A boron doped synthetic diamond material with at least a portion of an exposed surface layer comprising boron doped synthetic diamond material having a boron content in a range 1 x 10 20 boron atoms cm -3 to 7 x 10 21 boron atoms cm -3, characterized in that the boron doped synthetic diamond material also comprises no sp 2 carbon as detectable by Raman spectroscopy when performed at room temperature with a Raman microscope using an excitation wavelength of 514.5nm and wherein the boron doped synthetic diamond material has the following characteristics: a solvent window meeting one or both of the following criteria as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode in a solution containing only deionised water and 0.1 M KNO 3 as a supporting electrolyte at pH 6: the solvent window extends over a potential range of at least 4.1 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 38 mA cm -2; and the solvent window extends over a potential range of at least 3.3 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 0.4 mA cm -2; a peak-to-peak separation ΔE p (for a macroelectrode) or a quartile potential ΔE 3/4-1/4 (for a microelectrode) of no more than 70 mV as measured by sweeping a potential of the boron doped synthetic diamond material at a rate of 100 mV s -1 with respect to a saturated calomel reference electrode in a solution containing only deionised water, 0.1 M KNO 3 supporting electrolyte, and 1 mM of FcTMA + or Ru(NH 3) 6 3+ at pH 6; and a capacitance of no more than 10 μF cm -2 as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode between 70 mV and -70 mV in a solution containing only deionised water and 0.1 M KNO 3 supporting electrolyte at pH 6, measuring resultant current, subtracting a current value at 0 V when sweeping towards negative potentials from a current value at 0 V when sweeping towards positive potentials, dividing the subtracted current value by 2, and then dividing the result by an area (cm 2) of the boron doped synthetic diamond material and by a rate at which the potential is swept (Vs -1) to give a value for capacitance in F cm -2. a solvent window meeting one or both of the following criteria as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode in a solution containing only deionised water and 0.1 M KNO 3 as a supporting electrolyte at pH 6: the solvent window extends over a potential range of at least 4.1 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 38 mA cm -2; and the solvent window extends over a potential range of at least 3.3 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 0.4 mA cm -2; the solvent window extends over a potential range of at least 4.1 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 38 mA cm -2; and the solvent window extends over a potential range of at least 3.3 V wherein end points of the potential range for the solvent window are defined when anodic and cathodic current density measured at the boron doped synthetic diamond material reaches 0.4 mA cm -2; a peak-to-peak separation ΔE p (for a macroelectrode) or a quartile potential ΔE 3/4-1/4 (for a microelectrode) of no more than 70 mV as measured by sweeping a potential of the boron doped synthetic diamond material at a rate of 100 mV s -1 with respect to a saturated calomel reference electrode in a solution containing only deionised water, 0.1 M KNO 3 supporting electrolyte, and 1 mM of FcTMA + or Ru(NH 3) 6 3+ at pH 6; and a capacitance of no more than 10 μF cm -2 as measured by sweeping a potential of the boron doped synthetic diamond material with respect to a saturated calomel reference electrode between 70 mV and -70 mV in a solution containing only deionised water and 0.1 M KNO 3 supporting electrolyte at pH 6, measuring resultant current, subtracting a current value at 0 V when sweeping towards negative potentials from a current value at 0 V when sweeping towards positive potentials, dividing the subtracted current value by 2, and then dividing the result by an area (cm 2) of the boron doped synthetic diamond material and by a rate at which the potential is swept (Vs -1) to give a value for capacitance in F cm -2.