The reason you get a surface that's all black is that the grid is too fine, so all you see are the black lines between grid cells. To fix it you can use a coarser grid, if that's an option, or you can turn the grid lines off by specifying 'EdgeColor','none' when the surface is created (or after it's created).
Here's your code but with 'EdgeColor','none' in second call to surf. (The first surface looks ok, but you could turn the grid lines off for that one as well, of course.)
t = readmatrix('time.xlsx');
c_so4_aem = readmatrix('concentration.xlsx');
d_aem = 0.562*10^(-3);
K_m = 25;
hours = 2.2;
x = linspace(0,d_aem,K_m)';
y = t;
z = c_so4_aem;
figure(9)
surf(x,y,z);
ylabel('Time (s)')
zlabel('Concentration (mol m^-^3)')
title('Concentration of SO_4 in AEM (mol m^-^3)')
[Xq,Yq] = meshgrid((0:d_aem/K_m:d_aem),0:10:3600*hours);
Zq = interp2(x,y,z,Xq,Yq,'linear');
surf(Xq,Yq,Zq,'EdgeColor','none');
colorbar
xlabel('Membrane layer (m)')
ylabel('Time (s)')
zlabel('Concentration (mol m^-^3)')
figure(10)
[C,h] = contourf(Xq,Yq,Zq,15);
h.LevelList=round(h.LevelList,0);
clabel(C,h);
colorbar
colormap('Jet')
xlabel('Membrane layer (m)')
ylabel('Time (s)')
title('Concentration of SO_4 in AEM (mol m^-^3)')
