Implementing molecular catalysts for hydrogen production in proton exchange membrane water electrolysers

Coordination Chemistry Reviews 256 (2012) 2435–2444

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Implementing molecular catalysts for hydrogen production in proton exchange membrane water electrolysers Minh Thu Dinh Nguyen a , Alireza Ranjbari a , Laure Catala a , Franc¸ois Brisset a , Pierre Millet a,∗ , Ally Aukauloo a,b,∗ a b

Institut de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO),UMR-CNRS 8182, Université Paris-Sud 11, 91405 Orsay, France CEA, iBiTec-S, Service de Bioénergétique Biologie Structurale et Mécanismes (SB2SM), F-91191 Gif-sur-Yvette, France

Contents 1. 2. 3.

4. 5.

6. 7. 8. 9.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2436 Water electrolysis technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2436 Basic electrochemical thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2437 3.1. Thermodynamics of water electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2437 3.2. Role of pH on cell voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2437 Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2438 Efficiency of electrochemical PEM water electrolyser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2438 5.1. Energetic efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2438 5.2. Faradaic efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439 Limitation and challenges for PEM cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439 Modified electrodes with non-noble metal complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439 Membrane electrode assemblies for PEM water electrolyser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2440 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2444 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2444 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2444

a r t i c l e

i n f o

Article history: Received 10 February 2012 Accepted 30 April 2012 Available online 6 May 2012 Keywords: Artificial photosynthesis Hydrogen Electrocatalysis Modified electrodes PEM water electrolyser

a b s t r a c t At the last COST (European Cooperation in Science and Technology) EU–US meeting held in May 2011 in Prague, one of the main questions raised was how can molecular chemistry with an emphasis on the use of non noble metal complexes contribute to water photolysis for the production of solar fuels. In general molecular chemistry can help not only in the understanding of the sequential steps of water oxidation with the design of sophisticated metal complexes but also in the catalytic reaction involving the reduction of protons to hydrogen to make a fuel. The water oxidation reaction stands as the grand challenge for molecular chemists as water has been recognised as the source of protons and electrons to be used in the synthesis of solar fuels. Based on recent advances, it seems that the development of molecular metal complexes with abundant and cheap metals holds the promises for their putative integration in functional devices for the hydrogen production reaction. However, for the majority of these metal complexes, the electrocatalytic activity towards the reduction of protons has been reported in organic solvents and only rarely in aqueous medium. Furthermore, these molecular catalysts also suffer from degradative processes upon catalytical activity in solution. Hence there is still much room for the design and preparation of molecular based catalysts capable to perform the H2 production in aqueous phase. In this review/article we give a brief overview on the state of the art on solid polymer exchange (SPE) membrane water electrolysers and their limitations regarding their widespread commercialisation, that is in part related to the expensive and rare noble metal catalysts used for both the hydrogen producing reaction and the water oxidation process. We focus herein on the recent advances made in preparing

∗ Corresponding authors at: Institut de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO),UMR-CNRS 8182, Université Paris-Sud 11, 91405 Orsay, France. Tel.: +33 169154756; fax: +33 169154755. E-mail addresses: [email protected] (P. Millet), [email protected] (A. Aukauloo). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ccr.2012.04.040

2436

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

modified carbon electrodes with molecular based complexes and on their catalytic properties in heterogeneous medium. A challenging step in this research field is to couple the cathodic process to that of the water oxidation reaction. We report here on the implementation of fluroboryl dimethylglyoxime cobalt complexes supported on a carbonaceous material at the cathode of a proton exchange membrane (PEM) water electrolysers. electrocatalytic activity for the H2 production was observed with current densities in the range of 500 mA cm−2 for a cell efficiency of 80% using iridium at the anode. The durability of these systems was tested for several days upon on and off polarisation without noticeable loss in activity. These results therefore lead us to think that it should be feasible to reduce the cost of actual PEM water electrolyser by replacing platinum at the cathode. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Finding renewable sources of energy is an imperative in the coming environmentally and energetically constrained future [1–6]. A particularly appealing solution resides in splitting water using solar energy in order to liberate electrons and protons for the production of hydrogen [7–11]. Currently, the use of precious metals as electrodes to perform both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), impedes their implementation on a global scale [12–23]. A more cost-effective way of performing these reactions is currently under investigation by chemists. The development of molecular catalysts based on non-noble metal ions offers an alternative route to develop such materials [24–32]. We discuss here the recent advances in developing modified electrodes with molecular metal complexes and their reactivity pattern towards the HER. We also report on our first attempt to introduce cobalt based molecular complexes at the cathode of a PEM water electrolyser.

2. Water electrolysis technology There are several water electrolysis technologies. The materials used in the electrolysis cells depend mostly on the operating temperature. At low temperature (from zero up to 150 ◦ C), aqueous electrolytes are used [33–35]. The two main technologies are (i) the alkaline process (using a liquid KOH electrolyte) (ii) the acidic process (using a solid polymer electrolyte (SPE)). The acidic process (commonly known as PEM water electrolysis where PEM stands for Proton Exchange Membrane) is more specifically considered here. In a PEM water electrolysis cell (Fig. 1), there is no liquid electrolyte. Sulfonated tetrafluoroethylene based fluoropolymer–copolymer (W. Grot, E.I. DuPont Co., Nafion® products [36]) is used as solid electrolyte. Electric energy is used to split liquid water into gaseous oxygen and protons (anodic reaction a). Solvated protons migrate to the cathode where they are reduced into molecular hydrogen (cathodic reaction b) and where liquid water is released (electroosmosis drag). The anode and the cathode are separated by a thin (0.2 mm thick) membrane of proton-conduction polymer electrolyte. Such cells are very compact and can operate at high (several A cm−2 ) current densities. In conventional PEM water electrolysis technology, metallic platinum is used at the cathode for the promotion of the HER and metallic iridium (or iridium oxide) is used at the anode for the promotion of the OER. The use of such noble metals is made necessary due to the highly acidic environment encountered in the solid polymer electrolyte, which would cause the corrosion of less noble catalysts (first row transition metals). Cost considerations require a significant reduction of noble metal loadings, at both anode and cathode, from a few mg cm−2 down to ca. 0.1 mg cm−2 (current state-of-the-art). As in fuel cell technology, this can be achieved by using carbon-supported nanoparticles. Membrane Electrode Assemblies (MEAs) have been developed by coating thin (a few microns thick) layers of various catalytic inks

Fig. 1. Schematic diagram of a PEM water electrolysis cell.

on each side of Nafion® SPEs using spray techniques. Mixtures of platinum black and solutions of Nafion® ionomers have been used at the cathode for the HER. While platinum or palladium nanoparticles deposited at the surface of carbon carriers of large surface areas have also been developed [37]. Mixtures of iridium black and solutions of Nafion® ionomer have been used at the anode for the oxygen OER. A particular effort has been devoted to plating conditions to obtain homogeneous and adherent catalytic layers of constant thickness onto hydrated SPE membranes. Another handle for optimising the PEM electrolyser was to develop millimetre thick plates made of sintered titanium spherical particles (mean diameter 100 ␮m). These have been used as porous current collectors (PCC) which play a key role in the PEM cell: they transfer electricity from the bipolar plates to the interfaces. Concomitantly, they permit the transport of liquid water to the anode and the expulsion of oxygen gas away from the anode. In the cathodic compartment, they also allow the transport of a biphasic mixture (liquid water resulting from the electro-osmotic drag across the SPE and gaseous hydrogen) away from the cathode. Insufficient thickness and/or porosity result in excessive ohmic losses and inefficient water splitting reaction. The design of these current collectors has been optimised to obtain high cell efficiencies [38,39]. During electrolysis, these porous current collectors are firmly pressed against the catalytic layers to obtain good electrical contacts. As can be seen from Fig. 1, gas evolution does not take place homogeneously over the surface of the catalytic layers. Instead, gas is collected through

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

2437

cracks located in the neighbourhood of sintered titanium particles, at contact points with the current collectors, and transferred away from the interface through the porous current collectors. When current collectors are pressed too firmly against the MEA, the catalytic layer can be locally crushed and even destroyed (direct contact between current collector and SPE), and this can reduce the lifetime and efficiency of the MEA. 3. Basic electrochemical thermodynamics For an electrochemical reaction at equilibrium, the amount of electricity (n·F·E) needed to perform that reaction is equal to the change of Gibbs free energy Gd of this reaction: Gd − n F E = 0 where

Gd > 0

(1)

n = 2 (number of electrons exchanged during the reaction). F≈96 485 (C mol−1 ). E = thermodynamic voltage (V) required to perform the reaction. Gd = free energy change of the reaction (J mol−1 ). Gd is a function of both operating temperature and pressure and thus: Gd (T, P) = Hd (T, P) − T Sd (T, P) > 0

(2)

Hd (T,P) and Sd (T,P) are respectively the enthalpy change (J mol−1 ) and entropy change (J mol−1 K−1 ) associated with the reaction. Gd (J mol−1 ) of electricity and T·Sd (J mol−1 ) of heat are required to perform that reaction. The thermodynamic electrolysis voltage E in Volt is defined as: E(T, P) =

Gd (T, P) nF

(3)

The thermo-neutral voltage V in Volt is defined as: V (T, P) =

Hd (T, P) nF

(4)

3.1. Thermodynamics of water electrolysis Under standard conditions (T = 298 K, P = 1 bar), the splitting reaction of liquid water is a non-spontaneous transformation: H2 O(liq) → H2 (g) + ½O2 (g)

(5)

Standard free energy, enthalpy and entropy changes for reaction (5) are: Gd ◦ (H2 O) = 237.22 kJ mol−1 ⇒ E ◦ = Gd ◦ (H2 O)/2F

Fig. 2. G(T), H(T) and T. S(T) of the water splitting reaction at P = 1 bar. (—) data for liquid water up to 250 ◦ C.

of the operating temperature facilitates the dissociation of water by decreasing the electrolysis voltage. At room temperature, 15% of the energy required for electrolyzing water comes from heat and 85% from electricity. At 1000 ◦ C, one third comes from heat and two third from electricity. This is why high temperature water electrolysis is interesting when heat is available at such temperatures: the process requires less electricity. Usually, water vapour is electrolysed at medium (250–500 ◦ C) and high (500–1000 ◦ C) temperatures. In the low temperature domain (0–250 ◦ C), liquid water is electrolyzed. Operating temperatures in the 100–250 ◦ C are obtained by increasing the operating pressure. 3.2. Role of pH on cell voltage In acidic media, the following half-cell reactions take place: anode(+) : H2 O(liq) → ½O2 (g) + 2H+ + 2e− (a)

= 1.2293 V ≈ 1.23 V

Hd ◦ (H2 O) = 285.840 kJ mol−1 ⇒ V ◦ = Hd ◦ (H2 O)/2F = 1.4813 V ≈ 1.48 V

Sd ◦ (H2 O) = 163.15 J mol−1 K−1 Hd ◦ (H2 O) kJ of energy are required to electrolyze one mole of water in standard conditions according to reaction (5). This energy is provided as electricity (Gd kJ) and heat (T·Sd kJ). In other words, a cell voltage E = Gd /(2F) = 1.23 V is required and an additional voltage term T·Sd /(2F) = 0.25 V must be added to the thermodynamic voltage E to provide the heat required by reaction (5). Main thermodynamic functions and cell voltages are plotted as a function of temperature in Figs. 2 and 3 respectively. The discontinuity at 100 ◦ C is due to the vaporisation of water. Above 100 ◦ C, the entropy change of the water splitting reaction is reduced and the slope of T·S(T,1) is less than for T < 100 ◦ C. An increase

Fig. 3. Role of operating temperature on the thermodynamic E () and the enthalpy V () cell voltage of electrolysis of water at P = 1 bar.

2438

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

Table 1 Tafel parameters for oxygen evolution reaction. danodic /d(log i) (mV)

˛

Measured i0 A cm−2 (geometrical area)

Measured i0 A cm−2 (real area)

i0 A cm−2 Ref. [40]

25 ◦ C 80 ◦ C

110 130

0.54 0.45

1 × 10−7 2 × 10−5

6 × 10−10 1 × 10−7

1 × 10−9 –

25 ◦ C 80 ◦ C

110 130

0.54 0.45

2 × 10−4 6 × 10−3

1 × 10−6 4 × 10−5

1 × 10−6 –

Electrode Pt

Ir

According to Fig. 5, the anodic overvoltage associated with the OER is larger on Pt than on Ir and Ru. In commercial systems, Ir is preferred as anodic catalyst because of its higher activity over Pt and its higher stability over Ru. At the cathode, Pt is the most commonly used electrocatalyst for the HER. The kinetics are fast and the associated overvoltage is lower (by a factor of 3) compared with that of the OER.

cathode(−) : 2H+ + 2e− → H2 (g) (b) fullreaction : H2 O(liq) → H2 (g) + ½O2 (g) From the Nernst equation, it follows: 1/2

0 E + = EH

2 O/O2

+

2 RPG T (aH+ )(fO2 ) Ln nF aH2 O

(6) 5. Efficiency of electrochemical PEM water electrolyser

At 298 K, when the pressure of oxygen is one bar (ideal gas), E+ ≈ 1.23 − 0.06 pH 0 E − = EH

+ 2 /H

+

a2 + RPG T Ln H ≈ −0.06 pH nF fH2

(7)

At 298 K, when the pressure of hydrogen is one bar (ideal gas), E− ≈ − 0.06 pH Therefore, under the same conditions, the cell voltage Ecell = E+ − E− = 1.23 V. Consequently, the thermodynamic voltage required to split water into hydrogen and oxygen is independent of pH (Fig. 4). 4. Kinetics

5.1. Energetic efficiency The cell efficiency is defined as: ε=

Wt Wr

(8)

where Wt is the theoretical amount of energy required to split water into H2 and O2 , Wr is the real amount of energy, Wr = U cell .I.t

(9)

Ucell is the potential applied to the cell (V), I is the current (A) and t is the duration (s).

PEM water electrolysis technology is used over the −10 to +100 ◦ C temperature range. Over this limited temperature range, the different terms of the cell voltage (anodic and cathodic overvoltages and ohmic drop across the SPE vary by ca. 30% (Fig. 5) [12]. Tafel parameters for the OER are compiled in Table 1. When the current densities are corrected for the roughness of the electrodes, their values are similar to those measured on polished electrodes.

Fig. 4. Electrode potential versus pH for the water splitting reaction.

Fig. 5. Role of operating temperature on the different terms of the cell voltage in a PEM water electrolysis cell.

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

2439

Fig. 6. Cyclic voltammograms obtained using a GC electrode modified with [Co(dmgBF2 )2 ] in aqueous phosphate-buffered solutions at pH 2, 3, 4 and 7 (thanks to L. Berben and J. Peters).

Wt can be defined from the thermodynamic voltage E or from the thermoneutral voltage V: Wt,G = E · I · t

(10)

Wt,H = V · I · t

(11)

Consequently, two different ways are commonly used to express the efficiency of an electrochemical cell. The efficiency based on the thermodynamic voltage and the efficiency based on the thermoneutral voltage: εG = εH =

E U cell V U cell

(12) (13)

In fact, E is based on the low heating value of the water splitting reaction and V on the high heating value. Therefore, it makes sense to calculate the reaction efficiency on V and not E. This is not because the % is higher. This is because, in order to complete the reaction, G J mol−1 of electricity are required and T·S J mol−1 of heat, since heat must be supplied to the cell [41]. In standard conditions, E = 1.23 V and V = 1.48 V. 5.2. Faradaic efficiency The Faradaic efficiency εF relates the quantity of electricity (i) transferred at the electrode to the gas production during electrolysis (dn/dt): εF = 2F

dn/dt i

(14)

At atmospheric pressure, the Faradaic efficiency of a PEM cell is unity because cross-permeation of hydrogen and oxygen is negligible. At high pressure, gas cross-permeation tends to reduce the Faradaic efficiency [42]. 6. Limitation and challenges for PEM cells PEM water electrolysis technology is an efficient process: efficiencies of 85% (calculated from the higher heating value of hydrogen) have been demonstrated at the lab-scale at 1 A cm−2 . The proton exchange membrane (PEM) water electrolysis has a number of advantages over the traditional water-alkaline such as smaller design devices for the production of hydrogen fuel for domestic end-uses, a higher degree of purity of the gases and the possibility

to compress gases within the installation [43]. The main drawback comes from the fact that this is an expensive technology. The replacement of platinum-family catalysts and expensive solid polymer electrolyte remain challenging issues. Some recent advances have been made at the cathodes of PEM cells using non-precious metal electrocatalysts which are presented in the following section. 7. Modified electrodes with non-noble metal complexes Recently, a cornucopia of molecular based metal complexes has been reported as potential candidates for the hydrogen evolution reaction (HER). Herein we focus on the reports where molecular catalysts have been adsorbed on a surface of an electrode together with their electrocatalytic activity rather than providing an extensive review of homogeneous molecular catalytic systems. The reader is conveyed to the following excellent reviews in this field [25,30,44–53]. Peters et al. have recently shown that adsorbed bisdifluoroboryldimethyl glyoximato, ([Co(dmgBF2 )2 ], see below) on glassy carbon electrode, is active towards hydrogen production in aqueous solution at pH < 4.5 with quite low overpotential (240 mV at pH = 2) [51,54,55]. As depicted in Fig. 6, while lowering the pH from 7 to 2, the authors observed an increase in the current density by two orders of magnitude when compared with the bare glassy carbon (GC) electrode. It is worth noticing here that the bulk electrolysis performed on [Co(dmgBF2 )2 ] in aqueous solution at pH 4 using phosphate as buffer resulted in a low Faradaic yield (around 15%) suggesting a probable degradation of the molecular complex under these conditions. The fact that controlled potential electrolysis when the catalyst is adsorbed on GC surface lead to a constant evolution of H2 over a period of more than 7 h allowed the authors to conclude on the gain in efficacy and stability of the molecular catalyst once attached to the surface of the GC electrode. Artero et al. have reported on a sophisticated covalently anchored Dubois’s catalyst on multi walled carbon nanotubes (MWCNT) [48b]. The modified electrode consists of a Nafion® membrane with a gas diffusion layer (GDL) on which was deposited the Ni-functionalised MWCNT at a GC electrode (see Fig. 7). The HER was tested in aqueous medium. The authors observed a current density of 4 mA cm−2 at 300 mV overpotential in a 0.5 M H2 SO4 aqueous solution. The same group recently published a similar system where the catalyst was no longer covalently linked but ␲-stacked on MWCNT through pyrene conjugates [48c]. Comparable catalytic activity towards the hydrogen producing reaction was reported as for the covalently connected catalyst.

2440

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

Fig. 7. Artero’s supported Ni based catalyst on MWCNT.

However, the latter strategy, that is to say, adsorbing the catalyst to the surface of the MWCNT, is more easily achieved rather than the long synthetic route to covalently attach the catalyst to the MWCNT. This is an important issue in the design of cheap devices for the replacement of expensive noble metal catalyst in the electrolyser. Nadjo and coworkers have developed modified surface electrodes based on PolyOxoMetallates (POMs) [H7 P8 W48 O134 ]33− and on cobalt containing silicotungstates [Co6 (H2 O)30 {Co9 Cl2 (OH)3 (H2 O)9 (␤-SiW8 O31 )3 ]5− and [Co3 (B␤-SiW9 O33 (OH))(B-␤-SiW8 O29 OH)2 ]22− [56]. These molecular complexes were fixed on Vulcan® XC72 and deposited on a glassy carbon electrode or entrapped in polyvinylpyridine films on the electrode. The electrocatalytic activity of the [Co6 (H2 O)30 {Co9 Cl2 (OH)3 (H2 O)9 (␤-SiW8 O31 )3 ]5− compound for the HER is depicted in Fig. 8. As we can notice, there is a subsequent shift of the hydrogen evolution onset potential towards more positive potentials with time and a remarkable increase in the current density. Such behaviour was attributed to microenvironment effects that favour both electron and proton accumulation on the framework of the POM’s. The authors also report that such modified electrodes are stable for several months upon storage in normal laboratory conditions or even under acidic conditions and more importantly display similar catalytic activity as freshly prepared electrodes. The authors also raised an important question on the mechanistic issues that still need to be realised to elucidate the nature of the reactive species. Kaneko et al. have developed several graphite coated electrodes

Fig. 9. MoS3 -modified electrode on GC. Polarisation curves of drop-casted MoS3 modified glassy carbon electrodes. Curve a: MoS3 only (loading: 21 ␮g/cm2 ). Curve b: MoS3 blended with MWCNTs (loading: 21 ␮g/cm2 [MoS3 ] and 42 ␮g/cm2 [MWCNTs]). The measurements were conducted at pH = 0 (1.0 M H2 SO4 ); scan rate: 5 mV/s. Thanks to X. Hu.

with molecular complexes such as metalloporphyrins of cobalt, iron and platinum bipyridine complexes dispersed in a Nafion® polymer membrane for the HER [57]. These systems function as competitively as the conventional Pt catalyst. Since the decisive work of Chorkendorff et al. to elucidate the active site of MoS2 nanoparticulate catalytic materials [58], different groups have recently reported on the use of molybdenum and tungsten sulphide materials loaded on glassy carbon electrodes as HER catalysts. Although these materials are not molecular based per se, we took this as an example of a highly active research area in material science with the common goal to replace Pt at the cathode of PEM water electrolysis cells. Hu and coworkers have elegantly described in a series of reports on the electrocatalytic activity of different types of molybdenum sulphide materials for the HER [59,60]. Amorphous molybdenum sulphide (MoSx ) are active catalysts used to elaborate modified electrodes. A highly interesting result from this group is related to MoS3 -modified electrode on GC (Fig. 9). As we can notice the presence of MWCNT greatly improves electrocatalytic performance by enhancing the electron transfer to the catalytic species. The observed current density was 4.8 mA cm−2 with an overpotential of 200 mV when the measurements were conducted at pH = 0 (1.0 M H2 SO4 ) and scan rate 5 mV/s made by drop-casting. Other preparative methods for the fabrication of modified electrodes are also discussed and their catalytic properties are compared. 8. Membrane electrode assemblies for PEM water electrolyser

Fig. 8. Evolution with time of the cyclic voltammograms of [Co6 (H2 O)30 {Co9 Cl2 (OH)3 (H2 O)9 (␤-SiW8 O31 )3 ]5− on Vulcan® XC72, loaded on a glassy carbon electrode in 0.5 M H2 SO4 (initial t = 0 and final t = 3 h). Reprinted with permission from Bineta Keita et al., Langmuir, 2007, 23, 9531–9534. Copyright 2007 American Chemical Society.

Encouraging results have been obtained in designing modified surface electrodes for the HER, but almost no attempt has been made to test these molecular materials in a PEM electrolyser cell under operating conditions. This is a necessary step to show the actual potential of molecular catalysts in a real device. In the following section, we report on our effort to replacing the platinum based electrode at a proton exchange membrane (PEM) cell. For this study, we have used the bisdifluoroboryl-dimethyl glyoximato, [Co(II)(dmgBF2 )2 ] [48,51,54,61–64] and bisfluoroboryltrisdimethyl glyoximato [Co(III)(dmg)3 (BF)2 ]BF4 cobalt complexes [37,65] (Scheme 1). These molecular complexes are molecular precursors for the HER.

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

F

F

F B

O

N

Me

N

N

Me

O

O

Me

N

Me

O

O

Me

N

N

Me

N O

Co

O Me

N Me

BF4-

Co

B F

+

B O

F

[Co(dmgBF2)2]

2441

N O B

N Me

Me

O

F [Co(dmg)3(BF)2]BF4 Scheme 1.

The modified electrodes were prepared by fixing the complexes on glassy carbon electrode (GCE) mixed with Vulcan® XC72 (Cabot Co.) in an acetonitrile solution. After solvent evaporation, the disc was smeared with a perfluorosulfonate ion-exchange polymer (Nafion® 117) to prevent leaching of the complex. Fig. 10 shows the variation of the current density as a function of the applied potential under an argon-saturated solution and in an 0.5 M aqueous solution of H2 SO4 . Traces (a) and (b) are the controlled experiments with the GCE and GCE + Nafion® + Vulcan® respectively indicating the absence of any relevant catalytic current in the scanned potential window. Curves (c) and (d) correspond to the modified electrodes with [Co(dmg)3 (BF)2 ]BF4 and [Co(dmgBF2 )2 ] respectively in presence of Vulcan® and differ distinctively from the previous curves. They exhibit a net increase in the current density, indicating the reactivity towards the reduction of protons to H2 . These values were obtained upon optimisation of the amount of loaded catalyst on the surface of the electrode. This enhancement of the cathodic current was reached after activation of the modified electrode through continuous cycling of the electrode potential between +0.0 and −0.9 V vs SCE and a stable regime was reached after several hours. This observation is reminiscent of the electrocatalytic activity reported by Nadjo et al. for cobalt containing POMs on the same carbon support. Following these interesting electrocatalytic results obtained on the chemically modified electrode in quite harsh acidic conditions, we reasoned that we could implement these complexes at the

Fig. 10. Electrochemical behaviour of (a) a clean glassy carbon electrode (GCE), (b) GCE modified with carbon black (Vulcan® XC72) and Nafion® 117, (c) GCE modified with Vulcan® XC72 (70 wt.%), [Co(dmg)3 (BF)2 ]BF4 (30 wt.%) and Nafion® 117, (d) GCE modified with Vulcan® XC72 (70 wt.%), Co(dmgBF2 )2 (30 wt.%) and Nafion® 117, in a 0.5 M H2 SO4 aqueous solution, scan rate: 10 mV/s.

Fig. 11. Schematic view of the PEM water electrolyser. 1: proton exchange membrane, 2: catalytic layer, 3: porous current collector, 4: bipolar plate (a: anodic, c: cathodic).

cathode of PEM water electrolyser. The main components of PEM water electrolysis cells are (Fig. 11): (i) the proton exchange membrane onto which are platted; (ii) two electrocatalytic layers; (iii) the porous current collectors (mm thick plates of sintered titanium powder); (iv) titanium bipolar plates. In a PEM cell, electric current flows from one bipolar plate (4a in Fig. 11) to the next one (4c in Fig. 11). At the same time, mass transfer phenomena are taking place across the porous current collectors (3a and 3c in Fig. 11). In particular, the two gases formed during the electrochemically driven OER and HER (hydrogen and oxygen) are transferred from the catalytic layers to the collecting channels previously machine-made in the thickness of bipolar plates. From a qualitative viewpoint, current collectors of large porosity will facilitate gas removal from the interfaces but will also increase the ohmic resistance of the plates and introduce additional parasitic ohmic losses at contact points between current collector and catalytic layers (front sides) and between current collectors and channels (backsides). In this study, we have designed a 7 cm2 experimental cell, using characteristic features of a PEM water electrolyser. A thin proton-conducting membrane played the role of the solid polymer electrolyte sandwiched between two thin layers of catalyst, on the one side the HER catalyst and the OER catalyst on the other. The carbonaceous substrate used as electronic carrier for the cobalt complexes was Vulcan® XC72 while the porous current collectors were millimetre thick titanium plates made of sintered titanium spherical particles (mean diameter 100 ␮m) [38]. H2 and O2 are evacuated through the backside of the cell. Different membraneelectrode assemblies (MEAs) were prepared and tested. Typical catalyst loading was ca. 2.5 mg cm−2 for the cobalt complexes. The anode was prepared by mixing black iridium with a solution of Nafion® 117 ionomers and was directly applied on the surface of the current collector (Table 2). Prior to running the electrocatalytic experiments, the carbonaceous materials were imaged by Scanning and Transmission Electronic Microscopy (SEM). The SEM images of the carbonaceous/cobalt complexes mixture show a homogeneous deposit without crystal formation due to cobalt complexes at the surface of Vulcan® XC72 (Fig. 12).

2442

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

Fig. 12. SEM image of (a) Vulcan® XC72, (b) [Co(dmg)3 (BF)2 ]BF4 -Vulcan® XC72 and (c) [Co(dmgBF2 )2 ]-Vulcan® XC72.

Fig. 13. EDS spectra of (a) [Co(dmg)3 (BF)2 ]BF4 -Vulcan® XC72 and (b) [Co(dmgBF2 )2 ]-Vulcan® XC72. Reference is superimposed (in blue) showing the presence of iron and copper due to the sample holder and grid.

Transmission electron microscopy (TEM) was further used to characterise the carbonaceous material. The samples were prepared by suspension in hexane under sonication to ensure no desorption of the catalyst prior to the deposition on the TEM grids, and observed at 80 kV in a Philips TE microscope. Nanoparticles of carbon black are observed, as in the reference Vulcan® XC72 sample, with two populations around 50–100 nm and below 20 nm. The adsorbed molecular cobalt complexes were not imaged due to the weak content in metal and the fine division of the deposition at the surface of the carbon particles. However, Energy dispersive X-ray Spectroscopy (EDS) performed on the same TEM grids indicates the presence of the corresponding complex on the Vulcan® particles where cobalt and fluorine are detected. Energy Dispersive X-ray Spectroscopy confirmed the presence of cobalt species in the materials. Similar experiments after the catalytic runs also show the presence of cobalt (see Fig. 13). Fig. 14 shows the measured polarisation curves during water electrolysis on different MEAs (curves are corrected for the ohmic resistance of the measurement cell). For comparison we have

plotted the curve (a) which represents the established electrochemical performance obtained with black iridium as catalyst for the OER and with platinum for the HER catalyst. Curve (d) corresponds to the case where iridium at the anode is being replaced by platinum (nanoparticles) and clearly indicates the higher overpotential for the OER at the Pt catalyst. For this reason, we have used the iridium based catalyst at the anode. Curves (b) and (c) attest that the water electrolysis is achieved when the platinum based cathode has been replaced by the MEAs

Table 2 Catalyst mixtures at each electrode in PEM configuration cell. Curve

Anode

a b c

Ir 2.5 mg cm−2

e d

Pt 2.5 mg cm−2

Cathode Pt 1 mg cm−2 Vulcan® 1.5 mg cm−2 [Co(dmg)3 (BF)2 ]BF4 2.5 mg cm−2 Vulcan® 1.5 mg cm−2 [Co(dmgBF2 )2 ] 2.5 mg cm−2 Vulcan® 1.5 mg cm−2 Co(acac)3 3 mg cm−2 Vulcan® 1.5 mg cm−2 Pt 1 mg cm−2 Vulcan® 1.5 mg cm−2

Fig. 14. Current–voltage performances at a 7 cm2 single cell with different MEAs: (a) Ir(O2 )/Nafion® 117/Pt(H2 ), (b) Ir/Nafion® 117/[Co(dmg)3 (BF)2 ]BF4 -Vulcan® XC72, (c) Ir/Nafion® 117/[Co(dmgBF2 )2 ]-Vulcan® XC72, (d) Pt/Nafion® 117/Pt, (e) Ir/Nafion® 117/[Co(acac)3 ]-Vulcan® XC72. Experiments were carried at 60◦ and P = 1 atm.

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

with [Co(dmg)3 (BF)2 ]BF4 and [Co(dmgBF2 )2 ] complexes respectively. As we can notice, the electrochemical performance of these systems is lower than that with the nanoparticles of platinum (a) when iridium is used as catalyst at the anode. The shift of the curves (b) and (c) to higher voltage corresponds to the higher overpotential of the cobalt catalysts when compared with platinum. However, the observed current densities are very interesting: at 60 ◦ C, current densities of ca. 640 and 500 mA.cm−2 for the MEAs with [Co(dmg)3 (BF)2 ]BF4 and [Co(dmgBF2 )2 ] respectively when the cell voltage corresponds to a, enthalpic efficiency of εH = 80% (at 60 ◦ C, V = 1.42 V), under similar experimental conditions. Hence, the observed values are to be compared with the best electrolysis efficiencies (εH = 80% at 1.2 A cm−2 , using noble metal loading of ca. 0.5 mg cm−2 of Pt for the HER and ca. 2 mg cm−2 of Ir for the OER. The hydrogen flow of the PEM water electrolyser can be directly obtained from the Faraday’s Law. As mentioned above, it is well established that the faradaic efficiency of a PEM water electrolyser under atmospheric pressure is unity. This is confirmed by our measurement of the amount of H2 evolving at the cathode with an Ucell 1.7 V [42]. We also performed a control experiment using the cobalt(III) acetylacetonate complex to prepare a MEA. As depicted in Fig. 14 curve (e), no significant water electrolysis was found within the applied voltage range, thus pertaining to the fact that the electrocatalytic activity for the hydrogen production stems from the initial use of cobalt oxime complexes. The current measured below 2 V for the Co(acac)3 case, is mainly due to the large specific area of the carbon carrier. This is a clear indication that Co(acac)3 has no significant catalytic activity. Based on these facts, we can conclude that the activity of curves (b) and (c) is directly related to the use of cobalt glyoxime compounds and not only to the presence of cobalt. Moreover, from the Pourbaix diagram of cobalt, we can rule out the presence of pure cobalt particles or cobalt oxides as the origin for such electrocatalytic activity, as these species would dissolve under our experimental conditions. Electrochemical Impedance Spectroscopy (EIS) was used to characterise the behaviour of the PEM electrolyser [66]. This technique permits to discriminate between the different contributions brought about by the individual processes i.e. HER and OER towards the determination of the overall performance of PEM electrolyser. The experimental impedance data were analysed using Nyquist plots and provide in situ information about the PEM cell operating in real water electrolysis conditions. For the analyses of the EIS data, we modelled our cell as being constituted of two RC circuits (resistor and capacitance which represent the charge transfer resistance and the double layer capacitance at each electrode) connected in series with a resistance [67]. The Zi (imaginary impedance) versus Zr (real impedance) Nyquist plot contains two semi-circles. The first semi-circle with the smaller time constant ( 1 = R1 ·C1 ) can

2443

Fig. 15. (i) Experimental data (dotted line) and simulation (solid line) for kinetic parameters identification, Nyquist plot was obtained for Ucell = 1800 mV. Simulation gives RHF = 0.672  cm2 , R1 = 0.777  cm2 , R2 = 1.773  cm2 (ii) Nyquist plots of impedance diagrams measured at 60 ◦ C on a Ir/Nafion® 117/[Co(dmg)3 (BF)2 ]BF4 Vulcan® XC72 MEA at different cell voltage (mV): (a) 1900, (b) 1850, (c) 1800, (d) 1750, (e) 1700, (f) 1675, and (g) 1650.

be attributed to the HER given the faster kinetic while the second one to the OER ( 2 = R2 ·C2 ). The cell impedance was measured as a function of cell voltage (Fig. 15 (ii)) and was predominantly determined by the second semi-circle (OER) at low cell voltages. Such behaviour can be related to the classical Ir/Pt based electrolyser. The charge transfer resistance R2 gradually decreases as the cell voltage increases whereas R1 remains almost constant. This is clear evidence that the cobalt containing carbonaceous materials behave as competitive systems for the catalytic property for the HER, although kinetics of the reaction are faster on Pt. The impedance at high frequency (HF) is real and is attributed to the total ohmic resistance of the setup (the electrolyser cell, the polymer electrolyte, the cable connections and wires). Nyquist plots obtained during impedance measurements show that at low frequencies, impedances are real, supporting the fact that there is no limitation of the electrocatalytic reactions due to mass transfer [66]. This implies that even the transport of H2 and O2 away from the interfaces is taking place in the process, and does not impact on the overall kinetics.

Fig. 16. Change in cell voltage during continuous operation at 200 mA cm−2 . The cell (b) Ir/Nafion® 117/[Co(dmg)3 (BF)2 ]BF4 -Vulcan® XC72, (c) Ir/Nafion® 117/[Co(dmgBF2 )2 ]-Vulcan® XC72.

was

run

on

an

average

7 h/day:

(a)

Ir/Nafion® 117/Pt,

2444

M.T. Dinh Nguyen et al. / Coordination Chemistry Reviews 256 (2012) 2435–2444

In order to assess the durability of these cells, electrolyses were run for 7 h periods over several days. Fig. 16 shows the evolution of the functioning cell voltage of the electrolysers at 200 mA cm−2 over a period of 6 days. The performance of a Pt/Ir cell within the same configuration is given for comparison. As an overall consideration, we found that the MEAs with the cobalt complexes are less efficient than the platinised cathode. However, our results clearly show that the electrochemical performance of the electrolysers remains stable and maintains their activity even after repeated on and off polarisation. 9. Conclusion We have reported here on the recent development of modified electrodes with molecular based complexes for the HER. The target behind this research is to furnish low cost and highly efficient modified electrodes to replace the expensive and nonsustainable actual technology for water electrolysis. Promising results have already been obtained on the electrocatalytic properties of modified electrodes. We also report on our attempts to implement on non-noble metal molecular complexes at the cathode of proton exchange membrane water electrolysers. We found that cobalt based complexes show interesting current densities in the water electrolyser where the anode still consists of iridium. These electrolysers endured several hours of work without loss of the electrocatalytic activity. We believe that this is the start of a challenging task to bring molecular based compounds as precursors to replace the current technology. However, much work is still needed to understand the real nature of the catalytic species at the surface of the electrodes. Work along this line is ongoing in our labs. Acknowledgements This work was supported by ANR-TechBiophyp and CNRS PR Commerce H2 . References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

http://www.sd-commission.org.uk/publications.php. R. Eisenberg, D.G. Nocera, Inorg. Chem. 44 (2005) 6799. N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci U.S.A. 103 (2006) 15729. D.G. Nocera, Daedalus 135 (2006) 112. H.B. Gray, Nat. Chem. 1 (2009) 7. T.K. Mandal, D.H. Gregory, Proc. IMEC J. Mech. Eng. Sci. 224 (2010) 539. J.A. Turner, Science 305 (2004) 972. L. Duan, L. Tong, Y. Xu, L. Sun, Energy Environ. Sci. 4 (2011) 3296. C. Herrero, A. Quaranta, W. Leibl, A.W. Rutherford, A. Aukauloo, Energy Environ. Sci. 4 (2011) 2353. S.Y. Reece, J.A. Hamel, K. Sung, T.D. Jarvi, A.J. Esswein, J.J.H. Pijpers, D.G. Nocera, Science 334 (2011) 645. D. Gust, T.A. Moore, A.L. Moore, Faraday Discuss. (2012) 9. P. Millet, R. Durand, M. Pineri, Int. J. Hyd. Energy 15 (1990) 245. A. Michas, P. Millet, J. Membr. Sci. 61 (1991) 157. P. Millet, T. Alleau, R. Durand, J. Appl. Electrochem. 23 (1993) 322. A.J. Bard, J. Am. Chem. Soc. 132 (2010) 7559. T.R. Cook, D.K. Dogutan, S.Y. Reece, Y. Surendranath, T.S. Teets, D.G. Nocera, Chem. Rev. 110 (2010) 6474. F. Li, Y. Jiang, B. Zhang, F. Huang, Y. Gao, L. Sun, Angew. Chem. Int. Ed. 51 (2012) 1. J.J. Concepcion, J.W. Jurss, M.K. Brennaman, P.G. Hoertz, A.O.v.T. Patrocinio, N.Y. Murakami Iha, J.L. Templeton, T.J. Meyer, Acc. Chem. Res. 42 (2009) 1954. W.J. Youngblood, S.-H.A. Lee, K. Maeda, T.E. Mallouk, Acc. Chem. Res. 42 (2009) 1966.

[20] M. Hambourger, G.F. Moore, D.M. Kramer, D. Gust, A.L. Moore, T.A. Moore, Chem. Soc. Rev. 38 (2009) 25. [21] S. Roeser, P. Farràs, F. Bozoglian, M. Martínez-Belmonte, J. Benet-Buchholz, A. Llobet, ChemSusChem 4 (2011) 197. [22] Y. Xu, L. Duan, L. Tong, B. Akermark, L. Sun, Chem. Commun. 46 (2010) 6506. [23] Y. Xu, A. Fischer, L. Duan, L. Tong, E. Gabrielsson, B. Åkermark, L. Sun, Angew. Chem. Int. Ed. 49 (2010) 8934. [24] M. Yagi, M. Kaneko, Chem. Rev. 101 (2000) 21. [25] A.J. Esswein, D.G. Nocera, Chem. Rev. 107 (2007) 4022. [26] P. Kurz, G. Berggren, M.F. Anderlund, S. Styring, Dalton Trans. (2007) 4258. [27] K. Sanderson, Nature 452 (2008) 100. [28] S. Romain, L. Vigara, A. Llobet, Acc. Chem. Res. 42 (2009) 1944. [29] M. Yagi, A. Syouji, S. Yamada, M. Komi, H. Yamazaki, S. Tajima, Photochem. Photobiol. Sci. 8 (2009) 139. [30] H.I. Karunadasa, C.J. Chang, J.R. Long, Nature 464 (2010) 1329. [31] J.L. Fillol, Z. Codolà, I. Garcia-Bosch, L. Gomez, J.J. Pla, M. Costas, Nat. Chem. 3 (2011) 807. [32] L.L. Tinker, N.D. McDaniel, S. Bernhard, J. Mater. Chem. 19 (2009). [33] C. Fan, D.L. Piron, A. Sleb, P. Paradis, J. Electrochem. Soc. 141 (1994) 382. [34] I. Abe, Energy Carriers and Conversion Systems, vol. 1, EOLSS, 2008, p. 131. [35] Y. Choquette, H. Ménard, L. Brossard, Int. J. Hyd. Energy 15 (1990) 21. [36] K.A. Mauritz, R.B. Moore, Chem. Rev. 104 (2004) 4535. [37] E. Anxolabehere-Mallart, A. Aukauloo, P. Millet, O. Pantani, C.n.d.l.r. Scientifique, vol. 11, U.P.S., France, 2009. [38] S.A. Grigoriev, P. Millet, S.A. Volobuev, V.N. Fateev, Int. J. Hyd. Energy 34 (2009) 4968. [39] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Int. J. Hyd. Energy 31 (2006) 171. [40] A. Damjanovic, A. Dey, J.O.M. Bockris, J. Electrochem. Soc. 113 (1966) 739. [41] K. Onda, T. Kyakuno, K. Hattori, K. Ito, J. Power Sources 132 (2004) 64. [42] J. Zhang, L. Zhang, H. Liu, A. Sun, R.-S. Liu, Electrochemical Technologies for Energy Storage and Conversion, Wiley-VCH, 2012. [43] S.A. Grigoriev, I.G. Shtatniy, P. Millet, V.I. Porembsky, V.N. Fateev, Int. J. Hyd. Energy 36 (2011) 4148. [44] P. Connolly, J.H. Espenson, Inorg. Chem. 25 (1986) 2684. [45] M. Schmidt, S.M. Contakes, T.B. Rauchfuss, J. Am. Chem. Soc. 121 (1999) 9736. [46] F. Gloaguen, J.D. Lawrence, T.B. Rauchfuss, J. Am. Chem. Soc. 123 (2001) 9476. [47] S. Ott, M. Kritikos, B. Åkermark, L. Sun, R. Lomoth, Angew. Chem. Int. Ed 43 (2004) 1006. [48] (a) M. Razavet, V. Artero, M. Fontecave, Inorg. Chem. 44 (2005) 4786; (b) A. Le Goff, V. Artero, B. Jousselme, P.D. Tran, N. Guillet, R. Métayé, A. Fihri, S. Palacin, M. Fontecave, Science 326 (2009) 1384; (c) P.D. Tran, A. Le Goff, J. Heidkamp, B. Jousselme, N. Guillet, S. Palacin, H. Dau, M. Fontecave, V. Artero, Angew. Chem. 123 (2011) 1407. [49] L. Sun, B. Åkermark, S. Ott, Coord. Chem. Rev 249 (2005) 1653. [50] A.D. Wilson, R.H. Newell, M.J. McNevin, J.T. Muckerman, M. Rakowski DuBois, D.L. DuBois, J. Am. Chem. Soc. 128 (2005) 358. [51] X. Hu, B.S. Brunschwig, J.C. Peters, J. Am. Chem. Soc. 129 (2007) 8988. [52] S. Losse, J.G. Vos, S. Rau, Coord. Chem. Rev. 254 (2010) 2492. [53] Y. Sun, J.P. Bigi, N.A. Piro, M.L. Tang, J.R. Long, C.J. Chang, J. Am. Chem. Soc. 133 (2011) 9212. [54] X. Hu, B.M. Cossairt, B.S. Brunschwig, N.S. Lewis, J.C. Peters, Chem. Commun. (2005) 4723. [55] L.A. Berben, J.C. Peters, Chem. Commun. 46 (2010) 398. [56] B. Keita, U. Kortz, L.R.B. Holzle, S. Brown, L. Nadjo, Langmuir 23 (2007) 9531. [57] F. Taguchi, T. Abe, M. Kaneko, J. Mol. Catal. A: Chem. 140 (1999) 41. [58] T.F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff, Science 317 (2007) 100. [59] D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2 (2011) 1262. [60] D. Merki, X. Hu, Energy Environ. Sci. 4 (2011) 3878. [61] C. Baffert, V. Artero, M. Fontecave, Inorg. Chem. 46 (2007) 1817. [62] O. Pantani, E. Anxolabéhère-Mallart, A. Aukauloo, P. Millet, Electrochem. Commun. 9 (2007) 54. [63] J.L. Dempsey, B.S. Brunschwig, J.R. Winkler, H.B. Gray, Acc. Chem. Res. 42 (2009) 1995. [64] V. Artero, M. Chavarot-Kerlidou, M. Fontecave, Angew. Chem. Int. Ed. 50 (2011) 7238. [65] O. Pantani, S. Naskar, R. Guillot, P. Millet, E. Anxolabéhère-Mallart, A. Aukauloo, Angew. Chem. Int. Ed. 47 (2008) 9948. [66] A. Lasia, B.E. Conway, J.O.M. Bockris, R.E. White, Electrochemical impedance spectroscopy and its applications, in: Modern Aspects of Electrochemistry, Springer, US, 2002, p. 143. [67] P. Millet, N. Mbemba, S.A. Grigoriev, V.N. Fateev, A. Aukauloo, C. Etiévant, Int. J. Hyd. Energy 36 (2011) 4134.