Paulo Casal

Javier Castro-Jiménez

Mariana Pizarro

Athanasios Katsoyiannis

Jordi Dachs

Seasonal soil/snow-air exchange of semivolatile organic pollutants at a coastal arctic site (Tromsø, 69°N)

teaching and research institutions in France or abroad, or from public or private research centers.


Emmanuel Boss

Anya Waite

Frank Muller-Karger

Hidekatsu Yamazaki

Rik Wanninkhof

Julia Uitz

Sandy Thomalla

Heidi Sosik

Bernadette Sloyan

Anthony Richardson

Patricia Miloslavich

Johannes Karstensen

Gérald Grégori

Katja Fennel

Herve Claustre

Marcela Cornejo

Ilana Berman-Frank

Sonia Batten

Silvia Acinas

Beyond Chlorophyll FluoresCenCe The Time is right to expand Biological Measurements in ocean observing programs

A new Scientific Committee for Ocean Research (SCOR, http://www.scor-int.org/) working group has been formed, entitled SCOR WG-154 "Integration of Plankton-Observing Sensor Systems to Existing Global Sampling Programs (P-OBS, http://www.scor-int.org/SCOR_WGs_WG154. htm.)." The working group (P-OBS WG) is reviewing biological sensing technologies and measurements that are ready for integration into existing regional and global ocean observing programs. Multidisciplinary sets of measurements, whose choice is guided by research and societal benefit goals, will transform our understanding of ocean biology and its impacts on Earth systems.

The working group is currently considering measurements from two globally co-ordinated ocean observing networks: The Global Ocean Shipped-Based Hydrographic Investigations Program (GO-SHIP), http://www.go-ship.org/ and OceanSITES, http://www.oceansites.org/. GO-SHIP co-ordinates trans-basin ship surveys that are repeated at least once every 10 yr per transect. OceanSITES co-ordinates full ocean depth time series observations from moorings and repeat ship visits. Both programs are represented in the P-OBS WG, along with the Continuous Plankton Recorder (CPR) program, https://www.sahfos. ac.uk/ (the longest plankton observing system in the world), and the BGC-Argo-Biogeochemical Argo program (BGC-Argo program). BGC-Argo is implementing a global network of profiling floats equipped with bio-optical and biogeochemical sensors. Other P-OBS WG members cover areas such as remote sensing, in situ plankton measurement by imaging, genomics approaches, and biogeochemical modeling.

The P-OBS WG will help the community document the many recent advances in ocean sampling. For example, a global focus on genomic sampling of the oceans began with Venter's Global Ocean Sampling Expedition, which targeted surface waters of the North-West Atlantic andEastern Tropical Pacific (2004-2006). It was followed by the Tara Oceans Expedition (2009)(2010)(2011)(2012)(2013), which standardized sampling protocols and genomic analysis. Together with the Malaspina expedition (2010-2011), these programs dramatically expanded the geographical extent of genetic sampling of the ocean. They sampled temperate coastal and open oceans, the polar oceans, and both surface and deep waters, including mesopelagic and bathypelagic depths. Ocean time-series of genomic data have been expanding rapidly (e.g., Bermuda Atlantic Time Series, http://bats.bios.edu/; Integrated Marine Observing System, http://imos.org.au/; Hawaii Ocean Time series, http://hahana.soest.hawaii. edu/hot/; Carbon Retention In A Colored Ocean, http://imars.marine.usf.edu/cariaco; FRAM, https://www.awi.de/en/expedition/observatories/ocean-fram.html; and Marine Biodiversity Observation Network, https://ioos.noaa.gov/ project/bio-data/). These and other programs have accelerated the availability of genetic data for the oceans. Standardizing, managing, and interpreting these genomic data is an emerging challenge for biologists and oceanographers.

Like genomics, in situ plankton imaging holds great promise. Yet, this approach brings major data demands and a requirement for community consensus. Commercial imaging systems have matured to the point where they are deployed regularly on profiling rosettes, on moorings, and in flow-through systems. These measurements allow high-resolution mapping of organisms and particles which are transforming our understanding of their distribution in the ocean. Pigment data and particulate organic carbon samples are also measured routinely and are being collected in conjunction with validation of ocean color remote sensing and calibration of sensors on autonomous platforms, such as the floats of the BGC-Argo program (many of these floats are deployed from GO-SHIP cruises).

Bio-acoustics and bio-optical sensors are mature technologies used on both moorings and research vessels. While the collection of such An example of a flow-through system that could be incorporated on GO-SHIP cruises as one component to study plankton. It includes an automated microscope to capture the community of chlorophyll containing particles (McLane's IFCB), a spectro-fluorometer and fast-repetition-rate-fluorometer (WETLabs's ALFA) to study phytoplankton physiology, a particle sizer (Sequoia Scientific's LISST-100X) and a series of bio-optical instruments (WETLabs' AC-S, ECO-BB3, and CDOM fluorometer) to study the signature of upper-ocean plankton on the ocean color signals measured from space as well as constrain photosynthesis. Ocean water to this system is pumped with a diaphragm pump to minimize the impact on particles and is passed through a thermos-salinograph (SeaBird's TSG) to provide environmental information. This system has been installed around a sink of the R/V Atlantis as part of the NASA-funded NAAMES project (Photo credit: Lee Karp-Boss).

data are now standard, the quality control and interpretation of these data still require significant expertise and no global standards exist. By co-ordinating within these global sampling programs, relevant expertise can be shared to compile Best Practices for plankton observations. Finally, flow-cytometric analysis of marine water samples is a very powerful tool to detect, discriminate, and quantify auto-and heterotrophic bacteria, eukaryotic micro-organisms, and viruses. It has expanded to include robust commercial ocean-going instruments, including some that can be deployed in situ (with remote control from the lab as-needed), including on moorings and in flowthrough systems. Some flow cytometers can sort cells based on optical properties related to their size, granularity, structure (nucleic acid, proteins, lipid content) and physiological characteristics, allowing for further quantification of sub populations of particles. Imaging cells is also possible with some flow cytometers-these pictures can provide morphological characterization of the particles (cells) and enhance identification capabilities, especially for microplankton. Conversion of flow cytometry data to biovolume provides a direct pathway to quantify group-specific biomass.

The advances detailed above motivated the formation of P-OBS WG. Its main charge is to identify best practices (technologies and sampling protocols) allowing the incorporation of planktonrelated measurements into global ocean observing platforms (initially GO-SHIP and subsequent expansion into the mooring array of OceanSITES).

Among the challenges that P-OBS WG has identified is the lack of standardization and protocols to perform automated quality control, data formatting, and immediate delivery of validated observations to open-use databases (e.g., OBIS). A major challenge is to prepare such global databases for a quantum jump in data volume once these protocols and data pipelines can be streamlined to deliver data.

Together, we hope to facilitate biological sampling of the oceans and promote more robust, systematic, and routine analyses. These data will establish a baseline from interoperable and comparable datasets, and facilitate the identification of spatial gradients and temporal trends in biodiversity and other key biological parameters. The data are needed to help in the calibration of autonomous platforms, contribute to the validation of remote-sensing algorithms, and constrain biogeochemical and ecosystem models of the ocean. Finally, these data are required to understand and quantify the benefits that the ocean provides to society, and how these benefits are being impacted by global change.

The P-OBS WG seeks help in identifying practical and ready-to-use plankton observation methods that use acoustics, including use of the established Acoustic Doppler Current Profiler on the GO-SHIP and OceanSITES platforms, and other programs. More generally, we invite the oceanographic community to provide information to ensure our findings represent existing and ready-to-use methodologies for plankton observations that could be readily integrated into global sampling programs. Information can be provided to any member of our WG.

ACKNOWLEDGMENT

The idea to establish the P-OBS SCOR working group was conceived following discussions at the Global Ocean Observing System (GOOS) workshop on implementation of Multi-Disciplinary Sustained Ocean Observations (IMSOO, http://www.goosocean.org/index.php? option=com_oe&task=viewEventRecord&even tID=1825), funded by NSF, NASA, and NOAA.


J Come

M Naguib

P Rozier

M W Barsoum

Y Gogotsi

P.-L Taberna

M Morcrette

P Simon
email: simon@chimie.ups-tlse.fr

A Non-Aqueous Asymmetric Cell with a Ti 2 C-Based Two-Dimensional Negative Electrode

AnewMXene phase, Ti 2 C, obtained by aluminum extraction from Ti 2 AlC and exfoliation of the reaction product, was electrochemically studied vs. Lithium. Li-ions insertion into the 2-D structure was characterized by in situ XRD measurements. Additional electrochemical kinetic characterizations of Ti 2 C, using a cavity micro-electrode, showed that the electrochemical reactions involve two different phenomena: one diffusion-limited, the other not. A Ti 2 C/activated carbon asymmetric cell was assembled to highlight the high rate performance of the MXene. The cell was cycled between 1.0 V and 3.5 V, and showed good capacity retention during 1000 galvanostatic charge/discharge cycles at rates up to 10C.

Among energy storage devices, electrochemical double layer capacitors (EDLCs) and Li-ion batteries (LIBs) are considered to be the best in terms of performance, durability and safety. 1,[START_REF]Linden's Handbook of Batteries[END_REF] LIBs possess energy densities of up to 200 Wh • kg -1 but at the expense of limited power capability and cyclability. [START_REF]Linden's Handbook of Batteries[END_REF] EDLCs, on the other hand, are characterized by power densities of more than 10 kW • kg -1 , for a quasi-infinite number of charge/discharge cycles, [START_REF] Conway | Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications[END_REF][START_REF] Simon | [END_REF] EDLC's however, suffer from low energy densities (∼5 Wh • kg -1 ), because they are limited by the charge storage at active materials' surface. As a result, tremendous efforts are currently being devoted for increasing the energy density of supercapacitors, defined as:

E = 1 2 • C dl V 2 ( 1
)
where E is the energy (J), C dl is the double layer capacitance (F) and V is the operating voltage (V). Tailoring novel carbon nanostructures, [5][6][7] matching ion size with pore size, [8][9][10][11] using pseudocapacitive materials with fast surface redox reactions, [12][13][14][15] and increasing the voltage window using ionic liquid electrolytes 9,16 are ways being explored to improve EDLCs' performance. Non-aqueous asymmetric electrochemical capacitors have been proposed as an alternative way to overcome the low energy density of conventional EDLCs. [START_REF] Conway | Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications[END_REF][17][18][19][20][21] This technology is based on the association of two materials involving distinct charge storage mechanisms, namely an activated carbon capacitive electrode and a Li-ion insertion faradaic electrode, both immersed in an organic electrolyte. Amatucci and et. al. 17 were the first to demonstrate that such devices possess higher power densities than LIBs, while showing higher energy densities than EDLCs. They used nanocrystalline lithiated titania (LTO) as the active negative electrode and activated carbon as the positive electrode. The cell was cycled between 1.5 V and 3.0 V in an acetonitrile electrolyte up to a rate of 50C relative to LTO, and was able to deliver 25 Wh • kg -1 , with excellent cycling stability. JM Energy commercializes a 3.8 V carbon graphite/activated carbon asymmetric cell exhibiting 10 Wh • kg -1 of packaged cell. 18 Commercially available Li-ion capacitors have gravimetric energy densities of about 20 Wh • kg -1 , which is about three times higher than that of conventional carbon/carbon EDLCs. 19 These results opened the door to a wide choice of Li ions insertion materials exhibiting high performance rates.

Obviously the choice of the faradaic electrode is of highest importance since it sets the kinetics and rate performance of the cell. Herein we report results on the design of a non-aqueous asymmetric cell -based on a new two-dimensional (2-D) transition metal carbide material as the negative electrode, combined with a positive electrode of activated carbon, operating between 1.0 and 3.5 V.

Recently, 2-D nanocrystals of transition metal carbides were synthesized by exfoliation of ternary carbides with a M n+1 AX n formula, where M is an early transition metal, A is an A-group element and X is carbon and/or nitrogen, so-called MAX phases. 22 The exfoliation process was carried out by selective etching the A layer using hydrofluoric acid, HF, at room temperature and ending up with 2-D layers of M n+1 X n . We labeled these new phases MXene to emphasize the loss of A from MAX, and their similarity to graphene. The material of interest in this study is obtained by the exfoliation of Ti 2 AlC. The post-exfoliation material obtained contained oxygen, hydroxide and fluoride groups at the surface of the Ti-layers. 23 Hence, the formulae of the material is assumed to be Ti 2 CO x H y F z , and will henceforth be refered to as Ti 2 C for convenience, keeping in mind that oxygen/OH/F surface groups are present.

More recently, MXene showed promising performance as an anode material in LIBs. 24 Ti 2 C was used in that study vs. lithium; the electrochemical Li + ions insertion gave stable capacities of 160 mAh • g -1 at C/10 rate and 70 mAh • g -1 at 10C rate for more than 200 cycles. It was proposed that the lithiation and delithiation reaction was:

Ti 2 CO x + yLi + + ye -↔ Li y Ti 2 CO x (2)

Experimental Details

The Ti 2 C powders were obtained by immersing commercially purchased -325 mesh, Ti 2 AlC powders (3-ONE-2, Voorhees, NJ, > 92 wt.% purity) in 10% HF (Fisher Scientific, Fair Lawn, NJ) for 10 h at room temperature, as described elsewhere. 25 The obtained Ti 2 C material was subsequently mixed with carbon black (TimCal super C65) and polyvinylidene fluoride (PVDF) binder in a 80:10:10 weight ratio in acetone. The slurry was then cast onto a copper foil current collector, from which 12 mm diameter disks were punched.

The electrode loading was comprised of between 2.5 and 3.5 mg • cm -2 . Commercially available microporous activated carbon powder (YP17 Kuraray Chemical Co., LTD, Japan) -obtained from carbonization of coconut shell 9 -with a specific surface area of 1700 m 2 • g -1 was used as positive electrode. The powder was mixed with 5 wt.% polytetrafluoroethylene (PTFE) binder (60 wt.% in H 2 O, Aldrich) in ethanol. The paste was cold rolled and 14 mm diameter disks were punched. The mass loading was then adjusted depending on the negative active material mass. The freestanding electrode was placed onto a carbon-treated aluminum current collector to decrease the contact resistance. 26 Finally, the electrodes were dried at 120 • C in vacuum for 12h. Two-electrode CR2016 coin cells, and three-electrode nylon Swagelok cells consisting of a Ti 2 C negative electrode and an activated carbon positive electrode separated by a glass fiber separator (Whatman, GF/A) saturated with 1M LiPF 6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) 1:1 electrolyte (LP30, Merck) were tested. For the three-electrode cells, a Li metal foil was used as the reference.

High rate electrochemical kinetic characterizations were performed using a cavity microelectrode (CME) obtained from the French network "CNRS microelectrode à cavité". The CME was described elsewhere. 27 In this case, the Ti 2 C powder was mixed with the same conductive carbon black (Timcal Super C65) -in a 30 wt.% ratio to ensure good electrical contact -and manually pressed inside the microcavity.

All cells were assembled in an Ar-filled glove box (MBraün, O 2 < 0.1 ppm, H 2 O < 0.1 ppm) to avoid any moisture contamination. Charge/discharge galvanostatic cycling and cyclic voltammetry tests were performed using a VMP3 cycler (Biologic, S.A.) at different C rates, considering 1 Li + exchanged per n hour at C/n.

Electrochemical in situ X-ray diffraction, XRD, patterns of the Ti 2 C powder were collected on a Brucker D8 diffractometer using a Cu Kα radiation (λ = 1.5406 Å) in the range 2θ = 7-40 • with a step of 0.02 • . The sample was placed in a Swagelok-type cell equipped with a beryllium window which also acted also as current collector, allowing in-situ XRD recording (cell from LRCS, Amiens University). The scans were recorded during a relaxation period in steps, every y = 0.2 (from Li y Ti 2 CO x in equation ( 2)), upon charge (reduction) and discharge (oxidation), using a rate of C/10.

Results and Discussion

Electrochemistry of the negative electrode: Ti 2 C.-Two-electrode cells.-Figure 1 shows the cyclic voltammetry (CV) curves of Ti 2 C and carbon black at 0.1 mV • s -1 . To determine the contribution of carbon black to the total capacity of the film, currents are normalized to the same amount of C-black present in the Ti 2 C:carbon black electrode. The Ti 2 C material showed broad lithiation peaks at 1.8 V and 1.0 V and delithiation peak at 2.0 V both vs. Li + /Li. The CV of the carbon black alone (dotted curve in Fig. 1) showed increased current, with no peaks, at potentials below 1.2 V vs. Li + /Li. This is attributed to lithiation of the disordered structure of the carbon black in the electronically non-equivalent insertion sites. It is known that carbon blacks have a structure made of graphene sheets and amorphous carbon, that lead to irreversible capacities and high rate performances. 28 Below 0.5 V vs. Li + /Li, it is clear that no electrochemical faradaic reaction takes place in Ti 2 C since most of the capacity measured can be attributed to the carbon black. Between 0.05 V and 2.7 V vs. Li + /Li, the electrode capacity changes from 172 mAh • g -1 to 160 mAh • g -1 . Since the difference comes from the carbon black, the capacity between 0.05 V and 0.5 V can be neglected. It follows that the reduction cutoff potential was set to 0.5 V vs. Li + /Li.

The XRD patterns collected during a relaxation period every 0.2 Li + per unit formulae of Ti 2 C are reported in Figure 2. Before cycling the pattern is characteristic of a multiphasic system including wellcrystallized un-reacted Ti 2 AlC and Ti 3 AlC 2 together with a set of broad diffraction peaks that corresponds to exfoliated MXene based compound. During lithiation, the Bragg peaks position characteristic of un-reacted Ti 2 AlC and Ti 3 AlC 2 do not change, confirming that they are not electrochemically active. These peaks however, are useful as internal references for the determination of the positions of other peaks.

An examination of the XRD pattern shows that during the electrochemical process, no extra peaks appear; instead a reversible shift of the broad diffraction peaks is observed. This result is consistent with an insertion process occurring in the exfoliated MXene phases. A careful examination of the XRD pattern collected during Li insertion shows that the overall behavior can be divided into two domains. In the 0 < y < 0.4 domain the main broad peak located at 11 • is not affected, while, for example, the one located at 25 • is shifted to lower diffraction angles. In the 0.6 < y < 1.4 domain, the opposite is observed. The same observations can be made for other diffraction peaks indicating that the broad diffraction peaks probably correspond to the sum of two independent sets. Those two different sets may be related to different termination groups, e.g. O and F, involving different d hkl lengths. The relative intensity of the two sets indicates that Ti 2 C is the main phase; the minor phase has not yet been identified. As the domains are clearly separated, only the one corresponding to the electrochemical activity of Ti 2 C is detailed.

During lithiation, the progressive downshift of (002) peak from 2θ = 11.4 • to 2θ = 9.4 • corresponds to an increase of c lattice parameter from 15.46 Å to 18.72 Å for Ti 2 C (17.4% expansion). During delithiation the c parameter decreases down to 17.96 Å, a value higher than the value obtained during lithiation for the same Li content (Fig. 2c). This can be related to the large irreversibility observed in the 1 st cycle and interpreted by the trapping of Li + ions between the layers of MXene concomitant with SEI formation. Based on the XRD study it is reasonable to reach the following important conclusion: the charge storage in this Ti 2 C-based material (plus an unidentified minor phase) is due to the intercalation of Li + ions and not due to a conversion reaction. This comment notwithstanding, more research is needed and ongoing to further understand the details of the lithation and delithiation mechanisms.

Figure 3 shows the dependencies of the galvanostatic charge and discharge capacities vs. cycle number between 0.5 V and 2.7 V vs. Li + /Li as a function of different C-rates. As shown in the inset of Fig. 3, and consistent with the aforementioned conclusion, the material does not exhibit a flat faradaic plateau typical of a two-phase insertion process, meaning that lithiation and delithiation take place over a large range of potentials. This is consistent with the broad peaks observed in the diffractograms. Although the gradually inclined voltage profile might be considered as a drawback if this material is used as an anode in LIBs, it is worth investigating its behavior in an asymmetric EDLC as done here.

Another current drawback with the use of this material as an anode in LIB is the 40% irreversible capacity on the first cycle. 24 The latter is believed to be due to the SEI formation, in addition to, possibly, the partial irreversible trapping of Li + ions in the Ti 2 C structure, as well as, some remaining surface functional groups such as hydroxides or fluorides present in the interlayer spaces of the 2-D structure. At potentials below 1.0 V vs. Li + /Li, SEI layers form. [START_REF]Carbons for Electrochemical Energy Storage and Conversion Systems[END_REF][START_REF] Schalkwijk | Advances in Lithium-ion batteries[END_REF] It is known that the SEI layer formation on carbonaceous anodes is of the highest importance in stabilizing the lithiation/delithiation process despite the Li + ions consumption by the electrolyte and there is no reason to believe that is not true here.

Electrochemical kinetics study of Li insertion in Ti 2 C.-Cyclic voltammetry can be used to accurately distinguish between different insertion processes. The current, I (A), dependence on the potential scan rate, v (V • s -1 ), are related according to:

I = av b (3)
where a and b are adjustable parameters. In Eq. ( 3), the b-coefficient is related to the rate limiting step of the overall kinetics, i.e. whether the lithiation and delithiation reactions are diffusion-limited (b = 0.5) or not (b = 1). [START_REF] Bard | Electrochemical Methods: fundamentals and applications[END_REF] Figure 4a shows CV curves from 0.05 mV • s -1 to 5 mV • s -1 . As noted above, two main lithiation and delithiation peaks are observed at 1.8 V and 2.0 V vs. Li + /Li, labeled as peaks A and B, respectively. When the current dependencies are plotted vs. scan rate on a log scale (Fig. 4b), it is obvious that the relationship is linear with a slope of unity. In other words, b = 1, which, in turn, implies that Li + diffusion is not the limiting step of the overall electrochemical reaction kinetics.

When the capacity is plotted as a function of scan rate, again on a log-log plot (Fig. 4c) it is clear that the maximum capacity of 160 mAh • g -1 occurs at a scan rate of 0.05 mV • s -1 and is more or less constant up to 0.2 mV • s -1 .

The inset in Fig. 4a shows the emergence of additional reductive and oxidative peaks at around 1.0 V at low scan rates, which can be attributed to different intercalation stages of Li + ions in the Ti 2 C structure. Note that the signal at 5 mV • s -1 starts to be distorted due to ohmic losses, which limits the maximum scan rate to 5 mV • s -1 .

Cavity microelectrode, CME.-CME is a convenient and efficient tool for qualitative kinetic analysis of various electroactive materials. [START_REF] Portet | [END_REF][33][34][35] The few micrograms of active material needed for measurements lead to low current and hence to small ohmic drops. This in turn allows electrochemical measurements over a wide range of scan rates. However, because the amount of material inserted inside the microcavity is not accurately known, the analysis from here on is qualitative.

Figure 5a shows cyclic voltammetry curves, obtained with a CME from 5 mV • s -1 to 100 mV • s -1 . Note that the potential window was extended to 4.0 V vs. Li + /Li to observe the complete oxidation reaction. The Ti 2 C electrochemical signature is observed with the ∆ peaks A and B at 1.8 V and 2.0 V vs. Li + /Li during reduction and oxidation, respectively with a diffusion current plateau at potentials below 1.2 V vs. Li + /Li during cathodic scans. As expected, the peaks broaden and the overpotential increases with increasing scan rates due to polarization effects (kinetic and ohmic).

When log I is plotted vs the log v (Fig. 5b), a change in slope is observed around 5 mV • s -1 for both anodic and cathodic scans. The calculated slopes, or b-coefficients, are listed in Table I. Confirming the conclusion reached above, it is again clear that the lithiation/delithiation of Ti 2 C is not mass transport limited at rates below 5 mV • s -1 . However, for v values higher than 5 mV • s -1 , the bcoefficient decreases to around 0.7, implying that at the higher scan rates, mass transport in the bulk of the Ti 2 C structure becomes important.

When the charge and discharge capacities are plotted as a function of log v (Fig. 5c), the maximum capacity is reached at scan rates below 2 mV • s -1 , before decreasing at higher rates. The drop in capacity corresponds to the same scan rates at which the b-coefficients change. Those observations are consistent with the values obtained from the two-electrode cell measurements, even though the shapes of the electrochemical signals are different.

These results can be treated to a more sophisticated analysis. The total specific current is assumed to be the sum of two contributions: one part deriving from a diffusion-controlled process (I proportional to v 1/2 ), while the other from a capacitive-like charge storage process (I proportional to v). This assumption leads to:

I p (V ) = k 1 • v + k 2 • v 1/2 (4)
which can be rearranged to read:

I p (V ) • v -1/2 = k 1 • v 1/2 + k 2 (5)
where Ip(V) is the current density at a given potential, k 1 and k 2 are coefficients, respectively, in μA • V -1 • s and μA • V -1/2 • s 1/2 , related to the relative contributions of the diffusion-controlled and capacitive-like parts to the total current as described elsewhere. 36 Plotting Ip(V) • v -1/2 vs. v 1/2 (Fig. 6) it is possible to determine both Table I. b-coefficients obtained from Fig. 4 for anodic and cathodic peaks.

v < 0.5 mV • s -1 v > 0.5 mV • s -1
Anodic peak (peak B) 0.95 0.65 Cathodic peak (peak A) 0.90 0.68 k 1 and k 2 from the slopes and the y-intercepts, respectively. Note that both k 1 and k 2 are rate-dependent. The non-diffusion limited contribution to the total current can be calculated assuming:

%k 1 (v) = k 1 • v • (k 1 • v + k 2 • v 1/2 ) -1 (6)
The inset in Fig. 6, plots the k 1 contribution to the total current, as a function of log v. As expected, the trend is the same as the Q vs. log v plot shown in Fig. 5c, viz. showing a decrease in the k 1 contribution with increasing scan rates. This highlights once again that at the higher rates, mass transport limitation become important. Note that this contribution does not decrease to zero at high scan rates. Instead the%k 1 reaches a steady value around 50% for both cathodic and anodic peaks. This phenomenon has already been observed in Ref. 27 and is attributed to contribution to the total current of the fast Li + ion redox reactions of the active sites located on the surface of the particles. In parallel, the capacity loss in the corresponding rate range reveals that these active sites become less accessible with increasing rates. Likely, the active surface layer becomes thinner. At Figure 6. plot of Ip.v -1/2 vs. v 1/2 from 5 mV • s -1 to 100 mV • s -1 . Inset: contribution of the non-diffusion limited current to the total current%k 1 as a function of the potential scan rate. andb) constant current charge and discharge curves at 300 mA • g -1 , 500 mA • g -1 , and at 1 A • g -1 between 3 V and 4 V vs. Li + /Li. rates below 5 mV • s -1 , the%k 1 values are 100% and 85% for anodic and cathodic processes, respectively (inset in Fig. 6). The differences in the rate limiting steps arise from faster electrochemical kinetics during oxidation than during reduction. Altogether, for both coin cells and the CME measurements, the Ti 2 C material presents similar ratedependence as a capacitive electrode up to 2-5 mV • s -1 , meaning that no diffusion limitations are observed, allows fast Li + ions transport to the active sites and gives high rate capability.

Positive electrode: activated carbon YP17.-The capacities of activated carbon materials depend on the potential windows, V, in which they are cycled, according to:

C(F/g) = Q (mAh/g) * 3600/( V * 1000) (7)
Figure 7 shows cyclic voltammetry and galvanostatic charge/discharge curves of YP17 in LP30 in different voltage ranges. It shows typical capacitive behavior and a capacitance of 100 F • g -1 at 300 mA • g -1 is obtained. As expected, the capacity increases as V increases. However, the electrolyte oxidative stability limits the extension. Above 4.3 V vs. Li, the electrolyte starts oxidizing, which could result in a fade in cell capacity upon cycling. A capacity of 27 mAh • g -1 for YP17 between 3 V and 4 V vs. Li was calculated. Ti 2 C-YP17 asymmetric cell.-For the electrochemical study of an asymmetric cell, the positive and negative electrode masses must be balanced. The balance is based on the capacities of both materials. Here the Ti 2 C negative electrode capacity at 1C rate was chosen. The reason we focused on the higher rates, is to render them relevant for the performance of an asymmetric supercapacitor. Lower rates were thus not considered.

Since the capacity of the Ti 2 C during the first cycle is 200 mAh • g -1 -including the 90 mAh • g -1 of irreversible capacity -between 3 V and 0.5 V vs. Li at 1C rate, and the YP17 activated carbon capacity between 3.0 V and 4.0 V vs. Li is 27 mAh • g -1 , the mass ratio of activated carbon to Ti 2 C was fixed to 7.4. This ratio ensures that during the first charge, the activated carbon electrode and the negative electrode would cycle between 3.0 V and 4.0 V vs. Li and between 3.0 V and 0.5 V vs. Li + /Li, respectively, giving a maximal cell voltage of 3.5 V. Taking into account that the irreversible capacity during the first cycle is 45% of the first charge capacity, the cell voltage discharge cutoff was fixed to 1.0 V. It is worth mentioning here that for these experiments the cell was cycled from 1.0 V to 3.5 V, and the positive and negative electrodes' potentials were recorded vs. the Li reference.

Galvanostatic charge-discharge cycling curves of the asymmetric cell are shown in Fig. 8, where the specific capacity is based on the Ti 2 C weight. The first charge/discharge cycle (dotted line) shows capacity for SEI formation at the surface of the Ti 2 C particles. The subsequent cycles show slight potential drift related to the irreversible capacity of Ti 2 C, but stabilizes at 110 mAh • g -1 after 20 cycles. Considering the mass of both electrodes, the specific capacity of the device would be 14.2 mAh • g -1 .

In order to characterize its power capability, the cell was then cycled from rates of 2C to 20C. As expected, the voltage window of the negative electrode increased with increased current densities. Indeed, a 20C rate corresponds to 5 A • g -1 of Ti 2 C, and 680 mA • g -1 of activated carbon, the latter becoming much more overcapacitive at such high current densities. As a result, the negative electrode operating voltage window increases from 0.5 V-2.5 V to 0.3 V-2.7 V vs. Li, as shown in the inset of Figure 8. The cell was subsequently charged and discharged at a rate of 10C, with no relaxation time between cycles. Figure 9 shows capacity vs. cycle number of the cell. The initial capacity was 70 mAh • g -1 based on the weight of Ti 2 C, in accordance with previous values. After 1000 cycles, the capacity loss was ∼15% at 60 mAh • g -1 . The potential drift upon cycling was also found to be important, as it decreased to 0.1 V vs. Li at the end of charge after 1000 cycles, due to the intrinsic capacity loss of the material. Considering a maximum cell voltage of 3.5 V, the stored energy density was measured at to be 15 Wh • kg -1 of active materials (positive and negative) at 600 W • kg -1 after 1000 cycles. Given that the maximum energy and power densities are 50 Wh • kg -1 and 190 kW • kg -1 , respectively, it follows that the Ti 2 Cbased material explored herein is suitable for use as an electrode in high energy-density supercapacitors.

Conclusions

Electrochemical lithiation and delithiation of Ti 2 C were studied by in-situ XRD and electrochemical measurements. The c-parameter expansion confirmed that the Li + ions are inserted between the Ti 2 C layers. The material exhibited a capacity of 65 mAh • g -1 at 10C rate despite a sizable irreversible capacity on the first cycle. Electrochemical kinetic analysis using both composite electrodes and a cavity microelectrode showed that no diffusion limitation at rates below 5 mV • s -1 . This implies that up to that rate the Li + ions have easy access to the electroactive sites.

An asymmetric electrochemical cell was assembled and exhibited a maximum energy density of 30 Wh • kg -1 at 930 W.kg -1 of active materials for 1000 cycles. These results highlight the potential of using exfoliated MAX phases in devices in which high stable rates are required.

Figure 1 .

 1 Figure 1. Cyclic voltammetry curves at 0.1 mV • s -1 of Ti 2 C, and carbon black.

θ °Figure 2 .

 2 a) in-situ XRD of Ti 2 C collected each 0.2 Li + at a C/10 rate for charge (blue curves) and discharge (orange curves), *: unreacted Ti 2 AlC, •: unreacted Ti 3 AlC 2 , b) corresponding constant current charge/discharge curve, and c) c parameter calculated from (002) peak shift during charge and discharge.

Figure 3 .

 3 Figure 3. charge and discharge specific capacities of Ti 2 C from C/10 to 10C rate. Inset shows the capacity loss between the 1 st and 2 nd galvanostatic charges.

Figure 4 .

 4 Figure 4. a) cyclic voltammetry curves from 0.05 mV • s -1 to 0.2 mV • s -1 (inset), and from 0.5 mV • s -1 to 5 mV • s -1 , b) plot of peak current (in A.g -1 ) as a function of the potential scan rate, and c) specific capacity as a function of scan rate.

Figure 5 .

 5 Figure 5. a) cyclic voltammetry curves obtained with a CME from 5 mV • s -1 to 100 mV • s -1 , b) plot of peak current (in mA) as a function of the potential scan rate, and c) capacity as function of the potential scan rate.

Figure 7 .

 7 Figure 7. a) cyclic voltammetry curves at 5 mV • s -1 of a YP17 activated carbon in 1M LiPF 6 EC:DMC (1:1) electrolyte with different voltage window,and b) constant current charge and discharge curves at 300 mA • g -1 , 500 mA • g -1 , and at 1 A • g -1 between 3 V and 4 V vs. Li + /Li.

Figure 8 .

 8 Figure 8. three-electrode constant current charge and discharge curves of a Ti 2 C/YP17 asymmetric cell cycled between 3.5 V and 1.0 V from 1C to 20C rate. Specific capacity is represented based on the Ti 2 C electrode. Inset: corresponding curves of the Ti 2 C electrode.

Figure 9 .

 9 Figure 9. charge and discharge specific capacity vs. cycle number of a Ti 2 C/YP17 asymmetric cell at 10C charge and 10C discharge with no relaxation between cycles. The capacity is based on the Ti 2 C electrode. Inset: corresponding curves of Ti 2 C electrode at different cycles showing the potential drift of the negative electrode.

Acknowledgments

The authors thank the French network "CNRS MicroElectrode à Cavité" (MEC) for the kind supply of the CME and fruitful formation. J.C. was supported by a French Délégation Générale pour l'Armement grant. This work has been done within the French network on electrochemical energy storage (RS2E). P.S thanks the FUI ("Récupéner" Project). The authors also thank Dr. M. Dolle (CEMES, UPR 8011) for the kind supply of the in-situ XRD cell. The work at Drexel University was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract 6951370 under the Batteries for Advanced Transportation Technologies (BATT) Program.