface functional groups, oxygen reduction reaction, hydrogen evolution reaction

The influence of the bipolar electrode on the voltammetry observed with a closed bipolar electrochemical cell (CBPEC) goes far beyond simply conducting electrons between the two electrolyte solutions. The surface of each pole of the bipolar electrode may contain redox active functional groups that generate misleading or interfering electrochemical responses. Herein, a 4-electrode CBPEC configuration was studied with the opposite poles of the bipolar electrode resting in separate aqueous and organic electrolyte solutions. Using gold mesh wire electrodes as the poles, we systematically investigated the many experimental variables that influence the observed voltammetry upon addition of a reductant (decamethylferrocene) to the organic phase. External bias of the driving electrodes forced electrons released by decamethylferrocene at the organic pole to flow along the bipolar electrode and reduce redox active surface functional groups at the aqueous pole, such as oxide or hydroxide groups, or carry out the oxygen reduction reaction (ORR) or hydrogen evolution reaction (HER). The 4-electrode CBPEC configuration diminishes capacitive currents, permitting observation of voltammetric signals from electron transfer processes related to surface functional groups at the aqueous pole at much lower scan rates than possible with working electrodes in conventional 3-electrode electrochemical cells. Surface modification, by oxidative or reductive electrochemical pretreatment, changes the potential window experienced by the aqueous pole in the 4-electrode CBPEC in terms of its position versus the standard hydrogen electrode (SHE) and dynamic range. In a related observation, the electrochemical responses from the surface functional groups on the aqueous pole completely disappear after oxidative pre-treatment, but remain after reductive pre-treatment. The flow of electrons from decamethylferrocene to the surface of the aqueous pole is limited in magnitude, by the decamethylferrocene concentration, and kinetically limited, due to decamethylferrocene diffusion to the organic pole, in comparison to the infinite supply of electrons delivered to the surface of a working electrode in a 3-electrode cell. This unique feature of the 4-electrode CBPEC facilitates a very gradual evolution of the surface chemistry at the aqueous pole, for example from fully oxidised after oxidative pretreatment to a more reduced state after repetitive cyclic voltammetry cycling. Perspective applications of this slow, controlled release of electrons to the electrode surface include spectroelectrochemical analysis of intermediate states for the reduction of metal salts to nanoparticles, or conversion of CO2 to reduced products at catalytic sites. The use of indium tin oxide (ITO) electrodes in CBPEC experiments for specific reactions is recommended to avoid misleading or interfering electrochemical responses from redox active functional groups prevalent on metallic surfaces. However, the electronic bridge to implement entirely depends on the reaction under study, as ITO also has drawbacks such as a lack of electrocatalytic activity and the requirement of an overpotential due to its semiconducting nature.

Introduction

Bipolar electrochemistry entails oxidation and reduction reactions proceeding simultaneously at spatially separated sites on a single electrode, the bipolar electrode [START_REF] Loget | Bipolar electrochemistry: from materials science to motion and beyond[END_REF]. The redox reactions on the bipolar electrode are driven by a potential gradient in solution, generated by an external bias of the driving electrodes. The bipolar electrode is electrically floating or "wireless" [START_REF] Mavré | Bipolar electrodes: A useful tool for concentration, separation, and detection of analytes in microelectrochemical systems[END_REF]. As the electric field varies in strength along the length of the bipolar electrode, so too does the magnitude of the interfacial potential difference between the bipolar electrode and solution (in parallel to the driving electric field) [START_REF] Duval | Bipolar electrode behaviour of the aluminium surface in a lateral electric field[END_REF]. This interfacial potential difference initiates the electrochemical reactions at the surface of the bipolar electrode. The charge is conserved on the bipolar electrode, and therefore the reduction current is equal to the oxidation current [START_REF] Koefoed | Bipolar electrochemistry-A wireless approach for electrode reactions[END_REF].

In a closed bipolar electrochemical cell (CBPEC), the two poles of the bipolar electrode rest in two separate electrolyte solutions [START_REF] Plana | The voltammetric response of bipolar cells: Mechanistic investigations of electroless deposition[END_REF][START_REF] Plana | The voltammetric response of bipolar cells: Reversible electron transfer[END_REF]. Each of these electrolytes have their own driving electrode. Thus, if a reduction reaction is to occur at the surface of the pole of the bipolar electrode in one electrolyte, and oxidation at the other, electroneutrality must be maintained in each electrolyte as electrons flow along the bipolar electrode. This is achieved by the opposite redox reactions occurring at the driving electrodes in each electrolyte [START_REF] Cox | Steady-state voltammetry of a microelectrode in a closed bipolar cell[END_REF][START_REF] Guerrette | Coupled electrochemical reactions at bipolar microelectrodes and nanoelectrodes[END_REF]. To optimise the potential applied to each driving electrode, in order to precisely tune the interfacial potential difference between the bipolar electrode and solution, reference electrodes may also be introduced into each electrolyte. This is known as a CBPEC in a 4-electrode configuration and may conceptually be compared to two back-to-back 3-electrode electrochemical cells connected by a conducting electronic bridge, i.e., the bipolar electrode [START_REF] Gamero-Quijano | Closed bipolar electrochemistry in a four-electrode configuration[END_REF]. The opposite poles of the bipolar electrode are effectively working electrodes in each electrolyte, with the driving electrodes operating as the counter electrodes.

The CBPEC configuration is highly versatile, with the experimental conditions experienced by each pole of the bipolar electrode often diverging widely, e.g., in terms of the nature of the solvent and solute, pH, temperature, exposure to light, etc. In particular, the CBPEC configuration has proved very popular for the development of novel (spectro)electroanalytical sensors [START_REF] Ibañez | Bipolar spectroelectrochemistry[END_REF][START_REF] Karimian | The principles of bipolar electrochemistry and its electroanalysis applications[END_REF]. The basic premise of such devices is that the signal generated at one pole (known as the reporting pole) is an accurate reflection of the processes occurring at, and the state of, the pole resting in the analyte (the sensing pole) [START_REF] Fosdick | Bipolar electrochemistry[END_REF]. The reporting signal has typically been the detection of an optical signal generated by the oxidation half-reaction (electrochemiluminescence [START_REF] Bouffier | Advances in bipolar electrochemiluminescence for the detection of biorelevant molecular targets[END_REF], electrochromism [START_REF] Xu | Electrochromic sensor for multiplex detection of metabolites enabled by closed bipolar electrode coupling[END_REF], and electrofluorescence [START_REF] Xing | Bipolar electrode based reversible fluorescence switch using Prussian Blue/Au nanoclusters nanocomposite film[END_REF]), with the sensing being through the reduction half-reaction.

Herein, we discuss the 4-electrode CBPEC with aqueous-organic electrolyte solutions [START_REF] Hotta | Electron-conductor separating oil-water (ECSOW) system: A new strategy for characterizing electrontransfer processes at the oil/water interface[END_REF][START_REF] Gründer | Solids at the liquid-liquid interface: Electrocatalysis with pre-formed nanoparticles[END_REF][START_REF] Uehara | Au Electrodeposition at the liquid-liquid interface: mechanistic aspects[END_REF], with one pole of the bipolar electrode immersed in various aqueous electrolytes and the other in the organic solvent -trifluorotoluene (TFT) containing a highly hydrophobic salt. Electron transfer between redox couples at opposite poles of a bipolar electrode, immersed in aqueous and organic electrolytes, respectively, is thermodynamically equivalent to the corresponding interfacial electron transfer event at the interface between two immiscible electrolyte solutions (ITIES) [START_REF] Hotta | Electron-conductor separating oil-water (ECSOW) system: A new strategy for characterizing electrontransfer processes at the oil/water interface[END_REF].

The interpretation of voltammetry for a 4-electrode CBPEC is non-trivial as the electrochemical features are determined by thermodynamic and kinetic factors that influence the redox processes at both poles of the bipolar electrode simultaneously [START_REF] Gamero-Quijano | Closed bipolar electrochemistry in a four-electrode configuration[END_REF]. The bipolar electrode is not simply an electron conductor between the two electrolytes, its surface may contain redox active functional groups that influence the voltammetryjust as the surface chemistry of any working electrode influences the voltammetry observed in a conventional 3electrode cell. In this regard, we provide a systematic understanding of the voltammetry of the 4-electrode CBPEC with aqueous-organic electrolyte solutions when a reductant, consisting of a ferrocene-derivative, is introduced to the organic phase and the bipolar electrode is metallic.

The number of factors which required careful consideration when evaluating the voltammetry of this seemingly simple system was extensive, such as the relative diffusion coefficients of species in the aqueous and organic electrolytes, the nature of the redox reaction (diffusioncontrolled vs. surface-active) in each electrolyte, the redox potential of the ferrocene derivative, the nature of the aqueous anions (protonated or non-protonated, strongly or weakly adsorbing), the electrochemical pre-treatment of the aqueous pole of the bipolar electrode (reductive vs.

oxidative pre-treatment), the presence of oxygen in the aqueous electrolyte, the nature of the bipolar electrode (metallic vs. non-metallic), the concentration of reductant in the organic phase, and the width of the potential window applied during the 4-electrode CBPEC experiment.

The experimental findings emphasise the ease with which misleading electrochemical signals can be generated in a 4-electrode CBPEC once an electron donor (or acceptor) species is introduced to either electrolyte, especially if the poles of the bipolar electrode have redox active surface functional groups. The systematic nature of our study provides a guide to avoid such experimental artefacts and permit the widespread use of this under-utilised, but hugely versatile, electrochemical technique.

Experimental methods

Materials

All chemicals were used as received without further purification. All aqueous electrolyte solutions were prepared with ultra pure water (Millipore Milli-Q, specific resistivity The pH of the latter was fixed to pH 7 by dissolving a PBS tablet in ultra pure water and diluting to achieve final concentrations of 2 mM phosphate buffer, 0.54 mM potassium chloride (KCl), and 27.4 mM NaCl.

Electrochemical measurements

The configuration of the 4-electrode CBPEC used in this study is shown in Scheme 1, and an image of the setup is provided in Fig. S1, ESI. The poles of the bipolar electrode consisted of two gold mesh electrodes, one immersed in the aqueous phase (Pw) and the other in the organic phase (Po). Each pole was connected through a copper wire. Two Ag/AgCl wires were used as pseudo-reference electrodes, one immersed in the aqueous phase and the other in the organic reference solution, i.e., an aqueous solution of 10 mM LiCl and 1 mM BACl, through Luggin capillaries. Platinum wire electrodes, one in each phase, were used as counter electrodes (driving electrodes). Prior to each electrochemical measurement, to eliminate the presence of any trace impurities, a flame anneal procedure was implemented [START_REF] Climent | Thirty years of platinum single crystal electrochemistry[END_REF][START_REF] Clavilier | Electrochemical adsorption behaviour of Pt(100) in sulphuric acid solution[END_REF]. Briefly, the gold mesh electrode was carefully heated in a butane-oxygen flame and protected with a drop of ultrapure water while the electrode was at relatively high temperatures. The gold mesh electrode was then carefully transported and immersed in the aqueous or organic compartment of the CBPEC, respectively. Additional studies were carried out replacing the gold mesh with either platinum mesh or ITO electrodes for both Pw and Po. at the ITIES can also be used to calibrate all voltammetry data obtained in the CBPEC configuration to the Galvani potential scale. This procedure has been outlined in detail previously [START_REF] Gamero-Quijano | Bioelectrochemistry of Cytochrome c in a Closed Bipolar Electrochemical Cell[END_REF].

Results and Discussion

Interpreting voltammetry in a 4-electrode CBPEC experiment

In a conventional 3-electrode cell, the peak on the forward sweep with a positive current indicates the species in solution is being oxidised at this potential on the working electrode.

However, in the 4-electrode CBPEC with immiscible aqueous-organic solutions, the peak on the forward sweep with a positive current indicates the flow of negative charge, i.e., electrons, from the organic to aqueous compartment through the progress of two half-reactions [START_REF] Gamero-Quijano | Closed bipolar electrochemistry in a four-electrode configuration[END_REF]. As outlined in eqn ( 1) and ( 2), simultaneously, an aqueous electron acceptor species (Aw) is reduced at Pw and an organic electron donor species (Do) is oxidised at Po.

D o 0 + P o 0 → D o + + P o - (1)
A w 0 + P w -→ A w -+ P w 0

(2)

When the sweep direction is reversed and scanned negatively, the back-reactions of eqn [START_REF] Loget | Bipolar electrochemistry: from materials science to motion and beyond[END_REF] and ( 2) take place, and electrons flow from the aqueous to organic compartment producing a peak on the reverse sweep with a negative current. Overall, assuming Aw and Do exchange a single electron, the heterogeneous redox reaction is:

D o 0 + A w 0 → D o + + A w - (3)
Thus, to summarise simply, a positive current during a 4-electrode CBPEC experiment means a species in the aqueous electrolyte is being reduced at that potential, while a negative current means a species in the organic phase is being reduced. Therefore, important aqueous reactions at Pw, such as the oxygen reduction reaction (ORR) or hydrogen evolution reaction (HER), take place at positive potentials and produce positive currents in a 4-electrode CBPEC experiment.

By comparison, the ORR and HER at the working electrode in a conventional 3-electrode cell appear at negative potentials and produce negative currents.

Voltammetry in a 4-electrode CBPEC with a low concentration of a reductant in the organic phase

Cyclic voltammograms (CVs) were obtained with the 4-electrode CBPEC outlined in Scheme 1 with a phosphate buffer solution. This potentiodynamic mode controls the potential difference between the two half-cells along the selected potential window on the Galvani potential scale ( o w ). A blank CV, with only the aqueous and organic electrolytes present in each phase, displayed a featureless double-layer with no faradaic process recorded within the potential window (Fig. 1a, dashed line). A further CV upon addition of micromolar concentrations of the reversible redox probe DcMFc in the TFT organic phase, acting as Do, gave a reversible redox response with a classic diffusional profile (Fig. 1a, solid line). The appearance of a positive peak centered at -0.280 V as the potential was swept positively indicated the flow of electrons from Po to Pw. Thus, an unidentified species was reduced at Pw (thereby acting as Aw), as the DcMFc species were oxidised at Po. When the direction of the scan was reversed, the back-reactions occurred with a negative peak centered at -0.350 V.

The standard Galvani potential difference for the heterogeneous electron transfer (HET)

between Aw and Do with a 4-electrode CBPEC, assuming Aw and Do exchange a single electron, is given by: = +0.584 V) in the TFT organic phase [START_REF] Gamero-Quijano | Bioelectrochemistry of Cytochrome c in a Closed Bipolar Electrochemical Cell[END_REF]. The change of the electron donor in the 4-electrode CBPEC led to a shift of the reversible redox response towards more positive potentials (Fig. 1b). From a thermodynamic point of view, a potential shift of 0.477 V was expected. However, the experimentally observed shift was 0.570 V. Although, the latter was proportional to the difference of the standard redox potentials of both electron donors employed, an overpotential of 93 mV was required. This overpotential is inherent to the 4-electrode CBPEC, with further detailed explanations on this topic given in section 3.9. A scan rate study was performed with the 4-electrode CBPEC, across the range 10-200 mVs -1 , with 16 M DcMFc in the TFT organic phase and a 5 mM NaCl aqueous electrolyte (Fig. 1c). The plot of the peak current (ip) versus the square root of the scan rate ( 1/2 ) was linear, indicating that the HET recorded with the 4-electrode CBPEC was limited by a diffusion-controlled electrochemical reaction (Fig. 1d, black triangles). The limiting reaction is likely to be that in the organic phase, involving the diffusion of micromolar concentrations of DcMFc towards Po to compensate the charge of surface processes at Pw, a high-surface area mesh electrode. Furthermore, the peak-to-peak separation increased with fast scan rates (Fig. 1d, red dots), as expected for a diffusion-controlled reaction. Nevertheless, as noted in Eqn ( 1) and ( 2), in the 4-electrode CBPEC, two half-reactions occurs simultaneously at Pw and Po.

 o w  HET ⊖ = [𝐸 D o + /D o 0 ⊖ ] SHE o -[𝐸 A w 0 /A w - ⊖ ] SHE w (4)
 o w  1/2 =  o w  HET ⊖ +  o w  ET +  o w iR (5)
Thus, if one of the reactions is diffusion-controlled, the overall bipolar response may also be diffusion-controlled even if the second reaction involves a diffusionless surface process. In this regard, it is noteworthy that at lower scan rates (10 and 20 mV•s -1 ) the peak shape and the peak-to-peak separation became less characteristic of a diffusion-controlled process (Fig. 1c). This is the first evidence that the electroactive species acting as Aw may be attributed to a diffusionless surface process on Pw. The diffusion of DcMFc to Po is less dominant at slow scan rates and the features of the observed CVs in the 4-electrode CBPEC change, being influenced more by the redox reaction at Pw.

Reversible gold oxide or hydroxide formation and reduction at the aqueous pole of the bipolar electrode in a 4-electrode CBPEC

In order to identify the unknown aqueous redox process acting as Aw in Fig. 1, the electrochemistry of the gold mesh in the aqueous compartment with a 2 mM phosphate buffer solution was probed using a conventional 3-electrode cell (Fig. 2). Electron transfer processes, such as gold oxide or hydroxide formation and their reduction, respectively, take place across a wide potential window ranging from +0.100 to +0.700 V vs. SHE. In a 3-electrode cell, certain electron transfer processes are obscured by capacitive currents at low scan rates (20 and 100 mVs -1 ) and only become apparent at high scan rates (200 to 2000 mVs -1 ) [START_REF] Ferro | The formation and dissolution of an electrochemical oxide film of up to a monolayer thickness on gold[END_REF][START_REF] De Tacconi | Dynamic ageing of an oxygen-containing monolayer at platinum and gold electrodes in aqueous acid solutions[END_REF][START_REF] Kirk | The electrochemical formation of Au(I) hydroxide on gold in aqueous potassium hydroxide[END_REF], as indicated by the asterisks in Fig. 2 at 2000 mVs -1 . However, the 4-electrode CBPEC configuration diminishes such capacitive currents by a mechanism that is still unclear and currently under investigation. Despite the fact that Pw and Po are high surface area gold mesh electrodes, capacitive currents on the order of 10 -7 A were measured (Fig. 1a). Thus, electron transfer processes involving surface functional groups on Pw, typically only seen at high scan rates (200 to 1000 mVs -1 ) on gold working electrodes in a 3-electrode cell, may appear at 20 mVs -1 using a 4-electrode CBPEC configuration in the presence of micromolar concentrations of an electron donor such as DcMFc in the organic phase. The reversibility of the redox process observed with the 4-electrode CBPEC in Fig. 1 suggests that hydroxide formation and reduction on the surface of the gold mesh is the leading candidate to act as Aw, over the less reversible oxide formation and reduction surface process.

The redox process is unlikely to be hydrogen adsorption. The latter is not a surface process observed with gold electrodes, and is not seen in the voltammetry of the gold mesh electrode in a 3-electrode cell (Fig. 2). In the following sections, the possibility that alternative redox processes, such as anion adsorption, oxygen reduction, and hydrogen evolution, give rise to the redox response with

[𝐸 A w 0 /A w - ⊖ ] SHE w = +0
.422 V are all explored and, ultimately, ruled out.

Voltammetry in a 4-electrode CBPEC in the presence of different aqueous electrolyte anions

The possibility of anion adsorption being responsible for the aqueous redox process occurring at Pw, and acting as Aw, was investigated by changing the aqueous phase in the 4electrode CBPEC from a 2 mM phosphate buffer solution to either 5 mM NaCl, 10 mM Li2SO4, or 10 mM LiClO4 (Fig. 3). The reversible redox response in the presence of DcMFc in the TFT organic phase was seen once more with NaCl, Li2SO4 and LiClO4 as the aqueous electrolyte (Fig. 3), in each case reasonably near the  o w  1/2 value initially observed with the phosphate buffer solution. These small shifts in  o w  1/2 and minor variations in the shape of the redox response between aqueous electrolytes are attributed to influence of the different anions on the adsorption (or re-structuring) of the water on the surface of Pw [START_REF] Briega-Martos | Effect of pH and water structure on the oxygen reduction reaction on platinum electrodes[END_REF]. Furthermore, slight experimental errors associated with the calibration of the pseudo-reference Ag/AgCl reference electrodes in each compartment of the 4-electrode CBPEC are possible. Surface oxides or hydroxides on gold may be reduced or oxidised using the exchange of protons with adsorbed phosphates in the phosphate buffer solution, such as H2PO4 -and HPO4 2- [START_REF] Yaguchi | Speciation of adsorbed phosphate at gold electrodes: A combined surface-enhanced infrared absorption spectroscopy and DFT study[END_REF][START_REF] Weber | In situ vibrational spectroscopy analysis of adsorbed phosphate species on gold single crystal electrodes[END_REF], and hence go through a proton-coupled electron transfer mechanism (PCET) mechanism at Pw giving rise to the redox response seen in Fig. 1a. However, such a PCET mechanism cannot occur if the absorbing species is a chloride (Cl -), sulfate (SO4 2-), or perchlorate (ClO4 -) anion. Thus, the role of adsorbed phosphates in a PCET mechanism giving rise to the redox response in Fig. 1a is unlikely. Furthermore, ClO4 -adsorbs weakly to a gold electrode, whereas SO4 2-adsorbs strongly [START_REF] Prieto | The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes[END_REF]. As a result, for CVs obtained using a 3electrode cell with a gold mesh working electrode, additional voltammetric features appear with a 10 mM Li2SO4 aqueous electrolyte in comparison to using 10 mM LiClO4 (Fig. S2, ESI). One of these additional peaks on the CV appears in the potential region matching the potential window experienced by Pw in a 4-electrode CBPEC experiment (+0.630 to +0.410 V vs. SHE). Nevertheless, no extra peaks were observed for CVs with the 4-electrode CBPEC across the potential range of -0.600 V to +0.200 V using 10 mM Li2SO4 as the aqueous electrolyte in comparison to using 10 mM LiClO4 (Fig. 3). Again, this indicates that the redox response at  o w  1/2 = -0.315 V is unlikely to involve anion adsorption. The first CV cycle produced a sharp positive peak centered at -0.070 V due to the oxide/hydroxide-free gold mesh. However, for the second CV cycle, the surface of Pw was already partially modified with oxide/hydroxide monolayers [START_REF] Kirk | The electrochemical formation of Au(I) hydroxide on gold in aqueous potassium hydroxide[END_REF]. Therefore, two positive The potential windows of both reductively and oxidatively pre-treated gold mesh electrodes acting as Pw were monitored using a multimeter and an auxiliary Ag/AgCl pseudoreference as an electric contact while the Galvani potential difference in the 4-electrode CBPEC was scanned from -0.600 to +0.200 V. The configuration used is detailed in Scheme 1, with a 2 mM phosphate buffer solution and 16 µM DcMFc present in the TFT organic phase. When

Voltammetry in a 4-electrode CBPEC after oxidative or reductive electrochemical pretreatment of the aqueous pole of the bipolar electrode

Pw was reductively pre-treated, the potential window varied from +0.600 to +0.390 V vs. SHE during potentiodynamic cycling (indicating a scan rate of ~5 mVs -1 at Pw), in line with the potential window observed for an untreated Pw, as discussed in section 3.3. However, when Pw was oxidatively pre-treated, the potential window varied only slightly during potentiodynamic cycling, from +0.980 to +0.940 V vs. SHE, a mere 40 mV (indicating a very slow scan rate of just 1 mVs -1 at Pw). Thus, the potential window of Pw is relatively dynamic when reductively pre-treated, varying across a 210 mV range as the CV is scanned from -0.600 to +0.200 V in the 4-electrode CBPEC. On the other hand, the potential window of Pw is almost static when oxidatively pre-treated, varying only 40 mV during the same 4-electrode CBPEC experiment.

Futhermore, depending on the type of electrochemical pre-treatment for Pw, the potential window experienced by Pw differs during the 4-electrode CBPEC experiment.

Reductive pre-treatment leads to a potential region of the gold mesh being scanned (+0.600 to +0.390 V vs. SHE) that is rich in electron transfer processes related to oxide or hydroxide formation or reduction, respectively, as seen in Fig. 2, that can act as Aw. Thus, the reversible redox response in the 4-electrode CBPEC seen in Fig. 4 after reductive pre-treatment is due to the same surface process with a predicted redox potential of +0.422 V vs. SHE on Pw, capable of acting as Aw, as seen for the untreated Pw in Fig. 1. However, oxidative pre-treatment leads to a tiny potential region of the gold mesh being scanned at far more positive potentials (+0.980 to 0.940 V vs. SHE). After oxidative pre-treatment, the electrode is coated in a thick passivating oxide film that is kinetically difficult to reduce (in comparison to the much more electrochemically reversible oxide or hydroxide groups present on Pw after reductive pretreatment). Thus, the small potential variation experienced by Pw is simply too short and at too positive potentials to reduce the passivating gold oxide film on Pw by promoting electron transfer from DcMFc, acting as Do, to the surface of Pw. The passivating gold oxide film on the mesh is not capable of acting as Aw as the CV is scanned from -0.600 to +0.200 V in the 4electrode CBPEC, electroneutrality cannot be reached in either compartment, and therefore electrons cannot flow along the bipolar electrode as described in Eqn ( 1) and ( 2).

Thus, a key conclusion is that any surface modification, such as oxide formation or possibly even modifying a gold electrode with a self-assembled monolayer of thiol molecules, may lead to different potential regions of Pw being scanned during the 4-electrode CBPEC experiment and even affect the scan rate experienced by Pw. As a consequence, misleading experiment results may be easily generated.

Use of other metallic electrodes and non-metallic electrodes as the poles of the bipolar electrode

The nature of Pw was also investigated by replacing the gold mesh in the aqueous compartment with a platinum mesh electrode. By changing the nature of Pw from gold to platinum, the redox response with a phosphate buffer solution and micromolar concentrations of DcMFc in the TFT organic phase was positioned at a more negative  o w  1/2 = -0.473 V both without pre-treatment and also after reductive pre-treatment (Fig. 5a). For a direct comparison of the shifts in the redox responses using platinum instead of a gold mesh (both without pretreatment) as Pw, see Fig. S5, ESI. This is clear evidence that the redox response on Pw, acting as Aw, is attributed to the surface chemistry of Pw, and not Po. Indeed, if the gold mesh in the organic compartment acting as Po was replaced with a platinum mesh, but Pw remained unchanged as a gold mesh, no change in the voltammetry was observed (data not shown), i.e., the same reversible redox process centered at -0.320 V was recorded as seen when both Pw and Po were gold mesh electrodes. In other words, DcMFc diffused to Po and acting as Do underwent electron transfer with the metal. This half-reaction, described in Eqn (1), was not influenced by the nature of Po once it was a conductive material (e.g., a metallic electrode or ITO).

In comparison to the electrochemistry of gold or platinum electrodes, the electrochemistry of non-metallic indium tin oxide (ITO) electrodes is devoid of any electron transfer processes due oxide or hydroxide formation and their reduction, respectively. In this regard, ITO electrodes were used as Pw and Po. As shown in Fig. 5b, no reversible redox responses were observed while the Galvani potential difference in the 4-electrode CBPEC was scanned from -1.200 to +0.200 V with a phosphate buffer solution and an excess (>1 mM) of DcMFc present in the TFT organic phase. Only an increase in current at positive potentials was seen, attributable to the irreversible reduction of the ITO [START_REF] Liu | Important implications of the electrochemical reduction of ITO[END_REF], beginning at a Galvani potential difference of ~0 V.

Observing the ORR in a 4-electrode CBPEC with a high concentration of a reductant in the organic phase

Studies of the aqueous phase purged with nitrogen, and maintained under anaerobic No such peak is seen at this potential with such low DcMFc concentrations for any of the other aqueous electrolytes tested, as shown in Fig. 3. This peak is attributed to the ORR on Pw. The reason the ORR is only seen at this relatively negative potential with LiClO4 in the 4-electrode CBPEC is attributed to the weak interaction of ClO4 -anions with the surface of the gold mesh [START_REF] Prieto | The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes[END_REF]. As demonstrated in 3-electrode experiments with Au(100) electrodes at neutral and basic conditions, the presence of a strongly adsorbing anion in solution, such as Cl -, SO4 2-, H2PO4 - and HPO4 2-, herein, may compete with HO2 -for the adsorption sites and force its desorption, hindering the reduction to OH - [START_REF] Prieto | The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes[END_REF]. Therefore, the weaker the adsorption of the anion, the more positive the potential required to initial the ORR on the working electrode in a 3-electrode cell (and conversely the more negative a potential required in a 4-electrode CBPEC, as noted in section 3.1). Upon increasing the concentration of DcMFc in the organic phase five-fold, within the potential window of -0.600 to +0.200 V in the 4-electrode CBPEC, the ORR appears as a constant increase in current with an onset at ~0 V as the transfer of electrons from DcMFc to the surface of Pw is no longer limiting (Fig. 6b, solid red line).

Observing the HER in a 4-electrode CBPEC with a high concentration of a reductant in the organic phase at a positively extended potential window

The HER demands a significant overpotential on a gold electrode immersed in 2 mM phosphate buffer solution. Extending the potential window with a 3-electrode cell, the onset potential for the HER on the gold mesh electrode occurred at -0.800 V vs. SHE (Fig. 7a). Also, before any hydrogen production, the ORR was observed as a broad, asymmetric peak centered at 0 V vs. SHE (Fig. 7a). Referring back to section 3.1, while the HER occurs at more negative potentials than the ORR on the working electrode in a 3-electrode cell, the HER will occur at more positive potentials than the ORR on Pw in a 4-electrode CBPEC. Furthermore, as shown in Fig. 6b, to clearly observe the ORR, and therefore also the HER, large concentrations of DcMFc in the TFT organic phase are required.

To demonstrate the potentials at which the HER takes place in a 4-electrode CBPEC experiment with a gold mesh electrode (reductively pre-treated at -0.9 V vs. SHE) as Pw and a phosphate buffer solution, the positive edge of the potential window was extended significantly, and a large excess (> 1 mM) of DcMFc was dissolved in the TFT organic phase (Fig. 7b). During the forward scan, from negative to positive potentials, the onset of the current due to the ORR appeared at ~0 V and reached a maximum current at ~0.8 V in the extended potential window tested. During the backward scan, the ORR displayed an asymmetric CV profile, in accordance with the voltammetry recorded in a conventional 3-electrode cell (Fig. 7a). The onset of the HER with the 4-electrode CBPEC began at ~1.2 V (Fig. 7b), and displayed a resistive current profile. The latter is likely due to the large iR drop in the organic compartment and the diffusion of DcMFc to Po. At micromolar concentrations of DcMFc the classic irreversible redox response of the HER is not observed with the 4-electrode CBPEC in the extended potential window tested in Fig. 7b.

Fig. 7. Observing the hydrogen evolution reaction (HER). (a)

Extending the potential window of CVs from a 3-electrode cell, obtained with a 2 mM phosphate buffer solution, to observe the H2 evolution reaction at the gold mesh working electrode. The scan rate used was 20 mVs -1 .

(b)

The CV was obtained with a 2 mM phosphate buffer solution and excess (>1 mM) DcMFc present in the TFT organic phase at a positively extended potential window. The scan rate used was 100 mVs -1 . The full configuration for the 4-electrode CBPEC experiment is shown in Scheme 1. Pw was reductively pre-treated at -0.9 V vs. SHE prior to the 4-electrode CBPEC experiments.

Slow reduction (at fast scan rates) of the aqueous pole of the bipolar electrode after oxidative electrochemical pre-treatment at a negatively extended potential window in a 4electrode CBPEC

As noted in section 3.5, after oxidative pre-treatment of Pw, when the CV is scanned from -0.600 to +0.200 V in the 4-electrode CBPEC, only a tiny 40 mV potential variation takes place on Pw. This small potential variation is insufficient to reduce the passivating gold oxide film by promoting electron transfer from DcMFc to the surface of Pw. Thus, to enhance the magnitude of the potential variation on Pw after oxidative pre-treatment, the potential window in the 4-electrode CBPEC was extended negatively by 600 mV (from -1.200 to +0.200 V), with Li2SO4 as the aqueous electrolyte and 50 M DcMFc in the TFT organic phase (Fig. 8a).

Starting at -1.200 V and scanning positively, a large peak with a positive current attributed to the reduction of the gold oxide layer was observed with a peak maximum of -0.600 V. On the reverse sweep, oxide formation took place with a a very negative potential of -1.000 V. With each successive CV cycle, the characteristics of the voltammogram changed dramatically (see CV cycles 1, 3, 8 and 10 in Fig. 8a), as the electrode surface became gradually more reduced due to the oxide not fully re-forming on the reverse sweep. In other words, with each cycle Pw exhibited more and more features typical of a reductively pre-treated gold mesh (Fig. 8b, solid line). Specifically, by the 10 th CV cycle, the peaks at very negative potential due to the reduction and re-oxidation of the passivating oxide layer almost disappeared, a positive peak at -0.235 V appeared due to the reduction of hydroxide or oxide groups on the reduced gold mesh surface, and a peak with an onset potential at ~0 V appeared attributable to the ORR (Fig. 8b, dotted line). CVs after reductive pre-treatment of Pw, as shown in Fig. 8b, solid line, did not vary with successive CV cycles. This indicates that the nature of the surface chemistry of the gold mesh did not change during the course of the 4-electrode CBPEC experiment.

For the experiment shown in Fig. 8a, the potential window of the oxidatively pre-treated gold mesh electrode acting as Pw was monitored using a multimeter and an auxiliary Ag/AgCl pseudo-reference as an electric contact while the Galvani potential difference in the 4-electrode CBPEC was scanned from -1.200 to +0.200 V. Extension of the potential window negatively by 600 mV led to an increase in the magnitude of the potential variation on Pw after oxidative pre-treatment, from +1.159 to +0.787 V during the first CV cycle. This 372 mV variation is significantly larger than the 40 mV variation seen for CVs scanned from -0.600 to +0.200 V in the 4-electrode CBPEC, as described in section 3.5. In line with the variation of the CVs shown in Fig. 8a, the potential window experienced by Pw with each CV cycle also changed due to the gradual reduction of the gold mesh surface with each successive CV cycle, as summaried in Table S1, ESI, for the first 6 CV cycles. The latter was due to the larger potential variation on Pw being sufficient to reduce the passivating gold oxide film by promoting electron transfer from DcMFc to the surface of Pw. Indeed, with each cycle the potential window experienced by Pw approached that typical of a reductively pre-treated gold mesh (Table S1, ESI). Also, in line with the invariant nature of the CVs after reductive pre-treatment of Pw (Fig. 8b, solid line), the potential window experienced by Pw did not vary with each CV cycle, measured as +0.725 to -0.062 V (Table S1, ESI). Furthermore, as observed using the smaller potential window in section 3.5, the potential variation on the reductively pre-treated Pw was more wide-ranging, changing by a substantial 787 mV compared with the 372 mV experienced by the oxidatively pre-treated Pw. The bipolar electrode in the 4-electrode CBPEC is an equipotential body and is typically considered to have a floating potential along Pw and Po [START_REF] Gamero-Quijano | Closed bipolar electrochemistry in a four-electrode configuration[END_REF]. However, for the experiments described herein, the fact that the potential window on Pw barely changed after oxidative pretreatment when applying the short potential window (-0.600 to +0.200 V), means that "the floating potential" concept is not an entirely accurate description in this case. An understanding of how the potentiostat measures the difference of potential between the aqueous (𝐸 ref. We speculate that the very small potential window experienced by Pw is a consequence of how the potentiostat in a 4-electrode configuration measures the potential of the aqueous compartment. When applying the short potential window (-0.600 to +0.200 V), after reductive pre-treatment of Pw, the potential measured between the aqueous and organic compartments in the 4-electrode CBPEC (𝐸 measured ) can be expressed as follows,

𝐸 measured = 𝐸 ref. o -(𝐸 ref. w + 𝑖𝑅 dynamic ) (6)
After oxidative pre-treament 𝐸 measured is expressed as

𝐸 measured = 𝐸 ref. o -(𝐸 ref. w + 𝑖𝑅 constant ) (7)
The surface chemistry, and iR drop (𝑖𝑅 dynamic ), is constantly evolving after reductive pretreatment of Pw, whereas the oxide film and iR drop (𝑖𝑅 constant ) are stable after oxidative pretreatment of Pw unless a very negative potential (-1.200 to +0.200 V) is applied in the 4electrode CBPEC. The latter leads to a subsequent evolution of the potential window experienced by Pw after oxidative pre-treatment with successive CV cycles, as shown in Fig. 8 and Table S1, ESI.

The experiment shown in Fig. 8a, monitoring the slow reduction of a passivating gold oxide layer and the resultant gradual appearance of redox responses due to electron transfer processes related to surface functional groups or the ORR, at a relatively fast scan rate of 20 mVs -1 , is impossible to achieve using a gold mesh as a working electrode in a 3-electrode cell.

For the latter, the metallic working electrode acts as a source or sink of electrons. Thus, by potentiostatically tuning the Fermi level of the metal towards reduction, for example, the number of electrons that may be supplied to the surface of the metallic electrode is effectively unlimited and occurs instanteously. This means that in a single CV sweep all of the passivating gold oxide layer would be reduced, and the response of the working electrode instantly switched from that of a fully oxidised surface to that of a fully reduced surface. The detail of the processes occuring in the interim between the two extremes is lost. However, with a 4electrode CBPEC, the reduction of the surface functional groups at Pw was severely delayed, being dependent on the concentration of DcMFc in the organic phase for the number of electrons available to be shuttled to Pw and also by the diffusion of DcMFc to Po. Therefore, the 4-electrode CBPEC allows the very gradual evolution of the surface chemistry of Pw, from that characteristic of a fully oxidised surface to that of a fully reduced surface (or vice versa if an organic oxidant is introduced to the organic phase), to be monitored electrochemically.

Perspective work in this area will involve combining the 4-electrode CBPEC with in situ spectroscopic techniques, such as infra-red or Raman spectroscopy, to open a new line of research into the slow reduction (or oxidation) of electrode surfaces using relatively fast scan rates applied by the potentiostat. In this manner, surface processes such as gold nanoparticle nucleation and growth may be slowed down and controlled with exceptional precision by precisely feeding the number of electrons required to slowly reduce metallic salts at the surface of Pw through tuning the concentration of the reductant in the organic phase. Monitoring such processes in situ using 4-electrode CBPECs integrated into scanning or transmission electron microscopy (SEM or TEM) holders is a particularly exciting prospect. Furthermore, if catalytic sites are present on Pw, the slow flow of electrons to these sites may provide a unique mechanism to study the slow evolution of catalytic intermediates, such as during electrocatalytically activated CO2 reduction.

Conclusions

The primary focus of this article is to highlight the significant contribution of redox active functional groups on, and electrochemical modification of, the surface of the aqueous pole of a bipolar electrode on the observed voltammetry in a closed bipolar electrochemical cell (CBPEC) with a 4-electrode configuration and aqueous-organic electrolyte solutions. With low (< 20 M) concentrations of electron donating decamethylferrocene in the organic phase, redox active surface functional groups on the aqueous pole of the gold mesh bipolar electrode, such as hydroxide or oxide reduction and formation, appeared at 20 mVs -1 . Typically these redox responses are only seen at high scan rates (200 to 1000 mVs -1 ) on gold working electrodes in a 3-electrode cell, being obscured by capacitive currents at lower scan rates.

However, the 4-electrode CBPEC configuration diminishes such capacitive currents.

The width of the potential window experienced by the aqueous pole is far shorter than that set by the potentiostat in the 4-electrode CBPEC. This is due to the assymmetric scan rates experienced at each pole of the bipolar electrode. Due to the far smaller iR drop in the aqueous phase relative to the organic phase, the scan rate at the aqueous pole is significantly slower than that applied by the potentiostat to the 4-electrode. The potential window experienced by the aqueous pole in the 4-electrode CBPEC, in terms of its position versus the standard hydrogen electrode and dynamic range, were also heavily influenced by oxidative or reductive pretreatment. The electrochemical responses from the surface functional groups on the aqueous pole were seen to completely disappear after oxidative pre-treatment, but remained after reductive pre-treatment. This was due to the inability of the passivating gold oxide film formed after oxidative pre-treatment to act as an electron acceptor in the aqueous compartment (and maintain electroneutrality upon the flow of electrons along the bipolar electrode). By comparison, the surface functional groups on the reductively pre-treated gold mesh electrode were more electrochemically reversible and within the potential window experienced by the aqueous pole during the 4-electrode CBPEC experiment. The oxygen reduction reaction was typically only seen at higher (> 80 M) decamethylferrocene concentrations and at positive potentials. Weakly adsorbing perchlorate anions did permit oxygen reduction to be observed electrochemically at less positive potentials. The hydrogen evolution reaction also required significant concentrations of decamethylferrocene (> 1 mM) and a highly positive extension of the potential window to be observed. The voltammetry was resistive in nature due to the iR drop in the organic phase and slow diffusion of decamethylferrocene to the organic pole of the bipolar electrode.

In a 4-electrode CBPEC, reduction of the surface of the aqueous pole was kinetically slow, being dependent on the decamethylferrocene concentration for the number of electrons shuttled to the aqueous pole and the diffusion of decamethylferrocene to the organic pole. A negative extension of the potential window was required to initiate the reduction process for an oxidatively pre-treated gold mesh. From the viewpoint of perspective applications, and future fundamental studies, this observation is key. In effect, the slow flow of electrons from the organic electron donor to the surface of the aqueous pole facilitates a very gradual evolution of the surface chemistry, from fully oxidised to fully reduced. This slow reduction of the surface of the gold mesh, at relatively fast applied scan rates of 20 mVs -1 , is not possible by potentistatically polarising a working gold mesh electrode in a 3-electrode cell. Thus, changes in the surface chemistry, such as the nucleation and growth of metallic nanoparticles, reduction of redox active surface functional groups, or catalytic conversion of adsorbed molecular species (such as in CO2 reduction), may be monitored spectroelectrochemically with exceptional precision by combining the 4-electrode CBPEC with aqueous-organic electrolyte solutions and techniques such as infra-red and Raman spectroscopy.

The electrochemistry of non-metallic indium tin oxide (ITO) electrodes is devoid of any redox active surface functional groups due to oxides or hydroxides, respectively. Thus, for 4-electrode CBPEC experiments that probe the electrochemistry of redox species in solution diffusing to the aqueous or organic poles, the use of ITO is highly recommended to avoid interfering electrochemical responses from metallic redox active functional groups.

Alternatively, the aqueous pole may be metallic if it is electrochemically pre-oxidised and used with either an oxidant in the oil phase, or a reductant as long as the potential window in the 4electrode CBPEC is not scanned too negatively.

The following is the Supplementary data to this article: Comparisons were made when the the aqueous and organic poles were either platinum, or gold mesh electrodes (both without pre-treatment). Fig. S6. CVs were obtained with a 2 mM phosphate buffer solution, degassed with N2, and maintained under anaerobic conditions, and 10 µM DcMFc present in the TFT organic phase. The aqueous pole was investigated without pre-treatment, and after reductive and oxidative electrochemical pre-treatment. Table S1.

Monitoring the potential window of the oxidatively or reductively pre-treated gold mesh electrode acting as the aqueous pole as the potetial window in the 4-electrode CBPEC was scanned from -1.200 to +0.200 V.

Scheme 1 .

 1 Scheme 1. Configuration of the 4-electrode closed bipolar electrochemical cell (CBPEC) used in this study. Each pole of the bipolar electrode consisted of gold mesh wires (unless stated otherwise), with one immersed in each compartment, denoted Pw in the aqueous phase and Po in the organic phase, and connected with an electric wire. The electron donating ferrocene derivative, either decamethylferrocene (DcMFc) or dimethylferrocene (DiMFc), was dissolved in the α,α,α-trifluorotoluene (TFT) organic phase. The organic electrolyte salt was 5 mM bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BATB). Several aqueous electrolytes were studied; 2 mM phosphate buffer solution, 5 mM sodium chloride (NaCl), 10 mM lithium sulfate (Li2SO4) and 10 mM lithium perchlorate (LiClO4). The 4-electrode CBPEC experiments were carried out under aerobic conditions and the scan rate used was 20 mVs -1 (unless stated otherwise).

  redox potentials of the organic electron donor and aqueous electron acceptor redox couples, respectively, on the SHE scale. [𝐸 D o + /D o /DcMFc redox couple in TFT has been determined previously as +0.107 V[START_REF] Gamero-Quijano | Bioelectrochemistry of Cytochrome c in a Closed Bipolar Electrochemical Cell[END_REF].Thus, measuring  o w  1/2 as -0.315 V from Fig.1aand assuming the overpotential and iR drop were negligible during the CV measurement, [𝐸 A w 0 /A w - redox process at the surface of Pw was +0.422 V. It should be noted at this point that, as will be detailed throughout this article, the overpotential and iR drop are not negligible and thus the value of [𝐸 A w 0 /A w - 422 V should be considered as a close guideline to the true value and not an absolute value.As detailed in Eqn (4) and (5), if the HET occurs between Do and the unknown redox process at the surface of Pw on the gold mesh in the aqueous compartment,  o w  1/2 for the redox response in the 4-electrode CBPEC should depend on the standard redox potential of Do. Thus, the strongly reducing DcMFc redox couple ([𝐸 DcMFc o + /DcMFc o weaker DiMFc redox couple ([𝐸 DiMFc o + /DiMFc o

Fig. 1 .

 1 Fig. 1. (a) Redox response using a 4-electrode CBPEC in the presence of micromolar concentrations of a reversible redox species in the organic phase acting as a reductant (Do). Cyclic voltammograms (CVs) were obtained with a 2 mM phosphate buffer solution, with or without 16 µM DcMFc present in the TFT organic phase. (b) Nature of the organic electron donor species. CVs were obtained with a 2 mM phosphate buffer solution, and either 14 µM DiMFc or DcMFc present in the TFT organic phase. (c) Scan rate studies. CVs were obtained with scan rates across the range 10-200 mVs -1 using a 4-electrode CBPEC with a 5 mM NaCl aqueous electrolyte, and 16 µM DcMFc present in the TFT organic phase. (d) Plots of peak current (ip / A) versus the square root of the scan rate ( 1/2 / (Vs -1 ) 1/2 ) and peak-to-peak

Fig. 2 .Fig. 2 .

 22 Fig.2. CVs from a conventional 3-electrode cell obtained with a 2 mM phosphate buffer solution and a gold mesh working electrode. Electrochemical electron transfer processes related to surface functional groups concealed by capacitive currents at the gold surface at lower scan rates (20 and 100 mVs -1 ) become apparent at higher scan rates (200, 500, 1000 and 2000 mVs -1 ), as indicated by the asterisks.

Fig. 3 .

 3 Fig. 3. Varying the aqueous electrolyte anion. CVs were obtained using a 4-electrode CBPEC with either a 5 mM NaCl, 2 mM phosphate buffer solution, 10 mM Li2SO4 or 10 mM LiClO4 aqueous electrolyte, and micromolar concentrations of DcMFc present in the TFT organic phase. The scan rate used was 20 mVs -1 . The full configuration for the 4-electrode CBPEC experiments is shown in Scheme 1.

Further experiments were performed

  to confirm the impact of the surface chemistry of Pw on the electrochemical responses observed with the 4-electrode CBPEC. Pw was electrochemically pre-conditioned potentiostatically, acting as the working electrode in a 3electrode cell configuration, prior to being used in the 4-electrode CBPEC, either at +1.2 V vs. SHE (oxidative pre-treatment) or -0.9 V vs. SHE (reductive pre-treatment), respectively, for 100 seconds. In the absence of DcMFc in the TFT organic phase, CVs with a featureless double-layer were observed for both electrochemical pre-treatments (Fig.S3, ESI), in accordance with the blank CV recorded in Fig.1afor an unconditioned gold mesh as Pw.However, with micromolar concentrations of DcMFc present in the TFT organic phase, markedly different electrochemical responses were observed with the 4-electrode CBPEC depending on the nature of the electrochemical pre-treatment, as shown with NaCl as the aqueous electrolyte in Fig.4. Reductive pre-treatment led to the observation of a reversible redox response (Fig.4, solid lines) at a near identical  o w  1/2 to that with the unconditioned gold mesh as Pw. At least three CV cycles were needed to stabilise this redox response. The sharp positive peak in the first CV cycle broadened and shifted towards more negative potentials over the course of the next two CV cycles. Reductive pre-treatment of the gold mesh leads to the removal of oxide/hydroxide monolayers from the gold surface which typically ages after repetitive cycles[START_REF] De Tacconi | Dynamic ageing of an oxygen-containing monolayer at platinum and gold electrodes in aqueous acid solutions[END_REF][START_REF] Nicol | The anodic behaviour of gold -Part II -Oxidation in alkaline solutions[END_REF][START_REF] Burke | The electrochemistry of gold: I. The redox behaviour of the metal in aqueous media[END_REF]. As Pw in the 4-electrode CBPEC during a CV, oxidation and subsequent reduction of surface oxides occurs with a particular kinetic profile on the gold mesh.

  peaks were observed during the second CV cycle, indicating the presence of two processes with different kinetic profiles. These two processes may have been oxide formation either within patches of pristine gold or on sites where oxide formation had already occurred. By the third cycle, the surface of Pw was more uniform and produced a single broad positive peak. The broad nature of this peak indicated a much larger heterogeneity of absorption sites than was the case during the first CV scan. These trends during the first three CV cycles were highly reproducible and observed after reductive pre-treatment of Pw and subsequent immersion into any of the aqueous electrolytes investigated. For example, the experimental data for Li2SO4 as the aqueous electrolyte in the 4-electrode CBPEC is shown in Fig.S4, ESI. On the other hand, oxidative pre-treatment of the gold mesh acting as Pw led to all of the voltammetric features disappearing, again irrespective of the aqueous electrolyte, as shown for 4-electrode CBPEC experiments with NaCl (Fig.4, dashed line) and Li2SO4 (Fig.S4, ESI) as the aqueous electrolytes.

Fig. 4 .

 4 Fig. 4. Reductive and oxidative pre-treatment of the aqueous pole of the bipolar electrode (Pw). CVs were obtained with a 5 mM NaCl aqueous electrolyte and 16 µM DcMFc present in the TFT organic phase after electrochemical oxidative (+1.2 V vs. SHE) or reductive (-0.9 V vs. SHE) pre-treatment of the gold mesh electrode acting as Pw. For the latter, the first 3 CV scans are shown. The scan rate used was 20 mVs -1 . The full configuration for the 4-electrode CBPEC experiments is shown in Scheme 1.

Fig. 5 .

 5 Fig. 5. Nature of the aqueous pole of the bipolar electrode. (a) Platinum mesh electrodes were used as both Pw and Po. CVs were obtained with a 2 mM phosphate buffer solution and 10 M DcMFc present in the TFT organic phase. Pw was investigated without pre-treatment, and after reductive and oxidative electrochemical pre-treatment. (b) Indium tin oxide (ITO) electrodes were used as both Pw and Po. A CV was obtained with a 2 mM phosphate buffer solution and excess (> 1 mM) DcMFc present in the TFT organic phase at an extended potential window. Scan rate used was 20 mVs -1 . The full configuration for the 4-electrode CBPEC experiments is shown in Scheme 1.

  Scheme 1, with a 10 mM LiClO4 aqueous electrolyte and either 20 or 100 µM DcMFc present in the TFT organic phase. The charge transfer recorded on the forward and reverse sweeps was directly proportional to the increase of DcMFc concentration (Fig.6a). Integration of the charge under the peak on the forward sweep revealed a 5-fold increase in charge (from 0.276 mC to 1.401 mC) upon increasing the concentration of DcMFc from 20 M (Fig.6a, solid blank line) to 100 M (Fig.6a, solid red line). Thus, the aqueous redox process (Aw) on the gold mesh at  o w  1/2 = -0.315 V is limited by the concentration of DcMFc (Do).

Fig. 6 .

 6 Fig. 6. Concentration of the organic electron donor species. (a) CVs were obtained with a 10 mM LiClO4 aqueous electrolyte and either 20 or 100 µM DcMFc present in the TFT organic phase. Extending the potential window with 20 M DcMFc present in the TFT organic phase revealed a diffusion limited peak attributed to the oxygen reduction reaction (ORR). (b) On extending the potential window with 100 M DcMFc present in the TFT organic phase, the ORR was no longer diffusion limited and appeared as a constant increase in current. Scan rate used was 20 mVs -1 . The full configuration for the 4-electrode CBPEC experiments is shown in Scheme 1. Pw was reductively pre-treated at -0.9 V vs. SHE prior to the 4-electrode CBPEC experiments.

Fig. 8 .

 8 Fig. 8. Slow reduction of the oxidatively pre-treated aqueous pole of the bipolar electrode. (a) The CV was obtained with a 10 mM Li2SO4 aqueous electrolyte and 50 M DcMFc present in the TFT organic phase at a negatively extended potential window. Pw was oxidatively pretreated at +1.2 V vs. SHE prior to the 4-electrode CBPEC experiments. The features on the CV changed with each successive CV cycle, as shown for CV cycles 1, 3, 8 and 10. (b) The same experiment as in (a) was performed except that Pw was reductively pre-treated at -0.9 V vs. SHE prior to the 4-electrode CBPEC experiments (solid line). The scan rate used was 20 mVs - 1 . The CV scan direction is indicated by the solid black arrows. The full configuration for the 4-electrode CBPEC experiments is shown in Scheme 1.

  w ) and organic (𝐸 ref.o ) reference electrodes is key. The organic reference electrode is sensing the aqueous solution through Pw, which undergoes modification during oxidative or reductive pretreatment, thus giving a different potential window in each instance. Furthermore, any dynamic changes in the surface chemistry of Pw during a 4-electrode CBPEC experiment will lead to dynamic changes in how the organic reference electrode senses the aqueous phase.

Fig. S1 .

 S1 Image of the 4electrode closed bipolar electrochemical cell (CBPEC) used in this study. Fig.S2. CVs obtained using a 3-electrode cell with 10 mM Li2SO4 and 10 mM LiClO4 aqueous electrolytes and a gold mesh working electrode. Fig.S3. CVs obtained with a 2 mM phosphate buffer solution and no electron donor species present in the TFT organic phase, after either electrochemical oxidative (+1.2 vs. SHE) or reductive (-0.9 V vs. SHE) pre-treatment of the gold mesh electrode acting as the aqueous pole for 100 seconds. Fig.S4. CVs obtained with a 10 mM Li2SO4 aqueous electrolyte and 20 µM DcMFc present in the TFT organic phase after electrochemical oxidative (+1.2 V vs. SHE) or reductive (-0.9 V vs. SHE) pre-treatment of the gold mesh electrode acting as the aqueous pole for 100 seconds. Fig.S5. CVs were obtained with a 2 mM phosphate buffer solution and 10 µM DcMFc present in the TFT organic phase.

  18.2 Mcm). The organic solvent ,,-trifluorotoluene (TFT, 99+%) was received from

	Acros Organics. Dimethylferrocene (DiMFc, 95%) and decamethylferrocene (DcMFc, 97%)
	were purchased from Sigma-Aldrich. Bis(triphenylphosphoranylidene)ammonium chloride
	(BACl, 97%), potassium ferricyanide (K3Fe(CN)6, ≥99%), potassium ferrocyanide
	(K4Fe(CN)63H2O, ≥99.95), lithium chloride (LiCl, ≥ 95%), sodium chloride (NaCl, ≥99%),
	lithium sulfate (Li2SO4, ≥99%) and lithium perchlorate (LiClO4, ≥95%) were purchased from
	Sigma-Aldrich. Lithium tetrakis(pentafluorophenyl)borate diethyletherate ([Li(OEt2)2]TB)
	was received from Boulder Scientific Company. BACl and [Li(OEt2)2]TB were used to prepare
	the	organic	electrolyte	salt	bis(triphenylphosphoranylidene)ammonium

tetrakis(pentafluorophenyl)borate (BATB) by metathesis of equimolar solutions of BACl and [Li(OEt2)]TB in a methanol-water (2:1 v/v) mixture. The resulting precipitates were filtered, washed and recrystallized from acetone. Phosphate buffered saline (PBS) tablets were purchased from Sigma-Aldrich and used in the preparation of the phosphate buffer solution.

Acknowledgments

This publication has emanated from research by M. D. S. supported by the European Research Council through a Starting Grant (agreement no. 716792) and in part by a research grant from Science Foundation Ireland (SFI) (grant number 13/SIRG/2137). A. G.-Q. acknowledges funding received from an Irish Research Council Government of Ireland Postdoctoral Fellowship Award (grant number GOIPD/2018/252). A. G.-Q. and G. H. are grateful to the Agence Nationale pour la Recherche (Grant No. ANR-14-CE14-0002-01) for the partial financial support of this work. The authors are grateful to the support of the Irish Research Council and Campus France for travel support between the French and Irish groups through their joint ULYSSES programme. The authors would like to thank Dr Pekka Peljo (Aalto University, Finland) for fruitful conversations on the intricacies of operating a fourelectrode closed bipolar electrochemical cell. Last but not least thanks to D. G. H. for being the best father ever. Appendix A. Supplementary data

Conflicts of Interest

There are no conflicts of interest to declare.


George Tanski

Helena Bergstedt

Alexandre Bevington

Philip Bonnaventure

Frédéric Bouchard

Caroline Coch

Simon Dumais

Alevtina Evgrafova

Oliver W Frauenfeld

Jennifer Frederick

Michael Fritz

Denis Frolov

Silvie Harder

Ingo Hartmeyer

Joanne Heslop

Elin Högström

Margareta Johansson

Gleb Kraev

Elena Kuznetsova

Josefine Lenz

Alexey Lupachev

Florence Magnin

Jannik Martens

Alexey Maslakov

Anne Morgenstern

Alexandre Nieuwendam

Marc Oliva

Boris Radosavljevic

Justine Ramage

Andrea Schneider

Julia Stanilovskaya

Jens Strauss

Erin Trochim

Daniel Vecellio

Samuel Weber

Hugues Lantuit

The Permafrost Young Researchers Network (PYRN) is getting older: The past, present, and future of our evolving community

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The imminence of the International Polar Year (IPY) prompted the need for visible representation of the young permafrost community [START_REF] Krupnik | Understanding Earth's Polar Challenges: International Polar Year 2007-2008[END_REF]. From the onset, the IPY emphasized the development of 'the next generation of polar scientists'. This translated into a record involvement of young scientists in IPY projects. A rough estimation by IPY organizers showed that the entire IPY endeavor involved a greater number of young researchers than 'senior' ones [START_REF] Baeseman | Early Career Researchers Activities during IPY[END_REF]. In anticipation of the IPY, the Permafrost Young Researchers Network (PYRN) was established in November 2005 at the 2 nd International Conference on Arctic Research Planning as an IPY education and outreach activity of the International Permafrost Association (IPA) and to represent permafrost young researchers within the Youth Steering Committee of the IPY. At the time, the Youth Steering Committee was the overarching program for youth and "early-career" activities in the IPY framework. PYRN was focused on young permafrost researchers (i.e., scientists and engineers), but was integrated into the overall IPY "early-career" networking effort from the beginning.

PYRN is an "early-career" network that fosters innovative collaboration and seeks to recruit, retain, and promote future generations of permafrost researchers. From the beginning, PYRN disseminated permafrost science and engineering information to the public and especially to schools. Since PYRN's initiation in 2005, its membership, visibility, and activities steadily increased to 1,200 members at the time of its 10 th anniversary in 2015. PYRN reports regularly to the IPA, its members and interested researchers on a regular basis at geoscientific conferences, through a news bulletin, website (https://pyrn.arcticportal.org/), and social media (e.g., Facebook, Instagram).

The strength of PYRN is based on its interdisciplinarity and its presence in virtually all regions of the world. PYRN members are interested in the frozen ground regions of the Earth, including alpine, submarine and polar latitudes. In the Northern Hemisphere alone, 23 million km 2 of the landmass is characterized by permafrost occurrence [START_REF] Romanovsky | Permafrost thermal state in the polar Northern Hemisphere during the international polar year 2007-2009: a synthesis[END_REF]Zhang et al., 2008); a vast region with an area almost twice the size of Antarctica. This large area is impacted by climate change, which fundamentally affects environmental and socio-economic systems [START_REF] Hope | Economic impacts of carbon dioxide and methane released from thawing permafrost[END_REF]IPCC, 2013). With its global presence, PYRN has the ability to mobilize its members beyond borders and disciplines to tackle these issues in a progressive and innovative manner.

PYRN represents permafrost science and engineering within broader international and regional young researcher assemblies, such as the Association of Polar Early Career Scientists (APECS).

The development of PYRN

PYRN has continuously evolved since its establishment at the 2 nd International Conference on Arctic Research Planning 2005and IPY 2007[START_REF] Grosse | PYRN-Bib 3.1: The Permafrost Young Researchers Network Bibliography of Permafrost-Related Theses[END_REF] At its 1 st anniversary in 2006, it already had 250 members from 20 countries [START_REF] Lantuit | The Permafrost Young Researchers Network (PYRN)[END_REF]. During IPY, registrations rapidly increased, with 620 members by the end of IPY in 2008 [START_REF] Lantuit | PYRN news[END_REF]. PYRN largely benefited from the momentum of IPY with its members expanding to 1,200 at its 10 th anniversary in 2015 [START_REF] Tanski | PYRN Report[END_REF]. Since 2005 there have been seven executive committees with each coordinating PYRN for two years, except of the initial committee that governed the network from 2005-2009.

In the following section, the evolution of PYRN and its main activities are summarized and its milestones synthesized in Table 1. All major activities and events since 2005 are listed in the Supporting material Table S1.

PYRN activities from 2005 to 2010

After the launch at the 2 nd International Conference on Arctic Research Planning in 2005, PYRN focused on developing the network. It organized its internal structure consisting of an executive committee and national representatives. It also established an online presence, a monthly newsletter, a thesis bibliography (Grosse & Lantuit, 2006), and an annual PYRN award for permafrost conferences. During PYRN's kick-off meeting in Abisko, Sweden (February 2007), the first PYRN mandate was drafted, progress of ongoing initiatives was reviewed, and future activities were planned (Fig. S1). PYRN's first meeting during IPY was the 2 nd national representatives meeting at the Otto Schmidt Laboratory in St. Petersburg, Russia (November/December 2007). The meeting set the stage for numerous 'IPA-IPY-PYRN' activities that took place until 2009 [START_REF] Lantuit | PYRN news[END_REF], with one of the major initiatives being the PYRN Thermal State of Permafrost project [START_REF] Christiansen | Report from the International Permafrost Association: Education and outreach for the International Polar Year[END_REF] and the establishment of PYRN Thermal State of Permafrost boreholes in Scandinavia [START_REF] Christiansen | The thermal state of permafrost in the nordic area during the international polar year 2007-2009[END_REF]. PYRN's IPY activities were endorsed and supported from the beginning by both IPY and IPA [START_REF] Baeseman | Early Career Researchers Activities during IPY[END_REF], with PYRN officially representing the IPA during the IPY opening in Paris, France (March 2007) [START_REF] Lantuit | PYRN news[END_REF]. In addition to PYRN's IPY activities, new efforts were initiated such as the creation of a list of senior scientist mentors [START_REF] Bonnaventure | Report from the International Permafrost Association: The Permafrost Young Researchers Network (PYRN)[END_REF], as well as the organization of young researcher activities at the 9 th International Conference on Permafrost (NICOP 2008) in Fairbanks, Alaska (June/July 2008).

PYRN activities from 2010 to 2012

The period from 2010 to 2012 was characterized by a refocusing of the PYRN mission and objectives and an internal restructuring process, following on the initial successful IPY years.

This meant adapting the network to create the conditions of sustainability and move on from the "one-off" momentum provided by the IPY. Early in 2011, PYRN identified its major priorities, which included an update of the PYRN bylaws, organizing a young researchers' workshop at the 10 th International Conference on Permafrost (TICOP 2010) in Salekhard, Russia (June 2012), and strengthening its ties with IPA and APECS. A major event was the PYRN workshop at TICOP 2010, which kicked-off the tradition of holding workshops before major biennial gatherings of young permafrost scientists. It also acted as the new transition point for the incoming executive committee to meet in person and identify their priorities for the coming twoyear term. Emphasis was placed on creating a more inclusive and team orientated approach to governing the PYRN network and the new executive committee had triple the number of members as the departing one. The large gathering of PYRN and polar researchers at TICOP was leveraged to support the implementation of a memorandum of understanding between PYRN and its official host IPA and APECS. The memorandum of understanding clearly established PYRN as the primary organization for the coordination of permafrost young researchers for committees and events surrounding IPA and APECS activities.

PYRN activities from 2012 to 2014

In 2012 PYRN's activities intensified. This invigoration was fostered by the new motivation and future avenues for permafrost science from the perspective of "early-career" researchers led by PYRN members [START_REF] Fritz | Brief Communication: Future avenues for permafrost science from the perspective of early career researchers[END_REF]. This strategy paper has been formulated in collaboration with the IPA as a contribution to the 3 rd International Conference on Arctic Research Planning 2015.

PYRN activities from 2014 to 2016

The 2014-2016 period was characterized by a professionalization of the network. A 4-year agenda and long-term strategy were developed to enhance the network's visibility, transparency, and cooperation with IPA. In turn, the IPA granted PYRN an annual and independent budget to support its activities and achieve its long-term objectives. Various strategic initiatives were achieved, including a major update of the member list because there had been no mechanisms to 'remove' young researchers that became 'senior'. This membership overhaul uncovered that east Asian colleagues were strongly underrepresented within PYRN. To tackle this issue, outreach activities were specifically designed to better engage permafrost research institutes in Korea, Japan, and China, and the network advertised at the Arctic Science Summit Week in Toyama, Japan (April 2015).

PYRN also improved its overall online outreach by better utilizing its website and social networks, in particular its Facebook following, which rose from almost zero in June 2014 to more than 600 in June 2016. Furthermore, PYRN expanded its partners by establishing an information exchange program with the Global Terrestrial Network for Permafrost. The executive term ended and a new executive committee was introduced (see Supporting material

PYRN activities from 2016 to 2018

Following the great success of the 11 th International Conference on Permafrost and with the increasing importance of young researchers within the research community, recent activities have focused on strengthening the national representation of PYRN, establishing an improved presence on social media as well as preparing the PYRN workshop at the 5 th European Conference on Permafrost (EUCOP5 2018) in Chamonix, France (June/July 2018). The Facebook following further increased during this period to more than 800 people. Many workshops, events, and meetings were convened thanks to the continuous efforts of the executive committee and national representatives (see Supporting Table S1). To better promote the participation of PYRN members on a national scale, a coordinator position of the national representatives was created and national representatives are encouraged to apply for funding and to organize local, national, and regional events. To strengthen the social media presence, the PYRN Facebook page (facebook.com/PYRNofficial), Twitter (@pyrn_official) and Instagram (@pyrn_official) were improved. A major improvement was the update and launch of the fully redesigned PYRN website. The increased social media and web presence throughout 2016-2018 helped to strengthen PYRN's role as a key promoter of international collaboration in permafrost research. In addition to these media improvements and to better engage our members and society, a major outreach and educational effort was devisedthe 'Frozen-Ground Cartoons' project (see Bouchard et al., this special issue). This project was officially launched in early 2016 as an IPA-funded Action Group, and later received financial support from many organizations, including PYRN. Most of the contributors to the project are indeed PYRN (and APECS) members. This international and interdisciplinary scientific outreach initiative aims at making permafrost science accessible and fun for children, youth, their parents, and teachers through the creation of cartoons and illustrated products [START_REF] Nääs | Frozen-Ground Cartoons: An international collaboration between artists and permafrost scientists[END_REF].

PYRN's structure and the first census in 2015 3.1 PYRN's international structure

Since being founded in 2005, PYRN has undergone many changes in its executive structure to become more dynamic and to more rapidly adapt to the evolving needs of its members. Whereas the first executive committees (2005 until 2012) consisted of only two to three members that had multifold tasks, the executive committees since 2012 consisted of 12 members that had more specified tasks. Currently PYRN is led by an executive committee, supported by a Council and several national representatives (Fig. 1). The executive committee directs PYRN's activities and consists of 12 positions, including president, vice-president, secretary, newsletter-editor, treasurer, web manager, representative for regional and international permafrost conferences, and ordinary members. The Council is an advisory panel responsible for providing feedback from

PYRN's regional branches

PYRN Russia is the oldest branch within PYRN. PYRN Russia's goal is to improve the qualifications of young permafrost researchers in Russia [START_REF] Kraev | Permafrost young researchers network of Russia[END_REF]. The network maintains an independent web portal (http://vk.com/pyrn_russia) for Russian-speaking researchers and engineers, raises funding, and provides solutions to overcome language-barrier problems. PYRN D-A-CH is a conglomerate of German-speaking countries and is the successor Its main objective is to improve networking of young permafrost researchers based in Canada and the US, which was lacking so far.

PYRN census and member survey

The PYRN constitution and bylaws state that 'a young researcher is defined as early in their career and is any individual currently enrolled as a post-secondary student or having completed their most recent post-secondary degree within the last six years'

(https://pyrn.arcticportal.org/about-us/constitution-bylaws). Since the establishment of PYRN, memberships cumulated without formerly young 'seniors' members bailing out. Thus, the first membership survey (in English) since PYRN's establishment was carried out. The purpose of the census was to review the PYRN membership and to improve the performance and networking capabilities of PYRN. The objectives were to obtain a better sense of how many PYRN members are currently active (i.e, receiving information and responding), where they are located, and what research topics they study. The census identified areas where PYRN has been successful, and likewise areas that need improvement. In addition to disseminating it to its registered members, it was sent through to various media channels (e.g., Cryolist and Facebook) to promote the network and gain new members. The survey was conducted with support from the United States Permafrost Association, and survey questions were revised through several internal and external reviews and feedback from APECS and IPA.

In total, 200 members completed the survey (via Google online Forms) between June and November 2015, of which 79 were female and 121 male, representing 26 countries across the world (see Supporting material Fig. S4 andS5). The majority of participants was young (mean = 30 years old; Fig. 2). Most of the members were from the academic sector and had a master's degree, followed by PhDs and undergraduates (Fig. 2). Of the 1,200 officially registered members, we assume that a large portion does not meet the requirement of 'young researcher' anymore, and another large portion is inactive. However, the survey gives a representative sample of the active and enthusiastic PYRN members on which the network relies, and this number roughly matches the ~280 young researchers that gathered at the 11 th International Conference on Permafrost (see Supporting material Fig. S3). In the survey, several questions were asked about potential improvements to the network, which have been considered and implemented in PYRN's plans and strategies for the future (see following section). A summary of all questions can be found in Table S2 in the Supporting material.

Future plans and visions in the legacy of IPY

PYRN continuously improved its visibility, outreach, and impact during the years by constantly exchanging with its own members and partners. The use of social media and Facebook in particular improved the visibility of the network with more than 800 followers in 2018. The PYRN membership survey helped to consolidate its own members and thus made the network One of the main legacies of PYRN is the creation of a community of practice, a strong networked group of individuals committed to permafrost research who remain connected to PYRN and its activities after their departure from the organization. This is visible through the mentoring from former members taking place during workshops, through the informal commenting and involvement in social media activities or through the involvement of many of these individuals in large research projects. This community of practice has been very efficient at articulating a strong vision for permafrost research [START_REF] Fritz | Brief Communication: Future avenues for permafrost science from the perspective of early career researchers[END_REF] and at proposing future avenues for collaboration between early career and senior scientists. Ultimately, a PYRN Alumni network would give a forum for PYRN members from all generations, many of whom connected as 'young' researchers during the IPY 2007-2009. However, this project remains in its infancy.

In response to the survey results, PYRN will enhance its online media presence through various channels (e.g., YouTube, Instagram, Facebook) that have been under development since 2014.

Although PYRN members indicated that social media is not necessarily a development priority for PYRN and that the dissemination of information on funding and positions should be the focus of the network, social media is nonetheless an effective mechanism to disseminate this information, and offers opportunities to reach out beyond PYRN membership. It is clearly an important tool for outreach to society and promotes permafrost as popular science. A good example is the 'Frozen-Ground Cartoons' project (https://frozengroundcartoon.com/home/), led by PYRN members.

Conclusions

This research note demonstrates that PYRN has successfully grown and evolved since its establishment in 2005, and has become a strong and effective network within the polar research community today. This development was catalyzed by the original IPY project, which helped its membership grow and promoted exchange of PYRN and other polar research network members.

An important legacy of the IPY 2007-2009 is that it successfully united a young emerging international community of researchers into PYRN, a network that promotes permafrost young researchers in their "early-career" stages. PYRN today (data from 2015), as a self-governing body supported by the IPA, relies on approximately 200 active members, which are mostly responsible for the success of the network. Many of the first generation PYRN members that became 'senior' researchers are still affiliated with PYRN and pass on the legacy of IPY to the new young generation. In the future, PYRN will strive to maintain its role within the polar research community. Regional PYRN branches will foster stronger and independent support of young researchers 'on site'. PYRN will promote the establishment of a PYRN Asia branch to further thrive, and to complement its international diversity of members. Finally, new technological developments and media tools will help PYRN improve its outreach and education projects to promote the important role of permafrost that it deserves in the Earth system and for society. Table 1. PYRN milestones since its establishment in 2005. PYRN assemblies mostly included 450 workshops and awards (see Supporting material Table S1). 451

Milestone Year

Establishment of PYRN at ICARP II in Copenhagen, Denmark

  Both networks were developed during IPY and have a memorandum of understanding to coordinate efforts. Further strong bonds exist with the Arctic Portal, which hosts PYRN's website and database, the ArcticNet Student Association, the United States Permafrost Association, Polar Educators International, Climate and Cryosphere, the Global Terrestrial Network for Permafrost, the working group 'Permafrost' of the Deutsche Gesellschaft für Polarforschung, the Bolin Centre for Climate Research, and various large research projects such as the Horizon 2020 project Nunataryuk. The main aim of this research note is to illustrate that the momentum of IPY has promoted and sustained this young researcher network for a decade, which in turn facilitates intercultural and interdisciplinary exchange, reflecting the original charge of IPY. More specifically, the objectives are (i) to review the development of PYRN since IPY, (ii) to present the first membership survey conducted since the IPY, and (iii) to set a vision for PYRN as crucial part of the polar research community.

  energy gained from the activities at 10 th International Conference on Permafrost (TICOP 2010), executing the memorandum of understanding with APECS that fostered and multiplied information exchange, and the forthcoming 4 th European Conference on Permafrost (EUCOP4 2014) in Évora, Portugal (June 2014). The executive period 2012-2014 culminated in the Permafrost Young Researchers Workshop at EUCOP4, co-organized with APECS, Changing Permafrost in the Arctic and its Global Effects in the 21 st Century, and Arctic Development and Adaptation to Permafrost in Transition. Aside from bringing young researchers together, the workshop aimed at determining future avenues of permafrost research from a young researcher's perspective. This part of the workshop was mentored by the International Arctic Science Committee, which was leading the implementation towards the 3 rd International Conference on Arctic Research Planning in 2015. A major outcome of the workshop was a strategy paper about

Fig

  Fig. S2) at the 11 th International Conference on Permafrost (ICOP 2016) in Potsdam, Germany

  PYRN members to the executive committee. National representatives are regular PYRN members representing a specific country and are part of the Council. PYRN has implemented a Constitution and Bylaws in 2012 to structure and organize itself, which have been first formulated in 2007 and reviewed with every new executive period since the IPY. PYRN also hosts independently operating national branches with the most active ones being PYRN Russia, PYRN D-A-CH (Germany, Austria and Switzerland), and PYRN NA (North America, i.e., USA and Canada).

  of PYRN Germany that existed until 2013. PYRN D-A-CH's main aim is to bring the polar and the alpine permafrost community closer together. It has gathered annually since 2009, at the meetings of the 'Permafrost' working group of the Deutsche Gesellschaft für Polarforschung -German Polar Society. PYRN North America (PYRN NA) is the latest established branch and was launched simultaneously in Canada (at Arctic Change 2017) and the USA (at AGU 2017).

  as our partnersthe ArcticNet Student Association, United States Permafrost Association, International Arctic Science Committee, Climate and Cryosphere, Horizon 2020 project Nunataryuk, Arctic Development and Adaptation to Permafrost in Transition, Changing Permafrost in the Arctic and its Global Effects in the 21 st Century, Deutsche Gesellschaft für Polarforschung, Université franco-allemande -Deutsch-Französische Hochschule, and Petzl Foundation. Last but not least we thank senior researchers and organizers of the IPY to promote young researchers during their early stages of their career. Tables 449

1Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Bar-chart (A) showing the distribution of PYRN members by age and stacked bar-

  Figure1

Acknowledgements

We would like to thank all the encouraged and motivated PYRN members that contributed to the success of the network and all national representatives that spread the word to local PYRN communities. We also thank members that have run the executive committees since 2005 for their outstanding contributions to the networks success. PYRN is grateful for the enduring support of the IPA. We thank the Arctic Portal for hosting and maintaining the PYRN website and support with the data management. We further thank APECS for fruitful cooperation as well

also more visible for its own members. This survey was part of a long-term strategy supported by the IPA with a first independent annual budget in PYRN's history. This money, in return, was used to support further outreach activities such as PYRN marketing for new members in general and East Asian members in particular. Apart from that PYRN activities started to have a direct impact on scientific decision making. With joint efforts of APECS and PYRN, which both had one of its members as liaison on the other ones executive committee, it was possible to establish PYRN members as session co-chairs at conferences and to develop its own priorities for the future of permafrost research, which fed directly into research planning exercises such as the 3 rd International Conference on Arctic Research Planning (ICARP III, 2015).

The outcomes of the membership survey served as a guideline for PYRN's future developments.

The survey revealed that to PYRN members, information on funding as well as workshops and meetings is most important. Less important are activities from social media, access to literature, pictures and videos, or education and outreach. In the future PYRN is planning to establish further branches such as PYRN Europe or PYRN Northern Europe (Scandinavia and Fennoscandia) to facilitate efforts and promote regional gatherings, which was one of the recommendations of the survey. As improved interactions with Asian members are an ongoing priority, PYRN also seeks to capitalize on the upcoming 12 th International Conference on Permafrost in Lanzhou, China to establish a PYRN Asia or East Asia branch for its Asian members (i.e., China, Japan, and South Korea). Recent PYRN activities during the Arctic Science Summit Week 2015 in Toyama and the 2 nd Asian conference on permafrost 2017 in Sapporo, Japan certainly reiterated the interest for an Asian regional branch.