documentclass[preview]{standalone}
usepackage{tikz}
usepackage{pgfplots}
usepackage{graphicx, float}
usepackage{subcaption}

deffigone{%
    begin{tikzpicture}
        begin{axis}[%
             width = 1.2textwidth,
             height = textwidth,
             grid = both,
             xtick distance = 1,
             ymin = 0,
             ymax = 2,
             ytick distance = 0.4,
             xlabel = $t/tau$,
             ylabel = $V/V$
            ]
            addplot[red, ultra thick]{exp(x)};
        end{axis}
    end{tikzpicture}
    }

deffigtwo{%
    begin{tikzpicture}
        begin{axis}[%
             width = 1.2textwidth,
             height = textwidth,
             grid = both,
             xtick distance = 1,
             ymin = 0,
             ymax = 2,
             ytick distance = 0.4,
             yticklabels={,,},
             xlabel = $t/tau$
            ]
            addplot[blue, ultra thick]{exp(2*x)};
        end{axis}
    end{tikzpicture}
    }


begin{document}

begin{figure}[ht]
    centering
    begin{subfigure}[t]{0.45textwidth}
        figone
    end{subfigure}
    hspace{5.5mm}
    begin{subfigure}[t]{0.45textwidth}
        figtwo
    end{subfigure}
    caption{Caption}
    label{fig:my_label}
end{figure}





end{document}