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@INPROCEEDINGS{Schatz:1008981,
      author       = {Schatz, Michael and Kochs, Johannes Florian and Jovanovic,
                      Sven and Eichel, Rüdiger-A. and Granwehr, Josef},
      title        = {{I}n {O}perando {M}agnetic {R}esonance {I}maging {R}eveals
                      {L}ocal p{H} and {I}on {C}oncentration {P}rofiles {D}uring
                      {C}u-{C}atalyzed {CO}2 {E}lectroreduction},
      reportid     = {FZJ-2023-02565},
      year         = {2023},
      abstract     = {Cu-based electrode materials for the CO2 reduction reaction
                      (CO2RR) are of special interest in current research, as Cu
                      is the only elemental metal that catalyzes the
                      electrochemical reduction of CO2 towards short-chain
                      hydrocarbons [1]. This provides the possibility of
                      converting the greenhouse gas CO2 into e-fuels or base
                      chemicals while simultaneously acting as storage for
                      fluctuating renewable energy sources. However, tuning the
                      selectivity in CO2RR towards desired products is still a
                      challenging task. Not only are the reaction mechanisms not
                      entirely understood, but also the complex interactions of
                      local conditions in electrode proximity, i.e. local pH,
                      buffer capacity or cation effects, are also subject of
                      ongoing discussions [2].The common use of diluted KHCO3
                      electrolyte in CO2RR demonstrates how this interplay can be
                      employed to one’s advantage: Low buffer capacity leads to
                      high pH in electrode proximity, which in turn favors C2+
                      product reaction pathways over undesired hydrogen formation
                      [3]. Moreover, potassium as counterion has a more favorable
                      effect on pH buffering than smaller cations and has been
                      proven to accelerate CO2 activation [2]. For the
                      investigation of local pH in electrochemical cells several
                      in operando methods have been developed, e.g. optical
                      methods like infrared or Raman spectroscopy or scanning
                      probe microscopy techniques [4]. In our previous work, we
                      presented a new method for spectroelectrochemical pH
                      determination by in operando 13C Nuclear Magnetic Resonance
                      (NMR), where we operated an electrochemical cell for CO2RR
                      in a standard 5 mm NMR tube and evaluated the local pH using
                      the CO2(aq)/HCO3-/CO32- equilibrium [5].The present work
                      expands this method with magnetic resonance imaging (MRI)
                      techniques to spatially resolve local pH measurements. The
                      working electrode was placed perpendicular to magnetic field
                      gradients used for MRI (cf. Figure 1a) to obtain NMR spectra
                      as a function of distance to electrode. Phase-encoded
                      chemical shift imaging using spin echoes was applied to
                      subdivide the sensitive volume into slices, each of which
                      contributes a single spectrum. The results of 13C chemical
                      shift imaging of 0.5 M KHCO3 electrolyte are depicted in
                      Figure 1b and 1c. The chemical shift of the coalesced
                      HCO3-/CO32- resonance serves as pH sensor, as it is highly
                      pH dependent [6]. Increasing chemical shifts near the
                      electrode were assigned to an increase in pH value from 7.2
                      at the beginning up to a pH of ca. 9 after 3 h of
                      electrolysis at constant current of 2.1 mA/cm². The CO2
                      concentration decreased below the detection limit shortly
                      after start of the experiment due to the pH shift. Still,
                      typical CO2RR products were detected by ex situ 1H NMR using
                      water suppression, confirming that CO2 was reduced from
                      equilibrium with HCO3- [7]. When investigating NaHCO3 as
                      electrolyte in a broadband NMR probe, the 23Na and 13C
                      spectra could be acquired alternatingly. Using this method,
                      local pH profiles can be directly correlated with
                      concentration profiles of dissolved species such as CO2(aq)
                      or the HCO3-/CO32- anions as well as Na+ cations.In
                      conclusion, this study shows the evolution of pH profiles
                      over time and how this local pH effect depends on the
                      applied potential. Furthermore, this interaction will be put
                      into relation to cation concentration profiles as well as ex
                      situ product analysis to give new insights into the
                      electrolyte chemistry of Cu-catalyzed CO2RR.References[1] Y.
                      Hori, Electrochemical CO2 Reduction on Metal Electrodes, in
                      Modern Aspects of Electrochemistry, edited by C. G. Vayenas,
                      R. E. White, M. E. Gamboa-Aldeco (Springer-Verlag, s.l.,
                      2008), 89[2] B. Deng, M. Huang, X. Zhao, S. Mou, F. Dong,
                      ACS Catal. 12, 331 (2022)[3] T. Burdyny, W. A. Smith, Energy
                      Environ. Sci. 12, 1442 (2019)[4] M. C. Monteiro, M. T.
                      Koper, Current Opinion in Electrochemistry 25, 100649
                      (2021)[5] M. Schatz, S. Jovanovic, R.-A. Eichel, J.
                      Granwehr, Sci Rep 12 (2022)[6] S. Moret, P. J. Dyson, G.
                      Laurenczy, Dalton Trans. 42, 4353 (2013)[7] M. Dunwell, Q.
                      Lu, J. M. Heyes, J. Rosen, J. G. Chen, Y. Yan, F. Jiao, B.
                      Xu, J. Am. Chem. Soc. 139, 3774 (2017)},
      month         = {May},
      date          = {2023-05-28},
      organization  = {243rd ECS Meeting, Boston (USA), 28
                       May 2023 - 30 Jun 2023},
      subtyp        = {After Call},
      cin          = {IEK-9},
      cid          = {I:(DE-Juel1)IEK-9-20110218},
      pnm          = {1232 - Power-based Fuels and Chemicals (POF4-123) / DFG
                      project 390919832 - EXC 2186: Das Fuel Science Center –
                      Adaptive Umwandlungssysteme für erneuerbare Energie- und
                      Kohlenstoffquellen (390919832) / HITEC - Helmholtz
                      Interdisciplinary Doctoral Training in Energy and Climate
                      Research (HITEC) (HITEC-20170406)},
      pid          = {G:(DE-HGF)POF4-1232 / G:(GEPRIS)390919832 /
                      G:(DE-Juel1)HITEC-20170406},
      typ          = {PUB:(DE-HGF)6},
      url          = {https://juser.fz-juelich.de/record/1008981},
}