Oxygen Tolerance of a Molecular Engineered Cathode for Hydrogen

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Oxygen Tolerance of a Molecular Engineered Cathode for Hydrogen Evolution based on a Cobalt Diimine-Dioxime Catalyst Nicolas Kaeffer, Adina Morozan, and Vincent Artero J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03136 • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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The Journal of Physical Chemistry

Oxygen Tolerance of a Molecular Engineered Cathode for Hydrogen Evolution Based on a Cobalt Diimine-Dioxime Catalyst

Nicolas Kaeffer, Adina Morozan, Vincent Artero*

Laboratoire de Chimie Biologie des Métaux, Univ. Grenoble Alpes, CNRS, CEA, 17 rue des Martyrs, 38000, Grenoble, France *Corresponding author: Vincent Artero, [email protected] Tel: +33 4 38 78 91 06

Abstract We report here that a bio-inspired cobalt diimine-dioxime molecular catalyst for hydrogen evolution immobilized onto carbon nanotubes electrodes proves tolerant towards oxygen. The cobalt complex catalyzes O2 reduction with onset potential of +0.55 V vs. RHE. In this process, a mixture of water and hydrogen peroxide is produced in a 3:1 ratio. Our study evidences that such side-reductions have little impact on effectiveness of proton reduction by the grafted molecular catalyst which still displays good activity for H2 evolution in the presence of O2. The presence of O2 in the media is not detrimental towards H2 evolution under the conditions used, which simulate turn–on conditions of a water-splitting device.

Keywords: proton reduction, bio-inspired chemistry, Carbon nanotubes, surface functionalization, ORR, RRDE

Introduction Hydrogen is an efficient and one of the cleanest energy carrier so far.1 In addition, sustainable hydrogen production through water splitting technologies appears as a promising pathway for 1 ACS Paragon Plus Environment


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large-scale growth in carbon-free energy.2-3 However, such processes always require catalysts to increase the reaction rate and lower the overpotential of the hydrogen evolution reaction (HER). Platinum-group metals are the most active electrocatalysts for HER but their large scale utilization is restricted by their limited supply and the cost issue.4 The development of non-precious alternative catalysts is thus a key scientific challenge. Nature provides examples of such catalysts in the form of hydrogenases having iron and nickel-based active sites for fast and reversible interconversion of protons into hydrogen.5 The resolution of the tridimensional structure of their active site and further insights into their catalytic mechanism allowed synthetic chemists to develop a series of molecular bioinspired HER electrocatalysts composed of inexpensive, earth-abundant elements.6-8 These compounds have recently been subjected to the benchmarking of their catalytic performances.9 However, while good activity and robustness are the primary figures of merit for assessing their possible implementation in technological devices, HER catalytic materials should also exhibit a certain tolerance to oxidative conditions. Specifically, these catalysts have to prove resistant to degradation by molecular oxygen either penetrating into the device during turn-off phases or produced in situ as a result of water splitting. This concern is reminiscent from hydrogenases, the most active of which are extremely oxygen sensitive and susceptible to inactivation by even traces of oxygen,5 even if some of them exhibit good tolerance to atmospheric oxygen thanks to a specific iron-sulfur cluster.10-13 Similarly, O2 inhibition has been identified as a main barrier in producing viable molecular proton reduction catalysts.14 However, there has been very little progress in the development of transition metal-based molecular H2-generating catalysts that operate in the presence of O2. To the best of our knowledge, all known examples shown to be functional under air are based on cobalt, including cobaloximes,15-16 a cobalt–corrole catalyst,17 a cobalt-microperoxidase18 and cobalt diimine-dioxime complexes.19 Significant progress has been made in our laboratory during the last ten years regarding the development

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of highly active molecular catalysts for HER6, 20-30 and their integration within electrocatalytic nanomaterials24, 31-32 based on carbon nanotubes (CNTs).14e, 15 The morphology and electrical conductivity of CNTs make them ideal supports to promote the electrocatalytic activity of the molecular complexes.33 In particular, we recently reported on an efficient cathode active for HER from aqueous solution and based on a cobalt diimine-dioxime complex30 immobilized on amino-functionalized carbon nanotubes.34 Here we report on the behavior of this H2evolving cathode material in the presence of oxygen.

Experimental part Materials Solvents,

starting

materials

and

supporting

electrolyte

salt

(tetrabutylammonium

tetrafluoroborate, nBu4NBF4) were purchased in the highest purity from Sigma-Aldrich and used as received, unless otherwise stated. 2,6-lutidine was purchased from Lancaster. Acetonitrile and dichloromethane were distilled on CaH2. Nafion® 117 solution (5 wt. % in a mixture of lower aliphatic alcohols and water, Sigma-Aldrich) was used. Oxygen gas (Air Products ultrapur quality) was of 99.995% purity. UP-NC7000WT (purity >90%) multi-wall carbon nanotubes (MWCNTs) were obtained from Nanocyl (Belgium). The dichloro cobalt complex of a diimine-dioxime ligand bearing an activated ester group, [Co(DO)(DOH)C8pnCl2]

(labelled [Co]) was prepared as previously described.34 (4-aminoethyl)benzene

diazonium tetrafluoroborate was prepared according to literature procedures.35

Preparation of the GC/MWCNT electrode The glassy carbon (GC) disk (0.196 cm2) of the rotating ring-disk electrode (RRDE, Pt ring, Pine Instruments) was polished on a MD-Nap polishing pad (Struers) with 1 µm synthetic diamond suspension (Struers, DP-Suspension M) and lubricant (Struers, DP-Lubricant Blue),



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then was rinsed with ethanol and dried in the air. The MWCNTs (2 mg) were dispersed in a mixture of ethanol (200 µL) and 5 wt. % Nafion® solution (20 µL) under sonication (30 min). A GC/MWCNT electrode was obtained by dropping 100 µL of the MWCNTs suspension onto the GC disk and drying in the air.

Functionalization of the GC/MWCNT electrode with [Co] The method is adapted from the procedure described in the literature.34 The GC/MWCNT working electrode was used in a three-electrode electrochemical cell setup connected to a potensiostat (SP-300 Bio-Logic) and a MSR rotator (Pine Instruments). The counter electrode and reference electrode were a Ti wire and an Ag/AgCl/3M KCl electrode (abbreviated as Ag/AgCl), respectively. (4-aminoethyl)benzene diazonium tetrafluoroborate (10 mg, 31 µmol) was dissolved in acetonitrile (25 mL) together with nBu4NBF4 (0.1 M). The solution was degassed with argon prior to use. The functionalization of the GC/MWCNT electrode was performed using cyclic voltammetry through the recording of 3 scans between –1.0 and +0.5 V vs. Ag/AgCl. The amino-functionalized GC/MWCNT electrode (GC/MWCNT-amino) was then rinsed by rotation of the electrode dipped in a pristine MeCN solution at 200 rpm for few minutes and subsequently dried under a flow of argon. The GC/MWCNT-amino electrode was dipped into a solution of [Co] (3.7 mg, 5 µmol) in distilled dichloromethane (5 mL) added with 2,6lutidine (5 µL). The reaction was held overnight under an argon atmosphere with continuous rotation of the electrode at 200 rpm. The resulting GC/MWCNT-amino-[Co] electrode was then rinsed with dichloromethane and dried under a flow of argon.

Electrochemical measurements


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Electrochemical analysis was performed using a BioLogic SP300 bi-potentiostat. The electrochemical experiments were carried out in three-electrode electrochemical cells under nitrogen atmosphere using the modified GC/Pt RRDE described above as the working electrode. A titanium wire was used as auxiliary electrode. The Ag/AgCl reference electrodes were independently calibrated with 5 mM [Fe(CN)6]4–. The [Fe(CN)6]3–/[Fe(CN)6]4– couple (E0 = 0.20 V vs. Ag/AgCl in phosphate buffer at pH=7) has then been used for the standardization of the measurements.36 The electrolyte used was either an aqueous acetate buffer (0.1 M, pH 4.5) or acetonitrile with n

Bu4NBF4 (0.1M) as supporting electrolyte. Voltammograms were recorded by scanning the

disk potential between –0.6 and +0.7 V vs. Ag/AgCl at 10 mV⋅s–1 in N2- and O2-saturared electrolytes. RRDE measurements were conducted at 10 mV⋅s–1, with a rotation speed of 1500 rpm in an O2-saturared electrolyte; the potential of the Pt ring was set at +1.1 V vs. Ag/AgCl. The HER polarization curves were measured at a scan rate of 10 mV⋅s–1 in both N2- and O2saturated electrolytes.

Results In order to have a better control on oxygen amounts, the molecular engineered cathode for hydrogen evolution based on a cobalt diimine-dioxime catalyst was constructed onto a glassy carbon (GC) electrode instead of the previously used porous gas diffusion layer (GDL). This design indeed allows a straightforward degassing of the electrolyte solution and avoids entrapment of O2 in the porous electrode substrate. The cathode was prepared according to Figure 1, following the method previously described.34 MWCNTs were suspended in an ethanol/Nafion® mixture and drop casted onto the GC electrode. Subsequent surface functionalization by amino groups was performed through the electroreduction of (4aminoethyl)benzene diazonium tetrafluoroborate using cyclic voltammetry.32, 37 The dichloro 5 ACS Paragon Plus Environment


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cobalt complex of a diimine-dioxime ligand bearing an activated ester group, [Co(DO)(DOH)C8-pnCl2] (hereafter labelled [Co]) was covalently grafted at the surface of MWCNTs through amide coupling between the activated ester moiety in [Co] and an amine function at the surface of MWCNTs.

Figure 1. Synthetic approach used to construct the GC/MWCNT-amino-[Co] electrode

The resulting GC/MWCNT-amino-[Co] electrode was characterized by cyclic voltammetry in acetonitrile. The grafting of [Co] is evidenced by the observation of two quasi-reversible systems at –0.11

and –0.67 V vs. Ag/AgCl (Figure 2) respectively assigned to the

Co(III)/Co(II) and the Co(II)/Co(I) couples of [Co] by analogy with previous studies.34 The integration of the wave assigned to the Co(II)/Co(I) couple allows to determine a catalyst loading of ca. 1.1×10–8 mol⋅cm–2


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Figure 2. Cyclic voltammogram of a GC/MWCNT-amino-[Co] cathode recorded in acetonitrile solution of nBu4NBF4 (0.1 M) at 20 mV⋅s–1

We then investigated the behavior of the GC/MWCNT-amino-[Co] cathode in the presence of oxygen. All following electrochemical studies were performed in an 0.1 M aqueous buffer acetate (pH 4.5) electrolyte which was shown to provide optimal conditions for proton reduction.34

Cyclic voltammograms recorded at GC/MWCNT-amino-[Co] cathode or

GC/MWCNT-amino electrodes in N2-saturated media do not display any noticeable electrochemical features (Figure 3). However, when the solution is saturated with oxygen, a reduction wave appears for both electrodes and is assigned to a catalytic oxygen reduction reaction (ORR). This wave shifts by 170 mV to more positive potentials for the GC/MWCNT-amino-[Co] cathode compared to the GC/MWCNT-amino electrode used as a control (Figure 3). This observation suggests that the grafted cobalt complex plays an active role in the ORR process.



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Figure 3. Cyclic voltammograms recorded at GC/MWCNT-amino (green lines) and GC/MWCNT-amino-[Co] (blue lines) electrodes in O2-saturated (plain lines) or N2-saturated (dashed lines) aqueous acetate buffer electrolytes (0.1 M, pH 4.5) at a scan rate of 10 mV⋅s–1.

To gain more insights into the ORR process, we then used the rotating ring-disk electrode (RRDE) technique which allows quantification of the relative amounts of H2O2 and H2O resulting from the two-electron (Eq.1) and four-electron (Eq. 3) reductions of O2, respectively.

O2 + 2H+ +2e- H2O2

(Eq. 1)

H2O2 + 2e- + 2H+ H2O

(Eq. 2)

O2 + 4H+ + 4e- 2H2O

(Eq. 3)

Indeed, in such an experiment, H2O2 produced at the rotating disk electrode is ejected radially and oxidatively detected at the Pt ring maintained at a potential of 1.1 V vs. Ag/AgCl (1.55 V vs. RHE). The current measured at the ring thus directly relates to the amount of H2O2 produced at the disk during ORR. The number of electrons (n) transferred to O2 during ORR


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and the relative amount of hydrogen peroxide (% H2O2) produced can be determined using the following equations:

n = (4Id)/[Id+(Ir/N)]

(Eq. 4)

% H2O2 = [(4-n)/2]×100

(Eq. 5)

where Id is the faradic current at the disk and Ir the faradic current at the ring (Sring = 0.110 cm2). The value of the collection efficiency of the rotating ring disk electrode was measured as N = 0.27 using the one-electron [Fe(CN)6]3–/4– redox couple, according to the manufacturer’s instructions.

The RRDE curves for GC/MWCNT-amino and GC/MWCNT-amino-[Co] electrodes are shown in Figure 4a and the potential dependence of n is shown in Figure 4b. The reduction of O2 on the GC/MWCNT-amino-[Co] electrode commences at +0.55 V vs. RHE while the reduction wave on the GC/MWCNT-amino electrode starts at +0.40 V vs. RHE, reproducing the behavior observed on CVs (Figure 3). The reduction current continuously increases with increasing cathodic potential. No current plateau is observed, as expected for ORR reaction only controlled by O2 diffusion. The observation of positive ring currents indicates production of hydrogen peroxide at the disk. The calculated number of electrons involved in ORR on GC/MWCNT-amino-[Co] electrode was found at ca. 3.5 (Figure 4b) and the ORR is essentially assigned to a combined O2 reduction via 2e– and 4e– reduction pathways with ca. 25 % H2O2 production.



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Figure 4. (a). RRDE measurements at GC/MWCNT-amino (green lines) and GC/MWCNTamino-[Co] (blue lines) electrodes with disk and ring currents shown as plain lines and dashed lines, respectively (Ering=1.55 V vs. RHE, 1500 rpm, scan rate: 10 mV⋅s–1); (b). The potential dependence of n (black dots, left scale) and of the ratio of H2O2 produced (red dots, right scale).

The GC/MWCNT-amino-[Co] cathode catalyzes HER at more negative potentials than ORR. Figure 5 shows the corresponding polarization curve measured in acetate buffer in the absence of dissolved oxygen and provides a comparison with the GC/MWCNT-amino material.38 A 150 mV positive shift of the polarization curve is observed upon covalent grafting of the [Co] complex, as previously reported.34 The GC/MWCNT-amino-[Co] cathode produces H2 from an onset potential of –0.35V vs. RHE and reaches a current density of 1 mA⋅cm–2 at ca. –0.55 V vs. RHE.


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Figure 5. Linear sweep voltammograms for GC/MWCNT-amino (green line) and GC/MWCNT-amino-[Co] (blue line) electrodes recorded in N2-saturated aqueous acetate buffer electrolyte (0.1 M, pH 4.5) at 10 mV⋅s–1 scan rate.

To further determine if the presence of dissolved oxygen impacts the HER activity of the GC/MWCNT-amino-[Co] cathode, we studied its behavior towards proton reduction in an O2-saturated electrolyte. As shown on Figure 6, a first reductive event starting from +0.55 V vs. RHE is observed, assigned to ORR by comparison with RRDE experiments (Figure 4). While shifting potential downward, a catalytic current raises from ~ –0.35 V vs. RHE. This current overlaps very well with the one observed in the absence of oxygen (Figure 5) and assigned to H2 evolution. The GC/MWCNT-amino electrode behaves differently with ORR taking place at more negative potentials and catalytic proton reduction polarization curve shifted 80 mV more cathodic than for GC/MWCNT-amino-[Co].



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Figure 6. Linear sweep voltammograms of GC/MWCNT-amino (green lines) and GC/MWCNT-amino-[Co] (blue lines) electrodes in O2-saturated (plain line) or N2-saturated (dashed lines) aqueous acetate buffer electrolytes (0.1 M, pH 4.5) at 10 mV⋅s–1 scan rate.

Discussion Current efforts to integrate the MWCNT-amino-[Co] H2-evolving cathode34 within a complete photoelectrochemical (PEC) water-splitting cell lead us to investigate the impact of residual O2 on the HER. As we were manipulating our material under atmospheric conditions before experiments, we went to notice that a reductive side-reaction occurred during cathodic polarization. This event, found at more positive potentials than proton reduction, was assigned to the reduction of residual oxygen. Actually, oxygen can be present in the cathodic compartment for, at least, two reasons. It can first be trapped inside the porous gas diffusion layer (GDL) used as electrode support,34 as a consequence of atmospheric handling of the cathode material. Alternatively, O2 can diffuse from the anodic part where it is produced as a result of water oxidation. The impact of O2 on catalytic proton reduction activity hence appears as a general issue when dealing with integration of HER cathodes within full PEC devices. In the present study, we report on the impact of O2 on the production of H2 with a MWCNT-amino-[Co] cathode deposited on a compact glassy carbon (GC) electrode support, 12 ACS Paragon Plus Environment

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thus allowing for an easy and complete purge of the cell when needed. The GC/MWCNTamino-[Co] architecture was successfully designed by the covalent approach previously developed in our group (Figure 1).34 The catalyst loading, estimated around 1.1×10–8 mol⋅cm– 2

, is in good agreement with the one previously determined for the GDL/MWCNT-amino-

[Co] cathode (4.5×10–9 mol⋅cm–2). It follows that changing the electrode substrate from GDL to GC does not influence further surface functionalization and grafting of the cobalt diiminedioxime complex. As shown in Figure 2, the presence of [Co] on the amino-functionalized MWCNTs has a direct impact onto the ORR, with a 150 mV anodic shift of the reduction wave. This observation supports an active role of the immobilized [Co] complex for catalyzing O2 reduction. Cobalt complexes are known to be ORR catalysts and have been intensively studied in that context.39-40 In particular, Shamsipur et al. made similar observations with a cobalt diimine-dioxime complex [Co(DO)(DOH)pnCl2], adsorbed onto the surface of a preanodized GC electrode and dipped into a 0.1 M aqueous acetate buffer solution at pH 4.19 In line with this report, we also observe that catalytic ORR reaction develops at a potential corresponding to the Co(III)/Co(II) couple of the immobilized cobalt complex (Figure 2). Of note, [Co] sites are not equally distributed within the bulk of the MWCNT film but rather in a thin enveloping coating.37 This explain why we do not see any effect of the nanostructure of the MWCNT film to limit O2 diffusion, as it has been found by Armstrong and co-workers for a hydrogenase-based 3D electrode based on compact porous carbon material.41 ORR is a multielectronic process. Hence its mechanism is quite complicated and involves many intermediates, primarily depending on the nature of the electrode material, catalyst and/or electrolyte. Two pathways are possible in acidic media:42 (i) one is a 2e– mechanism generating the intermediate hydrogen peroxide, H2O2 (Eq. 1) with further reduction (Eq. 2) or its dismutation; (ii) the other is a 4e– mechanism directly producing H2O (Eq. 3), an ideal



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pathway since it avoids the formation of reactive oxygen species (ROS). RRDE measurements showed that 3.5 electrons in average are involved in O2 reduction at the GC/MWCNT-amino-[Co] electrode over a wide range of potentials (–0.1 to +0.4 V vs. RHE). This is an indication of mixed 2e– and 4e– processes with a predominantly 4e– transfer pathway and ca. 25 % H2O2 production. A likely mechanism for ORR involves first coordination of O2 to a Co(II) species, generating a Co(III)-superoxo intermediate,43 immediately reduced at the electrode to give a Co(II)-peroxo/hydroperoxo species44,45,46 from which the two pathways diverge. Generation of H2O2 at the vicinity of the electrode surface may damage the structure of the cobalt-based molecular catalyst. To address this issue, we then investigated the HER properties of the GC/MWCNT-amino-[Co] electrode both under N2-degassed and O2saturated conditions. In deaerated conditions, H2 evolution is observed from an onset overpotential of 350 mV, consistent with a catalytic cycle involving the Co(II)/Co(I) couple of the grafted [Co] complex.47 The overpotential corresponding to a current density of 1 mA.cm–2 is 550 mV for the GC/MWCNT-amino-[Co] electrode. Such value is comparable with that (580 mV) measured for the same material deposited onto GDL34 if the variation in [Co] loading is taken into consideration. This point demonstrates the ability of the GC/MWCNT-amino-[Co] cathode to evolve hydrogen at low overpotential. Within the accuracy of the measurement, the HER activity of the [Co]-based cathode is unaffected by the presence of O2, as far as both the overpotential corresponding to a current density of 1 mA.cm–2 and the onset overpotential are concerned. The only notable difference is the reduction process observed at potentials below 0.55 V vs. RHE assigned to ORR. Of note, on the control GC/MWCNT-amino electrode, the catalytic wave attributed to HER appears as superimposed onto the catalytic plateau ORR current, indicating that O2 and H+ reduction processes can occur simultaneously at the electrode surface. Such a phenomenon is not


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observed on the GC/MWCNT-amino-[Co] cathode. By contrast, the HER polarization curves recorded in the presence of O2 is superimposed with that measured in its absence, which speaks for both processes competing for the same [Co] catalytic sites. It is possible that HER and ORR take place in geographically distinct regions. The [Co] sites exposed at the surface of the film may achieve ORR preventing access of O2 to the bulk of the MWCNT-amino-[Co] film where only H2 evolution takes place. This dual behavior would mirror the mechanism at play in a recently reported enzymatic hydrogel.48-49 However, we found it unlikely because of the relatively low thickness of the polyphenylene film deposited at the external surface of the MWCNT film during functionalization.37 Anyhow, our data show that the [Co] sites are O2tolerant HER catalysts since they are able to achieve H2 evolution in the presence of O2. A similar conclusion was previously reached by Reisner’s group on a parent [Co(dmgH)2]-type (dmgH2 = dimethylglyoxime) catalyst when 21% O2 was introduced in the headspace of the electrochemical cell.15 We note that the conditions used in our experiments are quite harsh and do not directly reflect more realistic O2 concentration in electrolyte when a 21% O2 in air atmosphere is used as headspace. Still, we postulate that the conditions used to obtain the data shown in Figure 6 simulate in first approximation the events at stake just after turning on a PEC device. Back to the results previously described in ref 34 (Figure 4b and c), it is obvious that the deviation of faradic yield from unity mainly occurs at the start of the controlled potential coulometry experiment, which we assigned to reduction of traces of O2 trapped within the film with 495±25 turnovers achieved with the experiment. Several reasons might be invoked to support the high O2 tolerance of the HER catalyzed by grafted [Co] complexes. A reason for the disconnection between H+ and O2 reduction processes could first arise from the fact that ORR develops at potentials corresponding to the Co(III)/Co(II) couple of the grafted complex while HER involves its Co(II)/Co(I) couple. Then, the solubility of O2 in aqueous solutions is quite low (0.27 mmol⋅L–1) compared to the



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protons concentration. It is thus not surprising that protons binding takes over O2 activation at potentials where Co(I) is formed. Yet, should oxygen bind to a grafted Co(I) complex, it would directly generate the same Co(III)-peroxo/hydroperoxo intermediate as discussed above. From this species, H2O2 shall evolve before degradation could occur through intramolecular attack50 on nearby unsaturated imino positions of the diimine-dioxime ligand. In any case, formation of H2O2, even as a minor product of ORR, might be an issue for long term operation if it accumulates in the electrolyte. Solutions to overcome long term degradation may include the use of catalysts such as catalase mimics for H2O2 dismutation into O2 and H2O. Another strategy has been recently developed by Plumeré and coworkers. The catalyst, an O2-sensitive hydrogenase, was integrated into a methyl-viologen containing hydrogel. In the reduced state formed under catalytic H+/H2 conditions, this redox mediator reduces O2 at the extremity of the catalytic layer in contact with the electrolyte and thus prevents degradation of buried hydrogenase enzymes.48 It would be very interesting to extend such a strategy, by which an immobilized HER catalyst is shielded from undesired ORR, to synthetic molecular systems such as our MWCNT-amino-[Co] electrode.

Conclusion We report here on O2 tolerance of an inexpensive cobalt diimine-dioxime catalyst covalently grafted on carbon nanotubes which evolves H2. Electrochemical studies demonstrate good selectivity for electrocatalytic reduction of protons over oxygen in O2-saturated conditions. Oxygen is mainly reduced to water through a 4e– process, but significant amounts of H2O2 are formed anyway, which may cause long term degradation of such a cathode material integrated in a PEC device. To overcome this issue, alternative O2 protection strategies are required.


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Acknowledgements N. Kaeffer thanks the Solar Fuel Institute (SOFI, www.solar-fuels.org) for an exchange fellowship in the group of D. Gust at Arizona State University. This work was supported by the French National Research Agency (Labex program, ARCANE, ANR-11-LABX-000301), the FCH Joint Undertaking (ArtipHyction Project, Grant Agreement n.303435).

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Graphical Abstract


Oxygen Tolerance of a Molecular Engineered Cathode for Hydrogen

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